The concealed Kopperå fault: A half-graben bounding structure located along the eastern flank of the Trøndelag depression, mid-Norway
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Seismic-reflection data acquired along a traverse from Stjørdal, east of Trondheim, to Storlien in J\u00E4mtland, Sweden, clearly indicate the presence of a concealed, moderately steep-dipping fault which we have named the Kopperå fault. The fault is located along the eastern flank of the 8-10 km deep Trøndelag depression, and effectively delimits a half-graben in its hanging-wall. The Kopperå fault truncates the westward extension of the lower-angle, extensionally reactivated, Steinfjell thrust that marks the base of the Caledonian Upper Allochthon in this area. The top-to-the-WNW reactivation is considered to relate to late-Scandian, Devonian extension, but no robust dating of the Steinfjell fault is yet available. Although the timing of generation of the blind Kopperå fault is clearly speculative, some comparable, steep, normal faults transecting low-angle, ductile, Mid Devonian, detachment faults elsewhere in the Mid-Norwegian Caledonides have radiometrically dated Early Carboniferous ages. The Kopperå fault may thus conceivably fall into this age category.