interpretation of the western gawler craton section of

Interpretation of the western Gawler

Craton section of seismic line 13GA-EG1

M.P. Doublier, R.A. Dutch, D. Clark, M.J. Pawley, G.L. Fraser, T.W.

Wise, B.L.N. Kennett, A.J. Reid, C.V. Spaggiari, A.J. Calvert, S. van

der Wielen, H. Dulfer, B.R. Bendall, S. Thiel, and J. Holzschuh

Western Gawler seismic workshop line 13GA-EG1E

Project Partners

Contributors:

Australian National University Canberra

Simon Fraser University Vancouver

Western Gawler seismic workshop, 10/12/2015 Adelaide

Introduction:

~ 834 km long deep

reflection seismic

transect between Haig

(WA) to Tarcoola (SA):

‘Eucla Gawler Survey’

or ‘13GA-EG1’

links with line 12GA-

AF3 in the west and

08GA-OM1 (GOMA) in

the east

transects 3 provinces:

Mandura, Coompana,

western Gawler


Aim

define province

boundaries

investigate contact

relationships between

provinces, and internal

crustal architecture

image faults/shear

zones

implications for mineral

systems and

exploration strategies


Aim

define province

boundaries

investigate contact

relationships between

provinces, and internal

crustal architecture

image faults/shear

zones

implications for mineral

systems and

exploration strategies

here: eastern part of line (~360 km), covering western Gawler

Craton and easternmost Coompana Province – line 13GA-EG1E


Interpretation workflow

Approach: combination of hardcopy interpretation and interpretation in

digital environment (ArcGIS)

Benefits:

zooming in and out

digital linework allows for easy comparison between different

interpretations

during each stage linework can be visualized in 3D (e.g. GoCAD), or

used for potential field modelling

overlay and compare different image types


Image types used

DMO stack

Final migrated DMO stack with 20 m CDP spacing and 150 nominal

fold

Migrated NMO stack with 100 m CDP spacing and a nominal fold of

750 - improvement for imaging of the lower crust

Image processed to increase contrast and enhance the distinction

between regions of differing seismic character

Image of the stack section processed by coherency enhancement

processing and CDP trace equalisation

Image of the migrated section processed by coherency

enhancement processing and CDP trace equalisation


13GA-EG1E – seismic image

Final migrated DMO stack

quality of seismic reflection data is good

main image used for interpretation


13GA-EG1E – seismic image

Final migrated DMO stack; some zones with low reflectivity:

• under W-Wilgena Domain

• at contact Christie and Nawa Domains

• under Eucla Basin

reasons: sand dunes, weathering, Nullarbor Limestone


13GA-EG1E – zone of low seismic reflectivity

Zones of low seismic reflectivity are best visible in DMO stack(top; as

bland zones) and coherency enhanced stack images (bottom; as

vertical ‘corridors’)


Different image types - example granite

intrusions Hiltaba Suite intrusions in the

Wilgena Domain:


(top); and an image processed

to increase contrast and

enhance the distinction

between regions of differing

seismic character (bottom)


Different image types – example shear zones

Anastomosing shear zones in

Central Gawler Seismic

Province:


(top); and image of the

migrated section processed by

coherency enhancement

processing and CDP trace

equalisation (bottom)


Different image types – example Moho and lower

crust Final migrated DMO stack (top); and image of the migrated NMO

stack (bottom) with 100 m CDP spacing and a nominal fold of 750


Interpretation – regional context: two provinces

Gawler Craton: Wilgena, Christie, Nawa – Fowler not observed

Coompana Province: eastern boundary poorly defined


Interpretation - Moho

reasonably well imaged in most parts of line 13GA-EG1E

discontinuity between a reflective lower crust and underlying largely

non-reflective material (mantle)

two regions of crust > 52 km thick

conservative interpretation – could be up to 1 sec (~3 km) deeper


Interpretation – crustal architecture

Three layered crust under Christie and western Wilgena Domains:

a reflective lower crust

middle crust that is weakly to non-reflective

an upper crust with some variation, but overall moderately reflective


Interpretation – major faults and shear zones

all orientations are apparent

crustal scale structures, which may sole into lower crust or Moho

typically listric, westerly dipping shear zones

exception is the Wilgena Domain, which shows east dipping

structures


Interpretation – major faults and shear zones

domain and seismic province boundaries in red

some shear zones are difficult to trace in the non-reflective middle

crust, and shown as ‘inferred’ (stippled lines)

structures have polyphase history, involving major strike slip

displacements – hence offsets will often be apparent


Interpretation – form lines

delineate coherent reflectors

illustrate the westerly dipping structural grain in lower and middle

crust

show open folding in the upper crust, which is not observed in

middle and lower crust – decoupling at some stage

form lines


Gawler Craton – lower and middle crust detail

within the reflective lower crust we can distinguish reflective

lowermost crust, with reflectors parallel to the Moho

the distribution of reflective lower crust and poorly reflective middle

crust is controlled by shear zones, which often show a ramp like

geometry (example Coorabie Shear Zone)


