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Tectonophysics 464 (2009) 30–42 Contents lists available at ScienceDirect Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o Continuing evolution of the Pacific–Juan de Fuca–North America slab window system —A trench–ridge–transform example from the Pacific Rim Patricia A. McCrory a,⁎, Douglas S. Wilson b, Richard G. Stanley a a b US Geological Survey, Menlo Park, California USA Department of Earth Science, University of California, Santa Barbara, California, USA a r t i c l e i n f o Article history: Received 18 July 2007 Accepted 30 January 2008 Available online 20 February 2008 Keywords: Slab windows Plate kinematics San Andreas fault Forearc volcanism Microplate capture Plastic deformation a b s t r a c t Many subduction margins that rim the Pacific Ocean contain complex records of Cenozoic slab-window volcanism combined with tectonic disruption of the continental margin. The series of slab windows that opened beneath California and Mexico starting about 28.5 Ma resulted from the death of a series of spreading ridge segments and led to piecewise destruction of a subduction regime. The timing and areal extent of the resultant slab-window volcanism provide constraints on models that depict the subsequent fragmentation and dispersal of the overlying continental margin. The initial Pioneer slab window thermally weakened the overlying western Transverse Ranges and California Borderlands region starting about 28.5 Ma. A second thermal pulse occurred in the same region starting about 19 Ma during growth of the Monterey slab window. This additional heating, combined with the capture of a partially subducted Monterey plate fragment by the Cocos plate, initiated the pulling apart and rotation of the adjacent continental margin. Similarly, the capture of Guadalupe and Magdalena plate fragments by the Pacific plate and initiation of the Guadalupe–Magdalena slab window about 12.5 Ma are coeval with Baja California pulling away from the Mexico continental margin, with the break along the Comondú arc, in crust already thermally weakened by about 10 My of volcanism. In coastal California, distributed crustal extension and subsidence accompanied the new transform plate boundary, and continued until the slab windows cooled and plate motion coalesced along a through-going system of strike-slip faults. The transform boundary continues to evolve, and forward modeling predicts an instability with the current configuration as a result of convergence between the Sierra Nevada and Peninsular Ranges batholiths, starting about 2 My in the future. The instability may be resolved by a shift in the locus of transform motion from the San Andreas fault to the eastern California shear zone, or by breaking off another fragment of the Mojave or southern Sierra Nevada crustal blocks and translating it northward. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The continental margins that rim the Pacific Ocean are notable for the sustained presence of subduction plate boundaries over Cenozoic time (Atwater and Severinghaus, 1989). Subduction during this 65-My interval has taken many different geometries. Sometimes oceanic plates disappear entirely as when the Resurrection plate was subducted beneath Alaska by ca. 50 Ma (Haeussler et al., 2003). The passage of such subducted plates is recorded in marginal accreted terranes (e.g., Thorkelson and Taylor, 1989) and half pairs of sea-floor magnetic anomalies (Atwater, 1989). Other times subducting plates are captured by adjacent plates as when the Kula plate was captured by Pacific plate ca. 40 Ma (Lonsdale, 1988). In the case of plate capture, relict plates are recorded by fossil spreading ridges (e.g., Stock and Hodges, 1989; Hole et al., 1991). Some subduction geometries create slab gaps or windows beneath a continental margin that are filled ⁎ Corresponding author. Tel.: +1 650 329 5677; fax: +1 650 329 5163. E-mail address: [email protected] (P.A. McCrory). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.01.018 with upwelling asthenosphere (Dickinson and Synder, 1979). The passage of slab windows is recorded by temporally and spatially focused heat sources in the overlying continental margin. The hypothesis that numerous tectonic and volcanic events recorded in Pacific Rim margins reflect the passage of slab windows (e.g., Thorkelson and Taylor, 1989; Thorkelson, 1996) has emerged from the accumulation of geologic knowledge about processes associated with active and fossil slab windows. Cenozoic slab windows around the Pacific Rim margin have been documented on the Antarctic Peninsula (Hole et al., 1991), Patagonia (Gorring and Kay, 2001), Mexico (Bourgois and Michaud, 2002), Baja California (Bellon et al., 2006), California (Wilson et al., 2005), British Columbia–Yukon (Breitsprecher et al., 2003; Madsen et al., 2006), Alaska (Haeussler et al., 2003; Cole et al., 2006), and Japan (Kinoshita, 1999; Okudaira and Yoshitake, 2004). Some of these regions currently host active slab windows associated with ongoing or recent ridge subduction (Fig. 1), such as the Antarctic–Scotia ridge beneath the Antarctica Peninsula, the Nazca–Antarctic ridge beneath Patagonia (Bourgois and Michaud, 2002), the Cocos–Nazca ridge beneath Costa Rica (Johnston and Thorkelson, 1997), and the Pacific–Cocos ridge beneath Mexico. P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 