Myra Keep

Models of lithospheric-scale deformation during plate collision: effects of indentor shape and lithospheric thickness.

Abstract Analogue models designed to test the effects of lithosphere thickness and indentor shape during continental collision indicate that strength and density differences between lithosphere types control structural vergence during collision. Models presented here include layers of putty and sand to form four-, three- and two-layer lithosphere types (craton, arc and oceanic equivalent), respectively, which collide during unidirectional shortening. In most cases, the stronger, thicker four-layer lithosphere does not deform, but simply acts to bulldoze weaker material aside. Deformation, concentrated in the thinnest layers, consistently verges away from the indenting continent, irrespective of indentor shape. Lateral (strike-slip component) deformation occurs in the early stages of most models, even with simple unidirectional shortening, and is enhanced by highly oblique indentor shapes. The effects of extrusion during collision, tested by repeating models with and without a free edge, which allows extrusion, consistently indicate that with the ability to extrude, structures are smaller and more numerous. Without extrusion, fewer, larger structures tend to develop. Two geological examples, from the Precambrian part of the Tasman orogenic belt and the modern-day collision at the leading edge of Australia, illustrate aspects of the models.

Early-stage orogenesis in the Timor Sea region, NW Australia

Neogene collision between the Australian, Eurasia and Pacific plates was coeval with the growth of major depocentres in the Timor Sea. Seismic cross-sections across these troughs identify a characteristic architecture: all three lie inboard of diffuse horst-like structural highs and all mimic older depressions in the top-Permian and upper-Jurassic surfaces. Distortion of pre-tectonic (Aptian–Oligo-Miocene) sequences indicates apparent trough subsidence was coupled to uplift of outboard highs. Rudimentary cross-section restoration identifies negligible strike-normal length change. We interpret apparent Neogene subsidence to reflect amplification of basement topography. This was not accommodated by discrete structural inversion. Rather, landward propagation of contractional strain caused continuous vertical amplification of basement topography. At shallow levels, normal faulting accommodated flexure and thin-skinned collapse of detached sedimentary cover, permitting it to shed away from structural highs and pond in adjacent troughs. We infer that shortening of the North West Shelf accommodated oblique convergence between Australia and the Eurasia–Pacific arc and speculate that the transcurrent component of this deformation was partitioned well outboard. Neogene modification is interpreted to reflect the earliest stages of collisional orogenesis.

The Neogene tectonic history of the North West Shelf, Australia

Neogene deformation along the northern margin of the Australian Plate changes character, from a collisional margin to the east in the Timor Sea to a passive margin further west in the Carnarvon Basin. Changes in the deformation style reflect changing regional stress. The continental collision currently underway in the vicinity of Timor Island (the Banda Orogen) influences Neogene deformation in the adjacent Timor Sea, but has little effect in the Carnarvon Basin. The location of deformation also changes, from outboard in the Timor Sea, to inboard in the Carnarvon Basin, with neotectonic events strongly controlled by basement boundaries in the Carnarvon Basin. We propose that Neogene deformation in the Timor Sea and Browse Basin can be explained by an elastic flexure model, whereas in the Carnarvon Basin Neogene deformation is strongly controlled by competency contrasts between basement and basinal rocks. We present a comprehensive model for Neogene deformation across the North West Shelf, based on seismic interpretation and mapping.

NW Australian intraplate seismicity and stress regime

[1] The interrelations between stress orientation, regional deformation, and seismic activity in the context of intraplate seismicity continue to pose difficult questions. Stress models of the Australian continent are largely based on nonseismic information. Our study of 26 microearthquakes in NW Australia shows that they are good indicators of the seismogenic stress field. By inverting focal mechanisms calculated from the events we obtained deviatoric stress tensors in full agreement with the known in-situ stress field. The stress tensors corroborate the elastic modeling of the continent. The methods used here have widespread applicability in determining crustal stress field parameters in regions where in-situ stress data are sparse or absent.

Tectonic evolution of the northern Bonaparte Basin: impact on continental shelf architecture and sediment distribution during the Pleistocene

The Bonaparte Basin (NW Australia) forms a rare, recent example where Neogene deformation shaped a very wide platform (630 km wide) in which a mixed carbonate-silliciclastic sedimentary sequence developed. This study combines structural and stratigraphic analysis and provides new insights as to the role of tectonics in controlling platform shape and sediment distribution in wide shallow water settings. Detailed analysis of the structure and stratigraphy of the northern part of the Bonaparte Basin allowed identification of the main regimes and phases of deformation and their control on sedimentation during the Neogene. The results reveal that the distribution of Neogene sediments across the northern Bonaparte Basin is mainly controlled by flexure-induced deformation mechanisms associated locally with extensional faults and low-strain, left-lateral strike-slip. These processes ultimately shaped the geometry and sedimentary architecture of the wide continental shelf. They led to the development of two different types of tectonically induced shelf depocentres that controlled the gross distribution of Quaternary sediments. In particular, deformation processes enhanced the formation of the carbonate-dominated, ∼200 m-deep Malita intra-shelf basin. The Bonaparte Basin is a prime natural laboratory to describe the links between tectonics and sedimentation along a very large, mixed carbonate/clastic platform and could be used as a modern analogue to similar settings in the past Earths history.

