Gravity anomalies, crustal structure and isostasy associated with the Proterozoic Capricorn Orogen, Western Australia

Present knowledge of the Proterozoic Capricorn Orogen in Western Australia is largely based on surface geological studies. Geophysical studies of the crust of the orogen are limited to a few regional seismic refraction profiles and regional potential field interpretations. The aim of this paper is to present a simple, gravity-constrained crustal model for the Capricorn Orogen. Anomalous lows immediately north of the Yilgarn Craton are prominent features of the Capricorn Orogen gravity field and geoid. These anomalous lows do not correlate with surface geology or elevated topography. Their character suggests that they reflect thickened crust in the core of the Capricorn Orogen, preserved in a state of mechanical equilibrium for about two billion years. The location of the anomalous lows and associated crustal thickening, south of the inferred location of the Pilbara-Yilgarn suture, suggest that the Pilbara Craton was subducted southwards beneath the previously combined Yilgarn Craton and Southern Gascoyne Complex. In a simple two-dimensional crustal model, features such as the Bangemall, Bryah and Yerrida Basins are assumed to be dominantly supported by the rigid lithosphere of this region (effective elastic thickness 50-100km). The Yerrida Basin and adjacent Marymia Inlier are modelled as north-dipping features, the Bangemall Basin as a southward-deepening basin and the lower boundary of the Gascoyne Complex is best modelled as a north-dipping contact. At the northern margin of the orogen, the Sylvania Inlier appears to lie at shallow depths (∼1km) beneath the eastern Ashburton Basin. The rocks of the south-eastern Hamersley Province reach a maximum thickness of about 7km and thin toward the north. A notable feature of the model is that it does not maintain isostatic equilibrium. The prolonged erosion of this region and the prominent upper-crustal density anomalies mean that it is difficult to develop gravity models that maintain isostatic balance. A simple isostatic analysis suggests that the present-day topography of the Capricorn Orogen is over-compensated. This over-compensation can be explained by resistance of the rigid lithosphere of Western Australia to rebound in response to Mesozoic or Cainozoic erosion. Mantle flow acting to pull the crust downward from its equilibrium position could also lead to the apparent lack of isostatic equilibrium.