The Arnaboll Thrust - its type area

The Arnaboll thrust is famously exposed in a 10m high cliff at the NW end of the plateau, also a spectacular viewpoint onto the Heilam imbricates further north (see below). The hanging-wall to the Arnaboll thrust is marked by 2-3m (the bottom of about 50m of basement in the thrust sheet) of Lewisian gneisses and acid sheets that have been sheared during thrusting. The amphibolite-facies mineral assemblages and textures of the protolith are recrystallised into a very fine-grained phyllonite comprised of epidote, chlorite and quartz. Thin pegmatite veins within the host gneisses become increasing deformed down towards the thrust plane. Deflection of these early markers clearly indicates top to WNW shearing, as do minor shear bands and asymmetric fabric boudinage. The footwall is in the upper part of the Pipe Rock. Here the pipe structure defined by Skolithos and rare Monocraterion show systematic deflections inicating top to the WNW shear senses. Shear strains are variable (Fischer and Coward, 1982), being especially high along phyllosilicate-rich bedding surfaces, but generally attain g values between 1-2.