Resolving basement crustal architecture and extensional tectonics using 3D inversion modelling of airborne gravity data in the Otway Basin region, Victoria.

Seismic reflection surveys are effective for imaging the crustal architecture of sedimentary basins including the top-of-basement, which is an important interface for understanding rift tectonics, structural controls imposed on basin fill, and basin system prospectivity. However, seismic data can be less effective where the basin is thick, logistics limit acquisition, or where vintage seismic are noisy. 3D inversion modelling of airborne gravity gradiometry data can be an effective complement or alternative. This study provides a logical, stepwise approach that tackles this problem by focussing on separating superimposed geophysical anomalies. We use the Otway Basin in southeast Australia as a 'laboratory' to demonstrate a systematic approach for understanding basin and basement geometry, using airborne gravity data in conjunction with other geoscientific datasets. The forward response of a starting model, built using only existing seismic interpretations, was a poor fit to the observed gravity data because gravity provides the net response of all rocks down into the mantle. Therefore, in areas where geology in the deep crust is complex - like the Otway Basin – the entire crust must be modelled to account for gravity signatures caused by basement geological heterogeneity, before the smaller gravity variations attributable to the sedimentary basin fill can be discriminated. This process targets anomalies in the residual gravity, speculates on their likely causes, builds a geological structure into the model to explain them, and then considers the next unexplained miss-match in the residual. Forward modelling and inversion results suggest gravity anomalies are caused not only by geological diversity in the basement, but also by Moho shape. Accounting for the gravity response of these deeper structures not only improves understanding of basement structures, but also improves the recovered geometry of the top-of-basement interface. Specifically, inversion results suggest the Portland Trough in the southwestern region is significantly deeper than previously thought. • Airborne gravity gradiometry data was used to resolve the 3D geological architecture of the Otway Basin in SE Australia. • A logical, stepwise approach focuses on separating superimposed gravity anomalies using 3D forward and inversion modelling. • Full crustal modelling was necessary to account for gravity signatures caused by the Moho, basement geology, and basin fill. • Inversion results suggest the Portland Trough in the western Otway Basin is significantly deeper than previously thought. [ABSTRACT FROM AUTHOR]