Late Paleozoic-Cenozoic fault correlation and characterization of fault rocks in western Troms, North Norway

摘要

The present work focuses on the mapping and description of onshore brittle fault zones on the SW Barents Sea Margin, within gneisses and granitic intrusions belonging to the West Troms Basement Complex. The description of the brittle structures includes the geometry, kinematics and fault rock characteristics, using DEM satellite imaging, structural field work and a microstructural analysis of the fault rocks. As a result of the present study,two major sets of onshore brittle faults have been observed, trending NNE-SSW and ENE-WSW,and they are tentatively correlated in two, NE-SW trending fault complexes: the Rekvika and the Vestfjorden-Vanna fault complexes. These fault complexes run parallel to the Troms-Finnmark fault complex that borders major offshore sedimentary basins in the NW. On the one hand, the Vestfjorden-Vanna fault complex defines the southeastern boundary of the West Troms Basement Complex towards the Caledonian nappes, and is made of wide fault zones that have experienced major displacement (ca. 1-3km). On the other hand, the Rekvika fault complex is considered as an intra-horst fault system composed of narrow fault zones that are thought to have accommodated low amounts of displacement (> 250 m). On a local scale, the fault zones display similarities in attitude (trend and dip) with the Caledonian and Precambrian fabrics, indicating a possible influence of pre-existing zones of weakness on brittle faulting. The dominant deformation mechanism (cataclastic flow) indicates pressure range about 0.2-0.3 GPa, i.e. 5-10 km depth for the formation of the cataclastic fault rocks, and these fault rocks contain mineral assemblages that generally indicate temperatures about 350-500ᵒC. This suggests that the West Troms Basement Complex has been largely uplifted later on, until present level. The NNE-SSW and ENE-WSW trending fracture sets are believed to have formed synchronously due to WNW-ESE extension, during an early stage of rifting in the Permian-Early Triassic. The NNE-SSW trending faults likely represent the main fault system, orthogonal to the extension direction, and the ENE-WSW trending faults may correspond to oblique transfer zones that link the NNE-SSW fault segments. An alternative model implies NW-SE trending transfer zones to link the NNE-SSW trending faults. A late stage of minor reverse reactivation of the NNE-SSW and ENE-WSW trending faults, and the development of NW-SE striking fractures are tentatively correlated to ridge-push forces during the opening of the North Atlantic in the Eocene.