A new tectonic model for the Bass Basin

Abstract

The majority of the Bass Basin lies offshore between mainland Australia and Tasmania. Detailed mapping of the top of Eastern View Coal Measures and four shallower horizons within the non-prospective Torquay Group led to the identification of regional north-south and east-west transfer/wrench zones. Mapping of deeper horizons has confirmed the existence of these trends and has resulted in a new tectonic model for the Bass Basin. The basin was initiated during the Early Cretaceous as a northwest trending rift characterised by steeply dipping normal faults. Australia separated from Antarctica at the end of the Early Cretaceous, causing crustal extension within the Bass Basin. This led to the rotation of existing normal faults and development of north-south transfer faults with wrench components. A period of basin sag with limited basin extension followed. A second major phase of crustal extension was associated with the opening of the eastern margin of Australia. This event caused further rotation of earlier normal faults and development of east-west transfer faults with associated wrenching. Basin sag resumed during the remainder of the Late Cretaceous and Palaeocene. Early Eocene re-activation of two north-south transfer/wrench zones controlled the development of the prospective Cormorant and Pelican troughs. Late Miocene re-activation of north-south transfer/wrench zones is marked by 'pop up' structural features and igneous activity. Concurrently east-west transfer/wrench zones acted as hinge lines for uplift and erosion along the southern and northern parts of the basin.