Structure and timing of the Austerfjord thrust and related shear zones, Hinnøy, north Norway: Implications for late-stage Caledonian tectonic evolution

Abstract

The Austerfjord thrust on central Hinnøy, north Norway is reexamined in its context as the structurally lowest Caledonian thrust preserved in the arctic Norwegian Caledonides. The discovery of the Vassvika group metasedimentary sequence in central Hinnøy, which connects the Austerfjord thrust to the even more internal, and, thus, structurally lower Gullesfjord shear zone 8 km to the west, results in the recognition of the Gullesfjord-Austerfjord shear zone (GASZ). The GASZ is a tops-east-directed thrust duplex that incorporates metasedimentary cover rocks and slivers of granitic basement gneiss. The GASZ has decapitated the domal crest of the Austerfjord antiform, implying that the latter is a pre-Caledonian structure that controlled the somewhat odd geometry of this segment of the basal Caledonian thrust. A high-temperature, tops-west shear zone, the Sørfjorden shear zone (SSZ) is recognized as a counterpart to a macroscopic northwest-vergent sheath-style back fold that affected the upper and lower plates of the GASZ. 40Ar/39Ar thermochronology reveals that muscovite from the tops-east GASZ records cooling from 420-380 Ma (Scandian), and muscovite from the tops-west SSZ records cooling from 380-370 Ma (Devonian extension). This temporal relationship documents that Caledonian contraction was followed closely by extensional movement in this area of north Norway. Connection to the synchronously developed tops-east Øse thrust requires reassessment of Devonian extension here in the northernmost terminus of the orogen-wide system of Devonian extensional faults. Observations and data presented herein strongly support that Devonian extension in Lofoten-Vesterålen was accommodated by gravity-driven, foreland- and hinterland-directed movement away from a thermal dome created beneath over-thickened Caledonian crust.