Linking the upper crust to the upper mantle: comparison of teleseismic tomography with long-wavelength features of the gravity and magnetic fields of southeastern Australia

Abstract

Acquisition of teleseismic data in south-western New South Wales during 2007 formed the latest stage of a rolling deployment of seismometers over south-eastern Australia, and allowed a revised tomographic model to be constructed for the lithospheric mantle under Victoria and southern NSW. Our aim here is to link the observed distribution of upper-mantle P-wave velocity to the major geological features of the upper crust, which here comprise terranes of the Delamerian and Lachlan orogens. We have extended the definition of the boundaries of these terranes under cover by the use of the tilt-filter of total magnetic intensity, which provides an image with detailed resolution for sources at depths down to ~5 km. We proceed to infer the distribution of deeper sources in the middle and lower crust by two approaches to the use of potential-field images: we exploit the relationship between wavenumber and source depth, through the application of a 20-km low-pass filter to the total magnetic intensity grid; and we take advantage of the lower sensitivity of gravity anomalies to depth of source, compared to magnetic anomalies with dipolar sources, by defining broad features in the isostatic gravity grid. Our interpretation of the low-pass magnetic and isostatic gravity imagery confirms the relationship between high mantle velocity and the Proterozoic Delamerian Orogen, and indicates that a salient of high mantle velocity under the Palaeozoic Stawell Zone results from an underthrust wedge of Delamerian basement. High mantle velocity under the Palaeozoic Wagga-Omeo Zone may be a result of lithospheric thickening that is a corollary of mid- to lower crustal thrust faulting indicated by the potential field data. Low mantle velocity under part of the Melbourne Zone may result from thermochemical resetting of its Proterozoic microcontinental basement by the thermal event responsible for the extensive Cainozoic volcanism in western Victoria; low mantle velocity under the Hay-Booligal Zone, which also appears to be anomalous, may similarly be related to a heat pulse that engendered the swarm of diatremes that is distributed across the zone.