1.1. Slab-window formation Slab windows are transient phenomena that typically develop in triple junction settings where a spreading ridge system “subducts” beneath a continental margin and a gap or window opens between two down-going oceanic plates. Hot, anhydrous sub-slab asthenosphere wells upward to fill the void or gap (Fig. 2) and heat flow increases within the overlying continental lithosphere, often triggering magmatic activity that reflects a mix of lithospheric and asthenospheric sources (e.g., Thorkelson, 1996; Johnston and Thorkelson, 1997; Breitsprecher et al., 2003; Cole et al., 2006). The asthenosphere that fills the slab window then slowly cools over several million years to ambient temperatures. Some continental margins such as California (Wilson et al., 2005) and British Columbia (Madsen et al., 2006) have undergone repeated episodes of slab-window formation during the Cenozoic, resulting in overprinted phases of volcanism. Over the past several years, the link between slab windows and magmatism has been confirmed in multiple locations through geochemical analyses of volcanic and intrusive products that identify magma sources (e.g., Benoit et al., 2002; Calmus et al., 2003; Breitsprecher et al., 2003) combined with radiometric analyses that date magmatic activity (e.g., Cole and Basu, 1995; Cole et al., 2006; Bellon et al., 2006), and plate kinematic reconstructions that constrain the timing and location of ridge–trench interaction (e.g., Johnston and Thorkelson, 1997; Bourgois and Michaud, 31 2002; Madsen et al., 2006). These integrated studies are able to rule out other potential sources of focused thermal perturbations such as mantle plumes or lithospheric attenuation. 1.2. Ridge death complexities When a ridge system parallels the trench, convergence sometimes slows as the ridge approaches the subduction zone. Rather than subducting, the ridge stalls just seaward of the trench and continues to feed young oceanic plate into the subduction zone. Kinoshita (1999) and Okudaira and Yoshitake (2004) infer this configuration along southwest Japan during the Cretaceous, based on reconstruction of the Pacific, Kula and Eurasian plates, and a linear track of granitic plutons that decrease in age with distance from the trench. As a subducting plate decreases in size, propagating rifts can serve to pivot and fragment the plate, creating a complex isochron pattern before the ridge dies. Such complex plate histories can be inferred from the isochron patterns of Juan de Fuca (Wilson, 1988a), Rivera (Bourgois and Michaud, 2002), and Nazca (deBoer et al., 1995; Johnston and Thorkelson, 1997) plates. If the ridge dies adjacent to the trench rather than subducting, the oceanic plates on either side of the spreading ridge or bounding fracture zone can fuse. The larger of the plate pair in effect “captures” the smaller plate, forming a new plate configuration. The smaller plate Fig. 1. Plate tectonic map of Pacific Rim area showing locations of spreading ridges that currently intersect subduction trenches and actively form slab windows beneath the adjacent continental margin. Mercator projection. Coastline from ESRI. Plate boundaries modified from Bird (2003). Spreading ridges denoted by double lines; fracture zones denoted by single lines; trenches denoted by dashed lines; other plate boundaries denoted by dark gray lines. 32 P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 Fig. 2. Simplified side-views of the Monterey subduction system before, (a), and after, (b), microplate capture and slab-window formation. No vertical exaggeration. separates or tears from the subducted slab beneath the continent effectively eliminating slab-pull forces and begins to move with the larger plate, reversing its relative motion. Such captures by the Pacific plate have been identified adjacent to Baja California (Magdalena microplate; Stock and Hodges, 1989) (Fig. 3) and in the northern Pacific (Kula plate; Lonsdale, 1988). When a plate boundary dies by subduction or capture of a spreading ridge, relative motion of the new plate system can abruptly terminate a subduction regime. 1.3. Neogene example from western North America Ridge deaths, by both plate capture and ridge subduction occurred along the late Paleogene–early Neogene California continental margin as the segmented Pacific–Farallon ridge system (East Pacific Rise) encountered the North America trench during subduction (Atwater, 1970; Atwater and Stock, 1998). As the Farallon plate decreased in size, it broke into northern (Juan de Fuca) and southern Farallon plates ca. 30 Ma (Menard, 1978) (Fig. 4b), with an intervening Monterey microplate (Lonsdale, 1991; Fernandez and Hey, 1991) (Fig. 4c). The southern Farallon plate subsequently broke into the Cocos (Fig. 4e) and Nazca plates ca. 25 Ma. The northern Cocos plate in turn broke into two microplates (Lonsdale, 1991), the Guadalupe microplate ca. 15 Ma, followed by the Magdalena microplate ca. 14 Ma. The three microplates pivoted and subsequently stalled adjacent to central California and Baja California (Fig. 3) precluding subduction of their ridge segments. The step-wise foundering of this long-lived subduction boundary fundamentally changed the tectonic regime within the overlying North America plate as direct relative motion between the North America and Pacific plates initiated a broad disorganized transform