Download Late Cretaceous–early Cenozoic tectonic evolution of

Late Cretaceous–early Cenozoic tectonic evolution
of the southern California margin inferred from
provenance of trench and forearc sediments
Carl E. Jacobson1,†, Marty Grove2, Jane N. Pedrick1, Andrew P. Barth3, Kathleen M. Marsaglia4,
George E. Gehrels5, and Jonathan A. Nourse6
1
Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212, USA
Department of Geological and Environmental Sciences, Green Earth Sciences, Rm. 225, Stanford University, Stanford, California
94305-2115, USA
3
Department of Earth Sciences, 723 West Michigan Street, SL118, Indiana University–Purdue University, Indianapolis, Indiana
46202-5132, USA
4
Department of Geological Sciences, California State University–Northridge, 18111 Nordhoff Street, Northridge, California 913308266, USA
5
Department of Geosciences, 1040 E. 4th Street, Gould-Simpson Building, University of Arizona, Tucson, Arizona 85721, USA
6
Department of Geological Sciences, California State Polytechnic University, 3801 W. Temple Avenue, Pomona, California 91768,
USA
2
ABSTRACT
During the Late Cretaceous to early Cenozoic, southern California was impacted by
two anomalous tectonic events: (1) underplating of the oceanic Pelona-Orocopia-Rand
schists beneath North American arc crust
and craton; and (2) removal of the western margin of the arc and inner part of the
forearc basin along the Nacimiento fault. The
Pelona-Orocopia-Rand schists crop out along
a belt extending from the southern Sierra Nevada to southwestern Arizona. Protolith and
emplacement ages decrease from >90 Ma in
the northwest to <60 Ma in the southeast.
Detrital zircon U-Pb ages imply that metasandstones in the older schists originated
primarily from the western belt of the Sierran–Peninsular Ranges arc. Younger units
were apparently derived by erosion of progressively more inboard regions, including
the southwestern edge of the North American craton. The oldest Pelona-OrocopiaRand schists overlap in age and provenance
with the youngest part of the Catalina Schist
of the southern California inner continental
borderland, suggesting that the two units are
broadly correlative. The Pelona-OrocopiaRand-Catalina schists, in turn, share a common provenance with forearc sequences
of southern California and the associated
†
E-mail: [email protected]
Salinian and Nacimiento blocks of the central Coast Ranges. This observation is most
readily explained if the schists were derived
from trench sediments complementary to the
forearc basin. The schists and forearc units
are inferred to record an evolution from
normal subduction prior to the early Late
Cretaceous to flat subduction extending into
the early Cenozoic. The transition from outboard to inboard sediment sources appears
to have coincided with removal of arc and
forearc terranes along the Nacimiento fault,
which most likely involved either thrusting
or sinistral strike slip. The strike-slip interpretation has not been widely accepted but
can be understood in terms of tectonic escape
driven by subduction of an aseismic ridge,
and it provides a compelling explanation for
the progressively younger ages of the PelonaOrocopia-Rand schists from northwest to
southeast.
INTRODUCTION
The geology of California is dominated by
NNW-trending lithotectonic belts produced
during late Mesozoic–early Cenozoic subduction of the oceanic Farallon plate beneath the
western edge of North America (Hamilton,
1969; Dickinson, 1970; Ernst, 1970). The
Franciscan subduction complex, Great Valley
forearc basin, and Sierra Nevada batholith of
central California east of the San Andreas fault
are particularly well-known components of this
system (Fig. 1). Similar rocks also characterize
southern California and its displaced correlatives
within the central Coast Ranges west of the San
Andreas. In the latter regions, however, the convergent margin belts are severely disrupted by
the Nacimiento fault, which separates the Salinian block on the northeast from the Nacimiento
block to the southwest (Page, 1970, 1981, 1982;
Dickinson, 1983; Hall, 1991; Saleeby, 2003;
Ducea et al., 2009). The Salinian block is cored
by granitoid plutons similar in age (100–75 Ma)
and composition to those of the central belt of
the Sierra Nevada and regions to the east, yet
it lacks an analog to the western foothills belt
of the Sierra (Mattinson, 1978, 1990; Silver,
1983; Ross, 1984). The Nacimiento block, in
contrast, is underlain by the Franciscan Complex and remnants of sedimentary sequences
equivalent to the distal parts of the type Great
Valley Group, but it is missing forearc units
indicative of a proximal setting (Hart, 1976;
Hall et al., 1979; Page, 1981; Seiders, 1982;
Vedder et al., 1983; Hall, 1991). By comparison with the Franciscan–Great Valley–Sierran
triad east of the San Andreas fault, the juxtaposition of the Salinian and Nacimiento blocks
along the Nacimiento fault implies the removal
of a width of 150 km or greater of formerly
intervening westernmost arc and inner to central forearc basin. This truncation appears
to have occurred sometime between 75 and
59 Ma (see following), although the mechanisms
GSA Bulletin; March/April 2011; v. 123; no. 3/4; p. 485–506; doi: 10.1130/B30238.1; 10 figures; 1 table; Data Repository item 2011005.
