The northern Perth Basin is an under-explored part of the southwest continental margin of Australia. Parts of this basin have proven hydrocarbon potential. The basin is extensively covered by mostly 2D seismic reflection data and marine gravity and magnetic data. The seismic data helps to resolve the structural framework of the basin, but in deepwater regions, the basement-cover contact and deeper basement structure are generally not well imaged. To help overcome this limitation, integrated 3D gravity modelling was used to investigate crustal structure in onshore and offshore parts of the basin. Such modelling also relies on knowledge of crustal thickness variations, but these variations too are poorly constrained in this area. Multiple models were constructed in which the seismic data were used to fix the geometry of sedimentary layers and the fit to observed gravity was examined for various different scenarios of Moho geometry. These scenarios included: 1) a Moho defined by Airy isostatic balance, 2) a Moho based on independently-published Australia-wide gravity inversion, and 3) attempts to remove the Moho gravity effect by subtracting a long-wavelength regional trend defined by GRACE/GOCE satellite data. The modelling results suggest that the best fit to observed gravity is achieved for a model in which the thickness of the crystalline crust remains roughly constant (i.e. deeper Moho under sediment depocentres) for all but the outermost parts of the basin. This finding has implications for understanding the evolution of the Perth Basin, but remains susceptible to uncertainties in sediment thickness.

Absract for Indonesian Geophysics Conference (HAGI)

For many basins along the western Australian margin, knowledge of basement and crustal structure is limited, yet both play an important role in controlling basin evolution. To provide new insight into these fundamental features of a continental margin, we present the results of process-oriented gravity modelling along a NW-SE profile across the Browse Basin through the Brecknock field. Process-oriented gravity modelling is a method that considers the rifting, sedimentation and magmatism that led to the present-day gravity field. By backstripping the sediment load under different isostatic assumptions (i.e. range of flexural rigidities), the crustal structure associated with rifting can be inferred. Combining the gravity anomalies caused by rifting and sedimentation and comparing them to observed gravity provides insight into the presence of magmatic underplating, the location of the continent-ocean boundary and the thermal history of a margin. For an effective elastic thickness of 25 km, backstripping syn- and post-rift sediments (Jurassic and younger) along the Browse Basin profile suggests moderate Jurassic stretching (beta-1-2) and shows that rifting and sedimentation generally explain the observed free-air gravity signature. The gravity fit is reasonable for most of the Scott Plateau and Caswell Sub-basin, but over the Leveque Shelf and Wilson Spur, predicted gravity is less than observed and predicted Moho is also shallower than suggested by seismic refraction data. These misfits suggest the presence of magmatic underplating beneath the Leveque Shelf and outermost parts of the basin, an inference that has mixed support from refraction and crustal-scale seismic reflection data.

Paleogeographic reconstructions of the conjugate Australian and Antarctic rifted continental margins based on geological versus plate tectonic considerations are rarely, if ever, fully compatible. Possible exceptions include a recently published plate tectonic reconstruction combining ocean floor fabrics and magnetic anomalies with revised rotational poles for successive extensional events in the region that coincidently brings about a match between the Kalinjala Mylonite Zone in South Australia and Mertz Shear Zone in Antarctica (Whittaker et al., 2007). A match between these two crustal-scale shear zones has been previously proposed on isotopic and geological grounds (Di Vincenzo et al., 2007; Goodge and Fanning, 2010). However, whereas the Mertz Shear Zone marks the western limits of ca. 500 Ma magmatic activity in Antarctica (Delamerian-Ross Orogen), the Kalinjala Mylonite Zone lies well to the west of this magmatic front and is bounded either side by rocks of the Mesoarchean-Mesoproterozoic Gawler craton. An alternative geological match for the Mertz Shear Zone in Australia is the hitherto unrecognised Coorong Shear Zone in South Australia (Fig. 1), tracts of which have been intruded by gabbro and granite of Delamerian-Ross age and west of which such rocks are either completely absent or greatly reduced in volume. The north-south-trending Coorong Shear Zone lies directly along strike from the (Spencer-) George V Fracture Zone and is clearly visible in aeromagnetic images and offshore deep seismic reflection data as a steep to subvertical crustal-penetrating basement structure across which there is an abrupt change in the orientation of magnetic fabrics and sedimentary basin fault geometries. An equally conspicuous change of direction is evident in ocean floor fabrics immediately offshore, inviting speculation that the along-strike George V Fracture Zone originated through reactivation of the older Coorong Shear Zone and shares the same orientation as the original basement structure. Correlation of this basement structure with the Mertz Shear Zone leads to a reconstruction of the Australian and Antarctic continental margins in which Antarctica and the entrained Mertz Shear Zone are located farther east than some recent restorations allow (Fig. 1). These restorations commonly fail to take into account an episode of NE-SW to NNE-SSW-directed extension preserved in the sedimentary and seismic record of the neighbouring Otway Basin and which is intermediate in age between initial NW-SE directed rifting in the Bight Basin and later N-S rifting that affected all of the continental margin and produced most of the ocean floor fabrics, including all of the major oceanic fracture zones. The Coorong basement structure was briefly reactivated as a sinistral strike-slip fault during this phase of NE-SW extension, but failed to evolve into a continental transform fault as was the case farther east off the southwest coast of Tasmania. There, an analogous pre-existing north-south-trending basement structure identified as the Avoca-Sorell Shear Zone was optimally oriented for reactivation as a strike-slip faulting during north-south rifting (Gibson et al., 2011). This reactivated structure is continuous along strike with the Tasman Fracture Zone and shares many similarities with the Coorong Shear Zone, separating not only basement domains with opposing magnetic fabrics but sedimentary rift basins with differently oriented sets of normal faults. Together, these two basement structures constitute an important first order constraint on palaeogeographic reconstructions of the Australian and Antarctic margins, and serve as a critical test of future palaeogeographic reconstructions based on ocean floor fabrics and plate tectonic considerations.

