structural analysis of inversion features of the barents sea

Ruhr-Universität Bochum

Institute of Geology, Mineralogy and Geophysics

Structural analysis of inversion features of the Barents Sea

Doctoral thesis

by

Muhammad Armaghan Faisal Miraj

Bochum, 2017


Institute of Geology, Mineralogy and Geophysics

Structural analysis of inversion features of the Barents Sea

Doctoral thesis by Muhammad Armaghan Faisal Miraj

Faculty of Geoscience


Prof. Dr. Christophe Pascal

Prof. Dr. A. Immenhauser

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Acknowledgement

I owe a debt of gratitude to my supervisor Prof. Christophe Pascal for his supervision

and valuable discussions. Throughout the research work he provided me sound

advices, good teaching and constructive ideas.

I am greatly thankful to Prof. Adrian Immernhauser and Prof. Rebecca Harrington for

their reviewes.

I would also like to thank the Department of Geosciences, Univesity of Oslo, Norway

for providing me seismic data. In particular, many thanks to Prof. Roy Helge

Gabrielsen and Prof. Jan Inge Faleide for valuable discussions and encouragements.

Special thanks to DAAD (Deutscher Akademischer Austauschdienst) and HEC

(Heigher Education Commision) Pakistan for funding.

Thanks to my friends and Colleagues especially Richard, Caroline, Nicole, Henrick,

Kathi and Sara for their moral support.

It is difficult to find adequate words to express my many thanks and tremendous

gratitude to my parents and specially my wife who rode side by side with me in this

tiresome journey of completing my studies at Ruhr University Bochum.

Finally, I would like to thank all the teaching and administration staff of the Institut of

GMG, RUB.

Muhammad Armaghan Faisal

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Abstract

Basin inversion is a common phenomenon worldwide. Inversion/reactivation of normal fault

systems within a basin has a significant impact on its final structure and modifies reservoirs

and fluid paths. The study of inversion is prone to shed lights on second or third order tectonic

phenomena that have escaped the overall framework of plate tectonics theory until now. In

addition, the understanding of inversion structures is of prime importance for e.g. oil

exploration. However, in most cases the causes and mechanisms related to inversion remain

enigmatic. The present work aims to address these issues by means of studying the particular

case of the western Barents Sea.

Inversion structures including folds, reverse faults are observed along the Bjørnøyrenna Fault

Complex and the Ringvassøy–Loppa Fault Complex in the western Barents Sea, although

both fault complexes are extensional in origin and developed in mid-Jurassic to Early

Cretaceous. Subsidence along the fault complexes was interrupted in Early Cretaceous

(Valanginian to early Barremian) because of syn-rift localized tectonic inversion, itself related

to the uplift of the Loppa High. The Early Cretaceous inversion caused dextral transpression

along the boundary faults adjacent to the Loppa High. The second phase of inversion is

interpreted to be Late Cretaceous (mid-Cenomanian) in age, coeval to the deposition of the

Kolmule Formation in the Bjørnøyrenna Fault Complex and the Ringvassøy–Loppa Fault

Complex. The later phase of compression is of regional significance and related to NW-SE

directed far field stresses in Late Cretaceous which caused head-on inversion in the study

area.

The results of structural restoration of Cretaceous inversion events in the Bjørnøyrenna Fault

Complex, western Barents Shelf, are presented. The aim of the study is to identify the

structures related to inversion (anticlines, reverse faults) by means of identifying and locating

null point positions. 2D MOVETM (structural modeling and analysis software by Midland

Valley Exploration Ltd) is used to restore three key seismic profiles located in the central and

northern segments of the Bjørnøyrenna Fault Complex. Key profiles 1 and 2 reveal null point

positions at the base of the Cretaceous (Hekkingen Formation). Null point positions show

progressive compressional inversion of syn-rift Early Cretaceous deposits (Knurr Formation).

Below and above null points the geometries of the restored faults show normal and reverse

faulting respectively. The results of the restored key profiles 1 and 2 confirm reverse faulting

at the Lower Cretaceous triggered by inversion of the study area. The restored sections also

show positive inversion features associated with folding of the hangingwall of the base of the

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Upper Cretaceous (Kolmule Formation). The reconstruction of the amount of eroded material

on the footwall block also suggests reverse faulting of the base of the Upper Cretaceous.

In key profile 3 the footwall block is eroded up to the base of the Upper Cretaceous (Kolmule

Formation) due to the uplift of the Loppa High. The corresponding restored section shows a

compressional anticline associated with both Early and Late Cretaceous inversion events.

The results of numerical modeling of inversion of faults induced by Late Triassic to Miocene

tectonic stress fields in the western Barents Sea are presented. The aim is to test the potential

for fault reactivation under such circumstances. A finite-element numerical code (ANSYS™)

is used to simulate stress and fault slip patterns based on four 2-D thin plate modeling setups.

Following previous works, four major regional inversion events are assumed: Late Triassic to

Early Jurassic (E-W contraction, Model 1), Late Cretaceous (NW-SE contraction, Model 2),

dextral megashear plate margin in Early Eocene (Model 3) and NW-SE Atlantic ridge push

starting in Miocene (Model 4).

Model 1 confirms the potential for compressional conditions in the western Barents Sea and,

hence, contractional reactivation of master fault systems like the Thor Iversen Fault and

Troms-Finnmark Fault complexes. Compressive regimes in the Måsøy and Hoop fault

complexes favor the development of inversion structures in the study area during Late

Triassic to Early Jurassic.

Simulated stress patterns in Model 2 (inducing a NW-SE compressional stress) suggest a

clockwise stress rotation in the Bjørnøyrenna Fault Complex and the Ringvassøy – Loppa

Fault Complex and pronounced stress deflections in the Asterias Fault Complex. These

modeled stress deflections support tectonic inversion during Late Cretaceous in the

corresponding fault complexes. The analyses suggest that significant strike-slip is to be

expected to have occurred along some segments.

The results obtained in Model 3 suggest that the interior of the western Barents Sea was not

severely influenced by Early Eocene North Atlantic opening/shearing. The results suggest that

Early Eocene sea floor spreading caused stress partitioning along the Senja Fracture Zone.

The observed inversion structures in previous studies may be related to local effects. The

results of Model 4 appear to be in agreement with the observed NW-SE contraction, expressed

as folds and reverse faults in the study area (e.g. Ringvassøy – Loppa, Bjørnøyrenna,

Leirdjupt and Asterias fault complexes). The results of the four models suggest the presence

of compressive structures along the major fault complexes of the western Barents Sea during

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Late Triassic to Miocene but do not favor the development of inversion structures during

Eocene.

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Contents

1. Introduction ............................................................................................................................ 1

1.1 The western Barents Shelf ................................................................................................ 1

1.2 Aims and thesis statement ................................................................................................ 2

1.3 Thesis outline and framework .......................................................................................... 4

2. Basin inversion tectonics........................................................................................................ 5

2.1 Inversion tectonics ............................................................................................................ 5

2.2 Geometry and kinematics of basin inversion ................................................................... 6

2.3 Driving forces ................................................................................................................... 9

2.3.1 Compression-related inversion (examples)................................................................ 9

2.3.2 Strike slip related inversion...................................................................................... 11

2.3.3 Transpressive inversion ........................................................................................... 12

2.4 Basin inversion in the western Barents Sea.................................................................... 13

2.4.1 Regional geological setting...................................................................................... 13

2.4.2. Inversion phases and evidences in the western Barents Sea ................................... 20

3. Methodology ........................................................................................................................ 31

3.1 Numerical modeling ....................................................................................................... 31

3.1.1 Finite Element Method ............................................................................................ 31

3.1.2 ANSYS™................................................................................................................. 31

3.2 Seismic reflection survey................................................................................................ 38

3.2.1 Kingdom Suit 8.82.................................................................................................... 39

3.3 Structural restoration3 ..................................................................................................... 40

3.3.1 MOVETM .................................................................................................................. 41

4. Cretaceous inversion of the western Barents Shelf: integrated seismic interpretation of theBjørnøyrenna and the Ringvassøy–Loppa fault complexes ..................................................... 46

4.1 Introduction .................................................................................................................... 46

4.2 Geological setting ........................................................................................................... 47

4.3 Data and methodology.................................................................................................... 51

4.4 Seismic interpretation ..................................................................................................... 52

4.4.1 Snadd Formation (Upper Triassic)........................................................................... 54

4.4.2 Fruholmen Formation (Base Jurassic) ..................................................................... 54

4.4.3 Tubåen Formation (Lower Jurassic) ........................................................................ 54

4.4.4 Nordmela Formation (Upper Jurassic)..................................................................... 55

4.4.5 Fuglen Formation (upper Middle Jurassic).............................................................. 55

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4.4.6 Hekkingen Formation (Base Cretaceous) ................................................................ 56

4.4.7 Knurr Formation (Lower Cretaceous) ..................................................................... 56

4.4.8 Kolmule Formation (Base Upper Cretaceous)......................................................... 57

4.5 Results and discussions .................................................................................................. 57

4.5.1 Early Cretaceous inversion ...................................................................................... 57

4.5.2 Late Cretaceous inversion........................................................................................ 62

4.6 Conclusions .................................................................................................................... 65

5. Structural restoration of Cretaceous inversion events in the Bjørnøyrenna Fault Complex,western Barents Shelf. .............................................................................................................. 66

5.1 Introduction ........................................................................................................................ 66

5.2 Geometry and structural evolution of the Bjørnøyrenna Fault Complex........................... 67

5.3 Data and Methodology ....................................................................................................... 70

5.4 Structural restorations ........................................................................................................ 72

5.5 Results ................................................................................................................................ 73

5.5.1 Restoration of key profile 1 ......................................................................................... 73

5.5.2 Restoration of key profile 2 ......................................................................................... 79

5.5.3. Restoration of key profile 3 ........................................................................................ 84

5.6 Discussions......................................................................................................................... 87

5.7 Conclusions ........................................................................................................................ 88

6. Numerical modeling of multi stage basin inversion in the western Barents Shelf............... 89

6.1. Introduction ................................................................................................................... 89

6.2. Numerical modeling ...................................................................................................... 91

6.2.1 The Finite Element Method (ANSYS™)................................................................. 91

6.3 Model set up ................................................................................................................... 91

6.4 Boundary conditions....................................................................................................... 94

6.5 Results ............................................................................................................................ 96

6.5.1 Late Triassic – Early Jurassic .................................................................................. 96

6.5.2 Late Cretaceous........................................................................................................ 97

6.5.3 Early Eocene .......................................................................................................... 100

6.5.4 Miocene.................................................................................................................. 101

6.6 Discussion..................................................................................................................... 103

6.6.1. Origin of the Cenozoic stress field........................................................................ 107

6.6.2. Modeled stress patterns compared to observations............................................... 108

6.7 Conclusion .................................................................................................................... 111

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7- SUMMARY....................................................................................................................... 113

References .............................................................................................................................. 116

CURRICULUM VITAE ........................................................................................................ 134

Declaration ............................................................................................................................. 135

List of published work............................................................................................................ 136

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1. Introduction

1.1 The western Barents ShelfThe western Barents Shelf represents the area of the epicontinental Barents Sea extending N-S

between the archipelago of Svalbard and mainland Norway and E-W from the Norwegian-

Russian border to the Atlantic Ocean (Fig. 1.1). The area involves a mosaic of basins,

numerous fault complexes and intra-basinal highs, which formed in response to various Late

Paleozoic to Cenozoic tectonic events (Doré et al. 1995). The structure of the central and

eastern parts of the western Barents Shelf is dominated by NE-SW to ENE-WSW-striking

fault complexes, whereas the western part incorporates mostly NNW-SSE and N-S structural

elements (Gabrielsen et al. 1990; Fig. 1.1).

Several fault complexes of the western Barents Shelf were tectonically inverted to varying

degrees from Mesozoic to Cenozoic (Faleide et al. 1988, 1993, Gabrielsen et al. 1997,

Grunnaleite 2002, Bergh and Grogan 2003, Fitryanto 2011). Reactivation of pre-existing

major fault zones was directly or indirectly emphasized by early investigators (Rønnevik and

Motland 1979, Rønnevik et al. 1982, Faleide et al. 1984, Rønnevik and Jacobsen 1984) and

was later on attributed to wrenching (Riis et al. 1986, Brekke and Riis 1987, Gabrielsen and

Færseth 1988, Faleide et al. 1993a, 1993b) or head-on inversion (Gabrielsen et al. 1992,

1997). Strike-slip related inversion was dated to Mesozoic and Cenozoic (Gabrielsen et al.

1997, 2011). Inversion structures including upright open folds, deformed fault planes, reverse

faults and deformation of footwall blocks were reported, in particular, from Turonian

throughout Late Cretaceous and into Early Cenozoic (Gabrielsen et al. 1997). Grunnaleite

(2002) conducted a regional seismic interpretation study of inversion structures on the

Norwegian Shelf, including the western Barents Sea and suggested reactivation and inversion

of pre-existing extensional faults during Mesozoic (Cretaceous) and Cenozoic (Eocene and

Miocene). Inverted normal faults are distributed in the whole region accompanied by reverse

faulting and folding. Thus inversion seems to be common and associated to most of the

significant fault systems of the western Barents Sea (Gabrielsen et al. 1997).

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Figure 1.1. Regional setting and major structural elements of the study area (modified from

Google Maps and NPD fact maps http://gis.npd.no/factmaps/html_20/). (AFC = Asterias Fault

Complex, BB = Bjørnøya Basin , BFC = Bjørnøyrenna Fault Complex, BP = Bjameland Platform, CFZ = Central Fault Zone,

COB = Continental Oceanic Boundary, FSB = Fingerdjupet Sub-Basin, HB = Harstad Basin, HfB = Hammerfest Basin, HFC

= Hoop Fault Complex, HrFC = Hornsund Fault Complex, KFC = Knølegga Fault Complex, LFC = Leirdjupt Fault

Complex, LH = Loppa High, MB = Maud Basin, MFC = Måsøy Fault Complex, MH = Mercurius High, NB = Nordkapp

Basin, NFC = Nysleppen Fault Complex, NH = Norsel High, OB = Ottar Basin, PSB = Polhem Sub-Platform, R-LFC =

Ringvassøy – Loppa Fault Complex, SB = Sørvestsnaget Basin, SFZ = Senja Fracture Zone, SH = Stappen High, SR = Senja

Ridge, TB = Tromsø Basin, T-FFC = Troms-Finnmark Fault Complex, TIFC = Thor Iversen Fault Complex, VH = Veslemøy

High, VVP = Vestbakken Volcanic Province).

1.2 Aims and thesis statement

The goal of the present research is to investigate the causes and effects of Mesozoic and

Cenozoic inversion events in the western Barents Shelf. For this purpose, four main scenarios

have been advanced for orientations and timings of stress states responsible for fault

reactivation in the western Barents Shelf. These include: (1) westward motion of Novaya

Zemlya in Late Triassic-Early Jurassic (Buiter and Torsvik 2007), (2) Late Cretaceous Alpine

inversion (Gabrielsen et al. 1997, Vågnes et al. 1998), 3- NW-SE Early Eocene North Atlantic

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opening (Tsikalas et al. 2002, Doré et al. 2008) and 4- NW-SE directed North Atlantic ridge

push (Doré and Lundin 1996).

The present thesis work consists in three different research phases involving seismic

interpretation, structural restoration and numerical modeling.

Phase I considered seismic interpretation of 142 2D seismic lines along the Bjørnøyrenna

Fault Complex and the Ringvassøy-Loppa Fault Complex. The aim of this study was to detect

and interpret eventual inversion structures (folds and reverse faults) of Early and Late

Cretaceous. IHSTM Kingdom 8.8 seismic and geological interpretation software was used.

In Phase II, 2D structural restoration of Cretaceous inversion structures present in the

Bjørnøyrenna Fault Complex was carried out. Three seismic key-profiles with different

orientations (WNW-ESE and ENE-WSW) were selected for this purpose. The main objective

of Phase II (i.e. kinematic modeling) was to restore the seismic sections backward, to locate

null point positions and to investigate Early Cretaceous and Late Cretaceous inversion events.

For this purpose 2D MOVETM (structural modeling and analysis software by Midland Valley

Exploration Ltd) was used and the 2D kinematic module with different restoration techniques

(i.e. 2D unfolding, flexural slip and 2D move on fault, simple shear) was dopted. In general,

two main results can be obtained from restoration or backward modeling of a particular

structure. The approach can validate the interpreted geometry in cross section and can provide

information about the processes linked to regional progressive deformation.

In Phase III, numerical modeling techniques were used to investigate inversion induced by

Late Triassic to Miocene tectonic stress fields. The aim is to test the potential for fault

reactivation under such specific tectonic stress fields. A finite-element numerical code

(ANSYS™) was used to simulate stress patterns and fault slip based on four 2-D thin plate

modeling setups. Following previous works, four major regional inversion events were

assumed: Late Triassic to Early Jurassic (E-W contraction, Model 1), Late Cretaceous (NW-

SE contraction, Model 2), dextral megashear in Early Eocene (Model 3) and NW-SE Atlantic

ridge push from Miocene to present-day (Model 4).

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1.3 Thesis outline and framework

The thesis consists in seven chapters including theoretical background, methodology and

results of the research. Chapter 2, ‘Basin Inversion Tectonics’, gives an account on geometry

and kinematics of basin inversion, main driving forces responsible for basin inversion and

examples of inverted basins in world. The last sections of the chapter address previous studies

of Mesozoic and Cenozoic inversion in the western Barents Sea.

Chapter 3, ‘Methodology’, introduces the different techniques used in this thesis (i.e.

numerical modeling, seismic interpretation and structural restoration) and presents the chosen

computer tools (i.e. AnsysTM, Kingdom suit and MOVETM).

Chapter 4, ‘Mesozoic inversion in the western Barents Shelf: Integrated seismic

interpretation of the Bjørnøyrenna Fault Complex and the Ringvassøy–Loppa Fault

Complex’, presents the results of the 2D seismic interpretation.

Chapter 5, ‘Structural Restoration of Cretaceous inversion events in the Bjørnøyrenna Fault

Complex, western Barents Shelf’, details the results of the structural restoration of the

Bjørnøyrenna Fault Complex, western Barents Shelf.

Chapter 6, ‘Numerical modeling of multi stage basin inversion in the western Barents Shelf’,

includes results of the numerical modeling of Mesozoic and Cenozoic inversion events. Four

different models were constructed in order to predict stress patterns and to explore the

conditions for tectonic inversion during four specific tectonic configurations spanning from

Late Triassic to Miocene.

Chapter 7 gives a brief summary of the main results of the three individual studies.

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2. Basin inversion tectonics

2.1 Inversion tectonics

The term inversion is refes to be reversal of sedimentary basin record in the sense of motion

during different stages of basin evolution (Glennie and Boegner 1981, Mitra 1993, Coward

1994). Basin inversion can be defined as the process of shortening of extensional basins

which is accommodated by compressional reactivation of pre-existing normal faults (Turner

& Williams 2004). According to Cooper et al. (1989) ‘A basin controlled by a fault system

that has been subsequently compressed-transpressed producing ‘uplift’ is defined as basin

inversion.

Figure 2.1. Schematic diagram (not to scale) showing basin inversion (modified after

Williams et al. 1989)

Inversion can be caused by various mechanisms and can have different origins. Many authors

believe that the main cause for the compressional reactivation of faults are external horizontal

stresses related to plate movements (Coward 1994) e.g. continent - continent or arc-continent

collision, that can cause compression, uplift and reactivation of pre-existing faults or ocean-

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continent convergent margins where the changes in the rate and dip of subduction may cause

basin inversion. Isostatic rebound of sediments or caused by removal of glacial overburden,

flexural and thermal mechanisms and salt tectonics may also cause basin inversion (Voigt

1962).

Basin inversion can occur at differen scales (e.g. from basin to sub-basin scale) and is widely

documented in different tectonic settings, e.g. continental rifts, rifted continental margins,

backarcs, orogenic foredeeps, intracratonic basins, and in regions of strike slip faulting (e.g.

Cámara and Klimowitz 1985, Gillcrist et al. 1987, Tucker and Arter 1987, Daly et al. 1989,

Letouzey et al. 1990, Turner and Hancock 1990, Boldreel and Andersen 1993, Roure and

Colletta 1996, Guiraud and Bosworth, 1997, Butler 1998, Underhill and Paterson1998,

Gasperini et al. 2001, Morley et al. 2001, Benkhelil et al. 2002, Turner et al. 2003).

2.2 Geometry and kinematics of basin inversion

Glennie & Boegner (1981) documented tectonic inversion as positive (uplift) and negative

(subsidence) relative to a fault system. Positive inversion occurs when extensional faults

reverse their sense of motion during compressional tectonics which causes the basin to turn

inside out and to become a positive feature (Fig. 2.1 and 2.2; Williams et al. 1989). As a result

each fault may show net extension at deep levels and depicts net contraction associated with

an anticline in the upper portion of the faulted rocks. In positive inversion the areas which

initially underwent subsidence were uplifted afterwards.

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a

b

Figure 2.2. Schematic diagram showing classical positive inversion structure. a) Extension

and b) basin inversion (modified after Williams et al. 1989).

