Application of open-source software and high-resolution geophysical ...

Application of open-source software and highresolution geophysical images to explore the plate tectonic evolution of Australia 1School

Simon Williams1, Robert Musgrave2, R. Dietmar Müller1

of Geoscience, University of Sydney, NSW, Australia, [email protected] 2Geological Survey of New South Wales, NSW, Australia, [email protected]

INTRODUCTION The Australian continent, as with rest of the world, looked very different hundreds of millions of years ago compared to today. We describe a powerful method to explore spatio-temporal relationships within geological and geophysical data sets by analyzing the data within the context of tectonic reconstructions. GPlates is part of a new generation of plate reconstruction software that incorporates functionality familiar from GIS software with the added dimension of geological time. We use GPlates to reconstruct geological terranes, geophysical grids, and paleomagnetic data and so construct more robust models for the tectonic evolution of Australia. With the ability to rapidly visualize a diverse range of geological and geophysical constraints, users can easily investigate the implications of different tectonic models for reconciling a variety of observations, for example the mechanisms controlling the formation and present-day distribution of mineral deposits within Australia.

WHY WE NEED PLATE RECONSTRUCTION SOFTWARE Geoscientists have come to rely heavily on Geographic Information System (GIS) software to store and work with their data. They also realize that the rocks they study have often reached their present day locations as a result of complex tectonic histories – there are often important spatio-temporal relationships between samples which are now located far apart, but were once much more proximal to each other. To explore these issues, we need software tool that load geospatial data, whether in point, vector or raster in format (akin to standard GIS software), and then visualizes these data in their plate tectonic context based on available plate motion models and time-varying properties. We discuss the application of one such tool. The GPlates software [1,2], is an open-source, cross platform Geographic Information System (GIS). This software has been developed to include tools enabling plate models to be evaluated and revised, and to enable users to reconstruct data relevant to a broad range of subdisciplines within the Earth Sciences. GPlates is a virtual globe in the same manner as Google Earth, with the added dimension of geological time, thus enabling 4D visualizations to refine and improve the quality of tectonic reconstruction. Importantly, GPlates interoperates with a wide variety of data types, with the reconstruction of high-resolution geophysical images of particular importance in aiding the optimization and critical assessment of plate motion models.

APPLICATION 1: EASTERN AUSTRALIA DURING THE PALAEOZOIC The geology of Eastern Australia records a series of orogenic events along the margin of the supercontinent Gondwana. Musgrave and Cayley [3] used GPlates to develop a new model for the tectonic evolution of this area during the Silurian-Devonian. Magnetic anomaly maps [4] help to define the major Paleozoic-age geological provinces and the shear zones that disrupt them – reconstructions are constrained by bringing the anomalies that define these provinces into their original alignment.

Figure 1: Reconstruction of Eastern Australia in the Silurian (left), and revised APWP (right). Sydney, Australia

28 Oct- 1 Nov 2012 6th eResearch Australasia Conference

Previous Apparent Polar Wander Paths (APWPs) for Gondwana during the mid-Paleozoic have relied on paleomagnetic data from the tectonically active parts of Eastern Australia. By incorporating available paleomagnetic data into the GPlates reconstruction, Musgrave and Cayley (2011) revised this APWP taking these tectonic motions into account.

APPLICATION 2: PROTEROZOIC MOTIONS BETWEEN AUSTRALIAN CRATONS The older parts of Australia are generally described in terms of three 'cratons', the North, West and South Australian Cratons [5], from here on abbreviated to NAC, WAC and SAC respectively. Various authors have proposed large relative motions between these cratons during the Proterozoic (>542 Ma ago), drawing evidence from paleomagnetic data and mapped geology. For example, Li and Evans [6] recently proposed a 40˚ rotation of the NAC relative to SAC and WAC took place around 600 Ma, while Giles et al [7] suggested an alternative scenario where the available observations were best explained by a large rotation of the SAC relative to the NAC around 1300 Ma. Figure 2 shows the magnetic anomaly map for Australia [4], reconstructed using the different proposed reconstruction scenarios for Proterozoic Australia. The reconstructed configuration of Giles et al [7] bring into closer juxtaposition the distinctive anomaly patterns in the regions of the Mount Isa and Curnamona Provinces. The Li and Evans [6] model (Figure 2b) leaves Mount Isa and Curnamona widely separated. The magnetic anomalies also reveal the structural grain within the Gawler Craton and Arunta Inlier. Giles et al [7] argue that these two provinces formed a continuous orogenic belt along Australia’s southern margin during the late Paleoproterozoic. Figure 2c illustrates how the rotation of the South Australian Craton yields continuity in the grain of the magnetic anomalies between the Gawler (Ga) and Arunta (Ar) provinces.

