Palaeozoic arc growth, deformation and migration across the Lachlan Fold Belt, southeastern Australia

Abstract

The Palaeozoic Lachlan Fold Belt (LFB) is a low-pressure metamorphic belt characterized by greenschist-facies rocks and infolded volcanic sequences, which indicate that the belt has not undergone substantial uplift at any stage. Porphyroblast/ matrix relationships in one of the highest-grade metamorphic zones of the LFB, the Omeo Complex, indicate that the thermal peak was reached before, or was synchronous with, the earliest deformation, similar to that in high-grade metamorphic zones of the northern Arunta Inlier, central Australia. In both areas, the metamorphism is of low- P, high- T type, centred on major granite intrusions as regional aureoles, and characterized by anti-clockwise P- T- t-paths. However, in the outer (lower-grade) parts of the aureoles, peak metamorphic conditions post-date the earliest deformation. This diachronous relationship indicates that (1) outward migration of a thermal front was centred on major granitoid intrusions (batholiths), (2) deformation propagated outward more rapidly than migration of the thermal front, and (3) plastic and ductile deformation began only after heating around the batholiths. Thus, orogeny was initiated after, and localized by, heat-focusing in the mid-crust associated with batholith emplacement. Therefore, the early deformation is a result of thermal softening: it is typically subhorizontal ductile shear at mid-crustal levels, but is characterized by upright to inclined folds at upper crustal levels. In the latter environment, the typically discordant “contact-aureole” plutons are often interpreted as post-tectonic granitoids, but they were part of the ongoing mid-crustal thermal perturbation that induced regional greenschist-facies metamorphism, which overprinted the early-formed, upright folds as it migrated to upper crustal levels. Orogeny migrated generally eastward through the LFB. It began with the Early Silurian Benambran Orogeny, centred on the Wagga Metamorphic Belt (WMB), and terminated with the Carboniferous Kanimblan Orogeny, which was most intense in the Hill End Trough. Eastward migration of orogeny across central Victoria, part of the West LFB, began in the Ballarat-Bendigo Zone during the Early Devonian and terminated in the Melbourne Zone in the Middle Devonian. Orogeny was centred on the meridional batholiths of the LFB, each of which are considered to represent the transitory axis of an ancient magmatic arc. Stepwise, but generally eastward, arc migration caused the eastward migration of orogeny. A tectonic model, based on the modern southwest Pacific arc system, can be applied to the LFB. Following Cambrian intra-oceanic arc growth associated with west-dipping subduction, an Ordovician marginal sea developed, partly on stretched Cambrian crust, and was flooded by an extensive turbiditic wedge. Closure of the eastern part, the East LFB, began in the Early Silurian after a magmatic arc developed over the WMB, possibly associated with an east-dipping subduction zone. After re-establishment of west-dipping subduction in the Middle Silurian, absolute-motion retreat of the upper (Australian) plate, caused by oblique plate convergence, resulted in dextral transtension and the generation of Siluro-Devonian inter-arc and back-arc basins. Transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard (eastward) arc system. Periodic compression within the evolving arc and back-arc system resulted in a series of east-younging, east-verging, linear, Siluro-Devonian fold-and-thrust belts, which were localized adjacent to elongate batholiths that represent the relict arc systems. Local westward jumps in deformation and plutonism occurred. The most significant is the $ ̃ 600 km westward jump into the West LFB, caused by closure of the remainder of the passive Ordovician back-arc basin (Melbourne and Bendigo-Ballarat zones). West-dipping subduction beneath the basin produced a low-pressure fold-thrust belt similar to the East LFB, though the granitoids have a more primitive isotopic character, indicating closure of thin, dominantly oceanic crust in the Early to Middle Devonian. Localized deformation, induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins, is likely to produce anticlockwise P-7- t-paths and may be a typical feature of southwest Pacific-style tectonism.