The geometry and kinematics of flow perturbation folds.

Alsop, G.I.; Holdsworth, R.E.

Abstract

Minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow. Using structures associated with WNW-directed Caledonian thrusting in N Scotland, we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction. These define a series of larger-scale fold culminations (reflecting ‘surging flow’) or depressions (reflecting ‘slackening flow’) that are bisected by transport-parallel culmination and depression surfaces. The fold patterns suggest a dominance of layer-normal differential shearing due to gradients in shear strain normal to transport. Culmination surfaces are marked by along-strike reversals in the polarity of structural facing and vergence of minor folds which, contrary to classic fold patterns, define reverse asymmetric relationships. Culmination surfaces separate folding in to clockwise (Z folds) and anticlockwise (S folds) domains relative to the transport lineation. The dip of fold axial planes systematically increases as their strike becomes sub-parallel to transport resulting in a 3D statistical fanning arrangement centred about the transport direction. Thus, mean S- and Z-fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved. Culminations display convergent fold patterns with fold hinges becoming sub-parallel to transport towards the culmination surface and underlying detachment, whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface. Thus, around culminations and depressions there are ordered, scale-independent relationships between transport direction, shear sense, fold facing, vergence and hinge/axial plane orientations. The techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow.

Citation

Alsop, G., & Holdsworth, R. (2002). The geometry and kinematics of flow perturbation folds. Tectonophysics, 350(2), 99-125. https://doi.org/10.1016/s0040-1951%2802%2900084-7

• Journal Article Type: Article
• Publication Date: 2002-05
• Journal: Tectonophysics
• Print ISSN: 0040-1951
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 350
• Issue: 2
• Pages: 99-125
• DOI: https://doi.org/10.1016/s0040-1951%2802%2900084-7
• Keywords: Folding; Vergence; Shear sense; Shear zone; Mylonite; Caledonides.