The Hunter - Bowen Orogeny in eastern Australia

Abstract

The last phases of orogenesis that affected eastern Australian during the Permian and Triassic are collectively referred to as the HunternBowen Orogeny. This deformation was the expression of subduction-related processes along the entire southern margin of Gondwana (Gondwanide Orogeny). It involved folding and thrusting in the New England Orogen, and the emplacement of voluminous subduction-related magmatism along the continental margin. Importantly, the HunternBowen Orogeny was characterised by distinct episodes of deformation and magmatism, intermitted by periods of tectonic relaxation. The episodic character of the HunternBowen Orogeny, and the geodynamic mechanisms that controlled this behaviour, are still poorly understood and are the focus of this thesis. The thesis aims at providing new constraints on the style and timing of HunternBowen deformation, integrating a wealth of available data into a regional spatio-temporal synthesis, and ultimately, providing new insights into the fundamental processes that controlled late Paleozoic and early Mesozoic deformation in eastern Australia and elsewhere along the Gondwanan margin.The onset of HunternBowen Orogeny is addressed here by investigating the stratigraphy, sedimentology, and geochronology of detrital zircon from early syn-orogenic strata associated with the initiation of foreland loading at the margin of the Bowen Basin and proximal orogenic hinterland. The Middle Permian strata display evidence of rapid base-level fall, coeval with increasingly abundant and more proximal mass-wasting deposits which reflect a change in subsidence and a shift from tectonic quiescence to syn-tectonic deposition ahead of the approaching orogenic front to the east. Detrital zircon provenance records an increasing contribution of detritus from exhumed and uplifted basement rocks along with abundant syn-depositional magmatism from 275n260 Ma. These results confirm previous suggestions that tectonic forcing of the orogenic hinterland affected subsidence and sedimentation patterns in the easternmost part of the basin during the Capitanian, recording the earliest stages of foreland loading 5n9 Myr before major effects were felt in the western part of the foreland basin. nAn insight into the second pulse of HunternBowen deformation is obtained from a study of the South Island Shear Zone. The kinematics and magmatic evolution of this deformational zone were studied by structural geology, whole-rock geochemistry, and geochronology (40Ar/39Ar and UnPb). The results show that this segment of the New England Orogen was subjected to deformation and magmatism at 258n248 Ma. During this period, the South Island Shear Zone operated as one part of a major out-of-sequence thrust system in the most outboard part of the central New England Orogen. The results demonstrate the heterogeneous nature of finite strain and strain rates both along and across the orogen, thus enabling a broader discussion on the role of strain partitioning during the HunternBowen Orogeny and its relationship to specific tectonic processes that may have affected the eastern Gondwana margin during the Permian and Triassic.The third and final pulse of HunternBowen deformation is examined in the Gympie Terrane, whose accretion has been previously suggested to play a major role in driving orogenesis. The structural style, kinematics, and timing of deformation in the Gympie Terrane are elucidated through detailed field mapping and structural analysis. The new data reveal that the earliest major deformation in the Gympie Terrane occurred at the final stages of the HunternBowen Orogeny (235n230 Ma). The results do not support the idea that HunternBowen Orogeny was driven by an arc-continent collision represented by the Gympie Terrane. The results are integrated in a larger-scale tectonic synthesis that reveal continental-scale spatial and temporal variations in the timing and intensity of the three major episodes of HunternBowen Orogeny.One major conclusion of this thesis is that HunternBowen Orogeny is a component in a much larger orogenic system. Thus, larger-scale plate tectonic reconstructions may provide the key to understanding the fundamental mechanisms that controlled orogenesis. Tectonic processes involving, for example, island arc accretion or slab buoyancy anomalies, may locally enhance orogenic processes due to an increase in plate coupling. However, using the case study of the HunternBowen Orogeny, it appears that the first-order behaviour of the orogen was governed by processes capable of affecting the entire southern margin of Gondwana. In a similar way to the Cenozoic evolution of the central and southern Andes, it is suggested that plate coupling and orogenic cycles in the late Paleozoic to early Mesozoic Gondwanide Orogeny have resulted from the superposition of mechanisms acting at a range of scales, which contributed to the along- and across-strike variations in the intensity, timing, and duration of deformation phases within the orogenic belt.n