GEOLOGY | Volume 44 | Number 10 | www.gsapubs.org 795 Zipper junctions: A new approach to the intersections of conjugate strike-slip faults John P. Platt 1 * and Cees W. Passchier 2 1 Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0742, USA 2 Department of Earth Sciences (Institut für Geowissenschaften), Johannes Gutenberg Universität, Becherweg 21, 55128 Mainz, Germany ABSTRACT Intersecting pairs of simultaneously active faults with opposing slip sense present geometrical and kinematic problems. Such faults rarely offset each other but usually merge into a single fault, even when they have displacements of many kilometers. The space prob- lems involved are solved by lengthening the merged fault (zippering up the conjugate faults) or splitting it (unzippering). This process can operate in thrust, normal, and strike-slip fault settings. Examples of conjugate pairs of large-scale strike-slip faults that may have zip- pered up include the Garlock and San Andreas faults in California (USA), the North and East Anatolian faults (Turkey), the Karakoram and Altyn Tagh faults (Tibet), and the Tonale and Giudicarie faults (southern Alps). Intersecting conjugate ductile shear zones behave in the same way on outcrop and micro-scales. Zippering may pro- duce complex and significant patterns of strain and rotation in the surrounding rocks, depending on the angle between the faults and the relative strength of the blocks they bound. A zippered fault will have a slip rate equal to the vector sum of the slip rates on the merg- ing faults, unless that displacement is transferred into or out of the system by distributed strain in the surrounding rocks. INTRODUCTION Intersecting pairs or sets of simultaneously active faults or shear zones are common on all scales and in all tectonic settings, and the mechanics of their formation and the nature of their interactions have been discussed by numerous workers (e.g., Lamouroux et al., 1991; Froitzheim et al., 2006; Schwarz and Kilfitt, 2008; Carreras et al., 2010; Yin and Taylor, 2011). Nevertheless, their kinematic evolution poses a significant problem. If faults of one set offset faults of the other, the offset faults are likely to be deactivated. If the faults terminate without offsetting one another, displacement is limited by the requirement that it be converted into a more distributed form of deformation (e.g., Kelly et al., 1998; Watterson et al., 1998). If the faults merge, a combination of strain and rotation is required in the area of the intersection to accommodate the difference in displacement vector. A wide variety of possible configurations for merging faults or shear zones is possible (Fig. 1, top) (Passchier and Platt, 2016). The most interesting case is where two faults with opposite senses of slip merge, zippering up to form a single fault (Fig. 1, bottom). We refer to the two faults in this situation as conjugate faults, without any dynamic or mechanical implications. The configuration shown in Figure 1 does not entirely resolve the problems noted above, but it does remove the appar- ent limitations on the amount of slip. Pure zipper junctions produce an inactive, fossil branch (Fig. 1B), while shear zipper junctions have slip on the merged segment (Fig. 1C). Fault zippering and unzippering has already been described in vari- ous contexts. One example is the so-called blind front developed in some thrust belts, where a backthrust branches off from a décollement surface at depth (Vann et al., 1986). The décollement progressively unzippers so that its hanging wall becomes the hanging wall of the backthrust and its footwall becomes the footwall of the active décollement. The opposite situation may arise in extensional settings, where conjugate low-angle normal faults merge to form a single detachment; in this case the two faults zipper up (Mohn et al., 2012; Fossen et al., 2014). This situation was described by Froitzheim et al. (2006), who referred to the merged fault as an extraction fault. Zippering of transform faults in a forearc set- ting has also been proposed by Authemayou et al. (2011). Here we focus on examples of pairs of active or recently active large-scale intraconti- nental strike-slip faults that we suggest have zippered up over significant distances. All of these examples occur in complex tectonic settings; the rate and amount of slip, the geometric evolution, and the mechanics are not fully known and are controversial. Our purpose is not to resolve all of these issues, but to point out the implications of the zippering process, which may help solve the underlying paradox presented by these fault systems in both active and ancient settings. THE SAN ANDREAS–GARLOCK FAULT INTERSECTION The right-slip San Andreas fault (SAF) and the left-slip Garlock fault (GF) meet on the northern side of the Transverse Ranges in southern Cali- fornia (USA; Fig. 2). Both faults are active; the SAF is generally regarded as the transform boundary between the Pacific and North America plates (Atwater, 1970), with a current slip rate of ~25 mm/yr in central California (e.g., Platt and Becker, 2010) and an accumulated slip of ~315 km since the middle Miocene (Crowell, 1979; Dickinson and Wernicke, 1997). The GF is reported to have a total slip of ~64 km (Davis and Burchfiel, 1973; Monastero et al., 1997), but both the amount and rate of slip change along its length (e.g., Platt and Becker, 2013). Meade and Hager (2005) estimated the slip rate on the western GF from geodetic data at 3.2 ± 1.5 *E-mail: jplatt@usc.edu GEOLOGY, October 2016; v. 44; no. 10; p. 795–798 | doi:10.1130/G38058.1 | Published online 17 August 2016 © 2016 Geological Society of America. For permission to copy, contact editing@geosociety.org. closing zipper sinistral freeway dextral freeway opening zipper sinistral closing zipper sinistral opening zipper dextral closing zipper dextral opening zipper zipper junctions freeway junctions shear zipper junctions extraction fault A B C Figure 1. Top: Main types of zipper and freeway junctions. Bottom: Evolution of zipper and shear zipper junctions. A: Initial state. B: Zipper junction, with equal and opposite displacements on conju- gate faults. Zippered section (extraction fault) has no displacement. C: Sinistral shear zipper junction. Extraction fault has sinistral dis- placement. Both types of junction can bring previously non-adjacent volumes of material into juxtaposition.