Palaeozoic suturing of eastern and western Tasmania in the west Tamar region: Implications for the tectonic evolution of southeast Australia

Abstract

A new tectonic model for Tasmania incorporates subduction at the boundary between eastern and western Tasmania. This model integrates thin‐ and thick‐skinned tectonics, providing a mechanism for emplacement of allochthonous elements on to both eastern and western Tasmania as well as rapid burial, metamorphism and exhumation of high‐pressure metamorphic rocks. The west Tamar region in northern Tasmania lies at the boundary between eastern and western Tasmania. Here, rocks in the Port Sorell Formation were metamorphosed at high pressures (700–1400 MPa) and temperatures (400–500°C), indicating subduction to depths of up to 30 km. The eastern boundary of the Port Sorell Formation with mafic–ultramafic rocks of the Andersons Creek Ultramafic Complex is hidden beneath allochthonous ?Mesoproterozoic turbidites of the Badger Head Group. At depth, this boundary coincides with the inferred boundary between eastern and western Tasmania, imaged in seismic data as a series of east‐dipping reflections. The Andersons Creek Ultramafic Complex was previously thought of as allochthonous, based mainly on associations with other mafic–ultramafic complexes in western Tasmania. However, the base of the Andersons Creek Ultramafic Complex is not exposed and, given its position east of the boundary with western Tasmania, it is equally likely that it represents the exposed western edge of autochthonous eastern Tasmanian basement. A thin sliver of faulted and metamorphosed rock, including amphibolites, partially separates the Badger Head Group from the Andersons Creek Ultramafic Complex. Mafic rocks in this package match geochemically mafic rocks in the Port Sorell Formation. This match is consistent with two structural events in the Badger Head Group showing tectonic transport of the group from the west during Cambrian Delamerian orogenesis. Rather than being subducted, emplacement of the Badger Head Group onto the Andersons Creek Ultramafic Complex indicates accretion of the Badger Head Group onto eastern Tasmania. Subsequent folding and thrusting in the west Tamar region also accompanied Devonian Tabberabberan orogenesis. Reversal from northeast to southwest tectonic vergence saw imbricate thrusting of Proterozoic and Palaeozoic strata, possibly coinciding with reactivation of the suture separating eastern and western Tasmania.

SHRIMP U–Pb detrital zircon ages from Proterozoic and Early Palaeozoic sandstones and their bearing on the early geological evolution of Tasmania

“…Two other Proterozoic sandstones that were sampled cannot be readily correlated with those above. The Port Sorell Formation is a complex, faulted association of sedimentary and volcanic rocks in central‐north Tasmania, in faulted contact to the east with the Badger Head Group (Gee & Legge 1979; Reed et al . 2002).…”

Linking mainland Australia and Tasmania using ambient seismic noise tomography: Implications for the tectonic evolution of the east Gondwana margin

“…Palaeomagnetic evidence in the form of magnetic remanence (Li et al, 1997) has also been used. Geochemical investigations include apatite fission track (O'Sullivan et al, 2000) and detrital zircon age analysis (Berry et al, 2001), U-Th-Pb monazite dating (Berry et al, 2005(Berry et al, , 2007(Berry et al, , 2008) and a variety of other geochemical measurements (Reed et al, 2002;Clemens and Benn, 2010). Only a few interpretations rely on an integrated approach whereby multiple datasets are used to provide a tighter set of constraints (Cayley et al, 2002;Direen and Crawford, 2003;Teasdale et al, 2003).…”

Lithospheric structure of Tasmania from a novel form of teleseismic tomography

“…Second, the so‐called Tamar Fracture System (TFS), previously regarded as the surface expression of crustal‐scale suturing, may be a thin skinned feature [ Leaman , 1994; Reed et al , 2002] unrelated to the formation of a major lithospheric boundary. Finally, there are several competing plate tectonic models for the early mid‐Phanerozoic evolution of Tasmania [ Corbett et al , 1972; Crawford and Berry , 1992; Reed et al , 2002], which involve various combinations of subduction, obduction, deposition, extension and volcanism. The detailed 3‐D seismic images of the lithosphere beneath northern Tasmania presented in this paper have the potential to make valuable contributions toward resolving each of these issues.…”