Structure of the Australian Lithosphere

Abstract:

Cratons (ancient continental cores) are thick, cold, depleted, and rheologically strong. It is believed their seismic profiles are almost entirely thermally controlled, without rapid velocity decreases at lithosphere-asthenosphere boundary (LAB) depths. Yet globally there are observations of pronounced negative velocity gradients in cratons at depths of 60-90 km, shallower than the estimated LAB for these regions. Such velocity gradients are termed midlithospheric discontinuities (MLDs). In 2010, Ford et al. used Sp and Ps receiver functions to image the presence of MLDs in cratonic Australian lithosphere.

In the decade since, a number of permanent, additional stations have been added to the Australian National Seismograph Network (AU). We have carried out an updated Sp and Ps receiver function analysis of Australia using 34 stations, an increase of 20 since 2010. Most results are in agreement with the earlier work of Ford et al., with eastern Phanerozoic stations having thin lithosphere and obvious negative phases associated with the LAB; cratonic stations have much thicker lithosphere, generally with MLD phases and a weak or absent negative LAB phase. At some stations on the cratonic western margin of the continent we observe LAB phases once thought to be MLDs. At many cratonic stations we observed multiple discrete negative phases within the lithosphere, suggesting complex lithospheric stratification.

The origin of MLDs has been debated, although several studies invoke an explanation involving the presence of hydrous minerals formed from metasomatic processes. Mantle xenoliths from kimberlite pipes in cratonic Australia show evidence of metasomatism, supporting this hypothesis. Other proposed mechanisms of MLDs (e.g., thermal effects, anisotropy, etc.) do not appear to adequately explain our observations. The oldest of the mantle xenoliths is roughly 2.025 Ga, with other evidence suggesting subduction as early as 2.7 Ga outboard of the Yilgarn craton (Graham et al., 2004; Choi et al., 2020). If MLDs are evidence of metasomatic processes within the lithosphere from subduction, it suggests that the MLDs (and subduction) in Australia are at least as old as ~2 Ga and may be as old as 2.7 Ga.