Unveiling the Gawler Craton: New insights from passive seismic deployments in South Australia

ASEG ACT Branch Tech Talk given on 26 September 2024 by Caroline Eakin. Abstract: Craton margins are often associated with (i) mineralisation, and (ii) heightened intra-plate seismicity. Both phenomena are closely tied to the underlying crustal/lithospheric structure. A good example of this is the eastern margin of the Gawler Craton in South Australia. Like most of Australia, however, this craton margin is largely hidden beneath a blanket of sediment, and so much of the deeper crustal/lithospheric architecture is poorly known. In recent years, several passive-seismic arrays have been deployed over the eastern Gawler region to investigate the structure and dynamics below. Several key findings have been made. Firstly, relocated earthquakes illuminate a left-lateral fault system that appears to define the previously poorly resolved eastern boundary of the Gawler Craton. The intra-plate seismicity appears to be fluid assisted with a trans-crustal fault system connected to the mantle below. Secondly, new constraints indicate that the crust beneath the Gawler Craton is distinct from that beneath Kati Thanda-Lake Eyre (which was previously obscured by sediment), but that the difference in crustal thickness is not as large as initially thought. Lastly, we find strong evidence for coherent fossilized anisotropy within the cratonic lithosphere. Considering the geological history, the pattern of anisotropy is consistent with a major and widespread deformational event 1.6 billion years ago, coeval with a period of extensive volcanism.