Download Continuing evolution of the Pacific–Juan de Fuca–North America

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. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tecto.2008.01.018.
References
Atwater, T.M., 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of
western North America. Geol. Soc. Amer. Bull. 81, 3513–3536.
Atwater, T.M., 1989. Plate tectonic history of the northeast Pacific and western North
American. In: Winterer, E.L., Hussong, D.M., Decker, R.W. (Eds.), The Eastern Pacific
Ocean and Hawaii. . Geology of North America, vol. N. Geological Society of America,
Boulder, Colorado, pp. 21–72.
Atwater, T.M., Severinghaus, J.P., 1989. Tectonic maps of the northeast Pacific. In:
Winterer, E.L., Hussong, D.M., Decker, R.W. (Eds.), The Eastern Pacific Ocean and
Hawaii. . The Geology of North America, vol. N. Geological Society of America,
Boulder, Colorado, pp. 15–20.
Atwater, T.M., Stock, J.M., 1998. Pacific–North America plate tectonics of the Neogene
southwestern United States—an update. Int. Geol. Rev. 40, 375–402.
Aguillón-Robles, A., Calmus, T., Benoit, M., Bellon, H., Maury, R.D., Cotton, J., Bourgois, J.,
2001. Late Miocene adakites and Nb-enriched basalts from Vizcaino Peninsula,
Mexico: indicators of East Pacific Rise subduction below southern Baja California.
Geology 29, 531–534.
Barbeau Jr., D.L., Ducca, M.N., Gehrels, G.E., Kidder, S., Wetmore, P.H., Saleeby, J.B., 2005.
U–Pb detrital-zircon geochronology of northern Salinian basement and cover rocks.
Geol. Soc. Amer. Bull. 117, 466–481.
Bellon, H., Aguillón-Robles, A., Calmus, T., Maury, R.C., Bourgois, J., Cotten, J., 2006. La
Purísima volcanic field, Baja California Sur (Mexico): Miocene to Quaternary
volcanism related to subduction and opening of an asthenospheric window. J.
Volcanol. Geotherm. Res. 152, 253–272.
Bennett, R.A., Friedrich, A.M., Furlong, K.P., 2004. Codependent histories of the San
Andreas and San Jacinto fault zones from inversion of fault displacement rates.
Geology 32, 961–964.
Benoit, M., Aguillón-Robles, A., Calmus, T., Maury, R.C., Bellon, H., Cotten, J., Bourgois, J.,
Michaud, F., 2002. Geochemical diversity of late Miocene volcanism in southern
Baja California, Mexico: implication of mantle and crustal sources during the
opening of an asthenospheric window. J. Geol. 110, 627–648.
Bird, P., 2003. An updated digital model of plate boundaries. Geochem. Geophys.
Geosystems 4, 1027. doi:10.1029/2001GC000252.
Bohannon, R.G., Parsons, T.E., 1995. Tectonic implications of post-30 Ma Pacific and
North American relative plate motions. Geol. Soc. Amer. Bull. 107, 937–959.
P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42
Bourgois, J., Michaud, F., 2002. Comparison between the Chile and Mexico triple
junction areas substantiates slab window development beneath northwestern
Mexico during the past 12–10 Myr. Earth Planet. Sci. Lett. 201, 35–44.
Breitsprecher, K., Thorkelson, D.J., Groome, W.G., Dostal, J., 2003. Geochemical
confirmation of the Kula–Farallon slab window beneath the Pacific Northwest in
Eocene time. Geology 31, 351–354.
Calmus, T., Aguillón-Robles, A., Maury, R.C., Bellon, H., Benoit, M., Cotten, J., Bourgois, J.,
Michaud, F., 2003. Spatial and temporal evolution of basalts and magnesian
andesites (“bajaites”) from Baja California, Mexico: the role of slab melts. Lithos 66,
77–105.
Cloos, M., 1993. Lithospheric buoyancy and collisional orogenesis: subduction of oceanic
plateaus, continental margins, island arcs, spreading ridges, and seamounts. Geol.