For permission to copy, contact [email protected]
© 2011 Geological Society of America
485
Jacobson et al.
PSP
121°W
Faults
Ef
EHf
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AT
Atascadero
BL Ben Lomond Mountain
CA Cambria
CL Coalinga
37°N
CP Cajon Pass
DP Del Puerto Canyon
IR
Indians Ranch
LP La Panza Range
NB Northern Baja California
OR Orocopia Mountains
PL Point Lobos
PM Pine Mountain Block
PP Pigeon Point
PS Point Sur
PSP Point San Pedro
SA Northern Santa Ana Mts.
SC Santa Catalina Island
SD San Diego
35°N
SG San Gabriel block
SL NW Santa Lucia Range
SM Santa Monica Mts-Simi Hills
SMI San Miguel Island
SMM Sierra Madre Mountains
SR San Rafael Mountains
SY Santa Ynez Mountains
ev
ad
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ian
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Pelona-Orocopia-Rand Schists
BR Blue Ridge
CD Castle Dome Mountains
CH SE Chocolate Mountains
EF East Fork
MP Mount Pinos
NR Neversweat Ridge
OR Orocopia Mountains
PR Portal Ridge
RA Rand Mountains
SC Santa Catalina Island
SE San Emigdio Mountains
SP Sierra Pelona
SS Sierra de Salinas
TR Trigo Mountains
Elsinore
East Huasna
Garlock
Nacimiento
Rinconada
San Andreas
San Gabriel
San Gregorio-Hosgri
San Jacinto
Santa Ynez
SMI
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MP
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WTR
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Accretionary complex
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McCoy Mountains Formation
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Ef
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100 km
y Mts.
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Pelona-Orocopia-Rand Schists
McCo
ETR
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AZ
115°W
Figure 1. Generalized late Mesozoic–early Cenozoic geology of central to southern California and adjoining areas, based largely on Jennings
(1977). Some outcrops of arc rocks and older wall rocks were omitted from southeasternmost California and southwesternmost Arizona
to emphasize outcrops of Pelona-Orocopia-Rand schist and McCoy Mountains Formation. Inset delineates the Salinian and Nacimiento
blocks. The Nacimiento fault is indicated by a heavier line weight than the other faults. We follow the interpretation of Hall et al. (1995) and
Dickinson et al. (2005) that the Nacimiento fault has been offset ~45 km in a dextral sense by slip on the combined Rinconada–East Huasna
fault. Sample localities for both Pelona-Orocopia-Rand schists and forearc units are indicated. Repeated labels (SG, SY, and SR) indicate
the grouping of samples collected over relatively large areas. Abbreviations not defined in the figure: AZ—Arizona; CA—California;
CTR—central Transverse Ranges; ETR—eastern Transverse Ranges; MEX—Mexico; WTR—western Transverse Ranges.
involved remain unclear. Previous interpretations include thrusting of 150–200 km (Silver,
1983; Hall, 1991; Barth and Schneiderman,
1996; Saleeby, 1997; Ducea et al., 2009), dextral strike slip of 2000 km or more (Page, 1982;
Champion et al., 1984), and sinistral strike slip
of 500–600 km (Dickinson, 1983; Seiders and
Blome, 1988; Dickinson et al., 2005).
