Late Devonian–Carboniferous faulting and controlling structures and fabrics in NW Finnmark

Abstract

In the SW Barents Sea, Devonian–Carboniferous collapse led to the formation of major basins and faults, e.g., the Hammerfest Basin bounded by the Troms–Finnmark Fault Complex, and rhomboid- to sigma-shaped (half-)grabens on the Finnmark Platform. High-resolution aeromagnetic and bathymetry data from the shallow shelf show that analogue fault systems are present in coastal and onshore areas of NW Finnmark. We provide new documentation for the Langfjorden–Vargsundet fault, a post-Caledonian, NW-dipping, zigzag-shaped, margin-parallel fault complex consisting of alternating linear to sub-linear, NNE–SSW- and ENE–WSW-striking, down-NW normal fault segments. This fault formed as an extensional splay-fault of inverted, Caledonian, brittle–ductile thrusts, e.g, the Talvik and Kvenklubben faults, which the fault eventually truncated and offset. In northern Finnmark, the Langfjorden–Vargsundet fault is offset laterally by up to 28 km by a system of WNW–ESE-trending faults notably including potential segments and splays of the Trollfjorden–Komagelva Fault Zone, a reactivated Neoproterozoic, margin-orthogonal transfer fault, separating NW Finnmark from the eastern Finnmark Platform. This fault system likely dies out westwards, and portions of the system’s process zone may crop out on the island of Magerøya. Similarly, the WNW–ESE- to ENE–WSW-striking Akkarfjord fault offsets the Langfjorden–Vargsundet fault by c. 2 km left-laterally. This fault may have formed as a conjugate to the Trollfjorden–Komagelva Fault Zone in Neoproterozoic (Timanian?) times. Steeply NW-plunging, upright and gently NE-plunging, inclined folds in Archaean–Palaeoproterozoic basement rocks and margin-parallel Caledonian thrusts may have controlled the formation and geometry of post-Caledonian faults. A Late Devonian–Carboniferous age for the Langfjorden–Vargsundet fault is supported by geochronological dating of onshore dykes and fault gouge, and by syn-tectonic sedimentary wedges along the offshore extension of the Langfjorden–Vargsundet fault. Mini-basins bounded by the Langfjorden– Vargsundet fault on the Finnmark Platform and on the shallow shelf, e.g., the Ryggefjorden trough, may represent analogues to deep, offshore, Devonian–Carboniferous basins, like the Nordkapp Basin, prior to the deposition of late Palaeozoic evaporites.