Late Caledonian extensional reactivation of a ductile thrust in NW Scotland

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Abstract
Evidence is given for ductile extension across a fault zone located on the western seaboard of the Northern Highlands of Scotland. The fault zone lies within the metamorphic Caledonides and extensional movement occurred during the waning stages of Caledonian orogenic activity. An earlier phase of westerly directed overthrusting took place on the same fault zone at higher metamorphic temperatures but after the peak of Caledonian metamorphism. The compression may be broadly coeval with early displacements on the Moine Thrust Zone which probably lies offshore some 10 km to the northwest. The extensional reactivation could relate to local crustal instability resulting directly from overthrusting; it may have been due to local gravitational collapse following overthickening of the crust. Alternatively the extensional movement may witness the onset of regional gravitational collapse of the orogen during the late Silurian to early Devonian. Reactivation accords with interpretations of the movement histories of similarly eastward dipping offshore faults recognized on seismic reflection profiles.