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305NORWEGIAN JOURNAL OF GEOLOGY Structural geology of the southeastern part of the Trollfjorden-Komagelva Fault Zone

The peninsula is divided into two distinctive northern and southern regions by the NW-SE- to WNW-ESE- trending Trollfjorden-Komagelva Fault Zone (TKFZ) (Siedlecka & Siedlecki 1967) (Fig. 1). The southern, Tanafjorden-Varangerfjorden Region (TVR) comprises what is mostly an autochthonous/parautochthonous, pericratonic/platformal, diagenetic to mid-anchizone, sedimentary succession ranging in age from Late Riphean (Tonian-Cryogenian) to Early Cambrian (Banks et al. 1971, Siedlecka et al. 1995a) (Fig. 2). Close to Tanafjorden, however, these same rocks occur in allochthonous position, as part of the anchizone Gaissa Nappe Complex (Lower Allochthon), and the succession reaches up to Early Tremadoc on the nearby Digermul Peninsula (Reading 1965, Nikolaisen & Henningsmoen 1985). In innermost Varangerfjorden the diagenesis-grade succession lies unconformably upon a Neoarchaean crystalline basement (Føyn 1937, Siedlecki 1980, Rice et al. 2001), and is disrupted by several internal unconformities. In contrast, the northern Barents Sea Region (BSR) – also called the Northern Varanger Region by some authors

Introduction

The geology of Varanger Peninsula in East Finnmark, northern Norway, is now well known through the systematic mapping, and stratigraphic and sedi-mentological studies of Siedlecka & Siedlecki (1967, 1971, 1972), Banks et al. (1971, 1974), Siedlecka (1972, 1985), Siedlecki (1980), Pickering (1981, 1982, 1985), Edwards (1984), Rice (1994) and Røe (2003). Complementing this work there have been diverse investigations on metamorphism (Bevins et al. 1986, Rice et al. 1989a), micropaleontology (Vidal 1981, Vidal & Siedlecka 1983), palaeomagnetism (Kjøde et al. 1978, Bylund 1994a, b, Torsvik et al. 1995) and mafic dykes (Beckinsale et al. 1975, Roberts 1975, Rice & Reiz 1994, Rice et al. 2004), as well as structural geology and tectonics (Roberts 1972, Rice et al. 1989b, Karpuz et al. 1993) and 40Ar/39Ar geochronology (Dallmeyer & Reuter 1989, Guise & Roberts 2002, Rice & Frank 2003, Kirkland et al. 2008). A detrital zircon study of several of the formations occurring on Varanger Peninsula is also in progress (Roberts et al. 2008).

Tore Herrevold, Roy H. Gabrielsen & David Roberts

Herrevold, T., Gabrielsen, R.H. & Roberts, D.: Structural geology of the southeastern part of the Trollfjorden-Komagelva Fault Zone, Varanger Peninsula, Finnmark, North Norway. Norwegian Journal of Geology, vol. 89. pp. 305-325, Trondheim 2009, ISSN 029-196X.

Structural studies in the Late Riphean to Vendian succession of the southeastern part of the Trollfjorden-Komagelva Fault Zone on Varanger Peninsula, Finnmark, have revealed the presence of two main sets of contractional structures, here grouped into D1 and D2 events. These folding and cleavage-forming episodes are superseded by two, separate extensional events, D3 and D4. Although only one of these four events (D3) has been directly dated in the actual study area, the probable ages of the disparate, contractional deformation episodes can be deduced from published geochronological data in neighbouring parts of Varanger Peninsula or Northwest Russia.The deformation history of this area shows the following sequence of main events: (1) Inversion of the pre-existing, Neoproterozoic, basinal regime occurred during an episode of SW-directed shortening, folding and associated cleavage development, with reverse faulting and minor sinistral strike-slip movements along the Trollfjorden-Komagelva Fault Zone. By comparison with the situation on the nearby Rybachi and Sredni Peninsu-las in NW Russia, this D1 deformation and low-grade metamorphism is likely to have occurred during the Timanian orogeny at about 570-560 ± 10 Ma. (2) The D2 event represents the main Caledonian deformation and is characterised by SE-directed shortening (NE-SW-trending folds and NW-dipping cleavage) that gave rise to a moderate dextral strike-slip displacement along the Trollfjorden-Komagelva Fault Zone. Based on recent geochronology from NW Varanger Peninsula, this major contractional, epizone event is considered to be late-Finnmarkian, c. 470-460 Ma. It is likely that Siluro-Devonian Scandian deformation in the study area and along the fault zone was largely of brittle character. (3) A regional, extensio-nal event, D3, is reflected in N-S fracturing and dolerite dyke intrusion. The dyke intrusion is dated to Late Devonian time at c. 375-370 Ma. (4) A younger extensional event, D4, is manifested in dip-slip normal faulting along and adjacent to the main fault zone, with breccias, gouge and quartz-calcite veining. The age of this episode is open to speculation – from Carboniferous to Cretaceous – but a Mesozoic reactivation seems highly likely based on our knowledge of offshore geology.

Tore Herrevold, Total E & P AS, PO Box 168, 4001 Stavanger, Norway ([email protected]). Roy H. Gabrielsen, Department of Geosciences, University of Oslo, P.O. Box 1047, NO-0316 Oslo, Norway. David Roberts, Geological Survey of Norway, NO-7491 Trondheim, Norway.

Structural geology of the southeastern part of the Trollfjorden-Komagelva Fault Zone, Varanger Peninsula, Finnmark, North Norway

306 T. Herrevold et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 1. Principal structural elements of the Varanger Peninsula, Finnmark, northern Norway (mainly after Siedlecki (1980) and Karpuz et al. (1993)). BSR – Barents Sea Region (Barents Sea and Løkviksfjellet groups, and Berlevåg Formation). TVR – Tanafjorden-Varangerfjorden Region (Vadsø, Tanafjorden and Vestertana groups). The grey ornamented area in the extreme northwest is the Berlevåg Formation of the Tanahorn Nappe. The lined area in the extreme southwest is the Archaean crystalline basement. TKFZ – Trollfjorden-Komagelva Fault Zone. The inferred eastward extension of the sole thrust-fault to the Parautochthon is taken from Sygnabere (1997). The inset map in the upper right corner shows the Trollfjorden-Komagelva Fault Zone and its southeastern continuation (SRFZ) onto the Rybachi and Sredni Peninsulas in Northwest Russia. SRFZ – Sredni-Rybachi Fault Zone. The study area is indicated by the rectangle.


revealed that structures ascribed to the Vendian-age Timanian orogeny are almost certainly represented in the northeastern parts of Varanger Peninsula (Roberts 1995, 1996, Roberts & Olovyanishnikov 2004). The TKFZ extends southeastwards along the coastline of Kola Peninsula, NW Russia (known there as the Karpinskiy Line), and farther southeast into the Timans as the Central Timan Fault (Olovyanishnikov et al. 2000). To the northwest of Varanger, the fault zone can be traced offshore north of Magerøya as a system of WNW-ESE-trending fault segments transecting the Finnmark Platform, and passing across the Hammerfest Basin towards the southern flank of the Loppa High where it seems to terminate (Lippard & Roberts 1987, Gabrielsen & Færseth 1989, Jensen & Sørensen 1990). Clearly, this fundamental crustal lineament has been multiply reactivated from Precambrian to Cenozoic time, and along segments of the fault in parts of NW Russia it is seismically active even today. The principal phase of dextral strike-slip translation along the TKFZ is now considered to be of Ordovician age (Rice & Frank 2003).

