The three dimensional shape and localisation of deformation within multilayer sheath folds G.I. Alsop a, * , R.E. Holdsworth b a Dept. of Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK b Reactivation Research Group, Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK article info Article history: Received 21 March 2012 Received in revised form 22 August 2012 Accepted 24 August 2012 Available online 6 September 2012 Keywords: Sheath fold Deformation Detachment Self-similarity abstract Sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non-coaxial deformation. Where layering becomes rheo- logically significant, then active sheath folds may form in which the inner nose of the sheath decouples along detachments, and undergoes a translation relative to the outer folded layers that surround and envelope the nose. We present a systematic 3-D analysis of multiple folded layer geometries in a serially- sectioned natural sheath fold. Weak layers, which are reactivated as detachments, define different- shaped sheath folds relative to other layers, with the sense of cut-off along detachments reversing across the axial surface as the inner fold nose has “protruded” into the surrounding sheath envelope. Detachment layers are more tightly folded meaning that such active sheath folds are non-similar shapes. The obliquely oriented, bifurcating, en-echelon nature of the fold hinges developed in adjacent layers suggest that pre-cursor folds formed with pronounced 3-D obliquity relative to the subsequent shear plane. Mineral lineations folded around sheath closures display asymmetric “star-burst” patterns consistent with recrystallisation during active folding and hinge rotation. We show that the eye-fold shapes exposed in any 2-D y-z slice can be used to predict the geometry of marker horizons back along the x-axis in the third dimension. This self-similarity may be of value when tracing stratiform mineralised horizons in large-scale sheath folds. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Highly curvilinear sheath folds are most simply defined as folds displaying more than 90  of hinge-line curvature and are typically considered to form by the gradual rotation of fold hinges towards the shear direction during progressive deformation (e.g. Carreras et al., 1977; Cobbold and Quinquis, 1980). Folds are considered to initiate with gently curving hinges broadly normal to the shear direction, and may subsequently undergo opposing senses of rotation at either end of the hinge ultimately resulting in sheath fold geometries (see Alsop et al., 2007 and references therein). Sheath folds generated during homogeneous non-coaxial deformation in which the folded layering is rheologically no different from it’s surroundings, and plays no part in the mechanics of the folding process, have been termed passive sheath folds by Cobbold and Quinquis (1980, p. 120). Shear sense during such homogeneous deformation will be consistent across the entire sheath fold, the dimensions of which provide minimum constraints on the thickness of the enveloping shear zone (see Alsop et al., 2007; Bonamici et al., 2011). Although sheath folds are widely assumed to have formed passively (e.g. experiments of Cobbold and Quinquis, 1980, and Reber et al., 2012), there is clear evidence in many cases for buckle folding and boudinage indicating that mechanical heterogeneity may have played a role (e.g. Alsop and Carreras, 2007). Indeed, it has long been recognised that models in which active folding is completely replaced by passive hinge rotation during progressive fold evolution may be unrealistic (e.g. Treagus and Treagus, 1981). Sheath folds that are generated during heterogeneous deformation in which rheologically distinct layering results in the development of local discontinuities about the sheath closure may be termed active sheath folds. The shear couple generated in detachments around such sheath fold closures consistently suggest that during development of the sheath geometry, the nose has protruded into the outer folded layers that envelope the inner fold core (e.g. Alsop, 1994). Note that we here use the term “envelope” in the sense of Alsop (1994) where outer folded layers surround and entirely enclose the sheath nose. * Corresponding author. E-mail addresses: Ian.Alsop@abdn.ac.uk (G.I. Alsop), R.E.Holdsworth@ durham.ac.uk (R.E. Holdsworth). Contents lists available at SciVerse ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsg.2012.08.015 Journal of Structural Geology 44 (2012) 110e128