Preserved within the Glenelg River Complex of SE Australia is a sequence of metamorphosed late Neoproterozoic-early Cambrian deep marine sediments intruded by mafic rocks ranging in composition from continental tholeiites to mid-ocean ridge basalts. This sequence originated during breakup of the Rodinia supercontinent and is locally host to lenses of variably sheared and serpentinised mantle-derived peridotite (Hummocks Serpentinite) representing the deepest exposed structural levels within the metamorphic complex. Direct tectonic emplacement of these rocks from mantle depths is considered unlikely and the ultramafites are interpreted here as fragments of sub-continental lithosphere originally exhumed at the seafloor during continental breakup through processes analogous to those that produced the hyper-extended continental margins of the North Atlantic. Subsequent to burial beneath marine sediments, the exhumed ultramafic rocks and their newly acquired sedimentary cover were deformed and tectonically dismembered during arc-continent collision accompanying the early Paleozoic Delamerian Orogeny, and transported to higher structural levels in the hangingwalls of west-directed thrust faults. Thrust-hosted metasedimentary rocks yield detrital zircon populations that constrain the age of mantle exhumation and attendant continental breakup to be no later than late Neoproterozoic-earliest Cambrian. A second extensional event commencing ca. 490 Ma overprints the Delamerian-age structures; it was accompanied by granite magmatism and low pressure-high temperature metamorphism but outside the zone of magmatic intrusion failed to erase the original, albeit modified, rift geometry. This geometry originally extended southward into formerly contiguous parts of the Ross Orogen in Antarctica where mafic-ultramafic rocks are similarly hosted by a deformed continental margin sequence.

Processed Stacked and Migrated SEG-Y seismic data and uninterpreted and interpreted section images for the Capricorn Deep Crustal Seismic Survey. This survey was a collaborative ANSIR project between AuScope, the Geological Survey of Western Australia and Geoscience Australia. Funding was through AuScope and the Western Australian Government royalites for Regions Exploration Incentive Scheme. The objectives of the survey were use deep seismic profiling to improve the understanding of the Western Australian continent by imaging the subsurface extent of Archean crust beneath the Capricorn Orogen and determining whether the Pilbara and Yilgarn Cratons are in direct contact or separated by one of more elements of Proterozoic crust. Raw data for this survey are available on request from clientservices@ga.gov.au

Crustal magnetism is predominantly caused by the abundantly distributed ferrimagnetic mineral magnetite which posses the property of spontaneous magnetisation. Such magnetisation is dependent on temperature, which if high enough, will cause magnetite minerals to lose their magnetic property of spontaneous magnetisation and become paramagnetic. This temperature, known as the Curie point isotherm, occurs at ~580oC for magnetite. As temperature increases with depth in the crust, the Curie point can be taken as the depth at which the crustal magnetism ceases to be recorded. Using power spectral analysis of aeromagnetic data, we have generated a Curie point depth map for the Olympic Dam region in South Australia, host to the world's largest iron oxide-copper-gold-uranium deposit. The map shows an approximately 55 km long by 35 km wide and 40 km deep hemispherical depression in the Curie point depth beneath Olympic Dam, from a background average of around 20 km. Olympic Dam is notable for its large iron and uranium content, and it is located in a region of unusually high heat flow (av. 73 mWm-2). With such high heat flow one would expect the Curie point depth to be shallow. The paradox at Olympic Dam is that the Curie point depth is deep, raising questions about the geothermal gradient, depth-integrated abundance of heat-producing elements, and the source of the iron. A possible solution to the paradox is to interpret the deep Curie point depth as a giant hydrothermal alteration zone, where the heat-producing elements have been scavenged and concentrated into the upper crust, along with the gold and copper. The iron must have a significant mantle source as it is measured throughout the full crustal column. As iron is electrically conductive, such an interpretation is supported by the high conductivity measured deep beneath Olympic Dam.

