Thrust, extensional and strike-slip tectonics in an evolving Palaeozoic orogen—a structural synthesis of the Lachlan Orogen of southeastern Australia

Abstract

The Lachlan Orogen preserves the best and most accessible evidence of the Palaeozoic development of the eastern margin of Gondwanaland. Its history is marked by a raising of freeboard from generally deep marine (Cambrian to Early Silurian), through mixed deep and shallow marine, volcanics and granite-emplacement (mid-Silurian to mid-Devonian), to fluviatile in the mid-Devonian to Early Carboniferous. A long deformation history from latest Ordovician to mid-Carboniferous is marked by formation of: (1) (near-)meridional structures—folds, cleavage and faults—in response to latitudinal shortening, and (2) (near-)latitudinal structures—folds, cleavage and faults—together with strike-slip movement on (near-)meridional faults in response to N-S and NE-SW shortening. Formation of these two sets of structures reflects a long history of oblique compression within the developing orogen. Partitioning of these deformation effects across the Lachlan Orogen leads to recognition of four meridional structural belts, each characterised by different structural histories. The common feature linking all four belts is the thin-skinned nature of deformation: steep surface contractional faults flatten with depth, to become parts of linked thrust systems soling into detachments lying at different levels in the upper and middle crust. The Lachlan Orogen is thus characterised by thin-skinned tectonics. The Southwestern Belt is allochthonous and preserves an Early to Middle Devonian east-vergent fold/thrust system which soles into the base of the Ordovician and also into the base of underlying Cambrian greenstones. Together with later strike-slip movement on major faults, this deformation records transpressional insertion of a terrane from the southeast. The Central Belt is characterised by N-S shortening, latitudinal folds and thrusts together with strike-slip movement on NNW-trending faults, and en echelon granites. Southward translation as a separate terrane occurred in the Early Silurian and mid-Devonian. Deep thrusting in the southern part of the belt has exhumed a metamorphic layer from depths of ⩾ 10 km. The Eastern Belt is a thin-skinned (near-)meridional thrust belt which has also experienced some strike-slip movement and N-S shortening. Thrusts detach in several major horizons: in the Late Ordovician/earliest Silurian (at a depth of ca. 4 km), within or at the base of the Ordovician, which is internally thickened with inferred duplexing and imbrication, and within a mid-crustal metamorphic layer at ca. ⩾ 10 km. Vergence reversals both parallel and perpendicular to the orogen indicate the presence of tear faults and tectonic wedges. Extensional faults and orogen-parallel extension are also features of the Lachlan Orogen, with the former able to account for the some of the “compressional” Silurian “orogenies” in the Eastern Belt. At deeper crustal levels, upper and mid-crustal shortening was balanced by over/underthrusting within and along boundaries of Precambrian continental basement terranes.