Negative inversion is the extensional reactivation of existing contractional faults. It gives a

useful and relevant concept to understand syn- and post orogenic extension (Williams et al.,

1989, Turner et al. 2003).

The geometry of inversion structures is very much influenced by the stratigraphic built up of

the extensional basin. The pre-rift sequence is deposited before any fault activity and can be

recognized by the equal thickness of stratigraphic units on hangingwalls and footwalls. The

syn-rift sequence is deposited during extensional faulting and stratigraphic thickness changes

from footwall to hangingwall i.e. growth faulting. The post-rift sequence is deposited when

extensional faulting ceased. This latter sequence may also be deposited on top of a marked

break-up unconformity reflecting erosion or non-deposition (Fig. 2.3).

8

Figure 2.3. Schematic diagram showing accumulation of pre-rift, syn-rift, and post-rift

stratigraphic units before, during and after extensional faulting.

A null point is an apparently unfaulted point on a fault plane (Fig. 2.4). In positive inversion,

the downward position of the null point evidences the progressive compressional inversion of

an extensional syn-rift sequence.

Figure 2.4. Schematic diagram showing the apparently unfaulted point on a fault plane, i.e.

the null point (modified after Cramez and Letouzey 2014).

Below the null point the fault is normal while above it the fault is reverse (Fig 2.5). The

position of the null point on a fault plane depends on the amount of inversion, i.e.the higher

the null point on the fault plane the smaller the inversion. The position of the null point at the

bottom of the fault plane shows total inversion.

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a) b)

Figure 2.5. Downward movement of the null point evidencing positive inversion (modified

after Cramez and Letouzey 2014).

2.3 Driving forces

Far-field stresses transmitted within tectonic plates can cause tectonic inversion (Coward

1994, Lowell 1995). Basins can be inverted by compression, strike-slip or combination of

both (e.g. transpression).

2.3.1 Compression-related inversion (examples)

The orientation of horizontal forces responsible for inversion ranges from 0 to 90° with

respect to pre-existing faults. Inversion caused by compression at 90° to existing faults is

highly effective (Letouzey et al. 1990), in case of reverse stress regimes (Fig. 2.6a).

Figure 2.6. Schematic diagram depicting the orientation of the three principal stresses (σ1, σ2

and σ3) and related stress regimes: (a) reverse and (b) strike slip (modified from

http://www.see.leeds.ac.uk/structure/faults/stress/stress.htm).

10

The world class example of tectonic inversion caused by direct compression are the Atlas

Mountains in Morocco, where the ENE-WSW Triassic-Jurassic Atlas Rift inverted due to

NNW-SSE compression caused by Miocene convergence of Africa and Iberia (Fig 2.7). The

direct compression resulted into the formation of low-angle thrusts on both sides of the Atlas

(Bennett et al. 1992, Brede et al. 1992).

Figure 2.7. Map of morocco showing the direction of maximum principal compressive stress

(σ1) (Modified after Lowell 1995).

Another example of inversion due to almost direct compression is the Uinta Mountains in

northeast Utah which are situated in the foreland of the Wyoming-Utah-Idaho thrust and fold

belt. An E-W trending Uinta Basin which developed during Proterozoic rifting was inverted

during the N-S (Gries 1982) or NE-SW (Stone 1989) late Laramide (Early-Middle Eocene)

compression (Fig. 2.8). The inversion of the Uintas is evidenced by reactivation of normal

faults and development of km-scale thrusts.

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Figure 2.8. N-S cross section of the Uinta Mountains showing inversion caused by direct

compression (Hansen 1986). (Cz = Cenozoic rocks, MzPz = Mesozoic and Paleozoic)

2.3.2 Strike slip related inversion

Inversion caused by strike slip require a dominance of lateral motion where σ1 (maximum

principal stress) and σ3 (minimum principal stress) lay in the horizontal plane and the σ2

(intermediate principal stress) is vertical (Fig. 2.6b). Strike slip related inversion has also been

observed in different parts of the world (e.g. offshore northeast Brazil, western Barents Sea).

The offshore Ceara Piaui Basin in northeast Brazil experienced inversion. Aptian rift

sediments were inverted due to the convergent right-lateral movement caused by the

separation of South America and Africa (Ponte and Asmus 1978). The inversion resulted in

the reactivation of high angle pre-existing normal faults (Fig. 2.9).

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Figure 2.9. Cross section (based on reflection seismic lines) showing the evolution of the

northeast Brazilian continental margin under transpression. (a) Aptian rifting between South

America and Africa; (b) inversion, reactivation of high angle pre-existing normal faults and

truncation of the upper Aptian below a post-Aptian unconformity (modified after Lowell

1995).

Strike slip related inversion has also been identified in the western Barents Sea and dated to

Late Paleozoic, Mesozoic and Cenozoic (Gabrielsen et al. 2011). Inversion structures

including upright open folds, reverse faults, deformation of footwall blocks and deformed

fault planes were reported from Turonian throughout Late Cretaceous and into Early

Cenozoic in particular (Gabrielsen et al. 1997). However issues concerning the exact timings

and stress directions responsible for the multiple reactivations of main fault complexes in the

western Barents Sea are still under consideration.

2.3.3 Transpressive inversion

Inversion can be caused by the combination of compression and strike slip, i.e. transpression.

Although compression at 90° to pre-existing faults is more effecient (Letouzey et al. 1990),

but it is not the general case (Fig. 2.10).

13

Figure 2.10. Possible angles of incidence of compression to pre-existing normal faults.

(Modified after Lowell 1995).

The analysis of the relative contribution of compression and strike slip in an inverted region is

always a difficult task. The azimuth of slip can be determined from measurements of strain in

the field but precise assessment of fault slip using 2D seismic data is not possible (Lowell

1995).

2.4 Basin inversion in the western Barents Sea

2.4.1 Regional geological setting

The complex mosaic of platform areas and basins of the Barents Sea formed mainly through

continental collisions in Paleozoic, e.g. Caledonian and Uralian orogenies (Doré 1996, Gee et

al. 2008), rifting events during Paleozoic and Mesozoic (Smelror et al. 2009, Tsikalas et al.

2012) and opening of the North Atlantic Ocean in Cenozoic (Gabrielsen et al. 1990, Faleide et

al. 2008).

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The series of orogenies and rifting events divided the study area into three distinct provinces

(Fig. 2.11): (1) the Svalbard Platform to the north, (2) a rifted domain between the Svalbard

Platform and the Norwegian coast and (3) the continental margin to the west (Faleide et al.

1993a). The regional geology of these provinces has been published by various authors

(Faleide et al. 1984, Gabrielsen et al. 1990 and the reference therein).

The metamorphic basement of the Barents Sea was consolidated during the Caledonian

Orogeny which includes closure of the Iapetus Ocean and the consequent collision of

Laurentia with Baltica in early Paleozoic (Dengo and Røssland 1992). The overall strike of

Caledonian structures in northern mainland Norway is NE-SW (Sturt et al. 1978, Townsend

1987) whereas a NW-SE structural trend predominates in Spitsbergen (Harland 1985, Dengo

and Røssland 1992). The orientation of later extensional features, formed by subsequent

rifting phases, mimics the trend of pre-existing fracture systems. This stubbing similarity

shows that the orientation of younger extensional features was largely controlled by the pre-

existing structural grain (Gabrielsen 1984, Gabrielsen et al. 1990, Dengo and Røssland 1992).

The N-S to NNW-SSE and WNW-ESE to NW-SE structural strike in Svalbard and northern

Norway formed due to the Archean to late Precambrian deformation (Harland 1969, Harland

et al. 1974, Beckinsale et al. 1975, Kjøde et al. 1978, Berthelsen and Marker 1986, Rider

1988). In contrast, the Caledonian deformation resulted in ENE-WSW to NE-SW striking

structural features (Roberts 1971, 1972, Worthing 1984, Fig. 2.11).

According to Gabrielsen et al. (1990), the structural trend of all features in the western

Barents Sea cannot be directly linked to the Caledonian Orogeny because most of the major

structural trends may have been shaped by Devonian tectonics.

The rifting events responsible for the development of the post - Caledonian geological setting

of the western Barents Sea includes: Late (?) Devonian - Carboniferous, Middle Jurassic-

Early Cretaceous and Early Cenozoic events (Faleide et al. 1993a). All of these major rifting

phases involved several tectonic pulses.

15


google map and NPD fact maps http://gis.npd.no/factmaps/html_20/). (AFC = Asterias Fault

Complex, BB = Bjørnøya Basin , BFC = Bjørnøyrenna Fault Complex, BP = Bjameland Platform, CFZ = Central Fault Zone,

COB = Continental Oceanic Boundary, FSB = Fingerdjupet Sub-Basin, HB = Harstad Basin, HfB = Hammerfest Basin, HFC

= Hoop Fault Complex, HrFC = Hornsund Fault Complex, KFC = Knølegga Fault Complex, LFC = Leirdjupt Fault

Complex, LH = Loppa High, MB = Maud Basin, MFC = Måsøy Fault Complex, MH = Mercurius High, NB = Nordkapp

Basin, NFC = Nysleppen Fault Complex, NH = Norsel High, OB = Ottar Basin, PSB = Polhem Sub-Platform, R-LFC =

Ringvassøy – Loppa Fault Complex, SB = Sørvestsnaget Basin, SFZ = Senja Fracture Zone, SH = Stappen High, SR = Senja

Ridge, TB = Tromsø Basin, T-FFC = Troms-Finnmark Fault Complex, TIFC = Thor Iversen Fault Complex, VH = Veslemøy

High, VVP = Vestbakken Volcanic Province).

The change in stress system, from compressional to extension in Late Devonian to Early

Carboniferous caused the formation of Bjørnøya Basin, Fingerdjupet Basin, Hammerfest

Basin, Maud Basin, Nordkapp Basin, Ottar Basin and Tromsø Basin in the western Barents

Sea (Dengo and Røssland, 1992). According to Harland (1969), Faleide et al. (1984),

Rønnevik & Jacobsen (1984) and Ziegler (1988) the first rifting event (Late Devonian to

Early Carboniferous) in the western Barents Sea initiated along the sinistral strike-slip fault

and along a conjugate dextral strike-slip fault in the central Barents Sea. Dengo and Røssland

16

(1992) argued and suggested that the major structural elements in the Barents Sea mainly

developed due to dip-slip normal faulting with little evidence of strike-slip components.

Deformation along the western parts of the Barents Sea continued throughout the Mesozoic

and the Cenozoic whereas the eastern and northeastern parts remained tectonically less active

since the Late Carboniferous (Gabrielsen et al. 1990). The Permian is considered to be a

period of thermal subsidence in the Barents Sea (Dengo and Røssland, 1992). Major structural

elements which controlled the subsequent structural architecture of the Barents Sea may have

been established by the end of the late Paleozoic (Gabrielsen et al. 1990). In the eastern part

of the Barents Sea, closure of the Uralian Sea took place from Late Permian to Early Triassic

and the Barents Sea is assumed to be a distal foreland basin of the Uralian Orogeny (Dengo

and Røssland 1992). The eastern parts of the Barents Sea experienced subsidence during

Triassic and Early Jurassic whereas the western parts remained tectonically quiet; however

the Stappen High and the Loppa High were submitted to tilting (Gabrielsen et al. 1990).

The Middle Jurassic to Early Cretaceous rifting phase is believed to be the most significant

one in the western Barents Sea. It resulted in the formation of major basins and highs. The

tectonic process caused high rates of subsidence in the western part of the Bjørnøya Basin and

the Tromsø Basin in Early Cretaceous is merely complex, while evidences of local inversion

along the Ringvassøy-Loppa Fault Complex and Bjørnøyrenna Fault Complex are also

recorded (Gabrielsen et al. 1990, 1997).

The study area was also affected by Late Cretaceous-Early Cenozoic tectonic inversion

(Gabrielsen et al. 1997). Minor folds and thrust faults at the base of the Upper Cretaceous in

the central segment of the Bjørnøyrenna Fault Complex were interpreted by Gabrielsen et al.

(1997). During and after Early Cenozoic rifting and breakup (earliest Eocene), the western

margin of the Barents Sea was subject to tectonic dextral shear and associated folding with

NW-SE-striking fold axes (Faleide et al. 1996).

The western Barents Sea continental margin developed at the Paleocene-Eocene transition (~

55-57 Ma) as a result of continental breakup and opening of Norwegian-Greenland Sea which

was linked by the regional megashear system (De Geer Zone) to the Arctic Eurasian Basin

(Faleide et al. 1996, 2008). According to Libak et al. (2012), the De Geer Zone is a large

dextral continental strike-slip zone and was located between the western Barents Sea/Svalbard

and northeast Greenland, and extended from northern Norway to the Arctic Ocean (Fig. 2.12).

17

The Northeast Atlantic Basin and the Eurasian Basin were linked by this transform margin

(Doré 1995).

Initial spreading started along the Aegir and Mohns ridges in Early Eocene (Talwani &

Eldholm 1977, Czuba et al. 2011) and relative plate motions were parallel to the strike-slip

system (Libak et al. 2012, Fig. 2.12). As a matter of fact, the breakup did not propagate into

the southwestern Barents Sea, but shear motions along the De Geer Zone were developed and

this relative plate movement generated different margin segments, i.e. shear and rifted ones on

the western Barents Sea margin (Libak et al. 2012).

To the south, the Senja Fracture Zone (SFZ) marks the southern segment of the purely

sheared margin (Fig.2.12), developed due to the opening of the Norwegian-Greenland Sea

during the Eocene (Faleide et al. 2008). The generation of the Senja Fracture Zone (SFZ) was

initially related to continent-continent shear followed by continent-ocean shear and has been

passive since Oligocene imes (Faleide et al. 2008).

In the central part of the western Barents Sea continental margin, the strike-slip system

changed and resulted into a pull-apart setting (Faleide et al. 1993, Breivik et al. 1998, Ryseth

et al. 2003) which caused generation of a rifted segment associated with volcanism, i.e. the

VVP (Vestbakken Volcanic Province). The rifted margin (VVP) linked sheared margin

segments to the north and south (Fig. 2.12). A number of buried mounds interpreted as buried

volcanoes of Early Eocene to Early Oligocene age are reported in the northwestern parts of

the province (Faliede et al. 1988, Libak et al. 2012). The significant intrusion of dense

magmatic material at the VVP (Vestbakken Volcanic Province) was due to the transtension

which caused thinning of the crust (Sundvor and Eldholm 1979, Eiken and Austegard 1987,

Eldholm et al. 1987, Czuba et al. 2011). The Cenozoic evolution of the VVP (Vestbakken

Volcanic Province) includes several tectonic and volcanic events (Faleide et al. 1988,

Richardsen et al. 1991, Sættem et al. 1994, Eidvin et al. 1998, Jebsen 1998, Ryseth et al.

2003). In Early Eocene, only the western parts of the province experienced volcanic activity,

but later effected by erosion (Jebsen 1998, Faleide et al. 1988). The presence of middle – late

Eocene sediments above the volcanic flows in the VVP proves that the area was a major

sedimentary basin at that time. The main source of these sediments was the uplifted Stappen

High in the northeast (Fig. 2.11, Richardsen et al. 1991, Ryseth et al. 2003, Libak et al. 2012).

18

Figure 2.12. Schematic diagram (not to scale) showing tectonic setting of the Arctic region (a)

Early Eocene breakup and (b) Post Eocene extension. AR = Aegir Ridge, DGZ = De Geer

Zone, HFC = Hornsund Fault Complex, MR = Mohns Ridge, SFZ = Senja Fracture Zone,

VVP = Vestbakken Volcanic Province. (Modified after Doré et al. 2008)

The Cenozoic development of the VVP (Vestbakken Volcanic Province) was mainly

controlled by two major fault zones including; the Knølegga Fault Complex (KFC)

(Gabrielsen et al. 1990) and Central Fault Zone (CFZ). The N-S to NNW-SSE Knølegga Fault

Complex (KFC) lies in the east of the VVP and marks the western boundary of the Stappen

High (Fig. 2.11). The Central Fault Zone (CFZ) lies on the eastern limit of the VVP (Fig.

2.11) and marks a boundary between Eocene seabed sediments in the east and Pliocene-

Pleistocene seabed sediments in the west (Sættem et al. 1994). In contrast, Faleide et al.

(1988), observed volcanic products also in the east of the CFZ (Central Fault Zone). Many

previous authors (Eldholm et al. 1987, Faleide et al. 1988) believed that the VVP is underlain

by thick oceanic crust and the COB (Continent-ocean boundary) is located close to the CFZ

(Central Fault Zone). Faleide et al. (1991) suggested that the outer parts of the province

contain thick oceanic crust while the inner parts are composed of continental crust which

covered by sediments and volcanics. Later studies (Ryseth et al. 2003) showed that the eastern

parts of the province represent stretched continental crust covered by pre-breakup sediments.

Most recent studies (Breivik et al. 1999, Engen et al. 2008, Czuba et al. 2011) of the VVP

(Vestbakken Volcanic Province) suggest that the whole province is underlain by stretched

19

continental crust. From middle Miocene to middle Pliocene the western Barents Sea and the

VVP experienced regional uplift (Libak et al. 2012).

To the north, a margin segment developed along the Hornsund Fault Complex (Faleide et al.

1993, Faleide et al. 2008, Libak et al. 2012) which is affected by continent-ocean and oblique

continent-continent shearing with both transpressional and transtensional components during

Eocene (Gorgan et al. 1999, Berg and Grogan 2003). The restraining bend along north-

northwest trending faults between the northeast Greenland and Svalbard caused transpression

and as a result the Spitsbergen fold and thrust belt was formed (Czuba et al. 2011), while the

releasing bend between the Hornsund Fault Complex and the Senja Fracture Zone facilitated

by Oligocene rifting (Faleide et al. 1993). This rifting caused reactivation of NE – trending

normal faults in the Sørvestnaget Basin (Jebsen 1998).

In Early Oligocene spreading ceased in the Labrador Sea – Baffin Bay (Talwani & Eldholm

1977, Mosar et al. 2002) and the spreading direction between Greenland and Eurasia changed

from NNW-SSE to NW-SE (Oakey 2005, Faleide et al. 2008). The change in relative plate

movement caused the progressive development of the Mid Atlantic Ridge towards north

(Czuba et al. 2011). The propagation of the spreading axis into the Spitsbergen Shear Zone

resulted into the formation of an obliquely spreading and asymmetric the Knipovich Ridge

(Czuba et al. 2011). The opening of the Fram Strait and a deep water connection to the Arctic

Ocean basin in Early Miocene are also outcomes of the change in plate motion (Jakobsson et

al. 2007, Engen et al. 2008).

Cenozoic inversion in the western Barents Sea is assumed to be caused by North Atlantic

ridge push (Ranalli and Chandler 1975, Stephansson 1988, Talbot and Slunga 1989, Spann et

al. 1991). According to Doré and Lundin (1996), the renewed direction of plate motion caused

compression in the area due to the counterclockwise shift in the poles of rotation in the North

Atlantic, during A13–A7 chrons (35–25 Ma). NW-SE transfer of stress (ridge push) in

Miocene also caused development of inversion structures including anticlines and reverse

faults in the western Barents Sea (Doré and Lundin 1996). Vågnes et al. (1998) suggested that

the ridge push force will not be affected by shift in plate motion. According to Srivastava and

Tapscott (1986), no such changes in spreading axis are noticed in the North Atlantic in the

Early Oligocene.

20

2.4.2. Inversion phases and evidences in the western Barents Sea

Most of the fault complexes in the western Barents Sea were inverted due to different tectonic

processes from Late Triassic to Miocene (Eldholm 1977, Myhre and Eldholm 1988, Faleide et

al. 1988, 1993, Richardson 1992, Gabrielsen et al. 1990, 1997 and references therein,

Grunnaleite 2002, Bergh and Grogan 2003, Fig. 2.13).

Figure 2.13. Inversion direction in the western Barents Sea (modified from Grunnaleite 2002)

Reactivation of pre-existing major fault zones was evidenced by early investigators in the

form of wrenching (Riis et al. 1986, Brekke and Riis 1987, Gabrielsen and Færseth 1988,

Faleide et al. 1993 a, b) or head-on inversion (Gabrielsen et al. 1992, 1997).

Strike slip related inversion has also been recognized and dated to Late Paleozoic, Mesozoic

and Cenozoic (Gabrielsen et al. 2011). Grunnaleite (2002) conducted a regional study of

inversion structures including the greater part of the western Barents Sea. In the study, all the

major fault complexes, including the Knølegga Fault Zone, Bjørnøyrenna Fault Complex,

Leirdjupet Fault Complex, Ringvassøy – Loppa Fault Complex and Hoop Fault Complex

were found to show signs of inversion. Inverted normal faults are distributed in the whole

region accompanied with reverse faulting and folding.

