The kinematic and metamorphic history of the Sgurr Beag Thrust, Ross-shire, NW Scotland

Abstract

This paper presents a field and microstructural study of a Caledonian amphibolite-facies shear zone within the Moine rocks of Northern Scotland. The shear zone, the Sgurr Beag thrust, is an important tectonic break within the Moine nappe of Ross-shire and is the structurally highest and oldest of the foreland-propagating thrust system the youngest and lowest of which marks the Caledonian front in Britain. Microstructures and quartz c-axis analysis show that fabrics formed during nappe emplacement were thoroughly recrystallised before or during peak Caledonian metamorphic conditions. One segment of the shear zone was reactivated as a north-directed thrust with retrograde reworking of annealed metamorphic textures. Quartz-rich mylonites from the reactivated zone display asymmetric or single-girdle quartz c-axis fabrics consistent with north-directed overthrust shear, except within and immediately beneath an allochthonous orthogneiss sheet of Archaean basement. Here, quartz c-axis fabrics have orthorhombic symmetry, implying that the orthogneiss unit and the thin smear of psammitic mylonites accreted to its base extended coaxially as a rigid sheet while the softer quartz-rich mylonites of the enclosing Moine accommodated the late non-coaxial strain. The thrust zone was probably reactivated within the old footwall and by ductile extension of the hanging wall along localised zones of intense non-coaxial strain. This conclusion emphasises the complexity and longevity of Caledonian ductile deformation.