Low-temperature thermochronology of the northern Thomson Orogen: Implications for exhumation of basement rocks in NE Australia

Abstract

The Tasmanides of eastern Australia record much of the Phanerozoic tectonic development of the retreating Pacific-Australia plate boundary and are an oft-cited example of an orogen that has undergone “tectonic mode switching.” To begin to constrain the timing of exhumation of basement rocks that are now exposed in portions of the NE Tasmanides, we measured apatite and zircon (U–Th)/He ages from the Thomson Orogen and overlying Paleozoic strata in the back-arc of the New England Orogen in NE Australia. Zircon (U–Th)/He ages from basement samples (including those recovered from boreholes at depths of up to 1.1km) are characterized by large inter- and intra-sample variability and range from approximately 180Ma (Early Jurassic) to 375Ma (Late Devonian). (U–Th)/He zircon ages from several individual samples are negatively correlated with effective uranium (eU), a pattern that is also true of the dataset as a whole, suggesting that variations in U and Th zoning and radiation damage are partially responsible for the age variability. The oldest zircon (U–Th)/He cooling ages coincide with the formation of regionally extensive Late Devonian–early Carboniferous back-arc basins, suggesting that Late Devonian extension played a significant role in exhumation of parts of the northern Thomson Orogen. Apatite (U–Th)/He ages from a basement sample and a late Permian sandstone in the overlying Bowen Basin, which are also marked by intra-sample variability and age–eU correlations, span from the Early Cretaceous through Oligocene, in general agreement with previous apatite fission track data. In conjunction with observations of key geologic relationships and prior K–Ar and 40Ar/39Ar data, our results suggest four overall phases in the thermal history of the northern Thomson Orogen: (1) Cambrian–early Silurian metamorphism during the Delamerian and Benambran Orogenies; (2) protracted cooling during the Late Devonian through mid-Permian that likely resulted from extensional exhumation; (3) Permian–Triassic reheating during burial beneath thick sedimentary basins; and (4) Cretaceous and Paleogene cooling during uplift and erosion.