A second key feature of southern and coastal
central California is represented by the PelonaOrocopia-Rand schists (Fig. 1; Haxel and Dillon,
486
1978; Ehlig, 1981; Jacobson et al., 1988). These
rocks broadly resemble the Franciscan Complex, but instead of being positioned outboard
of the forearc basin, they sit in structural contact
directly beneath the Cretaceous marginal batholith and adjoining cratonal areas of southeastern
California and southwestern Arizona. Potentially equivalent rocks, recovered as xenoliths,
occur still farther east in the Four Corners region
of Arizona, New Mexico, Utah, and Colorado
(Usui et al., 2006). The origin of the Pelona-
Orocopia-Rand schists has long been debated
(Haxel et al., 2002; Grove et al., 2003), although
most workers now favor a model involving lowangle subduction of the Farallon plate during the
Laramide orogeny (Crowell, 1968, 1981; Yeats,
1968; Burchfiel and Davis, 1981; Dickinson,
1981; Hamilton, 1988; May, 1989; Malin et al.,
1995; Jacobson et al., 1996, 2002, 2007; Wood
and Saleeby, 1997; Yin, 2002; Grove et al.,
2003; Saleeby, 2003; Kidder and Ducea, 2006;
Saleeby et al., 2007).
Geological Society of America Bulletin, March/April 2011
Late Cretaceous–early Cenozoic tectonic evolution of southern California
In the past, detailed understanding of the
Pelona-Orocopia-Rand schists has been hampered by a lack of good control on either the
depositional age of the protolith or the time
of underthrusting and metamorphism. Recent
40
Ar/39Ar thermochronologic analyses and U-Pb
dating of detrital zircons, however, demonstrate that sedimentation and underplating occurred from >90 Ma to <60 Ma (Jacobson et al.,
2000, 2002, 2007; Barth et al., 2003; Grove
et al., 2003). This point is critical, because it
indicates that underthrusting of the PelonaOrocopia-Rand schists began at least 15 m.y.
prior to the 75 Ma or younger initiation of slip
on the Nacimiento fault. Hence, formation
of the schists was at least in part a separate
phenomenon from the juxtaposition of the
Salinian and Nacimiento blocks. On the other
hand, both events appear to have been occurring
simultaneously sometime between ca. 75 and
59 Ma. It is hard to imagine that they were not
linked in some fashion during this period.
Detrital zircon ages from the schists also
reveal a major shift in provenance from relatively outboard to inboard source areas at about
the time of movement on the Nacimiento fault
(Grove et al., 2003). However, because the
schists are allochthonous, relatively limited in
geographic distribution, and represent only a
narrow time range in any given area, they do not
provide tight control on the nature and areal extent of this event.
To better understand the interconnections
between Nacimiento fault slip and underplating of the Pelona-Orocopia-Rand schists, we
conducted a regional detrital zircon and sedimentary petrologic analysis of unmetamorphosed units of the forearc basin that overlap
in age and geographic extent with the schists.
As part of this study, we expanded the data set
of Grove et al. (2003) for the Pelona-OrocopiaRand schists and treated these analyses along
with the results from Grove et al. (2008) for the
broadly similar Catalina Schist of the inner continental borderland of southern California. The
combined schist and forearc data provide important constraints on this critical period in the
paleogeographic and plate-tectonic evolution of
southern California and environs.
GEOLOGIC BACKGROUND
Palinspastic Reconstruction
The Late Cretaceous–early Cenozoic assemblages considered in this study have been
disturbed by younger tectonic events, including middle Cenozoic extension and middle to
late Cenozoic San Andreas–related deformation (Crowell, 1962, 1981; Frost et al., 1982;
Luyendyk et al., 1985; Crouch and Suppe, 1993;
Dillon and Ehlig, 1993; Powell, 1993; Nicholson et al., 1994; Dickinson, 1996; Fritsche et al.,
2001). To help correct for these complications,
we utilize a schematic pre-Miocene reconstruction of southern California and adjoining areas
(Fig. 2) that builds upon a similar analysis presented by Grove et al. (2003; see also the GSA
Data Repository item for details1). This map
serves as a base for plotting our results and provides context for discussing the regional geologic setting of the study area.
In reconstructing southern California at the
level of detail shown here, it is relatively easy to
restore northwest-southeast–trending strike-slip
faults near the continental margin and to backrotate the western Transverse Ranges, which lie
along a “free edge” of the map (Fig. 2). In contrast, compatibility issues make it far more difficult to (1) correct for rotations of regions that
lie entirely within the map area (e.g., the Sierran
“tail” or eastern Transverse Ranges); (2) restore
offset on strike-slip faults at a high angle to the
San Andreas fault or those parallel to the San
Andreas but located relatively far inboard; or
(3) undo extension associated with detachment
faults. Such corrections were not attempted here
but must be kept in mind when considering the
paleogeology.
