The structural evolution of the Tollie antiform—a geometrically complex fold in the Lewisian north-east of Gairloch, Ross-shire

Abstract

Five phases of deformation D1–5 are recognized in the Tollie gneisses. D1–2 are probably Scourian (s.l.) and ante-date a set of basic dykes. The pre-dyke metamorphism was highgrade, in part granulite facies. D3 was accompanied by almandine–amphibolite facies metamorphism and caused minor folding F3 on nnw–sse axes with an associated penetrative foliation of variable dip. The basic dykes were intruded either before or during D3. D4 produced widely-developed steep penetrative foliation S4 with associated folds F4 and lineation L4 having a nw–se trend slightly oblique to that of F3 and causing widespread transposition of S3. The associated metamorphism was retrogressive over part of the area to lower almandine–amphibolite facies. D5 comprises late- or post-metamorphic structures without penetrative foliation. The Tollie antiform hinge is an F4 fold superimposed on a pre-existing major structure the nature of which is investigated by analysing the variation in L4 attitude using lineation-pitch isogons, and by studying the geometry of the dykes. It is deduced that the F4 folds were superimposed on an upright box-shaped anti-form described by the S3 foliation in the gneisses and by the dykes. Alternative hypotheses are proposed to account for the pre-F4 variation in attitude of the dykes.