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Tectonics of strike-slip restraining and releasing bends

W. D. CUNNINGHAM1 & P. MANN2 1Department of , University of Leicester, Leicester LEI 7RH, UK (e-mail: [email protected]) 2Institute of , Jackson School of Geosciences, 10100 Burnet Road, R2200, Austin, Texas 78758, USA (e-mail: [email protected])

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

From:CUNNINGHAM,W.D.&MANN, P. (eds) of Strike-Slip Restraining and Releasing Bends. Geological Society, London, Special Publications, 290, 1–12. DOI: 10.1144/SP290.1 0305-8719/07/$15.00 # The Geological Society of London 2007. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

2 W. D. CUNNINGHAM & P. MANN

Contractional horsetail splay Transtensional Extensional relay ramp strike-slip duplex

Oblique deformation Pull-apart belt – transpressional orogen Narrow push up

Positive Contractional flower Releasing strike-slip duplex structure bend Paired bend Restraining bend Sharp rest raining bend Paired bend bypass fault Gentle restraining bend Extensional Double restraining bend horsetail Single restraining bend splay

Transtensional rift Dilational stepover En echelon folds Contractional stepo Negative flower ver structure

Fig. 1. Tectonic features associated with strike-slip restraining and releasing bends. geometries in plan view, with strike-slip displace- lowest topographic depressions, such as the Dead ments terminally accommodated by oblique-slip Sea (ten Brink et al. 1999), Death (Christie- and dip-slip faulting (McClay & Bonora 1997). Blick & Biddle 1985) and submarine basins under- Adjacent restraining and releasing bends called lying the Gulf of Aqaba (Elat; Ben-Avraham 1985), ‘paired bends’ by Mann (this volume) are com- the (Leroy et al. 1996, AAPG) and monly described from strike-slip systems in all tec- the Gulf of California (Persaud et al. 2003). tonic settings and may reflect a volumetric Restraining and releasing bends along both con- balancing between crustal thickening and uplift at tinental and oceanic strike-slip faults may act as restraining bends, and crustal thinning and basin barriers to propagation (King & formation at releasing bends (Woodcock & Nabelek 1985; Sibson 1985; Barka & Kadinsky- Fischer 1986). Cade 1988) or conversely, they may provide Restraining bends are sites of topographic uplift, nucleation sites for major (e.g. Shaw crustal shortening and of crystalline 2006). There are also documented cases of large basement (Segall & Pollard 1980; Mann & fault bend earthquakes (M . 7) having complex Gordon 1996; McClay & Bonora 2001), whereas rupture mechanisms with multiple faults being acti- releasing bends are sites of , crustal vated within the bend, as well as major faults rup- extension, significant basin , high turing through the bend (Bayarsayhan et al. 1996; fluid flow, and possible volcanism (Aydin & Nur Harris et al. 2002). Because the length of fault 1982; Mann et al. 1983; Hempton & Dunne 1984; segment rupture is proportional to earthquake mag- Dooley & McClay 1997). Restraining bends and nitude (Scholz 1982), identification of fault bends releasing bends are commonly elongate, lazy-S- or between parallel strike-slip fault segments that Z-shaped features in plan view, and they may may act as seismic propagation barriers is important form the dominant topographic and structural in assessing the potential severity of future earth- feature within a deforming region. With increased quakes in areas of active strike-slip faulting. Docu- strike-slip offset, S- and Z-shaped pull-apart menting three-dimensional fault connectivity and basins may evolve into more rhomboidally shaped kinematics within an individual fault bend is features (Mann et al. 1983). important for assessing whether the bend may act Restraining bends produce elongate, individual as a future earthquake propagation barrier massifs with anomalously high topographic (Graymer et al. this volume). elevations such as the Denali Range in Alaska In addition to earthquake hazards, tectonically (Fitzgerald et al. 1993), the Lebanon and Anti- bends have other societal relevance. Lebanon ranges of the (Gomez et al. Restraining bends may: this volume), or the Cordillera Septentrional on the of Hispaniola (Mann et al. 1984, 2002). Releas- 1. exhume crystalline basement rocks that ing bends produce pull-apart basins and fault- contain important deposits (e.g. bounded troughs that comprise some of Earth’s Pinheiro & Holdsworth 1997); Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

