Patterns of curvilinear folding associated with shear zones

G.I. Alsop1 and R.E. Holdsworth2

1CGG, School of Geography & Geosciences, University of St. Andrews, KY16 9ST UK

2RRG, Department of Geological Sciences, University of Durham, DH1 3LE UK

gia@st-andrews.ac.uk

Deformation within mylonites related to major faults and shear zones may be both temporally and spatially variable, resulting in localised perturbations in flow associated with diverse folding. Continued progressive deformation may subsequently modify and reduce angular relationships between folds and fabrics, but the robust nature of the system will preserve the original geometric obliquities. Deformation cells thus 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. Two main associations of shear zone folds are typically recognised with a single, early phase of tight to isoclinal folds, often with mylonitic limbs that are usually preserved in low-strain augen. These structures may be cross-cut at low angles by the associated mylonitic shear zones. In addition, one or more local generations of later, syn-shearing folds are usually preserved within (or root downwards into) mylonitic high strain zones. These locally fold the mylonitic foliation and lineation whilst displaying geometric characteristics that are kinematically compatible with the main movement regime of the major shear zone. 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. 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. 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.

 

Schematic 3-D cartoon illustrating geometric relationships between asymmetric flow perturbations and sheath folds. Deformation and folding intensifies towards the underlying mylonitic detachment.