Formation and evolution of Australian passive margins: implications for locating the boundary between continental and oceanic crust

Abstract

Australian passive margins display a large variety of styles, including narrow, wide, volcanic and non-volcanic margins. Their tectonic history has been complicated by tectonic reactivation and anomalous subsidence/uplift, widespread at various times during the post-rift phase. Defining the exact location of the boundary between continental and oceanic crust is of key importance to understanding the structure and evolution of continental passive margins. Here, we review the history of Australian passive margins and the location of the continent-ocean boundary, and we assess rift-related magmatism and anomalous post-rift subsidence based on recently acquired seismic-reflection data, available industry well data, and current tectonic models for margin evolution. Tectonic subsidence and strain-rate analysis of well data from the largely volcanic North West Shelf and the non-volcanic inner South Australian Bight margin result in relatively small lithospheric stretching factors (β <1.3). Crustal thinning estimates from seismic reflection and refraction data indicate extension factors of β ∼3-4 for the wide outer Bight margin. A lack of correlation between lithospheric strain rates and stretching factors with the occurrence or absence of syn-rift volcanics on the North West Shelf indicates that elevated mantle temperatures were the primary control for rift-volcanism, rather than high temperature gradients and small-scale convection created by large lateral variations in stretching or stretching rates. We find that phases of uplift and permanent, anomalous subsidence in the early post-rift phase are common, especially on the North West Shelf. This may be attributed to small-scale convection triggered by passive stretching leading to an unstable lithospheric configuration.