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.

Constraints on the morphology of the Moho are essential to establish reliable models of the subsurface and infer the evolution of the Australian crust. Reliable information on crustal thickness variations is important for thermal modelling and structural mapping, for both energy and mineral system studies. Here, we combine information from both passive seismic deployments and full-crustal reflection seismic profiling to produce a new representation of the character of the Moho in northern Australia. Data coverage has been dramatically improved by investments, under the Exploring for the Future program, in new deployments of passive seismic instrumentation and expansion of the network of reflection seismic profiles in the South Nicholson and Barkly regions. Using a new approach to combining results from different classes of seismic analysis, different spatial sampling associated with the various types of data have been taken into account. The resulting Moho surface reveals small-scale features not seen in previous models. New data reveal that some Moho discontinuities are clearly associated with known structures such as the Willowra Suture. Similar ~100 km wavelength undulations are visible in areas under cover that may indicate the presence of unknown major structures. Significant base metal mineral deposits appear to be localised along the edges of thicker crustal block. <b>Citation:</b> Gorbatov, A., Medlin, A., Kennett, B.L.N., Doublier, M.P., Czarnota, K., Fomin, T. and Henson, P., 2020. Moho variations in northern Australia. 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.

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.

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.

New compilations of levelled marine and onshore gravity and magnetic data are facilitating structural and geological interpretations of the offshore northern Perth Basin. Multi-scale edge detection helps the mapping of structural trends within the basin and complements interpretations based on seismic reflection data. Together with edge detection, magnetic source polygons determined from tilt angle aid in extrapolating exposed basement under sedimentary basins and, therefore, assist in the mapping of basement terranes. Three-dimensional gravity modelling of crustal structure indicates deeper Moho beneath the onshore and inboard parts of the Perth Basin and that crustal thinning is pronounced only under the outboard parts of the basin (Zeewcyk Sub-basin).

The Australian earth sciences have been recognized as part of Australia's key scientific capability to understand the structure and evolution of the Australian continent. Over the last five years, Geoscience Australia, through its Onshore Energy Security Program (OESP), in conjunction with the State and Territory Geological Surveys, the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC), the AuScope Earth Imaging (under Australian Government's National Collaborative Research Infrastructure Strategy) and the Australian National Seismic Imaging Resource has acquired over 6,500 line kilometres of new world-class seismic reflection data and over 3,700 kilometres of magnetotelluric (MT) data from more than 640 stations. Geoscience Australia acquires high quality deep seismic reflection data in most of Australia's economically significant geological regions, by collecting at least one deep seismic reflection traverse across the key structures. The acquisition parameters for regional vibroseis surveys have been selected from broad experience in hard rock environments and experimental programs prior to seismic acquisition. Three IVI HEMI-50 or 60 peak force vibrators are used with three 12 s varisweeps with 80 m between vibration points, 40 m group interval, and 20 s listening time to image down to approximately 60 km in depth. Geoscience Australia continues to provide expertise in deep crustal seismic reflection processing and mineral province interpretation to collaborative research programmes which focus on understanding the 3D crustal architecture and mineral systems within `hard-rock' mineral provinces. As part of this program , broadband and long period MT data have been acquired along 12 deep seismic reflection transects across potential mineral provinces and frontier sedimentary basins.

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.

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.

<div><strong>Output Type: </strong>Exploring for the Future Extended Abstract</div><div><br></div><div><strong>Short Abstract: </strong>The continental crust directly hosts or underlies almost all mineral resources on which society depends. Despite its obvious importance its structure is poorly characterised. In particular, its density is surprisingly poorly constrained because it is difficult to directly image from the surface. Here we collate a global database of crustal thickness and velocity constraints. In combination with a compilation of published laboratory experimental constraints on seismic velocity at a range of pressures, we develop a scheme with which to convert seismic velocities into density as a function of pressure and temperature. We apply this approach to the Australian crystalline basement. We find that the Australian crust is highly heterogeneous, ranging in bulk density from 2.7—3.0 g cm-3. Finally, we explore the utility of our database for testing hypotheses about the location and endowment of mineral resources using porphyry copper deposits as an example. Our results provide an improved framework with which to explore the subsidence and thermal evolution of sedimentary basins, as well as probing relationships between deposit types and crustal architecture.</div><div><br></div><div><strong>Citation: </strong>Stephenson, S.N., Hoggard, M.J., Haynes, M.W., Czarnota, K. & Hejrani, B., 2024. Constraints on continental crustal thickness and density structure. In: Czarnota, K. (ed.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, https://doi.org/10.26186/149336</div>