CONVERGENCE AND INTRAPLATE DEFORMATION IN THE LACHLAN FOLD BELT OF SOUTHEASTERN AUSTRALIA

The Lachlan Fold Belt of southeastern Australia is dominated by a widespread oceanic association including Cambrian submarine mafic volcanics and an overlying widespread Ordovician quartz-rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres. These rocks are overlain by, or in fault contact with, deep-marine to continental Silurian to Early Carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks. Major deformation, achieved by thrusting and folding, affected the fold belt during the Silurian to Middle Devonian and the Early Carboniferous. Shortening estimates vary throughout the belt but the widespread Ordovician quartz turbidite succession usually has at least 60% shortening which implies an original width of the fold belt of about 1700 km. Strike-slip faulting was important on a local to regional scale, but large-scale strike-slip displacements appear unlikely. The large values of shortening are consistent with development of the belt in a convergent margin setting at least for the Silurian to Carboniferous. The nature of the lower crust of the Lachlan Fold Belt has always presented a problem of interpretation but we favour a Late Proterozoic quasi-continental lower crustal layer, that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during Silurian-Carboniferous convergence, and provided a source for Silurian-Devonian granites.