Upper-mantle shear velocity beneath eastern Australia from inversion of waveforms from SKIPPY portable arrays

Abstract

In 1993, the Research School of Earth Sciences of the Australian National University commenced a nationwide seismometry project, SKIPPY, for a comprehensive study of the upper mantle beneath the Australian continent. We applied a waveform inversion technique to broad-band data recorded while the SKIPPY portable arrays were positioned in eastern Australia in order to construct a 3-D model of shear velocity in the upper mantle and transition zone beneath eastern Australia and the adjacent oceanic regions. The SKIPPY data were augmented by data from the permanent seismological observatories in the region. The first step of the waveform inversion used involved the matching of the waveforms of fundamental- and higher-mode Rayleigh waves with waveforms synthesized from radially stratified models; in the second stage the linear constraints on radial variations in shear velocity were combined in a tomographic inversion for aspherical variations in shear velocity. The preferred model reduces the data variance by 90 per cent. Owing to the dense data coverage, structural features with dimensions larger than 250 km laterally and 50 km vertically are resolved. The major structural features inferred from the tomographic images are (1) a substantial increase in the thickness of the high-velocity ‘lid’ from the oceanic region to the Phanerozoic continental and Proterozoic continental regions, (2) a pronounced low-velocity zone centred at 140 km depth beneath the part of eastern Australia often referred to as the Tasman fold belt, which confirms previous inferences from Rayleigh-wave dispersion curves, (3) a prominent zone of low wave speeds beneath the eastern margin of the continent that coincides with locations of Cenozoic volcanism and regions of enhanced heat flow, (4) deep low-wave-speed anomalies beneath both the Coral and Tasman seas, and (5) localized high-wave-speed perturbations in the mantle transition zone (410-660 km depth) beneath eastern Australia, in particular beneath the New England fold belt and the Mount Isa block. From the images we infer that the eastern boundary of the Precambrian shields does not coincide with a sharp seismic contrast, unless this boundary is located further to the east than is commonly assumed. The structure of the upper mantle and transition zone beneath eastern Australia as inferred from the images is more complex than that beneath the Precambrian cratons of central and western Australia, which could be due to subduction beneath the eastern continental margin prior to opening of the Tasman sea. If correct, this interpretation would imply that part of the upper mantle has moved along laterally with the lithosphere during the relatively fast northward motion of the plane.