The metamorphic environment of the Sgurr Beag Slide; a major crustal displacement zone in Proterozoic, Moine rocks of Scotland

Abstract

New evidence from the SW Northern Highlands of Scotland establishes the southerly extension of the Sgurr Beag Slide. Interpretation of changes in the mineralogy of calcsilicate rocks, particularly the use of the anorthite content of plagioclase feldspar as an index of metamorphic grade, together with consideration of the texture and mineralogy of pelitic rocks, suggests that the slide juxtaposes crustal segments of different metamorphic grade and history.Movement on the slide post-dates a major phase of regional metamorphism and two phases of regional deformation of Precambrian age, was synchronous with a regional fold-forming event and regional metamorphism at or before c. 467 Ma (Caledonian), and was followed by regional folding. Models of the thermal effects of displacement across syn-metamorphic shear zones are investigated with particular reference to the roles of thermal relaxation and levels of shear strain. A model based on the development of a ductile, syn-metamorphic, asymmetric shear zone best explains the metamorphic patterns associated with the Sgurr Beag Slide. An original low to moderate easterly dip is implied, with north-westerly upthrusting of the eastern crustal block.