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.

ABSTRACT The Scandinavian Caledonides formed during the continental collision between Baltica and Laurentia. During the collision, a complex nappe stack was thrust over the Baltican continental margin. The orogen can be subdivided into segments based on architectural differences within the Scandian nappes. The southern and central segments of the orogen link up in the Gudbrandsdalen area in south-central Norway. Alpine-type metaperidotite-bearing metasedimentary complexes occur in the southern and central segments and can be traced continuously along the strike of the orogen from one into the other segment. Traditionally, these units have been assigned to different tectono-stratigraphic levels, one below the Middle Allochthon and one above the Middle Allochthon. Here, we trace the Alpine-type metaperidotite-bearing units from Bergen to Esandsjøen and show that these units exhibit a common geologic and metamorphic history, consistent with the metaperidotite-bearing units representing a single tectonic unit. We suggest that the metaperidotite-bearing units can be used as a “marker level” to revise the tectono-stratigraphy of the Gudbrandsdalen and adjacent areas. The tectono-stratigraphic revisions imply that the Scandian nappe stack consists of seven tectono-stratigraphic levels that can be traced throughout the southern and central segments of the Scandinavian Caledonides. Moreover, the revision of the tectono-stratigraphy and new U-Pb geochronology data also suggest a revision of the timing of the succession of tectonic events leading up to the Scandian continental collision. The available evidence indicates that Baltica-derived tectonic units collided with the Iapetan/Laurentian subduction complexes as early as ca. 450 Ma. The initial collision was followed by in-sequence nappe formation of Baltican-derived units, which occurred contemporaneously with the opening of a marginal basin in the upper plate. After the arrival of thick, buoyant, unthinned Baltican crust at the trench, the main zone of convergence stepped outboard, the marginal basins closed, and those basins were thrust out-of-sequence over the previously assembled nappe stack.