Various aspects of isostasy concept are intimately linked to estimation of the elastic thickness of lithosphere, amplitude of mantle-driven vertical surface motions, basin uplift and subsidence. Common assumptions about isostasy are not always justified by existing data. For example, refraction seismic data provide essential constraints to estimation of isostasy, but are rarely analysed in that respect. Average seismic velocity, which is an integral characteristic of the crust to any given depth, can be calculated from initial refraction velocity models of the crust. Geoscience Australia has 566 full crust models derived from the interpretation of such data in its database as of January 2012. Average velocity through velocity/density regression translates into average density of the crust, and then into crustal column weight to any given depth. If average velocity isolines become horizontal at some depth, this may be an indication of balanced mass distribution (i.e., isostasy) in the crust to that depth. For example, average velocity distribution calculated for a very deep Petrel sedimentary basin on the Australian NW Margin shows no sign of velocity isolines flattening with depth all the way down to at least 15 km below the deepest Moho. Similar estimates for the Mount Isa region lead to opposite conclusions with balancing of average seismic velocities achieved above the Moho. Here, we investigate average seismic velocity distribution for the whole Australian continent and its margins, uncertainties of its translation into estimates of isostasy, and the possible explanations for misbalances in isostatic equilibrium of the Australian crust.

The Antarctic Ice Sheet plays a fundamental role in influencing global climate, ocean circulation patterns and sea levels. Currently, significant research effort is being directed at understanding ice sheet dynamics, ice mass balance, ice sheet changes and the potential impact on, and magnitude of, global climate change. An important boundary condition parameter, critical for accurate modelling of ice sheet dynamics, is geothermal heat flux, the product of natural radiogenic heat generated within the earth and conducted to the earths surface. The total geothermal heat flux consists of a mantle heat component and a crustal component. Ice sheet modelling generally assume an 'average' crustal heat production value with the main variable in geothermal heat flux due to variation of the mantle contribution as a function of crustal thickness. The mantle contribution is typically estimated by global scale seismic tomography studies or other remote methods. While the mantle contribution to the geothermal heat flux is a necessary component, studies of ice sheet dynamics do not generally consider local heterogeneity of heat production within the crust, which can vary significantly from global averages. Heterogeneity of crustal heat production can contribute to significant local variation of geothermal heat flux and may provide crucial information necessary for understanding local ice sheet behaviour and modelling.

Australia's North West Margin (NWAM) is segmented into four discrete basins which have distinct rift and reactivation histories: Carnarvon, offshore Canning (Roebuck), Browse and Bonaparte. Bonaparte Basin incorporates Vulcan and Petrel sub-basins. The Bonaparte Basin stands out as an extensive sedimentary basin which has a geological history spanning almost the entire Phanerozoic, with up to 20 km of sediment accumulation in the centre. Browse Basin has considerably less thick sediment accumulation ? 12 km at maximum, which is still high for general hydrocarbon potential estimation. The structural architecture of the region is the product of a number of major tectonic events, including: ? Late Devonian northeast-southwest extension in the Petrel Sub-basin; ? Late Carboniferous northwest-southeast extension in the proto-Malita Graben, Browse Basin and proto-Vulcan Sub-basin; ? Late Triassic north-south compression; ? Early-Mid Jurassic development of major depocentres in the Exmouth, Barrow and Dampier sub-basins, and extension in the Browse Basin; ? Mid-Late Jurassic breakup in the Argo Abyssal Plain, onset of thermal sag in the Browse basin and extension in the Bonaparte Basin; ? Valanginian breakup in the Gascoyne and Cuvier abyssal plains, and onset of thermal sag in the Bonaparte Basin; and ? Late Miocene reactivation and flexural downwarp of the Timor Trough and Cartier Sub-basin Many of these events have involved processes of lower crustal extension and are strongly controlled by the pre-existing regional structural fabrics and basement character. Most reliable information on basement and deep crustal structure in the region comes from combined ocean-bottom seismograph (OBS) and deep reflection profiling along several regional transects (including Vulcan and Petrel transects in the Bonaparte Basin, and one transect in the Browse Basin). Average spacing between the OBSs of 30 km and shot spacing of 100 m with data recording to maximum offsets of 300 km enabled development of accurate crustal-scale seismic velocity models. Deep reflection data along the coincident profiles were recorded as part of Geoscience Australia?s regional grid of seismic lines. Consistent interpretation of several key horizons tied to petroleum exploration wells through the entire grid created the basis for co-interpretation of the OBS and deep reflection data supplemented by gravity field modelling.