plate boundary and differential motion of fault blocks within the former forearc. Subduction of the segmented ridge system has been characterized by episodic ridge deaths interspersed with intervals of fracture zone subduction. Each ridge death initiated a new phase of slab-window formation and intensified forearc volcanism. Temporally and spatially clustered volcanism is associated with (1) subduction of the ridge segment just south of the Pioneer fracture zone ca. 29–27 Ma (Fig. 4c– d); (2) subduction of the ridge segment between the Mendocino and Pioneer fracture zones ca. 26–25 Ma (Fig. 4e–f); (3) capture of the Monterey ridge system and subduction of the northern Cocos ridge segments ca. 20–16 Ma (Fig. 4g–h); and (4) capture of the Guadalupe and Magdalena ridge systems ca. 12.5–11.5 Ma (Fig. 4). The initially clustered volcanic centers in California were subsequently dispersed by strike-slip faulting during evolution of the Pacific–North America (San Andreas) transform boundary (Dickinson, 1997; Wilson et al., 2005), whereas volcano centers in Baja California remain largely intact on the peninsular fault-block. After establishing a relationship between the volcanic pulses and the predicted location of slab windows, Wilson et al. (2005) exploited their utility to serve as paleogeographical markers. These markers, by constraining the motion of fault-bounded blocks whose slip histories were heretofore poorly resolved from field studies, allowed a comprehensive and quantitative reconstruction of margin geometry through Cenozoic time. This quantitative model provides a new tool to investigate questions about how the fragmented style of forearc deformation and its evolution through time might have been influenced by slab-window heating and microplate capture. In this contribution, we accept the Wilson et al. (2005) reconstruction model as a reasonably accurate depiction of California and Baja California kinematic history characterized by ‘punctuated equilibrium’ or long intervals of steady-state motion punctuated by short intervals of rapid reconfiguration. We examine plate and slab-window interactions P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 33 Fig. 3. Map showing location of magnetic anomalies (Atwater and Severinghaus, 1989; Lonsdale, 1991), fault blocks, and modern distribution of late Oligocene through middle Miocene forearc volcanic centers (Stanley et al., 2000; Wilson et al., 2005) used in the kinematic reconstruction. Major changes in plate motion are recorded by changes of anomaly strike at 28.5 Ma (C10), though only between Pioneer and Murray fracture zones, and at 19 Ma (C6). Spreading west of Baja California ceased about 12.5 Ma. Anomalies C6B–C6 are approximately radial about a pole near the upper right corner of the figure. Monterey, Guadalupe, and Magdalena microplate fragments denoted by gray shading. See Fig. 5 for key to age of volcanic centers. during the intervals of rapid reconfiguration with the aim of identifying recurring fault patterns that might illuminate underlying tectonic processes that drive deformation and underlying rheologic controls on deformation style. We combine these observations with a new forward kinematic model that forecasts unstable fault-block configurations by modeling future fault activity and resultant margin geometries. 2. Kinematic model of San Andreas transform plate boundary Numerous volcanoes erupted in the California forearc when ridge segments of the East Pacific Rise encountered the North America subduction zone (Fig. 4). Wilson et al. (2005) previously correlated this volcanism with slab windows predicted from a new analysis of magnetic anomalies on the Pacific plate (Fig. 3), to develop an internally consistent model of North American deformation since 30 Ma. This comprehensive reconstruction of North America combined (1) kinematic histories of the oceanic plates derived from Pacific plate isochrons (e.g., Lonsdale, 1991) and global plate circuits (e.g., Klitgord and Schouten, 1986; Lemaux, 2000), and (2) kinematic histories of the continental blocks derived from various piercing point offsets (e.g., Matthews, 1976; Graham et al., 1989; Dickinson, 1996), paleomagnetically-constrained block rotations (e.g., Hall, 1981; Luyendyk, 1991), and Basin and Range extensional fault offsets (e.g., Wernicke and Snow, 1998; Snow and Wernicke, 2000). The kinematic model specifies finite rotations for numerous fault blocks within the San Andreas transform boundary relative to the Pacific plate (reproduced in Appendix Table A1; Figures A1 and A2). For those faults with poorly known offsets and those blocks with large uncertainties in vertical axis rotation, finite rotations were selected somewhat subjectively—guided by avoiding large overlaps or gaps between fault blocks through time. Changes in fault-slip rates are also often poorly known, so for simplicity the model presumes that relative motion is constant whenever possible, but changes at known platemotion reorganizations at 19.0 and 12.5 Ma. By restoring continental blocks to their initial positions, forearc volcanism—now dispersed by 34 P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 Fig. 4. Geometry of subducted oceanic plates beneath North America predicted from the Wilson et al. (2005) reconstruction model shows the change in slab-window configurations through time. Subducted plate geometry predicted