TECTONICS OF STRIKE-SLIP BENDS 3

2. host hydrocarbons in their interiors and flank- Bonora 2001). Because fault bends typically form ing basins (Christie-Blick & Biddle 1985; in mechanically heterogeneous crust, pre-existing Escalona & Mann 2003; Decker et al. faults and basement fabrics may be reactivated 2005); and instead of new faults generated. The orientations of 3. form major topographic uplifts that provide a reactivated older structures are unlikely to be ideal locally significant rain catchment area and for either pure strike-slip or pure dip-slip motions, potential resources (Gobi Altai thus oblique-slip displacements on reactivated and Altai restraining bends, Cunningham faults are typically important within fault bends, et al. 1996; Cunningham 2005, this volume). and workers should therefore be aware of field cri- teria that indicate fault reactivation (Holdsworth The societal significance of releasing bends et al. 1997). includes the following:

1. pull-apart basins form depressions containing Strain magnitude and distribution significant sedimentary accumulations that may host hydrocarbons, metalliferous deposits, Strike-slip displacements along master faults that evaporites and other industrial (e.g. enter a fault bend will be partially or wholly accom- the Vienna Basin, Hamilton & Johnson 1999; modated by deformation within the bend (Segall & Hinsch et al. 2005); Pollard 1980). Thus, large displacement strike-slip 2. releasing bends may be zones of high heat flow faults are capable of producing the largest restrain- and crustal dilation that can be exploited as ing and releasing bends. However, small restraining sources of geothermal energy, such as the and releasing bends may also exist along major Coso geothermal area of California (Lees strike-slip faults, especially when early formed 2002) and the Cerro Prieto geothermal area bends are bypassed as the system evolves (Bennett of Mexico (Glowacka et al. 1999); and et al. 2004; Mann et al. this volume), or when 3. releasing bends may create large valleys that fault bends nucleate late in the history of a strike-slip provide fertile agricultural land and flat-lying fault system (Sieh & Natawidjaja 2000), or when the urbanized areas, such as the Imperial Valley releasing stepover and basin depocentre has pro- of southern California, the Silicon Valley of gressively migrated along the master strike-slip northern California, the Vienna Basin of system, instead of maintaining a fixed position rela- Austria, and the –Sea of Galilee tive to the adjacent sliding blocks (Wakabayashi Valley in the Middle East. et al. 2004; this volume; Lazar et al. 2006). Depend- ing on the angle between the master strike-slip fault and the far-field displacement direction, the degree Origin and evolution of strike-slip fault of of oblique deformation within bends the bend into separate thrust, normal and strike-slip displacements will control bend evolution. Kin- Factors that influence and control the origin and ematic partitioning of non-coaxial strike-slip and progressive development of restraining and releas- coaxial strains is common when the far-field displa- ing bends are complex and numerous, but can be cement direction is strongly oblique (,208) to the grouped into several major research themes. deformation zone boundary (Dewey et al. 1998). In addition, three-dimensional strain in strike-slip Fault geometry and reactivation settings typically involves vertical-axis rotations (e.g. Jackson & Molnar 1990). Thus, the progressive The shape, topography and internal architecture of a evolution of a fault bend may involve local vertical fault bend is fundamentally controlled by several axis rotations within the bend, and vertical axis factors, including the orientation of the plate rotations in the larger region that the bend occurs motion vector relative to the master strike-slip within (Luyendyk et al. 1980; Westaway 1995; fault; the original width of the stepover; and Cowgill et al. 2004b). This may be indicated by whether the bend is a strike-slip fault termination; changes in strike trends, and can be proven palaeo- a double bend along a single continuously linked magnetically (Luyendyk et al. 1985). Progressive strike-slip fault; or a stepover where parallel strike- vertical-axis rotations within a fault bend will slip fault segments are offset and may or may not result in changing fault kinematics as the faults overlap. For example, wide stepovers may contain rotate relative to the external field. fewer faults that bridge the gap between master Vertical-axis rotations may thus lead to fault aban- strike-slip faults, whereas narrow stepovers may donment and propagation of new faults. In addition, have greater linkage between major faults within strain hardening processes may operate locally the bend (Dooley & McClay 1997; McClay & within the bend and may influence whether old Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

