SHEAR ZONE ASSOCIATED FOLDS: READING RECORDS OF FLOW IN NATURAL SHEAR ZONES

Using examples from the Moine metasediments of N Scotland, we show that syn-shearing (local F3) folds display predictable geometric patterns that can be related to the development of flow perturbation cells during mylonitisation associated with Caledonian ductile thrusting under mid-crustal conditions. Fold axes and axial surfaces display consistent changes in asymmetry and sense of obliquity relative to local, transport-parallel mineral lineations that can be used to map out a series of flow culminations and depression zones. Earlier tight to isoclinal (local F2) folds are locally refolded by syn-shearing F3 folds and cross-cut by the mylonitic ductile thrust zones. However, these folds preserve more acute, but almost identical geometric patterns compared to the later syn-shearing folds, with culmination and depression zones often coinciding in location and scale. These observations suggest that the F2 and F3 folds are linked to the same kinematic regime of deformation, i.e. the F2 folds are not an earlier, unrelated phase of structures that have been passively overprinted by the ductile thrust-related high strain zones. In addition, a flow perturbation model can be applied to the earliest phases of folding that are regionally associated with ductile thrusting. Structures formed by such flow perturbations are remarkably resilient to the affects of ductile strain and are preserved even in those areas where the overprinting intensity of deformation is very high. The factors controlling the development of culmination and depression zones are similar at all stages of the deformation process. This suggests a long-lived control possibly linked to the presence of material anisotropies that either pre-dated, or were generated at an early stage of ductile thrusting. Consistent and ordered scale-independent relationships between the transport direction and fold hinges, fold asymmetries and axial surfaces thus provides a record of the transient perturbations in mylonitic flow generated within the coherent kinematic system. These predictable patterns of folding are likely to be repeated in all terrains where folds and shear zones are found in close association.