Geochronological data indicate that the Caledonian thrust sheets of NW Scotland were assembled in <18 Ma, yet regional Barrovian metamorphism to amphibolite facies and local melting occurred syn-thrusting. From theoretical studies, it is clear that there is insufficient time to generate the high metamorphic temperatures during the thrusting event. We suggest that a plausible explanation for the heat source is that it resulted from a 20–25 Ma period of crustal heating in a back-arc setting between the Grampian and Scandian orogenic events. Most of the orogenic heat therefore came from the pre-existing back-arc rather than the orogenic process itself.

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