Soc. Amer. Bull. 105, 715–737.
Cole, R.B., Basu, A.R., 1992. Middle Tertiary volcanism during ridge–trench interactions
in western California. Science 258, 793–796.
Cole, R.B., Basu, A.R., 1995. Nd–Sr isotope geochemistry and tectonics of ridge
subduction and middle Cenozoic volcanism in western California. Geol. Soc.
Amer. Bull. 107, 167–179.
Cole, R.B., Nelson, S.W., Layer, P.W., Oswald, P.J., 2006. Eocene volcanism above a
depleted mantle slab window in southern Alaska. Geol. Soc. Am. Bull. 118, 140–158.
Crouch, J.K., Suppe, J., 1993. Late Cenozoic tectonic evolution of the Los Angeles basin
and inner California borderland: a model for core complex-like crustal extension.
Geol. Soc. Am. Bull. 105, 1415–1434.
Crowell, J.C., 1987. Late Cenozoic basins of onshore southern California—complexity is
the hallmark of their tectonic history. In: Ingersoll, R.V., Ernst, W.G. (Eds.), Cenozoic
Basin Development of Coastal California, Rubey, vol. VI. Prentice-Hall Inc.,
Englewood Cliffs, pp. 207–241.
Davies, J.H., Von Blanckenburg, F., 1995. Slab breakoff: a model of lithosphere
detachment and its test in the magmatism and deformation of collisional orogens.
Earth Planet. Sci. Lett. 129, 85–102.
deBoer, J.A., Drummond, M.S., Bordelon, M.J., Defant, M.J., Bellon, H., Maury, R.C., 1995.
Cenozoic magmatic phases of the Costa Rican Island Arc. In: Mann, P. (Ed.), Geologic
and Tectonic Development of the Caribbean Plate Boundary in Southern Central
America. Geol. Soc. Am. Spec. Paper, vol. 295, pp. 35–56.
Dickinson, W.R., 1983. Cretaceous sinistral strike slip along Nacimiento Fault in coastal
California. Am. Assoc. Pet. Geol. Bull. 67, 624–645.
Dickinson, W.R., 1996. Kinematics of transrotational tectonism in the California
Transverse Ranges and its contribution to cumulative slip along the San Andreas
transform fault system. Geol. Soc. Am. Special Paper, vol. 305. Boulder, p. 46.
Dickinson, W.R., 1997. Tectonic implications of Cenozoic volcanism in coastal California.
Geol. Soc. Amer. Bull. 109, 936–954.
Dickinson, W.R., Synder, W.S., 1979. Geometry of subducted slabs related to San Andreas
transform. J. Geol. 87, 609–627.
Dokka, R.K., Travis, C.J., 1990. Role of the eastern California shear zone in accommodating Pacific–North American plate motion. Geophys. Res. Lett. 17, 1323–1326.
Faulds, J.E., Henry, C.D., Hinz, N.H., 2005. Kinematics of the northern Walker Lane: an
incipient transform fault along the Pacific–North American plate boundary.
Geology 33, 505–508.
Fernandez, L.S., Hey, R.N., 1991. Late Tertiary tectonic evolution of the seafloor spreading
system off the coast of California between the Mendocino and Murray Fracture
Zones. J. Geophys. Res. 96, 17955–17979.
Fletcher, J.M., 2003. Tectonic restoration of the Baja California microplate and the
geodynamic evolution of the Pacific–North American plate margin. Geol. Soc. Am.,
Abst. Programs 35, p. 9.
Gorring, J.L., Kay, S.M., 2001. Mantle processes and sources of Neogene slab window
magmas from southern Patagonia, Argentina. J. Petrol. 42, 1067–1094.
Graham, S.A., 1978. The role of the Salinian block in evolution of San Andreas fault
system, California. Am. Assoc. Pet. Geol. Bull. 62, 2214–2231.
Graham, S.A., Stanley, R.G., Bent, J.V., Carter, J.B., 1989. Oligocene and Miocene
paleogeography of central California and offset along the San Andreas fault. Geol.