by rotating Pacific plate isochrons according to the stage-rotation history of Wilson et al. (2005). Approximate western edge of North America is indicated by the dashed line; blue dashes where subduction is active; gray dashes where subduction has ceased. Gray shading with paired relative motion arrows shows presumed diffuse boundaries between and within oceanic plates. The reference frame is fixed to the Pacific plate. (a) Prior to chron C13, Juan de Fuca–Farallon plate motion is in slow rotation about a local pivot. (b) As the East Pacific Rise approaches the trench, faster right-lateral relative motion develops. (c) During chron C10n, the Monterey microplate separates. (d) Both Pacific–Juan de Fuca and Pacific–Farallon relative motion shift clockwise during chron C9n. (e) A slab window develops just south of the Pioneer fracture zone during chron C8. (f) By chron 6B, Cocos–Pacific motion has slowed to the point where a Cocos–Monterey boundary is no longer required. (g) At about chron C6y, Monterey–Pacific spreading stops, and the Monterey–Cocos slab tear position parallels to the continental margin. (h) Continued Cocos–Pacific spreading opens a slab window east of the stalled Monterey plate. (i) Slab window extends southward when spreading between Morro and Shirley (not shown) fracture zones stops. Neogene fault motion (Fig. 3)—coalesces into three regionally restricted phases of activity in coastal California from late Oligocene through middle Miocene time (Fig. 5). With finite rotations specified for fault blocks within the San Andreas transform boundary, we examine possible future configurations by running these finite rotations forward in time. We test two models: for Model A, we simply assume continued steady-state motion on the various fault blocks; for Model B, we develop a plausible alternative scenario by adjusting some of the fault-block velocities. These kinematic scenarios, with specified finite rotations of fault blocks, allow us to explore when and where potentially unstable configurations such as large gaps or overlaps will arise if current block P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 35 Fig. 5. Model of initial distribution of volcanic centers and of slab-window configurations at selected times beneath North America to show the reheating of coastal California by overprinted slab-window transients. The Pioneer window started forming about 28 Ma, followed by the Monterey window about 19 Ma, and the Cocos window about 17 Ma. (The Guadalupe and Magdalena slab windows, which started forming about12.5 and 11.5 Ma, respectively, are not shown in this figure.) The reference frame is fixed to the Pacific plate. The 26.5 Ma (C10) configuration, denoted by dashed blue lines, shows the initial slab gap adjacent to the Pioneer fracture zone; the 23.0 Ma (C6B) configuration, denoted by dashed green lines, shows a new phase of slab-window growth adjacent to the Mendocino fracture zone; the 14.5 Ma (C5AD) configuration, denoted by dashed red lines, shows continued northern growth of the Pioneer slab window, plus a new Monterey slab window beneath the previous window and a new Cocos window to the south. The gray ridge fragment at 14.5 Ma denotes a fossil Monterey ridge segment. Note, extension, rotation, and translation of coastal California starting about 20 Ma are not depicted in this figure. motions continue into the future. To test these conjectures, we run the kinematic scenarios forward 5 My. 3. Tectonic patterns based on kinematic model Our kinematic model depicts continental margin deformation as long periods of steady-state motion punctuated by abrupt shifts in the location of the primary trace of the San Andreas transform boundary. Initially we place a short primary trace at the base of the central California continental slope ca. 28.5 Ma, adjacent to the subducted ridge segment just south of the Pioneer fracture zone (Fig. 4c). The locus of transform motion jumps inland ca. 19 Ma, forming two traces offset laterally by about 70 km (Fig. 6a). Clockwise rotation of intervening fault blocks such as the western Transverse Ranges begins at this time as part of a large-scale, restraining step-over. The locus of transform motion jumps inland again ca. 12.5 Ma (Fig. 6b), at the same time as a new fault trace forms within the incipient Baja California peninsula to the south. With this progressive growth and reorganization of the transform boundary, previously disconnected fault traces become a continuous fault system, albeit with significant structural complexity. In addition, the Sierra Nevada batholith in California and the Peninsular Ranges batholith in Baja California—which had previously been located on the same (east) side of the transform boundary—are now located on opposite sides (Fig. 6b). With this new configuration, two major crustal blocks, comprising the Sierra Nevada Mountains and the Peninsular Ranges, began to converge at a rate of about 20 mm/year. 3.1. Tectonic effects of proximal plate capture The concentration of strike-slip motion near the coastline starting ca. 19 Ma occurred at the same time as an abrupt change in motion of the adjacent Monterey microplate (Fig. 3). The Cocos plate captured the remnant microplate at about this time across the Morro fracture zone (the remnant was subsequently transferred to the Pacific plate; Wilson et al., 2005) and the Monterey slab tore