Seismic structure of the Yilgarn Craton, Western Australia

Abstract

The deep crustal and upper mantle structure of the Yilgarn Craton is investigated in this study using receiver-function analysis of teleseismic earthquake records from temporary stations. Two lines of stations were deployed, the main transect ran between Perth and Kalgoorlie, and a second line of stations ran across the east Yilgarn Craton 200 km to the north of Kalgoorlie. The broadband instrumentation records high-fidelity waveform data allowing the signal from the near-receiver structure to be separated from the influence of the earthquake source. The nature of the crust and upper mantle structure under each station is determined from seismic-velocity models that match the observed receiver-function waveforms and the resulting coarse-scale transect provides new, independent controls on the structure of the lithosphere. Mechanisms for the evolution of the Yilgarn Craton, previously put forward to explain surface geological and geochemical observations, and seismic velocity structure from reflection and refraction studies, may be classified as favouring: (i) predominantly accretionary lithospheric evolution; (ii) mixed accretion and other influences; or (iii) no accretionary-style influence. Characteristics of the deep seismic structure enable the evolutionary mechanism to be inferred. From the teleseismic data, we find that the seismic Moho is sharp in character under stations in the middle of the proposed terranes and more gradational near the proposed terrane boundaries. The Moho dips gently eastward and the seismic velocity of the upper mantle increases moving from west to east across the whole craton. An anomalous region exists under the Southwest terrane that shows a thick high-velocity gradient zone at the base of the crust and a Moho dipping to the west. The nature of the lateral heterogeneity in structure and its correspondence with proposed terrane boundaries suggest that accretionary processes are significant in the evolution of the Yilgarn Craton.