Gawler Craton – lower and middle crust

Reflective lower crust is present throughout the seismic section, and

becomes thicker and occurs at higher structural levels towards both ends

Middle crust is largely absent under eastern Wilgena Domain

Interpretation under western Wilgena non-conclusive – low reflectivity zone

Middle to lower crust does not reach the surface


Gawler Craton – Seismic Provinces

Middle and lower crust represents a ’seismic province’: a discrete volume

of middle-lower crust, which cannot be traced to the surface and whose

seismic reflectivity is different from that of vertical or horizontally adjoining

provinces (Korsch et al., 2010)

We distinguish the Western Gawler Seismic Province and the Central

Gawler Seismic Province, which in comparison is (i) slightly more

reflective, (ii) not overlain by a weakly to non-reflective middle crust, and

(iii) contains a number of anastomosing shear zones.


Gawler upper crust – Wilgena and Christie

Both domains are up to

~24 km thick, and show

a similar, moderately

reflective seismic

character interpreted to

represent Mulgathing

Complex (purple).

Differences:

structural trend: Wilgena shows east dipping structures

Hiltaba Suite intrusions absent from Christie Domain

Tarcoola Formation overlies Wilgena Domain only


Gawler upper crust – Wilgena and Christie detail

Both domains are up to

~24 km thick, and show

a similar, moderately

reflective seismic

character interpreted to

represent Mulgathing

Complex (purple).

Form lines reveal

open folds


Gawler upper crust – Nawa Domain

upper part of Nawa Domain low to

moderately reflective package,

thickens into Karari Shear Zone

drillholes intersect

Palaeoproterozoic metasediments,

including the Moondrah Gneiss

basement to these metasediments

shows differences in seismic

character, and allows to

distinguish two seismic

subdomains, the Karari Seismic

Subdomain (KSS), and the

Wirinjinna Seismic Subdomain

(WSS)

30 k

m

30 k

m

WSS KSS


Gawler upper crust – Nawa Domain

30 k

m

30 k

m

KSS WSS

upper part of Nawa Domain low to

moderately reflective, thickens into

Karari Shear Zone

drillholes intersect

Palaeoproterozoic metasediments,

including the Moondrah Gneiss

basement to these metasediments

shows differences in seismic

character, and allows to

distinguish two seismic

subdomains, the Karari Seismic

Subdomain (KSS), and the

Wirinjinna Seismic Subdomain

(WSS)


Gawler upper crust – Nawa Domain detail

the Karari Seismic Subdomain

(KSS) consists of an upper

reflective package, and a lower

non- to poorly reflective package

the Wirinjinna Seismic

Subdomain (WSS) consists of a

package of poorly reflective crust

Questions:

do the non- to poorly reflective parts of the seismic subdomains

represent the middle crust observed further east?

does the reflective upper part of the Karari Seismic Subprovince

represent Mulgathing Complex?


Coompana Province

weak seismic reflectivity, with

reflectors dipping shallowly to the

west

westward thickening wedge

bounded by the (planar) Jindarnga

Shear Zone

second order structure: listric

Maralinga Fault System


Coompana Province - detail

overlain by Eucla Basin (yellow) and Officer Basin (orange)

new constraints from data processing further to the west

suggests that lower part of the Officer does represent multiples

Coompana Province will be assessed during interpretation of the

western part of the line (13GA-EG1W)


Summary

The western Gawler Craton is dominated by a three layered crust

The seismic section is dominated by westward dipping, crustal scale shear

zones, which control the distribution of the middle and lower crust

An exception is the Wilgena Domain, which shows and eastward dipping

structural grain

Structures have a protracted history involving significant strike slip

movement – offsets are apparent


Summary

The mid-to lower crustal Western Gawler

Seismic Province extends beyond the

western end of 13GA-EG1E, underlying

the Coompana Province

The Coompana Province is bounded by

the Jindarnga Shear Zone, has a weak

seismic reflectivity, with reflectors dipping

shallowly to the west

Underneath the Paleoproterozoic

metasediments, the Nawa Domain shows

a heterogeneous seismic character

(similar to GOMA)

Phone: +61 2 6249 9697

Web: www.ga.gov.au

Email: [email protected]

Address: Cnr Jerrabomberra Avenue and Hindmarsh Drive, Symonston ACT 2609

Postal Address: GPO Box 378, Canberra ACT 2601

Thank you!

interpretation of the western gawler craton section of

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