Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Abstract

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison with backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.

KEY WORDS:

• contractional deformation
• convergence
• Devonian
• extension
• Lachlan Fold Belt
• plate tectonics
• Silurian

Acknowledgements

I have benefited over the years from many discussions on the geology of aspects of eastern Australia with many colleagues. My thanks go to members of the Geological Survey of New South Wales including Gary Burton, Gary Colquhoun, Ian Percival, Lawrie Sherwin and Carol Simpson. Discussions with colleagues at the University of Wollongong including Sol Buckman, Paul Carr, Bruce Chappell, Brian Jones, Penny Williamson and Tony Wright are gratefully acknowledged. I have also had a recent useful collaboration with Ian Graham. I thank Evan Leitch, Bill Collins and, in particular, Bob Henderson, for discussions and inspiration over many years. Evan Leitch made many suggestions that improved the manuscript. Reviews by Peter Cawood and particularly Fons VandenBerg resulted in substantial revision of the initially submitted manuscript. Support has been provided by the University of Wollongong (GeoQuEST research centre).