Yilgarn crust and mantle lithosphere framework: geodynamic insights and interpretations

Transcription

1 Australian Government Geoscience Australia Yilgarn crust and mantle lithosphere framework: geodynamic insights and interpretations Minerals Exploration Seminar Perth, 29 November 2004

2 Acknowledgements Geoscience Australia: Kevin Cassidy, David Champion, Bruce Goleby, Tanya Fomin,, Paul Henson, Russell Korsch,, Barry Drummond, Ed Chudyk, Leonie Jones, Malcolm Nicoll,, Terry Brennan Research School of Earth Sciences (ANU): Brian Kennett, Anya Reading Geological Survey of Western Australia: Bruce Groenewald pmd*crc (Predictive Mineral Discovery) AMIRA P437/P437A, P482, P624 Australian National Seismic Imaging Research (ANSIR) facility mineral exploration companies

3 Mineral Systems Jon Hronsky (Western Mining Corporation) presented at SEG 2004 Ore deposits are focal points of large-scale energy and mass flux systems Generally, larger and more energetic systems equals bigger ore deposits The system may be recognised even though the precise genetic links with mineralisation are unclear

4 Talk Outline What signature has this energy and mass flux left on the Yilgarn s crust and mantle lithosphere? Use seismic tools to examine the possible signatures Slabs, subduction, delamination? Plumes? Examining hypotheses Yilgarn Craton Terranes? Minerals Exploration Seminar, Perth, 29 November 2004 Conclusions Geoscience Australia

5 Seismic tools: Broad Band Location of Yilgarn recorders Minerals Exploration Seminar, Perth, 29 November 2004 P-wave, S-wave and surface wave velocity from distant earthquakes Tomographic image of velocity structure of the Australian lithosphere (to 350 km) Geoscience Australia

6 Red/Brown slower than the world average Blue faster than the world average. B.L.N. Kennett Seismic tools: Broad Band Shear (S) wave velocity at 75 km WEST fast depleted refractory cold less dense dry strong buoyant Next slide EAST slow undepleted fertile dense warm wet weak less buoyant

7 S-wave velocity model (transect) Moho Leonora 01AGSNY1 350 km Mantle lithosphere stratified by a fast velocity layer Blue S-wave velocities slower than the world average, Pink S-wave velocities faster than the world average View NNW oblique

8 S-wave velocity model (3D slices) Fast velocity layer 150 km View WNW oblique

9 S-wave velocity model (4.8 km/s) View NW oblique

10 S-wave velocity model (3D slice) W M Y (SC) EGP E 120 km Ida F Yamarna F 350 km View to north looking up from ~100 km depth Intriguing SE-dipping fast-velocity body, what is it?

11 Minerals Exploration Seminar, Perth, 29 November 2004 Geoscience Australia Seismic tools: Receiver Functions WT WV A. Reading Yilgarn transect (WT and WV) Non-linear inversion for crustal velocity Map Moho Characterising provinciality Location of Yilgarn recorders

12 Seismic tools: Receiver Functions WV NE WT Next slide W Reading et al. (2003)

13 Seismic tools: Receiver Functions Youanmi Kalgoorlie Kurnalpi E Velocity generally increases with depth (~ density) depth Moho Characteristic patterns in some terranes (eg Kalgoorlie) S-wave velocity increase Australian average velocity profile Calculated crustal velocity profile

14 Seismic tools: Reflection seismology NY1 EGF1 Deep seismic reflection explosives & vibs ( ) Information on crustal architecture Some velocity and lithological information Quantum leap in 3D understanding and geodynamics

15 Seismic tools: Reflection seismology Kalgoorlie ( $ ( $ ( Ñ ( ( ( ( 91EGF1 213x45km ( ( $ ( $ $ Ida Shear Ñ Ñ ( ( Ñ $ $ Ñ $ Ñ$ $ $ Ñ $ $ ( Ñ Ñ ( ( Ñ X( Ñ Ñ $ Ñ ( $ $ $ & ( $ ( Ñ Ñ ( Ñ Ñ Leonora Laverton Yamarna Ockerberry Shear Laverton Shear Yamarna Shear 01AGSNY1 384x48km Moho deepens to east three broad crustal layers Leonora to Lake Yeo prominent low-angle east dip (foreland fold-thrust belt) 4 crustal-penetrating??? shear zones

16 Subduction, slabs, delamination etc? velocity Yilgarn subduction at various times (geochemistry) Both east- and westdipping slabs Can we see the signature of subduction? Slabs characterised by fast S-wave velocities Tomography of slabs under Japan (Fukao et al. 2001)

17 SE-dipping subduction zones?? N Kalgoorlie? Felsic volcanism Ma Young east? Cassidy & Champion (2004) Youanmi Felsic volcanism? Ma Zircon inheritance Postulated SE-dipping slab ca Ga

18 W-dipping subduction zones? Kalgoorlie Terrane Kurnalpi Terrane Ca Ga Together? Fault EAST Continental crust Oceanic crust High-HFSE Slab Mantle upwelling produces komatiites High-Ca Mafic/calc-alkaline Champion & Cassidy (2001) Morris & Witt (1997) Models for Westdipping subduction at ~2.7 Ga

