Formation of the New England oroclines: insights from Permian sedimentary basins in Eastern Australia

Abstract

The origin of the late Paleozoic New England oroclines in eastern Australia is an open question that is here investigated using structural geology, geochronology and paleomagnetism. The study is focussed on Permian sedimentary and volcanic rocks, which provide new constraints on the timing, kinematics, paleogeography and tectonic setting during the formation of the New England oroclines. Ultimately, the results provide insights into the geodynamic mechanisms associated with oroclinal bending. Paleomagnetic investigation of Early-Middle Permian (~272 Ma) volcanic rocks was conducted in order to constrain the timing of oroclinal bending and to determine the relative role that Early Permian extension and Middle Permian (Hunter-Bowen) contraction played during oroclinal bending. The targeted volcanic formation is found in a structural basin that is situated in the southernmost part of the oroclinal structure (Myall Block), close to the hinge of the Manning Orocline. A positive paleomagnetic fold test, in conjunction with indications for low-grade prehnite-pumpellyite metamorphism (T<300°C), suggest that the magnetisation of these rocks is primary. The measured magnetisation was calculated to yield a ~272 Ma paleopole for the Myall Block, which is found to be consistent with coeval data from cratonic Gondwana. This indicates that the Myall Block was not subjected to major rotations relative to Gondwana after 272 Ma, meaning that the orocline must have formed prior to the initiation of Middle Permian contractional deformation. Rather oroclinal bending was likely contemporaneous with the wholesale extension that affected eastern Australia during the Early Permian. Structural mapping in an Early Permian succession (Nambucca Block) shows that rocks were subjected to four phases of deformation. Previously published and recently obtained time constraints for the second generation of structural fabrics (S2), associate the first generation (S1) and possibly the second one (S2) with the formation of the New England oroclines and therefore provide kinematic constraints for the New England oroclines. The first stage (<300 Ma) was intimately linked to backarc extension, which was likely triggered by variations in the rates of trench rollback. This was followed by a second stage of oroclinal bending, which involved ~N-S contraction (F1) and by subsequent deformation at 275–265 Ma, which involved formation of nappe-style structures (F2) and tightening of the fully developed orogenic curvatures. The formation of these F2 nappe-style structures possibly mark the onset of the Hunter-Bowen contractional event. In order to constrain the timing of deposition in the Nambucca Block, and to unravel the provenance and tectonic setting of the sedimentary basin, 452 concordant U-Pb ages were obtained, coupled with a morphological analyses of detrital zircons. The youngest populations of zircon ages are 299 Ma (n=22) and 285 Ma (n=7), and a large fraction of pre-Devonian morphologically matured zircons indicates that detritus were sourced from cratonic Gondwana. The results suggest a backarc setting during deposition, associated with a bimodal input of detritus, which received sediments from both the continent and the arc. A similar investigation was conducted on detrital zircons from an adjacent sedimentary succession (Dyamberin Block). The results of 754 concordant ages and corresponding morphological analyses of detrital zircons were used to assess the relationship of the Dyamberin Block with the Nambucca Block and other Early Permian successions in the New England oroclinal structure. Average values of abrasion were used as a proxy for distance of transportation, thus highlighting possible sources of detritus, and a comparison of data from different samples was done by separately plotting cumulative proportion curves for periods that correspond to plausible source regions. It is shown that the usage of this innovative data handling approach, when complemented by a statistical analysis of zircon morphologies, provides insights into the paleogeography of sedimentary basins. The results show that in eastern Australia during the Early Permian, a regional fluvial system transported detritus from continental Gondwana across the landscape of the simultaneously developing Sydney-Gunnedah-Bowen Basin System and the former (Devonian-Carboniferous) magmatic arc. In addition, a local transportation system mobilised detritus within the New England Orogen. The source of Early Permian zircons was likely associated with an oceanward positioned magmatic arc. These results support the idea that the New England Orogen was positioned in the backarc region during the Early Permian, thus receiving sediments from both the continent and an oceanward positioned Permian magmatic arc. The evidence for the development of Early Permian backarc basins contemporaneously with oroclinal bending, and the fact that rocks that originally formed in a forearc setting (during the Carboniferous) were subsequently moved to a backarc position (in the Early Permian), suggest that subduction rollback was the primary mechanism responsible for the formation of the New England oroclines. The demonstrated link between oroclinal bending, backarc extension and subduction rollback, may have operated similarly to form other orogenic structures in ancient and modern orogens throughout the world.