4 W. D. CUNNINGHAM & P. MANN faults remain active or lock up (Cowgill et al. releasing bends through normal faulting and 2004a). , will lead to changes in the thermal evol- ution of the fault bend. This can be demonstrated by fission-track and other geothermometric data Stress field considerations which reveal the timing and rates of exhumation The orientation of the maximum horizontal stress (Fitzgerald et al. 1995; Blythe et al. 2000; Batt S et al. 2004). If heat flow increases within a restrain- ( Hmax) relative to the deformation zone boundary will strongly influence the degree of ing bend region, then it may lead to increased buoy- or within the fault bend region ancy and topographic uplift. This may lead to (Tikoff & Teyssier 1994). Fault bends that form positive feedback between uplift and erosion and S progressive exhumation of mid-crustal rocks where Hmax is at a high angle to the deforming zone will tend to have large dip-slip displacements, similar to the crustal aneurysm model proposed thus forming large restraining bend (e.g. for structural culminations in the Himalayan syn- Karlik Tagh Range, China, Cunningham et al. taxes (Zeitler et al. 2001). In releasing bend set- 2003) or wide and deep releasing-bend basins tings, high extensional strains in pull-apart basins (e.g. Sea of Japan, Jolivet et al. 1994) When may lead to increased heat flow and possibly vol- regional plate-motion changes lead to stress-field canism; extrusive rocks may then constitute volu- changes in transform boundary settings, ratios of metrically significant basin fill (Dhont et al. 1998). strike-slip to dip-slip displacements within fault Because all of these factors will be different for bends will change and the fundamental architecture every fault bend, it follows that restraining and and topographic development of the bend will releasing bends should be diverse in – with reflect that change. In addition, fault bends may each one having unique topographic, geomorpholo- switch from transtensional to transpressional gical, architectural and evolutionary characteristics. systems or vice versa, if the original fault Analogue models of fault bends have recreated S some of the fault patterns and topographic charac- bend was at a low angle relative to Hmax (Tikoff & Teyssier 1994). Thus, transtensional basins may teristics of natural examples, and have documented become inverted and restraining bends may be progressive stages of evolution (Hempton & Neher cross-cut by overprinting transtensional faults 1986; Dooley & McClay 1997; McClay & Bonora (Legg et al. this volume). Stress fields within 2001); however, they have not included many of individual fault bends may also evolve with pro- the factors considered here, and so they must be gressive faulting, structural compartmentalization regarded as somewhat generic. and increased mechanical interaction between inter- With this previous work in mind, and in order to secting faults, resulting in fault motions that are bring together workers from around the world who internally guided (Muller & Aydin 2004; are actively investigating strike-slip fault bends, an Waldron et al. this volume; Fodor this volume). international meeting on the tectonics of strike-slip restraining and releasing bends in continental and oceanic settings was convened in London on 28– Feedback between climate, topography, 30 September 2005, under the auspices of the Geo- faulting and thermal history logical Society of London. This volume includes contributions that were presented at the conference, Long-term climate patterns and mountain erosion and new results by others whose research connects rates compete with mountain uplift and influence with the conference theme. the extent of topography generation or destruction for all mountain ranges, including restraining bends (Anderson 1994; Willett et al. 2001). If a This volume restraining bend achieves a steady state between uplift and erosion, then its dimensions will stabilize, The 17 papers included in this volume cover a and thus individual faults will tend to remain active variety of topics and regions related to tectonics, and new faults may not form (Beaumont et al. 1991; geology and geophysics of restraining and releasing Norris & Cooper 1997; Willett 1999). In addition, bends. The papers are organized into three major larger releasing bends that evolve into marine themes: (1) bends, sedimentary basins and earth- basins may also influence local climate, driving quake hazards; (2) restraining bends, transpres- changes in precipitation, erosion and rates of sedi- sional deformation and basement controls on ment deposition – thus influencing the overall development; and (3) releasing bends, transten- dimensions of the releasing bend basin (e.g. Sea sional deformation and fluid flow. Many papers of Marmara, Turkey). have multiple emphases, and these subdivisions Progressive exhumation of deeper crustal rocks are general guides to subject content only. The in restraining bends by uplift and erosion, and in brief discussion below is meant to summarize key Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