beneath the continental margin (Nicholson et al., 1994; Wilson et al., 2005). Death of the Monterey ridge system was preceded by pivoting of its 36 P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 Fig. 6. Margin reconstruction through time with fastest slipping fault trace in continental crust highlighted by heavy orange line (modified from Wilson et al., 2005). Reconstruction in Pacific-fixed coordinates. Major reorganizations are interpreted to be simultaneous with the capture of the Monterey microplate at 19 Ma and of the Guadalupe and Magdalena microplates at 12.5 Ma. In this model, the big bend in the San Andreas fault system formed during the 12.5-Ma reorganization and has been straightening since. Green triangles denote correlation (piercing) points used to constrain fault offsets (see Wilson et al., 2005 for discussion of data). The Sierra Nevada–Great Valley block and the Peninsular Ranges (Baja) block, denoted by grid pattern, are modeled as rigid crustal blocks. P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 ridge axis and slowing of its spreading rate (Fernandez and Hey, 1991). The detached down-dip portion of the slab continued to subduct, whereas, up-dip, the partially subducted Monterey microplate (Fig. 3) reversed direction and began to move with the Pacific plate away from coastal California (Bohannon and Parsons, 1995). The captured fragment apparently coupled with the overlying continental margin and the locus of transform motion shifted eastward to the inland edge of the stranded microplate at this time (Fig. 6). The ca. 12.5 Ma jump in locus of strike-slip motion (Fig. 6b) occurred at the same time as an abrupt reversal in motion of the adjacent Guadalupe and Magdalena microplates due to death of their ridge systems, and their capture by the Pacific plate (Stock and Hodges, 1989). Capture of the partially subducted Magdalena microplate was preceded by pivoting of the Magdalena–Pacific ridge axis (Lonsdale, 1995; Michaud et al., 2006), and slowing of its spreading rate (Atwater and Severinghaus, 1989; Michaud et al., 2006). Baja California tore away from the Mexico mainland along the Comondú volcanic arc (Oskin et al., 2001; Fletcher, 2003), as the partially subducted Guadalupe and Magdalena microplates reversed direction and began to move with the Pacific plate away from coastal Mexico. The location of the tear in the underlying Guadalupe and Magdalena plates is not well constrained, but is assumed to parallel the coastline just west of slab-window volcanism that began erupting ca. 11.5–10 Ma along the Baja peninsula (Aguillón-Robles et al., 2001; Michaud et al., 2006; Pallares et al., 2007), similar to the orientation and coastal location inferred for the Monterey plate. Similar to California, the captured fragments coupled with the overlying continental margin resulting in transtensional motion localizing along the inland edge of the microplate fragments. 3.2. Tectonic effects of shallow slab tears (Wilson et al., 2005) located the Monterey slab tear near the coastline (Fig. 2b) based on seismic imaging of the fossil plate fragment beneath the continental margin (Tréhu, 1991) and on the distribution of forearc volcanism. Theoretical studies that model slab tears typically assume a much deeper location for detachment (e.g., Davies and Von Blanckenburg, 1995), at depths of 50–100 km where “dehydration embrittlement” of the slab is thought to occur (Peacock, 1993; Peacock et al., 2002). Apparently the shallower slab in this case proved mechanically weaker than the deeper slab. If we project Monterey isochrons beneath the margin, the age of the slab below the coastline would have ranged from 5 to 10 My old at the time subduction ceased. Similarly, the age of the Guadalupe and Magdalena slabs below the western Baja California coastline would have ranged from about 5 to 15 My old at the time subduction ceased there. Oceanic lithosphere younger than 10 My is buoyant relative to the underlying asthenosphere (Cloos, 1993; Gutscher et al., 2000), and resistant to slab-pull forces. In addition, active subduction faults are most strongly coupled up-dip of a 450 °C thermal threshold that marks the brittle–ductile transition for quartzo-feldspathic-rich rocks in the accretionary prism. This transition is found at a depth of about 25 km (e.g., Peacock et al., 2002; Wang et al., 2003; McCaffrey et al., 2007) in settings where the ridge is near the trench so that the downgoing slab is relatively young and hot. Thus, where hot, buoyant oceanic slab resists subduction, and slab-pull forces exceed the strength of a relatively thin slab, we propose that the tear can occur at much shallower depths than current theoretical models investigate. 