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.

Absract for Indonesian Geophysics Conference (HAGI)

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.

Crustal structure associated with the northern Perth Basin is largely unknown. To help address this uncertainty, we constructed 3D gravity models. We adopt an approach whereby 'flawed' models are used to provide insight into basin thickness and crustal structure by highlighting areas where computed gravity does not fit measured gravity anomalies. The initial flawed models incorporate no arbitrary adjustments to geometry or density. In these models, two different Moho geometries are used, one based on Airy isostasy, the other incorporating an independently-computed Moho model for the Australian region. The resulting flawed models show that the crust of the northern Perth Basin is not in Airy isostatic equilibrium. A reasonable fit to long-wavelength observed gravity data is achieved for a model incorporating the Australia-wide Moho model. The deep Moho beneath the onshore Dandaragan Trough is interpreted to be the result of crustal-scale block rotation on the Darling Fault about a pivot point close to the Beagle Ridge. Flawed model results in the outboard Zeewyck Sub-basin suggest that the thickness of low-density sediment interpreted from seismic reflection data is underestimated. However, by making minimal adjustments to the model geometry, the gravity field over the Zeewyck Sub-basin can be explained by a deep and steep-sided depocentre associated with large variations in Moho depth over short distances. This geometry is suggestive of a transtensional formation mechanism. The flawed models do not explain the gravity field over the Turtle Dove Ridge, where computed gravity is less than observed. The results of our modelling highlight the benefits of considering 'flawed' gravity models that do not necessarily generate a good fit between observed and calculated gravity anomalies. These models help to more clearly identify areas with insufficient constraints and also provide impetus for re-assessing the interpretation of seismic reflection data.

The Onshore Energy Security Program was funded by the Australian Government from 2006 to 2011 to reduce risk in energy exploration. The program was delivered by Geoscience Australia, in collaboration with state and territory geological surveys, the National Research Facility for Earth Sounding (ANSIR) and AuScope. During this program approximately 6,500 line kilometres of deep crustal seismic reflection data were acquired and processed. The seismic images provide an understanding of the crustal architecture of sedimentary basins and their tectonic relationship to older basement terrains. Deep crust and upper mantle structures were also imaged and the Moho boundary could often be interpreted. The 2D seismic reflection data were acquired using three vibroseis trucks, with three 12 s variable frequency sweeps at each vibration point, usually with frequencies from 6 to 96 Hz. Correlated 20 s data were recorded, imaging to approximately 60 km depth. 300 geophone groups at 40 m intervals and 80 m source intervals provided 75 fold data. Data processing included imaging shallow sedimentary basins and also complex, deep, steeply dipping crystalline rock structures with high stacking velocities and out of plane energy. The seismic data, complemented by other geophysical and geological data, helped constrain and develop geological models. These models improved the understanding of crustal architecture in known hydrocarbon and metalliferous provinces as well as in frontier geological terrains.

The New Caledonia Trough is a bathymetric depression 200-300 km wide, 2300 km long, and 1.5-3.5 km deep between New Caledonia and New Zealand. In and adjacent to the trough, seismic stratigraphic units, tied to wells, include: Cretaceous rift sediments in faulted basins; Late Cretaceous to Eocene pelagic drape; and ~1.5 km thick Oligocene to Quaternary trough fill that was contemporaneous with Tonga-Kermadec subduction. A positive free-air gravity anomaly of 30 mGal is spatially correlated with the axis of the trough. We model the evolution of the New Caledonia Trough as a two-stage process: (i) trough formation in response to crustal thinning (Cretaceous and/or Eocene); and (ii) post-Eocene trough-fill sedimentation. To best fit gravity data, we find that the effective elastic thickness (Te) of the lithosphere was low (5-10 km) during Phase (i) trough formation and high (20-40 km) during Phase (ii) sedimentation, though we cannot rule out a fairly constant Te of 10 km. The inferred increase in Te with time is consistent with thermal relaxation after Cretaceous rifting, but such a model is not in accord with all seismic-stratigraphic interpretations. If most of the New Caledonia Trough topography was created during Eocene inception of Tonga-Kermadec subduction, then our results place important constraints on the associated lower-crustal detachment process and suggest that failure of the lithosphere did not allow elastic stresses to propagate regionally into the over-riding plate. We conclude that the gravity field places an important constraint on geodynamic models of Tonga-Kermadec subduction initiation.

Paleoproterozoic-earliest Mesoproterozoic sequences in the Mount Isa region of northern Australia preserve a 200 Myr record (1800-1600 Ma) of intracontinental rifting, culminating in crustal thinning, elevated heat flow and establishment of a North American Basin and Range-style crustal architecture in which basin evolution was linked at depth to bimodal magmatism, high temperature-low pressure metamorphism and the formation of extensional shear zones. This geological evolution and record is amenable to investigation through a combination of mine visits and outcrop geology, and is the principal purpose of this field guide. Rifting initiated in crystalline basement -1840 Ma old and produced three stacked sedimentary basins (1800-1750 Ma Leichhardt, 1730-1670 Ma Calvert and 1670-1575 Ma Isa superbasins) separated by major unconformities and in which depositional conditions progressively changed from fluviatile-lacustrine to fully marine. By 1685 Ma, a deep marine, turbidite-dominated basin existed in the east and basaltic magmas had evolved in composition from continental to oceanic tholeiites as the crust became increasingly thinned and attenuated. Except for an episode of minor deformation and basin inversion at c. 1640 Ma, sedimentation continued across the region until onset of the Isan Orogeny at 1600 Ma.

Stations on the Australian continent receive a rich mixture of ambient seismic noise from the surrounding oceans and the numerous small earthquakes in the earthquake belts to the north in Indonesia, and east in Tonga-Kermadec, as well as more distant source zones. The noise field at a seismic station contains information about the structure in the vicinity of the site, and this can be exploited by applying an autocorrelation procedure to the continuous records. By creating stacked autocorrelograms of the ground motion at a single station, information on crust properties can be extracted in the form of a signal that includes the crustal reflection response convolved with the autocorrelation of the combined effect of source excitation and the instrument response. After applying suitable high pass filtering the reflection component can be extracted to reveal the most prominent reflectors in the lower crust, which often correspond to the reflection at the Moho. Because the reflection signal is stacked from arrivals from a wide range of slownesses, the reflection response is somewhat diffuse, but still sufficient to provide useful constraints on the local crust beneath a seismic station. Continuous vertical component records from 223 stations (permanent and temporary) across the continent have been processed using autocorrelograms of running windows 6 hours long with subsequent stacking. A distinctive pulse with a time offset between 8 and 30 s from zero is found in the autocorrelation results, with frequency content between 1.5 and 4 Hz suggesting P-wave multiples trapped in the crust. Synthetic modelling, with control of multiple phases, shows that a local Ppmp phase can be recovered with the autocorrelation approach. This approach can be used for crustal property extraction using just vertical component records, and effective results can be obtained with temporary deployments of just a few months.

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.