David A. Foster

Strain rate in Paleozoic thrust sheets, the western Lachlan Orogen, Australia : strain analysis and fabric geochronology

David Foster and David Gray

It is possible to calculate average orogenic strain rates when mica cleavage or syndeformational veins can be dated, and finite strain can be estimated. Deformation of accretionary-style thrust sheets in the western Lachlan Orogen occurred by chevron folding and faulting over an eastward propagating decollement. Based on 40Ar/39Ar dates of white micas, that grew below the closure temperature, this deformation started at ~457 Ma in the west and ended at ~378 Ma in the east, with apparent “pulses” of deformation at about 440, 420 and 388 Ma.

The 40Ar/39Ar data from thrust sheets in the Bendigo structural zone show that deformation progressed from early buckle folding, which started at 457-455 Ma through to chevron fold lock-up and thrusting at 441-439 Ma. Based on retrodeformation, the total average strain for this thrust sheet is -0.67, such that the bulk shortening across the thrust sheet is 67%. This amount of strain accumulated over a duration of ~16 Ma gives a minimum strain rate of 1.3 x 10-15 s-1 and a maximum strain rate of 5.0 x10-15 s-1, based on fan thickness considerations. The total shortening on this thrust sheets is between ~310 km and ~800 km, which gives a decollement displacement rate between ~19 mm a-1 (minimum) and ~50 mm a-1 (maximum). If deformation occurred in pulses ~457-455 and ~441-439 Ma then the calculated strain rate would be on the order of 1 x 10-14 s-1. These strain rates are similar to convergence rates in western Pacific back arc basins, and shortening rates in accretionary prisms, and turbidite-dominated thrust systems like Taiwan.

Transformation from Oceanic Back-Arc Basin to Continental Crust: The Turbidite-Dominated Lachlan Orogen

The Tasmanides of eastern Australia preserve one of the most complete records of Paleozoic accretion because they are not overprinted by a terminal continent-continent collision like many other classic accretionary orogens (e.g., the Pan African in NE Africa or Paleozoic central Asia), reworked by younger active tectonics (e.g., the Andean margin), or largely covered by ice (e.g., parts of the Antarctic margin). In this project we are investigating the setting and origin of a Cambrian volcanic arc exposed in the southern Lachlan Orogen aimed at understanding the processes involved in transforming a large oceanic back-arc basin, filled with continent-derived turbidite into stable continental crust.The Tasmanides of eastern Australia preserve one of the most complete records of Paleozoic accretion because they are not overprinted by a terminal continent-continent collision like many other classic accretionary orogens (e.g., the Pan African in NE Africa or Paleozoic central Asia), reworked by younger active tectonics (e.g., the Andean margin), or largely covered by ice (e.g., parts of the Antarctic margin). In this project we are investigating the setting and origin of a Cambrian volcanic arc exposed in the southern Lachlan Orogen aimed at understanding the processes involved in transforming a large oceanic back-arc basin, filled with continent-derived turbidite into stable continental crust.

The process of stepwise structural thickening combined with accretion of sediment-filled back-arc-basins and island arcs, followed by thermal and chemical maturation by magmatism, converts material in the oceanic realm into continental crust, and this can be accomplished without the incorporation or intercalation of previous continental crust. To test this general hypothesis for continental growth and recycling in accretionary orogens we are assessing whether the lower crust of the Lachlan Orogen is entirely oceanic or if any significant Precambrian continental fragments were involved in the amalgamation of the Lachlan Orogen. We are analyzing trace element geochemical, and isotopic (Sr, Nd, Pb, Hf) compositions of Cambrian volcanic arc rocks, which were erupted within the proto-Lachlan back-arc basin above either (1) Cambrian or latest Neoproterozoic suprasubduction zone oceanic crust or (2) on a large fragment of Precambrian continental lower crust.