A second complication pertains to longstanding controversies regarding the magnitude of
slip on various branches of the San Andreas
system. Our reconstruction follows interpretations of Crowell (1962, 1981), Dillon and Ehlig
(1993), Hall et al. (1995), Dickinson (1996), and
Dickinson et al. (2005). Contrasting estimates
of slip for a number of major faults within the
study area have been presented by Sedlock and
Hamilton (1991), Powell (1993), Underwood
et al. (1995), and Burnham (2009), among
others. However, these disagreements, while
important for a complete understanding of the
geology of California, are secondary in nature
at the scale considered here and have no significant impact on our conclusions.
By restoring middle Cenozoic and younger
deformations as best we can, Figure 2 more
clearly highlights the impact of the Nacimiento
fault on the geologic framework of southern California. In addition to the ~150 km of
stratigraphic omission of forearc and batholithic rocks that occurs across the fault, note
that Proterozoic cratonal basement crops out
anomalously close to the margin in the cor1
GSA Data Repository item 2011005, explanation
of the palinspastic reconstruction, geologic summaries
of forearc sampling areas and notes on individual samples, supplementary figures, and data tables, is available at http://www.geosociety.org/pubs/ft2011.htm or
by request to [email protected].
ridor between the Sierra Nevada batholith to
the north and the Peninsular Ranges batholith
to the south. The location of the southeastern
extension of the Nacimiento fault is unknown
(queried in Fig. 2), but it likely occurs outboard
of Proterozoic basement of the central Transverse Ranges and southeasternmost California.
The omission of forearc and western batholithic
rocks and out-of-position outcrops of cratonal
basement are central to understanding the displacement history and tectonic significance of
the Nacimiento fault.
Pelona-Orocopia-Rand and
Catalina Schists
Pelona-Orocopia-Rand Schists
The Pelona-Orocopia-Rand schists consist of
90% or more quartzo-feldspathic schist presumably derived from turbidite sandstone (Ehlig,
1958, 1981; Haxel and Dillon, 1978; Jacobson
et al., 1988, 1996; Haxel et al., 1987, 2002).
The schists also include up to 10% metabasite,
along with minor amounts of Fe-Mn metachert, marble, serpentinite, and talc-actinolite
rock. Metamorphism occurred under conditions
of moderately high pressure relative to temperature. Mineral assemblages lie mostly within
the greenschist and albite-epidote amphibolite
facies but locally extend into the epidote-blueschist and upper amphibolite facies (Ehlig, 1981;
Haxel and Dillon, 1978; Graham and Powell,
1984; Jacobson and Sorensen, 1986; Jacobson
et al., 1988; Kidder and Ducea, 2006). Inverted
metamorphic zonations are typical. Initial emplacement of the schists is attributed to a Late
Cretaceous–early Cenozoic low-angle fault system referred to as the Vincent–Chocolate Mountains thrust (Haxel and Dillon, 1978). However,
most present-day contacts between the schists
and North American upper plate appear to be
low-angle normal faults related to exhumation
of the schists, either shortly after underthrusting or during middle Cenozoic extension (Frost
et al., 1982; Hamilton, 1988; Jacobson et al.,
1988, 2002, 2007; Malin et al., 1995; Wood and
Saleeby, 1997; Haxel et al., 2002).
Outcrops of the Pelona-Orocopia-Rand
schists define a northwest-southeast–trending
belt extending from the southern Sierra Nevada
to southwestern Arizona (Figs. 1 and 2). The
northwestern schists underlie the outboard to
central parts of the Sierran batholith of Early
to middle Cretaceous age. The southeastern
schists, in contrast, sit beneath Proterozoic, Triassic, Jurassic, and latest Cretaceous to early
Cenozoic rocks situated inboard of the main
Cretaceous arc. As noted by Grove et al. (2003),
and discussed in more detail herein, protolith
and emplacement ages of the schists decrease
Geological Society of America Bulletin, March/April 2011
487
Jacobson et al.
Sierra
Nevada
PSP
BL
RA
Gf
NV
AZ
CA
SE
Mojave
Desert
PR
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H
f
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f
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Igneous Rocks
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EH SR
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SG
55–85 Ma
CA
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?
