Mid/Late Devonian-Carboniferous extensional faulting in Finnmark and the SW Barents Sea

Abstract

In northern Norway, late/post-Caledonian extension initiated along inverted, brittle-ductile thrusts, e.g. Sørøya-Ingøya shear zone, which truncated existing, margin-oblique, late Neoproterozoic, Timanian faults and margin-parallel, latest Mesoproterozoic-mid Neoproterozoic faults related to the opening of the Asgard Sea and Iapetus Ocean. Inverted thrusts were active through the Devonian and early Carboniferous, accommodating the deposition of thick, Mid/Upper Devonian growth strata and of subsequent, lower Carboniferous sedimentary rocks in spoon-shaped basins on the Finnmark Platform and in the southwesternmost Nordkapp basin. Simultaneously, NE-SW trending basement ridges exhumed as part of a regional metamorphic core complex along bowed portions of the Sørøya-Ingøya shear zone and in the footwall of high-angle, zigzag-shaped normal faults following pre-existing basement fabrics, such as the Troms-Finnmark Fault Complex. Late/post-Caledonian exhumation is also documented onshore adjacent areas in NW Finnmark by retrograde fault-rock mineral assemblages and K/Ar faulting ages along the Langfjord-Vargsund fault, indicating rapid exhumation from > 10 km to 2-8 km depth in the Late Devonian-early Carboniferous. Inherited, margin-oblique brittle faults such as the Trollfjorden-Komagelva Fault Zone acted as minor strike-slip transfer faults, segmenting the margin and offsetting major zigzag-shaped fault complexes like the Langfjord-Vargsund fault. Bathymetry data further show sigma-shaped mini-basins with dense, internal, fault-fracture networks (e.g. Ryggefjorden trough), potentially representing small-scale analogs to major offshore basins such as the Nordkapp Basin. In the late Carboniferous, faulting activity declined and extension localized along a few major faults. In addition, Caledonian rocks were slowly exhumed to depths < 3.5 km., and thick, upper Carboniferous evaporites deposited in the southwesternmost Nordkapp basin. The margin was tectonically quiet by the end of the Carboniferous and was only subjected to minor, Permian-Cenozoic tectonic adjustments. Alternatively, late Carboniferous-mid Permian K/Ar ages obtained in NW Finnmark reflect an episode of margin weathering.