21

2.4.2.1 Asterias Fault Complex

The Asterias Fault Complex also known as the Southern Loppa High Fault System (Faleide et

al. 1984, Gabrielsen et al. 1984, Berglund et al. 1986), is located between 71°50’N, 20°E and

72°20’N, 24°E and separates the Hammerfest Basin from Loppa High (Fig. 2.11). The

western limit of the Asterias Fault Complex connects to the Ringvassøy - Loppa Fault

Complex (Gabrielsen et al. 1990). According to Gabrielsen et al. (1984, 1990), the E-W

trending Asterias Fault Complex is a first- or second- order basement-involved extensional

structure initiated in Triassic to Jurassic (Gabrielsen et al 1984). Presence of inversion

structures including reverse faults, half – flower structures and local domes at the Jurassic –

Cretaceous boundary are evidenced at the western segment of fault complex (west of

21°15’E, Fig. 2.14) and at its junction with the Ringvassøy - Loppa Fault Complex (Berglund

et al. 1986, Brekke and Riis 1987). A (dextral) strike-slip fault forming half flower structures

at the end of Jurassic time is also suggested by Rønnevik and Jacobsen (1984). According to

Indrevær et al. (2016), head-on contraction is suggested in the early Barremian – early Aptian

and early Barremian – middle Albian along the Asterias Fault Complex. The formation of

inversion structures during the above mentioned time periods was caused by uplift of the

Loppa High due to space accommodation problems (Indrevær et al. 2016).

22

Figure 2.14. Uninterpreted and interpreted seismic lines (BSS01-107 and NBR07RE09-

231068) crossing the Hammerfest Basin, from the Finnmark Platform in the south and the

Loppa High in the north (modified after Indrevær et al. 2016). See figure 2.11 for location.

Several models have been proposed for the development of inversion structures along the

Asterias Fault Complex including e.g. regional strike slip movement (Rønnevik et al. 1982,

Rønnevik and Jacobsen 1984, Riis et al. 1986, Gabrielsen et al. 2011), gravity induced dextral

shear of the Hammerfest Basin sedimentary fill (Ziegler et al. 1986), largescale horizontal

rotation of the Hammerfest Basin relative to the Loppa High (Gabrielsen and Færseth 1988)

and uplift and clockwise rotation of the Loppa High (Indrevær et al. 2016).

2.4.2.2 Bjørnøyrenna Fault Complex

The Bjørnøyrenna Fault Complex strikes mainly NE-SW and is situated between 72º N, 19' E

and 73º 15' N, 22º E. Rønnevik and Jacobsen (1984) described the fault complex as the south-

eastern boundary fault of the Bjørnøya Basin whereas Gabrielsen et al. (1984), defined it as

the northeastern extension of the Ringvassøy - Loppa Fault Complex (Fig. 2.11). In general

the fault complex marks the boundary between the Bjørnøya Basin and the Loppa High in the

southwest and it separates the Loppa High from the Fingerdjupet Subbasin in the north east

(Rønnevik et al. 1975, Hinz and Schlüter 1978, Rønnevik and Motland 1981, Gabrielsen et al.

1990).

The Bjørnøyrenna Fault Complex is of extensional origin with sets of normal faults having

large throws and was active in the Late Jurassic to Early Cretaceous. Signs of tectonic

inversion including domal features, deformed fault planes and reverse faults are observed

affecting Cretaceous to Cenozoic sedimens along the fault complex (Gabrielsen et al. 1997).

Gabrielsen et al. (1997) suggested that the Early Cretaceous reactivation of the normal faults

wass caused by dextral shear (Fig. 2.15a) and that they experienced NW-SE compressional

inversion in Late Cretaceous – Early Cenozoic (Fig. 2.15b).

23

N

ba

Figure 2.15. Structural development of the Bjørnøyrenna Fault Complex indicating (a) Early

Cretaceous dextral shear and (b) Late Cretaceous-Early Cenozoic NW-SE compression

(modified from Gabrielsen et al. 1997).

2.4.2.3 Hoop Fault Complex

The Hoop Fault Complex is located between 72º 50’ N, 21º 50’ E and 74º N, 26º E (Fig. 2.11)

and is considred to be an old zone of weakness which cuts across the Bjarmeland Platform

and Loppa High (Gabrielsen et al. 1990). It also separates the Mercurius High from the Maud

Basin in the SW of the fault complex. According to Gabrielsen et al. (1990), the NE-SW to N-

S trending fault complex is characterised by normal faulting and further subdivided into three

main segments. Its northern segment strikes N-S and consists of swarm of normal faults that

cut the Bjarmeland Platform. The NE-SW central segment of the fault complex is related to

the development of the Maud Basin and the Svalis Dome. The southern NE-SW segment of

the Hoop Fault Complex comprises of narrow graben which is part of minor grabens arranged

in an en echelon pattern in the northern Loppa High. The arrangement of the system defines

the transition between the Hoop Fault Complex and the Bjørnøyrenna Fault Complex.

24

Figure 2.16. Inversion structure (minor fold) at the level of the MT (Middle Triassic) on the

southern segment of the Hoop Fault Complex (modified after Fiytriano 2011). MT (Mid

Triassic), ET (Early Triassic), P (Permian). See figure 2.11 for location.

Activity along the fault complex started in Late Carboniferous to Permian and was followed

by thermal subsidence in Early-Late Permian. Growth faulting was active during Early-

Middle Triassic and was followed by mild inversion (minor folds) due to head-on contraction

(Fig. 2.16) in Middle-Late Triassic (Gabrielsen et al. 2016).

Subsidence along the fault complex occurred during Early-Middle Jurassic, which was

interrupted by the Late Jurassic - Early Cretaceous NW-SE rifting. The Late Cretaceous was

marked by regional uplift and erosion, which was followed by Paleogene subsidence and

Neogene glaciation uplift and erosion (Fiytriano 2011).

2.4.2.4 Knølegga Fault Complex

The Knølegga Fault Complex is part of the Hornsund Fault Complex and defines the western

boundary of the Stappen High (Sundvor and Eldholm 1976, Myhre et al. 1982, Gabrielsen et

25

al. 1984). The fault complex strikes NNE-SSW to NS (Fig. 2.11 and 2.13) and is referred to

as the Bjørnøya-Sørkapp fault zone by Faleide et al. (1988) and Myhre and Eldholm (1988).

According to Gabrielsen et al. (1990) the Knølegga Fault Complex has a listric geometry and

the main phase of movement was in Cenozoic times.

The contractional structures observed by Ur-Rehman (2012) along the Knølegga Fault

Complex including synclines and anticlines (Fig. 2.17) are suggested to be the result of

compression in Oligocene. The intra-Oligocene, intra-Miocene and intra-Pliocene reflectors

are eroded on the anticline (Fig. 2.17).

Ridge push direction changed from NW-SE to WNW-ESE at the Eocene-Oligocene

boundary, as response to the adjustment of the rotation poles in the North Atlantic (Boldreel

and Andersen 1993 in Vågnes et al. 1998). The contractional stresses have deformed both the

hanging wall and the footwall. This contraction is suggested to be older than the Pliocene-

Pleistocene glacial sediments as no contractional structures are observed in the younger

Pliocene-Pleistocene sediments along the fault complex.

Figure 2.17. Observed inversion structures (syncline and anticline) at the southern segment of

the Knølegga Fault Complex (modified after Ur-Rehman 2012, see figure 2.11 for location).

UN (Upper Neogene), IP (Intra Pliocene), IM (Intra Miocene), IO (Intra Oligocene), IE (Intra

Eocene), NBE (Near Base Eocene).

26

2.4.2.5 Leirdjupet Fault Complex

The N-S Leirdjupet Fault Complex extends from the Loppa High towards the Stappen High

between 73° - 73°55'N at 21°E (Fig. 2.11). The fault marks the transition zone that divides the

Bjørnøya Basin into deep western and shallow eastern (Fingerdjupet Subbasin) parts

(Rønnevik and Jacobsen 1984, Gabrielsen et al. 1990).

The fault complex is believed to had been an extensional feature and was active during

different time periods but the main tectonic activity took place in Carboniferous, Mid Jurassic

and Early Cretaceous (Gabrielsen et al. 1990). A change in structural style along strike was

avidenced by Gabrielsen et al. (1990) and the fault complex is further subdivided into three

segments from south to north. The southern part (segment 3) has in general a NW-SE trend

and consists of several rotated smaller normal faults. The central part (segment 2) is

characterised by a single normal fault (N-S) with a large throw towards Bjørnøya Basin and

the NE-SW northern part (segment 1) is composed of horst and graben structures (Fig. 2.11).

The Leirdjupet Fault Complex is considered to be a northern continuation of the Bjørnøyrenna

Fault Complex (Gabrielsen et al. 1990). The Leirdjupet Fault Complex experienced a phase of

Early Cretaceous dextral shear and Late Cretaceous-Early Cenozoic NW-SE contraction

(Gabrielsen et al. 1997, Bjørnestad 2012). Inversion structures (folds) are reported by earlier

investigators (Gabrielsen et al. 1990, Bjørnestad 2012) along the central and northern parts of

the fault complex. The orientation of the observed folds with axes almost parallel to the fault

strike suggests head-on inversion in Late Cretaceous – Early Cenozoic (Fig. 2.18).

27

Figure 2.18. Folding along the Leirdjupet Fault Complex indicating inversion in Late

Cretaceous – Early Cenozoic (modified after Bjørnestad 2012). See figure 2.11 for location;

BUC (Base Upper Cretaceous), EC (Early Cretaceous), BC (Base Cretaceous), UMJ (Upper

Mid Jurassic).

2.4.2.6. Måsøy Fault Complex

The Måsøy Fault Complex is the southern marginal fault of the Nordkapp Basin and is located

between 71°27’N, 24°45’E and 72°15’N, 28°40’E (Fig. 2.11). This NE-SW fault complex

marks the structural division between the Nortkapp Basin and the Finnmark Platform and is

considered to be an extensional structure with an en echelon pattern and mainly dip slip

components (Gabrielsen et al. 1990). Tectonic activity is observed during Early Carboniferous

but also recorded in Mesozoic and Cenozoic (Gabrielsen et al. 1990). The hanging wall of the

major fault is reported to be severely damaged; showing minor folds, and may indicate

inversion at the Jurassic-Cretaceous transition (Gabrielsen & Faerseth 1989). Glørstad-Clark

et al. (2010) also suggested inversion related to (dextral) strike-slip movements during Early –

Middle Jurassic.

28

2.4.2.7 Ringvassøy – Loppa Fault Complex

The N-S Ringvassøy-Loppa Fault Complex is an extensional fault complex involving old

zones of weakness. The northern part of the fault complex defines the western boundary of

the Loppa High and in the south it merges into the Troms-Finnmark Fault Complex. It

separates the Tromsø Basin in the west from the Hammerfest Basin in the east (Fig. 2.11).

The southern part of this fault complex is dominated by normal faulting (Øvrebø and

Talleraas 1977, Faleide et al. 1984, Gabrielsen 1984, Berglund et al. 1986). The geometry of

the complex is interpreted as two levels of detached listric normal faults and a possible deeper

zone of weakness (Gabrielsen 1984). The main subsidence along the southern part of the

complex was from Mid Jurassic time to Early Cretaceous (Gabrielsen, 1990). This was due to

large-scale rifting (Talleraas 1979). According to Braut (2012) and Zalmstra (2013) the

Ringvassøy-Loppa Fault Complex was reactivated during the Late Cretaceous but Cenozoic

strata have also been affected by faulting. Compression is suggested during the Late

Cretaceous period and caused the formation of the observed inversion structures along the

fault complex (Fig. 2.19).

29

Figure 2.19. Inversion along the western margin of the Ringvassøy – Loppa Fault Complex

affecting the Cretaceous (after Zalmstra 2013). IP = Intra Permian, IC = Intra Cretaceous, BT

= Base Tertiary.

The summary of main tectonic events in the western Barents Sea is given in figure 2.20.

30

Figure 2.20.S

umm

ary of the main tectonic events in the w

estern Barents S

ea

31

3. Methodology

3.1 Numerical modeling

A number of techniques have been applied to predict stress/strain distribution and fracture

patterns (Bourne and Willemse 2001, Maerten et al. 2002, Lunn et al. 2008). In general,

widely used approaches for numerical modeling are the finite element method (FEM) and

discrete element method (DEM).

3.1.1 Finite Element Method

A very basic concept of the finite element method (FEM) is to divide the structural body into

elements which are connected by nodes. In that way, equilibrium equations for each element

can be calculated and solved simultaneously.

The system of equilibrium equations can be written as:

KD=F

Where D is the displacement vector which contains displacements with all degrees of

freedom. F is the force vector and K is rigidity matrix.

FEM computations begin after having created geometry (surface bodies) and after having

assigned material properties and boundary conditions. Bodies are then divided into elements

and equilibrium equations are set. The K matrix is built according to material properties and

elements geometries and the nodal displacements d for each element are solved. For each

element, displacement fields u can be calculated by using an interpolation method u =

N d. The interpolation functions in N are called shape functions. The solver then can

calculate strain fields according to the constituve laws related to the selected material

properties. Material parameters i.e Young’s modulus (E) and Poisson’s ratio (v) can be used

for linear elastic materials to describe the stress-strain relation. The FEM can calculate

stresses and strains for heterogeneous structures with very complex geometries.

3.1.2 ANSYS™

ANSYS™ is a multiphysics FEM program based on advanced engineering simulation

technologies involving various analysis systems, e.g. fluid dynamics, structure mechanics,

thermal, electromagnetics. With the purpose of calculating horizontal stress in this thesis

32

work, 2D linear elastic models were generated using the ANSYS Workbench. For the detailed

mathematical description of the modeling approach, the reader is referred to ANSYS

Mechanical APDL Theory Reference, Release 15.0, Inc., 20131. The basic workflow

involved in the analyses is given in the following.

3.1.2.1 Analysis system

A number of analyses (e.g. Electric, Fluid Flow, Rigid Dynamics, Static Structural, Thermal-

Electric, and Transient Structural) are available (Fig. 3.1) and can be performed by using any

component system (e.g. Mechanical APDL, Fluent, Polyflow). Each analysis includes the

individual components of the analysis, e.g. geometry and model properties. Static structural

analysis which determines the displacements, stresses, strain and forces in structures caused

by loads is adopted here.

Figure 3.1. List of available analysis systems in ANSYS 15.

1 ANSYS Mechanical APDL Theory Reference, Release 15.0, Inc., 2013

33

3.1.2.2 Engineering data

A very important parameter during analysis is defining the engineering data. As the response

of each analysis result is determined by the assigned material properties. Depending on the

application and analysis type, material properties can be linear or nor-linear, as well as

temperature dependent. Linear material properties can be constant and isotropic (Fig. 3.2). A

part from defined ones, user can also assign material properties (elastic, inelastic etc.) for each

analysis.

Figure 3.2. List of engineering data and material properties.

3.1.2.3 Geometry creation or attachment

There are two ways to work on geometry in the ANSYS workbench, either to import an

existing mesh file (.cdb) or create a new geometry using DesignModeler. It is designed to be

used as a geometry editor of existing CAD models. The application allows the user to draw

2D lines, arcs and splines (Fig. 3.3) and convert into 3D models.

34

Figure 3.3. Example of geometry construction.

3.1.2.4 Coordinate systems

When a model is imported into the ANSYS workbench, the coordinate system object and its

sub section object called Global Coordinate System is automatically added to the working tree

with default location of 0, 0, 0. For solid parts and bodies, Global Coordinate System is used

by default but the user can apply a local coordinate system to any part or body.

3.1.2.5 Material properties

After creation of geometry and coordinate systems, the user can choose predefined material

for the simulation or can add new material properties. Different options including e.g. create a

new material definition, import a material, edit the characteristics of the current material or

assign a material from the list of available materials can be selected (Fig. 3.4).

35

Figure 3.4. Screen shot showing different options for assigning material properties.

3.1.2.6 Define connections

Once the material properties of the model have been assigned, one should apply connections

to the bodies in the model so that they are connected as a unit while sustaining the applied

loads for the analysis. The contact defines an area where two or more bodies are in contact

with each other. That connection could be e.g. bonded, frictional, frictionless, rough or no

separation, according to the analysis need and required results.

During the analysis, the application should prevent the two bodies from passing through each

other. When the application does so, it is said to enforce ‘contact compatibility’ (Fig. 3.5).

Different contact formulations e.g. pure penalty, augmented Lagrange, MPC and Normal

Lagrange can be used in order to enforce compatibility at the contact interface.

36

Figure 3.5. Schematic diagram showing penetration of contacts due to non-enforcement

3.1.2.7 Meshing

During meshing, the geometry is divided into elements and nodes depending on the mesh

method (triangular or quadrilateral). The meshed body along with the material properties

shows the mass distribution and stiffness of the structure. The analysis allows the user to

define the mesh sizing and refinement along certain body contacts (Fig. 3.6). The default

element size depends on different factors e.g. model size, body curvature and complexity of

the geometry.

Figure 3.6. Example of triangular mesh with refinement along the contact between bodies

regions.

37

3.1.2.8 Setting up boundary conditions

Boundary conditions include applied loads and support types which depend on the analysis.

For static structural stress analysis, pressure and force for loads, and displacement for

supports can be applied. These boundary conditions constrain or act upon the model by

exerting forces, rotations or by fixing the model in such a way that it cannot be deformed (Fig.

3.7).

Figure 3.7. Assumed boundary conditions and applied force

3.1.2.9 Analysing results

For structural analysis, equivalent stress, total displacement (Fig. 3.8) and vector principal

stress can be reviewed. The results can be visualised in the form of contour, vector, probe,

chart or animation.

38

Figure 3.8. Model results showing total displacement.

3.2 Seismic reflection survey

Seismic reflection survey is based on the principle that the sound waves reflect off the

interfaces between layers within the earth (Fig. 3.9). The earth is composed of different layers

having different physical properties (i.e. porosity, density). When sound waves travel through

the earth and found change in physical properties of layers, they penetrate into the earth or

reflect back to the surface. The physical properties responsible for reflection of the waves are

density and seismic velocity.

The seismic data acquired in the field contains noise and has to be processed before final

interpretation. The seismic data processing is basically consists of chain of operations to

refine the raw data. A pre-defined program is used to eliminate noises for better data

presentation. After processing the data has to be interpreted. The seismic interpretation

process involves its geological expression. Seismic reflection is displayed in two way travel

time (TWT) and the interpretation is basically the transformation of the data into a geological

structure using time depth conversion (Dorbin and Savit 1976).

39

Figure 3.9. Cartoon showing basic seismic reflection methodology (geosphere inc. n.d. 12May 2017 < http://www.geosphereinc.com/seis_reflection.html).

3.2.1 Kingdom Suit 8.82

Kingdom Suite is easy to use fully integrated geoscience software which includes different

modules including geophysical interpretation–2d/3dPAK, geological interpretation–

EarthPAK and Geosteering etc.

The software package is used for seismic analysis including generation of horizons and fault

on seismic lines and slices in both time and depth domains. It can also produce seismic-based

interpretation maps by combined utilization of horizon and fault picking tools2.

2 The KINGDOM Suit, Seismic Micro-Technology, Inc., 2015

40

3.3 Structural restoration3

Deformation is assumed to neither create nor destroy rock volume; therefore reconstruction of

deformed structure to its pre-deformed state is possible. Structural restoration is basically the

validation of an interpreted section through back stripping of depositional and tectonic events

by applying specific geometric rules. The process of restoration includes different techniques

i.e. removal of faulting effects, folding associated with faulting and flexural slip and volume

loss caused by compaction or erosion.

According to Chamberlin (1910, 1919), the foremost use of balancing cross section was to

estimate the depth to the décollement underlying concentric folds. The section can be restored

back in time to place the beds into their depositional and pre-deformed position. The process

can link the deformed and undeformed positions of beds and finite strain analysis can be

performed which can predict fracture distribution and orientation. The outcome of the restored

model can be used as a key to validate the seismic interpretation and can give a better idea to

understand the geological history of the area and can enhance the quality of the work.

There are certain rules for balancing which have to be followed when performing the

restoration techniques. It assumes that there is in general conservation of rock volume during

deformation (Hossack 1979, Goguel 1952). To build an accurate restored model of a

particular area, the model must account for erosion, sediment compaction (Sanderson 1976,

Wood 1974), over all tectonic compaction (Wood 1974), pressure solution (Plessmann 1964)

and elongations along orogenic strike (Ramsay and Wood 1973).

There are number of recently developed techniques for 2D/3D modeling of geological

structures including interpolation methods based on Geostatistical approaches (Goovaerts

1997, Chilès and Delfiner 1999, Wellmann et al. 2010), interface and orientation approaches

like in Calcagno et al. (2008), implicit function described in Frank et al. (2007) etc. The

results of these techniques are available in the form of 2D/3D modeling software packages

e.g. MOVETM, GOCAD®, GeoSec, EarthVision. The main motivation of all these approaches

is to generate a digital model showing subsurface structural geometry and to perform tests

using different modules (according to the needs of the project).

3 Help manual of Move TM © 2016 Midland Valley Exploration Ltd

41

3.3.1 MOVETM

2D MOVETM is mainly used for structural restoration and balancing of cross-sections. The

product suite belongs to Midland Valley Exploration Ltd which is designed for complete

geological structural modeling and analysis4. The suite comprises a full digital environment

which provides better results in structural modeling and reduces the risk and uncertainty in

geological models. It also provides a platform for integration and interpretation of data,

construction of cross-section, 2D/3D kinematic modeling, 2D/3D restoration and validation.

