STRAIN RATE IN A PALEOZOIC ACCRETIONARY OROGEN: THE WESTERN LACHLAN OROGEN, AUSTRALIA

Quantifying displacement rates and orogenic strain rates is more challenging than determining the timing of geological events in ancient orogens. It is possible to determine time-averaged strain and displacement rates when the timing of mica growth in cleavage and veins, along with finite strain estimates can be quantified. Deformation of a turbidite dominated, accretionary thrust wedge in the western Lachlan Orogen occurred by chevron folding and faulting over an eastward propagating decollement. Based on 40Ar/39Ar dates of white micas, 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 the 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 440-439 Ma. The total average strain for this thrust sheet is -0.67, based on retrodeformation and strain removal. This amount of strain over a duration of ~16 Ma gives a strain rate of 1.3 x 10-15 s-1. The total shortening on this thrust sheet is at least 310 km, which gives a decollement propagation rate of ~20 mm a-1. A similar strain rate is calculated from the thrust sheets in the Melbourne structural zone. 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 like Taiwan.