On Varanger Peninsula, the only published, structurally oriented study undertaken along the TKFZ is that of Karpuz et al. (1993) from central parts of the fault zone. This involved a synergistic approach integrating multiple datasets from the entire peninsula. In addition to this work, discussion of the evidence for dextral strike-slip was presented by Rice et al. (1989b), and structural information from a small area along the southern margin of the TKFZ inland from the present study area has been reported by Sygnabere (1997). The present contribution serves to report the results of an investigation of folds, cleavages, fractures and vein arrays concentrated in the southeastern part of the fault zone in the Komagelva-

(e.g. Rice 1994) – consists largely of slightly higher-grade (epizone), allochthonous, basinal to deltaic successions of Late Riphean-Vendian (Ediacaran) age in western and central areas (Siedlecka & Roberts 1995, Siedlecka et al. 1995b) (Fig. 2). These rocks are considered to represent a thrust sheet (Rákkočearru thrust sheet: Roberts 2009) of the Lower Allochthon, and are overlain by middle greenschist-facies, mostly turbiditic rocks of the Tanahorn Nappe (part of the Middle Allochthon) in the extreme northwest (Laird 1972, Levell & Roberts 1977, Rice et al. 1989a, Rice & Frank 2003). These thrust units are clearly imaged immediately offshore on newly acquired aeromagnetic data (Gernigon et al. 2008, Barrère et al. 2009).

The TKFZ thus constitutes a major, crustal lineament and there is evidence to suggest that its deep-seated precursor fault may have originated in Archaean time and formed a major terrane boundary (Karpuz et al. 1995, Roberts et al. 1997). Subsequently, it functioned as a normal fault during a long period of Late Mesoproterozoic to Neoproterozoic, passive-margin sedimentation, and was later reactivated several times in contractional, strike-slip and extensional regimes (Siedlecka & Siedlecki 1967, Rice et al. 1989b, Karpuz et al. 1993). The fault is most commonly cited as a dextral strike-slip lineament by virtue of the difference in metamorphic grade and intensity of folding across the structure, and both duplexes and positive flower structures have been reported by Lippard & Roberts (1987), Rice et al. (1989b) and Karpuz et al. (1993). Although the metamorphism and folding had earlier been considered as wholly Caledonian (e.g., Roberts 1972, Bevins et al. 1986, Rice et al. 1989a), work on the Rybachi and Sredni peninsulas in nearby Northwest Russia during the 1990s

Fig. 2. Lithostratigraphic successions, Varanger Peninsula. Columns 1 and 2 are from the Tanafjorden-Varangerfjorden Region, southwest of the TKFZ, and column 3 from the Lower Allochthon of the Barents Sea Region. Note that two amendments have been proposed for the strati-graphy shown in Column 1; the Ekkerøya Formation may possibly be raised to group status (Rice & Townsend 1996); and it has been suggested that the Veidnesbotn Formation may be equivalent to the Gamasfjellet Formation (Røe 2003).


based on regional structural correlations, to suggest the likely orogenic ages of particular fold generations and associated cleavages.

Komagnes area (Fig. 3), and thus aims to provide a better coverage and understanding of the entire fault zone. Here, it has been possible in many outcrops to discern the relative ages of certain structures and, moreover,

Fig. 3. Principal structural features of the Komagelva-Svartnes area (the study area), based on the present fieldwork with some information from Siedlecki (1980). Symbols for some of the structures are enlarged, and are therefore not to scale. Encircled numbers are localities referred to in the text.


main sets oriented ENE-WSW and WNW-ESE, with a subordinate set trending NNE-SSW (Fig. 4c). On outcrop scale, the lineaments of the two main sets are commonly seen to represent either a close jointing or shear zones which generally show minor dextral displacements.

Mesoscopic and even smaller-scale folds are best developed along and close to the southwestern escarpment to the main fault zone. Open to close, upward-facing folds trending between NE-SW and ENE-WSW with southeasterly vergence and an axial planar, spaced cleavage in pelitic lithologies are the most common structures, but a few NW-SE-trending folds verging southwest are also present. Some of these folds (both NE-SW and NW-SE trending fold axes) have associated low-angle or high-angle reverse faults with small offsets. Also present along this fault zone margin are quartz- and calcite-filled vein arrays, the most common being a steep to moderately dipping set striking parallel to the main trace of the TKFZ (Fig. 5b). Calcite veinlets along the surfaces of the fold-related, NE-SW-oriented reverse faults show slickensides and steps indicative of top-to-the-southeast transport. There are also lenses of intensely brecciated rock trending NW-SE and up to 15 m in length, with steeply dipping sheared margins. The comminuted matrix to these lenses is cemented by baryte and iron oxides. Here it can be mentioned that baryte mineralisation has been reported from similar cataclasites and breccias in northwestern parts of the trasé of the TKFZ (Sandstad 1987).

The coastal exposures and raised cliffs in the vicinity of Komagnes (Fig. 2) offer the best opportunities for studying meso-scale shear zones, cleavages and a few small-scale folds. There, several ENE-WSW-striking shear zones are exposed, with local micro-brecciation and rare development of duplex and flower structures (Fig. 6). Both transpressional and transtensional dextral displacements have been recorded, and some of the faults are reactivated by dip-slip normal movements. Along this c. 600 m-long shore section, the most prominent structures in the thin-bedded, maroon mudstones and grey silty sandstones are spaced cleavages, one oriented NE-SW to ENE-WSW and dipping at a moderate angle to the northwest (Fig. 7a), and the other striking c. WNW-ESE and dipping north-northeast. At this shoreline locality, the two cleavages show clear intersection lineations on bedding surfaces (Fig. 7b) with the c. NE-SW-trending cleavage being the more penetrative of the two. This feature is particularly noticeable at low tide. These cleavages clearly correlate with the folds and cleavages noted above from outcrops along the southwestern escarpment. Moreover, at Komagnes the NW-dipping cleavage is axial planar to small-scale, SE-verging folds, some of which carry cleavage-parallel reverse faults with offsets of a few centimetres.

The Komagnes locality is also well known from the presence of 3-4 mafic dykes, most clearly seen in the

Structures associated with the Trollfjorden-Komagelva Fault Zone

On Varanger Peninsula, the Trollfjorden-Komagelva Fault Zone defines a regional lineament approximately 75 km in length and varying between 1 and 5 km in width (Fig. 1). Its internal structure is characterised by an anastomosing or braided lineament pattern (Gabrielsen & Ramberg 1979, Gabrielsen 1984, Rice et al. 1989b, Karpuz et al. 1993). The main lineaments are mostly topographic, and most of the escarpments along the fault zone coincide with fracture sets or faults which are parallel or slightly oblique to the main fault (Karpuz et al. 1993), producing lensoid, contractional duplex structures (Rice et al. 1989b).

In the study area (Fig. 3) the river Komagelva flows roughly parallel to the trend of the main fault zone, and deflections along its course are partly structurally controlled (Karpuz et al. 1993). The fault zone is topographically defined by a broad, approximately 5 km-wide, alluvial plain, narrowing inland to the northwest, and bordered by escarpments both to the northeast and to the southwest. Along these opposing escarpments the degree of exposure is fairly good, particularly along the northeastern margin. In contrast, exposures are rare along the river course, which defines the assumed central part of the fault zone.