Soc. Amer. Bull. 101, 711–730.
Gutscher, M.-A., Spakman, W., Bijwaard, H., Engdahl, R.E., 2000. Geodynamics of flat
subduction: seismicity and tomographic constraints from the Andean margin.
Tectonics, 19, 814–833.
Haeussler, P.J., Bradley, D.W., Wells, R.E., Miller, M.L., 2003. Life and death of the
Resurrection plate: evidence for its existence and subduction in the northeastern
Pacific in Paleocene–Eocene time. Geol. Soc. Amer. Bull. 15, 867–880.
Hall Jr., C.A., 1981. San Luis Obispo transform fault and middle Miocene rotation of the
western Transverse Ranges, California. J. Geophys. Res., 86, 1015–1031.
Hirth, G., Kohlstedt, D.L., 1996. Water in the oceanic upper mantle: Implications for
rheology, melt extraction and the evolution of the lithosphere. Earth Planet. Sci.
Lett. 226, 101–116.
Hole, M.J., Rogers, G., Saunders, A.D., Storey, M., 1991. Relation between alkalic
volcanism and slab-window formation. Geology, 19, 657–660.
Humphreys, E.D., Coblentz, D.D., 2007. North America dynamics and western U.S.
tectonics. Rev. Geophys. 47. doi:10.1029/2005RG000181.
Ingersoll, R.V., Rumelhart, P.E., 1999. Three-stage evolution of the Los Angeles basin,
southern California. Geology, 27, 593–596.
James, E.W., 1992. Cretaceous metamorphism and plutonism in the Santa Cruz
Mountains, Salinian block, California, and correlation with the southernmost Sierra
Nevada. Geol. Soc. Amer. Bull. 104, 1326–1339.
Johnston, S.T., Thorkelson, D.J., 1997. Cocos–Nazca slab window beneath Central
America. Earth Planet. Sci. Lett. 146, 465–474.
Kinoshita, O., 1999. A migration model of magmatism explaining a ridge subduction,
and its details on a statistical analysis of the granite ages in Cretaceous Southwest
Japan. The Island Arc 8, 181–189.
41
Klitgord, K.D., Schouten, H.S., 1986. Plate kinematics of the Central Atlantic. In: Vogt, P.
R., Tucholke, B.E. (Eds.), The Geology of North America, The Western North Atlantic
Region, vol. M. Geol. Soc. Am., Boulder, pp. 351–378.
Lachenbruch, A.H., Sass, J.H., 1980. Heat flow and energetics of the San Andreas fault
zone. J. Geophys. Res. 85, 6185–6223.
Lemaux, J. II, 2000. The motion between Nubia and Somalia from magnetic anomaly and
fracture zone crossings flanking the Southwest Indian Ridge. M.S. thesis, Rice
University, Houston, p. 146.
Lonsdale, P.F., 1988. Paleogene history of the Kula plate: offshore evidence and onshore
implications. Geol. Soc. Amer. Bull. 100, 733–754.
Lonsdale, P.F., 1991. Structural patterns of the Pacific floor offshore of Peninsular
California. In: Dauphin, J.P., Simoneit, B.R.T. (Eds.), The Gulf and Peninsular Province
of the Californias, Memoir. Am. Assoc. Pet. Geol., vol. 47, pp. 87–125.
Lonsdale, P.F., 1995. Segmentation and disruption of the East Pacific Rise in the mouth of
the Gulf of California. Mar. Geophys. Res. 17, 323–359.
Luyendyk, B.P., 1991. A model for Neogene crustal rotations, transtension, and
transpression in southern California. Geol. Soc. Amer. Bull. 103, 1528–1536.
Madsen, J.K., Thorkelson, D.J., Friedman, R.M., Marshall, D.D., 2006. Cenozoic to Recent
plate configurations in the Pacific Basin: ridge subduction and slab window
magmatism in western North America. Geosphere 2, 11–34.
