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.

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.

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.

Australia's southern magma-poor rifted margin extends for over 4000 km, from the structurally complex region south of the Naturaliste Plateau in the west, to the transform plate boundary adjacent to the South Tasman Rise in the east (Figure 1a). The margin contains a series of Middle Jurassic to Cenozoic basins-the Bight, Otway, Sorell and Bass basins, and smaller depocentres on the South Tasman Rise (Figure 1b). These basins, and the architecture of the margin, evolved through repeated episodes of extension and thermal subsidence leading up to, and following, the commencement of seafloor spreading between Australia and Antarctica. Break-up took place diachronously along the margin, commencing in the west at ~83 Ma and concluding in the east at ~ 34 Ma. The Australian southern margin exhibits a gross 3-fold segmentation that is the product of basement geology and a prolonged and diachronous extension and breakup history. The basins that developed on the margin reflect those influences. Analysis of the stratigraphic evolution of those basins provides valuable constraints on the nature and timing of breakup processes in the absence of drilling on distal parts of the margin.

To improve understanding of basins and basement structures, and of the energy, mineral and groundwater resource potential of northern Australia, deep crustal seismic surveys were conducted, totalling 2787 line-km, between June 2017 and November 2019 as a part of Exploring for the Future program. Reflection seismic profiles provide the highest fidelity imaging of crustal-scale subsurface architecture and therefore have become the industry standard for energy exploration, and their use in mineral and groundwater applications is growing. Here, we document the acquisition of composite deep reflection seismic profiles (20 sec, ~60 km depth). The focus is on imaging new terranes, and resolving frontier basin and crustal architecture. Seismic data were acquired stretching from the Beetaloo Sub-basin to the Mt Isa western succession in the Northern Territory and Queensland, as well as in the Kidson Sub-basin in Western Australia. Raw data for these surveys are available on request from clientservices@ga.gov.au, and processed data are publicly available from the Geoscience Australia website at https://www.ga.gov.au/about/projects/resources/seismic. <b>Citation:</b> Fomin, T., Holzschuh, J., Costelloe, R.D., and Henson, P., 2020. Deep northern Australian 2D seismic reflections surveys. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

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.

As a result of work undertaken by Geoscience Australia during the Australian Government's Energy Security Program (2006-2011), data-poor and little-known frontier basins around Australia's continental margin are receiving increased scientific and exploration attention. Marine and airborne geophysical surveys conducted by Geoscience Australia along the eastern, southern and southwest margins of the Australian continent have yielded new aeromagnetic data, relatively closely-spaced ship-track magnetic and gravity data, industry-standard seismic reflection data and swath bathymetry data. Geoscience Australia's strategy for integrated geophysical interpretation and modelling includes: depth-to-basement determination using spectral and analytic-signal techniques applied to magnetic data; enhancement of aeromagnetic data to facilitate onshore-offshore geological interpretation; use of 3D forward and stochastic inverse modelling of gravity data to guide seismic interpretation of sediment thickness and basement structure; 3D inverse modelling of magnetic and gravity data to constrain the physical properties of the crust; and use of levelled ship-track magnetic and gravity data integrated with onshore data for multi-scale edge-detection analysis to guide interpretations of basement structure. However, Geoscience Australia's efforts to understand frontier basins are not without challenges. Our work highlights the lack of constraints on sub-basin crustal structure that leads to significant ambiguity when determining maximum sediment thickness and basement architecture. These deficiencies indicate a need for seismic refraction surveys that focus on sub-basin crustal structure. Refraction surveys should be complemented by airborne magnetic and gravity surveys that link onshore and offshore areas, and regional 2D seismic reflection surveys designed for deep sedimentary basins.

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.

Absract for Indonesian Geophysics Conference (HAGI)