TECTONICS OF STRIKE-SLIP BENDS 5 results and conclusions of each study, without being explain the origin and evolution of bends are exhaustive. In addition to this volume, the reader is presented: referred to several other important volumes which cover related topics of strike-slip fault tectonics 1. progressive linkage of en echelon shears within (Sylvester 1988); continental transpressional and a young evolving zone; transtensional tectonics (Holdsworth et al. 1998); 2. formation of lenticular ‘sidewall ripout’ struc- intraplate strike-slip deformation belts (Storti tures at scales ranging from outcrop to regional; et al. 2003); strike-slip deformation, basin for- 3. interaction of propagating strike-slip faults mation and sedimentation (Biddle & Christie-Blick with pre-existing crustal structures such as 1985); and continental wrench tectonics and hydro- ancient rift basins; and carbon habitat (Harding 1985; Zolnai 1991). 4. concentration of regional maximum compres- The volume begins with a review paper by sive stress on pre-existing, basement structures Mann, which contains a global compilation of in intraplate continental regions. active and ancient releasing and restraining bends, with the aim of defining common modes of origin With over 225 modern bends in this global compi- and tectonic development. He identifies five main lation, it follows from uniformitarian principles tectonic settings for strike-slip faults and related that restraining bends and releasing bends must bends: also have been widespread in the geological past. Whilst most modern restraining and releasing 1. oceanic transforms separating oceanic crust bends have obvious topographic expression, and offsetting mid-oceanic spreading ridges; ancient examples may occur in regions that have 2. long and linear plate-boundary strike-slip fault been eroded flat, or are buried beneath sedimentary systems separating two continental plates cover, or are overprinted by younger orogenic whose plate-boundary kinematics can be quan- events and thus may be difficult to discern. tified for long distances along strike by a single pole of rotation (e.g. the system of western North America); Bends, sedimentary basins, and 3. relatively shorter, more arcuate, indent-linked, earthquake hazards strike-slip fault systems bounding escaping continental fragments in zones of continent– Many of the original ideas regarding continental continent or arc–continent collision (e.g. the transforms and restraining and releasing bends ); were developed by workers in California investi- 4. straight to arcuate trench-linked strike-slip fault gating the San Andreas system (e.g. Crowell systems bounding elongate fore-arc slivers gen- 1974). Early work focused on the on-land erated in active and ancient fore-arc settings by expression of the diffuse plate boundary, whereas oblique (e.g. Sumatra); and it is now recognized that some of the interplate 5. continental interior, intraplate strike-slip fault strain is also accommodated offshore in the systems removed from active plate boundaries, southern California borderland region. This formed on older crustal faults, but acting as subject is addressed by Legg et al., who provide a ‘concentrators’ of intraplate stresses. review of restraining bends and releasing bends formed during the last 20 Ma along the diffuse By far the most common, predictable and best- Pacific–North American transform boundary in studied setting for restraining and releasing bends the southern California borderlands. By combining occurs in continental plate boundary strike-slip multi-beam swath bathymetry, high-resolution fault systems, where arrays of two to eight en seismic imaging, earthquake and geological data, echelon pull-apart basins mark transtensional fault they describe two major strike-slip restraining segments, and single and sometimes multiple large bends in the largely submarine setting – structures restraining bends mark transpressional segments; that are beautifully imaged because of the dimin- fault areas of transtension versus transpression are ished effects of submarine erosion as compared to determined by the intersection angles between on-land examples. Pre-existing rift trends have small circles about the interplate pole of rotation influenced the stepping geometry of the major and the trend of the strike-slip fault system. These strike-slip faults, and there is a common association longer and more continuous boundary strike-slip of double restraining bends bounded by releasing systems also exhibit a widespread pattern of bends, an association which is also seen in ‘paired bends’ or ‘sidewall ripouts’, or adjacent on-land examples in southern California. The zones of pull-aparts and restraining bends that largest restraining bends approach 100 km in range in along-strike-scale from kilometres to hun- length, and could be sites of major (M7 or greater) dreds of kilometres. Four evolutionary models to earthquakes. Because of their offshore setting and Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