Natural seismicity and tectonic geomorphology reveal regional transpressive strain in northwestern Australia

A temporary seismic network deployed in northwestern Australia between October 2005 and March 2008 recorded 28 earthquakes ranging in magnitude between M L 2.0 and M L 5.3. Twenty-three of these events occurred in areas underlain by continental crust previously affected by Mesozoic continental rifting (Carnarvon and Perth basins), with five occurring within the area underlain by the Precambrian craton. Focal mechanisms for 26 of the earthquakes show dominantly strike-slip solutions, regardless of the type of underlying crust (craton or extended margin). Geomorphological investigations in the Gascoyne region have identified several youthful fault scarps, especially along the Mt. Narryer fault zone in west-central Western Australia. This fault zone lies in the epicentral region of the 1941M L 6.8 to 7.3 Meeberrie earthquake, the largest historical earthquake to have affected Australia. The 120 km-long north-trending Mt Narryer fault zone comprises five left-stepping en echelon segments. The two southern segments display west-side-up reverse displacements across the Roderick and Sanford river alluvial channels, and have captured and diverted active stream flow, formed sag ponds, and impounded Lake Wooleen. The geomorphic expression of the northern three segments, and the en echelon stepping nature of the fault zone as a whole, suggests a component of strike-slip deformation, which is consistent with the sense of faulting determined from the focal mechanism solutions in our seismicity analyses. The higher proportion of events recorded along the extended continental margin compared with the craton suggests that the dense network of late Mesozoic rift-related faults along thecontinental margin may be reactivating and responding to far-field stress conditions. However, the presence of the Mt Narryer fault system indicates that structures within the craton also accommodate some of the regional stress and can be sources of future large magnitude earthquakes, such as the1941 Meeberrie event. The differences in activity rates between the extended margin and cratonic provinces may reflect the differences in crustal architecture, and the way that faults in these two different terranes respond to far-field stresses.

Elastic flexure and distributed deformation along Australia's North West Shelf: Neogene tectonics of the Bonaparte and Browse basins

Abstract Neogene collision between Australia and the Banda Arc modified the adjacent Bonaparte and Browse basins of the North West Shelf of Australia. Modification comprised both continuous long-wavelength amplification of Permo-Carboniferous basement topography and flexure and normal faulting of Triassic–Recent sedimentary cover. Deformation was continuous across the Browse and Bonaparte basins, despite the basins beings separated by a rupture-barrier style accommodation zone, the Browse–Bonaparte Transition. The degree of basement control and mechanisms of fault linkage vary significantly across this transition, and reflect differences in the structural relief, amplitude and depth of rifted basement either side of the transition. Neogene collision amplified the architectural divide. Amplification of basement topography over wavelengths of several hundred kilometres was associated with negligible horizontal length change. The transcurrent component of Neogene deformation was partitioned outboard of any continuous flexural amplification.

A restraining bend in a young collisional margin: Mount Mundo Perdido, East Timor

Mount Mundo Perdido, a 1750 m-high, steep-sided massif situated in the Viqueque district of East Timor, comprises approximately 30 km2 of complexly juxtaposed rocks deriving from both sides of the collisional plate boundary between the Australian Plate and the Banda Arc. Lithologies include Triassic–Jurassic interior-rift basin deposits, Cretaceous–Oligocene pelagites of Australian passive margin origin, neritic Oligocene–Miocene limestones and volcanics of Asiatic affinity, and Pliocene–Pleistocene synorogenic deposits. Detailed structural mapping shows Mount Mundo Perdido to be dominated by recent, high angle, oblique-slip and strike-slip faults that have been active into the Pleistocene and control the present-day topography. The fault architecture and stratigraphic distribution in the study area are comparable to pop-up structures developed at restraining bends, in this case within an east–west oriented zone of sinistral strike-slip. Our observations, supported by comparisons to scaled sandbox models and to similar pop-up structures developed in strike-slip systems elsewhere in the world, suggest that plate boundary-parallel strike-slip deformation is an integral part of the kinematics within the collisional zone between the Australian and Eurasian/Pacific plates in the Timor region.