100–135 Ma
n
Hi
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AZ
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85–100 Ma
lo
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Cretaceous undifferentiated
N
135–300 Ma
SY
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Proterozoic
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Sedimentary (±Volcanic) Sequences
and Metamorphosed Equivalents
su
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re
gu
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Early Cretaceous Alisitos arc
ng
Fi
Jurassic-Cretaceous McCoy Mountains Fm.
f
Ra
Cretaceous-Eocene forearc basin
Accretionary complex
Pelona-Orocopia-Rand-Catalina Schists
SJ
SMI
NB
Paleozoic-Mesozoic
Figure 2. Pre–San Andreas palinspastic reconstruction of southern California, southwestern Arizona, and northwestern Mexico. See text
and GSA Data Repository item for explanation (see text footnote 1). Abbreviations for faults and sampling localities are as in Figure 1.
Inferred extension of Nacimiento fault west of the San Gregorio–Hosgri fault is based on figure 10 of Dickinson et al. (2005). Truncated
rectangular box indicates the approximate area of coverage of the panels in Figure 6.
from >90 Ma at the northwest end of the belt to
<60 Ma in the southeast. Significant variations
in provenance are also observed from one end
of the belt to the other.
In contrast to the northwest-southeast alignment of schist exposures as a whole, the northwestern bodies define a southwest-northeast
subbelt extending from the Sierra de Salinas
to the Rand Mountains. One possibility is that
this deviation is an artifact of our failure to correct for oroclinal bending of the Sierran tail
and adjacent parts of the Mojave Desert and
Salinian block (Kanter and McWilliams, 1982;
McWilliams and Li, 1985). In this case, the belt
of northwestern schists would originally have
been oriented approximately north-south (i.e.,
488
more nearly parallel to the strike of the margin). Alternatively, A. Chapman and J. Saleeby
(2009, personal commun.) argue that rotation
of the upper-plate, batholithic rocks did not affect the structurally underlying schists and that
the current locations of the northwestern schists
closely reflect their relative distributions at the
time of underthrusting. This is a further complication for reconstructing the Late Cretaceous–
early Cenozoic paleogeography of southern
California and vicinity.
Catalina Schist
The Catalina Schist is a moderately highpressure metamorphic terrane typically viewed
as the southern extension of the Franciscan
Complex. The unit is most widely exposed on
Santa Catalina Island, where it forms a series
of fault-bounded slices of metasedimentary,
metavolcanic, and ultramafic rocks (Woodford,
1960; Platt, 1975, 1976; Sorensen, 1988; Grove
and Bebout, 1995; Grove et al., 2008). Metamorphism ranges from upper amphibolite facies
in the structurally highest unit to lawsoniteblueschist, lawsonite-albite, and albite-actinolite
facies in the deepest parts of the section (Grove
et al., 2008). The Catalina Schist also occurs as
widespread submarine outcrops within the inner
continental borderland, as minor outcrops on the
Palos Verdes Peninsula, and in the subsurface
of the Los Angeles basin (references in Grove
et al., 2008).
Geological Society of America Bulletin, March/April 2011
Late Cretaceous–early Cenozoic tectonic evolution of southern California
Relationship of Pelona-Orocopia-Rand and
Catalina Schists
Strong lithologic and metamorphic similarities between the Pelona-Orocopia-Rand schists
and parts of the Catalina Schist have long been
noted (Ehlig, 1958; Woodford, 1960; Platt,
1976; Jacobson and Sorensen, 1986). Nonetheless, the two groups of rocks have generally
been treated separately because: (1) the Catalina
Schist is lithologically more diverse and exhibits a greater range of metamorphic facies than
the Pelona-Orocopia-Rand schists; (2) initial
geochronologic studies suggested an Early Cretaceous metamorphic age for the Catalina Schist
(Mattinson, 1986) versus a latest Cretaceous–
early Cenozoic age for the Pelona-OrocopiaRand schists (Jacobson, 1990); and (3) the
Catalina Schist occurs in a more outboard setting than the Pelona-Orocopia-Rand schists.
