Most inherited Mesozoic extensional faults in the French external Alps strike approximately NNE–SSW, perpendicular to Alpine thrusting directions. In the Muzelle region of the NW Pelvoux Massif, the Vallon compartmental fault is a Mesozoic WNW–ESE-trending basement fault which was reactivated as a dextral strike-slip fault during the Alpine orogeny. Because this fault is parallel to the emplacement direction of the thrust sheet, it divides the thrust sheet into regions of diVerent emplacement rates at diVerent times. A model is proposed for compartmental-fault reactivation in which a varying proportion of compressible cover to rigid basement either side of the fault results in a strain incompatibility during Alpine shortening, which is resolved by reactivation.

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