Basement architecture and formation of an ocean-continent transform boundary off western Tasmania: insights from aeromagnetic and seismic reflection data collected from a frontier petroleum province

Abstract

Basement architecture off western Tasmania is a legacy of late Neoproterozoic-Cambrian subduction-related processes, ocean basin closure and multiple accretionary events, culminating in formation of the Delamerian-Ross and western Lachlan Orogens. Structures associated with these fold belts were subsequently reactivated during late Mesozoic-Cenozoic Gondwana breakup and the separation of Australia from Antarctica, strongly influencing the pattern and geometry of offshore rifting, including formation of an ocean-continent transform boundary off western Tasmania. Seismic reflection profiles combined with recently acquired high resolution aeromagnetic data permit this boundary and its associated reactivated basement structures (Avoca-Sorell Fault system) to be mapped in greater detail than has hitherto been possible and point to a transform margin dominated by steeply outward-dipping structures and deep sedimentary basins similar to other transform margin ocean-continent boundaries. These include the highly prospective Côte d?Ivore-Ghana region off the west African coast. Basement highs and rotated pre-rift crustal blocks adjacent to the west Tasmanian transform margin incorporate significant volumes of granite as well as a lower crust of probable Mesoproterozoic age that is locally juxtaposed against lower Paleozoic sequences intruded and/or floored by basaltic and ultramafic material. Aeromagnetic anomalies sourced from these basement rocks change orientation from NW- to NE-trending across the Avoca-Sorell Fault system and can be traced laterally into regions of known onshore basement geology, highlighting both the tectonic significance of this structure and its origins during lower Paleozoic deformation accompanying the Delamerian-Ross Orogeny.