6 W. D. CUNNINGHAM & P. MANN rugged topography, the active faults within large may be through-ruptured. However, stepovers of restraining bends pose a potential hazard 4–5 km width always arrest fault rupture, regard- which is only now being appreciated. less of the amount of displacement. An important consideration for understanding Although all major continental transform bound- the evolution of restraining and releasing bends is aries tend to have restraining and releasing bends whether their location remains fixed with respect along them, the Scotia–Antarctic transform bound- to adjacent laterally moving blocks or whether ary is particularly interesting, because it has both they migrate along strike with increased fault dis- oceanic and continental crustal elements along its placements. Wakabayashi addresses this question length, and stepover nucleation and development by looking at numerous examples from the San are directly related to the distribution of the two Andreas system, where he documents the sedimen- different types of crust. Bohoyo et al. present new tary and structural record of stepover migration, geophysical data that are used to image and map including ‘wakes’ of deposits trailing behind the distribution of restraining and releasing bends migrating stepovers as well as the sedimentary along this remote submarine plate boundary. Their and structural expression of migrating of most important conclusion is that the distribution of former releasing bends. Importantly, he compares releasing bend basins, and other bathymetric the sedimentary wake length to the overall displace- troughs formed by transtension, is directly controlled ment of the master fault system, in order to quantify by the distribution and shape of rheologically weak the magnitude of stepover migration. Two end continental fragments which rift more easily than sur- members are possible: stepovers that migrate for rounding, stronger oceanic crust. In contrast, restrain- the entire duration of strike-slip displacement, and ing bends and areas of transpression occur at the those that remain fixed. Fixed restraining bends interface between crust types, where oceanic crust will tend to have the largest structural relief and underthrusts continental blocks. greatest erosional exhumation, and will form regionally significant topographic and structural culminations. However, other smaller restraining Restraining bends, transpressional bends with less relief may have existed for just as deformation and basement controls long, but their migration limits their topographic on development and structural development, because uplifted/ downdropped areas are soon abandoned as defor- One of the most interesting distant effects of the mation moves along strike. The implication is that Indo–Eurasia collision is the active intraplate trans- size of bend may not reflect longevity. pressional mountain building in western China and, Although fault bends are widely regarded as in particular, Mongolia. In a paper by Cunningham, earthquake propagation barriers for most major the geological and structural characteristics of 12 active strike-slip fault systems (Sibson 1985; separate restraining-bend mountain ranges from Barka & Kadinsky-Cade 1988), surface fault com- the Altai, Gobi Altai and easternmost Tien Shan plexity may mask relatively simple patterns at are reviewed and compared. All bends have individ- depth (.5 km deep). This is expected if the faults ual structural, topographic and dimensional charac- within the stepover define a flower structure and teristics, due to multiple factors which influence surface faults root into a singular master fault. In their characteristics, including: stepover width; pre- a paper by Graymer et al. the authors demonstrate existing basement structures, angular relation that carefully located hypocentres beneath both S between main and Hmax; and local tec- restraining and releasing bends in California, tonic setting. The significant structural and topo- where large earthquakes have occurred, define graphic diversity challenges simplistic models of singular or simple fault patterns. They conclude restraining-bend evolution. In addition, there is an that stepover zones provide less of an impediment orogenic continuum: from isolated restraining to through-going rupture than previously assumed. bends along major strike-slip faults, to thrust- Exceptions may be those large bends which have dominated transpressional ridges with only minor complex multilayered fault patterns reflecting both strike-slip transfer zones between thrust ridges. strike-slip and thrusting displacements and which The key point is that as restraining bends grow have master strike-slip displacements migrating along-strike and across-strike they may coalesce through the bend and eventually bypassing the with neighbouring ranges and lose their individual bend. Their conclusions complement results from identities. Mature oblique deformation belts may Lettis et al.’s (2002) compilation of 30 historical obscure the original restraining bends uplifts strike-slip earthquake ruptures involving 59 step- which were the initial nucleation sites for over basins, which indicated that strike-slip events mountain building. with small to large displacements usually propagate The island of Jamaica is essentially the through stepovers less than 1–2 km wide. With morphological expression of a large Late Miocene increasing displacements, 2–4-km-wide stepovers to Recent restraining bend along the North Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

