Field and Petrographic Investigation of the Austerfjord Group, Austerfjord Thrust, and Sørfjorden Shear Zone, Hinnøy, North Norway, Implications for Caledonian Tectonic Evolution

The Austerfjord thrust (AT) exposed on the island of Hinnøy, north Norway has been argued to be the most western and basal Caledonian thrust exposed at this latitude (68°N). Two mutually exclusive interpretations reported for the fault zone are 1) a suture separating the Lofoten microcontinental block from Baltica or 2) a fault zone internal to Baltica such that Lofoten is not an exotic terrane. The AT juxtaposes Precambrian granitic basement rocks above the Austerfjord Group (AG) metasedimentary sequence, argued to be the primary cover sequence to either Baltica or the Lofoten block. Field studies during summer 2007 revealed discrepancies between interpretations for the basal AG contact (depositional or intrusive and/or faulted), which has consequence for interpretations of timing of movement along the AT. We found the basal AG boundary to be intruded by a granitic pluton that has not been directly dated but closely resembles the 1.6Ga Lødingen Granite. Such a correlation would refute the interpretation of the AG as Baltic cover and implies it had mantled an older Proterozoic craton, possibly Lofoten. We also discovered a km-wide zone of L-tectonite that we call the Sørfjord shear zone (SSZ), which merges with or is cut by the AT. Microstructures indicate that the AT formed at much higher crustal levels than did the SSZ (upper vs. lower middle crust, respectively), implying that movement along the AT postdated the SSZ. As this was written, we had initiated U-Pb and ⁴⁰Ar/³⁹Ar analyses of primary and detrital zircon and muscovite grains, respectively, to bracket the ages of deposition of the AG and movement along both the AT and the SSZ. Field relations in conjunction with the new absolute timing constraints will be presented in the context of a new tectonic synthesis of this part of the Scandinavian Caledonides.