ON THE DEEP CRUSTAL STRUCTURE OF THE LATE ARCHAEAN EASTERN YILGARN

CRATON: A COMPARISON WITH PALAEOZOIC AND MODERN ANALOGUES

Richard Blewett1, Paul Henson1, Bruce Goleby1, David Champion1,

Kevin Cassidy1, and Bruce Groenewald2

pmd*CRC c/o Geoscience Australia, GPO Box 378 Canberra ACT 2601(Richard.Blewett@ga.gov.au)

Geological Survey of Western Australia, PO Box 1664 Kalgoorlie WA 6433

SIZE-Mick flattens

Wallaby structure

SIZE-Mick flattens

Wallaby structure

‘Seis mic’ = crustal structure = orogenic process =mineral system =gold deposit (e.g., Wallaby)

Flatten structureSeismic > 1991

Pre-Seismic: Laverton 1:250K (Gower & Vogwill, 1972)

Seismic through Laverton 1:250K (GA, 2001)

3 km

10 km

5 km

Eastern Goldfields Lode Au

All Blacks strike and slip

Eagles expand their wings

Le Chook crumples under pressure

Golden Wallabies

All Blacks strike and slip

Strike-slip orogens

0-5 MaSteep fault, narrow orogenAfter Stern & McBride 1998

Strike-slip orogens

~400 Ma

Norembega FZ in AppalachiansAfter Stern & McBride 1998

Strike-slip orogens

• Steep structures

• Moho offset or

deflection?

• Narrow orogen

~400 Ma0 Ma

After Stern & McBride 1998

Eagles expand their wings

Extensional orogens

• Lower crustal reflectivity• Low-angle detachment• Half-grabens and basins in upper plate

~25 Ma

After Kruger et al., 1998

Extensional orogens

~250 Ma

• Big basins with or without apparent Moho effect• Question timing of reflectors

After Kruger et al., 1998

Le Chook crumples under pressure

Collisional orogens

Variscan of N France

~300 Ma• Broad orogenic belt

• Ramps and flats

• Thin-skinned style

• ‘Eroded’ mountain root in early Palaeozoic Bois et al., 1990

Collisional orogens

• ‘Crocodiles’

• Duplexes

• Deep penetrating low-angle faults (late)

• Undulating Moho

~300 Ma

Variscan of Germany

Dekorp Research Grp 1990

Vertical orogenesis

No known seismic comparisons possible ;)

Vertical orogenesis

Rey et al., in press

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Blue = gravity low

Golden Wallabies

~2650 Ma

91EGF1 213x45km

01AGSNY1 384x48km

Deep Crustal SectionKalgoorlie

Leonora to Lake Yeo

• Moho deepens east

• three broad crustal layers

• Prominent low-angle east dip

• Four crustal-penetrating shear zones

Tectonic mode?

• Strike-slip?

• Extension?

• Compression?

• Vertical?

Strike-slip structures

2 km

10 kmN=1200

dip

• Evidence for late (D3+) strike-slip

• Little evidence in seismic

Extensional structures

NW

CeliaV/h 3:1

3D model base of greenstone normal offset

Extensional structures

Yilgarn Yilgarn CratonCraton

MesoproterozoicMesoproterozoicsuccessionssuccessions

Extensional inversion of Archaean thrusts

Compressional structuresEW

• Duplexes

• A sense of bivergence

• ‘Y’ front

Compressional structuresEW

• Folds (ramp anticlines)

• ‘Crocodiles’

• Low-angle deep penetrating faults

Compressional structures

• Flat structures

• Strong sense of W-vergence

SE

NW

3D model sections

50 km

Tectonic mode in Goldfields?• Seismic geometry suggests compressional fold-

thrust belt

• Little strike-slip

• Late extension

• No evidence for diapirism

Seismic flattens the Wallaby structure

Questions?

Extensive regolith

Colours are outcrop (~10%) on magnetics

Seismic line

Yilgarn Craton

Seismic line

Crustal section of

Eastern Goldfields

Collisional orogensPyrenees

~30 Ma• Crustal penetration

• Bivergence

• Significant root and moho offsets

Bois et al., 1990