However, subsequent work on both units indicates that the structurally deepest, youngest
part of the Catalina Schist (lawsonite-blueschist
and lower-grade facies of Grove et al., 2008)
overlaps in both depositional and metamorphic
age with the oldest known part of the PelonaOrocopia-Rand schists (San Emigdio Mountains) at ca. 95–90 Ma (Grove et al., 2003,
2008). In addition, our work reveals that the coeval parts of the Catalina and Pelona-OrocopiaRand schists include similar assemblages of
detrital zircons. (Combined zircon results for
the Catalina Schist and Pelona-Orocopia-Rand
schist of the San Emigdio Mountains are presented herein. Individual plots for the two units
are included in Figure DR1 in the GSA Data Repository item [see footnote 1].) Finally, recent
workers have proposed that initial emplacement
of the Catalina Schist occurred beneath the
northern Peninsular Ranges, implying a tectonic setting analogous to that for the PelonaOrocopia-Rand schists (Crouch and Suppe,
1993; ten Brink et al., 2000; Fritsche et al.,
2001; Grove et al., 2008). According to this interpretation, the present location of the Catalina
Schist in the borderland is the result of being
exhumed from beneath the Peninsular Ranges
during Miocene extension. Based on these relations, we conclude that the Pelona-OrocopiaRand schists and youngest parts of the Catalina
Schist are broadly correlative. We do not include
the older elements of the Catalina Schist in this
grouping, because they may have originated in a
forearc, rather than accretionary wedge, setting
(Grove et al., 2008).
Forearc Sedimentary Units
We analyzed unmetamorphosed Upper Cretaceous to middle Eocene forearc units extending from northern Baja California to just
south of San Francisco. All sampling localities
can be classified into one of three tectonic domains: (1) Salinian block and central Transverse
Ranges, (2) Nacimiento block, and (3) western
Transverse Ranges and Peninsular Ranges.
Salinian Block–Central Transverse Ranges
The term “Salinian block and central Transverse Ranges” as used here also includes the
western fringes of both the Mojave Desert and
eastern Transverse Ranges. Basement rocks
of this region consist of Proterozoic to Jurassic magmatic and metamorphic units intruded
by Late Cretaceous plutons characteristic
of the central to eastern parts of the Sierran–
Peninsular Ranges arc (Mattinson, 1990;
Barth et al., 1995, 1997, 2000, 2001a, 2008a;
Kidder et al., 2003; Barbeau et al., 2005).
Even the youngest Cretaceous intrusive rocks
(ca. 75 Ma) are cut by the Nacimiento fault,
thus providing a maximum age for the structure. Cretaceous magmatism was followed by
a marine transgression that was in progress by
the middle Maastrichtian (time scale of Walker
and Geissman, 2009)(e.g., Santa Lucia and
La Panza Ranges; Howell et al., 1977; Vedder
et al., 1983; Saul, 1986; Seiders, 1986; Sliter,
1986; Grove, 1993) and that advanced southeast and inboard through the early Eocene (e.g.,
Pine Mountain block and Orocopia Mountains;
Crowell and Susuki, 1959; Chipping, 1972;
Advocate et al., 1988; Dickinson, 1995). The
marine flooding of the Salinian block was
presumably related to removal of the western
fringe of the arc and adjoining earlier phases of
the forearc basin and thus implies that significant slip had accumulated on the Nacimiento
fault by the middle Maastrichtian (ca. 68 Ma).
The transgressive sequences of the Salinian
block and central Transverse Ranges are distinctive in that many show evidence for relatively
continuous deposition across the CretaceousPaleocene boundary (Kooser, 1982; Saul, 1983).
Furthermore, Paleocene rocks, in general, are
widespread and relatively thick, although unconformities occur locally within this interval
(Chipping, 1972; Graham, 1979; Ruetz, 1979;
Saul, 1986; Nilsen, 1987a). Facies relations indicate that initiation of deposition in individual
areas, whether it occurred during the Maastrichtian, Paleocene, or Eocene, was commonly
accompanied by rapid subsidence of the basin
floor to bathyal depths (Kooser, 1982; Advocate
et al., 1988; Grove, 1993).
