Deep structure of the Mid Norway rifted margin
pp. 205-224

Interpretation of regional, long-offset seismic reflection and potential field data reveals new information on the deep structural framework of the Mid Norway area, and thus on the prolonged extensional history of the NE Adantic passive margin. The Trøndelag Platform is underlain by an array of deep, pre-Middle Triassic half-graben basins bounded by N-S and NE-SW-trending dip-slip or oblique normal faults that detach between 6s and 8s two-way time (twt). Comparison with onshore deformation patterns indicates that this fault system most likely originated as a result of late/post -orogeli.ic extension of the Caledonian nappe pile lasting from the Devonian in to the Early Carboniferous. Subsequent re-activation of this fault pattern controlled Permian to lower Triassic basin deposition. The relationship between the pre-Middle Triassic basins in the Trøndelag Platform area and the Jurassic basins on the Halten Terrace involves the successive development of two generations of extensional detachments and their interaction with a deep-seated, antiformal culmination interpreted here as a metamorphic core complex. Denudation of the core complex was related to large-magnitude extension in the Mid- to Late Palaeozoic, and was most likely responsible for the warping and de-activation of the Palaeozoic detachment that presendy underlies the Trøndelag Platform. Renewed activity along the distal flank of the core-complex controlled development of the platform/terrace boundary, as well as the ramp-flat geometry of the terrace-bounding Bremstein-Vingleia Fault Complex (BVFC) in the vicinity of the Njord field. West of the Frøya High, low-angle incision by a basin-bounding, large-magnitude, low-angle normal fault into the footwall of an older detachment zone produced a core-complex in the form of a mylonite front that was exhumed in the footwall of the low-angle normal fault. The extent of the Palaeowic-Mesozoic core-complexes are roughly indicated by strong positive magnetic anomalies in the Frøya High and Halten Terrace areas. These anomalies probably reflect the denudation of highly magnetic, high-grade metamorphic rocks that resided in the deep crust prior to extension. Our interpretation highlights the importance of the KFC and the BVFC as fundamental, large-magnitude border faults in the late Triassic-Jurassic rift architecture. West of the Klakk Fault Complex (KFC), the northemmost Møre and southern Vøring Basins are underlain at great depth by arrays of rotated fault blocks that are bounded by W-dipping normal faults and that contain wedge-shaped, pre-Cretaceous half-graben basins. In our interpretation, sedimentary rocks can be found down to at !east 9 s twt in the deepest parts of the rotated half-graben. We tentatively infer a Jurassic age for parts of the half-graben basin fill. Some of the faults clearly cut deep-seated reflection hands, whereas some are interpreted to detach on top of the reflective lower crust. The latter scenario applies to the most highly extended,' western parts of the northern Møre and southern V øring Basins. T he northern parts of the V øring Basin appears to be characterized by SE-dipping faults such as the o ne that flanks the Træna Basin in the NW. This fault was active at !east from the Early Cretaceous onwards. By the late Early Cretaceous, the large, roughly N-S trending basin-bounding faults such as the BVFC and the KFC had become extinct, with exception of subordinate, out-of-sequence reactivation. An extensional detachment under the western parts of the basin may have been active in to later parts of the Early Cretaceous, but was rendered extinct prior to the end of the Cenomanian. However, from this time and onwards in to the Early Tertiary, large, SE-dipping faults of the northern Vøring Basin were still active, providing accommodation for large volumes of Late Cretaceous sediments in areas such as the Træna Basin. The N-S-trending part of the earlier, mainly Jurassic system was characterized by stratigraphic overstepping of extinct fault planes, and by erosional degradation and draping of residual rift topography.

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