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

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