Subsidence History of Australia's Southern Margin: Constraints on Basin Models

Abstract

The subsidence history of Australia's southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation. The first phase is characterized by rapid tectonic subsidence (about 50 m/m.y.) which lasted for about 25-30 m.y. during the Early Cretaceous. The period of rapid subsidence represents the rifting phase of margin development. This early stage is correlated with associated rifting phenomena: faulting and extension of the brittle upper crust, formation of grabens and half-grabens, and the rapid accumulation of volcanogenic detritus (Otway Group). The amount of extension (20-50%) is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin The model includes uniform extension and thinning of the continental lithosphere, a prolonged extensional event followed by simple cooling, horizontal and vertical conduction of heat, and radioactive heat production by the crust. The rift phase may have begun earlier, but the most rapid and widespread subsidence occurred during the Early Cretaceous. The end of rapid and widespread subsidence occurred during the Early Cretaceous. The end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly (about 90 Ma) adjacent to the continent-ocean boundary (COB). The second stage of margin development is the post-rift (or drift) phase, a period of much slower rates of subsidence and relative tectonic quiescence. Subsidence during the post-rift stage is modeled as cooling and contraction of extended continental lithosphere. Estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the COB. Estimates of extension from the geometry of basement blocks near the COB do not agree with the analysis from subsidence modeling. Upper crustal extension seems to record only a small proportion of total lithospheric extension. Alternative or supplementary mechanisms of extension are explored, with emphasis on implications from a model that includes localized shear along a lithospheric detachment system.