Three-dimensional gravity models are a useful part of improving the geological understanding of large areas in various geological settings. Such models can assist seismic interpretation, particularly in areas of poor seismic coverage. In general, forward modelling and inversion are conducted until a single model is derived that fits well to the observed gravity field. However, the value of such a model is limited because it shows only one possible solution that depends on a fixed set of underlying assumptions. These underlying assumptions are not always clear to the interpreter and an arguably more useful approach is to prepare multiple models that test various scenarios under a range of different assumptions. The misfit between observed and calculated gravity for these various models helps to highlight flaws in the assumptions behind a particular choice of physical parameters or model geometry. Identifying these flaws helps to guide improvements in the geological understanding of the area. We present case studies for sedimentary basins off western Africa and western Australia. The flawed models have been used to rethink assumptions related to the geology, crustal structure and isostatic state associated with the basins, and also to identify areas where seismic interpretation might need to be revised. The result is a more reliable interpretation in which key uncertainties are more clearly evident.

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.

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.

In 2008, as part of the Australian Government's Onshore Energy Security Program, Geoscience Australia, acquired deep seismic reflection, wide-angle refraction, magnetotelluric (MT) and gravity data along a 250 km east-west transect that crosses several tectonic domain boundaries in the Gawler Craton and also the western boundary of the South Australian Heat Flow Anomaly (SAHFA). Geophysical datasets provide information on the crustal architecture and evolution of this part of the Archean-Proterozoic Gawler Craton. The wide-angle refraction and MT surveys were designed to supplement deep seismic reflection data, with velocity information for the upper crust, and electrical conductivity distribution from surface to the upper mantle. The seismic image of the crust from reflection data shows variable reflectivity along the line. The upper 2 s of data imaged nonreflective crust; the middle to lower part of the crust is more reflective, with strong, east-dipping reflections in the central part of the section.The 2D velocity model derived from wide-angle data shows velocity variations in the upper crust and can be constrained down to a depth of 12 km. The model consists of three layers overlying basement. The mid-crustal basement interpreted from the reflection data, at 6 km in depth in the western part of the transect and shallowing to 1 km depth in the east, is consistent with the velocity model derived from wide-angle and gravity data. MT modelling shows a relatively resistive deep crust across most of the transect, with more conductive crust at the western end, and near the centre. The enhanced conductivity in the central part of the profile is associated with a zone of high reflectivity in the seismic image. Joined interpretation of seismic data supplemented by MT, gravity and geological data improve geological understanding of this region.

Continental rifting and the separation of Australia from Antarctica commenced in the Middle-Late Jurassic and progressed from west to east through successive stages of crustal extension, basement-involved syn-rift faulting and thermal subsidence until the Cenozoic. Early syn-rift faults in the Bight Basin developed during NW-SE directed extension and strike mainly NE and E-W, parallel to reactivated basement structures of Paleoproterozoic or younger age in the adjacent Gawler craton. This extension was linked to reactivation of NW-striking basement faults that predetermined not only the point of breakup along the cratonic margin but the position and trend of a major intracontinental strike-slip shear zone along which much of the early displacement between Australia and Antarctica was accommodated. Following a switch to NNE-SSW extension in the Early Cretaceous, the locus of rifting shifted eastwards into the Otway Basin where basin evolution was increasingly influenced by transtensional displacements across reactivated north-south-striking terrane boundaries of Paleozoic age in the Delamerian-Ross and Lachlan Orogens. This transtensional regime persisted until 55 Ma when there was a change to north-south rifting with concomitant development of an ocean-continent transform boundary off western Tasmania and the South Tasman Rise. This boundary follows the trace of an older Paleozoic structure optimally oriented for reactivation as a strike-slip fault during the later stages of continental breakup and is one of two major basement structures for which Antarctic equivalents are readily identified. Some ocean floor fracture zones lie directly along strike from these reactivated basement structures, pointing to a link between basement reactivation and formation of the ocean floor fabrics. Together with the two basement structures, these fabrics serve as an important first order control on palaeogeographic reconstructions of the Australian and Antarctic conjugate margins.

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.

Interpretation of gravity and magnetic data in the vicinity of the deep seismic lines 10GA-CP1, 10GA-CP2 and 10GA-CP3, which cross the Capricorn Orogen of Western Australia. Interpretation techniques untaken include multiscale edge detection (worms), 2.5D forward modelling and unconstrained 3D inversion.