3.3.2 2D Kinematic modeling

There are several modules available in MOVETM including 2D kinematic modeling, 3D

kinematic modeling, goemechanical modeling, fracture modeling, stress analysis and fault

analysis. The 2D kinematic module comprises of a comprehensive range of tools (e.g. block

restoration, simple shear (unfolding and move on fault), flexural slip (unfolding), trishear

(planar and non-planar faults), fault-parallel flow (move on fault) and fault bend folding

(move on fault). It also includs sedimentation, erosion and salt movements along with de-

compaction, thermal subsidence and isostasy4.

All these modules are based on different algorithms which allow the user to fold/unfold and

fault/unfault geological models resulting in the pre-deformed tectonic setting of the study

area. Structural restoration or backward modeling has mainly two goals. (1) To validate the

geometry of the cross section and (2) to provide maximum information about the tectonic

processes that caused any kind of deformation. The workflow used in 2D structural

restoration involves application of different tools and algorithms. The details of some basic

tools from the MOVE product and help manual 2016 3, 4, are given in the following.

4 Product manual of Move TM © 2016 Midland Valley Exploration Ltd.

42

3.3.2.1 2D Unfolding

2D unfolding allows the user to unfold (restore) the geological beds to their pre-deformed

position. The pre-deformed position could be any target horizon or an assumed regional

datum. The beds can be unfolded by using either simple shear, flexure slip or line length

unfolding algorithm.

Simple Shear Unfolding3

The simple shear unfolding tool is used to unfold the horizons and the algorithm is

best for flattening a regional dip. The only limited factor with this algorithm is that

line length is not persevered (Fig. 3.11). The upper bed (blue horizon) is to be

restored to a horizontal datum (Green line). Vertical shear vectors are used to restore

the upper bed to target datum (3.11A). The restored geometry (3.11B) of the upper

and lower beds showed that the original length of the upper bed (before restoration)

is greater than the restored bed and also the original length of the lower bed (red

horizon) is greater before restoration.

The principle of the simple shear unfolding algorithm is that neither length nor area

is preserved. Line length in the unfolding directions and surface areas of beds vary

before and after deformation. Line length loss mainly depends on the dip, i.e. steep

dip of restored bed causes greater line length loss.

3 Help manual of Move TM © 2016 Midland Valley Exploration Ltd.

43

Figure 3.11. Folded horizon restoration using the simple shear unfolding tool (modified from

help manual of MoveTM © 2016 Midland Valley Exploration Ltd.).

Flexural Slip Unfolding3

The flexural slip unfolding tool can be used for concentric, layer-parallel folds. The

basic principle of the flexural slip unfolding algorithm uses a pin and a slip-system

parallel to the template bed to control the unfolding. The procedure involves the

rotation of limbs of a fold to a datum. The effects of flexural slip component are then

removed by applying the layer parallel shear to the rotated fold limbs. Unlike the

simple shear unfolding algorithm, this tool allows the user to maintain bed thickness

between the template horizon and other passive objects. The algorithm is built to

maintain the line length of the template horizon in the direction of unfolding and

maintain the area of the fold and the model. It can be used to validate complex thrust

deformations. Sequence of cross-sections showing how the flexural slip unfolding

works can be seen in figure 3.12.


44

Figure 3.12. Basic workflow for flexural slip unfolding. A) Folds with thickness variations.

B) Construction of a slip system parallel to the template bed using dip domain bisector. C)

Unfolded/restored template bed and passive beds about the pin. (modified from help manual

of MoveTM © 2016 Midland Valley Exploration Ltd.).

3.3.2.2 2D Move-on-Fault3

The move on fault module can be used for both forward models and to restore deformation. It

gives guidance and new ideas for seismic structural interpretation and allows the user to

model pre- and syn- tectonic successions and different displacements. The module is capable

to test and modify fault geometries within the stratigraphic framework and can directly create

balanced interpretation. There are different methods (simple shear, fault parallel flow, fault

bend fold, fault propagation, trishear, detachment fold) available within the capacity of the

module and can be used to control the deformation along the fault. Most of the algorithm tools

can be used for both compressional and extensional models, using either positive or negative

displacement3.


45

Simple Shear3

The simple shear algorithm is used to model the relationship between the hanging

wall deformational features and fault geometry. It extends the deformation along the

hanging wall rather than generating discrete slip between beds (i.e. flexural slip) 3.

The hanging wall is folded and maintains bed area during modeling. The tool is

useful in extensional systems e.g. half graben or rollover anticline developed on non-

planar normal fault. The algorithm can also be used for forward modeling of

restoration of growth faults and inverted basins. According to Yamada and McClay

(2003), the simple shear algorithm in MOVETM can only be used to restore inverted

basins and proposed to use a shear angle of 32° with respect to vertical.

3.3.2.3 2D Decompaction3

The 2 D decompaction module allows for modeling change in rock volume due to porosity

loss as a function of depth. The algorithm can also be used for isostatic effects and burial

history of any area. There are different methods within the range of the module to calculate

porosity change with depth, e.g. Sclater and Christie (1980), Baldwin and Butler (1985), and

Dickinson (1953).

According to Sclater and Christie (1980), porosity decreases with increasing depth

(compaction) and increases with decreasing depth (decompaction) and can be expressed as:

f = f0 (e -cy)

where:

f is present-day porosity

f0 is porosity at the surface

c is the porosity-depth coefficient (km-1)

y is depth


46

4. Cretaceous inversion of the western Barents Shelf: integrated seismic interpretation

of the Bjørnøyrenna and the Ringvassøy–Loppa fault complexes

4.1 Introduction

The Barents Sea represents a large part of the Arctic, it is located in the northwestern corner

of the Eurasian plate and bounded by the Norwegian–Greenland Sea to the west, Svalbard and

Franz Josef Land to the north, Novaya Zemlya to the east and the Norwegian– Russian

mainland in the south (Fig. 4.1). It covers a tectonically extended shelf which consists of

basins, highs and fault complexes (Gabrielsen et al. 1990, Indrevær et al. 2016). Most of these

fault complexes strike NE-SW and N-S in the eastern and central parts of the western Barents

Sea and developed as a result of multiple extensional events (throughout the Carboniferous to

Eocene) after the collapse of the Caledonian orogen (Faleide et al. 1984, 1993, 2008,

Gabrielsen et al. 1990, Gudlaugsson et al. 1998). The rifting episodes terminated with the

opening of the North Atlantic and Arctic oceans and the final stages of rifting characterized

by the transition from a rift system in the south to a dextral transform one connecting the

North Atlantic rift to the Arctic rift (Faleide et al. 2008, Indrevær et al. 2016).

In addition to several phases of extension, a number of authors have reported late Paleozoic,

Mesozoic and Cenozoic events of inversion related to strike-slip movements and head-on

compression which affected most of the fault complexes in the western Barents Sea (Ziegler

1978, Rønnevik et al. 1982, Riis et al. 1986, Berglund et al. 1986, Sund et al. 1986, Brekke &

Riis 1987, Wood et al. 1989, Gabrielsen & Færseth 1989, Gabrielsen et al. 1990, 1997, 2011,

Vågnes et al. 1998, Grogan et al. 1999, Henriksen et al. 2011, Glørstad-Clark et al. 2011,

Faleide et al. 2015, Indrevær et al. 2016).

The different phases of inversion included 1) NW-SE directed head-on inversion in the late

Cretaceous – Palaeocene caused by far field stresses (Gabrielsen et al. 1997, Vågnes et al.

1998), 2) dextral shearing due to Early Cenozoic (earliest Eocene) rifting and breakup

(Faleide et al. 1996,) and 3) NW-SE contraction related to ridge push in Miocene (Gabrielsen

& Færseth 1989, Engen et al. 2008, Faleide et al. 2015, Gac et al. 2016).

The current research focused on the Early and Late Cretaceous inversion events which

affected the Bjørnøyrenna Fault Complex and the Ringvassøy-Loppa Fault Complex

(Gabrielsen et al. 1990, 1997, Vågnes et al. 1998, Indrevær et al. 2016). Although Cretaceous

inversion has long been discussed, the exact timing and main source of driving force (s) are

47

not fully constrained. In the present study, inversion structures of the Early and Late

Cretaceous are interpreted along with the timing and mechanisms of development of these

structures.

4. 2 Geological setting

The Phanerozoic evolution of the Barents Sea involves series of orogenic events, subsequent

collapses and rifting (Gabrielsen et al. 1990, Worsley 2008, Henriksen et al. 2011, Gernigon

et al. 2014). The major tectonic phases responsible for the development of the geological

framework of the Barents Sea include the Timanian, Caledonian, and Uralian orogenies (Gee

et al. 2008) from Late Proterozoic to Late Paleozoic (Doré 1991) and are followed by proto-

Atlantic rifting events during Mesozoic (Smelror et al. 2009, Tsikalas et al. 2012) and opening

of the North Atlantic Ocean along the western margin of the shelf during Cenozoic

(Gabrielsen et al. 1990, Faleide et al. 2008).

The Barents Shelf is generally subdivided into two major geological provinces. The eastern

province was mainly affected by tectonics pertaining to Novaya Zemlya, the Timan-Pechora

Basin and the Uralian Orogeny (Worsley 2008), while the western province was mainly

controlled by major post-Caledonian rifting episodes (Fig. 4.1). The Late Paleozoic structures

of the western Barents Sea reflect mainly WNW-ESE and N-S to NE-SW structural grains

inherited from the Timanian (i.e. Ediacaran) and the Caledonian orogenies respectively

(Ritzmann and Faleide 2007, Gernigon and Brönner 2012, Gernigon et al. 2014).

48

Figure 4.1. Barents Sea major structural features including major faults, basins, structural

highs and platform areas. BF, Baidsratsky Fault Zone; BFC, Bjørnøyrenna Fault Complex;

BFZ, Billefjorden Fault Zone; HFZ, Hornsund Fault Zone; KFZ, Knølegga Fault Zone;

KHFZ, Kongsfjorden–Hansbreen Fault Zone; LFC, Leirdjupet Fault Complex; MFC, Masøy

Fault Complex; RLFC, Ringvassøy–Loppa Fault Complex; SJZ, Senja Fracture Zone; SKZ,

Sørkapp Fault Zone; SRFZ, Sredni–Rybachi Fault Zone; TIFC, Thor Iversen Fault Complex;

TFFC, Troms–Finnmark Fault Complex; TKFZ, Trollfjorden–Komagelva Fault Zone

(modified from Marello et al. 2013).

With respect to present-day geography, the Caledonian orogeny was characterized by east- to

southeast-directed convergent movement from Late Cambrian to Early Devonian (i.e. ~410

Ma) and involved both the closure of the Iapetus Ocean and the collision between Laurentia

and Baltica in the region under scope (Roberts 2003, Gee et al. 2006, Ritzmann and Faleide

2007, Gasser 2014, Gernigon et al. 2014). The post-Caledonian geological history of the

49

western Barents Sea was dominated by large-scale regional sinistral shear promoting both

transtension and transpression in Late Devonian – Early Carboniferous (Faleide et al. 1984).

Late Paleozoic tectonics resulted in the formation of three major sedimentary basins located in

the east of the studied region (Fig. 4.2): the ENE-WSW Nordkapp Basin, the NE-SW Ottar

Basin, located between the Loppa High and the Norsel High, and the NE-SW Maud Basin

located between the Loppa High and the Mercurius High (Jensen and Sørensen 1992).

In Early Triassic, the western Barents Sea was affected by rifting. This rifting phase is

recorded in many parts of the Arctic and North Atlantic regions (Tsikalas et al. 2012,

Gernigon et al. 2014). The Early Triassic extension continued until late Anisian-early

Ladinian and was characterized by normal faulting, tilting of fault blocks and erosion.

According to Gudlaugsson et al. (1998), Early Triassic faulting occurred mainly along N-S

striking structures in the western Barents Shelf. This was followed by Middle to Late Triassic

(post-rift) thermal subsidence in the North Atlantic and Arctic basins (Gernigon et al. 2014).

However, during the corresponding period of time the Barents Shelf was subject to

progressive uplift of its northern, eastern, and southern regions (Worsley 2008).

From Late Triassic to Early Jurassic, Late Paleozoic structural elements of the Barents Sea

were inverted following an E-W compressive regime (Otto and Bailey 1995, Buiter and

Torsvik 2007). Westward motion of Siberia has been advanced as a potential cause for this

compressional event (Buiter and Torsvik 2007). Otto and Bailey (1995) also interpreted

inversion structures on the eastern margin of the South Barents Sea Basin dated to Late

Triassic-Early Jurassic. Some other investigators (e.g. Gabrielsen et al. 1990, Gudlaugsson et

al. 1998, Fitryanto 2011, Gernigon et al. 2014 and references therein, Gabrielsen et al. 2016)

confirmed reactivation of major fault complexes (e.g. Troms-Finnmark, Måsøy, Thor Iversen

and Hoop fault complexes) in the western Barents Sea during the same period of time.

The Middle Jurassic to Early Cretaceous regional rifting affected mainly the western Barents

Shelf (Faleide et al. 2008). The Jurassic rifting was responsible for the development of major

deep basins, e.g. Hammerfest and Tromsø basins and fault complexes, e.g. Ringvassøy-

Loppa, Bjørnøyrenna, Leirdjupt and Asterias fault complexes (Faleide et al. 1993a, b). The

N-S-striking Ringvassøy-Loppa Fault Complex and its southern extension coincide with the

transition zone between the Hammerfest and Tromsø basins (Øvrebø and Talleraas 1977,

Gabrielsen et al. 1990) and merges with the Troms-Finnmark Fault Complex to the south. To

the north, the fault complex defines the western boundary of the Loppa High and defines the

50

southeastern margin of the Bjørnøyrenna Fault Complex (Fig. 4.2). The main subsidence

along the southern segment of the Ringvassøy-Loppa Fault Complex started in Mid Jurassic

and culminated in Aptian to Albian (Gabrielsen et al. 1990, Gudlaugsson et al. 1998).

Figure 4.2. Regional setting and major structural elements including major faults, basins,

structural highs and platform areas of the study area (modified from and google map and NPD

fact maps http://gis.npd.no/factmaps/html_20/).

The NE – SW Bjørnøyrenna Fault Complex is the northern continuation of the Ringvassøy –

Loppa Fault Complex and defines the western margin of the Loppa High, separating it from

the Bjørnøya Basin (Fig. 4.2). Rønnevik and Jacobsen (1984) described the fault complex as

the south-eastern boundary fault of the Bjørnøya Basin whereas Gabrielsen et al. (1984)

defined it as the north-eastern extension of the Ringvassøy - Loppa Fault Complex. In general

the fault complex defines the boundary between the Loppa High and the Bjørnøya Basin in

the southwest and in the northeast it separates the Loppa High from the Fingerdjupet Subbasin

(Rønnevik et al. 1975, Hinz and Schlüter 1978, Rønnevik et al. 1982). The area was affected

51

by tectonic inversion and presence of folds, reverse faults point to NW-SE directed Late

Cretaceous to Early Cenozoic contraction (Gabrielsen et al. 1997).

During and after Early Cenozoic rifting and breakup in earliest Eocene, the western margin of

the Barents Sea was subject to tectonic dextral shear and associated folding with NW-SE

striking fold axes (Faleide et al. 1993a). Cenozoic inversion in the western Barents Sea is

assumed to be caused by North Atlantic ridge push causing post Miocene shortening and

initiating folds with NE-SW-striking fold axes (Gabrielsen et al. 1990). Neogene uplift and

glaciations resulted in deep erosion of the western Barents Shelf (Faleide et al. 1996).

4.3 Data and methodology

The data used in the present study includes 142 2D seismic lines with different orientations

(N-S, E-W, NE-SW, NW-SE, Fig. 4.3). These seismic lines belong to different surveys

(BJRE, BJSY, NBR06, NBR07, NBR08, NBR10, TTR74R1 and TTR83R1) and are of

variable quality and variable depth resolution. The data were provided by the Department of

Geosciences, University of Oslo, Norway.

A total 162 wells have been drilled in the Barents Sea (NPD fact page,

http://factpages.npd.no/factpages/wells) and most of them are located in the Hammerfest

basin (Fig. 4.3). Well control was provided by ten exploration wells on the Loppa High (wells

7120/1-1 and 7120/2-1), the Hammerfest Basin (wells 7120/7-2, 7120/8-1, 7120/8-2 and

7120/8-4) and along the eastern margin of the Bjørnøya Basin (wells 7219/8-1, 7219/9-1,

7220/5-1 and 7220/6-1).

52

Figure 4.3. Base map of the study area showing the respective locations of 2D seismic lines

and wells. Red lines and dots indicate key profiles and exploration wells respectively.

IHS TM Kingdom 8.8 (seismic and geological interpretation software) is used for current work.

Kingdom Suit1 is easy to use fully integrated geoscience software which involves different

modules including geophysical interpretation–2d/3dPAK, geological interpretation–Earth

PAK and Geosteering.

4.4 Seismic interpretation

The first step while doing the seismic interpretation across the entire data sets (BJRE, BJSY,

NBR06, NBR07, NBR08, NBR10, TTR74R1 and TTR83R1) was to choose key reflectors of

different ages. Eight reflectors were chosen because of their continuity, prominence and

geological importance. The seismic tie to well 7219/9-1, located in the eastern margin of the

Bjørnøya Basin is shown in figure 4.4.

The next step is to extend these reflectors along the major fault complexes (i.e. Bjørnøyrenna

Fault Complex and Ringvassøy – Loppa Fault Complex). It was a particularly challenging

task to interpret the reflectors in all parts of the study area due to intense faulting. A detailed

description of interpreted reflectors and stratigraphy is given in the following.

1 The KINGDOM Suit, Seismic Micro-Technology, Inc., 2015

53

Figure 4.4. Seismic tie to well 7219/9-1, located in the eastern margin of the Bjørnøya Basin

(Data courtesy of TGS and Spectrum).

54

4.4.1 Snadd Formation (Upper Triassic)

The deepest reflector interpreted in the study area is the top Snadd Formation (Upper

Triassic). The Snadd Formation is of Ladinian to early Norian age and composed of basal

grey shales which coarsen up into shales with interbeds of grey siltstones and sandstones. The

lower and middle part of the formation consists of limestone and calcareous interbeds, while

thin coaly lenses are also developed in the upper part (Dalland et al. 1988). The reflector

exhibit strong amplitude and has been interpreted between 2180-2200ms TWT (Fig. 4.4).

4.4.2 Fruholmen Formation (Base Jurassic)

The base Jurassic reflector is interpreted as the top of the Fruholmen Formation between

1850-2150 ms TWT in the study area (Fig. 4.4). The formation is part of the Realgrunnen

Subgroup. The dominant lithologies of the group are sandstones, shales and coals of Late

Triassic to Mid Jurassic (early Norian to Bajocian, Dallmann 1999). The sandstones were

deposited in coastal plain and deltaic through shallow marine environments (Worsley et al.

1988). The base of the formation belongs to the Late Triassic (early Norian) and the top

corresponds to the Triassic/Jurassic transition. The formation consists of basal grey to dark

grey shales which gradually pass upwards into interbedded sandstones, shales and coals. The

lowermost Akkar Member (Norian) consists of open marine shales which pass up into a

coastal and fluvial sandstone dominated sequence of the middle Reke Member (Norian - ?

Rhaetian). Marine shales dominate the uppermost Krabbe Member (Rhaetian, Dalland et al.

1988, Larssen et al. 2002).

4.4.3 Tubåen Formation (Lower Jurassic)

The Lower Jurassic reflector is interpreted as top of the Tubåen Formation of the Realgrunnen

Subgroup which consists of various lithologies including sandstones and subordinate shales

with minor coals. The upper and lower part of the formation is dominated by a sand-rich unit

whereas a shaly interval exists in the middle part. The sand unit is believed to represent

stacked series of high energy marginal marine deposits (tidal inlet dominated, barrier complex

and/or estuarine) while marine shale characterizes more distal environments. Coal was

deposited in protected back-barrier lagoonal environments to the south-east (Dalland et al.

1988). The age of the formation is Late Triassic to Early Jurassic (late Rhaetian to early

Hettangian) and its top is found between 1790 and 2000 ms TWT (Fig. 4.4).

55

4.4.4 Nordmela Formation (Upper Jurassic)

The third formation of the Realgrunnen subgroup is the Nordmela Formation. In the present

study, the Nordmela Formation is interpreted as an Upper Jurassic reflector between 1750 and

2000 ms TWT (Fig. 4.4). The formation is relatively thick when measured in the reference

well (7119/12-2; 71°00'51.81"N, 19°58'20.81"E) but in contrast the thickness is only 62 m in

the type well (Fig. 4.3, 7121/5-1; 71°35'54.88"N, 21°24'21.78"E). The lateral thickness

variation between type and reference well suggests a southwest thickening wedge evidencing

early Kimmeridgian subsidence of the Ringvassøy-Loppa Fault Complex.

The upper part of the formation is mainly composed of sandstones but in general the

formation consists of interbedded siltstones, sandstones, shales and claystones with modest

amounts of coal. The variation in lithologies shows that the formation was deposited in tidal

flat to flood plain environments. Individual sandstone sequences represent estuarine and tidal

channels that dissected this low-lying area (Dalland et al. 1988). The Nordmela Formation is

Early Jurassic to early Mid Jurassic in age (Sinemurian - late Pliensbachian to Aalenian).

