Tectonic evolution of the Devonian Hornelen basin of western Norway

ABSTRACT

The Hornelen basin is the largest of several Devonian terrestrial basins in west-central Norway. The basin is filled by alluvial fan and stream deposits eroded from the Caledonian highlands. These deposits form shingled strata uniformly east-dipping with a total accumulation of sediment of ~25 km, and that today stand in positive relief. The little-metamorphosed sedimentary rocks are separated from the underlying Western Gneiss Region (WGR) and Scandian nappes (Lower, Middle, and Upper Allochthons) by the west-directed Nordfjord-Sogn Detachment Zone (NSDZ). The basin’s origin has been debated for more than 50 years. In the 1960s and 1970s several workers ascribed the unusual thickness and longitudinal shingling of the strata to strike-slip deformation comparable to the late Miocene–Pliocene Ridge basin of southern California. However, the recognition of extensional mylonites beneath the basin led others to propose a different model (the scoop or supradetachment model) in which extension and basin filling were due to west-directed displacement on a low-angle normal fault. An examination of kinematic indicators on the brittle fault surface atop the NSDZ reveals consistently N-S motion suggestive of late out-of-syncline thrusting rather than west-directed extension. The purpose of this paper is to make the case for a return to the Ridge basin model for the Hornelen and Kvamshesten basins, overprinted by later north-south shortening. An ~100 km long strike-slip fault, the Bortnen fault, close to the northern margin of the Hornelen basin, may be the structure responsible for the basin’s development.