Seismic imaging using earthquakes and implications for earth systems 123* 2 3 3 3 Huaiyu Yuan , Mike Dentith , Ruth Murdie , Simon Johnson and Klaus Gessner * Currently in a week-long meeting in San Francisco. 1 2 3 [email protected] CCFS, Macquarie University; CET-UWA; GSWA Email [email protected] for questions. Or spend a good 15 mintues reading it through! 1. Lithosphere control of the mineral systems and seismic imaging 6. Capricorn passive source project How giant magma-related ore systems form is vigorously debated. One view, displayed below, argues ascending magmas pick up ore-forming compo- The Capricorn orogen is a major tectonic unit that recorded the assembly of the Yil- nents (diamonds, gold) during their passage through the mantle lithosphere (Griffin et al., Nature Geoscience, 2013): e.g. garn and Pilbara cratons and the Proterozoic terranes to form the West Australian (a) diamonds, formed deep from metasomatically introduced carbon zones, are brought to surface in magmas that take advantage of lithospheric scale craton (a; e.g. Johnson et al. 2013). Numerous mineral deposit types have been rec- weak zones; and ognized throughout the orogen, and recent studies (Johnson et al. 2013; Aitken et (b) Au- and Cu-rich deposits are found in the back-arc and the mantle wedge, which are associated with low-degree and hi-degree melting, respectively. al. 2013) have illustrated the connection between known mineral deposits and the crustal scale fault systems and corresponding crustal blocks (b). In all cases lithospheric scale weak zones facilitate the movement of metal-bearing fluids to the surface. Therefore 3D structure of the lithosphere helps to focus deposition of the ore, and entire lithosphere architecture through seismic imaging is preferably needed. We illustrate below how seismic imag- An exciting seismic array of our own, the Capricorn passive source project is funded ing, mostly using earthquakes, can be utilized for understanding craton lithosphere tectonics and also for the purpose of mineral exploration. to cover the Capricorn area. This passive source project is part of a multidisciplinary (a) We focus in Sections 2 to 5 on imaging techniques that are at various resolution scales: from continent wide (500-km lateral and 20-km vertical resolu- project, “Distal footprints of giant ore systems: Capricorn case study”, which aims tion) surface wave tomography, to local fault scale (10s meters both later and vertical resolution) ambient noise imaging. These techniques will be ap- to characterize the Capricorn Orogen from the crust to mantle using approaches in (a) Elements of the Capricorn Orogen and surrounding cratons and basins, showing the location of plied to the Capricorn passive source deployment (Section 6), WA’s first ever large scale earthquake seismic deployment in Capriconn region which is geology and geochemistry and geophysics. The overall goal of the seismic project is the Capricorn Orogen seismic transect. Modified from Martin & Thorne (2004). The thick grey lines to delimit the northern and southern boundaries of the Capricorn Orogen. Inset shows location of the going to commence early next year. produce 3D multiple scale seismic images across the orogen, which will constrain Capricorn Orogen and Paleoproterozoic crustal elements (KC, Kimberley Craton; NAC, North Australian Craton; SAC, South Australian Craton;WAC,West Australian Craton) and Archean cratons (YC, Yilgarn geological models for the timing and kinematic evolution of faults and shear zones in Craton; PC, Pilbara Craton; GC, Gawler Craton). Other abbreviations: MI, Marymia Inlier; YGC, Yarlar- the region and its 4D metallogenic history. weelor Gneiss Complex. Figure and captions from Johnson et al. 2013. Red wiggle lines are active source lines, who results are summarized in b). Interactions between magmas and the SCLM. Overall there will be 64 broadband seismographs covering a500x500 km region of a, Plume triggers kimberlite formation and lows to area of thinner SCLM the orogen (c), providing unparalleled coverage in the region for the earthquake where melting is focused. Variable interaction of melts with crust and SCLM (b) influences Ni-Cu and PGE deposit genesis. b, Generalized convergent-margin seismology to uncover the lithosphere architecture: What is the spatial extent of setting. Au-poor magmatic-related deposits form from dominantly astheno- spheric or crustal melts (such as Cu-rich or W Sn porphyry, respectively). the major lithosphere domains? Where are their boundaries? Are the boundaries Low-degree melting of asthenosphere, particularly in retro-arc settings, can linked to the crustal faults? How do they fit the mineral deposits on the surface? produce Au-rich metasomatic refertilization of the SCLM. Subsequent melt- ing (which may be much later) contributes Au to magmatic systems, forming Are there signs of wedge tectonics and subduction polarity reversal? (b) Summary cross-section through the