Structure and internal deformation of the Arnaboll Thrust Sheet, NW Scotland: implications for structural styles and strain localization

Descriptions of structural evolution across thrust belts commonly assume a transition from ductile to brittle deformation, reflecting a progressive reduction in temperature accompanying exhumation. The universality of this model is challenged here using field relationships at Ben Arnaboll, in the northern part of the Moine Thrust Belt. Deformation in the Arnaboll Thrust Sheet, an allochthonous basement body of amphibolite-facies gneisses and pegmatite sheets, carried onto Cambrian sediments, includes widely distributed, low-displacement shears developed under greenschist facies with ingress of water. These ductile deformations post-date the emplacement of the thrust sheet as they link kinematically to breaching thrust structures emanating from the footwall of the Arnaboll Thrust. The thrust itself records a transition from mylonitic (ductile) to strongly localized (brittle) deformation that pre-dates the breaching thrusts and therefore the deformation within the thrust sheet itself. The structure of breaching thrusts charts an up-dip transition from localized slip to distributed shearing analogous to the trishear in fold-thrust complexes, Therefore deformation of the Arnaboll Thrust Sheet shows a return from strongly localized translation-dominated brittle deformation to more broadly distributed ductile deformation. This is likely to have been promoted by the ingress of water and the concomitant reaction-enhanced weakening of the basement.