title: Studies of fold tectonics in the Lewisian of the Durness Region, Sutherland
Creators: name: Findlay, Donald
Publisher: name: University of Glasgow
Year: 1970

description: The rocks of the region from Strath Dionard to Durness are a layered assemblage of amphibolitic, quartzo-feldspathic gneisses together with amphibolite units and smaller amounts of calc-silicate rook and muscovite gneiss, which, because of an efluivalence with and continuation of rocks to the south (Dash 1969), are referred to as the Rhiconich group. These rocks show the effects of a single (Laxfordian) deformation event and a history of structural development which is 6epend-ent on the variable resistance of individual rock types to deformation. The earliest fabric of the rock is a lithological layering, considered largely to represent original sedimentary variation modified by metamorphic differentiation, a parallel foliation and a strong oriented linear mineral growth in amphibolites. No folds are seen to be related to these fabrics and it is possible these represent the effects of an early component of homogeneous deformation in the area. A series of later N.W.-S.E. trending folds, essentially co-axial with the first and continuous with them are the most prominent structures developed in the area. Boudins and open warps of the layers follow the development of the major folds. Pegmatite occurs locally throughout the structural sequence, mostly as layers parallel to the axial planes of folds, but also associated with the formation of boudins as infilling the separation gap and associated cross-fractures. A set of late, N.E.-S.W. trending joints and as associated fracture cleavage mark the latest effects of deformation. This structural history represents the effects of a single deformation event in an inhomogeneous rock series at lower amphibolite facies. Crustal comprossion approximately in a N.E.-S.W. direction, together with large vertical stresses due to supracrustal overburden, are the main stresses controlling deformation. Differential shortening of the rocks in response to these stresses is expressed in the formation of early recumbent folds in more competent rocs, and a homogeneous deformation in less competent rocks in which shortening in N.W.-S.E. direction is matched by an extension and the development of a linear mineral growth in N.W.-S.E. direction. Consequent re-distribution of the competency pattern controls the location of the later main folds which develop as essentially upright structures. Flattening of these folds causes rotation of early recumbent folds resulting in overfolds, a rotation in the a-b plane an the layers eventually to attain a near-vertical attitude, essentially normal to the main compressive stress. At this stage boudins develop together with early cross-fracturing, and have their long axes parallel to the linear mineral growth in quartzo-feldspathic gneiss. This structural sequence is largely controlled by the pattern of competency variation in the rocks and re-adjustment of this pattern to the stress field causes local diachronism of the folding within major structures. These findings largely agree with previous work carried out to the south (Dash 1969; Chowdhary 1969), but differ in (a) the recognition of the early linear fabric in amphibolites; (b) the dependence of structural sequence on competency variation; (c) diachronsim of folding, and (d) the recognition of boudins as a major response of the rocks to late deformation.