Matthews III, V., 1976. Correlation of Pinnacles and Neenach volcanic formations
and their bearing on San Andreas fault problem. Am. Assoc. Pet. Geol. Bull. 60,
2128–2141.
McCaffrey, R., Qamar, A.I., King, R.W., Wells, R.E., Khazaradze, G., Williams, C.A.,
Steverns, C.W., Vollick, J.J., Zwich, P.C., 2007. Fault locking, block rotation and crustal
deformation in the Pacific Northwest. Geophys. J. Int. doi:10.1111/j.1365246X.2007.03371.x.
McCrory, P.A., Wilson, D.S., Ingle Jr., J.C., Stanley, R.G., 1995. Neogene geohistory analysis
of Santa Maria basin, California and its relationship to transfer of central California
to the Pacific plate. U.S. Geol. Surv. Bull. 1995-J, J1-J38.
McCrory, P.A., Blair, J.L., Oppenheimer, D.H., Walter, S.R., 2004. Depth to the Juan de Fuca
slab beneath the Cascadia subduction margin: a 3-D model for sorting earthquakes.
U.S. Geol. Surv. Data Series DS-91, 1 CD-ROM.
Meade, B.J., Hager, B.H., 2005. Block models of crustal motion in southern California
constrained by GPS measurements. J. Geophys. Res. 110, B03403. doi:10.1029/
2004JB003209.
Menard, H.W., 1978. Fragmentation of the Farallon plate by pivoting subduction. J. Geol.
86, 99–110.
Michaud, F., Royer, J.Y., Bourgois, J., Dyment, J., Calmus, T., Bandy, W., Sosson, M.,
Mortera-Guitiérrez, Sichler, B., Rebolledo-Viera, M., Pontoise, B., 2006. Oceanicridge subduction vs. slab break off: plate tectonic evolution along the Baja California
Sur continental margin since 15 Ma. Geology 34, 13–16.
Nicholson, C., Sorlien, C.C., Atwater, T.M., Crowell, J.C., Luyendyk, B.P., 1994. Microplate
capture, rotation of the western Transverse Ranges, and initiation of the San
Andreas transform as a low angle fault system. Geology 22, 491–495.
Okudaira, T., Yoshitake, Y., 2004. Thermal consequences of the formation of a slab
window beneath the Mid-Cretaceous southwest Japan arc: a 2-D numerical
analysis. The Island Arc 13, 520–532.
Oskin, M., Stock, J.M., Martin-Barajas, A., 2001. Rapid localization of Pacific–North
America plate motion in the Gulf of California. Geology 21, 459–462.
Oskin, M., Perg, L., Blumentritt, D., Mukhopadhyay, S., Iriondo, A., 2007. Slip rate of the
Calico fault: implications for geologic versus geodetic rate discrepancy in the
Eastern California Shear Zone. J. Geophys. Res. 112. doi:10.1029/2006JB004451.
Pallares, C., Maury, R.C., Bellon, H., Poyer, J.-Y., Calmus, T., Aguillón-Robles, A., Cotten, J.,
Benoit, M., Michaud, F., Bourgois, J., 2007. Slab-tearing following ridge–trench
collision: evidence from Miocene volcanism in Baja California, México. J. Volcanol.
Geotherm. Res. 161, 95–117.
Peacock, S.M., 1993. Large-scale hydration of the lithosphere above subducting slabs.
Chem. Geol. 108, 49–59.
Peacock, S.M., Wang, K., McMahon, A.M., 2002. Thermal structure and metamorphism
of subducting oceanic crust—insight into Cascadia intraslab earthquakes. U.S. Geol.
Survey Open-File Report 02-328, pp. 123–126.
Sandwell, D.T., Smith, W.H.F., 2006. 2-Minute Gridded Global Relief Data (ETOPO2v2).
National Geophys. Data Center. www.ngdc.noaa.gov.
Savage, J.C., Lisowski, M., Prescott, W.H., 1990. An apparent shear zone trending north–
northwest across the Mojave Desert into Owens Valley, eastern California. J.
