The FAST deep seismic reflection profile traverses the whole width of the Faroe–Shetland Trough. The principal target of the profile was the structure of the crust beneath the Faroe basalts. In this region, bright reflections are seen from between 7 and 9 km depth beneath the basalts, dipping westwards in the opposite direction to the dip of the basalts and reflections within the basalts. These sub-basalt reflections are regarded as originating from near top basement. The Moho has not been imaged beneath the basalts, possibly because of the absence of any impedance contrast at the base of the crust. The profile shows that the basement of the Faroe–Shetland Trough thins to c. 10 km beneath the centre of the trough. Thinning of the crystalline basement is probably the result of more than one phase of extension, the most recent of which occurred in the mid- to late Cretaceous. Extension appears to have been concentrated on a series of east-dipping normal faults cutting through the basement. These faults may have originated during a Precambrian rifting event. It is suggested that opening of the NE Atlantic occurred to the west of the Faroe Islands, as Mesozoic rifting in the Faroe–Shetland Trough had strengthened the lithosphere in this region.

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