Isostatic and decompensative correction of gravity data from New South Wales

Abstract

Gravity anomalies sourced from geological features of the upper crust are overprinted by the isostatic signal. Isostasy includes the familiar Airy-Heiskanen crustal-root response to topography, and an additional flexure of the base of the crust in response to geological loads. We have applied a two-step process to isolate the geological signal in gravity data from New South Wales. An isostatic residual (IR) anomaly map was generated by removing from the Bouguer anomaly the gravity arising from a modelled crustal root of topography, assuming Airy isostasy and purely local compensation. Reduction of the dynamic range in the IR map accentuates both local geological features and major boundaries between terranes. Discrimination of crustal features is particularly improved along the east coast and Great Dividing Range. Decompensative correction comprises removal of the gravity contribution of the crustal compensation for the geological load from the IR anomaly, to yield the decompensative anomaly (DA). We have approximated this correction by continuing the IR anomaly upward by 30 km to filter out the upper- and mid- crust contribution. Geological features are further highlighted by the DA map, especially in the south-west of the state below Murray Basin cover, although the signal of major features which penetrate the crust may be suppressed. Judicious use of either the IR or DA anomaly maps will aid interpretation of the geology of New South Wales.