Geophys. Res. 17, 2113–2116.
Snow, J.K., Wernicke, B.P., 2000. Cenozoic tectonism in the central Basin and
Range; magnitude, rate, and distribution of upper crustal strain. Am. J. Sci. 300,
659–719.
Snyder, W.S., Dickinson, W.R., Silberman, M.L., 1976. Tectonic implications of space–time
patterns of Cenozoic magmatism in the western United States. Earth Planet. Sci.
Lett. 32, 91–106.
Stanley, R.G., Wilson, D.S., McCrory, P.A., 2000. Locations and ages of middle Tertiary
volcanic centers in coastal California. U.S. Geol. Survey Open-File Report 00-154
(27 pp.).
Stock, J.M., Hodges, K.V., 1989. Pre-Pliocene extension around the Gulf of California and
the transfer of Baja California to the Pacific plate. Tectonics 8, 99–115.
ten Brink, U.S., Shimizu, N., Molzer, P.C., 1999. Plate deformation at depth under
northern California. Tectonics 18, 1084–1098.
ten Brink, U.S., Zhang, J., Brocher, T.M., Okaya, D.A., Klitgord, K.D., Fuis, G.S., 2000.
Geophysical evidence for the evolution of the California Inner Continental
Borderland as a metamorphic core complex. J. Geophys. Res. 105, 5835–5857.
Thorkelson, D.J., 1996. Subduction of diverging plates and the principles of slab window
formation. Tectonophysics 255, 47–63.
Thorkelson, D.J., Taylor, R.P., 1989. Cordilleran slab windows. Geology 17, 833–836.
42
P.A. McCrory et al. / Tectonophysics 464 (2009) 30–42
Tréhu, A.M., 1991. Tracing the subducted oceanic crust beneath the central California
continental margin: results from ocean bottom seismometers deployed during the
1986 Pacific Gas and Electric EDGE experiment. J. Geophys. Res. 96, 6493–6505.
Wallace, R.E., 1984. Patterns and timing of late Quaternary faulting in the Great Basin
Province and relation to some regional tectonic features. J. Geophys. Res. 89,
5763–5769.
Wang, K., Wells, R.E., Mazzotti, S., Hyndman, R.D., Sagiya, T., 2003. A revised dislocation
model of interseismic deformation of the Cascadia subduction zone. J. Geophys. Res.
108. doi:10.1029/2001JB001227.
Weldon, R.J., Sieh, K.E., 1985. Holocene rate of slip and tentative recurrence interval for
large earthquakes on the San Andreas fault, Cajon Pass, southern California. Geol.
Soc. Amer. Bull. 96, 793–812.
Wernicke, B.P., Snow, J.K., 1998. Cenozoic tectonism in the central Basin and Range:
motion of the Sierran–Great Valley block. Int. Geol. Rev. 40, 403–410.
Wessel, P., Smith, W.H.F., 1991. Free software helps map and display data. EOS Trans.
AGU 72, 441.
Wilson, D.S., 1988a. Tectonic history of the Juan de Fuca Ridge over the last 40 million
years. J. Geophys. Res. 33, 11,863–11,876.
Wilson, J.T., 1988b. Convection tectonics: some possible effects upon the Earth's surface
of flow from the deep mantle. Can. J. Earth Sci. 25, 27–53.
Wilson, D.S., 2002. The Juan de Fuca plate and slab: isochron structure and Cenozoic
plate motions. In: Kirby, S., Wang, K., Dunlop, S. (Eds.), The Cascadia Subduction
Zone and Related Subduction Systems-Seismic Structure, Intraslab Earthquakes and
Processes, and Earthquake Hazards. U.S. Geological Survey Open-File Report 02328, pp. 9–12.
Wilson, D.S., McCrory, P.A., Stanley, R.G., 2005. Implications of volcanism in coastal
California for the deformation history of western North America. Tectonics 24 p. 22.
Zandt, G., Furlong, K.P., 1982. Evolution and thickness of the lithosphere beneath coastal
California. Geology 10, 376–381.