Analysis of Late Paleozoic-Mesozoic brittle faults and fractures in West-Finnmark: geometry, kinematics, fault rocks and the relationship to offshore structures on the Finnmark Platform in the SW Barents Sea

Abstract

The present work focuses on the onshore/offshore analysis and correlation of brittle faults and fractures on the Porsanger Peninsula area in west-Finnmark, the shelf areas and the immidiate offshore areas (Finnmark Platform and Gjesvær low). Structural field observation, microstructural and SEM analysis provide the basis for the characterization of the geometry, kinematics and fault rocks. This data has been combined with interpreted DEM/bathymetric, aeromagnetic and seismic data to get a better understanding of the regional structural character.

This study demonstrates that Porsanger Peninsula area are characterized by two major fault-fracture trends: NW-SE and NE-SW, and a subsidary E-W fault-fracture trend. The NW-SE fault trending parallel to the Trollfjord-Komagelv Fault Zone and show dominantly oblique slip and lateral displacement. The NE-SW and E-W faults show mainly normal dip-slip movement and are likely related to the offshore Troms-Finnmark Fault Complex and Måsøya Fault Complex. The relative timing of the the brittle faults-fracture trends are uncertain, but are all possibly related to the WNW-ESE directed extension event that led to the opening of the NE Atlantic Ocean. The observed fault rocks indicate mostly mechanically frictional brittle deformation, and show greenschist and zeolite mineral assemblages indicative of shallow cataclastic fracturation. The juxtaposition of lower amphibolite facies host rock with greenschist facies fracturation and zeolite facies fracturation may indicate that the study area is part of a progressively exhumed margin.

Based on the onshore-offshore structural analysis, an evolutionary model is suggested for the structural development of Gjesvær low on the Finnmark Platform. Gjesvær low is possibly a Late Devonian- Early Carboniferous basin that likely initiated during fault linkage of the overlapping fault segments of the major NE-SW to ENE-WSW basin-bounding faults (Troms-Finnmark Fault Complex and Måsøya Fault Complex) in late Devonian times. The NW-SE branch fault segment connects Troms-Finnmark Fault Complex and Måsøya Fault Complex in the east end of Gjesvær low. This NW-SE fault appears to be related to the major Trollfjord-Komagelv Fault Zone. Fault activity continued along the major basin-bounding faults and the branching NE-SW trending segment. This resulted in further subsidence of Hammerfest Basin and Nordkapp Basin, while Gjesvær low is shallower due to the inactive termination of the Måsøya Fault Complex on the platform. This suggests that the NW-SE trending fault segment acted as transfer faults that decoupled the Gjesvær low from deep-basins such as the Nordkapp and Hammerfest basins.