Subsurface architecture of the Texas Orocline and its influence on in situ stresses in the overlying Surat Basin

Abstract

The New England Orogen in eastern Australia is a curved orogenic belt, with the Texas orocline being the most prominent curvature. The majority of the Texas Orocline lies beneath Permian to Mesozoic sedimentary basins (Bowen and Surat basins), which inhibit primary observation of the three-dimensional geometry of the orocline and its possible tectonic origin. The Bowen and Surat basins have recently been the target of coal seam gas exploration and production. Coal seam permeability is primarily influenced by fracture/cleat interaction with in-situ stresses. Permeability distribution appears primarily influenced by first order crustal scale structuring regardless of whether structures intersect actual coal bearing basins or whether structures terminate below the producing basins. The apparent influence of crustal structures, stress and ultimately coal permeability highlight the need for studies on the crustal architecture and its influence on the in situ stresses within the overlying basins. Consequently this thesis presents geophysical and well data that constrain the geometry and evolution of the Texas Orocline under the sedimentary cover, and characterize in situ stresses in the overlying basins. 2D seismic, aeromagnetic TMI and Bouguer Gravity are used to identify the depth to the New England “basement” and to locate significant faults intersecting it. These data are used to produce a structural interpretation of the curved New England Orogen below cover, as well as a tectonic model for its formation. Seismic interpretation reveals the presence of an early Permian sub-trough of the Bowen Basin, which lies approximately parallel to the curvature of the Texas orocline, suggesting that the inception of the oroclinal geometry was related to the formation of early Permian rift basins. The continuation of the Texas Orocline to the north is tracked by a characteristic association of serpentinite outcrops within fault zones and high gravity and magnetic anomalies. This relationship links two tectonic contacts (Peel Fault and Yarrol Fault) below cover, and shows the New England Orogen to be highly contorted between the Texas Orocline and the northern New England Orogen. These observations are incorporated into a tectonic model for the formation of the Texas Orocline, which attributes initial curvature of the Orocline to Early Permian extension, and later tightening of the orocline to Permian to Triassic transpression and compression events. The orientation and magnitude of in situ stresses with relation to basement structures are mapped across the Surat Basin using wireline log data. The stress state across the entire Surat Basin is observed as highly variable, with a mean E-W maximum horizontal stress orientation. Locally, stresses rotate from WNW-ESE over the Lachlan and Thomson Orogens on the western margin of the study area, to NW-SE over the New England Orogen on the eastern margin of the study area. This is in stark contrast to previous studies of the stress state in the Bowen Basin to the north, which exhibits a consistent NNE-SSW maximum horizontal stress orientation. Differential tectonic stress is observed to increase with proximity to basement intersecting faults (Leichhardt-Burunga Fault System) and areas of uplifted basement. Additionally, Andersonian tectonic regimes are observed to change with depth from reverse, to strike-slip, to normal where sedimentary cover is thickest. This stress field complexity is attributed to complex boundary forces associated with Indo-Australian Plate collision being preferentially transmitted within basement lithology over basin lithology due to basement exhibiting a higher rigidity. Local structure is more influential than plate boundary forces on the in situ stress state of the study area due to its location within an “area of divergence” in the Australian stress field. The results of this thesis refine the Permian-Cretaceous history of eastern Australia, and the lasting impact that this history has had on economically significant sedimentary basins.