Elsevier

Marine and Petroleum Geology

Volume 88, December 2017, Pages 1059-1077
Marine and Petroleum Geology

Research paper
Influence of lithosphere and basement properties on the stretching factor and development of extensional faults across the Otway Basin, southeast Australia

https://doi.org/10.1016/j.marpetgeo.2017.08.034Get rights and content

Highlights

  • The variation in stretching factor (β) and total amount of extensions across the whole crustal thickness in the Otway Basin is potentially controlled by lithospheric properties (age, thickness and strength).

  • The westernmost part of the Otway Basin, with the lowest stretching factor and amount of extension, is underlain by thicker lithosphere. The stretching factor and extension gradually increases towards the central and eastern parts of the basin, which are underlain by thinner and younger lithosphere of the eastern part of the Delamerian Orogen and western part of the Lachlan Orogen.

  • The values of the amount of extension and stretching factor derived from crustal thickness profiles are much greater than would be predicted from the extension accommodated by the extensional faults at the surface. This considerable discrepancy between equivalent transects might be due to the influence of the lower crustal rheology on the faults architecture in the upper crust and lower crustal flow.

  • The pattern of normal faults varies considerably along onshore and offshore components of the Otway Basin. In the westernmost and easternmost parts of the basin, a number of normal faults appear to follow the strike of major pre-existing basement faults (e.g. Coorong Shear Zone in the east and Bambra Fault in the west). However, in most regions, the basement structures have no control on the formation of the younger faulting pattern, and most normal faults have formed independently.

Abstract

The NW-SE striking Otway Basin in southeastern Australia is part of the continental rift system that formed during the separation of Australia from Antarctica. The development of this sedimentary basin occurred in two phases of Late Jurassic-Early Cretaceous and Late Cretaceous rifting. The evolution of this basin is mainly associated with extensional processes that took place in a pre-existing basement of Archean, Proterozoic to Paleozoic age. In this study, the total amounts of extension and stretching factor (β factor) have been measured for six transects across the entire passive margin of the Otway Basin region. The results show significant variation in extensional stretching along the basin, with the smallest stretching factors in the easternmost (β = 1.73, 1.9) and westernmost part of the basin (β = 2.09), and the largest stretching factors in the central part (β = 2.14 to 2.44). The domain with the lowest β factor is underlain mostly by thicker lithosphere of the Delamerian Orogen and older crustal fragments of the Selwyn Block. In contrast, the region with the largest β factor and amount of extension is related to younger and thinner lithosphere of the Lachlan Orogen. The main basement structures have been mapped throughout eastern South Australia and Victoria to examine the possible relationships between the younger pattern of extensional faults and the older basement fabrics. The pattern of normal faults varies considerably along onshore and offshore components of the Otway Basin from west to east. It appears that the orientation of pre-existing structures in the basement has some control on the geometry of the younger normal faults across the Otway Basin, but only in a limited number of places. In most areas the basement fabric has no control on the younger faulting pattern. Basement structure such as 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 from NW-SE to the NE-SW. Our results imply that the properties of the continental 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.

Introduction

Studying the relationship between pre-existing basement faults and extensional faults in sedimentary cover sequences is crucial for understanding extensional processes in the continental lithosphere, particularly as basement structures, such as terrain boundaries and shear zones, have been generally described to affect the architecture and structural geometries (location, dip and direction) of younger faults in the sedimentary cover (Bladon et al., 2015, Gibson et al., 2012, Holdsworth et al., 1997, Holdsworth et al., 2001; Butler, 1997, Watterson, 1975, Daly et al., 1989, Dewey et al., 1986, Sykes, 1978). These pre-existing basement faults commonly form weak zones and can therefore reactivate during a new tectonic deformation phase (e.g. Daly et al., 1989, Watterson, 1975).

The effect of pre-existing structures on the development of later fault architecture has also been examined in numerical and analogue models with results showing that the initial dip of thrust faults has a major influence on the growth of normal faults and that the interaction between pre-existing thrust faults and later normal faulting is strongly controlled by the obliquity of extension (Corti et al., 2007, Faccenna et al., 1995). A good example of the influence of inherited basement structures on rift faulting is the NE Brazilian rift margin, in which Precambrian (ca. 750-540 Ma) shear zones are thought to have controlled the rift geometry and fault architecture that formed during Mesozoic rifting (Kirkpatrick et al., 2013, Brito Neves et al., 2000, Matos, 1992).

The southern margin of Australia is an ideal example for assessing the influence of pre-existing basement properties on the formation and geometry of extensional faults, as this passive margin is characterised by Jurassic and Cretaceous rifting of different Archean, Proterozoic and Palaeozoic continental lithosphere. Also, the margin is composed of a series of sedimentary basins that formed during the break-up of Gondwana and the separation of Australia and Antarctica. Previous geological and geophysical studies indicate that the underlying basement of the Australian continental margin differs in lithospheric composition, age, thickness and strength from west to east (Huston et al., 2012, Teasdale, 2004, Teasdale et al., 2003). In the east, the Victorian passive margin is mostly underlain by Palaeozoic basement rocks of the Delamerian and Lachlan orogens and consists of Cambrian-Devonian sediments, granite intrusion and volcanic complexes.

