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1 Crustal Geodynamics Group, School of Geography & Geosciences, University of St. Andrew, St. Andrews KY16 9AL, UK (e-mail: gia{at}st-andrews.ac.uk)
2 Reactivation Research Group, Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK
Variable patterns of displacement in shear zones may result in arcuate fold and fault traces as recognized by Mike Coward and co-workers in the early 1980s. Within such deformation zones, localized perturbations in flow may undergo acceleration (surging flow) or deceleration (slackening flow) with respect to the adjacent regions. Such flow cells may govern the orientation and geometries of folds and fabrics and thereby provide evidence of the scale and nature of deformation associated with heterogeneous flow in the high-strain zones. The length/width ratio of individual flow cells (measured in the direction of flow) may vary from <1 to >1 for situations when flow cells are, respectively, dominated by layer-parallel shear (LPS) or layer-normal shear (LNS). Folds initiating at high angles to transport are associated with LPS, whereas LNS may generate folds with slight clockwise (sinistral LNS) or anticlockwise (dextral LNS) trends relative to flow. Continued progressive deformation may subsequently modify and reduce angular relationships between folds and fabrics, but these geometric obliquities are generally preserved. Using examples from the Moine metasediments of northern Scotland, we show that folding displays predictable geometric patterns that can be related to the development of flow perturbation cells associated with Caledonian ductile thrusting under mid-crustal conditions. The differing relative timing of folds and individual ductile thrusts reflects the complexity of flow cells within ductile imbricates and additionally highlights the progressive foreland-directed propagation of ductile thrusting. These geometric relationships developed during contractional shear are compared with those generated in extensional systems, to provide an overall framework for the study of perturbation patterns. The geometric arrangement of mean fold axial planes about the flow direction results in their intersection forming parallel to the transport direction. This relationship permits transport directions to be calculated via the axial-planar intersection method (AIM), and also allows comparison with other techniques devised primarily for the study of soft-sediment deformation and palaeoslope analysis.
