Structural framework of the gneiss–amphibolite–pegmatite assemblage of the Lewisian Complex south of Durness, NW Highlands

Abstract

After a gap of more than a century new mapping has established that the structural framework of the Lewisian Complex southwards from the north coast of Scotland consists of a pattern of shallowly-plunging upright folds with southeasterly trending axial planes resulting from Palaeoproterozoic deformation of the products of Neoarchaean crust formation. The lithologies and structural features are consistent with polyphase tectonothermal deformation at depth, including crustal shortening, of a flat-lying, mantle-derived protolith assemblage that consisted mainly of acidic and some basic igneous sheet-like intrusions and the products of their metamorphism (quartzofeldspathic gneiss and amphibolite) during crust formation. The crustal shortening is represented by penetrative planar and linear fabrics that largely replace those formed earlier in the less competent gneisses. Large-scale upright northeasterly trending folds formed subsequently are both a major feature of the structural framework and a major control of the emplacement of many pegmatite intrusions. Rotation of structural units with respect to one another was associated with the extension along fold axes, the common development of boudinage structures at various scales and the local reversal of the general southeasterly direction of fold plunge to northwesterly.