4.4.5 Fuglen Formation (upper Middle Jurassic)

The upper Mid Jurassic reflector corresponds to the Fuglen Formation between 1700 – 1800

ms TWT on seismic section (Fig. 4.4). The formation belongs to the Adventdalen Group

which includes sediments from Middle Jurassic (Bathonian) to Lower Cretaceous

(Cenomanian). The group is further subdivided into the Hekkingen, Knurr, Kolje and

Kolmule formations. The thickness of the group varies from 750-1600 m on Svalbard to

1000-1750 m on the Barents Sea Shelf. The dominant lithologies of the group are dark marine

mudstones with some deltaic and shelf sandstones along with carbonates of Late Jurassic to

Early Cretaceous in age. The group contains major hydrocarbon source rocks (Fuglen and

Hekkingen formationa) in the Upper Jurassic successions (Larssen et al. 2002).

The Fuglen Formation is composed of pyritic mudstone with interbedded thin limestones and

was deposited in marine environments during a high stand system tract. A relatively thick (48

m) unit of the formation was deposited in the western parts of the Hammerfest Basin

(7119/12-1). It thins to less than 10 m on the central highs of the basin. The age of the

formation is Late Callovian to Oxfordian (Dallmann 1999, Dalland et al. 1988).

56

4.4.6 Hekkingen Formation (Base Cretaceous)

The top of the Hekkingen Formation is represented in the study area by a reflector between

1680 and 1780 ms TWT (Fig. 5.4). The Hekkingen Formation is relatively thick (359 m) in

the type well (7120/12-1) but its thickness decreases down to 113 m in the reference well

(7119/12-1) showing northwards thinning to the Hammerfest Basin. The depositional pattern

shows the development of semi-grabens along basin margins. The formation consists of

brown-grey to very dark grey shales and claystones with thin interbeds of limestones,

dolomite, siltstones and sandstones (Larssen et al. 2002). The suggested age of the formation

is late Oxfordian/early Kimmeridgian to Ryazanian. The Hekkingen Formation is further

subdivided into the Alge and Krill members. The lower Alge Member consists of black shales

rich in organic material and the base of the member is defined by a transition from carbonate

cemented and pyritic mudstone to poorly consolidated shales deposited in restricted shelf

environments. The age of the Alge Member is late Oxfordian - Kimmeridgian. The upper

Krill Member of the Hekkingen Formation consists of brown-grey to very dark grey shales

and mudstone with some thin beds of limestone, dolomite, siltstone and sandstone. The

overall lithology of the Krill Member represents an open to restricted shelf environment. The

age of the member is Kimmeridgian – Volgian (Dalland et al. 1988).

4.4.7 Knurr Formation (Lower Cretaceous)

The top of the Knurr Formation is interpreted between 1600 and 1650 ms TWT in the study

area (Fig. 4.4). Based on dinoflagellates and foraminifera the age of the formation is

Ryazanian / Valanginian to early Barremian. The formation consists of dark grey to grey-

brown claystone with thin limestone and dolomites interbeds. A small amount of sandstone is

also present in the lower part of the unit but disappears laterally into the Hammerfest Basin.

The upper part of the formation consists of red to yellow brown claystones. The lithology of

the formation shows that the sediments were deposited in open and generally distal marine

environments with local restricted bottom conditions (Dalland et al. 1988). The thickness of

the formation recorded in the type well (7119/12-1) is 56 m and 285 m in the reference well

(7120/12-2).

57

4.4.8 Kolmule Formation (Base Upper Cretaceous)

The formation is mainly composed of dark grey to green claystone and shale. Limestone and

dolomite stringers along with minor thin siltstone beds are also present (Dalland et al. 1988).

Traces of glauconite and pyrite also occur in some places reflecting open marine

environments. The age of the formation is Aptian to mid-Cenomanian (Dalland et al. 1988).

4.5 Results and discussions

4.5.1 Early Cretaceous inversion

The study area was influenced by tectonic inversion (e.g. dextral strike slip) during Early

Cretaceous (Gabrielsen et al. 1997). The Bjørnøyrenna Fault Complex strikes mainly NE-SW

and is basically extensional, but the master fault plane is occasionally over-steepened. NNE-

SSW-striking structural highs (positive flower structures?) in the Bjørnøyrenna Fault

Complex affecting the assumed Lower Cretaceous (Hauterivian-Aptian?) sequence have been

interpreted to represent an event of dextral wrenching at that time (Riis et al. 1986, Brekke

and Riis 1987, Gabrielsen and Færseth 1988, Gabrielsen et al. 1992, 1997, Faleide et al.

1993a, 1993b, Vågnes et al. 1998).

In the present study, a lens-shaped structure affecting the Lower Cretaceous sediments is

detected in the central segment of the Bjørnøyrenna Fault Complex and interpreted as a

positive inversion one. The interpreted seismic section (Fig. 4.5), located at the central

segment of the Bjørnøyrenna Fault Complex (Fig. 4.2), depicts an inverted structure (red

square) affecting the EC (Early Cretaceous; Valanginian to early Barremian in age) and the

BUC (Base Upper Cretaceous; Aptian to Cenomanian). Both horizons have been

subsequently compressed and uplifted. The fault retains extension at a deeper level and

experienced net contraction associated with an anticline in the upper portion.

58

Figure 4.5. Interpreted inversion structure at the Early Cretaceous (Top Knurr Formation)

level. See figure 4.2 for location (Data courtesy of TGS and Spectrum).

The northernmost part of the central segment of the Bjørnøyrenna Fault Complex also shows

inversion structures affecting the Lower Cretaceous and Late Cretaceous sediments.

Interpreted seismic section (Fig. 4.6) showing inversion structure affecting LC (Lower

Cretaceous; Valanginian to early Barremian) and the BUC (Base Upper Cretaceous; Aptian to

Cenomanian) sediments on the northern segment of the Bjørnøyrenna Fault Complex. Small

wavy undulations can be imaged between the Base Cretaceous and the Lower Cretaceous

reflectors. The anticline imaged at the Lower Cretaceous level as well as the slight bulge on

the BUC (Base Upper Cretaceous) indicates Early and Late Cretaceous inversion phases.

Such positive structures are formed when extensional faults reverse their sense of motion

during compressional tectonics causing the basin to turn inside out and to become a positive

feature (Williams et al. 1989).

59

Figure 4.6. Interpreted inversion structure at the Early Cretaceous (Top Knurr Formation) and

Base Upper Cretaceous (Top Kolmule Formation) levels. See figure 4.2 for location (Data

courtesy of TGS and Spectrum).

Gabrielsen et al. (1997) and Hameed (2012) advance dextral strike-slip movements coeval to

Early Cretaceous inversion (Fig. 4.7 and 4.8). The contractional structures are overlain by a

set of extensional faults which affect the upper part of the section.

According to Gabrielsen et al. (1984, 1997), signs of tectonic inversion in the fault complex,

including deformed fault planes and reverse faults were dated to Cretaceous and Cenozoic.

These structures are developed above over-steepened fault branches in the hanging wall and

are associated with mild inversion (Fig. 4.7). The downward-steepening faults define

structures resembling positive (half) flower structures developed due to dextral strike-slip

movement (Gabrielsen et al. 1997).

60

Figure 4.7. Seismic line (NH8506-421) cutting the central segment of the Bjørnøyrenna Fault

Complex and depicting minor fold trains associated with assumed thrust faults. See Figure 4.2

for location. ILC (Intra Lower Cretaceous), BUC, (Base Upper Cretaceous). Modified after

Gabrielsen et al. 1997.

Hameed (2012) reported inversion structures in the central segment of the Bjørnøyrenna Fault

Complex. Based on the strike of the axis of the imaged fold, which is oblique to the strike of

the master fault, he interpreted the inversion to be caused by dextral strike-slip movement

(Fig. 4.8). Indrevær et al. (2016) also interpreted inversion structures of early Barremian to

mid-Albian age (ca. 131-105 Ma) along the margins of the Loppa High and concluded that

these inversion structures developed due to uplift of the Loppa High along its inclined

boundary fault (e.g. Bjørnøyrenna Fault Complex).

WNW ESE s twt

BUC

ILC

km

210

3

2

1

Northernseg

ment of Bjørøyrenna Fault Complex

61

Figure 4.8. Interpreted seismic section showing inversion structure (minor fold) at Lower

Cretaceous level in the central segment of the Bjørnøyrenna Fault Complex. See figure 4.2 for

location (modified after Hameed 2012).

The southern part of the Ringvassøy-Loppa Fault Complex is dominated by normal faulting

(Øvrebø and Talleraas 1977, Gabrielsen 1984, Faleide et al. 1984, Berglund et al. 1986). The

geometry of the complex is interpreted as two levels of detached listric normal faults and a

possible deeper zone of weakness (Gabrielsen 1984). Braut (2012) and Zalmstra (2013)

proposed that the two inversion events of the Ringvassøy-Loppa Fault Complex, during

Cretaceous (i.e. Early and Late Cretaceous), are of regional significance.

The present study also evidenced inversion in the eastern margin of the Ringvassøy-Loppa

Fault Complex, where a snake-head like structure affecting the Lower Cretaceous

(Valanginian to early Barremian) sediments in the hanging wall is observed and interpreted as

an inversion structure (Fig. 4.9). The interpreted seismic line (Fig. 4.9) shows inversion

structures in the eastern margin of the Ringvassøy – Loppa Fault Complex at lower and Late

Cretaceous levels. Folding at the Lower Cretaceous (Valanginian to early Barremian) and

62

Base Upper Cretaceous (Aptian to Cenomanian) sediments favors inversion in the Ringvassøy

– Loppa Fault Complex.

Figure 4.9. Interpreted inversion structures at the Early and Late Cretaceous level along the

Ringvassøy – Loppa Fault Complex. See figure 4.2 for location (Data courtesy of TGS and

Spectrum).

4.5.2 Late Cretaceous inversion

The study area was also affected by Late Cretaceous tectonic inversion (Gabrielsen et al.

1997). In the present study, an open fold affecting the Base Upper Cretaceous (Aptian to

Cenomanian) reflector is observed in the hanging wall of the central segment of the

63

Bjørnøyrenna Fault Complex (Fig. 4.5 and 4.6) and interpreted as an inversion structure. It is

assumed that the tilted and eroded Late Cretaceous horizon in the footwall was also involved

in folding or reverse faulting (Fig. 4.5). Inversion is also proposed for the eastern margin of

the Ringvassøy-Loppa Fault Complex at the base of Upper Cretaceous (Aptian to

Cenomanian) horizon. A Fold is detected affecting the Base Upper Cretaceous horizon and is

interpreted as an inversion structure (Fig. 4.9). It is suggested that the tectonic event that

affected the Bjørnøyrenna Fault Complex during Late Cretaceous is also responsible for the

inversion of the Ringvassøy – Loppa Fault Complex.

Minor fold trains associated with assumed thrust faults at the base of the Upper Cretaceous in

the central segment of the Bjørnøyrenna Fault Complex (Fig.4.7) were previously interpreted

by Gabrielsen et al. (1997) who suggested that the compressional event started to affect the

depositional geometry after the establishment of an intra-Cenomanian reflector (BUC;

Kolmule Formation) and continued throughout the Late Cretaceous and into the Early

Cenozoic. This compressional event caused inversion of local depocentres and development

of folds and reverse faults along the Bjørnøyrenna Fault Complex. The local erosion of Late

Cretaceous deposits is due to the thrusting which also affected the depositional pattern in the

area (Gabrielsen et al. 1997). The inversion features striking parallel to the fault complex

suggest that the Late Cretaceous inversion resulted from compression perpendicular to the

strike of the Bjørnøyrenna Fault Complex, i.e. SE-directed and presumably related to far field

stresses.

Riis et al. (1986) identified E W-trending fold axes affecting the Cenomanian along the south-

eastern margin of the Bjørnøya Basin and suggested that these structures were related to

wrenching.

Braut (2012) suggested inversion, based on the interpretation of folds in the hanging wall

block of the central segment of the Ringvassøy – Loppa Fault Complex where the base of the

Upper Cretaceous is affected by compression (Fig. 4.10).

64

Figure 4.10. Interpreted seismic section showing inversion structures (minor folds) at BUC

(Base Upper Cretaceous) level in the central segment of the Ringvassøy – Loppa Fault

Complex. See Figure 4.2 for location (modified after Braut 2012).

65

4.6 Conclusions

Kingdom 8.8 was used for the current research and 142 2D seismic lines were interpreted.

The main aim was to identify and interpret Cretaceous inversion structures in the

Bjørnøyrenna and the Ringvassøy – Loppa fault complexes. The results of the present study

suggest two different episodes of inversion which affected the fault complexes. The Early

Cretaceous inversion phase is related to strike-slip (dextral?) movement and the Late

Cretaceous relates to NW-SE far field stresses.

66

5. Structural restoration of Cretaceous inversion events in the Bjørnøyrenna FaultComplex, western Barents Shelf.

5.1 Introduction

The Barents Sea consists of a large epicontinental sea bounded by young passive continental

margins in the north and west (Faleide et al. 1984) and covers an area of approximately 1.4

million km2. It is bounded by Svalbard archipelago in the north and the Norwegian and

Russian coasts in the south. The Norwegian – Greenland Sea lies to the west and Novaya

Zemlya forms the eastern boundary of the Barents Sea (Fig. 5.1). The Barents Sea contains

some of the deepest basins in the world which developed due to different regional tectonic

events from Paleozoic to Cenozoic within the North Atlantic – Artic region (Faleide et al.

1993a). Its western part constitutes the northern Norwegian continental shelf and is located

between the mainland Norway and Svalbard, informally termed as southwestern Barents Sea

(Gabrielsen et al. 1997). Most of the fault complexes in the southwestern Barents Sea have an

overall NE-SW to ENE-WSW structural trend in its eastern and central parts, whereas the

western part of the area consists of NNW-SSE to N-S fault complexes (Gabrielsen et al. 1990,

1997; Fig. 5.1). These fault complexes formed from late Proterozoic to Cenozoic in response

to several rifting and collision events and indication of inversion structures were reported by

earlier investigators (Rønnevik and Motland 1979, Rønnevik et al. 1982, Faleide et al. 1984,

Rønnevik and Jacobsen 1984, Faleide et al. 1993a, Gabrielsen et al. 1997, Vågnes et al.

1998). The development of inversion structures has been suggested to be related to strike-slip

tectonics (Riis et al. 1986, Brekke and Riis 1987, Gabrielsen et al. 2016) or head-on inversion

(Gabrielsen et al. 1992, 1997). The aim of present study is to identify and decipher eventual

Cretaceous inversion structures in the Bjørnøyrenna Fault Complex by means of structural

restoration. To these aims, 2D MOVETM, a structural modeling and analysis software by

Midland Valley Exploration Ltd, is used and three key seismic lines crossing the central and

northern segments of the Bjørnøyrenna Fault Complex are restored.

67

5.2 Geometry and structural evolution of the Bjørnøyrenna Fault Complex

The NE-SW striking Bjørnøyrenna Fault Complex (Gabrielsen et al. 1990) is located in the

western Barents Sea between 72º N, 19 E and 73º 15' N, 22º E (Fig. 5.1) and is considered to

be the northern continuation of the Ringvassøy-Loppa Fault Complex (Gabrielsen et al.

1984). The fault complex marks the boundary between the platform-like Loppa High to the

southeast and deep Cretaceous basins to the northwest (Hinz and Schlüter 1978 in Gabrielsen

et al. 1997).


Google Maps and NPD fact maps http://gis.npd.no/factmaps/html_20/).

The Bjørnøyrenna Fault Complex is an extensional structural feature (Gabrielsen et al. 1990,

1997) which lies over a crustal zone of weakness (Gabrielsen et al. 1984) and displays

complex geometries due to multiple phases of deformation, including reactivation

(Grunnaleite 1991) and inversion phases (Gabrielsen et al. 1997). Based on fault geometry

and structural trend, Gabrielsen et al. (1997) subdivided the fault complex into four major

68

segments (Fig. 5.2). The southern segment (NNE-SSW and NE-SW), central segment (NE-

SW) and northern segment (NNE-SSW) are separated from the northwestern shoulder of the

Loppa High (segment 4), which consists of narrow horsts and grabens (Gabrielsen et al.

1997). According to Gabrielsen et al. 1992, these grabens have flower-like structures

(segment 4; Fig. 5.2).

Figure 5.2. Subdivision of the Bjørnøyrenna Fault Complex based on structural trend and

geometry. Modified after Gabrielsen et al. (1997).

Regional extension and minor strike-slip adjustments along old lineaments in the western

Barents Sea started again in Middle – Late Jurassic (Faleide et al. 1993a). At regional scale,

the Jurassic extensional structures of the Barents Sea belonged to the larger Arctic-North

Atlantic rift system (Doré 1991). The Middle – Late Jurassic extensional phase created the

Hammerfest and Bjørnøya basins following pre-existing structures and caused block faulting

(Faleide et al. 1993a). This was also coeval with renewed subsidence in the Tromsø and

Bjørnøya basins (Faleide et al. 1993a). The Bjørnøyrenna Fault Complex is of extensional

origin (Gabrielsen et al. 1990, 1997) and presents listric fault geometries (Faleide et al.

69

1993a). The main phase of subsidence along the fault complex started in Callovian

(Gabrielsen et al. 1990, Faleide et al. 1993a). The more complex structuration occurred in the

western Barents Sea at the end of Jurassic due to the development of regional scale

extensional fault blocks and the influence of shear movements in parts of the North Atlantic

(Håkansson and Stemmerik 1984, Riis et al. 1986).

In Early Cretaceous, subsidence went on in the western Barents Sea and, in particular, rapid

subsidence occurred along the Bjørnøyrenna and Ringvassøy-Loppa Fault complexes

(Gabrielsen et al. 1997). Faleide et al. (1993a) described three tectonic rifting phases affecting

the major basins of the western Barents Sea (e.g. Hammerfest, Tromsø and Bjørnøya Basin)

during Early Cretaceous. The first two phases (Berriasian/Valanginian and

Hauterivian/Barremian) strongly affected the Tromsø and Bjørnøya basins. The last phase

included thermal subsidence in the Tromsø Basin which is evidenced by gradually increased

thickness of the Barremian sediments (i.e. Kolje Formation) westwards in the Hammerfest

Basin and into the Ringvassøy-Loppa Fault Complex. In the western Hammerfest Basin,

uplift and thinning of the Aptian sequences (i.e. Kolmule Formation) towards the Ringvassøy-

Loppa Fault Complex evidence further the occurrence of an Aptian tectonic event in the area

(Faleide et al. 1993a). In general, the development of Early Cretaceous structures in the

western Barents Sea were coeval with the opening of the Amerasian Basin and the North

Atlantic rifting and mainly characterized by extensional faults with large downthrow to the

west with minor wrench component. Faleide et al. (1993a) described sinistral transtensional

strike-slip along the Bjørnøyrenna Fault Complex which caused formation of the Senja Ridge

and the Veslemøy High as positive structural elements. Riis et al. (1986) also suggested

sinistral shear which caused compressional faulting and folding in the Senja Ridge during

Early Cretaceous. Gabrielsen et al. (1997) suggested that the local thinning of the Hauterivian

– Aptian sequence along the Bjørnøyrenna Fault Complex is associated with mild inversion of

local depocentres and reverse faults associated with minor hanging wall folds. Gabrielsen and

Færseth (1988) also suggested dextral transpressional strike-slip movement during Early

Cretaceous. According to Gabrielsen et al. (1992) the fault pattern in the central segment of

the Bjørnøyrenna Fault Complex are similar to positive (semi)-flower structures. Indication of

local Early Cretaceous inversion is also observed along the Ringvassøy Fault Complex and its

junction with the Asterias Fault Complex (Gabrielsen et al. 1990).

During Late Cretaceous, opening of the Labrador Sea started and regional subsidence

occurred along the North Atlantic rift basins. These deep and broad basins ended at the Dee

70

Geer Zone where they were prolonged by pull-apart basins as a response to dextral oblique

slip (Faleide et al. 1993a). Most of the basins in the western Barents Sea (i.e. Tromsø and

Sørvestsnaget basins) continued to subside during the Late Cretaceous. Although extension

was dominant at regional scale during the Late Cretaceous, compressional deformation

evidenced by reverse faults and folds was also observed along the Bjørnøyrenna Fault

Complex and the Ringvassøy Fault Complex (Gabrielsen et al. 1990, 1997). Seismic

stratigraphy analysis suggested that compression started to act after the deposition of the

Intra-Cenomanian reflector and the Late Cretaceous strata, which was also involved in folding

and reverse faulting was eroded and uncomformably sealed by the Cenozoic strata

(Gabrielsen et al. 1997). The compressional phase continued into Cenozoic and early

Cenozoic sequences also experienced head-on inversion with NW-SE compression direction

(Figure 11c in Gabrielsen et al. 1997). The Late Cretaceous inversion phase is characterised

by e.g. upright open folds, close to tight inclined to recumbent folds and compressional

footwall shortcuts. In contrast, Rønnevik and Jacobsen (1984) and Riis et al. (1986) suggested

sinistral shear movement due to the opening of the North Atlantic Ocean, causing reactivation

of the Bjørnøyrenna Fault Complex in the Late Cretaceous and Cenozoic. The Ringvassøy

Fault Complex was also reactivated during the Late Cretaceous (Brekke and Riis 1987,

Gabrielsen et al. 1990).

