Evidence for crustal extension and inversion in eastern Tasmania, Australia, during the Neoproterozoic and Early Palaeozoic

The island of Tasmania in southeast Australia consists of a number of strat otectonic elements. The relationships between these elements are largely ob scured by younger cover of the Tasmania Basin, which contains extensive dol erite sills that limit the ability of potential field techniques to map bas ement. Therefore the development of a robust tectonic model for Tasmania ha s been inhibited. To assist in the development of a tectonic model, a deep seismic reflection program undertaken offshore around the entire island was designed to map the large-scale structures of Tasmania at depth. The airgu n seismic energy was also recorded at a number of seismographs deployed acr oss the island, allowing low resolution 3D tomographic imaging. Short refle ction profiles were recorded onshore across structures which could nor be i maged by the offshore profiling. This paper focuses on eastern Tasmania. In the seismic sections, the Proterozoic basement in the southeast is mostly featureless, except for large rotated blocks with weakly reflective boundar y faults, indicating extension of the Tyennan Element by block faulting. Th e deposition of the sedimentary succession of the Adamsfield-Jubilee Elemen t was related to this extensional event. In the northeast, a reflective low er crust is interpreted to represent thrust slices of previously highly ext ended continental crust and possibly fragments of oceanic crust. The Early Palaeozoic sedimentary succession of the Northeast Tasmania Element formed across the inverted margin. The apparently complex geology of eastern Tasma nia therefore fits into an extensional model where continental extension ev entually led to the formation of very thin continental crust and possibly o ceanic crust to the east. The extension was probably related to Late Neopro terozoic extension recorded elsewhere in Australia. The region was subseque ntly shortened, probably in a northeast-southwest direction, with most shor tening accommodated in the seismically reflective, probably oceanic part of the crust, and little or no shortening in the block-faulted, and extended continental crust. (C) 2000 Elsevier Science B.V. All rights reserved.