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Average orogenic strain rates may be calculated when it is possible to date mica cleavage or syndeformational veins and estimate finite strain. Deformation of accretionary-style thrust sheets in the western Lachlan Orogen occurred by chevron folding and faulting over an eastward propagating décollement. Based on 40Ar/39Ar dates of white micas, which grew below the closure temperature, this deformation started ca. 457 Ma in the west and ended ca. 378 Ma in the east, with apparent “pulses” of deformation ca. 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 m.y. gives a minimum strain rate of 1.3 × 10−15 s−1 and a maximum strain rate of 5.0 × 10−15 s−1, based on fan thickness considerations. The total shortening is between ∼310 km and ∼800 km, which gives a décollement displacement rate between ∼19 mm yr−1 (minimum) and ∼50 mm yr−1 (maximum). If deformation occurred in pulses ca. 457–455 and ca. 441–439 Ma, then the calculated strain rate would be on the order of 1 × 10−14 s−1. These strain rates are similar to convergence rates in western Pacific backarc basins and shortening rates in accretionary prisms and turbidite-dominated thrust systems as in Taiwan.

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