Deformation mechanisms in the Lewisian Gneiss and Cambro-Ordovician sediments at Heilam, Scotland


Heilam, a 10 km2 area on the east coast of Loch Eriboll near the Moine Thrust in NW Scotland, is composed of three structural areas: 1) the Arnaboll Nappe, 2) the coastal schuppen zone and 3) the intermediate imbricate zone. Samples of each lithology taken from the zones show a progressive sequence of deformation. The least deformed rocks in the Arnaboll Nappe and in the intermediate imbricate zone have strain shadowed quartz grains and feldspars with deformation lamallae. Near the thrusts of the imbricate zone and in the schuppen zone, micas exhibit a strong preferred orientation, quartz grains show discontinuous undulatory extinction and recrystallization about the margins. Pressure shadows in the quartzites and styolites in the dolomites post-date earlier recrystallization in these areas. Mylonites, notably those along the Arnaboll Thrust, contain micas and quartz with a strong preferred orientation. All minerals show reduction in grain size by either recrystallization or brittle fracture. A sequence of deformation mechanisms affecting the lithologies is 1) initial recrystallization, 2) pressure solution, and 3) minor recrystallization. This sequence is best observed in the mylonites and schuppen zone.


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