Internal evolution of the major Precambrian shear belt at Torridon, NW Scotland

J. Wheeler, B. F. Windley and F. B. Davies
Geological Society, London, Special Publications, 27, 153-163, 1 January 1987, https://doi.org/10.1144/GSL.SP.1987.027.01.13

Summary

The Torridon shear belt is a NW-striking region of heterogeneous reworking within the Lewisian complex. Mapping of the deformation fabric intensity reveals a geometry of low-strain, lozenge-shaped blocks surrounded by an anastomosing set of shear zones which form a linked system of flats and lateral ramps. These various orientations of shear zones share a common movement direction but show both sinistral-reverse and dextral-normal shear sense. This is true both in the Inverian, when the linked system was established, and in the Laxfordian, when shearing followed the established network but was localized along the ductile Scourie dykes. Possible explanations for the occurrence of opposed shear senses include local differential shear, and a modification of the Lister & Williams model of deformation. In the latter, shear zones generally show a sense of shear opposite to the overall displacement on the belt, and this underlines the need for a comprehensive analysis of the geometry of such shear belts.

The Inverian structure controlled emplacement of the Scourie dyke swarm, which in turn controlled the localization and orientation of the Laxfordian structures. Given the ductility of the dykes, this leads to a simple explanation for the coaxiality of Inverian and Laxfordian structures. A tentative model for the evolution of the Torridon shear belt is given, in which emphasis is placed on the importance of dykes as easy-slip horizons.

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