TECTONICS OF STRIKE-SLIP BENDS 7

America– boundary, and its origin Cowgill et al. (2004a, b) for the Akato Tagh bend and progressive development are described by on the Altyn Tagh Fault of Tibet. Smith et al. and Mann et al. By analysing geodetic, geological Morley et al. also link their results from Thailand and seismic data they document the continued to an evolving deformation regime initially driven uplift and evolution of the bend in eastern by collision, but later driven by escape tec- Jamaica (Blue Mountains), along with less well- tonics due to the Indo–Eurasia collision. expressed bends in central and western Jamaica. An unusual example of a restraining bend However, seismicity along the south coast of the formed within a is presented island suggests that a more linear short-cut fault is by Zampieri et al. who document a north–south developing, which will bypass the range, and that polydeformed relay zone cutting across the Italian the interplate strain may progressively transfer Alps, a zone that has localized transpressional to that fault system. Another important conclusion deformation at a prominent restraining stepover. of their study is that the restraining bends of Liassic and Palaeogene north–south extensional Jamaica were initially localized by older basement structures were reactivated during Alpine com- faults related to Palaeogene which trend pression as a strike-slip relay zone within the northwesterwards and oblique to the evolving thrust belt. Reactivation of normal faults and inver- plate boundary. sion of the older fill produced a complex Seyrek et al. provide a careful correlation of restraining bend with different degrees of shorten- Pleistocene across the northern Dead Sea ing on either side of the stepover, due to juxtaposed transform boundary, coupled with new age data to sequences with strongly contrasting rheological calculate slip rates along the plate boundary since properties. There are very few studies of restraining the Pliocene (c. 3.73 Ma). An important implication bends formed locally along transfer faults within of their work is that the calculated displacement major fold-and-thrust belts, especially where older vectors and slip rates require that the northern con- extensional structures have been reactivated; tinuation of the transform in southern Turkey must however, their study suggests that other examples be convergent, and that the entire Amanos Moun- await discovery. tain and Karasu Valley region constitutes a gentle In another example of polyphase deformation in restraining bend with active uplift in the Amanos an intraplate restraining bend, but in an older Mountains. The overall geometry and kinematics Palaeozoic setting, Waldron et al. document the are very similar to the Lebanon stepover, where detailed and complex internal structures within a Gomez et al., using geological and geomorphologi- portion of a flower structure that formed during cal fieldwork, cosmogenic dating, seismicity data, right-lateral Carboniferous strike-slip movement GPS results, and the analysis of relative plate along the Minas fault zone in Nova Scotia. Their motions, propose a two-stage history to the bend: study underscores the importance of detailed struc- an early wrench-dominated stage, followed by the tural analysis to unravel different phases of folding, modern strain-partitioned transpression-dominated thrusting and oblique deformation within a zone of stage. The switch was probably driven externally localized transpression. The structures that they by changes in relative plate motions. The recog- observe are consistent with a progressive change nition of strain-partitioned deformation within the in the local angle of convergence, increased strain modern bend has implications for the regional partitioning, and pure-shear dominated transpres- ; multiple strike-slip faults are sion. An important implication of their work is active within the bend region and growing anticli- that as the restraining bend developed, significant nes may hide seismically active blind thrusts. topography was generated, and the deformation A pair of papers by Smith et al. and Morley migrated laterally from inner, dominantly steep et al. addresses the structural evolution of transpres- transpressional zones, to outer, low-angle zones sional zones along the active, left-lateral Mae Ping enhanced by the presence of Late Palaeozoic eva- Fault Zone in central Thailand. By using satellite porite layers that promoted low-angle - images, geological maps and magnetic data, they ing and gravitational spreading. document a regional-scale strike-slip duplex within and adjacent to the Khlong Lan restraining bend. Importantly, Morley et al. review published Releasing bends, transtensional cooling-age data and provide new apatite and deformation and fluid flow zircon fission-track data to document the spatial and temporal evolution of uplift and exhumation Single restraining and releasing bends that form at within the restraining bend. Deformation and exhu- strike-slip fault terminations are usually sites of mation appear to have been focused at the corners oblique deformation where strike-slip displace- of the restraining bend, as blocks migrate out of ments are progressively transformed into dip-slip the bend during progressive strike-slip displace- displacements. The architecture and fault kin- ments in a manner similar to that described by ematics within such transitional zones can be very Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

