Seismic: A key tool in Seismic: A key tool in understanding crustal architecture understanding crustal architecture
and mineral systemsand mineral systems
pmd CRC
B.R. Goleby1, R.S. Blewett1, B.L.N. Kennett2, L.E.A. Jones1, R.J. Korsch1, B.J. Drummond1, T. Fomin1, A.M. Reading2,
K.F. Cassidy1, D.C. Champion1, P. Henson1, P.B.Groenewald3, M. Nicoll1 and T. Brennan1
‘‘Reduced Discovery Risk through Improved Targeting’Reduced Discovery Risk through Improved Targeting’
1 Geoscience Australia, pmd*CRC2 Research School of Earth Sciences (ANU)
3 GSWA, KalgoorlieANSIR AUSTRALIAN
NATIONALSEISMIC IMAGINGRESOURCE
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Why collect seismic in a Why collect seismic in a mineral province?mineral province?
• provides geometrical and depth constraints (architecture)
• provides velocity info and hence density information (infer lithology)
• Seismic has always shown up the unexpected
• Seismic provides a quantum leap in knowledge
Mineral Systems, a 5 Questions approach:Architecture, geodynamic inferences, fluid pathways, drivers, focus and deposition
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Talk OutlineTalk Outline
• Lithosphere velocityBroadband stations (distant earthquakes P-wave, S-wave & surface wave)
• Crustal velocityReceiver function (inversion of earthquake data)
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Talk OutlineTalk Outline
• Wide-angle refraction (long offset seismic energy from vibs)
• Deep seismic reflection (record near-vertical incident seismic waves)
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Broadband DataBroadband Data
• Broadband stations P-wave, S-wave and surface wave velocity from distant earthquakes
• Deep velocity structure beneath the Australian Continent
• Investigate characteristics of Australian Lithosphere.
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Shear wave velocity at 75 km (upper mantle).
B.L.N. KennettRed/Brown slower than the world averageBlue faster than the world average.
BROADBAND DATA
WESTWESTfastfast
depleteddepletedrefractoryrefractory
coldcoldless denseless dense
drydrystrongstrong
buoyantbuoyant
EASTEASTslowslow
undepletedundepletedfertilefertiledensedensewarmwarmwetwet
weakweakless buoyantless buoyant
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Shear wave velocity modelShear wave velocity model
Blue S-wave velocities slower than the world average, Pink S-wave velocities faster than the world average.
350 km
Leonora01AGSNY1
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Blue S-wave velocities slower than the world average, Pink S-wave velocities faster than the world average.
3D surfaces of S3D surfaces of S--wave velocitieswave velocities
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4.8 km/s S-wave isosurface
Blue S-wave velocities slower than the world average, Pink S-wave velocities faster than the world average.
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3D visualisation3D visualisation--Broad BandBroad Band
350 km
EEGPSCM
• Steps in 4.8 km/s isosurface• Intriguing geometry of the fast velocity body
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Broad Band data summaryBroad Band data summary•• Yilgarn mantle lithosphere: fast, Yilgarn mantle lithosphere: fast,
depleted, refractory, cold, light, dry, depleted, refractory, cold, light, dry, strong, buoyantstrong, buoyant
•• Heterogenous lithosphere and stratified Heterogenous lithosphere and stratified –– what would a fossil mantle plume do what would a fossil mantle plume do to this layering?to this layering?
500 km
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Broad Band data summaryBroad Band data summary•• Step changes in highStep changes in high--velocity layer velocity layer
correspond? with terrane boundaries?correspond? with terrane boundaries?•• EastEast--dipping fossil slab dipping fossil slab –– too shallow, too shallow,
wrong way?wrong way?
Gutscher et al. (2000)
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Receiver Function DataReceiver Function Data
• Good Yilgarn-wide coverage
• Broadband transect of Yilgarn
• Non-linear inversion for crustal velocity
• Characterising provinciality
WTWT
WVWV
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ANSIR AUSTRALIANNATIONALSEISMIC IMAGINGRESOURCE Reading, Kennett & Dentith, 2003
WV
WT
World average crustal velocity profile
Calculated crustal velocity profile
Receiver Function DataReceiver Function Data
W
NE
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Receiver Function DataReceiver Function Data
• Moho: shallow east dip– consistent with reflection
moho and eastward tilt of Yilgarn?
