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Australian Geodynamics Cooperative Research Centre, Department of Earth Sciences, Monash University, Melbourne, Vic. 3168, Australia
The Lachlan Fold Belt of southeastern Australia is a 700 km wide belt of deformed, Palaeozoic deep and shallow marine sedimentary rocks, cherts, and mafic volcanic rocks. Characterized by large areas of chevron-folded turbidite sequences, linked contractional and strike-slip faults, and superposed thrust-belts of different age and vergence, it has large volumes of granite and low pressure/high temperature metamorphic rocks. Surface structural elements suggest that it formed by massive telescoping and strike-slip translation within a continental margin sediment prism along the former margin of Gondwanaland during the mid-Palaeozoic. Migrating, sporadic compressional and extensional events over the 100 Ma deformational history of the belt produced a crustally thickened, high geothermal gradient orogen, involving non-craton directed thrusting in an inferred convergent margin setting. Compressional periods were marked by eastward-migrating deformation zones involving detachment-related folding and reverse faulting in the largely turbidite dominated sequences. High-grade metamorphic rocks are not part of a metamorphic hinterland, but are largely confined to a shear zone-bounded crustal wedge in the central part of the fold-belt. Extension was marked by localized development of rift basins and half grabens accompanied by extensive granitic magmatism and silicic volcanism. Subsequent basin collapse was by reactivation/inversion of the extensional faults and folding of the basinal sequence. Magmatic activity and high temperature/low pressure metamorphism are linked to a convergent margin setting over much of the Middle Palaeozoic, but with major convergence in the Silurian.
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