5.3 Data and Methodology

We used 2D MOVETM (structural modeling and analysis software by Midland Valley

Exploration Ltd) for structural restoration of Cretaceous inversion structures in the

Bjørnøyrenna Fault Complex, western Barents Sea. Three key seismic profiles with different

orientations (Fig. 5.3) have been selected for this purpose. These seismic lines belong to two

different surveys (NBR08, NBR10) and are of variable quality and variable depth resolution.

The data were provided by the Department of Geosciences, University of Oslo, Norway. The

main motivation for the modeling is to restore the sections backwards, to locate null point

positions and to isolate inversion events at Lower and Late Cretaceous levels. To these aims

the 2D kinematic module has been used and different restoration techniques (i.e. 2D unfolding

or flexural slip, 2D move on fault or simple shear) have been adopted. In general, two

objectives can be obtained from restoration or backward modeling of a particular structure.

The structural restoration can validate the interpreted geometry in cross section and can

provide information about the processes of progressive deformation in the region. Detailed

71

accounts on structural restoration, 2D kinematics and theory of the used modules are given in

section 3.3.

Figure 5.3. Base map of the study area showing locations and orientations of the restored

seismic profiles and locations of used well.

In order to identify inversion events and to locate null points the models need to be

decompacted and restored. The workflow used in the current restored models includes

digitizing of seismic cross sections. Different horizons including sea bottom and top basement

were digitized and polygons representing the different sediment packages were created. The

details of all digitized horizons, age and thickness (according to Well 7219/9-1) are given in

table 5.1. A brief description of all stratigraphic units can be found in section 5.4. The 2D

depth-conversion tool was used to convert the seismic sections into depth.

The 2D decompaction tool was used to decompact the rock units and footwall blocks were

unfolded using the flexure slip method. The hanging wall blocks were moved upwards along

the fault to restore the latest pre-compressional sedimentary layer, i.e. Hekkingen Formation,

to its original position.

72

Table 5.1. Digitized horizons representing colour and age.

5.4 Structural restorations

Structural restoration allows for validating an interpreted section through backstripping of

depositional and tectonic events by applying certain geometric rules. The robustness of the

restoration is checked by means of identifying space problems during unfolding restoration.

The 2D unfolding tool, involving flexural slip unfolding, to restore footwall block and the

move-on-fault technique for hanging wall block were selected in the present study.

The flexural slip unfolding algorithm maintains bed thickness between the template horizon

and other passive objects. The algorithm is also built to maintain the line length of the

template horizon in the direction of unfolding and maintain the area of the fold and the

model4. A detailed description of the 2D unfolding procedure is given in section 3.3.2.1. The

move-on-fault module is used to restore deformation and allows the user to model pre and syn

tectonic successions and different displacements. Simple shear maintains the relationship

between fault geometry and hanging wall deformational features.

73

It diffuses deformation throughout the hanging wall instead of partitioning it into discrete slip

between beds (i.e. flexural slip). A detailed description of the 2D move-on-fault procedure is

given in section 3.3.2.2.

5.5 Results

Based on the interpretation of seismic cross-sections, calibrated by exploration wells, the

structural restoration permits to investigate the Early and Late Cretaceous inversion events in

the western Barents Sea. Total three key seismic profiles located in the central and northern

segments of the Bjørnøyrenna Fault Complex have been used for this purpose (Fig. 5.3)

5.5.1 Restoration of key profile 1

The WNW-ESE seismic cross-section crossing the central segment of the Bjørnøyrenna Fault

Complex (Fig. 5.3), was imported in MOVETM (Fig. 5.4). Ten key horizons (Table 5.1) and

two faults were digitized using horizon and fault options (Fig. 5.5). Stratigraphy and rock

properties were assigned for each sedimentary package according to data available from well

7219/9-1 (Table 5.2).

The polygons of the different sediments packages, ranging in age from (Late Triassic to Late

Cretaceous, were created using the “create auto polygon tool”. The seismic image then

converted into depth using 2D depth conversion tool. The tool used for conversion of a

seismic section from time to depth or vice versa.

4 Product manual of Move TM © 2016 Midland Valley Exploration Ltd.

74

The equation implemented is:

Z= V0 (ekt - 1)/k

Where:

Z = depth in meters

v0 = initial velocity (m/s)

k = rate of change in velocity with increasing depth

t = one-way travel time(s)

If k is equal to zero then the formula becomes:

Z= V0 t

Table 5.2. Rock properties of sedimentary packages adopted from well 7219/9-1.

Stratigraphy Top

depth

(m)

Time

(TWT)

Porosity Depth

coefficient

Velocity

(m/s)

Thickness

(m)

Sea bottom 356 0.356 0.5600 0.00 2000 356

Kolmule

Formation

1468 1.397 0.5360 0.43 2101 368

Knurr Formation 1836 1.656 0.5280 0.44 2217 57

Hekkingen

Formation

1893 1.689 0.5220 0.41 2241 26

Fuglen

Formation

1919 1.706 0.6300 0.51 2250 32

Nordmela

Formation

2062 1.784 0.5020 0.34 2312 143

Tubaen

Formation

2206 1.856 0.5170 0.35 2377 99

Fruholmen

Formation

2305 1.905 0.5020 0.41 2420 572

Snadd 2877 2.193 0.5080 0.44 2624 1424

75

Figure 5.4. Interpreted seismic cross-section of the central segment of the Bjørnøyrenna Fault

Complex. See figure 5.4 for location (Data courtesy of TGS and Spectrum).

All the sediment packages (polygons) were then decompacted using Sclater and Christie’s

laws (Sclater and Christie 1980). In present study, most of the used decompacted parameters

(e.g. surface porosity and depth coefficient) are given by the program (Table 5.2). A detailed

description of the 2D decompaction procedure is given in section 3.3.2.3. The top-most

horizon (Earth surface) was deleted after decompaction.

76

Figure 5.5. Digitized key profile 1; seismic cross-section showing sediment packages and

major faults (see figure 5.4 for location).

The footwall block is then unfolded using the flexural slip method (Fig. 5.6). The base

Cretaceous reflector (top Hekkingen Formation) is used as a ‘template bed’ and unfolded to

an arbitrary depth of ~1550 m. The rest of all horizons are kept as ‘passive features’ which

follow the template bed and preserve interbed volumes. The resulted restored section shows

the unfolded footwall block with anticlockwise rotation and the hanging wall block on the left

side remains unaltered (Fig. 5.6). The restored section resulted in ~100-150 m gap between

the two fault blocks.

77

Figure 5.6. Unfolded footwall block at Base Cretaceous level (top of Hekkingen Formation).

The hanging wall block is moved along the fault to connect the base Cretaceous horizon (Fig.

5.7). A simple shear method is adopted and a shear angle of 32° with respect to vertical

(Yamada and McClay 2003) is applied. The hanging wall block was moved ~300m upwards

and the resultant horizontal elongation or restored shortening is ~200-250m. All the sediment

packages up to the Base Cretaceous level are well connected with each other (Fig. 5.8). The

thickness variations of Lower Cretaceous sediment package (Knurr Formation) from the

hangingwall block and to the footwall fault block indicate positive inversion. The extensional

fault reversed the sense of motion during compression which caused Lower Cretaceous

sediments to turn inside out and to become a positive feature (Fig. 5.8). As a result the fault

retains net extension at a deeper level and net contraction associated with an anticline in the

upper portion.

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The resultant horizontalelongation or restoredshortening is ~200-250m.

The hanging wall block moved ~300m upward.

Figure 5.7. Hanging wall block is moved along the fault up to the base Cretaceous level.

The restored section shows the null point at the base of the Cretaceous (Hekkingen Formation,

Fig. 5.8). The position of the null point evidences the progressive compressional inversion of

Lower Cretaceous syn-rift sequence (Knurr Formation). Below the null point, the geometry of

the restored fault shows normal faulting while above the null point the geometry points to

reverse. The eroded part of Lower Cretaceous (Knurr Formation) is interpolated which

suggests reverse faulting.

The restored section shows also positive inversion as a fold in the hangingwall at the base of

the Upper Cretaceous (Kolmule Formation). The eroded Kolmule Formation is interpolated

from the footwall towards the hanging wall (Fig. 5.8). The interpolation suggests reverse

faulting of the base of the Upper Cretaceous. The restored section is slightly modified and

space issues (~100-200m) between the hanging wall and the footwall are solved (Fig. 5.8).

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Figure 5.8. Position of null point indicates positive inversion affecting Lower and Upper

Cretaceous sediment packages.

5.5.2 Restoration of key profile 2

The WNW – ESE seismic cross section (Fig. 5.9) located north of key profile 1 and passing

through the central segment of the Bjørnøyrenna Fault Complex (see figure 5.4 for location) is

used for structural restoration. Eight horizons including top Fruholmen Formation (Base

Jurassic), top Tubåen Formation (Early Jurassic), top Nordmela Formation (Mid Jurassic), top

Fuglen Formation (Upper Mid Jurassic), top Hekkingen Formation (Base Cretaceous), top

Knurr Fomation (Lower Cretaceous), top Kolmule Fomation (Base Upper Cretaceous) and

seafloor are digitized (Fig. 5.10).

Assumed eroded part ofthe Early Cretaceous(Knurr Formation).

Position of null point at the baseCretaceous (Hekkingen Formation).

Assumed eroded part of thebase upper Cretaceous(Kolmule Formation).

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Figure 5.9. Interpreted seismic cross- crossing the central segment of the Bjørnøyrenna Fault

Complex (for location see figure 5.4; Data courtesy of TGS and Spectrum).

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Figure 5.10. Digitized key profile 2 (for location see figure 5.4).

After creating polygons of all sediment packages the seismic image is converted into depth

using the 2D depth conversion tool. The footwall is then unfolded using the flexural slip

method (Fig. 5.11). A similar procedure to the one used in the restoration of key profile 1 is

applied and the base Cretaceous reflector is used as a ‘template bed’. The footwall is unfolded

to the 1600 m depth level. The other horizons are kept as ‘passive features’. The restored

section shows an unfolded footwall with anticlockwise rotation and the hanging wall remains

unaltered (Fig. 5.11). The restored section resulted in ~150-200m gap between the two fault

blocks. The base of the Upper Cretaceous (Kolmule Formation) shows clockwise tilting on

the footwall as a response to the underlying folded Knurr Formation (Fig. 5.11).

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Fault gap (~150-200m) between twofault blocks.

Unfolded to datum ~ 1600m (Thepresumed depth of base Cretaceousused as reference).

Figure 5.11. 2D unfolding (flexure slip method) using the Base Cretaceous (top of Hekkingen

Formation) as template bed.

The hanging wall is moved ~700m along the fault in order to the base Cretaceous horizon in

the footwall block (Fig. 5.12). A simple shear method is adopted and a shear angle of 32°

from the vertical is applied as proposed by Yamada and McClay (2003). The resultant

horizontal elongation or restored shortening is ~500m. The eroded part of Lower Cretaceous

(Knurr Formation) and the Upper Cretaceous (Kolmule Formation) are interpolated. The

eroded sediment packages on the footwall (Knurr Formation and Kolmule Formatio) are

extended to the hangingwall (Fig. 5.12). In the upper part of the Lower Cretaceous sequence,

the extensional fault inverted its sense of motion during compression and, as a result, the

restored section suggests a positive inversion structure (anticline, reverse fault) at the level of

Lower Cretaceous (Fig. 5.12). The restored section also indicates a compressional structure

(reverse fault) at the base of the Upper Cretaceous (Kolmule Formation).

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Figure 5.12. Hanging wall block is moved along the fault up to the base Cretaceous (top of the

Hekkingen Formation).

Position of null point at the baseCretaceous level.

The hanging wall block moved ~ 650 - 700m upward.

The resultant horizontal elongationor restored shortening is ~ 500m.

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5.5.3. Restoration of key profile 3

The WSW-ENE seismic cross section (Fig. 5.13) crossing the northern segment of the

Bjørnøyrenna Fault Complex (figure 5.4) was used for structural restoration.

Figure 5.13. Interpreted seismic cross-section of the northern segment of the Bjørnøyrenna

Fault Complex (see figure 5.4 for location; Data courtesy of TGS and Spectrum).

Seven horizons including top Tubåen Formation (Early Jurassic), top Nordmela Formation

(Mid Jurassic), top Fuglen Formation (Upper Mid Jurassic), top Hekkingen Formation (Base

Cretaceous), top Knurr Fomation (Lower Cretaceous), top Kolmule Fomation (Base Upper

Cretaceous) and seafloor are digitized (Fig. 5.14).

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Figure 5.14. Digitized key profile 3 (for location see figure 5.4).

“Auto polygon tool” is used and polygons are created representing different sediment

packages ranging in age from Early Jurassic to Late Cretaceous. A similar procedure to the

one used for the depth conversion and decompaction of profile 1 and 2 is applied. In key

profile 3, the footwall block is eroded upto the base of the Upper Cretaceous (Kolmule

Formation) due to the uplift of the Loppa High, located in the east of the Bjørnøyrenna Fault

Complex. After depth conversion and decompaction, 2D unfolded technique is applied on

hanging wall block.

The base Cretaceous horizon (Hekkingen Formation) is used as a ‘template bed’ and the

section is unfolded at the depth of ~1430m. The resulted restored section shows

compressional structure (anticline) associated with Early and Late Cretaceous inversion

events. Both the Lower Cretaceous (Knurr Formation) and the base of the Upper Cretaceous

(Kolmule Formation) moved down ward showing compression. A minor gap (~50-75m)

occurred between the hanging wall block and the Loppa High due to unfolding (Fig. 5.15).

The base of the restored section is slightly modified (20-30m) to avoid further gaps.

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Figure 5.15. 2D unfolding (flexure slip method) using the Base Cretaceous horizon (top of

Hekkingen Formation) as templet bed.

The restored section shows downward movement of hangingwall depicting inversion structure

(anticline) at Lower Cretaceous (Knurr Formation) and at the base upper Cretaceous

(Kolmule Formation).

Fault gap (~ 50-75m).

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5.6 Discussions

The Bjørnøyrenna Fault Complex is defined as extensional by origin (Gabrielsen et al. 1990,

1997) with listric fault geometries (Faleide et al. 1993). The main phase of subsidence along

the fault complex started in late Middle Jurassic i.e. Callovian (Gabrielsen et al. 1990, Faleide

et al. 1993a). The subsidence along the Bjørnøyrenna Fault Complex is locally interrupted in

(Hauterivian-Aptian) which is evidenced by local thinning of Lower Cretaceous (Hauterivian-

Aptian) sediments (Gabrielsen et al. 1997). The inversional event is marked as a strike-slip

origin (dextral wrenching) on the basis of interpreted basin ward tilt of strata which define

half-flower-like structures (Riis et al. 1986, Brekke and Riis 1987, Gabrielsen and Færseth

1988, Gabrielsen et al. 1992, 1997). Indrevær et al. 2016 also interpreted inversion structures

of early Barremian to mid-Albian age (ca. 131-105Ms) along the margins of the Loppa High

and concluded that these inversion structures developed due to uplift of the Loppa High along

its inclined boundary fault (e.g. Bjørnøyrenna Fault Complex). The uplift of the Loppa High

may be due to the contemporaneous extreme lithospheric thinning going on in the Tromsø and

Bjørnøya Basin and somehow the heat created from this thinning caused the Loppa High to be

uplifted more than its surroundings and hence cause local tectonic inversion in the area as it

forced itself upwards like a wedge (Indrevær et al. 2016). The restored seismic sections along

the Bjørnøyrenna Fault Complex also showed inversion in Early Cretaceous. The unfolding of

footwall block helped to mark the position of null point in key profile 1 and 2, and restored

eroded parts of Lower Cretaceous (Knurr Formation). The position of ‘null pint’ is showing

the progressive compressional inversion of extensional syn-rift of Lower Cretaceous sequence

(Knurr Formation). Below the null point, the geometry of the restored fault showing normal

faulting while above the null point the geometry is reverse.

The Bjørnøyrenna Fault Complex also affected by Late Cretaceous tectonics and was

reactivated in compression in the Cenomanian, and again in the Late Cretaceous (Gabrielsen

et al. 1997, Vågnes et al. 1998). The Late Cretaceous sediments were also involved in folding

and reverse faults and were eroded and diconformably overlain by the early Cenozoic strata.

The Late Cretaceous inversional phase is resulted from head-on, southeasterly-directed

contraction and characterised by minor fold trains associated with assumed thrust faults at the

base of the Upper Cretaceous (Kolmule Formation) in the central segment of the

Bjørnøyrenna Fault Complex (Gabrielsen et al. 1997). Restoration of key profiles also

suggested folds and reverse faults associated with Late Cretaceous inversion event. The

results also suggested horizontal elongation or restored shortening ~200-250m in the central

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segment of the Bjørnøyrenna Fault Complex (Key profile 1) and ~500m shortening in the

northern part of the central segment (Key profile 2).

5.7 Conclusions

Inversion in the Bjørnøyrenna Fault Complex is evidenced by thickness vatiations of Lower

Cretaceous (Hauterivian-Aptian) sediments and by folds associated with assumed reverse

faults in Late Cretaceous. The restored seismic sections also confirmed two inversion events

i.e. Early and Late Cretaceous. The thickness variations of the Lower Cretaceous sediment

package (Knurr Formation) in the hangingwall block and footwall fault block (key profile 1

and 2) indicate positive inversion. The extensional fault reversed the sense of motion during

compression which caused Lower Cretaceous sediments to turn inside out and became

positive feature. As a result fault retains net extension at a deeper level and got net contraction

associated with anticline in the upper portion.

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6. Numerical modeling of multi stage basin inversion in the western Barents Shelf

6.1. Introduction

The western Barents Shelf covers the area between Svalbard in the north, mainland Norway in

the south, Novaya Zemlya in the east and the Norwegian – Greenland Sea in the west (Fig.

6.1). It is bounded by the Norwegian Greenland Sea in the west and the Eurasian Basin in the

north, which developed during the final continental breakup in Cenozoic (Faleide et al.

1993a). During the time of crustal breakup the western Barents Sea consisted in two mega-

lineaments including the North Atlantic rift zone between the present Charlie Gibbs and Senja

Fracture Zones and a shear zone, the De Geer Zone (Harland 1969, Faleide et al. 1993a). The

overall setting of the area showed a transcurrent to transform notion during the Late

Cretaceous - Paleogene rifting and early opening between Greenland and Eurasia.

The study area contains some of the deepest basins of the world separated by intra-basinal

structural highs and deep seated fault complexes (Gabrielsen et al. 1990, Faleide et al. 1993a).

Different regional tectonic events from Paleozoic to Cenozoic within the North Atlantic –

Arctic region are responsible for the development of these structures (Faleide et al. 1993a,

Gernigon et al. 2014).

The overall structural trend in the western Barents Sea is NE-SW to ENE-WSW in its eastern

and central parts, whereas the western part of the area consists of NNW-SSE to N-S fault

complexes (Gabrielsen et al. 1990; Fig. 6.1).

The existence of contractional structures (anticlines, synclines and reverse faults) of

Cretaceous – Cenozoic age and reactivation of major fault complexes in the western Barents

Sea were confirmed by numerous investigators (Rønnevik and Motland 1979, Rønnevik et al.

1982, Rønnevik and Jacobsen 1984, Faleide et al. 1984, Faleide et al. 1988, 1993a, b,

Richardson 1992, Vågnes et al. 1998, Bergh and Grogan 2003, Gabrielsen et al. 1990, 1997).

The development of inversion structures in the Barents Sea is suggested to be related to strike-

slip (Riis et al. 1986, Brekke and Riis 1987, Gabrielsen et al. 1990, 1997) or head-on

inversion (Gabrielsen et al. 1992, 1997). Possible mechanisms for the formation of Cenozoic

inversion structures in the Atlantic region could be: rifting and subaerial sea-floor spreading

(Vågnes et al. 1998), ridge push and mantle drag (Bott 1991, Boldreel and Andersen 1993,

Wilson 1993, Doré and Lundin 1996, Gölke and Coblentz 1996, Bird 1998, Vågnes et al.

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1998, Doré et al. 2008, Ziegler et al. 2001) and differential sediment loading (Stuevold et al.

1992).

Figure 6.1. Regional setting and major structural elements of the SW Barents Shelf (modified

from and google map and NPD fact maps http://gis.npd.no/factmaps/html_20/). AFC = Asterias

Fault Complex, BB = Bjørnøya Basin , BFC = Bjørnøyrenna Fault Complex, BP = Bjameland Platform, CFZ = Central Fault

Zone, COB = Continental Oceanic Boundary, FSB = Fingerdjupet Sub-Basin, HB = Harstad Basin, HfB = Hammerfest

Basin, HFC = Hoop Fault Complex, HrFC = Hornsund Fault Complex, KFC = Knølegga Fault Complex, LFC = Leirdjupt

Fault Complex, LH = Loppa High, MB = Maud Basin, MFC = Måsøy Fault Complex, MH = Mercurius High, NB =

Nordkapp Basin, NFC = Nysleppen Fault Complex, NH = Norsel High, OB = Ottar Basin, PSB = Polhem Sub-Platform, R-

LFC = Ringvassøy – Loppa Fault Complex, SB = Sørvestsnaget Basin, SFZ = Senja Fracture Zone, SH = Stappen High, SR

= Senja Ridge, TB = Tromsø Basin, T-FFC = Troms-Finnmark Fault Complex, TIFC = Thor Iversen Fault Complex, VH =

Veslemøy High, VVP = Vestbakken Volcanic Province.

