CONTINENTAL MARGIN TECTONICS AND THE EVOLUTION OF SOUTH EAST AUSTRALIA

Abstract

Following recent advances in geotectonics, a new approach can be applied to the study of the development of continental margins.

A continental margin begins to form as an older continental craton breaks up. The initial crack develops into a rift valley, which becomes filled with thick clastic and volcanic deposits. As separation continues a new mid-ocean ridge is formed, and the two plates begin to drift apart more rapidly. At this stage the structural evolution of the margins is virtually complete, and marine sediments are deposited unconformably across the fault troughs.

The continental fragments in the south west Pacific can be reassembled as a part of the ancient continent of Gondwanaland. Gondwanaland began to break up in the mid-Jurassic. A rift valley developed along the line of the present southern coast of Australia, through the Otway Basin. Two subsidiary tensional splays gave rise to the Elliston and Robe-Penola Troughs. Clastic sediments stripped from the cratonic highlands, and alkali basalts, occur in the rift grabens. Faulting and deposition continued throughout the Lower Cretaceous. About mid-Cretaceous a marine transgression from the west entered the subdividing rift valley. In the Eocene a new mid-ocean ridge formed and the Australian and Antarctic plates began to separate more rapidly. After this, quiet marine sedimentation occurred on the continental shelf and slope.

The Bass and Gippsland Basins began to develop in the Cretaceous as differential movement occurred between the main Australian plate and a partially detached Tasmanian sub-plate. In the Upper Cretaceous the Gippsland Basin became open towards the evolving Tasman Sea, as New Zealand detached. The Tasmanian sub-plate ceased fo exist after the Eocene, becoming firmly fixed to the Australian plate. Later readjustments have occurred giving rise to further limited movements, mainly in the Gippsland Basin.

The integration of detailed geological work and a regional tectonic analysis has been successfully applied to south east Australia and it is probable that a similar approach would yield fruitful results applied elsewhere.