The coastal section of the study area offers excellent exposure at every headland, but again, the central fault zone is unfortunately not exposed. North and south of the TKFZ in the study area, the topography is characterised by an undulating terrain that offers only scattered exposures. The inland part of the fault zone is separated from its less deformed northeastern and southwestern margins by prominent, topographic escarpments with elevations varying between a few tens of metres to more than 100 metres.

In the following, we first describe the structures and vein arrays observed in different parts of the study area, and then discuss the various structures in terms of stages, i.e., relative ages, of deformation in a kinematic framework. Finally, suggestions are made with regard to the likely ages of these structures in a more regional context.

The area south of the TKFZ

An E-W-trending, upright, gentle to open syncline, with thin-bedded, cleaved shales and silty sandstones of the Innerelva Member of the Stappogiedde Formation in its core dominates the geology of the southern part of the study area (Fig. 3). A smaller, parallel anticline is also present closer to the main fault zone. In this southern area, lineaments marking topographic features and mapped from aerial photographs are grouped into two


cleavage; and the dyke itself is cut by transverse joints. Various earlier investigations, geochronological and paleomagnetic, have resulted in differing ages for these dykes, either Devonian (Beckinsale et al. 1975) or Late Vendian (Torsvik et al. 1995), but a Late Devonian age has been confirmed by 40Ar/39Ar dating of plagioclase separates (Guise & Roberts 2002).

old raised cliff just north of the road but also exposed on the shore at low tide. The main dyke, which actually consists of at least two parallel dykes, trends c. N-S and dips 75º east, but in the intertidal zone it splits into several thinner dykes and dykelets. Adjacent to the main composite dyke in the raised cliff there is a prominent NNW-SSE-trending close jointing or fracture

Fig. 4. Contoured, equal-area, lower-hemisphere projection and azimuth-frequency rose dia-grams of fractures measured in the field; (a) the region northeast of the Trollfjorden-Komagelva Fault Zone, (b) the northeastern fault margin, (c) the southwestern fault margin, and (d) the region southwest of the fault zone.


Figure 6

Fig. 5. Contoured, equal-area, lower-hemisphere projection and azimuth-frequency rose diagrams of quartz- and calcite-filled veins found along (a) the northeastern margin and (b) the southwestern margin of the Troll-fjorden-Komagelva Fault Zone.

Fig. 6. Secondary shear zones with flower structures at the southern border of the Trollfjorden-Komagelva Fault Zone, developed in siltstones of the Innerelva Member of the Stappogiedde Formation at locality 2 (Fig. 3) just southwest of Komagnes. The two, vertical faults strike at N50°E and belong to one and the same strike-slip system. They can be followed along strike over a distance of more than 100 m. (a) The northern fault, looking northeast. Note the series of half-flowers developed at restraining bends. The half-flower structure in the central part of the photo is 30 cm in width. (b) Positive flower structure developed along the southern fault, looking northeast. This structure is not associated with a restraining bend, demonstrating the dextral transpressional nature of the strike-slip system. Side of compass in the foreground is 12 cm.


common folds strike between NNW-SSE and NW-SE; and the folds verge southwest with a steep, NE-dipping, tightly spaced cleavage parallelling their axial surfaces (Fig. 7c). In some dark grey to black mudstone beds this is more akin to a slaty cleavage. A few small, NE-SW-trending, open to close folds have also been recorded. These also carry an axial planar spaced cleavage and postdate the more common NW-SE folds. This deformation sequence, with the NW-SE folds predating the NE-SW folds, has also been reported from a wider area of NE Varanger Peninsula (Gjelsvik 1998). Several steeply dipping, WNW-ESE-striking faults show indications of sinistral shear, but some surfaces within the fault cores (some of which contain a 1-2 cm thickness of gouge) denote reactivation during later, extensional, dip-slip movements. Quartz and quartz-calcite veins are prominent features of this margin (Fig. 5a), and dilation veins, commonly with en échelon geometry, are seen in connection with flexural slip on macroscopic folds.

The area north of the TKFZ Regional maps of the northern part of the study area (Siedlecki 1980, Siedlecka & Siedlecki 1984) depict open, NNW-SSE- to N-S-trending folds that also extend farther north to the area between Vardø and Hamningberg (Fig. 1). Gentle, long-wavelength, cross folds of ENE-WSW trend are also present producing dome-and-basin outcrop patterns (Siedlecki 1980, Gjelsvik 1998). Along the eastern coast between Komagvær and Svartnes (Fig. 3), NNW-SSE to NW-SE-trending, close to tight folds with a steep NE-dipping axial planar cleavage are the most prominent structural features on macro- and meso-scale (Fig. 7c, d) and are associated with southwesterly directed reverse faults. Quartz- and calcite-filled veins are developed in many places, and are especially common along shear zones and reverse faults.

As compared with the southwestern margin, the northeastern escarpment margin is characterised by more extensive meso-scale folding. Axes of the most

Fig. 7. (a) Strongly cleaved (S2), thin-bedded and laminated pelitic rocks of the Stappogiedde Formation, Komagnes; looking northeast. (b) Photo looking down onto a near-horizontal bedding surface at the Komagnes locality showing intersections on bedding of the S1 (yellow pencil) and S2 (green pencil) cleavages. The narrow end of the white pen is pointing almost due north. (c) Penetrative spaced cleavage (S1) in interban-ded sandstones and silty claystones of the lower member of the Båsnæringen Formation, Grunnes; looking northwest. The notebook is lying on a bedding surface. (d) Strongly developed spaced cleavage (S1) in silty claystones in the hinge zone of a large-scale D1 fold, from the foreshore in the bay Laukvika, 2 km east of Grunnes; looking southeast. In the underlying sandstone, S1 is more of a fracture cleavage but the cleavage is poorly developed in, or swings around, the two ball-and-pillow type structures in the upper part of the sandstone unit.

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events, here designated D1 to D4. Although each of these may have involved more than one pulse of broadly coaxial deformation, any further subdivision into separate deformational phases based on trend analysis would seem inappropriate since the development of some of the minor structures may have been controlled by local factors rather than reflecting a regional stress regime.

D1 deformation

This is a prominent contractional deformation event that affected both the northern (BSR) and the southern (TVR) regions, though to differing degrees. Folds and associated cleavage ascribed to D1 are more common and extensively developed in the BSR than in the area south of the TKFZ.

NW-SE- to NNW-SSE-trending folds (F1). Folds of this generation and trend are ubiquitous along the northeastern fault margin and in the area to the north and northeast of the escarpment. Farther to the northeast, towards Vardø and Persfjord, there is a tendency for macrofolds and related cleavage to trend more NNW-SSE or, locally, even N-S (Roberts 1972, 1996, Siedlecki 1980, Rice et al. 1989b, Gjelsvik 1998). Across the northern part of the study area, the metasandstones and cleaved mudstones of the Båsnæringen Formation are deformed into open, parallel folds with wavelengths of up to 8-10 km. Whilst there is no pronounced asymmetry to the folds on a macro-scale, almost all mesoscopic folds are asymmetric and verge to the southwest, as reflected in the well developed, NE-dipping, axial planar cleavage (S1) described earlier (Fig. 7c, d). Locally developed SW-directed thrusts are found in the vicinity of ramp-and-flat structures (Fig. 8a) and minor, steeply dipping, reverse faults also indicate transport to the southwest. Along the northeastern fault margin, contractional duplexes in the Godkeila Member of the formation are characterised by gently E-dipping, layer-parallel flats and E-W to WNW-ESE-striking ramps dipping at 50-80º to the north. Fold axes here plunge gently WNW. Along the ramp faults the fine-grained sandstones are brecciated and impregnated with segregations and veins of quartz. Quartz and quartz-calcite veins are commonly developed across this northern area and appear to be associated with the F1 folding. The most conspicuous features are layer-parallel veins and conjugate sets of en échelon, rotated, sigmoidal gash veins (Fig. 8b). At Svartnes, low angle reverse faults show well developed slickensides indicating SW directed transport (Fig. 8c).

