An interpretation of the structural evolution of the Laxford Front, north-west Scotland

Abstract

Synopsis The Laxford Front is a zone of complex rotational deformation which separates intensely deformed Laxfordian migmatites from heterogeneously deformed pyroxene granulites. The dominant sense of movement shown by shear zones is sinistral and north side upwards, implying that the migmatites were moving upwards from beneath the Scourian granulites. The main rock types are, from south to north: a zone of relatively undeformed pyroxene granulite; a zone of regularly folded amphibolite facies gneisses with metagabbro-anorthosites and metasediments; a planar zone of granite sheets, and a zone of migmatites. The post granulite facies structural history of the Laxford Front can be adequately described in terms of three phases ( F 1 to F 3 ) of deformation. The first and second phases are separated by a phase of dyke intrusion, the ‘Scourie dykes’. The Scourian zone of granulites acted as a competent body throughout each phase of deformation, though the southern limit of ductile deformation occupied a more northerly position for each successive phase. A model for the F 2 strain trajectories is proposed. There is no evidence to indicate that the migmatites were folded into a basement of pyroxene granulites as suggested by Bowes (1962) and Holland (1966). The Laxfordian migmatites were on the same tectonic level as, or possibly lower than, the Scourian granulites prior to the F 1 deformation.