STRAIN RATE IN A TURBIDITE FOLD-THRUST BELT IN THE SOUTHWESTERN LACHLAN OROGEN, AUSTRALIA

Deformation of a turbidite dominated, accretionary-style thrust wedge in the southwestern Lachlan Orogen, Australia occurred by chevron folding and faulting over an eastward propagating decollement. Based on 40Ar/39Ar dates of white micas that grew below the isotopic 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. 40Ar/39Ar data from the thrust sheets in the western part of this fold-thrust belt show that deformation progressed from early buckle folding, which started at 457-455 Ma through to chevron fold lock-up and thrusting at 440-439 Ma. The total average strain for this thrust system is -0.67, based on retrodeformation and strain removal. 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 5.5 x 10-15 s-1, based on the calculated thickness of the sediment. The total shortening for western thrust sheets is ~310 km (minimum) and ~800 km (maximum), which gives a décollement propagation rate between ~19 mm a-1 (minimum) and ~55 mm a-1 (maximum). A similar, but less well defined, strain rate is calculated from the thrust sheets in eastern part of the fold-thrust belt. These strain rates are on the low end for plate tectonic rates but similar to convergence rates in western Pacific back arc basins, accretionary prisms, and in turbidite-dominated thrust systems.