Along the southwestern fault margin (locality 3 in Fig. 3), NW-SE to WNW-ESE-trending F1 folds in sandstones of the Lillevatnet Member are of parallel type with subhorizontal axes, wavelengths of 5-10 m and amplitudes of 3-4 m. Axial planes are generally vertical or dip steeply to the northeast. In map view, the axial traces display a left-stepping, en échelon geometry with

NORWEGIAN JOURNAL OF GEOLOGY Structural geology of the southeastern part of the Trollfjorden-Komagelva Fault Zone

Fracture and vein populations within and adjacent to the TKFZThe contrast in structural pattern between the Trollfjorden-Komagelva Fault Zone and its surrounding areas is clearly reflected by the contoured plots and rose diagrams of fractures (Fig. 4a, b). In the region to the south of the southwestern escarpment, NW-SE-striking fractures are far less common (Fig. 4d), whereas fractures parallel to the regional lineament are abundant along the southwestern fault margin and to the north of the fault zone (Fig. 4a, b, c). Of particular interest is the dissimilarity in the main fracture trends between the areas north and south of the fault margins, and the differences between these areas and the fault margins proper.

North of the fault zone, three principal sets of fractures are recognised, striking NW-SE, N-S and ENE-WSW, of which the first set is dominant (Fig. 4a). Along the northeastern fault margin, fractures striking NNW-SSE, ENE-WSW and NNE-SSW are the most abundant (Fig. 4b). It is particularly interesting to note the differences between the northeastern and southwestern fault margins where, in the latter case, ENE-WSW, NW-SE and NNE-SSW trends predominate (Fig. 4c). In the area south of the southwestern fault margin, three main sets of fractures are developed, striking NNE-SSW, ENE-WSW and WNW-ESE (Fig. 4d).

Stereographic plots of quartz- and calcite-filled veins registered along the northeastern fault margin (Fig. 5a) display mainly similar patterns to those of the fractures devoid of mineral growth (cf. Fig. 4b). Along this margin, veins striking NNW-SSE are extremely common and appear to follow the trend of the main folds and associated cleavage. One difference, however, is that the ENE-WSW fracture trend (Fig. 4b) is not represented in the vein population. In the case of the southwestern margin, the prevalent trend for veins is WNW-ESE (Fig. 5b), whereas the dominant fracture trend is ENE-WSW (Fig. 4c).

The common occurrence of quartz and/or calcite veins along and close to the TKFZ is generally indicative of the presence and strong influx of hydrothermal fluids along the trace of the fault. In general, the intensity of fracturing parallel to the fault zone diminishes markedly to the south of the southwestern margin, implying that the southern region has been relatively less affected by the increments of semi-ductile and brittle deformation associated with movement along this major crustal lineament.

Main deformational episodesBased on the field mapping of folds, cleavages and fracture sets, the observed superposition of folds and cleavages, and the analysis of these data in this southeastern part of the TKFZ, it is possible to ascribe these structures to at least four different deformational

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Stress tensors have been calculated based on fault plane orientation and slickenside information using the program Fault Kin v.3.25 (Allmendinger 1989-1992). Stereographic projections of contractional and extensional stress axes based on fault plane data from all localities in the investigated area are presented in Fig. 9. A contoured plot of shortening axes shows a bimodal distribution with a major cluster of mainly NE-SW-striking axes with horizontal to sub-horizontal plunges, and a minor cluster of mainly NNW-SSE-trending axes

respect to the TKFZ. In the mudstone units of the Vagge Formation, a well developed, axial planar, tightly spaced cleavage (S1) dips at moderate angles to the NE-NNE (Fig. 8d) and reflects the overall SW- to SSW-directed shortening in this particular subarea. In the Komagnes area (locality 2, Fig. 3), F1 folds are rare but there is a clearly identifiable spaced cleavage striking WNW-ESE and dipping NNE (Fig. 7b). Farther southwest, well away from the fault zone, folds and cleavage of this D1 generation appear to be lacking.

T. Herrevold et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 8. (a) Fault-propagation fold above a ramp-and-flat thrust surface in the Godkeila Member of the Båsnæringen Formation northeast of the Trollfjorden-Komagelva Fault Zone. Fold vergence indicates transport to the southwest. Brecciation and segregations of quartz can be seen along the steep ramp fault. View to the southeast. (b) Several sets of quartz veins in the Næringselva Member of the Båsnæringen Formation along the northeastern margin of the Trollfjorden-Komagelva Fault Zone. Two conjugate sets of rotated en échelon veins are cut by arrays of non-rotated veins. In the lower left part of the photo, the rotated quartz veins are deformed by several shear bands associated with layer-parallel movement, e.g. top to the left. View to the northwest. (c) Low-angle reverse fault (ramp in a ramp-and-flat structure) with quartz vein showing prominent slickensides. Båsnæringen Formation, Svartnes, looking southeast. The pencil is pointing towards 234°. (d) Equal-area, lower-hemisphere projections of bedding (great circles), fracture cleavage (dashed great circle) and fold axes from structures found in the Vagge Formation approximately 2 km northwest of Komagnes along the southwestern fault margin. The asymmetric, S- to SW-verging folds were generated during the D1 deformation episode (e) Structural data associated with D1 contractional faults from the study area. Poles to low-angle to steeply dipping, reverse-fault planes are indicated by triangles, and plunges of slickenside lineations by stars.


with moderate to steep plunges towards both NNW and SSE (Fig. 9a). The NE-SW-trending clusters reflect the D1 palaeostress situation, and the NNW-SSE clusters the D2 palaeostress situation (see description of D2 below). A similar plot of the stretching axes displays a cluster of steeply and partly southerly plunging axes (Fig. 9b).

D1 deformation: synthesis and age relations

Structures of the D1 event are most common along the fault margins and in areas to the north and northeast of the Trollfjorden-Komagelva Fault Zone. South of the fault zone, the F1 folds are confined to an area quite close to the trace of the TKFZ, but even there they are subordinate to structures ascribed to the D2 event. It is difficult to say with certainty whether these particular F1 folds have been rotated from their original orientation into sub-parallelism with the fault zone during a component of strike-slip movement along the TKFZ, or whether they reflect a contraction directly normal to the fault trace. However, the axes of F1 folds occurring along the southwestern fault margin display an en échelon configuration, which may suggest that anticlockwise rotation was associated with a small component of sinistral shear movement

along the fault zone at this time (Fig. 10a). North of the TKFZ, on the other hand, in the eastern part of the BSR, F1 macrofolds dominate the map pattern, thereby indicating the imposition of a more regional SW- to WSW-directed compression not confined to the proximity of the actual fault zone. Another explanation for the gradual swing in fold axial trend, from NW-SE in the Komagvær-Svartnes area to NNW-SSE or close to N-S between Vardø and Hamningberg, may be as a result of drag along the main fault zone associated with minor sinistral movements arising from a principal SW-directed compressive stress impinging on the WNW-ESE-trending TKFZ (Fig. 10b).