Introduction to the thematic issue on the evolution and dynamics of the Indo-Australian plate

The Australian Plate and the Indian plate, initially formed from the breakup of Gondwana, became the Indo-Australian Plate in the Middle Eocene some 45–40 million years ago as both plates amalgamated. This amalgamation was a consequence of the cessation of spreading at the mid-oceanic ridge in the Indian Ocean that separated the two plates (Liu et al. 1983; Royer & Coffin 1992). The amalgamation followed a long and complicated history of rifting with India separating from Antarctica–Australia at ca 130 Ma during the opening of the Indian Ocean (Royer & Coffin 1992; Gibbons et al. 2012). Australia and Antarctica went their separate ways in the Upper Cretaceous as the Southern Ocean started to form between the two continents (Royer & Sandwell 1989). The union seems destined to be short-lived, with evidence for breakup of the Indo-Australian Plate in the Indian Ocean west of Sumatra. Indeed, geodetic measurements point to very slow relative motion between India and Australia (e.g. Kreemer et al. 2003), while seismicity, folding and faulting below the northern Indian Ocean point towards the development of a diffuse plate boundary in the oceanic lithosphere separating the two continents (e.g. Weissel et al. 1980; Bull & Scrutton 1990; Sandiford et al. 2005). The MW 8.6 great earthquake that occurred on 11 April 2012 in the Indian ocean some 150 km southwest of the Sumatra–Andaman trench (GCMT catalogue), the largest strike-slip earthquake ever recorded, is a stark reminder that the Indo-Australian plate is actively deforming and breaking up into a separate Indian plate and Australian plate. Apart from its complicated history and its current tectonic development, the Indo-Australian plate is also a plate of extremes. It contains the fastest moving continent on Earth, Australia, moving *north-northeastward at *5.5–6.5 cm/yr (Schellart et al. 2011). The northern margin of the plate borders the highest mountain range on Earth, the Himalaya, with its highest peaks close to 9 km above sea level, and the Tibetan Plateau with an average elevation of 5 km (Fielding et al. 1994). The northeastern boundary consists of one of the largest subduction zones on Earth, the Sunda subduction zone. In the New Guinea region exhumation of the youngest high-pressure rocks is taking plate at plate tectonic rates (Baldwin et al. 2004) and at the eastern end of the plate the Tonga–Kermadec– Hikurangi subduction zone has the fastest subduction (up to 24 cm/yr) and backarc spreading (up to 16 cm/yr) observed anywhere on Earth (Bevis et al. 1995). Its very active tectonic nature, and its changes in plate geometry and configuration since the break-up of Gondwana, make the Indo-Australian plate an attractive and fascinating subject of study. This thematic issue presents an interesting series of studies of the various plate margins of the Indo-Australian plate as well as aspects of the plate interior. The thematic issue accompanies Symposium 15.3 at the 34 International Geological Congress in Brisbane (August 2012), entitled ‘‘Evolution and dynamics of the Indo-Australian Plate’’. Rather than invite papers for a special issue to be published some time after the Congress, we decided to produce a volume of papers to appear coincidently with the Symposium. As it happened, not all of the papers to be given at the Symposium ended up in this volume. However the end result is a geographically extensive set of papers that explore both the plate margins (from the Himalaya to New Zealand) and the plate interior, including both regional and local studies. Starting with the broadest regional perspective, Fishwick & Rawlinson (this issue, 809–826) explore the three dimensional structure of the Australian lithosphere using new seismic tomography models, identifying boundaries between cratons, blocks and ancient orogenic belts. Hall & Sevastjanova (this issue, 827–844) focus on the distribution of Australian crust in the Indonesian region, using radiometric dating, identifying several blocks and fragments that were added to the Indonesian core during the Mesozoic and Cenozoic. We then travel clockwise around the northern plate margin, from the Himalaya through to the New Zealand margin. Replumaz et al. (this issue, 845–858) use analogue modelling to explain east–west shortening in the northwest Himalayan syntaxis, reproducing known geometries with a variety of sandbox configurations. Further to the east, Benincasa et al. (this issue, 859–876) document a young strike-slip system in East Timor, which may be part of a far more extensive system. The strike-slip deformation is also found to the south in the Timor Sea, where Bourget et al. (this issue, 877–897) investigate the tectonic evolution of the northern Australian Journal of Earth Sciences (2012) 59, (807–808)

Basement reactivation and inversion mechanisms in the Timor and Norwegian seas

The North East Atlantic Margin and Australian North West Shelf share similar tectonic evolutions. Both are passive margins of similar age and size, both experienced significant periods of reactivation, inversion and volcanism, and both host significant oil and gas resources. The main differences between these two margins lie in their modern tectonic settings and hydrocarbon resources. The North West Shelf occurs at a present-day collisional plate boundary, the dynamics of which strongly control the location of oil and gas reserves, whereas the North East Atlantic has not yet entered a collisional phase. The North West Shelf tends to be gas prone, in contrast to its European counterpart. Recent mapping of Neogene to Recent modification across the North West Shelf highlights the influence of inherited basement architecture, long-wavelength strain partitioning and continuous flexural deformation. Similarities in the structural evolution of these two margins, especially in terms of the role of the basement structure, episodic modification, the location and extent of Cenozoic inversions and the role of inversion in controlling hydrocarbon distribution, illustrate the importance of these issues in passive margin development. Lessons learned from the Australian example include that structural inheritance has an overwhelming control on later reactivation, and that even small amounts of shortening strain (1–2%) can lead to significant modifications of structural traps and petroleum habitats.