3.3. Tectonic and rheologic effects of slab-window heating Asthenosphere that filled the initial Pioneer slab window supplied a shallow heat source in direct contact with the overlying accretionary prism and continental crust, thermally weakening the region through conductive heating. If the Neogene geometry of the subduction zone was similar to the active Juan de Fuca–North America (Cascadia) 37 subduction zone off Oregon and Washington (McCrory et al., 2004), the North America plate would only be about 20-km thick at the coastline, not reaching a thickness of 40 km until about 200 km from the trench axis (Fig. 2a). Thus, asthenosphere filling the slab window would impinge directly on the accretionary prism or lower crust in coastal California with no intervening continental mantle or supraslab asthenosphere (Fig. 2b). Slab-window volcanic rocks erupted in coastal California have mid-ocean ridge basalt (MORB) affinities (Cole and Basu, 1992, 1995) consistent with shallow (b50 km) asthenospheric upwelling (Davies and Von Blanckenburg, 1995). The temperature of sub-slab asthenosphere is estimated to be 1300–1400 °C (Wilson, 1988b; Hirth and Kohlstedt, 1996). Theoretical models suggest that a thermal front associated with the edge of a growing slab window would migrate upward over millions of years (Davies and Von Blanckenburg, 1995), raising temperatures in the lower crust into the ductile domain for quartzo-feldspathic rocks (i.e., above 450 °C; Hirth and Kohlstedt, 1996; Peacock et al., 2002). Lower crust temperatures would remain well above this ductile threshold for several million years (cf., Lachenbruch and Sass, 1980). Additional heating of the crust would occur advectively in regions where decompression melting of upwelled asthenosphere, combined with anatexis at the base of the lower crust, filled conduits through the crust with magma. Low p-wave velocities in the upper mantle (Zandt and Furlong, 1982; ten Brink et al., 1999) and elevated heat-flow measurements (Lachenbruch and Sass, 1980) support the presence of an anomalously shallow heat source beneath coastal California, consistent with emplacement of asthenosphere behind the leading edge of the Pioneer slab window (now bounded by the Mendocino fracture zone). A north–south profile of heat-flow measurements indicates a peak (twice ambient values) about 200 km south of the current northern edge of the Pioneer slab window, implying a 4-My lag time for the thermal front to migrate upward to the surface from the base of 20-km thick crust (Lachenbruch and Sass, 1980; Zandt and Furlong, 1982). East–west profiles of heat-flow measurements indicate an abrupt drop to ambient values about 80 km from the coastline (or 200 km from the trench axis), where continental crust reaches a thickness of 40 km (Lachenbruch and Sass, 1980) and lithospheric mantle begins to intervene between upwelled asthenosphere and the lower crust (Fig. 2a). This correlation suggests either buffering of the heat-flow gradient by lithospheric mantle or dissipation of the thermal front before it reaches the surface due to its deeper starting point. It is likely that the thermal perturbation of a strong brittle lower crust and its resulting shift to ductile crust profoundly changed the mechanical behavior of the entire continental lithosphere. In particular, tectonic regimes where deformation is dominated by fault-block motion tend to reflect plastic behavior (Humphreys and Coblentz, 2007). Thus, the fault-block deformation style of coastal California may reflect the plastic behavior of asthenosphere emplaced at shallow depth and the lack of either brittle lithospheric mantle or brittle lower crust. Subsequent Monterey and Cocos slab windows reheated much of the same thin continental crust (Fig. 5), further weakening it, and further promoting its breakup into multiple translating and rotating fault blocks. Most Neogene volcanic centers in California occur within 200 km of the former trench axis, above continental crust lacking lithospheric mantle (Fig. 2b). These slab-window volcanoes are also confined to regions that underwent extension or active faulting as manifest in the synchronous exhumation of metamorphic core complex rocks in the California Borderlands (e.g., Crouch and Suppe, 1993; ten Brink et al., 2000), and in the synchronous formation and rapid subsidence of marginal basins (e.g., Crowell, 1987; McCrory et al., 1995; Ingersoll and Rumelhart, 1999). The coincidence of volcanism and crustal extension suggests that the fragmentation of the crust created conduits for pooled melt to reach the surface. 38 P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 We propose that in similar fashion, the slab window associated with death of the Guadalupe and Magdalena ridge systems would have heated coastal Mexico accretionary prism rocks and continental crust—a region already thermally weakened from about 10 My of Comondú arc volcanism (Calmus et al., 2003; Pallares et al., 2007). In this case, the thermal pulse associated with the slab window further decreased the effective strength of the arc crust, allowing extensional strain to localize there (cf., McCaffrey et al., 2007), and in turn, promoting Baja tearing away from the Mexican continental margin. Neogene forearc volcanism in Baja occurs in a 100-km wide zone up to 200 km from the trench (Pallares et al., 2007), corresponding to inferred slab-window depths up to 40-km deep. The deeper portion of the slab window would promote mixing of magma sources from ecologized slab, sub-slab asthenosphere, and shallow supra-slab asthenosphere. The adakites and Nb-rich basalts volcanic rock suites mapped along the peninsula reflect such magma sources (AguillónRobles et al., 2001; Pallares et al., 2007). In contrast to Baja California where arc volcanism and slab-window volcanism occurred in close proximity during temporally distinct phases, the volcanic arc active just prior to death of the Pioneer, Monterey, and Cocos ridge segments was located several hundred kilometers to the east of the slab window, in eastern California and western Arizona (Snyder et al., 1976). Similar styles of margin deformation have been documented for the Paleogene western Canada and Pacific Northwest margin. For example, cessation of Resurrection plate subduction ca. 50 Ma, was marked by the initiation of transform motion on the Queen Charlotte fault, as well as extensional exhumation of high-grade metamorphic rocks and clockwise rotation of the Oregon forearc block (Haeussler et al., 2003). 