D2 deformation

The second episode of deformation identified in the study area is also contractional and represented mainly by folds and an associated axial-surface cleavage, reverse faults and dextral shear zones.

NE-SW to ENE-WNW-trending folds (F2). Folds of this generation are particularly common along the southwestern fault margin where they occur as gentle to open, buckle folds (Fig. 11a) with wavelengths of 2-4 m and amplitudes less than 30 cm. Tighter, chevron-type

Fig. 9. Contoured equal-area lower-hemisphere projection of estimated short-ening (a) and stretching axes (b) based on fault plane slickenside orientations and conjugate sets of shear fractures. Contour intervals are 2s.

A B

Fig. 10. Schematic representation of pos-sible fold development along the margins of the Trollfjorden-Komagelva Fault Zone during the D1 deformation event. (a) Rotated folds arising from roughly SW-directed compression with a component of sinistral strike-slip movement adding to the reverse slip along the fault zone. (b) Folds that show a gradual rotation into parallelism with the fault zone due to WSW- to SW-directed compression and with the fault zone acting as a barrier.


Fig. 11. (a) Siltstones of the Innerelva Member affected by layer-parallel buckling around NE-SW to ENE-WSW-striking axes (locality 2 in Fig. 3), with a weakly developed box-fold; looking northeast. (b) Tight to close similar folds of chevron type with a SSE vergence, in the Innerelva Member at Komagnes. (c) Large-scale asymmetric fold with a SE vergence in the Godkeila Member of the Båsnæringen Formation, at the nor-theastern margin of the Trollfjorden-Komagelva Fault Zone; locality along the river Komagelva, looking approximately north. (d) Equal-area, lower-hemisphere projections of structural data associated with the D2 contractional deformation from the above-mentioned localities. Circles – fold axes, squares – poles to dip-slip-reactivated, dextral strike-slip faults, great circles – axial planes of folds.

Fig. 12. Contoured equal-area lower-hemisphere projection of all bedding planes measured in the study area. Overlain are b-axes and poles, indicating statistical fold axes, based on four map-scale folds of the D1 (stars and dashed lines) and D2 (circles and simple lines) deformations.


folds with axial planes dipping steeply to the northwest and axes plunging gently towards northeast are also present (Fig. 11b), especially in the thin-bedded, more pelitic lithologies, and there are low- to high-angle thrust faults characterised by slickensides where calcite veins accompany the faults, thus confirming top-to-the-southeast transport. Top-to-the-southeast movement is also evident from quartz growth fibres and climbing arrays of calcite veins. A spaced cleavage (S2) axial planar to these F2 folds is a prominent feature just south of the TKFZ (Fig. 7a).

North of the fault zone, NE-SW-trending folds with related cleavage are uncommon, but there are SE-vergent, map-scale F2 folds along the northeastern fault margin where sandstones of the Båsnæringen Formation are weakly deformed into asymmetric, open, parallel folds (Fig. 11c). A stereographic projection of bedding planes, with a statistical best-fit great circle and b-axis, indicates a gentle NE-plunging statistical fold axis (Fig. 12). These particular folds are accompanied by steep conjugate fracture sets clearly coeval with the mesoscopic folding. Bedding-parallel quartz fibres associated with D2 flexural slip are mostly undeformed, in contrast to a wavy or crenulated, layer-parallel, mineral lineation associated with the SW-vergent folds of the D1 deformation event, and thus provides evidence that the F2 folds post-date the D1 structures.

NE-SW-striking reverse faults north of the TKFZ indicate transport to the southeast and are considered to belong to the same deformational event as the F2 folds. These meso-scale faults cross-cut and displace the mineral fibre lineation associated with the layer-parallel slip that occurred during the D1, SW-vergent transport.

NE-SW to ENE-WSW-trending dextral shear zones that can be ascribed to the D2 compressional regime are extensively developed along the southwestern margin of the TKFZ. These display rare duplexes, flower structures and calcite-filled breccias indicative of both transpressional and transtensional dextral shear. The shear zones are steeply dipping, segmented zones traceable for 40-50 m along strike. Farther south, vertical NE-SW-striking fractures are topographically well expressed in the sandstones of the Gamasfjellet Formation. Riedel-shear fractures and corrugation marks and steps on the master fracture surfaces suggest dextral strike-slip movements. These right-lateral faults of the D2 deformation were later reactivated by extensional dip-slip, as indicated by lineations defined by chlorite-coated growth fibres of calcite, and vertical offsets of bedding. This later extension is especially evident in outcrops along the southwestern fault margin. In addition to these reactivated faults, new faults were initiated and in some places there are weak flexures of bedding which may reflect the presence of blind normal faults at shallow depth.

D2 deformation: synthesis and age relationsWithin the study area, folds belonging to the D2 deformation are best developed along the fault margins of the TKFZ. The NE-SW to ENE-WSW-trending and SE- to SSE-verging folds and reverse faults are clearly indicative of a broadly SE-directed principal compressive stress. Seen in relation to the main NW-SE to WNW-ESE-trending fault trace, this compression would be expected to have generated dextral strike-slip movement along this major lineament. In the Komagelv area, there is evidence to suggest that this was of a transpressional character with local transport to the SE to ESE. There, the tightly folded siltstones were rotated clockwise with respect to the main fault trace and show easterly plunging fold axes, and thus support the idea of right-lateral transpression during the D2 deformation episode.

This compressional event seems to have been of a regional character across the Tanafjorden-Varangerfjorden Region, as structures ascribed to this deformation are not confined to the vicinity of the TKFZ but are also found over most of the region south of the fault zone (Siedlecka 1980, Chapman et al. 1985, Townsend 1986, Sygnabere 1998). In general, although mesoscopic and map-scale folds are far more common in the western half of the TVR, smaller-scale SSE- to S-vergent folds can be followed eastwards along and above one particular décollement horizon as far east as the coastline between Ekkerøya and Komagnes (Sygnabere 1998).

D3 deformation

An event which we designate as D3 is a phase of extensional deformation represented by features such as dolerite dykes, extensional dip-slip faults and associated master joints. In the study area, direct evidence of such features is confined to the area south of the TKFZ, but this does not preclude the possibility that D3-related structures may occur in parts of the BSR.

N-S-striking dolerite dykes are found along parts of the southwestern fault margin (Fig. 3) and another similar dyke occurs c. 20 km farther south on Ekkerøya (Beckinsale et al. 1975, Siedlecka & Roberts 1992, Guise & Roberts 2002, Rice et al. 2004). In the Komagnes area, three steeply east-dipping dykes (Fig. 3) display a right-stepping, en échelon geometry in map view (Fig. 13a). Locally where the middle dyke terminates towards the south, along the foreshore, a left-stepping geometry is seen. Close inspection reveals that some of the dykes are composite, dyke-in-dyke intrusions; and the thickest dyke is 2.5 m in thickness (by comparison, the Ekkerøya dyke is c. 16 m thick). In thin-section, the dolerite dykes show little signs of alteration and have an ophitic to subophitic texture dominated by plagioclase and clinopyroxene (Roberts 1975). The country rock (banded mudstones and siltstones of the Innerelv Member) shows a c. 4 cm-wide, baked contact zone.


