Showing papers by "Joanne M. Whittaker published in 2010"

Development of the Australian‐Antarctic depth anomaly

Abstract: The oceanic Australian-Antarctic Discordance (AAD) contains two unusual features: (1) N–S trending anomalously deep bathymetries and (2) rough basement morphologies in young ( 45° spreading obliquities.

Restoring the continent-ocean boundary: constraints from lithospheric stretching grids and tectonic reconstructions

Abstract: We present a revised set of Euler poles describing the relative motion between Australia and Antarctica from the onset of continental rifting at ~160-140 Ma to the reorganisation of the spreading system at ~50 Ma. Our revised reconstruction addresses two key issues that remain unresolved in current plate reconstructions. Firstly, we present new estimates of the pre-rift plate boundary locations for the conjugate Australian-Antarctic margins. These reconstructions are truly palinspastic, incorporating estimates of crustal thickness along these margins, derived from gravity inversions. Integrating the crustal thickness along tectonic flowlines results in the pre-rift location of the continental plate boundary. This integration relies on defining the present-day extent of stretched continental crust within the margin, which is the subject of the companion paper, Williams et al. [2010]. Once restored, we are then able to use the pre-rift plate boundary positions to compute ‘full-fit’ poles of rotation for Australia relative to Antarctica. This approach allows us to model a deforming passive margin, with implications for understanding and modelling the formation of basins and deposition of sediments along passive margins. Secondly, reconstructions for plate motions since ~83 Ma have been revised to obtain a better fit along the entire Australian-Antarctic conjugate margins, which extend from at least the Kerguelen Plateau and Broken Ridge in the west to Tasmania, Australia and Cape Adare, Antarctica in the east. Previously published reconstruction models for the period 83-50 Ma had resulted in a poor fit between the two plates at the extreme ends of the conjugate margins.

Mapping Tertiary mid-ocean ridge subduction and slab window formation beneath Sundaland using seismic tomography

Abstract: Based on tectonic reconstructions, Whittaker et al. (2007) proposed that a slab window formed beneath Sundaland between 70 and 43 Ma, due to subduction of the Wharton Basin spreading ridge between India and Australia. They suggest that extension in the Java Sea region at this time was exacerbated as a result of upwelling asthenosphere associated with the slab window. Active ridge subduction and subsequent slab window formation can severely affect basin formation, heatflow and petroleum systems development on the overriding margin. A slab window forms between diverging plates when a mid-ocean ridge is subducted, leading to anomalous thermal effects like increased mantle wedge temperatures and thermal gradients in the overlying crust. Whittaker et al.?s (2007) kinematic reconstructions rely on restoring now-subducted lithosphere based on preserved ocean crust, but the inherent uncertainties in this process call for an independent evaluation of this model. Mantle seismic tomography models provide qualitative boundary conditions for modelled tectonic histories. We compare seismic tomography models with the model of Whittaker et al. (2007) at a range of mantle depths to confirm the existence of a slab window, and obtain bounds for its maximum regional extent. We identify a break in the high-velocity, down-going Indian-Australian slab at depths between 950-1350 km and longitudes between 85° and 110°, supporting the presence of a slab window. However, we find that the window is located approximately 5° further north and 10°-15° further west than previously proposed, implying that the Wharton Ridge was subducted farther west than previously suggested.

What can potential field data really tell us about Continent-Ocean transitions?

Abstract: We investigate the ability of gravity and magnetic data to define the distribution of crustal types at continental margins, with specific focus on the conjugate Australia- Antarctica margins. Previous studies have used features in gravity maps as a proxy for the pre-rift location of plate boundaries. Instead, we discuss the use of potential field data to define the boundaries of stretched continental crust on a regional scale, and demonstrate this process for the conjugate margins of South Australia and Antarctica. In a companion paper (Whittaker et al, 2010) we show how these boundaries are used along with estimates of crustal thickness to determine the location of the plate boundaries prior to rifting, and ultimately to derive full-fit plate tectonic reconstructions.

Revised plate tectonic history of the west Australian margin reveals how the Gascoyne Terrane docked at West Burma

Abstract: Southeast Asia contains various continental fragments that sequentially rifted from East Gondwana. A number of these fragments were sourced from the northern margin of East Gondwana. East Gondwana's fragmentation was initialised with Late Jurassic rifting along the northwest Australian margin, forming the Argo Abyssal Plain, and followed by Early Cretaceous N-S oriented rifting along the entire western Australian margin, forming the Gascoyne, Cuvier and Perth Abyssal Plains. In a comprehensive study to address the issue of margin formation and terrane drift and docking, we revised the tectonic formation of the entire margin using an integrated analysis of marine magnetic and gravity anomaly data from all abyssal plains involved. Our model highlights the necessity for a new continental fragment, the Gascoyne Terrane, due to the presence of tectonic features, such as margin age offsets, spreading rates, ridge jumps and isochron orientations, that are distinctly different north and south of the Wallaby Zenith Fracture Zone (WZFZ). Our model suggests that the northern extent of Greater India was limited by the WZFZ. The relative motion between Greater India and Australia formed the Perth Abyssal Plain, while motion of the Gascoyne block, located north of the WZFZ, formed the Cuvier and Gascoyne Abyssal Plains. Following rifting from the northwest margin north of the Exmouth Plateau, Argoland accreted to Burma at ~75 Ma. Our model suggests that in contrast the Gascoyne Terrane was accreted to West Burma at ~55 Ma, following about 20 million years of Tethys seafloor subduction after Argoland's accretion. Our model also suggests that the eastern tip of Greater India collided with West Burma at around 35 Ma. This is consistent with the onset of extrusion of the North Indochina Block around 34 Ma, expressed by leftlateral strike-slip along the Ailao Shan - Red River shear zone between 34 and 17 Ma. Burma is rich in gems, lead, zinc, iron, tin, chromium, nickel and tungsten deposits, and has oil-bearing fields in its forearc basin. Our model has implications for the formation of the West Burmese margin, as it would also be comprised of a continental terrane once conjugate to the Bernier Platform and Exmouth Plateau, both proven hydrocarbon-rich resources. Advances in modelling the accretion history of Burma will help to distinguish possible continental fragments, which may reveal new resources.