4. Kinematic model of future plate boundary configurations When we run our finite-rotation model forward in time (see Appendix for animations of Model A and Model B), at about 2 My in the future (Myf), a spatial problem develops near the ‘big bend’ in the San Andreas fault in southern California. The Sierra Nevada Mountains, on the northeast side of the primary trace of the San Andreas transform, and the Peninsular Ranges, on the southwest side (Fig. 7), have converged since 12.5 Ma. Their incipient collision during this time has been accommodated in part by deformation and fragmentation within the intervening Mojave block (cf., Barbeau et al., 2005; Oskin et al., 2007). However, by about 2 Myf, the rigid granitic blocks will begin to interact directly (Fig. 7), in other words, collide. We resolve this anticipated collision in two ways: (1) Model A which breaks new faults through the southern end of the Sierra Nevada Mountains to allow the Peninsular Ranges to slip past their western side; or (2) Model B which jumps the locus of transform motion from the San Andreas fault system eastward onto the existing eastern California and Walker Lane shear zones (Fig. 8). The eastern California shear zone already carries about 25% of the 50 mm/year relative motion across the San Andreas transform boundary (Savage et al., 1990; Dokka and Travis, 1990) and Walker Lane carries about 15% (Faulds et al., 2005). However, large disparities between geodetic and geologic slip rates on specific faults and fault segments within this portion of the San Andreas transform boundary Fig. 7. Snapshots of reconstruction Models A and B from present to 5 My into the future with fastest slipping (primary) fault trace highlighted by heavy orange line. See Fig. 3 for key to age of magnetic anomalies; see animated versions of both models in the Appendix. P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 39 Fig. 8. Map showing relationship between San Andreas fault and proposed incipient plate boundary along east side of Sierra Nevada Mountains (Model B). Faults used in kinematic model denoted by red lines (from Wilson et al., 2005); eastern California shear zone and Walker Lane seismic belt (modified from Wallace, 1984) denoted in green. Plate boundaries modified from Wilson (2002). Moment tensors (1976–2006) from Global CMT Catalog. Mercator projection; base map created using GMT (Wessel and Smith, 1991). Shaded relief topography from GTOPO30 (USGS); bathymetry from ETOPO2v2 (Sandwell and Smith, 2006). have led workers to postulate that the locus of slip shifts rapidly between specific strands of the San Andreas fault and strands of the eastern California shear zone (Bennett et al., 2004; Oskin et al., 2007). In particular, a geodetic slip rate of approximately 5 mm/year on the San Bernardino segment of the San Andreas fault (Meade and Hager, 2005) contrasts sharply with a much higher geologic rate of 25 mm/ year for the same segment (Weldon and Sieh, 1985). This large discrepancy between instantaneous slip rate and an averaged longer term rate may be resolved by taking into account recent accelerated slip rates on the eastern California shear zone when compared with its Quaternary slip rates (Meade and Hager, 2005). We speculate that the pattern of mismatched instantaneous and Quaternary slip rates may be indicative of an ongoing shift in locus of transform motion from what has been the most active San Andreas trace over the past 12.5 My to a new, more favorably oriented trace in the near future. Alternatively, the disparity between short- and long-term slip rates could simply indicate that these particular fault segments are in different phases of the earthquake cycle. Geologic rates are averaged over many earthquake cycles that presumably smooth out short-term slip variations expected before or after major earthquakes. The geodetic rates may be sampling such short-term variations, and thus will not be sustained over longer time intervals. Our Model A results in the truncation and northwestward transport of a portion of the southern Sierra Nevada batholith. Graham (1978) and Dickinson (1983) postulated a similar episode during Paleogene oblique subduction when the granitic Salinian block broke from the 40 P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42 southern Sierra Nevada and began moving northwestward toward its current location in coastal northern California. In fact, the Salinian block apparently is composed of an amalgamation of several plutonic fragments with differing histories, suggesting episodic fragmentation and translations of multiple crustal blocks derived from the southern Sierra Nevada and Mojave area (e.g., James, 1992; Barbeau et al., 2005). If this mechanism has operated in the past, perhaps it will continue to resolve spatial instabilities as the Sierra Nevada and Peninsular Ranges continue to converge in the future. In contrast to scenario A, a jump in the locus of