Progressive deformation structures associated with ductile thrusts in the Moine Nappe, Sutherland, N. Scotland

Abstract

In the deeper parts of mountain belts, polyphase structural sequences observed at outcrop-scale can arise due either to overprinting of regionally distinct deformation phases (e.g. reworking, changes in orogenic boundary conditions), or to localized controls that bring about transient changes in the patterns of ductile flow. These are unlikely to be mutually exclusive processes, and examples from Scotland demonstrate that, once regionally separate events are delimited using radiometric evidence, it is possible to isolate complex deformation sequences arising due to local controls. In the western Moine Nappe of Sutherland, the dominant structures were formed during Caledonian ductile thrusting towards the WNW, whilst earlier (?Precambrian) phases are relatively minor in importance. Two groups of Caledonian folds and fabrics are recognized in many exposures: main phase ( D 2) structures which are broadly contemporaneous with ductile thrust fabrics, and later secondary phase ( F 3) folds. The latter can be divided into two geometric groups: sheath-fold types which formed initially as WNW-overturned buckles subsequently modified by ductile shearing; and asymmetric types, which are commonly open folds apparently formed with axes close or sub-parallel to the thrust transport direction. Secondary structures show a close spatial association with high strain zones along ductile thrusts, and can be shown to have formed during the later stages of thrusting in certain critical exposures. I propose that they may form due to strain perturbations resulting from variations in the relative rates of ductile flow within the mylonites. Where differential shearing occurs due to flow-normal perturbations, wrench-related asymmetric fold types may form. In contrast, secondary sheath-fold structures may result from localized compression phases caused by flow-parallel perturbations. Local flow-perturbation models may be appropriate in situations where the distribution of later structures is apparently related to spatial variations in strain intensity and/or there is a strong geometric and kinematic similarity between sets of folds and fabrics. If such features are not observed, a regional polyphase sequence is more likely, and it is therefore necessary to obtain radiometric dates (or some other independent source of data) to demonstrate a time separation.