MIDDLE PALEOZOIC THRUSTING IN THE EASTERN LACHLAN FOLD BELT, SOUTHEASTERN AUSTRALIA

Early to Middle Palaeozoic clastic and volcanic strata in the Goulburn-Bungonia region, 150 km southwest of Sydney, are divided by the N-trending Yarralaw Fault into two domains. In the western domain major E-W shortening formed folds, axial-planar cleavage and W-dipping contraction faults in all pre-Upper Devonian units. This deformation formed an anticlinorium cored by a W-dipping thrust system with an overturned E-younging limb. In the eastern domain upright flattened chevron folds occur in Ordovician strata with stratal repetition along steep contraction faults spaced at intervals of 1 km or less. Zones of tectonic mélange occur in the hangingwalls of these faults and have a well-developed scaly fabric, with a steeply plunging striation, overprinting a bedding-parallel cleavage in mudstone. In interbedded Silurian shale and limestone a classical stair-step trajectory is recognized for the Frome Hill Fault which duplicates the succession and has undergone back-rotation to its present steep dip. Overall, thrusting progressed from east to west with deformation in the eastern domain pre-dating intrusion of the Early Devonian Marulan Batholith, whereas farther west deformation continued into the Middle Devonian. The folds and thrusts in the Goulburn-Bungonia region are part of a major fold-thrust zone that extends throughout the eastern Lachlan Fold Belt, and which was formed in the Middle Silurian to Middle Devonian during a period of plate convergence between Gondwana and the palaeo-Pacific Ocean. The fold-thrust zone is inferred to have an arc-frontal arc setting above a W-dipping subduction zone, and the deformation relates to underthrusting of an allochthonous terrane with major shortening in the hangingwall block. © 1990.