The Influence of lithosphere and basement properties on the stretching factor and the development of extensional faults across the Otway Basin and eastern Bight Basin

The Influence of lithosphere and basement properties on the stretching factor and the development of extensional faults across the Otway Basin and eastern Bight Basin

1. KHARAZIZADEH*, W.P. SCHELLART, J.C. DUARTE

School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC 3800, Australia

Department of Earth Sciences, Vrije Universiteit Amsterdam, Amsterdam, Netherlands

Instituto Dom Luiz (ILD) and Geology Department, Faculty of Sciences of the University of Lisbon, Campo Grande, Lisbon, Portugal  

 

*nasim.kharazizadeh@monash.edu

*n.kharazi@hotmail.com

 

Abstract

The large southern continental margin of Australia, with a wide variety of sedimentary basins, formed during Mesozoic rifting. The evolution of sedimentary basins is mainly controlled by plate tectonic activity and the mechanism of continental extension. This work presents a comparative study between two main depocentres of the Bight Basin (Ceduna, Duntroon sub-basins) and the Otway Basin. Here, the total amount of extension (∆L) and stretching factor (β) have been measured across the Otway Basin and eastern Bight Basin. The results show significant variation in extensional stretching along the basins, with the smallest stretching factors in the Ceduna and Duntroon sub-basins (1.2<β<1.4), and the largest amount of extension (~ 177 km) and the largest stretching factor (β=1.85) in the eastern part of the passive margin. The regions with the lowest β factor are underlain mostly by thicker lithosphere, while the regions with the largest β factor and amount of extension are related to younger and thinner lithosphere. The main basement structures have been mapped throughout South Australia and Victoria to examine the possible relationships between the new pattern of extensional faults and old basement fabrics. The distribution pattern of normal faults varies considerably along onshore and offshore components of basins. It is proposed that in some regions fault strike varies due to changes in orientation of pre-existing structures in the basement. For example, the north-south Coorong Shear Zone seems to affect the geometry of normal faults by changing their strike from E-W to NW-SE and also, in the easternmost part of the basin, the Bambra Fault changes the strike of normal faults to the NE-SW. Also, the NE-SW basement structures in the western part of the Gawler Craton have some control on normal faults in the western Ceduna sub-basin. Normal faults in the easternmost and westernmost parts of the Otway Basin have a similar orientation to the basement faults. However, in most regions basement faults are perpendicular to the normal faults and there is a minor influence on the new pattern of faulting. Our results imply that the properties of the continental lithosphere (age, thickness and strength of lithosphere) exert a major influence on the β factor and amount of crustal extension but only a minor influence on the geometry of extensional faults.

Keywords: Otway Basin, Ceduna and Duntroon sub-basins, rifting, total amount of extension, β factor, normal faults, lithosphere properties