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Aziz Nasuti, Norwegian University of Science and Technology (NTNU) & Geological Survey of Norway (NGU) (Norway)
Jörg Ebbing, Norwegian University of Science and Technology (NTNU) & Geological Survey of Norway (NGU) (Norway)
Christophe Pascal, Geological Survey of Norway (NGU) (Norway)
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The ENE-WSW striking Møre-Trøndelag Fault Complex (MTFC) is a key element in the structuring of Mid Norway. Remote sensing studies show that the fault complex has a width exceeding 50 km. Within this zone, characteristic structures include folded, mylonitic km-wide sinistral detachment zones, truncative, steep, discrete fault strands and numerous smaller fracture-lineaments. The MTFC formed probably during the Scandian orogeny towards the end of the Caledonian cycle, since it slices through several of Scandian thrust sheets. The fault complex has been repeatedly reactivated from Devonian time to the Cenozoic and exhibits a great variety of fault rocks, exposed mostly on the Fosen Peninsula. Geological and geophysical observations demonstrate that the MTFC exerted a strong control in shaping the basins offshore but also in influencing the development of the landscape onshore, and continues today in modifying the regional stress pattern. However, determination of a kinematic history for the MTFC is complicated by the lack of well- preserved stratigraphic markers. In order to properly understand the control of the MTFC on the evolution of both offshore basins and onshore landscape, there is a crucial need to improve our knowledge on the geometry of the MTFC at depth.
Previous geophysical studies were unable to provide a clear image of the MTFC. In this study, different and independent geophysical methods are applied in order to image the MTFC structure. High-resolution seismic experiments will be combined with detailed gravity and magnetic analysis and resistivity profiles. Resistivity studies image structures in greater detail at shallower depths than seismic experiments and, therefore, will be of prime importance when welding surface mapping with deep geophysics. Rock sampling and petrophysical measurements on densities, magnetic susceptibilities and seismic velocities will furnish important constraints on the geophysical models. By using new techniques we are able to investigate the resistivity distribution down to 130 meters. Recent studies show that this method provides good information about fracture zone thickness, depth and dip. Fractures increase the secondary porosity of bedrock, and since the bedrock is normally water saturated, this will decrease the resistivity. We present five resistivity profiles carried out in the Tingvollfjorden area, where one of the main segments of the MTFC is evidenced by the topography. Data inversion revealed very low resistivity zones. The geometries of the low resistivity zones strongly suggest highly fractured bedrock. In the prolongation of one fault segments a shallow refraction seismic profile was also conducted. The results of this latter show that three low velocity anomalies do exist in the bedrock and are probably damage zones related to the MTFC. Forthcoming high-resolution gravity and magnetic surveys and deep seismic experiments will help to refine these preliminary results
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