Seismic structure of the crust and uppermost mantle of the Capricorn and Paterson Orogens and adjacent cratons, Western Australia, from passive seismic transects

Abstract

The seismic structure of the Proterozoic Capricorn and Paterson Orogens and adjacent Archaean Yilgarn and Pilbara Cratons, Western Australia, is determined using a passive seismic approach. We use recordings of distant earthquakes made along two transects of 3-component broadband stations. The stations were deployed for approximately 1. year (mid 2006-2007) during which time 70 earthquakes were recorded at each station with a suitable signal to noise ratio for receiver function analysis and hence the S wavespeed profiles of the crust and uppermost mantle beneath each recording station are determined. We investigate the deep crustal constraints on terrane boundary locations, the patterns of seismic discontinuities in the crust, and the variations in the depth and character of the Moho. This broad-scale information regarding the present day crustal architecture, and hence the crustal evolution, of Western Australia, complements previous surface geological and other geophysical studies. Western Australia is an exceptionally large, well preserved region of ancient crust and hence this work also adds to the body of knowledge regarding Proterozoic orogenic processes in general. The new passive seismic work shows a region of double crust or upper mantle discontinuities beneath the Glenburgh Terrane, adjacent to the northwest Yilgarn Craton. The upper crust of the orogens is always layered whereas the cratons have a simple upper crust. Both the Capricorn and Paterson Orogens are characterised by deeper Moho discontinuities with a lesser wavespeed contrast than the very sharp discontinuity observed beneath the adjacent Pilbara and Yilgarn Cratons. This is consistent with the weaker orogenic crust of the Capricorn and Paterson Orogens accommodating most of the horizontal deformation during assembly and reworking of the West Australian Craton while the Pilbara and Yilgarn Cratons acted as rigid crustal blocks.