The tectonic history of Adelaide’s scarp-forming faults

Abstract

A series of linear to arcuate fault scarps separate the Mount Lofty Ranges from the Cenozoic St Vincent and Murray basins of South Australia. Their tectonic, sedimentary and geomorphic evolution is traced from the oldest rock record through to present-day seismicity. The scarps are the latest manifestation of repeated compressive reactivation of ancient, deep-seated crustal faults and fractures whenever the stress field was of appropriate orientation. Formation of the basins and uplift of the ranges resulted from the same processes of repeated compressive reactivation. Continental crust was intensely fractured during three episodes of Neoproterozoic–Cambrian rifting that led to the formation of the Adelaide Geosyncline and break-up of Rodinia. Neoproterozoic eastward-dipping, listric extensional faults provided accommodation space for deposition of the Burra Group. Sediments of the Umberatana and Wilpena groups were deposited under mainly sag-phase conditions. In the early Cambrian, new extensional faults formed the deeply subsident Kanmantoo Trough. Cambrian rift faults swung from east–west on Kangaroo Island through northeasterly on Fleurieu Peninsula to north–south in the easten Mount Lofty Ranges, cutting across the older meridional rifts. These two sets of extensional faults were reactivated as basement-rooted thrusts in the ensuing Delamerian Orogeny. The Willunga Fault originated as a Cambrian rift fault and was reactivated in the Delamerian Orogeny as a thrust dipping southeast under a regional basement-cored antiform on southern Fleurieu Peninsula. Much of southern Australia, including the eroded remnants of the Delamerian highlands, was covered by a continental ice sheet in the Carboniferous–Permian. The preferential preservation of glacial sediments on Fleurieu Peninsula may have resulted from extensional reactivation of the Willunga Fault, possibly in the early Mesozoic. Fleurieu Peninsula was then warped into an open, southwest-plunging antiform, spatially coincident with the much higher amplitude Delamerian antiform. Glacial sediments were eroded from uplifted (up-plunge) areas before formation of a ‘summit surface’ across deeply weathered bedrock and preserved glacial sediments in the later Mesozoic. This surface was covered with fluvial to lacustrine sediments in the middle Eocene. Neotectonic movements under a renewed compressive regime commenced with reactivation of the Willunga Fault, restricting subsequent Eocene to Miocene sedimentation to the St Vincent Basin. The Willunga scarp was onlapped in the Oligocene–Miocene concomitant with continuing uplift and formation of a hanging-wall antiform. In the late Cenozoic, repeated faulting and mild folding, angular unconformities, ferruginisation and proximal coarse sedimentation took place on various faults at different times until the late Pleistocene.

Keywords:

• neotectonics
• Delamerian Orogeny
• St Vincent Basin
• Mount Lofty Ranges
• fault reactivation
• hanging-wall antiforms
• Carboniferous-Permian glaciation
• Neoproterozoic-Cambrian rifting
• summit surfaces

Acknowledgements

This paper draws on the knowledge of many geoscientists with whom the author has had the privilege of interacting. The author thanks especially the following researchers for productive discussions relevant to the geoscience disciplines it touches upon: Neville Alley, Tony Belperio, Bob Bourman, Andrew Burtt, Dan Clark, Ron Coats, Alan Collins, Colin Conor, Barry Cooper, Wayne Cowley, Grant Cox, Brian Daily, Bob Dalgarno, John Drexel, Bill Fairburn, Mark Fanning, Jon Firman, Peter Fleming, Thomas Flöttmann, John Foden, Bryan Forbes, Jim Gehling, Victor Gostin, Martin Hand, Wayne Harris, Steve Hill, Richard Hillis, Matt Hutchens, Jim Jago, Liz Jagodzinski, Pat James, Richard Jenkins, Nick Lemon, Murray Lindsay, David Love, Neil Mancktelow, Brian McGowran, Tony Milnes, Robin Offler, Jeff Olliver, Mark Quigley, Anthony Reid, Peter Reid, Stuart Robertson, Roye Rutland, Mike Sandiford, Malcolm Sheard, Reg Sprigg, Bill Stuart, Mike Szpunar, Victor Tokarev, Rowl Twidale, Malcolm Wallace, Malcolm Walter, George Williams, Martin Williams and Frances Williams. I am grateful to Brian McGowran and Steve Hill for their helpful comments on an early draft of the manuscript. Dan Clark and Simon Holford are thanked for their thoughtful reviews and suggestions. The paper is published with the permission of the Director of the Geological Survey of South Australia.

Disclosure statement

No potential conflict of interest was reported by the author.