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ItemThermochronology of Tasmania and the South Tasman Rise: implications for the dynamic evolution of a complex rifted continental block( 2013)The crustal architecture of previously adjacent basement terranes in SE Australia, Tasmania and northern Victoria Land, Antarctica is a legacy of late Neoproterozoic-Paleozoic subduction along the east Gondwana margin, highlighting the Cambro-Ordovician Delamerian-Ross orogeny. Structures in this ancient crust were reactivated during late Mesozoic-Cenozoic Gondwana breakup. Tasmania and the offshore South Tasman Rise (STR) lay in a crucial location at the centre of these continental fragments and potentially contain clues regarding the nature of Gondwana dispersal. This study reports results of a systematic thermochronological study that has been carried out on STR dredge samples and Tasmanian dolerites to uncover the history of the ancient subduction system and the post-break up thermal history recorded in their crust. Oceanic investigations (ODP, IODP, magnetic and seismic surveys and swath mapping, etc) have been conducted across the STR and the western transform boundary over the past few decades. Important results have been previously reported and form the foundation towards understanding the tectonic significance of this region. In particular, the interpreted seismic profiles along with basement dredge materials provide essential structural, stratigraphic and petrological controls as in situ investigations are not possible. Nevertheless, only limited geochronological data are available and the regional thermal history is still sparse. In this study a systematic low-temperature thermochronological study has been carried out, applying 40Ar/39Ar, AFT, ZHe and AHe dating techniques to the STR dredge samples to disclose the tectonic evolution of study area. Results suggest that the western and eastern terranes of the STR (W-STR and E-STR, respectively) evolved differently prior to east-Gondwana breakup. While the 40Ar/39Ar data from the W-STR suggest late Cambrian-early Ordovician ages (~495-460 Ma), results from the E-STR suggests a much younger early Carboniferous (Mississippian) age range (325-357 Ma). The data allow temporal comparisons to be made between existing 40Ar/39Ar and K-Ar datasets obtained from S-SE Australia and NVL, and results obtained here from the thermochronologically less studied offshore STR region. The data support reconstructions, which indicate that W-STR shared provenance with Wilson Terrane prior to the continental breakup, and furthermore, refine its paleo-position. Based on the age pattern as well as petrological evidence, it is proposed that it was most likely situated west of the Lanterman Fault Zone (LFZ) and extended further west to the western flank of the pop-up structure bounded by Wilson and Exiles Thrust. E-STR,40Ar/39Ar ages are synchronous with a phase of major granite emplacement and mineralization, which occurred in western Tasmania and are correlated with deformation post Tabberabberan Orogeny in Tasmania and the Victoria. Results from this study provide more solid time-temperature constraints for late Neoproterozoic-Cambrian subduction-related processes and the more recent evolution of the transform Tasman Fracture Zone during separation between Antarctica and Australia. The thermal history models suggest a strong correlation between rapid cooling and tectonic activity in the STR block. Since mid-Cretaceous time these can be summarized as follows: (1) onset of the opening of the Tasman Sea at ~80 Ma, (2) amalgamation of the W- and E-STR blocks and a shift in the relative motion between Australia and Antarctica and (3) final clearance of continental breakup and onset of the opening of the Tasman Gateway. This study is also reports results from the first application of apatite (U-Th-Sm)/He (AHe) thermochronometry to the Tasmanian region. The data not only provide further spatial and temperature constraints, but also examine the quality of AHe ages obtained from mafic lithologies with lower U and Th content than felsic rocks and yielding less age dispersion. Mid Jurassic (~175-180 Ma) dolerite is widely distributed across onshore Tasmania making such a study possible. The dolerite forms part of the Ferrar Group continental flood basalt (CFB) emplaced prior to eastern Gondwana breakup. Its spatial/temporal significance and chemical composition make it an ideal rock-type to aid in documenting the regional post-continental breakup history and to test the influence of different parameters such as -radiation damage and U and Th zonation on AHe age dispersion from a low eU perspective. Low-temperature thermal modelling reveals two distinct cooling episodes. (1) Mid-Cretaceous cooling, which involved km-scale denudation (~3-4 km) in response to continental extension prior to the actual seafloor spreading in the Tasman Sea. (2) Late Cretaceous-early Tertiary cooling, restricted to the west margin of Tasmania in response to transform margin tectonism to the west. In addition, no correlation could be found between AHe ages and potential factors influencing age dispersion such as radiation damage, grain size, U-Th zonation. However, zircons derived from Tasmanian dolerites having high eU suggest an effective upper dosage limit (0.2-0.31018/g) for obtaining meaningful ZHe ages when studying Tasmanian dolerites or possibly other similar mafic lithologies.