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_Journal of Structural Geology 32 (2010) 656e666_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Structure and development of an anastomosing network of ductile shear zones_ _Jordi Carreras a,*, Dyanna M. Czeck b, Elena Druguet a, Peter J. Hudleston c_ _a Departament de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain_ _b Department of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA_ _c Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA_ _article info_ _Article history: Received 23 July 2009_ _Received in revised form 3 March 2010_ _Accepted 30 March 2010_ _Available online 7 April 2010_ _Keywords: Anastomosing shear zones Conjugate shears Geometry Kinematics Metagabbro Mylonite_ _abstract_ _A detailed structural analysis of an anastomosing shear zone network in metagabbros from the Archean Rainy Lake zone (Canada) revealed the existence of prevalent dextral and minor sinistral conjugate shear zones with the obtuse angle (>130°) facing the main shortening direction. A typology of shear zone intersections, confluences, and other features shows that all shears formed during a single deformation event, with dextral and sinistral shears being active together or in an alternating fashion. In spite of the difficulty of establishing a complete kinematic sequence, early and late shears can be distinguished. The final angular pattern between dextral and sinistral shears is not an original feature. Dextral and sinistral shears formed at nearly right angles, and the angles progressively opened towards the extension direction as a result of increasing strain. The obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge-shaped domains of lesser-deformed rock bounded by the shears. Through time, there was an increasing prevalence of dextral shears over sinistral ones. The studied pattern and sequential analysis indicate that the bulk deformation was noncoaxial with a deformation regime evolving from a pure shear-dominated dextral transpression to a higher vorticity dextral transpression._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Ductile shear zones and brittle shear fractures accommodate deformation in a wide variety of rock types. Shear fractures often consist of many linked smaller fractures (Wilcox et al., 1973; Cundall, 1991; Olsson et al., 2004; Kim et al., 2004). Although individual shear zones are usually defined as planar bands of deformation surrounded by less deformed or undeformed rocks (e.g., van der Pluijm and Marshak, 2003), they may often be nonplanar and link together in a pattern of anastomosed networks with rather complicated geometries that bound undeformed or less deformed lozenges (Ramsay and Graham, 1970; Mitra, 1979, 1998; Ramsay and Allison, 1979; Bell, 1981; Choukroune and Gapais, 1983; Gapais et al., 1987; Burg et al., 1996; Corsini et al., 1996; Hudleston, 1999; Arbaret et al., 2000; Carreras, 2001; Czeck and Hudleston, 2003, 2004; Bhattacharyya and Czeck, 2008). These geometrical organizations of shear zones are of primary importance as they have been called upon to partially control both strain accumulation (e.g., Hudleston, 1999) and deformation mechanisms (e.g., Fusseis et al., 2006)._ _* Corresponding author. Fax: +34 935811263. E-mail address: jordi.carreras@uab.cat (J. Carreras)._ _0191-8141 $ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.03.013_ _Whereas the formation and development of networks of brittle shear fractures are well documented in the literature (e.g., Wilcox et al., 1973; Maerten et al., 2002), less information on the progressive development of ductile or brittle-ductile shear zones and the interaction between shear zone strands exists (Carreras et al., 2000; Pennacchioni and Mancktelow, 2007)._ _When evaluating a network of shear zones, field geologists often look at the second order features to determine the temporal relationships between the strands. For example, shear zone thickness may be considered roughly proportional to age (Mitra, 1979), although caution is necessary when using this approach because the rheology of the material (i.e., strain hardening vs. strain softening) will affect this relationship (Hull, 1988; Means, 1995; Schrank et al., 2008)._ _The pattern of interrelated natural ductile shear zones varies considerably, from fairly regular conjugate pairs (Lamouroux et al., 1991) to sets that may have an octahedral arrangement (Mitra, 1979), to more complicated and less regular patterns of interconnectedness (Bhattacharyya and Hudleston, 2001). Some studies have indicated a similarity between the temporal evolution of ductile shear zone strands and brittle shear zones (Lamouroux et al., 1991). However, the two types of structures may evolve differently due to the differences in the active deformation mechanisms and the strain compatibility