The Komagnes composite dykes can be followed in outcrop for 325-625 m along strike. Using a handborne proton-magnetometer, it was possible to trace the northernmost dyke for a further 250 m into the southwestern marginal part of the fault zone beneath Pleistocene and recent deposits, until its signature eventually faded (Herrevold 1993). No lateral offset or change in strike could be detected in tracing the dyke northwards into the fault zone. Whilst this suggests that its emplacement may have post-dated all significant strike-slip movements associated with the earlier D1 and D2 events, we cannot be absolutely sure about this as the dyke has not been detected in or adjacent to the actual fault core. The dykes may have intruded along either pre-existing or contemporaneous N-S-trending fractures, since these appear to be particularly abundant close to the dykes and some show dip-slip extensional offsets. Intersection between cleavages and fractures in the Komagnes area has given rise to the formation of ‘pencil cleavage’ with the longest axes of the horizontal, elongate ‘pencils’ trending E-W. This feature is found in steeply N-dipping zones with intense fracturing.

The dykes are cut by at least two sets of steeply dipping joints, locally faults, striking NW-SE and WNW-ESE. Many of these later, open fractures or minor faults are filled or smeared with calcite, and show evidence of normal, dip-slip movement. Just to the south of the study area, conjugate sets of steep, N-S-trending, normal faults have been reported by Sygnabere (1998). These have downthrows of up to 20 cm and would appear to relate to the D3 extensional event documented here. In this same

area, the N-S normal faults post-date shallower dipping reverse faults which are likely to have formed during the D2 contractional event.


The dolerite dykes clearly transect and post-date the earlier folds and cleavages related to the D1 and D2 deformations, but are themselves cut by two sets of steep, NE- to N-dipping, extensional, calcite-filled fractures and minor faults striking NW-SE to WNW-ESE. Since we now know that the main Komagnes and Ekkerøya dykes are almost certainly of Late Devonian age (Guise & Roberts 2002), confirming the K-Ar study of Beckinsale et al. (1975), then the dyke intrusion event may signify the imposition of a regional E-W to WNW-ESE-oriented extensional stress field at or just before this time, at least in the TVR. There is evidence to suggest, however, that this extensional regime also influenced the northeastern BSR (Guise & Roberts 2002) where NE-SW trending faults were reactivated (Rice et al. 2004), thus allowing the sporadic intrusion of mafic dykes. Farther to the northwest in the BSR, several such normal faults (some of which are conceivably rejuvenated oblique-slip faults or minor thrusts) are also indicative of a NW-SE-oriented extensional regime (Siedlecki 1980, Lippard & Roberts 1987), but a more detailed analysis of faulting and fault reactivation is required in this particular region.

The age of dyke emplacement and lack of any visible strike-slip offsets of the main Komagnes dyke along and adjacent to the southwestern margin of the fault

Fig. 13. (a) Steep, N-S-trending dolerite dyke (locality 2 in Fig. 3) cutting Innerelva Member siltstones at the southwestern fault margin of the TKFZ; view looking north. At the southern termination of this dyke along the foreshore, the dyke splits into thinner dyke segments displaying a left-stepping en échelon geometry, and where the relay zones are characterised by brecciation of the siltstone. (b) Equal-area, lower-hemisphere projections of structural data associated with the D3 extensional deformation in the Komagnes area. Squares – poles to extensional dip-slip fault planes; great circles – dolerite dykes.


Ages of deformationThe sequence of deformation events outlined above, as recorded in this southeasternmost part of the TKFZ and its surroundings, is clearly a relative chronology based on the long-standing notion of superposition of folds, cleavages and other structures and fault-rock products. In only one case – that of D3 – do we have a reasonably secure idea of the absolute age of an event (Fig. 15). In the other three cases we are forced to rely on structural correlations, partly within the confines of Varanger Peninsula but also with events registered in nearby areas of Northwest Russia. Biostratigraphic evidence also provides some lower constraints, and our knowledge of offshore geology along the northwestward prolongation of the TKFZ helps in suggesting a likely age, or ages, for our youngest D4 structural event. In the brief paragraphs that follow, we therefore attempt to pin actual ages, or age ranges, on the four main events which have affected

zone would also indicate that all major strike-slip displacement along the TKFZ is likely to have ceased by Late Devonian time. A brief discussion of the dyke intrusion in a wider regional context is presented later.

D4 deformation

In the study area, this phase of deformation is represented by extensional structures developed just south of the TKFZ and along the northeastern fault margin.

ENE-WSW-striking tensile fractures are present south of the fault zone, both along the escarpment, where they are common, and in the area farther south. In alternating siltstones and mudstones, in several places, these fractures are filled by calcite, whereas quartz provides the fracture fill in the coarser-grained, thick-bedded sandstones. Late joints of this trend form a subordinate population along the northeastern fault margin, but there they are devoid of either quartz or calcite mineralisation.

WNW-ESE-striking extensional faults. Along the northeastern fault margin, this extensional phase is more pronounced and reflected in reactivation of larger WNW-ESE-striking shear zones that cross-cut map-scale D1 folds. The late reactivation of these faults by extensional dip-slip movements (Fig. 14) is characterised by brittle deformation with incohesive breccia and gouge along the fault cores. Normal drag close to the fault planes indicates a consistent downthrow of the northern blocks. The mode of deformation here is in clear contrast to that recorded in the two, earlier, contractional events, in being much more brittle in character. Along the southwestern margin, the WNW-ESE fracture pattern is accompanied by quartz and calcite veining, with indications of minor, dip-slip normal movement in several cases. Dip-slip reactivation of NE-SW-trending, dextral faults is also quite common in these southwestern areas.


Effects of this latest recognisible deformation in the study area, reflecting NNE-SSW to NE-SW oriented extension, are most pronounced along the northern margin of the TKFZ. The nature of the deformation products along these dip-slip reactivated faults indicates that deformation occurred at a shallower crustal depth than was the case with any of the structures seen in connection with the earlier deformational events. This extension may be seen in terms of a late, post-Caledonian reactivation of the Trollfjorden-Komagelva Fault Zone, since most of the extensional structures are found in proximity to the fault zone and its margins. However, this does not necessarily imply that reactivation is localised preferentially along the TKFZ. Within the BSR as a whole, several of the map-scale normal faults are suspected to have earlier histories of reverse- to oblique-slip movement.

Fig. 14. A steep, WNW-ESE-striking, normal fault in the Næringselva Member of the Båsnæringen Formation along the nor-theastern fault margin of the Trollfjorden-Komagelva Fault Zone, looking west. Drag indicates downthrow to the north. Fault breccia and gouge are found along the fault plane, but no slickensides have been detected.


Stappogiedde Formation close to Komagnes was acquired at c. 570-560 Ma (Gorokhov et al. 2001). This age coincides with the time of a reversal in palaeocurrent vectors (Banks et al. 1971), with sediment suddenly deriving from a rising landmass in the northeast and filling a small foreland basin (Gorokhov et al. 2001). If the diamictites of the subjacent Mortensnes Formation are reflecting part of the Gaskiers glaciation (Halverson et al. 2005), i.e., at c. 580 Ma, then the 570-560 Ma illites in the Stappogiedde Formation may be recording the approximate age of the Timanian deformation on Varanger.

