Submitted: Late Devonian-Carboniferous faulting in NW Finnmark and controlling fabrics

The authors Jean-Baptiste Koehl (UiT), Steffen Bergh (UiT), Per-Terje Osmundsen (NGU), Thomas F. Redfield (NGU/UNIS/UiO), Halldis Lea (UiT), and Espen Bergø (UiT) have submitted the article “Late Devonian-Carboniferous faulting in NW Finnmark and controlling fabrics” to the Norwegian Journal of Geology. The paper is yet to be accepted, but is available for ARCEx members via the intranet.

Abstract: 

A Late Devonian (?) – Carboniferous episode of regional extension related to the collapse of the Caledonides triggered the formation of major basin-bounding faults, e.g. Troms-Finnmark Fault Complex, and offshore basins in the SW Barents Sea, such as the Nordkapp Basin and smaller triangular to sigma-shaped (half-) graben basins on the Finnmark Platform. New high-resolution aeromagnetic and bathymetry data from the shallow shelf (strandflat) show that analog fault systems are present in coastal and onshore areas of NW Finnmark. We investigate the Langfjord-Vargsund fault, a NW-dipping, zigzag-shaped, onshore-nearshore, coast-parallel, extensional brittle fault complex, trending parallel to the Vestfjorden-Vanna fault complex in Lofoten-Vesterålen and Western Troms. This fault complex consists of alternating NNE-SSW and ENE-WSW striking normal fault segments that merge into triangular basins on the Finnmark Platform. Nearshore coastal fjords show several triangular, sigmashaped mini-basins, e.g. the Ryggefjorden trough, bounded by zigzag-shaped fault segments of the Langfjord-Vargsund fault, and they may represent analogs to shallow and deep offshore, DevonianCarboniferous basins such as the Nordkapp Basin prior to the deposition of late Paleozoic evaporites and subsequent diapirism. Moreover, the Langfjord-Vargsund fault accommodated hundreds of meters to a few kilometers of down-to-the-NW normal movement during post-Caledonian extension. In northern Finnmark, the Langfjord-Vargsund fault is offset ca. 28 km right-laterally by steep, WNWESE trending, strike-slip fault segments of the Trollfjorden-Komagelva Fault Zone, a reactivated Neoproterozoic, margin-orthogonal transfer fault zone that segmented onshore and nearshore areas of NW Finnmark from the offshore Finnmark Platform east. The island of Magerøya is potentially located within the fault-tip process zone of the Trollfjorden-Komagelva Fault Zone and the fault most likely dies out to the west. Similarly, a ca. 2 km left-lateral offset of the Langfjord-Vargsund fault in Revsbotn is linked to sinistral strike-slip movement along the Akkarfjord fault, a steep, WNW-ESE to ENE-WSW trending brittle fault forming steep escarpments in nearshore fjords and in northeast Sørøya. We propose that the Akkarfjord fault may have initiated as part of an oblique, conjugate fault set to the Trollfjorden-Komagelva Fault Zone in Timanian (?) times and was reactivated synchronously with the LangfjordVargsund fault. Steeply dipping fault segments of the Langfjord-Vargsund formed as extensional splayfaults along inverted, Caledonian, brittle-ductile thrusts, such as the Talvik and the Kvenklubben (thrust) faults, which they eventually truncated and decapitated. Other controlling fabrics include favourably oriented, (folded) Precambrian fabrics and units depicted by new aeromagnetic data, e.g. steeply NWplunging, upright and gently NE-plunging, inclined folds, which may have provided preferential zones of weakness for Caledonian thrust faults and post-Caledonian normal faults to form along steeply dipping fold limbs and define major bends over fold hinges. In addition, large-scale, NW-SE trending belts of granite-gneiss and macrofolded volcano-sedimentary rocks delineated by matching aeromagnetic anomalies are possibly downthrown to the northwest by the Langfjorden-Vargsund fault. A Late Devonian-Carboniferous age for the Langfjord-Vargsund fault is supported by radiometric dating of dolerite dykes and fault gouge, and by seismic interpretation of syn-tectonic sedimentary wedges along the possible offshore extension of the Langfjord-Vargsund fault.

Reference:

Koehl, J.B.P., Bergh, S.G., Osmundsen, P.T., Redfield, T.F., Indrevær, K., Lea, H., Bergø, E. (submitted): Late Devonian-Carboniferous faulting in NW Finnmark and controlling fabrics. Submitted for publishing in Norwegian Journal of Geology, 2018. [intranet]