Capricorn Orogen, showing the spatial relationship between deposits of porphyry Cu-Au, epithermal Au, iron oxide Cu-Au, intrusive-relat- known mineral deposits and the location of suture zones and other major lithospheric-scale faults. ed orogenic Au, and possibly also Carlin-type Au and classic orogenic Au. Figure and captions taken from Griffin et al. 2013. This first ever orogen-scale 2D seismic array is going to roll in early 2014 (d). The first batch of results will be available a year from now. The three-year deployment will provide excellent opportunity for thesis projects for graduate students. Reference cited: Griffin, W.L., Begg, G.C., O'Reilly, S.Y., 2013. Continental-root control on the genesis of magmatic ore deposits. Nature Geosci 6, 905-910. 2. Layer-cake craton in the North American continent (d) A surface waveform tomography of the North American continent shows provocative craton scale layered lithosphere structure (Yuan and Romanowicz, Nature 2010), owning to the rich (c) data coverage provided by the USArray trans- (c) The Capricorn passive source project. The deployment is composed of 3 parts: 1) a 8-station back- portable array deployment, a 400-seismometer bone network (red squares) that will cover the whole orogen for 3-years to provide overall control for broadband deployment sweeping the continen- the path coverage; 2) a 26-station short-term deployment (red dimands) will cover the western side of the orogen for the first 1.5 years to magnify the local coverage; and 3) another 26-site short-term tal US in the past 10 years: Section 4 deployment in the eastern side, using pulling out the sites in part 2 and re-deploying in the east. Grey sites are Geoscience Australia’s permanent sites (gray boxes) and Australian National Universities earli- Section 3 er temporary deployment site, which are taken into consideration to maximize the area coverage. Inset (a) Craton lithosphere is seismically fast above shows the location of the Capricorn deployment. 250 km; References cited: Aitken, A.R.A., Joly, A., dentith, M.C., Johnson, S.P., Thorne, A.M., Tyler, I.M., 2013. 3D architecture, structural (d) The Capricorn passive source project field leaflet. Site permitting is currently undergoing with the evolution and mineral prospectivity of the Gascoyne Province. Geological Survey of Western Australia Report 123. Section 5 (b) Suture zones that welded Archean blocks are cooperation from the Geological Survey of Western Australia. The leaflet shows the passive source site Johnson, S.P., Thorne, A.M., Tyler, I.M., Korsch, R.J., Kennett, B.L.N., Cutten, H.N., Goodwin, J., Blay, O., Blewett, design adopted from the ANU’s earlier passive source deployment. (Photo courtesy of Professor Brian R.S., Joly, A., Dentith, M.C., Aitken, A.R.A., Holzschuh, J., Salmon, M., Reading, A., Heinson, G., Boren, G., Ross, J., characteristic of Vsv (vertically polarized shear Kennett from the ANU). Costelloe, R.D., Fomin, T., 2013. Crustal architecture of the Capricorn Orogen, Western Australia and associated wave) faster than Vsh (horizontally polarized metallogeny. Australian Journal of Earth Sciences 60, 681-705. shear wave) in the craton lithosphere (red dented into blue in b; see map for suture zone locations); 5. Long Beach has its own fault (c) A layer-cake like anisotropy structure is pres- Ambient noise can be pushed even further to reach a high resolution comparable to (a) ent (Layer 1, 2 and Asthenosphere). The asthe- active source seismic studies: tens of meters both vertically and horizontally. This exam- nosphere possess a direction of fast velocity ple (Lin et al. Geophysics 2013) shows how to use over 5000 geophones in a 8x6 km area symmetry axis that is parallel to the current ab- Layer 1 (f)(f) in Long Beach, California to image the Newport-Inglewood fault zone in the shal- solute plate motion (red in asthenosphere in c), Layer 2 Layer 1 low-most upper crust (above 1km depth). reflecting rock deformation in response to the asthenospheric flow. Layer 1 and 2 are two Asthenosphere Layer 2 (b) (c) lithospheric layers of frozen-in anisotropy, with layer 1 fast axis direction parallel to the suture strikes which are coincidentally parallel to the plate motion for the transect (red in layer 1 in (a) The array configuration and the regional fault lines in c). (e)(e) Southern California. The small circles show the 5204 stations used in this study. From Lin et al. 2013. Layer 1 Layer 1 is interpreted as the old “scums” in the (d)(d) Layer 2 References cited: early Earth history when cratonic blocks first Lin, F.-C., Li, D., Clayton, R.W., Hollis, D., 2013. High-resolution 3D Asthenosphere shallow crustal structure in Long Beach, California: Application of formed in the vigorously convecting mantle, ambient noise tomography on a dense seismic array. Geophysics 78, (b) Inverted 3D shear velocity model at 100-, 300-, and 650-m depths. Black lines show the Newport-Inglewood fault system on the surface.