In the eastern BSR, a dolerite dyke near Hamningberg provided a U-Pb zircon, upper-intercept age of c. 567 Ma and a lower intercept of c. 392 Ma (Roberts & Walker 1997). The dyke cuts a penetrative cleavage related to N-S-trending folds, a cleavage which, nearby at Finnvika, trending NNW-SSE, had yielded an imprecise, Rb-Sr whole-rock, isochron age of 520 ± 47 Ma (Taylor & Pickering 1981). At c. 2.5 km southeast of Finnvika, this same cleavage is strongly deformed through a c. 60° arc around later, ENE-WSW-trending, upright folds. The three fractions of zircon analysed from the Hamningberg dyke are discordant and the authors then favoured an interpretation that the upper discordia intercept indicated the approximate age of crystallisation (Roberts & Walker 1997), notwithstanding the presence of variable amounts of inheritance in the fractions. Later, based on a review of mafic dyke geochemistry in northern Finnmark, Rice et al. (2004) argued that the lower-intercept age of 392 +25/-36 Ma was probably closer to the true age of intrusion. Taking into consideration the geochemistry of the Hamningberg dyke and another, chemically similar, dolerite dyke at Finnvika (with a robust 40Ar-39Ar plateau age on plagioclase of 369 Ma: Guise & Roberts 2002), we consider that the upper intercept is likely recording an inheritance signature and now favour the re-interpretation of Rice at al. (2004), i.e., that the dyke is Late Devonian in age, similar to the dykes at Finnvika, Komagnes and Ekkerøya. The question of the age of the NNW-SSE-trending, pre-dyke cleavage in the Finnvika-Hamningberg area, and wider Vardø-Komagelva region, thus remains open. Currently, the only secure lower and upper constraints are provided, respectively, by the earliest Vendian age (Vidal & Siedlecka 1983) of the youngest rocks in the Barents Sea Group affected by the D1 folds and cleavage, and by the Devonian age of the mafic dykes. Accepting that the ENE-WSW-trending folds which locally deform the D1 cleavage and become dominant farther to the northwest are of Early Ordovician age, as in the northwestern BSR (Rice & Frank 2003), this narrows down the constraints; but they are still poor. Clearly, more precise geochronological data are needed before any safe conclusion can be reached. As things stand, however, and taking all regional evidence into account, i.e., including structural trends and isotopic dating in Northwest Russia

this multiply reactivated crustal lineament and adjacent areas.

D1: During a period of collaborative Norwegian-Russian fieldwork and research in the early 1990s, it soon became clear that the lithostratigraphies and structural development observed on the Rybachi and Sredni Peninsulas of NW Russia (Fig. 1, inset map) had much in common with what we see in eastern Varanger Peninsula. On Rybachi Peninsula in particular, a major Late Riphean turbidite system (Siedlecka et al. 1995b) revealed many of the features seen in the Kongsfjord Submarine Fan of the Barents Sea Group of the BSR (Siedlecka 1972, Pickering 1981, 1985). Moreover, the low-grade lithostratigraphic succession on Rybachi is strongly folded and cleaved along a NW-SE (to NNW-SSE) axial trend, with fold vergence towards southwest (Roberts 1995, Roberts & Karpuz 1995). These structures, part of the local (Rybachi) D1 deformation, abut against a major steeply dipping fault – the Sredni-Rybachi Fault Zone (SRFZ) – which is a southeastward continuation of the TKFZ. On Sredni Peninsula, in lower diagenesis-grade rocks, open NW-SE-trending folds are restricted to a narrow zone immediately adjacent to the SRFZ. Accordingly, the D1 deformation on Rybachi and close to the SRFZ has been interpreted as a form of basin inversion.

The structural scenario on Rybachi and Sredni is, thus, highly reminiscent of the situation observed in the eastern part of the Varanger Peninsula, notably in the BSR, in terms of lithologies, fold axial trend and vergence, cleavage development and metamorphic grade. In NW Russia, the NW-SE-striking folds and cleavage have long been known to constitute part of the Vendian-age, ‘Timanian mountain chain’ (Tschernyschev 1901) or ‘Timanides’ (Schatsky 1935), also referred to in Russian literature as the ‘Baikalian’ orogeny but now replaced by the more appropriate term ‘Timanian’ (Puchkov 1997, Gee et al. 2000, Roberts & Olovyanishnikov 2004). Isotope dating constraints indicate that the principal phase of the Timanian orogeny occurred during the time interval 590-560 Ma, generally oldest in the hinterland and youngest along the frontal ranges and their foreland basins (see review in Roberts & Olovyanishnikov 2004; also Grazhdankin 2004). New data from southernmost Novaya Zemlya, however, are suggesting that the latest pulses of Timanian accretionary collision probably continued well into Cambrian time (Pease & Scott 2009).

We therefore consider that the D1 contractional structures in eastern Varanger Peninsula are almost certainly Timanian, i.e., of probable Vendian age, and indicative of basin inversion. As yet it has not been possible to date the very low-grade S1 cleavage, but there is indirect evidence which serves to provide loose constraints. In the TVR, Rb-Sr studies on illite subfractions have suggested that an interpreted compactional fabric in the Innerelva Member of the


transpression and right-lateral strike-slip movement along the TKFZ. In western parts of Varanger Peninsula, folds of this trend, with NW-dipping slaty cleavage, are far more abundant and dominate the structural picture in the parautochthon and Gaissa Nappe Complex (Roberts 1972, Siedlecka 1980, Johnson et al. 1978, Rice et al. 1989b). Until quite recently, the precise age of this contractional deformation, involving southeastward thrusting, was not known. Earlier, in a Rb-Sr whole-rock study of cleaved mudstones from the Stappogiedde

(Gee & Pease 2004, and papers therein), we suggest that the D1 deformation and low-grade metamorphism in this eastern part of the Barents Sea Region of Varanger Peninsula is likely to have occurred in Vendian time, and probably at around 570-560 ± 10 Ma.

D2: In our study area, mesoscopic folds trending NE-SW to ENE-WSW and verging SE to SSE represent the second, main deformation episode, a regionally extensive, SE-directed shortening that involved

Fig. 15. Summary of the main tectonic events that have affected the rocks along and adjacent to the Trollfjorden-Komagelva Fault Zone in the study area, with their suggested approximate ages. The strain ellipsoids and double arrows illustrate subhorizontal contraction for D1 and D2, and extension arising from subvertical shortening for D3 and D4.


together with the ages accorded to the D1 and D2 events, would therefore infer that the suggestion of a Vendian to Early Cambrian age for the Komagnes dyke based on a palaeomagnetic study (Torsvik et al. 1995) must be rejected. The dykes are aligned parallel to a prominent N-S open fracture set, and the Komagnes dyke has been followed into the main fault zone without any detectable strike-slip offset, implying that all major strike-slip movement along the TKFZ had ceased by Late Devonian time.

The Devonian dykes, and the D3 extensional event (Fig. 13), fit into a well documented pattern of Mid Devonian to Early Carboniferous rifting and sporadic mafic magmatism reported from nearby parts of the Kola Peninsula and Timan-Pechora region (Ziegler 1988, Nikishin et al. 1996; see discussion in Guise & Roberts 2002). On the Kola Peninsula, there are also Pb-Zn mineralisations of Late Devonian age associated with some of the N-S trending dolerite dykes (Juve et al. 1995).

