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([email protected])
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
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