The influence of basement structure on the formation of Jurassic-Cretaceous rift basins along the southern margin of Australia

Abstract

The Australian southern margin, formed as a result of rifting between Australia and Antarctica during the Late Jurassic-Cretaceous, includes a series of Mesozoic sedimentary basins that are underlain by Archean, Proterozoic and, Paleozoic basement blocks. Basement blocks along the margin vary considerably in lithological and structural characteristics (thickness, composition, fabric), making the southern margin an ideal location to identify relationships between underlying basement and basin formation. In this study, the properties of the underlying basement along the southern margin of Australia, specifically the basement of the Otway Basin and the neighbouring Duntroon and Ceduna sub-basins, as well as the current geometry of the basins have been reviewed and analysed. The analyses and calculations were done by mapping major basement blocks and structures on gravity and magnetic maps, re-interpreting seismic sections and using Australia’s Moho depth model and a crustal thickness map of the Australian southern margin. The amount of extension and the stretching factor (β factor) have been measured along the southern margin from west to east to identify different modes of rifting and factors that may control the style of rifting. Results show that total amount of extension and stretching factor varies along the margin. Reduced extension and low to intermediate β factors are observed in regions predominantly underlain by the old, cold and thick Archean cratonic region, whilst larger amounts of extension and larger β factors are found in younger, thinner and warm Phanerozoic regions. In addition, the pattern of extensional faults, and the distribution of faults length and spacing demonstrates the strong influence of basement age and pre-existing basement properties on sedimentary basins. Regions that are underlain by cratonic blocks are characterised by a small number of large faults with wide spacing, while a large number of small, short faults are found in younger and warmer regions. In order to identify the possible relationship between the basement structures and extensional faults, the main pre-existing basement structures have been mapped using gravity and magnetic data. In a limited area, normal faults followed the orientation of pre-existing basement structures. Observations show that in most regions basement structures have no controls on the formation of younger faulting patterns, with most normal faults having an orientation that is highly oblique to perpendicular to the strike of basement faults.