Tectonics of strike-slip restraining and releasing bends

One of the remarkable tectonic features of the Earth’s crust is the widespread presence of long, approximately straight and geomorphically prominent strike-slip faults which are a kinematic consequence of large-scale motion of plates on a sphere (Wilson 1965). Strike-slip faults form in continental and oceanic transform plate boundaries; in intraplate settings as a continental interior response to a plate collision; and can occur as transfer zones connecting normal faults in rift systems and thrust faults in fold–thrust belts (Woodcock 1986; Sylvester 1988; Yeats et al. 1997; Marshak et al. 2003). Strike-slip faults also are common in obliquely convergent subduction settings where interplate strain is partitioned into arc-parallel strike-slip zones within the fore-arc, arc or back-arc region (Beck 1983; Jarrard 1986; Sieh & Natawidjaja 2000). When strike-slip faults initiate in natural and experimental settings, they commonly consist of en échelon fault and fold segments (Cloos 1928; Riedel 1929; Tchalenko 1970; Wilcox et al. 1973). With increased strike-slip displacement, and independent of fault scale (Tchalenko 1970), fault segments link, and the linked areas along the ‘principal displacement zone’ may define alternating areas of localized convergence and divergence along the length of the strike-slip fault system (Fig. 1; Crowell 1974; Christie-Blick & Biddle 1985; Gamond 1987). Typically, divergent and convergent bends are defined as offset areas where bounding strike-slip faults are continuously linked and continuously curved across the offset, whereas more rhomboidally shaped stepovers are defined as zones of slip transfer between overstepping, but distinctly separate and subparallel strikeslip faults (Wilcox et al. 1973; Crowell 1974; Aydin & Nur 1982, 1985). However, fault stepovers may evolve into continuous fault bends as the bounding faults and connected splays propagate and link across the stepover (e.g. Zhang et al. 1989; McClay & Bonora 2001). Thus, the two terms ‘stepover’ and ‘fault bend’ are often used interchangeably. Bends that accommodate local contraction are referred to as restraining bends, and those that accommodate extension are referred to as releasing bends (Fig. 1; Crowell 1974; Christie-Blick & Biddle 1985). Double bends have bounding strike-slip faults which enter and link across them, whereas single bends are essentially strike-slip fault-termination zones. Restraining and releasing bends are widespread on the Earth’s surface, from the scale of major mountain ranges and rift basins to sub-outcrop-scale examples (Swanson 2005; Mann this volume). Releasing bends have also been documented along oceanic transforms connecting spreading ridges (Garfunkel 1986; Pockalny 1997), and extra-terrestrial restraining bends have been interpreted to occur on Europa and Venus (Koenig & Aydin 1998; Sarid et al. 2002). Strike-slip restraining and releasing bends are sites of localized transpressional and transtensional deformation, respectively. Thus, bends are characterized by oblique deformation that is ultimately controlled by larger-scale relative plate motions either acting on relatively straight, long interplate boundaries (Garfunkel 1981; Mann et al. 1983; Bilham & Williams 1985; Bilham & King 1989) or acting across more complex zones of intraplate deformation where faults tend to be shorter, less continuous and more arcuate (Cunningham this volume). Within the bend, oblique deformation may be accommodated by oblique-slip faulting or partitioned into variable components of strike-slip and dip-slip fault displacements (Jones & Tanner 1995; Dewey et al. 1998; Cowgill et al. 2004b; Gomez et al. this volume). As seen in deeply eroded outcrop exposures or from subsurface geophysical surveys, double restraining bends and releasing bends commonly define positive and negative flower structures respectively, and strikeslip bends or ‘duplexes’ in plan view (Fig. 1; Lowell 1972; Sylvester & Smith 1976; ChristieBlick & Biddle 1985; Harding 1985; Woodcock & Fisher 1986; Dooley et al. 1999), although considerable structural variation and complexity occurs (Barka & Gulen 1989; May et al. 1993; Wood et al. 1994; Waldron 2004; Barnes et al. 2005; Decker et al. 2005; Parsons et al. 2005). Single bends commonly have horsetail splay fault