New Perspectives Workshop, September 2014
Presented by AIG Victoria and AusIMM Central Victoria Branch, Romsey Victoria
School of Earth, Amosphere, and Environment, Monash University, Clayton, Victoria 3800, Australia
School of Earth Sciences, University of Melbourne, Parkville, Victoria 3010, Australia.
Geological Survey of Victoria, Melbourne, Victoria 3001, Australia.
Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA.
Congested subduction zones develop when they attempt to consume buoyant, exotic crust. In this presentation we show several 3D geodynamic models of buoyant continental and oceanic crust collision to illustrate the behaviour of congested convergent margins during accretionary orogenesis. Several common phenomena are observed in the models. These include: (1) A period of crustal shortening and orogenesis in front of the accreted exotic terrane; (2) the lateral escape of a the subduction zone around the tip of the accreted terrane as the trench retreats during roll back; (3) the development of an highly arcuate orocline behind the accreted terrane as the trench migrates parallel to the plate margin; (4) syn-orogenic extension in the overriding plate away from the zone of accretion, which may be accompanied by basin formation in the overriding plate; (4) escape of the back arc region behind the accreted terrane; (5) lateral translation of crust in the overriding plate and orocline development behind the accreted terrane may be enhanced by toroidal mantle flow through a slab window created beneath the accreted terrane; and (6) re-establishment of the subduction zone well behind the accreted terrane, which effectively embeds the accreted terrane within the overriding plate.
These results of these geodynamic models have a remarkable similarity to the ‘Cayley’ model for the evolution of the southern Tasmanides. In this model the accretion of the Van Dieland micro- continent triggered a period of plate margin instability that resulted in coincident shortening and extension along the Late Ordovician to Early Silurian Gondwanan plate margin and the tectonic escape of the back-arc region around the northern tip of Van Dieland, resulting in the development of regional orocline behind the micro-continent. During tectonic escape, the Macquarie arc formed was “sucked” into the core of the orocline, which explains the present-day position. The Newell and Darling Basin systems formed in response to tectonic escape. Re-establishment of subduction outboard of the Gondwanan margin occurred during the Devonian. The back arc region to this subduction zone represents a complex mosaic of arc and back arc magmatic rocks that have been superimposed upon Silurian basins and Ordovician back arc sedimentary rocks and accretionary rocks.
The dynamic models illustrate how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. Importantly, the model shows that complex geological associations such as oroclines, embedded terranes, and complex subduction relationships do not require complex plate margin configurations and can be explained by a simple single subduction system rather than complex tectonic scenarios involving multiple subduction zones as commonly depicted for the evolution of the Tasmanides. Our models may also be relevant for more complex orocline architectures such as that preserved in the New England Orogen in the eastern Tasmanides.
Peter Betts is an academic from Monash University. His is an Associate Professor in structural geophysics in the School of Earth, Atmosphere, and Environment and is the Associate Dean Research Training in the Faculty of Science. Peter is also a Director of a consulting company, PGN Geosciences, which specializes is structural geology, geophysical modelling and interpretation, and 3D modelling.
Pete has been a geologist/geophysicist for more than 20 years and has a diversity of research and industry experience. He research interests include geophysical analysis of Proterozoic basin systems, Proterozoic tectonics and plate reconstruction, and geodynamic modelling and geophysical interpretation of modern tectonic settings. He is currently undertaking research in the Red Sea, geodynamical modelling of accretion at convergent plate margins and constructing a 3D model of the Australian continent.
Pete’s industry engagement has seen him undertake work in Australia, South America, North America, West Africa and northern Europe, targeting a variety of commodities including Au, Cu, Pb, Zn, U, and Ni.