19 Stratified lithosphere as a slab? W M Y (SC) EGP E 120 km Ida F Yamarna F 350 km 500 km SE-dipping (low-angle) fast km depth possibly a fossil slab (cf. Narryer at 2.75Ga) no evidence for the W-dipping 2.7 Ga slab? The age of the high-velocity layer is unknown

20 Flat subduction in Chile as analogue? W M Y (SC) EGP E 120 km Ida F Yamarna F 350 km 500 km Gutscher et al. (2000) 500 km

21 Yilgarn mantle plumes? Plume movie Plume Plumes have been hypothesised for komatiites (eg Morris & Witt, 1997; Champion etc), and many granites (eg Campbell & Hill, 1988 and later papers) Kalgoorlie Terrane crust Continental Together? Slab Fault Kurnalpi Terrane Mantle upwelling produces komatiites High-Ca Morris & Witt (1997) High-HFSE Mafic/calc-alkaline Ca Ga EAST Oceanic crust Champion & Cassidy (2001)

22 Impingement of a mantle plume? M Y (SC) EGP E 120 km Ida F Yamarna F 350 km 500 km If a high-level plume: it would disrupt this stratification (assuming stratification is old) Or, horizontal reworking was dominant after the plume Or, disruption is out of this plane If a deep plume we wouldn t see it in this section We can not rule plumes in or out on this evidence

23 Champion & Sheraton, 1997 Yilgarn delamination? Low-Ca granite greenstone Major types (High-Ca, Low-Ca) Crustal melts Intrude external granites (at base of greenstone) Late <2655 Ma

24 Fast layer as delaminated restite? melt Ida F Delamination Yamarna F Delamination 500 km Heat? Delamination: delivers heat across entire Yilgarn (Low-Ca are craton-wide) Initiated at ~2670 Ma (first D2a event in east) Low-Ca granites ~2655 Ma (E) to 2640 (W) Synchronous with Au ages? AMIRA P624; Blewett et al., 2004

25 Terranes of the Yilgarn Craton Narryer? Youanmi Terrane South West Terrane Next slide Kalgoorlie Terrane East Yilgarn terranes? 300 km Various subdivisions of Yilgarn Based on geology, geochemistry, isotopes, magnetics/gravity, structure Various amalgamation models: autochthonous allochthonous strike-slip What are the seismic signatures of the terranes and their boundaries?

26 Terranes of the Yilgarn Craton NY1 line 6 tectono-stratigraphic terranes Norseman Kalgoorlie Gindalbie Kurnalpi Laverton Edjudina after Myers (1997)

27 Terranes of the Yilgarn Craton: upper crust from seismic Kalgoorlie Ockerberry F Gindalbi Emu F Kurnalpi 6 km Folded detachment 01AGSNY1 Terrane boundaries appear thin-skinned and sole out on to the folded detachment

28 01AGSNY1 20 km Terranes of the Yilgarn Craton: upper crust from seismic Kurnalpi Laverton Laverton-Kurnalpi boundary may be a major crustal fault Prominent moderate eastdips suggest thrust amalgamation (cf. strike-slip)

29 Terranes of the Yilgarn Craton: the Low-Ca granites Ida Fault Next 2 slides Low-Ca granite greenstone Intrude external granites (base of greenstone) Relatively low density, relatively slow (seismically)

30 Terranes of the Yilgarn Craton: upper crust from receiver functions Youanmi Kalgoorlie Kurnalpi E Kalgoorlie has low-velocity layer at ~10 km = less density depth 10 km Moho Not simply reflecting overlying greenstones S-wave velocity increase Australian average velocity profile Calculated crustal velocity profile

31 Terranes of the Yilgarn Craton: upper crust from receiver functions Youanmi Kalgoorlie Kurnalpi E depth Moho Crustal melts Thick Low-Ca granite underplate under Kal from crustal anatexis (link to Au) Thick Low-Ca signature of Kal endowment?? S-wave velocity increase World average crustal velocity profile Calculated crustal velocity profile

32 Lots of arm waving is interesting, but how does it help us? Jon Hronsky (WMC) Focus of this talk

33 Help through. Better understanding of the minerals system process geodynamics is an integral component and driver of the mass and energy flux Cassidy & Hagemann (2003)

34 Help through. being predictive in space and time taking this understanding under cover Outcrop map (Pink - granite, Green greenstones, yellow - cover

35 Conclusions Signatures of the mass and energy flux at the largest scale are visible in various seismic methods (tomography, receiver functions, seismic reflection) We don t always understand the signatures a complex mantle lithosphere and crust, especially the temporal component Geodynamic implications for slabs, plumes and delamination; tectonic development in general

36 Conclusions The Yilgarn is a data-rich region and we need a better methodology (paradigm?) to understand the signatures The way forward is clearly a holistic systems approach across all lithospheric scales and dimensions Need to integrate in 3D with time Erect multiple hypotheses and apply suitable tests

37 Challenge for us all Challenge is being predictive with this new understanding and knowledge The system may be recognised even though the precise genetic links with mineralisation are unclear (Hronsky( Hronsky, 2004), but we are working hard on it (pmd*crc)

38 Thanks for listening