The geometry, kinematics, and reactivation history of major fault systems in eastern Australia: implications for deformation phases from the Permian to Cenozoic

Abstract

Eastern Australia has been affected by numerous fault systems, but their pattern, geometry, kinematics, and timing have hitherto remained relatively understudied. This thesis makes an effort to address this issue by investigating fault systems and their reactivation history. The results of this thesis shed new light on the tectonic history of eastern Australia since the Permian. The aim of this work is to unravel the role of major faults in the tectonic evolution of eastern Australia by studying geometry and kinematics of fault systems during the early Permian, Triassic to Jurassic, and late Mesozoic to Cenozoic. The role of fault systems in the formation of the Texas and Coffs-Harbour oroclines in the southern New England Orogen has been studied. The results show that layer-parallel faults with a main strike-slip component occur parallel to the curved structure of the Texas and Coffs-Harbour oroclines. It indicates that a flexural slip mechanism may have operated during oroclinal bending in the early-mid Permian. The orocline-parallel faults were reactivated during the Mesozoic and Cenozoic, as indicated by the recognition of displaced Triassic granitoids, Mesozoic and Cenozoic sedimentary rocks, and Cenozoic basalts. Structural analysis was conducted on faults in the Triassic-Jurassic sedimentary basins of eastern Australia. The results show that the intermittent phases of rifting events occurred during the Triassic. The early stage syn-sedimentary normal faults in the Nymboida Coal Measures suggest a rifting phase in the Early-Middle Triassic. This phase of rifting was followed by a contractional event that resulted in tilting, folding, and thrust faulting. Moreover, evidence of syn-sedimentary normal faults and bimodal volcanism during the early Late Triassic is indicative of another rifting phase, resulting in the development of the Ipswich Basin. The alternating episodes of rifting and contraction during the Triassic were possibly controlled by plate boundary migration and switches between trench retreat and advance. The N-striking strike-slip Demon Fault is recognised as four steeply-dipping reverse dextral segments. It is interpreted to be a post-Late Triassic fault because it has displaced some Triassic NW-striking faults and Early to Late Triassic magmatic rocks. The amount of dextral offset along the different parts of the Demon Fault is decreased towards the south. The major activity of the Demon Fault is construed to have occurred contemporaneously with the development of the Clarence-Moreton Basin in the Jurassic. The recent reverse dextral activity of the Demon Fault could have possibly been related to either a mid-Cretaceous contractional deformation or Cenozoic deformation associated with collisional processes at the northern boundary of the Australian plate. The NNW-striking North Pine Fault System (NPFS) has been studied. The NPFS has undergone sinistral reverse strike-slip movement with offsets ranging from ~3.4 to ~8.2 km. Based on a Triassic granophyre dyke parallel to the southeastern NPFS, and the contribution of parallel NNW-striking strike-slip and normal faults in the development of the Late Triassic-Early Cretaceous Maryborough Basin, the NPFS has likely been active during the Mesozoic. The NPFS is interpreted to have been reactivated with oblique sinistral-normal kinematics during the Late Cretaceous-early Eocene in response to regional oblique extension associated with the opening of the Tasman and Coral seas. The recent strike-slip reverse movement was likely due to far-field contractional stresses from collisional tectonics at the eastern and northern boundaries of the Australian plate in the Cenozoic. New evidence shows that numerous strike-slip faults with a reverse component have displaced Cenozoic volcanic rocks, ranging in ages from ~ 31 to ~21 Ma, in southeast Queensland. These ages point out that faulting must have occurred after the late Oligocene. The reactivation of major faults resulted in the occurrence of brittle subsidiary faults in Cenozoic volcanic rocks. Intraplate transpressional deformation resulted from far-field stresses transmitted from the collisional zones at the northeast and southeast boundaries of the Australian plate since the late Oligocene. In summary, results of this thesis highlight the role of fault systems in the deformation of eastern Australia from the Permian to late Cenozoic. Layer-parallel faults around the Texas and Coffs-Harbour oroclines reflects a possible role of flexural slip mechanism during oroclinal bending in the early-mid Permian. Faults have intermittently been reactivated during the Mesozoic to early Cenozoic oblique extensional phases. The reactivation of faults has also occurred during the late Cenozoic in response to far-field stresses transmitted from the Australian plate boundaries.