The objective of this study is to understand how the lithosphere properties (age, thickness and strength of lithosphere) and underlying Palaeozoic basement of the Otway Basin in western Victoria have affected the development and geometry of extensional structures of this basin. Several authors have investigated the control exerted by the pre-existing structural fabrics and rocks on basin architecture along the Australian rifted passive margin (e.g. Gibson et al., 2012, Bernecker and Moore, 2003, Miller et al., 2002). In these previous studies, many basement structures were identified and mapped from gravity and magnetic data. These authors focused their studies on the geological interpretation of major pre-existing basement shear zones and their structural trends. The significant variation in extensional direction of normal faults between the western and eastern parts of the Otway Basin has also been noted before (Blevin and Cathro, 2008, Miller et al., 2002).

In this study, we focus on the effect of lithospheric properties (thickness, age and strength) in combination with basement characteristics on the variation of the extension factor and extensional structures across the Otway Basin. To do so we estimate the total amount of extension (ΔL) and the stretching factor (β) across the Otway Basin. Furthermore, to assess the influence of the pre-Mesozoic basement characteristics and its structural geometry on the location and architecture of the Late Jurassic-Early Cretaceous and Late Cretaceous faults, we compare the distribution and geometric patterns of newly-formed extensional faults across the basin. The β factor and the amount of extension have been calculated by using a recent model of the Australian crustal thickness (Aitken et al., 2013), a model of syn-rift and post-rift sediment thickness of the southern margin of Australia (Teasdale, 2004), and through applying a simple procedure in which the thickness of the basement is compared to the initial thickness prior to rifting (using the pure shear stretching model of McKenzie, 1978). In addition, quantitative measurements of the length and spacing of extensional faults have been performed throughout the Otway Basin and dominant trends of faults have been plotted. With these measurements and observations, we have identified the influence of possible factors that may change the amount of extension and pattern of faulting in the Otway Basin from the west to the east.

Section snippets

Geologic setting and Pre Mesozoic basement characteristics of the Otway Basin

The Otway Basin is an extensional basin with onshore and offshore components, and a NW-SE to NE-SW strike. It extends along the southeastern Australian passive margin from eastern South Australia to western Victoria and includes the Torquay sub-basin (Fig. 1).

The Otway Basin formed as part of an intra-continental rift system associated with the break-up of Australia and East Antarctica while Australia was separating from Antarctica during two rifting episodes that occurred in the Late

Geometry and growth of Late Jurassic- Early Cretaceous and Late Cretaceous faults along the Otway Basin

Previous studies have shown that the evolution of the Otway Basin was affected by a variety of tectonic events during different times of rifting (e.g. Perincek and Cockshell, 1995) when a number of extensional depocentres were formed with trends that changed from the western part to the eastern part of the basin. The intra-continental rifting and break-up of Australia and East Antarctica occurred during two main episodes: 1) Late Jurassic-Early Cretaceous; and 2) Late Cretaceous-Eocene (Hall

Data set

In order to calculate the total amount of extension (ΔL) and the stretching factor (β) along the Otway Basin, a recent model of Australia's Moho (Aitken et al., 2013), a crustal thickness map (Aitken et al., 2013) and a sediment thickness map of the Australian southern margin (from OZ SEEBASE and Teasdale, 2004) were used (Fig. 7, Fig. 8). Also, a structural map of the top basement (Briguglio et al., 2015, Krassay et al., 2004, Duddy, 2003, Messent et al., 1999, Hill et al., 1994) was used to

Estimation of lithospheric extension across the Otway Basin

In order to understand the margin-parallel variation of the stretching factor (β) for the Otway Basin, we have constructed six NE-SW striking, sub-parallel transects that cross the basin at regular intervals (see Fig. 7, Fig. 8). Each transect starts approximately from the unstretched point of the continental crust toward the ocean-continent boundary. In each transect the geometry of the Moho, crustal thickness and Jurassic to present day sediment thickness has been included (Fig. 9). Our

Discussion

In order to investigate the role of lithospheric strength on the localisation of deformation, and the role of pre-existing structural fabrics and their possible control on the evolution and architecture of extensional faults across the Otway Basin, we have calculated the amount of extension and the stretching factor (β). The results show different values from west to east along the basin. Furthermore, the variation in normal fault patterns and trends suggests the possible control of the

Conclusion

Using a recent crustal thickness map of Australia and a sedimentary thickness map of the Otway Basin margin, the stretching factor (β factor) and total amount of extension (ΔL) were calculated across the Otway Basin. The results show that the values of stretching factor (β) vary from west to east. Additionally, the total amount of extension and the stretching factor (β factor) derived from mapped faults of the basin are considerably lower than those values derived from investigating the crustal

Acknowledgments

We would like to thank Alan Aitken and Brian Kennett for providing several datasets, without which the calculations would not have been completed. We thank two anonymous reviewers for their constructive comments on the paper. This research was financially supported by Monash University. Wouter Schellart was partly supported by a Future Fellowship from the Australian Research Council (FT110100560) and partly supported by a Vici Fellowship from the Dutch Science Foundation (NWO) (016.VICI.170.110

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