The thermal evolution of alpine magma-poor hyperextended rift margins : constraints from apatite U-Pb thermochronometry
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The Tasna and and Err-Platta areas of Eastern Switzerland expose one of the best-preserved fossil rifted margins in the world and have been critical to the development of tectonic models for magma-poor hyperextended rift margins. The fossil continental margins formed during Jurassic extension between the Adriatic and Europe plates, which resulted in the opening of the Alpine Tethys. The Cretaceous-Eocene closure of the Alpine Tethys and subsequent Alpine orogeny exhumed remnants of the Alpine Tethyan margins and emplaced them in the Alps where they are currently preserved. The exposed fossil margins provide an excellent opportunity to advance the understanding of the thermal evolution of magma-poor hyperextended margins, something relatively poorly constrained due to the inaccessibility of current deepwater margins and collision related thermal overprinting of common low-temperature thermochronometers at the Alpine Tethyan margins. However, more intermediate temperature thermochronometers may be used to bypass the Alpine thermal imprint and directly observe the thermal signature imparted on basement units of the Alpine Tethyan margins. Apatite U-Pb has a thermal sensitivity window of ~375-550 °C and is analytically feasible to measure using LA-ICP-MS due to technological and methodological advancements made over the past decade. Apatite and zircon U-Pb data collected in crustal transects across both Tasna and Err elucidates the thermal evolution of each margin and places new paleogeographic constraints on each unit. An abundance of Late Carboniferous to Early Permian zircon U-Pb ages from Tasna and Err indicate both units were derived from Late Variscan influenced domains. Predominantly Late Carboniferous and Triassic apatite U-Pb ages from the Err margin indicate that during rifting it remained in a primarily upper crustal position of the distal lower-plate margin of the Piemont-Liguria. In contrast a wide array of apatite U-Pb ages from Tasna, ranging from Early Permian to Middle Jurassic, indicate the primarily middle- and lower-crustal position of the unit until its’ exhumation during the Jurassic opening of the Piemont-Liguria. Based on both margins zircon and apatite U-Pb ages a new model detailing the paleogeographic and tectonic evolution of the Alpine Tethys is proposed to explain the data collected for this study.