Tectonics squeeze South Australian ranges
29 August 2012
Research into the history of large earthquakes in parts of South Australia's Mount Lofty and Flinders Ranges has provided further evidence that the southern part of the continent is being squeezed in an east-west direction by tectonic forces.
Geoscience Australia geologist, Dr Dan Clark said the ranges are bordered by faults on the east and the west which are forcing ancient basement rocks over more recent hill-slope sediment.
Dr Clark said that these faults range from tens of kilometres to almost one hundred kilometres in length, and tend to be closely spaced when compared to similar faults in other parts of Australia.
"Successive large earthquakes during the past five million years have resulted in around 100 to 200 metres of displacement across these faults. The slip, or movement rates on individual faults range from 0.02 to 0.17 millimetres a year," Dr Clark said.
"There is abundant evidence also for large surface-breaking earthquakes on many faults within 100 kilometres of central Adelaide, although the tectonic slip rates on fault lines in this region are relatively low with major events likely every 10 000 years or more," he said.
"Taking into account the intermittent nature of faulting in Australia, it is likely that 30 to 50 per cent of the present-day elevation of the Flinders and Mount Lofty Ranges has occurred during the past five million years," he said.
"Beach deposits which formed along the southern margin of the Mount Lofty Ranges around 120 000 years ago are elevated up to 11 metres above their expected positions, suggesting that the deformation which is building the ranges is continuing," Dr Clark said.
"The east-west squeezing of the Mount Lofty and Flinders Ranges is largely the result of resistance to the northward movement of the Australian Plate by forces being exerted at the plate’s margins. The mountain ranges in Papua New Guinea and New Zealand and the Himalayas are also the result of these resistive forces," Dr Clark said.
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