The distribution, geometry and kinematic significance of Caledonian buckle folds in the western Moine Nappe, northwestern Scotland

Within internal regions of orogenic belts, systems of ductile thrusts and folds may form which are geometrically and kinematically analogous to those observed in foreland thrust belts. The Moine Nappe of Sutherland, northwestern Scotland, developed during WNW-directed Caledonian displacements under amphibolite facies conditions. Broad zones of mylonite associated with the ductile Ben Hope and Moine thrusts both contain numerous, small-scale folds which can be attributed to transient flow perturbations. It is possible to isolate and differentiate generations of such folds based on their association with particular major thrust zones and/or their geometry. In regions of lower strain between these thrusts, there are a series of kilometre-scale buckle folds, the axial traces of which define arcuate trends. Those with hinges orthogonal to transport are northwest-vergence whilst those plunging subparallel to the mineral lineation display variable vergence. The spatial distribution of these geometries suggests a causal relationship with displacements on specific underlying thrusts. We propose that they reflect larger (kilometre-scale), and possibly longer-lived flow perturbations. The transport parallel folds are persistent features that often centre on earlier structural culminations, and are thought to reflect components of differential shear. The most prominent set of regional buckle folds root downwards into the ductile Moine Thrust, and openly refold the structurally overlying Ben Hope Thrust and its associated buckle folds, thus supporting a foreland propagating sequence of ductile displacements.