The evolution of the Hornelen Basin detachment system, western Norway: Implications for the style of late orogenic extension in the southern Scandinavian Caledonides

Abstract

The geology of the Devonian Hornelen Basin and the surrounding Western Gneiss Region (WGR), western Norway has been examined in detail. Structural, clast provenance and geothermobarometric data have been combined to reconstruct the motion history of part of a major late-orogenic extensional shear zone in the southern Scandinavian Caledonides, the Nordfjord-Sogn Detachment (NSD). The NSD in the Hornelen Basin area is a zone of mylonites 4–6 km thick, tectonostratigraphically constrained within the Lower and Middle Allochthons (LMA) and the uppermost part of the WGR. It separates eclogite-bearing gneisses of the WGR from greenschist and amphibolite facies rocks of the Middle and Upper Allochthons, marking a profound metamorphic break and indicating major excision of crustal section. Kinematic indicators give a consistent “top to the west” shear sense, indicating a reversal of the movement sense of the Scandian nappes, with the ductile strain being distributed through the entire LMA section. The WGR was exhumed from depths of up to 100 km in the footwall of the shear zone, partly aided by pure-shear vertical flattening below the NSD and extension and erosion of the hanging wall. The majority of the movements on the NSD in the Hornelen Basin area occurred under amphibolite facies conditions prior to 410 Ma. Subsequent shear caused limited greenschist retrogression along extensional crenulation cleavages. Depending on interpretation of mica 40Ar- 39Ar ages, this lower-grade shearing may be due to a continuation of movements, or renewal of motion associated with opening of the Hornelen Basin at about 385 Ma. The lack of clear evidence of penetrative greenschist facies mylonites in the Hornelen area may be due to the faulted omission of tectonites of this grade. Following Scandian A-subduction and eclogite facies metamorphism at 425 Ma, the P-T-t path of the WGR shows very rapid decompression associated with early slip on the extensional detachment horizons. Unroofing was initially near isothermal, followed by a phase of rapid near-isobaric cooling. After 410 Ma decompression and cooling slowed, as slip on the NSD slowed or even ceased. By the Middle Devonian, the WGR lay at about 10–15 km depth and the crust had almost returned to isostatic equilibrium. At this time, a brittle fault, splaying from the NSD, ramped upwards through the Upper Allochthon, opening a scoop-shaped, west-facing half-graben, the Hornelen Basin. A further 50 km of heave on the NSD occurred during alluvial infilling of the basin. Subsequently, the basin fill was folded by (Late Devonian?) N-S compression and the basin margins were modified by Mesozoic faulting. These features are consistent with an orogenic collapse model for southern Scandinavia that involves large-scale backsliding of the Scandinavian Caledonides within tectonostratigraphic units. By this mechanism the Baltic Shield could have been efficiently unloaded, with tectonic translations producing significant metamorphic breaks within a seemingly preserved tectonostratigraphic sequence. The model favours orogenic collapse driven by an external pull, rather than internal body forces.