• Characteristic velocity functions across terranes– cf Kalgoorlie upper crustal
(10 km) low velocity anomaly
Southern Cross Kalgoorlie
10 km
Velocity increase
World average crustal velocity profile
Calculated crustal velocity profile
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Receiver Function DataReceiver Function Data
• Drop in velocity means less density
• Low-Ca granite (Champion and Sheraton, 1997) ‘underplate’ or transfer to base of upper crust from mid crustal anatexis?
Southern Cross Kalgoorlie Laverton
10km
Velocity increaseWorld average
Calculated
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Receiver Function DataReceiver Function Data
• Laverton velocity function similar to Southern Cross?
• The Kalgoorlie terrane as a ‘rift’
Southern Cross Kalgoorlie Laverton
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WideWide--Angle Refraction Data Angle Refraction Data
• Pioneering work• Refraction recording
using portable short-period instruments. Passive technique
• Vibrator source to record refracted seismic energy to offsets over 50 km
• Velocity structure of crust
• Infer some lithological information
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Wide Angle Refraction Data Wide Angle Refraction Data
0 200
Dep
th, k
m
0
76-6.2km/s
6.2-6.4km/s
5.0-5.8km/s
6.4-6.6km/s
6-6.2km/s
Leonora Laverton
Velocity model for top 7 km.
Upper crustal velocity model from first arrivalsCoincident with deep seismic reflection traverse
3D viewof wide-angle
velocity functions
14 km
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Seismic Reflection DataSeismic Reflection Data
• Deep seismic reflection explosives & vibs (1991-2001)
• Resulting sections provide information on crustal architecture
• Some velocity and lithological information
• Quantum leap in 3D understanding and geodynamics
NY1NY1
EGF1EGF1
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213x45km
WESeismic Reflection DataSeismic Reflection Data
91EGF01
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Leonora Laverton Lake-YeoYamarna
16 s
EW
• crustal thickness change across the NE Yilgarn• subdivision of crust into three broad layers• prominent east dip• three east-dipping crustal-penetrating shear zones
Seismic Reflection DataSeismic Reflection Data
384 km in length , 48 km in depth01AGSNY1
http://www.ga.gov.au/pdf/RR0110.pdf
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Leonora Laverton Lake-YeoYamarna
384 km in length , 48 km in depth 01AGSNY1
EW
http://www.ga.gov.au/pdf/RR0110.pdf
• Major gold deposits associated with major structures• East-dipping crustal shear zones coincident with
known mineral fields• Fault zone reflectivity due to alteration
Leonora Mineral Field
Laverton Mineral Field
Possible New Mineral Field
Au bearing fluids?Au bearing fluids?
Seismic Reflection DataSeismic Reflection Data
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Seismic Reflection DataSeismic Reflection Data
• Excellent tool for architecture (underpins mineral systems analysis)
• Invaluable for 3D model building as potential field data (mag and gravity) below greenstone base (< 7 km) is useless
• Speculate about fluid flow and focussing from derived geometries and texture
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MineMine--scale seismic datascale seismic data
• Mine-scale seismic reflection profiling (2001 and 2004)
• Information on mine-scale architecture
• Good fit to drill hole and mine scale geology
• Leading to drill targets
NY1NY1
EGF1EGF1
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MineMine--scale seismic datascale seismic data
Sons of Gwalia (Leonora)
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ConclusionsConclusions
• Exciting and novel results• Successfully imaging
crustal structure = architecture
• Crust and uppermost mantle lithology variations and velocity distribution
• Providing inputs for the geodynamic and metallogenic history of Yilgarn terranes
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Future Yilgarn WorkFuture Yilgarn Work
• pmd*CRC Y-New project 2005-2008 (see poster)
• Magneto-telluric study across existing seismic reflection profiles (EGF01)
• Refine and integrate the characterisation of Receiver Functions with 3D models
• Explore questions raised by these intriguing techniques (geodynamics and mineral systems)
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Thanks for listening