D4: Within the confines of the study area, the D4 extensional event is manifested in dip-slip reactivation of earlier shear zones and faults, with fairly consistent northeastward downthrows. Calcite and quartz veining along fractures is common, and along the northeastern margin there are breccias and gouge in fault cores. Although there are no constraints on the timing of D4 extension, and no known mineral growth amenable to radiometric dating, there is evidence from the offshore prolongation of the TKFZ on the Finnmark Platform of crustal extension, rifting and faulting in Carboniferous, Permo-Triassic and Jurassic to Early Cretaceous time (Gabrielsen & Færseth 1989, Dengo & Røssland 1992, Roberts & Lippard 2005). In addition, on the island of Magerøya (Fig. 1), there are NW-SE-trending mafic dykes (Roberts et al. 1991) of Early Carboniferous age (Lippard & Prestvik 1997), and one WNW-ESE-trending dyke of similar geochemistry and age occurs on the Digermul Peninsula (Beckinsale et al. 1975, Rice et al. 2004). D4 deformation could therefore have occurred in any one of these rifting episodes, or even include increments of movement from more than one of these events.

ConclusionsBased on structural studies in the southeastern part of the Trollfjorden-Komagelva Fault Zone on Varanger Peninsula, two main sets of contractional structures can be identified, grouped into D1 and D2 events. These deformational episodes are superseded by two, separate, extensional events, D3 and D4, characterised by brittle structures, fault breccias and quartz-calcite veining. The actual ages of the two contractional events can be deduced from published geochronological data either in other parts of the Varanger Peninsula or in neighbouring areas of Northwest Russia. The first extensional event,

Formation in the southwestern part of the TVR, Pringle (1973) had reported an isochron age of 504 ± 7 Ma (see also Sturt et al. 1975), interpreted as dating the cleavage and folding and regarded as reflecting latest Cambrian to Early Ordovician Finnmarkian deformation. The reliability of this isochron, however, was questioned by Dallmeyer & Reuter (1994).

Subsequently, Bylund (1994b) reported an Early Ordovician remagnetisation event in rocks of the Gaissa Nappe Complex along the eastern coast of Tanafjorden, and Sundvoll & Roberts (2003) presented Rb-Sr data from strongly cleaved pelites in the Gaissa southwest of Tanafjorden pointing to a Sr-isotopic resetting event at around 500-480 Ma. In a 40Ar/39Ar study of cleaved rocks from northwestern Varanger Peninsula, Rice & Frank (2003) concluded that, in the Tanahorn Nappe, the penetrative (S2) cleavage formed at c. 460 Ma “during the last stages of the SE-directed Finnmarkian event”; and in rocks of the Løkviksfjellet Group in the footwall of the Tanahorn Nappe, inferred ages of equivalent folds and pervasive cleavage range from pre-470 to 450 Ma. A later crenulation cleavage in the Tanahorn Nappe yielded plateau ages of c. 425 Ma, indicating that this younger, ESE- to E-directed, brittle overprint is of Mid to Late Silurian, Scandian age (Rice & Frank 2003). Kirkland et al. (2008) have also provided Ar-Ar geochronological evidence for a Late Silurian-Early Devonian event in rocks of the Berlevåg Formation. Just to the southeast, in rocks of the Løkviksfjellet Group, plateau ages for a late spaced cleavage are c. 443 Ma, suggesting that a separate pulse of weak deformation may have occurred towards the very end of the Ordovician period. In a Rb-Sr investigation of illites in the TVR, Gorokhov et al. (2001) reported that finer (0.1 μm) subfractions of authigenic illite range in age from 440 to 390 Ma, and were interpreted by them to reflect phases of early- to late-Scandian deformation, fluid migration and uplift.

The geochronological evidence currently available from Varanger Peninsula is thus suggesting that our SE-verging, D2 structures are likely to have formed in a Finnmarkian-related, Early Ordovician event (Fig. 15). It also indicates that polyphase deformation in the Lower Allochthon of the BSR extended well into the Ordovician period, and that the main dextral strike-slip translation along the TKFZ also occurred during this prolonged, Early to Late Ordovician time interval. Scandian deformation, on the other hand, including possible late pulses of strike-slip, is likely to have been of a more brittle character in this particular study area.

D3: As noted above, the N-S-trending dolerite dykes that transect the F1 and F2 folds and cleavages have yielded Late Devonian, 40Ar/39Ar mineral ages (Komagnes 378 Ma, Ekkerøya 375 Ma; Guise & Roberts 2002). These ages correspond closely with earlier, K-Ar whole-rock ages of c. 360 Ma for these same dykes (Beckinsale et al. 1975, but recalculated after Dalrymple 1979) and,


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D3, has a fairly secure, Late Devonian age, based on Ar-Ar mineral data from dolerite dykes. The age of the latest D4 event, on the other hand, is unknown, with diverse options available ranging from Late Palaeozoic to Mesozoic.

The sequential deformation history in the study area may be summarised as follows (Fig. 15):

(1) Inversion of the Neoproterozoic basinal regime during a pre-Caledonian, low-grade, and inferred Timanian (Vendian), contractional deformation (D1) event. This earliest episode is characterised by SW-directed shortening, folding and coeval cleavage development, with reverse movements along the Trollfjorden-Komagelva Fault Zone. By comparison with the Timanides in NW Russia, this deformation and metamorphism is likely to have occurred at around 570-560 ± 10 Ma.

(2) Caledonian deformation (D2) with SE-directed shortening and dextral strike-slip displacement of unknown magnitude along the Trollfjorden-Komagelva Fault Zone. Based on recent geo-chronology from NW Varanger Peninsula, this major contractional to transpressional event is considered to be late-Finnmarkian, pre-470 to 460 Ma (Arenig-Llanvirn), but pulses of deformation are believed to have occurred throughout Ordovician time. Scandian deformation in the actual study area and along the fault zone was probably only of brittle character.

(3) A regional, E-W-oriented, extensional event (D3) with intrusion of dolerite dykes. Dyke emplacement is dated to Late Devonian time, at c. 375-370 Ma.

(4) A later, NE-SW-oriented, extensional event (D4) along and adjacent to the Trollfjorden-Komagelva Fault Zone, reflected in normal faults with breccias, gouge and quartz-calcite veins. The age of this episode is open to speculation, but Late Palaeozoic-Mesozoic reactivation along the fault zone is highly likely based on our knowledge of offshore geology.

AcknowledgementsThe first author would like to thank Signe-Line Røe for hospitality, help with identification of some of the lithostratigraphic units, and guidance in the field. During fieldwork, T.H. received financial support from Finnmark Fylkeskommune, and enthusiastic help from former Fylkesgeolog Sigmund Johnsen. Sadly, Sigmund Johnsen passed away during the course of our work, such that he never saw its final result. We would like to dedicate this article to his memory. Financial support for fieldwork for M.R.K. from the Norwegian Research Council, from a joint Norwegian-Russian project ”Lineament and fault system analysis in Finnmark and western Kola Peninsula, Russia”, led by D.R., is acknowledged with thanks. We would also like to thank Ridvan Karpuz for collaboration during fieldwork and for fruitful discussions of the manuscript. Rodney Gayer, Hugh Rice and Anna Siedlecka kindly commented on an early version of the manuscript. The final typescript has benefited from helpful and constructive reviews by Stephen Lippard and Hugh Rice. In particular, we thank Hugh Rice for his brusque yet correct insistence that we sort out ambiguities relating to questionable, published, isotopic dates in the section on ages of deformation.


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