TIMING OF UPLIFT OF AUSTROALPINE AND PENNINIC UNITS OF SE SWITZERLAND: IMPLICATIONS FROM ZIRCON (U-TH)/HE THERMOCHRONOLOGY

The late Cretaceous to Miocene Alpine orogen formed during the closure of the Alpine Tethys and subsequent continental collision of Adria and Europe. Locally Alpine tectonic processes are characterized by multiple deformation events, and despite more than a century of study the structural and thermal evolution of the orogen remains controversial. While the structural evolution of the Graubünden region of SE Switzerland and adjacent Italy has been studied in detail, there are relatively few studies of the low-temperature thermal evolution of the area. The application of the zircon (U-Th)/He method to the region provides an opportunity to comprehensively understand the temporal and thermal evolution of both Austroalpine units (structurally highest in the orogen) and Penninic units (derived from the European continental margin and Alpine Tethys). Analyzed samples are from: (1) a Middle Austroalpine unit (Campo), (2) Lower Austroalpine units (Bernina and Err), and (3) a Middle Penninic unit of the Engadine window (Tasna). The zircon (U-Th)/He results range from the late Paleocene to Oligocene. Analyses from the Middle Austroalpine Campo unit cluster in the Eocene, indicating exhumation during active convergence. The Lower Austroalpine unit of Err exhibits a positive correlation between grain size and age, indicative of an exhumed helium partial retention zone and are only partially reset. Samples of the Bernina unit west of the Engadine line are relatively elevation dependent and range from the late Paleocene to late Eocene, perhaps indicating accretion of the unit during orogenesis. Analyses from the Bernina unit east of the Engadine line exhibit relatively elevation invariant ages of 31-35 Ma, denoting rapid exhumation through the zircon helium thermal sensitivity window. A similar, but slightly younger trend at ~27 Ma occurs in the samples from the Penninic Tasna unit and further constrains formation of the Engadine window. While our data do not allow direct discernment of the mechanism for the rapid uplift of the Tasna and Bernina units, the timing roughly correlates with thrust duplexing of subjacent crust and an influx of sediment to the Northern Alpine Foreland Basin, related to backthrusting of the Alpine orogenic wedge along the Periadriatic fault.