The aim of the research is to investigate the causes and effects of Mesozoic and Cenozoic

inversion events from Late Triassic to Miocene in the western Barents Shelf through

numerical modeling. In order to model Mesozoic and Cenozoic stress pattern in the western

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Barents Sea, ANSYS™ (Work bench) was used and four different 2-D thin plate modeling

setups were made.

6.2. Numerical modeling

6.2.1 The Finite Element Method (ANSYS™)

A multiphysics FEM program, namely ANSYS™, an advanced engineering simulation tool

that is aimed at the analysis of various physical systems and problems like fluid dynamics,

structure mechanics, thermomechanical systems and electromagnetics was used. The tool can

also be used in the simulation of stress and strain. For detailed mathematical description the

reader is referred to section 3. Considering the specific geometry of the problem and the data

at hand, a 2-D thin-plate approach was adopted, assuming plane strain conditions. In the

models linear elasticity was used and contact elements were introduced to simulate major

faults.

6.3 Model set up

Four different models were constructed in order to predict stress patterns and to explore the

conditions for fault reactivation and eventually tectonic inversion during four specific and

regional tectonic events spanning from Late Triassic to Miocene. The Late Triassic to Early

Jurassic (i.e. Model 1), the Late Cretaceous (i.e. Model 2), Early Eocene (Model 3) and

Miocene to recent (Model 4) inversion events. Model 1 involved ~70250 triangular solid

elements with mid-nodes and ~ 2970 surface to surface body contact elements. Model 2, 3

and 4 included ~ 87225 triangular solid elements and approximately 3000 contact elements.

In the most refined parts of the models (i.e. along the simulated faults) element minimum size

is ~20 m and average size is ~100 m (Fig. 6.2).

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Figure 6.2. Numerical meshes with refinement along simulated faults. (a) Model 1 involved

~70250 triangular solid elements with mid-nodes and ~ 2970 surface to surface body contact

elements. (b) Model 2, 3 and 4 included ~ 87225 solid elements and ~3000 contact elements.

Elastic and isotropic elastic behavior was prescribed to the models and two domains with

distinct properties were defined: (1) an oceanic one with relatively high Young's modulus

(E=100 GPa) (Model 4) and (2) a continental one (E=60 GPa). The latter value remains

reasonable for continental crust (e.g. Pascal and Gabrielsen 2001). A standard Poisson's ratio

value of 0.25 was chosen for the whole modeled area. The major fault zones introduced into

the models represent the major structures bordering the sedimentary basins of the western

Barents Sea (Faleide et al. 1993a, b, Gabrielsen et al. 1990, 1997; Table. 6.1). In order to

explore what fault segments are the most likely to be reactivated during the four studied

inversion events, friction coefficient values of 0.1 to 0.6 were tested and 0.1 is set for all fault

segments in models because the modeled stress magnitudes are too low to generate

displacement along faults with standard friction coefficients. Normal stiffness (FKN) values

from 0.01 to 1 were tested. The lowermost FKN values (0.01 – 0.1) resulted in bending and

contact penetration problems. No difference was noted for FKN values from 0.1 to 1. A value

of 1 was finally retained for the modeling.

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Fault zones

Model 1Young Modulus

(E) = 60 GPa

Poisson’s ratio =0.25


(E) = 60 GPa



(E) = 60 GPa


Model 4Young Modulus(E) = 100 GPa


Frictioncoefficient

(µ)

Asterias FaultSystem √ √ √ 0.1

BjørnøyrennaFault Complex √ √ √ 0.1

Hoop FaultSystem

√√ √ √ 0.1

Hornsund FaultComplex √ √ 0.1

Leirdjupt FaultComplex √ √ √ 0.1

Måsøy FaultComplex √ √ √ √ 0.1

Nysleppen FaultComplex

√√ √ √ 0.1

Ringvassøy –Loppa Fault

Complex√ √ √ 0.1

Senja FractureZone √ √ 0.1

Troms-FinmarkFault Complex √ √ √ √ 0.1

Thor IversenFault Complex √ √ √ √ 0.1

Table 6.1. Major fault zones introduced into the models with their material properties.

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6.4 Boundary conditions

The imposed boundary stresses were taken in agreement with stress directions suggested in

the literature (Doré and Lundin 1996, Gabrielsen et al. 1997, Vågnes et al. 1998, Mosar et al.

2002, Tsikalas et al. 2002, Buiter and Torsvik 2007, Doré et al. 2008). Different stress

magnitudes were tested (10, 25 and 50 MPa) for each model. Finally, a stress magnitude of 50

MPa was selected (Fig. 6.3). In Model 1, Late Triassic to Early Jurassic E-W compression,

associated with westward motion of Novaya Zemlya (Buiter and Torsvik 2007), was assumed

(Fig. 6.3a). The western and northern boundaries of Model 1 were kept fixed in the x and y

directions respectively. For Model 2, NW-SE directed compression in Late Cretaceous was

assumed (Fig. 6.3b) with fixed western (in the x direction) and northern (in the y direction)

boundaries. In Model 3, NW-SE Early Eocene North Atlantic opening (Tsikalas et al. 2002,

Doré et al. 2008) was introduced (Fig. 6.3c). In the present modeling 1 km of opening was

imposed. Such a value allows for keeping the model numerically stable while simulating

stress directions. The northern boundary of the model was kept fixed according to y and the

eastern and western boundaries according to x. In addition, ~ 20 nodes on the Barents Shelf

side along the Senja Fracture Zone are fixed to prevent artefacts due to physical spreading of

the material towards the gap created during the opening. As a far field stresses, 50 MPa stress

is induced from the southern boundary. For Model 4, NW-SE Atlantic ridge push from

Miocene to present-day was introduced (Fig. 6.3d). All four models cover ~ 2000 km2 but

only ~ 900 km2 represent the area of interest. In order to avoid potential edge effects, the

external boundaries of the models were placed relatively far away (i.e. ~ 550 km) from the

area of interest (Fig. 6.3).

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Figure 6.3. Geometry and assumed boundary conditions used in the modeling (acronyms as in

caption of fig. 6.1). See text (section 6.4) for explanation.

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6.5 Results

6.5.1 Late Triassic – Early Jurassic

Simulated stress patterns are presented for the Late Triassic-Early Jurassic in figure 6.4. The

results indicate local stress rotation in the northern segment (1) of the Thor Iversen Fault

Complex. The NE-SW to ENE-WSW fault was reactivated as a dextral one in the model

implying that the maximum principal stress rotated counterclockwise (Fig. 6.4). The angle

between the strike of the fault and the regional maximum principal stress is suggestive of

transpression. Slight counterclockwise rotation of ϭHmax is also modeled in the central segment

(2) of the Thor Fault Complex. The change in direction for the maximum principal stress is

due to the NE-SW orientation of the fault. The simulated results show dextral reactivation in

particular for fault segment (2). The southern segment (3) of the Thor Iversen Fault Complex,

which lies parallel to the main applied stress, does not show any rotation due to E-W

alignment of the fault strike (Fig. 6.4).

Figure 6.4. Simulated stress patterns for Late Triassic-Early Jurassic

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In concert with the observations (Glørstad-Clark et al. 2010) reactivation as a dextral fault and

subsequent stress rotation is also predicted for the Måsøy Fault Complex (4) similarly to the

Thor Iversen Fault Complex. Large counterclockwise σ1 rotations were also modeled in the

southern tip of the Måsøy Fault Complex (5) depicting reactivation.

A very slight stress rotation is also observed in the southern part of the Hoop Fault Complex

(6) where maximum principal stresses are simulated with a counterclockwise rotation (Fig.

6.4). The simulated stress pattern shows dextral reactivation which is due to the NNE-SSW

strike of fault. The modeled maximum principal stress pattern shows rotation in the northern

part (N-S) of the Troms-Finnmark Fault Complex (7), where counterclockwise rotations at its

southern tip and clockwise rotations at its northern tip point to dextral reactivation.

No change in maximum stress pattern is modeled in the northern segment of the Hoop Fault

Complex (8) which lies perpendicular (N-S) to the applied stress direction (E-W). Also along

the Norwegian mainland (9) and on the western side of the model 1 (10) the simulated stress

pattern is almost regular due to the modeled uniform rheological properties.

6.5.2 Late Cretaceous

Simulation of stress patterns (Model 2) is presented in Figure 6.5, which was designed to

account for Late Cretaceous tectonic situation. Modeled stress rotations are observed in most

of the major fault complexes located in the study area (Bjørnøyrenna Fault Complex,

Ringvassøy – Loppa Fault Complex, Leirdjupet Fault Complex, Hoop Fault Complex,

Asterias Fault Complex, Nysleppen Fault Complex, Måsøy Fault Complex and Troms-

Finnmark Fault Complex).

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Figure 6.5. Simulated stress patterns for the Late Cretaceous.

Major stress rotation were modeled in the eastern side of the study area, where the NE-SW the

Nysleppen Fault Complex, shows a change in stress pattern (1). The southern segment of the

Nysleppen Fault Complex experienced clockwise stress rotation, depicting reactivation during

Late Cretaceous (Fig. 6.5).

A change in stress pattern is suggested in the northern part (2) of the Troms -Finnmark Fault

Complex (Fig. 6.5) where the maximum principal stress (regional strike NW-SE) is indeed

modeled with a N-S strike. A clockwise stress rotation at the northern tip (2) of the segment

suggests reactivation of the Troms -Finnmark Fault Complex.

Moderate stress rotation was simulated for the NE-SW-striking the Hoop Fault Complex (Fig.

6.5). The simulated maximum principal stress pattern becomes parallel in the northern

segment (NNW-SSE) of the Hoop Fault Complex (3). In contrast, simulated stress directions

in the southern segment (NE-SW) of the fault rotate counterclockwise (4). The change in

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direction for the maximum principal stress is due to the alignment of the fault segments and

suggests reactivation of both segments of the Hoop Fault Complex (Fig. 6.5; 3, 4).

The modeled Late Cretaceous tectonic situation shows clockwise stress rotations in the

eastern segment (5) of the Asterias Fault Complex. The direction of maximum horizontal

stress (Hmax) becomes perpendicular along the strike of the fault (ENE-WSW) in the central

segment (5) and rotated counterclockwise in the western segment of the Asterias Fault

Complex. Notable clockwise rotation in the eastern tip and counterclockwise stress rotations

in the western tip of the Asterias Fault Complex indicates reactivation of the segment. The

simulated stress pattern close to the central segment of the Asterias Fault Complex depicts

inversion (Fig. 6.5). It is noted that Model 2 displays a stress situation that would be

compatible with a compressional regime in the Asterias Fault Complex in the Late

Cretaceous.

Model 2 suggest stress rotations for the Leirdjupet Fault Complex (6). The simulated results

show a change in Hmax direction in the northern segment (6) of the Leirdjupet Fault Complex

which is oriented NNE-SSW (Fig. 6.5). Hmax rotated clockwise and aligned itself with the

fault (6). The maximum stress pattern shows reactivation and inversion of the Leirdjupet Fault

Complex.

Clockwise stress rotations were simulated for the Bjørnøyrenna Fault Complex and the

Ringvassøy – Loppa Fault Complex in Model 2 (Fig. 6.5). A change in direction for the

maximum principal stress was modeled in the central segment (7) of the Bjørnøyrenna Fault

Complex (Fig. 6.5). The results show that the direction of Hmax changed from NW-SE to

NNW-SSE (clockwise) depicting reactivation. The northern segment of the Bjørnøyrenna

Fault Complex (7) also shows a change in Hmax direction (Fig. 6.5). Maximum principal

stresses become almost perpendicular to the fault strike (NE-SW) suggesting inversion of the

northern segment of the fault. Compressional stress was also modeled for the Ringvassøy –

Loppa Fault Complex (Fig. 6.5). The counterclockwise stress rotation in the northern part (8)

here is in agreement with the presence of contractional structures (e.g. folds and reverse

faults) observed by Gabrielsen et al. (1997) and Hameed (2012) and the findings of the

seismic interpretation in this thesis.

Drastic Hmax rotations were also simulated in the Veslemøy High and the Senja Ridge (Fig.

6.5, 9). Triple junction stress rotations are due to the closely located edges of the structural

highs. Pronounced clockwise maximum principal stress rotation was modeled in the central

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part of the Dee Geer Zone (10) where Hmax became parallel to the strike of this latter zone

(NW-SE).

The uniform stress pattern orientation on the Finnmark Platform (11) and the western part

(12) of the Model 2 is a consequence of to choice of uniform rheological properties for these

areas.

6.5.3 Early Eocene

Figure 6.6 shows the simulated stress pattern caused by dextral shear between Greenland and

the Barents Sea Shelf in Eocene. Along the Senja Fracture Zone (Fig. 6.6a, 1), ϭHmax becomes

parallel to the strike of the fault (i.e. NW-SE). The modeled stress pattern shows shearing

along the southern segment of the De Geer Zone (Fig. 6.6b). A slight change in stress

direction on the western side of the southern segment is due to the imposed boundary

conditions (Fig. 6.6a and 6.6b). The simulated stress pattern shows rifting in the central

segment (2) of the De Geer Zone (Fig. 6.6b). The development of the rift segment follows the

NW-SE applied stress direction which acted perpendicularly to the NE-SW striking central

segment (Fig. 6.6b). Along the northern segment of the zone, the results of the modeling

suggest shearing and rifting (3). ϭHmax was modeled parallel to the fault strike (i.e. NW-SE) in

the northern part of the segment, depicting shearing/strike-slip movement (4). Along the

southern part, just above the rifted segment, the simulated stress pattern suggests moderate

extension and shearing (Fig. 6.6b, 3). A slight change in the stress pattern on the western side

of the model is due to the boundary conditions. On the eastern side (5) of the model 3, the

stress pattern is regular due to the applied far-field stresses. The results suggest that Early

Eocene sea floor spreading caused stress partitioning along the Senja Fracture Zone and did

not influence significantly the eastern side of it (Fig. 6.6).

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Figure 6.6. Simulated Eocene stress patterns

6.5.4 Miocene

Figure 6.7 shows the Miocene simulated stress pattern (Model 4). Stress rotations of ~30-40°

are modeled at the continent-ocean boundary (1) because of the imposed contrasting

rheologies between the two modeled crustal domains. Pronounced stress rotations are

simulated along the boundary segments which are oriented obliquely to the applied stress.

More drastic stress deflections are visible in the Hornsund Fault Complex (2), where ϭHmax

becomes perpendicular to fault strike (Fig. 6.7). Large stress rotations occurred at the tips of

the fault zone, where ϭHmax becomes parallel or perpendicular to the fault. A notable

counterclockwise rotation at the northern tip and clockwise stress rotations in the south of the

Hornsund Fault Complex indicate dextral reactivation of the segment. The southern part of

the continental oceanic boundary (i.e. Senja Fracture Zone; 3) also shows stress deflections

(Fig. 6.7). Maximum principal stresses become perpendicular to the strike of the fault.

The simulated maximum principal stress pattern becomes parallel to the NW-SE northern

segment (4) of the Knølegga Fault Complex (Fig. 6.7). In contrast, simulated stress directions

in the southern segment of fault (5), striking N-S and NNE-SSW, are perpendicular. The

change in direction for the maximum principal stress is due to the alignment of the fault

segments and may cause reactivation of the Knølegga Fault Complex.

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Clockwise stress rotations are modeled in the Leirdjupet Fault Complex. Maximum principal

stresses are parallel throughout the main fault (6) depicting a strike-slip component (Fig. 6.7).

Stress rotations are modeled by the Bjørnøyrenna Fault Complex. The modeling results in the

northern segment (7) of the Bjørnøyrenna Fault Complex show that σ1 is simulated

perpendicular to the fault (NE-SW), whereas it appears clockwise rotated and almost parallel

(NW-SE) to fault complex in its central part (8) (Fig. 6.7). The change in stress pattern

suggests reactivation and inversion in the particular segments of the fault and suggests the

development of strike-slip related inversion along the central and head-on inversion in the

northern segment of the fault complex. The southern part of the fault exhibits inversion, as the

modeled clockwise rotation of the maximum principal stress suggests (Fig. 6.7).

The maximum horizontal stresses are deflected and become almost parallel to the N-S

southern segment of the Ringvassøy – Loppa Fault Complex (Fig. 6.6, 9). Change in stress

directions were also modeled along the northern segment, where maximum principal stresses

rotated clockwise to become almost parallel to the fault. The simulated stress pattern suggests

strike-slip movement in the southern segment and head-on inversion in the northern segment

of the fault complex. The rotation of maximum principal stresses is due to the N-S and NE-

SW orientation of the fault segments.

The northern segment of the Ringvassøy – Loppa Fault Complex and the central part of the

Bjørnøyrenna Fault Complex showed stress deflection, where maximum principal stresses

aligned parallel to the NNE-SSW strike of the faults. The stress pattern is consistent with

strike-slip.

Major stress rotations were modeled in the eastern side of the study area, where the NE-SW

Hoop Fault Complex (10) and the Nysleppen Fault Complex (11) are associated with changes

in stress pattern. The southern segment of the Hoop Fault Complex experienced clockwise

stress rotation, suggesting reactivation during Miocene. The southern tip of the Nysleppen

Fault Complex was also reactivated like suggested by the clockwise deflection of the

maximum principal stresses (Fig. 6.7).

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Figure 6.7. Simulated Miocene stress patterns.

The Asterias Fault Complex, located almost by the middle of the study area (12) shows slight

stress rotations. The change in stress pattern suggests reactivation and inversion in the western

segment of the fault complex.

Maximum principal stresses were simulated parallel to the Måsøy Fault Complex (13), which

is the southern boundary fault of the Nordkapp Basin (Fig. 6.1). Stress rotation suggests

reactivation. Clockwise rotation is modeled in the southern segment of the fault. Along the

Norwegian mainland (14) the simulated stress pattern is almost regular due to the modeled

uniform rheological properties there (Fig. 6.7).

6.6 Discussion

Several models have been advanced to explain the probable causes for the reactivation of

different fault complexes and development of inversion structures in the western Barents Sea

including effect of tectonic forces, gravity loading and sliding, differential sediment loading,

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mantle drag and plate driving forces (Bott 1991, Stuevold et al. 1992, Wilson 1993, Ziegler

1993, Bird 1998, Vågnes et al. 1998, Ziegler et al. 2001, Mosar et al. 2002, Doré et al. 2008).

In concert with the regional model of Gabrielsen & Færseth (1988), Gabrielsen et al. (1997)

Vågnes et al. (1998) and Doré et al. (2008), the current research favors the influence of

different regional tectonic forces and proposes that the Late Triassic – Miocene stress

development was due to far-field (plate-margin) stress and was responsible for the

reactivation of pre-existing faults and inversion in the study area.

According to Letouzey et al. (1990) compression perpendicular to existing faults is highly

efficient for inversion to occur, but it is seldom to find compression exactly normal to pre-

existing normal faults. Analyzing the relative contribution of compression and strike-slip in an

inverted region is always difficult. The azimuth of slip can be determined from measurements

in the field but exact slip determination using 2D seismic data is difficult (Lowell 1995).

Experience shows that reactivation of normal faults occur where strike slip is dominant

whereas development of new contractional structures (reverse faults) is mainly related to

compressional components (Lowell 1995).

The stress pattern predicted for Model 1 (Late Triassic to Early Jurassic) shows

counterclockwise rotation of the maximum principal stress, σ1 having the tendency to become

perpendicular to the strikes of the major fault complexes (Troms-Finnmark Fault Complex,

Thor Iversen Fault Complex, Hoop Fault Complex and Måsøy Fault Complex). The simulated

stress pattern also favors the presence of inversion structures interpreted by earlier

investigators (Berglund et al. 1986, Gabrielsen et al. 1990, Glørstad-Clark et al. 2010,

Fitryanto 2011, Gabrielsen et a. 2016).

A counterclockwise rotation of σ1 in the Hoop Fault Complex suggests the presence of

inversion structures in Late Triassic – Early Jurassic. The results are in agreement with the

findings of Fitryanto (2011), who interpreted structures related to mild inversion (minor folds)

in the southern side of the Hoop Fault Complex dating back to mid-Late Triassic. In

agreement with Glørstad-Clark et al. (2010), reactivation as a dextral fault and subsequent

stress rotation is also predicted for the Måsøy Fault Complex. The area has been mostly under

E-W extension throughout its tectonic history. However, the modelling suggests that this

general stress situation was possibly interrupted by inversion from Late Triassic to Early

Jurassic, presumably associated with the westward push of the Novaya Zemlya (Buiter and

Torsvik 2007), as indicated on interpreted seismic sections (Fiytriano 2011, Otto and Bailey

1995, Glørstad-Clark et al. 2010; Fig. 6.8).

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Figure 6.8. Comparison between the numerical modeling results (a) and an interpreted seismic

section (b; modified after Fiytriano 2011). The seismic interpretation confirms inversion at

MT (Middle Triassic) to early Late Triassic levels on the southern segment of the Hoop Fault

Complex. See figure 6.1 for location.

