Geometry of folded and boudinaged pegmatite veins emplaced within a strike-slip shear zone: A case study from the Caledonian orogen, northern Scotland

Abstract

The deformation of sheet-like igneous intrusions in multiple orientations can give detailed insights into the kinematics of ductile flow in shear zones. Using the Torrisdale Vein Complex in the northern Scottish Caledonides as a case study, we examine the relationships between folded and boudinaged pegmatitic intrusions deformed by strike-slip dominated shear. Within the shear zone, intrusions that trend clockwise of the adjacent regional foliation were commonly (75%) folded, whereas >80% of pegmatites at anticlockwise angles were asymmetrically boudinaged, giving dextral senses of shear parallel to the dominant and gently-plunging mineral lineations. Pegmatite fold trains at high angles to foliation (Sn) display the greatest % shortening, marked by steeply plunging ‘Z’ shaped folds when viewed down plunge. Pegmatites typically form Class 1C folds and were therefore more competent than their host gneisses at the time of deformation, i.e. they were not in the magmatic state. Such fold geometries are consistent with flattened parallel folds and suggest that up to 0.5 homogenous flattening was superimposed on the folds. Fold styles also indicate viscosity contrasts between the pegmatites and host gneisses of between 50 and 250 at the time of folding. Fold hinges are dispersed about an arc within the steeply-dipping foliation, suggesting a component of hinge rotation towards the mineral lineation during non-coaxial deformation. Folded pegmatites may also display boudinage on their limbs indicating that fold limbs had rotated into the extensional field during progressive deformation. Pegmatite boudin trains are typically developed <45° anticlockwise of Sn and display the greatest % extension when trending sub-parallel to the shear zone foliation. Within such trains, individual shearband boudins are more anticlockwise than the overall train and show right-stepping relative to neighbouring bodies. More equant boudins have rotated less than elongate boudins (aspect ratios >3) and preserve the largest anticlockwise angles with the overall boudin train. Conversely, domino boudins are clockwise of overall boudin trains and have undergone clockwise rotation marked by left-stepping of adjacent boudins. Domino boudins display flanking folds which progressively open as the deformable boudin rotates towards the foliation and margins of the boudin lozenges locally become clockwise of flow. This ‘unfolding’ of deflected foliation, together with local reversals in the sense of shear around rotating boudins, is consistent with a clockwise vorticity associated with bulk dextral shear. These observations collectively indicate that bulk dextral shear with an additional component of pure shear operated across the shear zone and provides further insights into the kinematics of mid-crustal deformation.