The structure of the Glenelg-Attadale Lewisianoid Inlier and its relationship to the Moine Thrust Zone

Abstract

Synopsis The Glenelg-Attadale Lewisianoid Inlier lies adjacent to the Moine Thrust Zone. The inlier is surrounded and divided into Eastern and Western units by Moinian metasediments. The Eastern Unit was emplaced against the Western Unit in the Grenvillian Orogeny. Moinian sediments were deposited unconformably on both units in the Neoproterozoic. During the Late Neoproterozoic Knoydartian and Lower Palaeozoic Caledonian orogenies both the inlier and the overlying Moinian rocks were deformed and metamorphosed in the amphibolite facies. Deformation affected particularly the western and eastern margins of the inlier and a strip of infolded Moinian rocks between Eastern and Western units, with the formation of blastomylonites and isoclinal folding (F1). Above a major décollement in the Moine strip and along the western margin of the inlier, the rocks were folded by easterly plunging reclined (F2) folds. This phase of folding affected mainly the Lewisianoid rocks of the Eastern Unit and the adjacent Moine sediments of the Mountain Belt. Deformation was associated with the formation of a large-scale mushroom interference structure and the extrusion of the inlier northwestwards, towards the foreland, marked by a dominant ESE stretching lineation. In Avernish, reclined F2 folds are cut by randomly-oriented hornblende porphyroblasts, which are altered to biotite and smeared out in localized mylonitic shear zones similar to the mylonites in the Moine Thrust Zone. If these mylonitic shear zones were formed contemporaneously with the mylonites in the Thrust Zone, folding and thrusting events superimposed on the mylonites, must have occurred later than events in the Mountain Belt.