Shear zone folds: Structural analysis and interpretation

Abstract

Shear zones and associated folds form an important category of structures that are widely developed in the internal portions of orogenic belts. The analysis of such folds is crucial as they can be used to help determine kinematics, bulk strain and deformation histories of crustal-scale shear zones as well as influencing fluid flow and potential mineralisation. Our case study focuses on a Caledonian shear zone in NW Ireland that is marked by NW-directed crustal thickening (D1) that is followed by a later phase (D2) of SE-directed gravity-driven extensional collapse. We provide a detailed analysis of mesoscopic folds and fabrics formed during this greenschist-facies extension in order to answer some general questions including how to distinguish folds with hinges that initiate parallel to transport versus those folds that have rotated towards shear, the control that larger folds may have upon the geometry and orientation of smaller folds, and the influence that lithology plays in the evolution of shear zone folds. Our study comprises a mixed fold and fabric data set derived from a complex combination of i) flow perturbation folds with hinges that have initiated sub-parallel to transport, ii) sheath folds with hinges that have rotated towards transport during intense progressive shear, and iii) spiral folds with hinges that initiate normal to transport but fail to rotate as they roll and tighten. These mesoscopic folds form across a range of scales with smaller folds forming at a variety of angles, and in some cases transecting larger fold hinges indicating a non-Pumpellyan or incongruous relationship. Competent psammites host more open folds with hinges at higher angles to transport when compared to weaker pelites that contain tighter folds at variable angles to shear. Lithology and scale are therefore critical factors in controlling shear zone folds, with natural data sets commonly dominated by smaller-scale (<10 cm) folds that are more prevalent and easier to measure at outcrop. A consequence of this bias in recording data is that the amount of fold rotation (and hence bulk strain) may be over-estimated. A further complication is that fold pairs preserve more open upper hinges compared to tighter lower hinges that are closer to underlying detachments, meaning that the exact position of the hinge within fold pairs should be routinely recorded. The flattening of buckle fold hinges within competent psammites reflects the influence of gravity-driven deformation with an absolute sense of top-down-to the SE shear, which we interpret as being focussed into the shear zone during orogenic collapse.