requirements in ductile shear zones. Schwarz and Kilft (2008) conducted analogue experiments of conjugate fault systems and demonstrated that the kinematics become more complicated as deformation progresses. In particular, directions of fault propagation are influenced by motion along neighbouring faults, and dominant activity alternates between neighbouring faults. A similar progression of alternating active strands may occur in the ductile shear zones. In order to understand the evolution of shear zone sets with a numerical approach, Mancktelow (2002) tested the effect of different rheologic parameters and deformation geometries in the development of conjugate shear zone systems and found that typically, conjugate shear zones initiate at approximately perpendicular orientations, and rotate with increasing bulk deformation. These experimental results also elucidate some of the difficulties in understanding shear zone network evolution. In particular, the relative timing of different shear zones may be difficult to establish because, in some instances, much like in brittle fault networks, the displacement along different shears has a pulsating character, shifting from one to another._ _Overall strain accumulation will be controlled both by the shear zones themselves, the interactions between shear zones, and the amount and type of strain that is partitioned into the wall rocks and at the interior of shear-bounded lozenges. One of the main differences between brittle and ductile shear networks is that in the latter, the angle facing the mean shortening direction may increase with progressive deformation (Mitra, 1979; Ramsay and Huber, 1987). This angular evolution cannot be achieved merely by passive rotation of the shear zones because strain compatibility requires contemporaneous internal deformation of the shear-bounded domains. In most cases, strain compatibility is maintained by a combination of strain within the shear zones, strain localization producing new shear zones, and internal deformation of the lozenges. Simultaneous rotation of initial shears plus the propagation of new ones merging with preexisting ones results in the typical network consisting of anastomosing shear zones and lozenges._ _Several authors have considered geometrical configurations and strain accumulation in and near shear zone networks. Gapais et al. (1987) described shear zone patterns that they interpreted to reflect the symmetry of the bulk strain, orthorhombic in the case of coaxial deformation and lower symmetry for noncoaxial deformation. They suggested that the individual shear zones tend to track surfaces of no finite deformation. In contrast, Mitra (1979) treated individual shears as markers that initiated at 90° on principal planes and rotated during deformation of the surrounding rock mass. Hudleston (1999) examined three-dimensional arrays of shear zones and determined that the evolving network geometries are inextricably linked to the maintenance of strain compatibility._ _The intersection points of shear zones within networks are loci for complex strain accommodation and potential secondary structures. Lamouroux et al. (1991) developed a theoretical analysis of geometrical compatibility constraints at domains of intersecting conjugate natural shear zones. Based on their analysis, shear zone networks should evolve in the same sequence as brittle shear networks, but the strains at the intersection regions are more complex. Pennacchioni and Mancktelow (2007) further demonstrated with a field example the complex strains and the compatibility problems at conjugate shear zone intersection points. Harris (2003) documented further complexities that can occur when shear zones interact, explaining the geometry and the kinematics of back-rotated folds that occur within shear-bounded lozenges when the bounding shear zones come into close proximity. In fact, other studies have similarly concluded that many lozenges in ductile networks contain some type of internal deformation (Bell, 1981; Treagus and Lan, 2000; Pennacchioni and Mancktelow, 2007; Jessell et al., 2009)._ _Despite these advances, key questions concerning the structure and evolution of anastomosing patterns of shear zones remain: (i) What are the initial orientation and kinematics of the sets? (ii) How are strain and kinematic compatibility maintained at the domains of merging shear strands? (iii) How do the sets evolve geometrically and kinematically during progressive deformation? (iv) In what orientations are new shears or sets added? (v) What is the behavior (internal deformation and/or rigid body rotation) of shear-bounded lozenges? All of these questions could be addressed from the analysis of naturally deformed examples from the micro-to the macro-scale._ _This paper describes a process-oriented study based on..._ Ключевые слова: e, r, o