Structure and internal deformation of the Arnaboll Thrust Sheet, NW Scotland: implications for strain localization in thrust belts

Abstract

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 updip 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.

Structure of the Northern Moine thrust zone, Loch Eriboll, Scottish Caledonides

“…Mylonitisation and retrogression of the dominantly amphibolite facies gneiss to greenschist facies occurred during motion on the Arnaboll thrust, as seen in a band of fault rocks < 2 m thick constituting the thrust surface. Retrogression is evident in the development of phyllosilicates such as sericite at the expense of feldspars and chlorite at the expense of mafic minerals (Wibberley and Butler, 2010). The formation of these retrograde products requires fluid-present reactions.…”

“…The Lower Arnaboll imbricates are a series of thrusts that ramp up from a basal flat in the footwall of the Arnaboll thrust, and breach the Arnaboll thrust surface, locally emplacing Cambrian quartzite (Pipe Rock) on top of Lewisian gneiss. The individual thrusts are therefore named breaching thrusts (Wibberley and Butler, 2010). The breaching thrust with the largest offset has created a large asymmetrical antiform in the Pipe Rock and Lewisian above on Ben Arnaboll that explains the notch-like map expression of these two units (Wibberley and Butler, 2010).…”

“…The individual thrusts are therefore named breaching thrusts (Wibberley and Butler, 2010). The breaching thrust with the largest offset has created a large asymmetrical antiform in the Pipe Rock and Lewisian above on Ben Arnaboll that explains the notch-like map expression of these two units (Wibberley and Butler, 2010). The foremost of the Lower Arnaboll imbricates is the Lower Arnaboll Thrust.…”

“…McClay and Coward (1981) made a detailed map of the Heilam -Ben Arnaboll region and favoured the foreland-directed thrust sequence proposed initially by Elliott and Johnson (1980) for the Assynt region. Coward (1984) and Butler (1987) recognised that the Arnaboll thrust was folded and was breached by small-scale imbricates in the underlying thrust sheet, termed breaching thrusts by Wibberley and Butler (2010). Holdsworth et al (2006Holdsworth et al ( , 2007 recognised out-of-sequence thrusts that truncate structures in the footwall and also proposed extensional structures within the Moine thrust zone.…”

“…6. The Sole Thrust Sheet includes examples of breaching thrusts, where underlying imbricate thrusts have cut up through the Arnaboll Thrust surface (Butler, 1987;Wibberley and Butler, 2010).…”