Chronology of deformation within the turbidite-dominated, Lachlan orogen: Implications for the tectonic evolution of eastern Australia and Gondwana

Ar-Ar data from fabric-forming white mica in slates, syntectonic quartz vei ns and granitic mylonites constrain the timing of metamorphism, deformation , and exhumation in the Lachlan orogen, Australia. These data also help def ine the tectonic evolution of the Tasmanides during Paleozoic time. The Lac hlan orogen formed by the progressive accretion of a thick turbidite fan se quence and volcanic terrains to Gondwana during the closing of a small marg inal ocean or back are basin. This tectonic setting was similar to the pres ent western and southwestern Pacific region. Accretion of the Lachlan oroge n to Gondwana occurred by closing of the basin system by subduction-accreti on processes and some translation. The process is typified in the western L achlan orogen by a major eastward migrating deformation involving chevron f olding and faulting over an eastward propagating decollement/melange zone a nd is recorded by Ar-Ar mica ages ranging from similar to 455 Ma in the wes tern part to similar to 390 Ma in the eastern part. In the central Lachlan orogen, deformation migrated southwestward from similar to 440-430 to 405 M a away from the high-temperature Wagga-Omeo metamorphic complex, where defo rmation/metamorphism occurred between >440 and 400 Ma. In the north, simila r to 400 Ma mica ages record deformation and inversion of structures in the Cobar basin. In the eastern Lachlan orogen, Ar-Ar mica dates range from 45 0 to 340 Ma. Ages of 455-445 Ma are yielded by the Narooma accretionary com plex, 405-390 Ma ages are found along the major thrust faults bounding high -grade metamorphic complexes, and 360-340 Ma cooling ages are found in the inverted extensional basins (e.g., Hill End) and related structural zones. The Ar-Ar results also document periods of reactivation on early-formed str uctures during later deformation elsewhere in the orogen.