The eastern margin of the West Tasmania terrane

The West Tasmania terrane is well exposed and extensively studied along its western side. In contrast, the eastern half of the terrane is poorly exposed hindering both Cambrian and Devonian tectonic reconstructions of Tasmania and southeastern Australia. To better understand how the West Tasmania terrane was affected by early and mid-Paleozoic orogenesis, we review the geology of the eastern margin of the terrane and obtain new geochemical and geochronological constraints on outcrop and drillholes in central Tasmania. The Lake River Inlier is Cambrian in age and correlates with other synorogenic stratigraphy from this margin including the Blyths Creek Formation. Samples from the Hunterston drill core include a ­volcaniclastic interval that is <650 Ma and the associated dolostone contains several phosphatic layers with an early Cambrian U/Pb age. Further south, the Glenorchy drill hole near Hobart intersects a calc-alkaline volcaniclastic section with high Th which is chemically similar to Cambrian rocks from the Fossey Mountain Trough, western Tasmania. The chemistry and detrital zircon geochronology of samples from the basement in three drill holes south of Hobart suggest that the Permian, Triassic and Jurassic rocks that outcrop at the surface are underlain by Siluro-Devonian Eldon Group rather than the Proterozoic rocks as previously reported. These results indicate that the basement beneath southern Tasmania is much younger than previously reported. They support the tectonic models of Gray et al. (Citation2024) that the eastern margin of the West Tasmania terrane is composed of Paleozoic shallow water sedimentary rocks deposited on high-strain low metamorphic grade Neoproterozoic rocks such as those exposed in the Beaconsfield area. The terrane boundary formed by shortening of the early Paleozoic passive margin along the eastern edge of the West Tasmania terrane during the Tabberabberan Orogeny.