The Lofoten-Vesterålen continental margin: a multiphase Mesozoic-Palaeogene rifted shelf as shown by offshore-onshore brittle fault-fracture analysis
1, 2
pp. 29-58

Regional offshore-onshore research using 2D seismic data, gravity and magnetic data, satellite images, 3D terrain image models and detailed onshore structural analysis, reveals that the Lofoten archipelago and adjoining offshore shelf share a multiphase, Mesozoic to Palaeogene tectonic evolution. Our proposed model differs partly from previous models with respect to fault initiation and evolution relative to known rifting events, synchronous versus diachronous faulting and lateral segmentation of the shelf. The continental margin is characterised by NNE-SSW and NE-SW-striking lineaments having stepwise to zigzag attitudes. This lineament pattern is composed of at least three interacting and genetically related faults and fracture groups: (1) Population 1 defines ESE-dipping extensional detachments (1a) and right-stepping, WNW-dipping antithetic planar normal faults (1b) offshore, and combined extensional and dextral reactivated shear fractures (S1) onshore. (2) Population 2 represents NW-dipping normal faults bounding the margins of the main basins and the western Lofoten Ridge, and regional normal faults and dextral transtensional faults (S2) onshore. (3) Population 3 is made up of NW-dipping normal faults that mark the shelf-edge offshore, and conjugate WNW-striking extensional and Riedel shear fractures (S3) onshore. The interaction of population 1a-b and population 2 faults and fractures forms the regional zigzag pattern. A three-phase offshore-onshore tectonic model with fault initiation and events is suggested: (1) Permo-Jurassic to Early-Cretaceous proto-rifting event with WNW-ESE orthogonal extension followed by NNW-directed oblique-extension and dextral shearing (population 1a-b and S1 faults); (2) Early to Late Cretaceous major syn-rifting event with NNW-SSE oblique extension and lateral reactivation producing regional NE-SW to ENEWSW-striking oblique-normal faults (population 2 and S2 faults); and (3) Late Cretaceous to Palaeogene post-rifting with continued oblique extension in a regional NNW-SSE-extension field producing NW-dipping extensional faults offshore (population 3) and NW-directed shear fractures onshore (S3 faults). This multiphase and time-progressive fault evolution indicates rotation of the regional stress field from c. E-W to NNW-SSE and locally NNE-SSW. Offshore, the temporally different fault sets may be controlled by favorable basement grains (Caledonian depressions versus homogeneous Precambrian rocks) in conjunction with time-progressive change of the regional extension vector. The proto-rift normal faults probably influenced the syn-rift and post-rift transtensional faults and promoted later tectonic reactivation. The apparent lateral shelf segmentation is explained by temporal and spatial development of population 1 to 3 faults with fault-breaching linking older, oblique (Permo-Jurassic) basins and the younger, ridge-parallel (Cretaceous) basins and faults.

Steffen G. Bergh, Karsten Eig & John-Are Hansen, Department of Geology, University of Tromsø, N-9037 Tromsø, Norway.
Oddbjørn S. Kløvjan & Tormod Henningsen, Statoil ASA, P.O.Box 40, N-9481 Harstad, Norway.
Odleiv Olesen, Geological Survey of Norway, N-7491 Trondheim, Norway