slip to the east side of the Sierra Nevada Mountains would place the presently converging batholiths on the same (west) side of the main fault trace, resulting in a kinematically simpler transform boundary. Model B would also continue an observed fault pattern into the future. In this case, slip would shift onto eastern faults by exploiting an existing zone of crustal weakness rather than breaking a new fault zone through an intact, much stronger granitic basement block. Model B is consistent with the higher observed geodetic slip rates on the relatively young eastern fault strands provided those slip rates are sustained and not cyclic. Predicted increases in slip rates on faults east of the Sierra Nevada Mountains would be balanced by substantial decreases in slip rates on currently active faults to the west. This kinematic scenario suggests that the current slip distribution is caught mid-transition between favoring one primary trace to favoring another—a snapshot of a dynamic process—that appears as an abrupt transition in the geologic record. However, Model B cannot resolve how increased slip along the eastern side of the Sierra Nevada Mountains would be distributed northward, where the young Walker Lane shear zone is currently composed of short en echelon fault strands (Faulds et al., 2005) with small cumulative displacement. Currently slip on the Walker Lane shear zone appears to be converted to clockwise rotation of the Oregon forearc block (McCaffrey et al., 2007). But in the future, if the shear zone carries substantially more slip, perhaps the zone would continue to propagate northward by breaking multiple diffuse strands in eastern California and Oregon that ultimately merge with the northwestern edge of the Basin and Range extensional domain (Faulds et al., 2005). Alternatively, future propagating Walker Lane strands might trend more westerly to merge with the Mendocino fracture zone as does the northern San Andreas fault trace. Finally, the southern Juan de Fuca plate, adjacent to the Gorda ridge, is similar in size to the Guadalupe and Magdalena fragments that became stranded ca. 12.5 Ma. Perhaps, the Walker Lane shear zone would propagate more northwesterly to merge instead with the Blanco fracture zone, triggering death of the Gorda ridge segment and stalling of the Gorda fragment adjacent to the continental margin. 5. Conclusions The formation, growth, and healing of slab windows beneath coastal California and Mexico has been a transient process marked at the surface by overprinted pulses of forearc volcanism triggered by asthenospheric upwelling behind subducting slab edges. In the subsurface, where slab windows opened beneath thin forearc lithosphere, the shallow asthenosphere thermally weakened the overlying crust, and promoted a plastic mode of mechanical behavior characterized by fault-block deformation. This plastic mode of deformation resulted mainly from (1) weak asthenosphere directly underplating the lower crust instead of strong lithospheric mantle; and (2) normally strong lower crust shifting from brittle to ductile behavior as a result of conductive heating. While the shallow asthenosphere thermally weakened coastal California and promoted plastic deformation, the actual fragmentation into fault blocks occurred in concert with the reversal in motion of a partially subducted Monterey plate fragment following its capture by the Pacific plate ca. 19 Ma. This abrupt change from convergent to transtensional deformation initiated the pulling apart and clockwise pivoting of the adjacent western Transverse Ranges and California Borderlands region. A similar reversal in plate motion occurred adjacent to coastal Mexico following the capture of the Guadalupe and Magdalena plate fragments by the Pacific plate ca. 12.5 Ma. In this case, the transtensional strain that initiated Baja California pulling away from the Mexican continental margin was localized along the already thermally weakened Comondú volcanic arc crust. Finite-rotation models built from quantitative reconstruction of Farallon–North America slab windows and their magmatic signatures allow detailed examination of past and future fault-block kinematics for the evolving North America continental margin. In California, true forward modeling—run into the future—yields insights into how past margin behavior and observed tectonic patterns might play out in the future. In particular, our model suggests that the current locus of strike-slip motion may shift eastward again, this time to the east side of the Sierra Nevada Mountains, starting about 2 My from now as a result of convergence between the Sierra Nevada and Peninsular Ranges batholiths. Alternatively, the locus of fault slip may remain on the San Andreas fault, requiring new blocks to be broken from the southern Sierra Nevada and Mojave blocks, to make room for the Peninsular Ranges to slip past on their west side. Acknowledgments A special thanks to William Dickinson for his pioneering work in slab-window kinematics, quantitative basin analysis, and evolution of the San Andreas transform boundary. We thank Ron Cole and an anonymous reviewer for comments that improved this contribution. We thank John Boatwright, Tom Parsons, and Arthur Lachenbruch for reviewing an earlier version, and Luke Blair for providing the largescale maps. Appendix A. 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