8 W. D. CUNNINGHAM & P. MANN complex. Mouslopoulou et al. address this problem Cox 2005). Berger presents structural, lithological in a detailed study of the termination zone of the and geochronological evidence which reveals the North Island strike-slip fault system against the importance of fault bends in controlling the Taupo back-arc rift of northern . locations of volcanism and associated epithermal They find that the strike-slip fault system splays volcanic centre-related hydrothermal and into five main strands as it approaches the rift. silver systems in Nevada. Specifically, by analysing These strike-slip faults link with the rift margin the temporal and spatial evolution of volcanism and normal faults, but do not displace the normal the relationship between high-grade gold deposits faults. This geometry requires that the faults bend and faults that formed at the stepover, he concludes and that slip vectors on the strike-slip faults must that migrating corner zones where strike-slip faults progressively steepen to accommodate increasing link to oblique-slip and dip-slip faults are important dip-slip components near the active Taupo Rift. sites of hydrothermal veining, and that high-grade Data from displaced landforms, fault trenching, bonanza were deposited along abandoned gravity and seismic profiles are used to quantify dis- normal-fault systems following stepover migration. placements, and to evaluate rotational and non- Another complicating factor is that abandoned step- rotational mechanisms of displacement transfer. overs have locally experienced contractional defor- In a similar study investigating the kinematic mation and inversion, further affecting the linkage between strike-slip faults and extensional permeability structure within the stepover. The faults, but on a local outcrop scale, Fodor docu- author also documents a rarer case of hydrothermal ments the role of transtensional relay ramps in mineralization in extensional veins within a accommodating displacement transfer within restraining bend in the Excelsior Mountains of releasing bends along Upper Tertiary strike-slip Nevada. faults in the Vertes Hills of the Pannonian basin in Hungary. His study comes from a mined area with exceptional exposures. Normal and oblique- Summary normal faults change their strike, dip and slip vectors systematically to accommodate extension Because restraining and releasing bends often occur across the relay ramp. Fault slip inversion for differ- as singular self-contained domains of complex ent groups of faults demonstrates that inclusion of deformation, they provide appealing natural labora- transfer-zone faults modifies the results of palaeos- tories for Earth scientists to study fault processes; tress calculations, because displacements on the earthquake ; active faulting and sedi- transfer-zone faults are not governed by the regional mentation; fault and fluid-flow relationships; links stress field, but by their bounding strike-slip faults between tectonics and topography; tectonic and ero- (i.e. guided slip). This influence of bounding-strike- sional controls on exhumation; and tectonic geo- slip faults on local stress fields should be considered morphology. A major challenge for future workers by anyone attempting palaeostress calculations will be to untangle the deformational history of using fault stepover data. those regions where multiple fault bends have Releasing bends and dilational stepovers are grown large enough to interact, coalesce and struc- typically complex sites of fracturing, veining and turally interfere. Finally, the deep expression of fluid flow. In a theoretical and field-based study fault bends and the manner in which they are using outcrop data from the Carboniferous North- coupled to ductilely deforming lower crust and umberland basin in England, DePaola et al. docu- lithospheric remain poorly understood ment how deformation within dilational stepovers (Tikoff et al. 2004). with low angles of oblique divergence (,308) may This special publication stems from an international con- evolve from wrench- to extension-dominated trans- ference hosted by the Geological Society of London as strain increases. Veins, dykes, during September 2005, on the tectonics of strike-slip meshes and faults record progressive transtensional restraining and releasing bends in continental and deformation, including reactivation of earlier- oceanic settings. We are grateful to all authors for contri- formed structures. The complex pattern of struc- buting to the volume, and are indebted to the many con- tures within the stepover may inhibit development scientious referees who provided professional reviews of a through-going single fault. Therefore, the step- ensuring the high quality of the volume. over may be long-lived and persist as a site of sub- The following colleagues and friends kindly helped sidence, and provide long-term enhanced structural with the reviewing of the papers submitted for this volume: P. Barnes, Wellington, New Zealand; G. Batt, permeability favourable to fluid migration and London, UK; C. Burchfiel, California, USA; mineral precipitation. R. Burgmann, California, USA; C. Childs, Dublin, Repub- It is well known that many world-class mineral lic of Ireland; E. Cowgill, California, USA; S. Cox, Can- deposits have formed where fluid flow is focused berra, Australia; N. de Paola, Perugia, Italy; T. Dooley, in dilational sites along fault bends (Sibson 2001; Texas, USA; M. Gordon, Texas, USA; C. Henry, Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

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