For Model 2 a clockwise stress rotation is simulated by the major fault complexes. The

modeling result reveals that NNW-SSE re-orientation of the maximum principal stress occurs

at the Bjørnøyrenna Fault Complex and the Ringvassøy – Loppa Fault Complex. The

Bjørnøyrenna Fault Complex is of extensional origin but with signs of tectonic inversion

including deformed fault planes and reverse faults affecting Early Cretaceous to Early

Cenozoic strata (Gabrielsen et al. 1997). Earlier investigators (e.g. Rønnevik and Jacobsen

1984, Gabrielsen et al. 1984, 1997, Riis et al. 1986, Faleide et al. 1993, Braut 2012, Hameed

2012, Indrevær et al. 2016) suggested that the Early Cretaceous reactivation of normal faults

was caused by dextral shear and Late Cretaceous – Early Paleogene. The Ringvassøy – Loppa

Fault Complex was also reactivated during the Early and Late Cretaceous and Cenozoic strata

have also been affected by faulting. Results of seismic interpretation also confirm inversion in

the Bjørnøyrenna Fault Complex and the Ringvassøy – Loppa Fault Complex during the early

and Late Cretaceous (Chapter 4). A comparison of seismic and numerical modeling results for

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the Bjørnøyrenna Fault Complex and the Ringvassøy – Loppa Fault Complex is presented

figure 6.9.

Moderate stress deflections are also modeled for the Asterias Fault Complex (Fig. 6.5, 5). The

E-W trending Asterias Fault Complex is an extensional structure initiated from Triassic to

Jurassic (Gabrielsen et al. 1984, 1990) but reactivated during the Late Jurassic – Cretaceous

rifting (Indrevær et al. 2016). The presence of inversion structures include reverse faults, half-

flower structures and local doming at the Jurassic – Cretaceous transition and is evidenced at

the western segment of the Asterias Fault Complex (Berglund et al. 1986, Brekke and Riis

1987, Mongat 2010, Indrevær et al. 2016).

Figure 6.9. Comparison of numerical modeling results (a) and interpreted seismic section in

central segment of the Bjørnøyrenna Fault Complex (b) and the Ringvassøy – Loppa Fault

Complex (c) confirming Inversion structure at LC (Lower Cretaceous) and BUC (Base Upper

Cretaceous) level. See figure 6.1 for locations of seismic lines (Data courtesy of TGS and

Spectrum). BUC (Base Upper Cretaceous), LC (Lower Cretaceous), BC (Base Cretaceous),

UJ (Upper Jurassic), BJ (Base Jurassic), UT (Upper Triassic).

107

A change in maximum principal stress is modeled in the Troms-Finnmark Fault Complex

which is believed to be an extensional feature (Gabrielsen 1984) but inversion connected to

sinistral slip in the northeastern segment of fault complex was proposed by Rønnevik et al.

(1982) and Rønnevik and Jacobsen (1984). The fault complex runs parallel to the coastline of

the Troms and the Finnmark counties (Fig. 6.1). The Troms-Finnmark Fault Complex display

listric fault geometry with normal dip-slip while the hanging wall is associated with roll-over

anticlines and antithetic faults (Fønstelien and Horvei 1979, Faleide et al. 1984, Gabrielsen et

al. 1984, Berglund et al. 1986, Gabrielsen et al. 1990). The modeled stress pattern would

initiate reactivation in the northern segment of the fault complex which is also observed by

Gabrielsen et al. (1990) and Ahmed (2012). The modeling results support the conclusion of

these authors.

Change in ϭHmax direction in the southern segment of the Hoop Fault Complex is modeled.

The fault complex is characterized mainly by normal faulting (Gabrielsen et al. 1990) but

reactivated during different tectonic time periods. The modeling results also suggest

compression in the Hoop Fault Complex during Late Cretaceous.

The modeled maximum stress pattern suggests inversion in the Leirdjupet Fault Complex,

which also experienced a phase of Early Cretaceous dextral shear (transtension?) and Late

Cretaceous - Early Cenozoic contraction (inversion) (Bjørnestad 2012). Inversion structures

(folds) are observed by earlier investigators in the central and northern parts of fault complex.

The simulated results show compression in the northern segment of the Leirdjupet Fault

Complex which is oriented NNE-SSW (Fig. 6.5; 6). The resulted stress pattern favors the

conclusion of Gabrielsen et al. (1990) and Bjørnestad (2012).

6.6.1. Origin of the Cenozoic stress field

Two main sources of stresses were advanced to explain Cenozoic inversion in the western

Barents Sea: Early Eocene North Atlantic opening (Tsikalas et al. 2002, Doré et al. 2008) and

ridge push from the North Atlantic mid-oceanic ridge system (Ranalli and Chandler

1975, Stephansson 1988, Talbot and Slunga 1989, Spann et al. 1991, Doré and Lundin 1996).

Breakup in the North Atlantic started in Early Eocene (55 Ma; anomaly A24b, e. g. Talwani

and Eldholm 1977, Srivastava and Tapscott 1986, Eldholm et al. 1987, Vågnes et al. 1998).

The ocean spread northwards and reached the Barents Sea margin at the time of Anomaly

A23 (Eocene, 54 Ma). According to Faleide et al. (1996, 2008) the western Barents Sea

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continental margin developed at the Paleocene-Eocene transition (~ 55-57 Ma) as a result of

continental breakup and opening of Norwegian-Greenland Sea. The breakup and initial

seafloor spreading in the Norwegian-Greenland Sea was linked to the Arctic Eurasian Basin

by the regional De Geer Zone megashear system (Faleide et al. 2008). During and after Early

Cenozoic rifting and breakup in earliest Eocene, the western margin of the Barents Sea was

subject to dextral shear and associated folding with NW-SE-striking fold axes (Faleide et al.

1996). Vågnes et al. (1998) suggested that the topographic momentum associated with the

early phase of sub-aerial sea floor spreading may have contributed to initiate contractional

deformation.

Sea floor spreading reached the margin off southern Spitsbergen and a narrow oceanic basin

formed between the western Barents Sea and continental margin of NE Greenland at the end

of Eocene (Faleide et al. 2008). In Early Oligocene the spreading direction between

Greenland and Eurasia changed from NNW-SSE to NW-SE (Faleide et al. 2008). A

progressive development of the Mid Atlantic Ridge caused by a change in relative plate

movement resulted in the formation of an obliquely spreading Knipovich Ridge (Czuba et al.

2011). The main source of Cenozoic inversion in the western Barents Sea is assumed to be

caused by the Knipovich Ridge push (Ranalli and Chandler 1975, Stephansson 1988, Talbot

and Slunga 1989, Spann et al, 1991). Doré and Lundin (1996) also favor compression due to

the counterclockwise shift in the poles of rotation in the North Atlantic between anomalies

A13 and A7 (35–25 Ma). Reactivation of NE-SW trending faults in the Vestbakken Volcanic

Province is also due to the change in relative plate motion in Early Oligocene.

6.6.2. Modeled stress patterns compared to observations

In Model 3, the simulated stress pattern caused by dextral between Greenland and the Barents

Sea Shelf in Eocene is presented. Maximum principal stresses are NW-SE and aligned with

the Senja Fracture Zone. The simulated stress pattern shows shearing along the southern

segment (Senja Fracture Zone) of the De Geer Zone. Faleide et al. (2008) marked the Senja

Fracture Zone as a pure sheared margin which developed due to the opening of the

Norwegian-Greenland Sea during Eocene. The generation of the sheared segment was

initially related to continent-continent shear followed by continent-ocean shear and has been

passive since earliest Oligocene (Faleide et al. 2008).

The simulated stress pattern shows rifting in the central segment of the De Geer Zone. The

rifted central segment (VVP; Vestbakken Volcanic Province) of the De Geer Zone formed

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due to NW-SE opening. The modeling results show that the main rifting occurred along NE-

SW striking segment. Many previous authors (e.g. Faleide et al. 1993, Breivik et al. 1998,

Ryseth et al. 2003) advanced rifting along the central part of the western Barents Sea

continental margin, where the strike-slip system changed and resulted into a pull-apart, which

caused generation of a rifted segment associated with volcanism, i.e. the VVP (Vestbakken

Volcanic Province).

Simulated stresses depict the formation of a sheared and to some extent rifted margin along

the northern segment (Hornsund Fault Complex) of the De Geer Zone. The ϭhmax aligned with

the fault strike (NW-SE) in the northen part of the northern segment showing shearing/strike-

slip movement and along the southern part, just above the rifted segment, the modelted stress

pattern suggests modest rifting and shearing.

Previous studies (Faleide et al. 1993, Faleide et al. 2008, Libak et al. 2012) support the

development of an oblique continent-continent and partly continent-ocean shearing along the

northern margin segment (Hornsund Fault Complex) during Eocene (Gorgan et al. 1999, Berg

and Grogan 2003). The shearing exhibited both transtension and transpression where the

restraining bend along north-northwest trending faults between Svalbard and northeast

Greenland caused transpression and, as a result, formation of the Spitsbergen fold and thrust

belt (Czuba et al. 2011), while the releasing bend between the Senja Fracture Zone and the

Hornsund Fault Complex facilitated Oligocene rifting (Faleide et al. 1993).

The present study suggests that Early Eocene sea floor spreading caused stress partitioning

along the Senja Fracture Zone and any direct effect of NE Atlantic opening on the studied

area is deemed minor. The stresses are concentrated along the shear margins (Senja Fracture

Zond and Hornsund Fault Zone) and did not penetrate east of the De Geer Zone (i.e. the

interior of the Barents Sea). Inversion structures observed by previous authors (Gabrielsen et

al. 1984, Riis et al. 1986, Gabrielsen and Færseth, 1988, Bjørnestad, 2012) along the major

fault complexes (Hoop Fault Complex, Troms-Finnmark Fault Complex, Ringvassøy – Loppa

Fault Complex and Bjørnøyrenna Fault Complex) during Eocene are may be related to other

mechanisms (e.g. gravity loading and sliding, underplating and Iceland hotspot influence,

differential sediment loading, mantle drag).

For Model 4, it was inferred that the NW-SE directed Atlantic ridge push is the main source

of stresses since Miocene (Gölke and Coblentz 1996, Czuba et al. 2011, Ranalli and Chandler

1975, Stephansson 1988, Talbot and Slunga 1989, Spann et al 1991, Doré and Lundin 1996).

110

The results of Model 4 suggest counter-clockwise rotation of the stress field in the western

Barents Sea. The maximum principal stress strikes in general NW-SE but often rotates to

WNW-ESE. The stresses rotated along the ocean- continent transition because of the

implemented change in rheology (Gölke et al. 1996, Pascal and Gabrielsen 2001). Strong

deflections were modeled along the fault complexes which are oblique to the applied

boundary stress. Maximum principal stresses become perpendicular to the strike of the major

fault complexes (Knølegga Fault Complex, Bjørnøyrenna Fault Complex, Ringvassøy-Loppa

Fault Complex and Leirdjupt Fault Complex), potentially causing direct inversion. According

to Gabrielsen et al. (1990) the Knølegga Fault Complex was mainly inverted during Cenozoic.

The contractional structures observed by previous investigators (Gabrielsen et al. 1990) along

the Knølegga Fault Complex, including synclines and anticlines, were suggested to be the

result of compression in Oligocene. It was proposed that ridge push direction changed from

NW-SE to WNW-ESE at the Eocene-Oligocene boundary, caused by the adjustments of the

poles of rotation in the North Atlantic (Boldreel and Andersen 1993, in: Vågnes et al. 1998).

Pronounced stress rotations are also modeled in the Senja Ridge and the Veslemøy High.

The N-S Leirdjupet Fault Complex which extends from the Loppa High towards the Stappen

High in the north (Fig. 6.1) and divides the Bjørnøya Basin into a deep western part and a

shallow eastern part (Fingerdjupet Subbasin) (Rønnevik and Jacobsen 1984, Gabrielsen et al.

1990) were also affected by Early Cretaceous dextral shear and Late Cretaceous - Early

Paleogene inversion (Bjørnestad 2012). Inversion structures are also observed by earlier

investigators along the central and northern parts of the fault complex in Miocene. The

modeling results are also in agreement with previous studies. Maximum principal stresses are

parallel to the main fault line depicting a strike-slip component (Fig. 6.7).

Rotations of maximum principal stresses are modeled by the Bjørnøyrenna Fault Complex

depicting inversion. The fault complex is believed to be the southern continuation of the

Leirdjupet Fault Complex (Gabrielsen et al. 1997) and lies in the north of the Ringvassøy-

Loppa Fault Complex (Fig. 6.1). It was also reactivated during different tectonic episodes

(Gabrielsen et al. 1997, Vågnes et al. 1998).

The simulated stress pattern shows a change in ϭHmax direction, confirming head-on/strike-slip

inversion of the southern and northern segments of the Ringvassøy – Loppa Fault Complex.

The N-S to NE-SW trending Ringvassøy-Loppa Fault Complex is an extensional fault

complex reactivating old zones of weakness. The fault complex was reactivated during Late

111

Cretaceous but also Cenozoic strata have been affected by faulting (Gabrielsen 1984). Vågnes

et al. (1998) and Doré and Lundin (1996) suggested NW-SE compressional inversion. The

modeled stresses are in agreement with previous studies and favor inversion in Miocene due

to ridge push from the Knipovich Ridge.

A comparison between the present and previous studies is presented in figure 6.10.

6.7 Conclusion

A FEM numerical tool (ANSYS workbench) was used in the present research and four 2D

linear elastic models were generated to investigate the causes for the development of

inversion structures from Late Triassic to Miocene in the western Barents Sea. The results of

the modeling suggest that different tectonic stresses affected the study area during the

mentioned time period and reorientation of stress patterns at major fault complexes indicate

the presence of inversion structures. The modeled principal stresses orientations in Model 1, 2

and 4 are in agreement with findings from previous studies. In particular, Model 3 succeeds to

predict the observed deformation field and suggests that the opening of the NE Atlantic

during Early Eocene had no direct impact on the observed inversion of study area.

112

Figure 6.10. Comparison of present study with previous studies results showing different fault

complexes affected by inversion events in the western Barents Sea.

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7- SUMMARY

The different chapters of this thesis present the results of different methodologies used to

identify, interpret, analyse and model the inversion structures in the western Barents Sea.

Results constrained by interpretation of industrial 2D seismic profiles and well data (Chapter

4) using Kingdom 8.8 evidence inversion structures including folds and assumed reverse

faults in the Bjørnøyrenna Fault Complex and the Ringvassøy–Loppa Fault Complex during

Early and Late Cretaceous.

Positive inversion structures are interpreted in the central and northern segment of the

Bjørnøyrenna Fault Complex at Early Cretaceous and Late Cretaceous levels. Both horizons

have been subsequently compressed and uplifted. The fault retains extension at a deeper level

and experienced net contraction associated with an anticline in the upper portion. This type of

inversion structures are formed when extensional faults reverse their sense of motion during

compressional tectonics causing the basin to turn inside out and to become a positive feature.

In addition inversion structures are also interpreted during Early Cretaceous and Late

Cretaceous in the eastern margin of the Ringvassøy–Loppa Fault Complex.

Results of structure restoration along the Bjørnøyrenna Fault Complex are presented in

Chapter 5. 2D MOVETM was used to restore key seismic profiles crossing the central and

northern segments of the fault complex. The main goal of the study was to locate null point

positions and to identify reverse faults in the Bjørnøyrenna Fault Complex. Key profile 1 and

2 show null point positions at the base of the Cretaceous (Hekkingen Formation) after

restoration. The downward movement of null points confirms the progressive compressional

inversion of the Early Cretaceous syn-rift sequence (Knurr Formation). Below the null point,

the restored geometry shows normal faulting while above reverse faulting is found. The

results of restored key profiles 1 and 2 confirm reverse faults at the Early Cretaceous created

by inversion in the study area. The resulted restored sections also show positive inversion

features associated with folding of the hangingwall at the base of the Upper Cretaceous

(Kolmule Formation). The assumed thickness of eroded footwall block also shows reversere

faulting at the base of the Upper Cretaceous.

In key profile 3, the footwall block is eroded upto the base of the Upper Cretaceous (Kolmule

Formation) due to the uplift of the Loppa High. The resulted restored section shows a

compressional structure (anticline) associated with Early and Late Cretaceous inversion

events. Both the Early Cretaceous horizon (Knurr Formation) and the base of the Upper

114

Cretaceous (Kolmule Formation) had to be moved downwards during the restoration,

suggesting compression.

The results of the 2D numerical modeling of tectonic inversion of the western Barents Sea

from Late Triassic to Miocene are devised in Chapter 6. Four 2D thin plate modeling setups

using a finite-element numerical code, namely ANSYS™, was used to simulate stress and

fault slip patterns along the major fault complexes. Four major regional sourcea of stresses

responsible for the development of inversion structures in the western Barents Sea were

considered: Late Triassic to Early Jurassic E-W contraction (Model 1), Late Cretaceous NW-

SE contraction (Model 2), plate margin dextral megashear in Early Eocene (Model 3) and

NW-SE Atlantic ridge push starting in Miocene (Model 4). Model 1 confirms the potential for

compressional conditions in the western Barents Sea and, hence, contractional reactivation of

master fault systems like the Thor Iversen Fault and Troms-Finnmark Fault complexes.

Compressive regimes in the Måsøy and Hoop fault complexes favor the development of

inversion structures in the study area during Late Triassic to Early Jurassic.

Simulated stress patterns in Model 2 (inducing a NW-SE compressional stress) suggest a

clockwise stress rotation in the Bjørnøyrenna Fault Complex and the Ringvassøy – Loppa

Fault Complex and pronounced stress deflections in the Asterias Fault Complex. These

modeled stress deflections support tectonic inversion during Late Cretaceous in the

corresponding fault complexes. The analyses suggest that significant strike-slip is to be

expected along some segments.

The results obtained in Model 3 suggest that the interior of the western Barents Sea was not

severely influenced by Early Eocene North Atlantic opening/shearing. They suggest

furthermore that Early Eocene sea floor spreading caused stress partitioning along the Senja

Fracture Zone. The observed inversion structures in previous studies may be related to local

effects (e.g. contraction during post-subsidence cooling). The results of Model 4 are in

agreement with the observed NW-SE contraction, expressed as folds and reverse faults in the

study area (e.g. Ringvassøy – Loppa, Bjørnøyrenna, Leirdjupt and Asterias fault complexes).

The four models suggest the presence of compressive structures along the major fault

complexes of the western Barents Sea during Late Triassic to Miocene but do not favor the

development of inversion structure during Eocene.

Its is suggested that other mechanisms e.g. thermal heating, gravity loading, sliding and

spreading, reactivation of basement lineaments, continental breakup involving divergent

115

asthenospheric flow, isostatic flexuring etc. may also contributed in the development of

inversion structures in the study area and these aspects need to be further tested.

116

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CURRICULUM VITAE

Personal data

Name: Muhammad Armaghan Faisal Miraj

Date of Birth: 01.01.1978

Nationality: Pakistan

Marital Status: Married

Email: [email protected]

Education

PhD. Structural Geology. 10.2013 – 07.2017. Ruhr-University Bochum, Germany.

M.Sc. Geology. April 2009. Aarhus University, Denmark.

M.Sc. Geology. February 2003. University of the Punjab, Lahore, Pakistan.

B.Sc. Applied Geology. October 2000, University of the Punjab, Lahore, Pakistan.

Career History

Februay 2011 – Present.

Lecturer at the Institute of Geology, University of the Punjab, Lahore, Pakistan (on

study leave).

January 2010 – January 2011.

Worked as geologist at SPUD Energy Pty Ltd, Islamabad, Pakistan.

March 2003 – August 2005

Worked as field geologist at Geosciences Associates (PVT) Limited, Pakistan.

135

Declaration

Ich versichere an Eides statt, dass ich die eingereichte Dissertation selbstständig und ohne

unzulässige fremde Hilfe verfasst, andere als die in ihr angegebene Literatur nicht benutzt und

dass ich alle ganz oder annähernd übernommenen Textstellen sowie verwendete Grafiken,

Tabellen und Auswertungsprogramme kenntlich gemacht habe. Außerdem versichere ich,

dass die vorgelegte elektronische mit der schriftlichen Version der Dissertation übereinstimmt

und die Abhandlung in dieser oder ähnlicher Form noch nicht anderweitig als

Promotionsleistung vorgelegt und bewertet wurde. Ich versichere, dass digitale Abbildungen

nur die originalen Daten oder eine eindeutige Dokumentation von Art und Umfang der

inhaltsverändernden Bildbearbeitung enthalten. Ich versichere zudem, dass keine

kommerzielle Vermittlung oder Beratung in Anspruch genommen wurde.

Bochum, September 2017

Muhammad Armaghan Faisal Miraj

136

List of published work

Numerical modeling of main inverted strcutures in the Western Barents Sea.

Proceedings of GeoMod 2014 (31.08.-05.09.14, Potsdam, DE, doi:

10.2312/GFZ.geomod.2014.001).

Numerical modeling of multi-stage basin inversion in the western Barents Shelf.

Geophysical Research Abstracts Vol. 18, EGU2016-799, 2016 EGU General

Assembly 2016.

structural analysis of inversion features of the barents sea

Documents