Figure 2: Magnetic anomaly data for Australia (a) reconstructed within different models for the configuration of Australian cratons during the Proterozoic Modelling the Proterozoic tectonic evolution of Australia has important implications for the juxtaposition of the Mount Isa (MI) and Curnamona (Cu) regions, located on the NAC and SAC respectively. Today, these regions lie far apart, but a number of authors have emphasized the similarities in the age and geochemical compositions, suggesting that the rock units (and mineral deposits) within both regions formed at a time when the regions were much closer together. GPlates combines tools for the reconstructing high resolution imagery and vector data with data-mining functionality that allows us to explore spatio-temporal relationships between the present-day distribution of mineral deposits and the geodynamic processes through which they formed.

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Boyden, J.A., Müller, R.D., Gurnis, M., Torsvik, T.H., Clark, J.A., Turner, M., Ivey-Law, H., Watson, R.J. and Cannon, J.S., 2011, Next-generation plate-tectonic reconstructions using GPlates, in: Geoinformatics: Cyberinfrastructure for the Solid Earth Sciences, Keller G.R. and Baru, C., eds., Cambridge University Press, p. 95114. Gurnis, M., Turner, M., Zahirovic, S., DiCaprio, L., Spasojevich, S., Müller, R.D., Boyden, J., Seton, M., Manea, V.C. and Bower, D., 2011, Plate Reconstructions with Continuously Closing Plates, Computers and Geosciences, 2012. Musgrave, R. and Cayley, R., 2011. Unfolding an orocline restores Australian Siluro-Devonian paleomagnetic poles. AGU Annual Meeting, San Francisco, GP11A-0991. Milligan, P.R., Franklin, R., Minty, B.R.S., Richardson, L.M. and Percival, P.J., 2010, Magnetic Anomaly Map of Australia (Fifth Edition), 1:15 000 000 scale, Geoscience Australia, Canberra. Myers, J.S., Shaw, R.D., and Tyler, I.M., 1996, Tectonic evolution of Proterozoic Australia: Tectonics, v. 15, p14311446. Li, Z.X., and Evans, D.A.D., 2011, Late Neoproterozoic 40° intraplate rotation within Australia allows for a tighterfitting and longer-lasting Rodinia: Geology, v. 39, p39. Giles, D., Betts, P.G., and Lister, G.S., 2004, 1.8-1.5-Ga links between the North and South Australian cratons and the Early-Middle Proterozoic configuration of Australia: Tectonophysics, v. 380, p27-41.

Sydney, Australia

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ABOUT THE AUTHORS Simon Williams joined the School of Geosciences at the University of Sydney in January 2010. He obtained a PhD in geophysics from the University of Leeds, having completed a degree in geology at Liverpool University. From 2004 to 2009 he worked as a geophysicist at GETECH in the UK, a potential-field geophysics consultancy. Since arriving in Sydney, his research has concentrated on revising the way that plate deformation is described within global plate tectonic reconstructions. He was also chief scientist aboard a 2011 voyage of the CSIRO research vessel Southern Surveyor, which collected new magnetic profiles, swath bathymetry data and dredge samples in the Perth Abyssal Plain, eastern Indian Ocean. Robert Musgrave is Research Geophysicist at the Geological Survey of New South Wales. He obtained his PhD in Geophysics from the University of Sydney in 1987, after completing an undergraduate degree in Geology & Geophysics. After postdoctoral positions at Victoria University of Wellington and the Australian National University, and a Bureau of Mineral Resources Fellowship at the University of Tasmania, he joined the international Ocean Drilling Program as a Staff Scientist, based at Texas A&M University, and played a leading role in four research cruises. From 1993 to 2003 he was a Senior Lecturer at La Trobe University, Melbourne, where he established a paleomagnetic laboratory. His research has spanned the gamut of applications of paleomagnetism, including studies of tectonics, the remote detection of mineralization, climate-change related analysis of marine sediments, and magnetic records of the behavior of the Earth’s core. His current research concerns the integration of spatially distributed geophysical data, paleomagnetic records of the Earth’s magnetic field, geological observations, and plate tectonic reconstructions, to interpret the tectonic history and mineralization potential of New South Wales. Dietmar Müller is Professor of Geophysics at the University of Sydney, Australia. He obtained his PhD in Earth Science from the Scripps Institution of Oceanography in 1993 and his undergraduate degree at the University of Kiel, Germany. After joining the University of Sydney in 1993, he established the University of Sydney Institute for Marine Science and the EarthByte e-research group (www.earthbyte.org), pursuing collaborative development of open-source software and community digital data sets. One of the fundamental aims of his research is geodata and model synthesis through space and time, assimilating the wealth of disparate geological and geophysical data into a four-dimensional Earth model. His achievements have been acknowledged by winning the year 2000 Fresh Science Prize, awarded by the British Council and ‘ScienceNow!’, followed by the Carey Medal in 2004 for his contributions to the understanding of global tectonics. In 2009 he was awarded a 5-year Australian Laureate Fellowship to build a Virtual Geological Observatory.

Sydney, Australia

28 Oct- 1 Nov 2012 6th eResearch Australasia Conference