Lower Eocene marine deposits within the
southern Salinian block resemble units of similar age in the southernmost Nacimiento block
and eastern part of the western Transverse
Ranges (Chipping, 1972; Page, 1981, 1982;
Vedder et al., 1983; Dickinson, 1995). We inter-
pret these rocks as an overlap sequence, indicating that most or all of the slip on the Nacimiento
fault had accumulated by ca. 56 Ma. In a number of locations southwest of the fault, the
Eocene section sits conformably on Upper
Paleocene Sierra Blanca Limestone (Whidden
et al., 1995). This limestone has not been recognized in the southernmost Salinian block
northeast of the Nacimiento fault, although the
base of the Eocene section in the latter area is
exposed only on the inboard side of the basin,
where it onlaps crystalline basement. Hence, the
overlap sequence could be as old as late Paleocene (ca. 59 Ma). In addition, Hall (1991, p. 29)
and Dickinson et al. (2005, p. 15) concluded
from unconformities within the Salinian block
that slip on the Nacimiento fault was most likely
over by ca. 62.5–62 Ma. As a compromise, we
use 59 Ma as the minimum age bound (Fig. 3),
keeping in mind that the actual age for cessation of fault movement could be somewhat older
or younger. In fact, the absolute minimum age
of slip is indicated by a distinctive barnaclebearing unit of early Miocene age that overlaps
the central part of the Nacimiento fault (Dickinson et al., 2005, p. 15).
Nacimiento Block
The Nacimiento block (equivalent to the SurObispo belt of Page, 1981, 1982) is underlain
primarily by the Franciscan Complex. Locally,
the Franciscan rocks are structurally overlain
by (1) fragments of Coast Range ophiolite;
(2) uppermost Jurassic to Valanginian and
Upper Cretaceous (through Campanian) sedimentary rocks deposited nonconformably on
Coast Range ophiolite and resembling units of
the Great Valley Group east of the San Andreas
fault; and (3) sandstone-rich sequences of Campanian(?) age, which may have been deposited
directly on the Franciscan accretionary wedge
(Hall and Corbató, 1967; Gilbert and Dickinson, 1970; Page, 1970, 1981, 1982; Hart, 1976;
Howell et al., 1977; Seiders, 1982; Vedder et al.,
1983; Seiders and Blome, 1988; Hall, 1991;
Seiders and Cox, 1992; Dickinson et al., 2005).
Rocks of Maastrichtian to Eocene age have not
been recognized, except at the southernmost end
of the block. We view the forearc units of the
Nacimiento block as distal analogs of the proximal facies present within the Salinian block.
However, the scarcity of forearc units younger
than Campanian in the Nacimiento block but
older than Maastrichtian in the Salinian block
makes it difficult to constrain the exact paleogeography prior to slip on the Nacimiento fault.
Most of our samples from the Nacimiento
block come from two distinct sequences of
Campanian or inferred Campanian age. One
of these groups represents the uppermost part
Geological Society of America Bulletin, March/April 2011
489
Jacobson et al.
of the Jurassic–Cretaceous sequence that sits
depositionally upon the Coast Range ophiolite
(e.g., samples of the Cachuma and Atascadero
Formations and potentially the Pigeon Point
Formation; see GSA Data Repository item [see
footnote 1]). We also analyzed samples from
the Cambria slab, which is widely regarded as
a trench-slope-basin assemblage deposited on
top of the Franciscan Complex (Howell et al.,
1977; Smith et al., 1979; Dickinson et al., 2005),
and the Pfeiffer Slab of Hall (1991), which has
been interpreted as either a trench-slope deposit (Howell et al., 1977; Smith et al., 1979;
Hall, 1991) or part of the Franciscan Complex
(Underwood and Laughland, 2001; Dickinson
et al., 2005). Despite the potentially contrasting
depositional settings of the Cambria and Pfeiffer
slabs, our detrital zircon results are consistent
with a common provenance.
Peninsular Ranges–Western
Transverse Ranges
This area shows no evidence for the structural
dislocation that affected the Nacimiento and
Salinian blocks to the north; any Nacimientorelated slip at this latitude is presumed to lie well
inboard of the forearc basin. On the other hand,
the western Transverse Ranges are widely considered to have undergone 90° or more of clockwise rotation during Neogene development of
the San Andreas system (Luyendyk et al., 1985;
Nicholson et al., 1994; Dickinson, 1996). Prior
to rotation, the western Transverse Ranges are
believed to have been positioned alongside the
western margin of the Peninsular Ranges.
Sedimentary units exposed along the western
flank of the Peninsular Ranges were deposited
in the relatively inboard part of the forearc basin
(Schoellhamer et al., 1981; Bottjer and Link,
1984; Fritsche et al., 2001). The base of the sequence consists of nonmarine units of Turonian
age that pass upward into marine formations of
Turonian to Campanian or early Maastrichtian
age. These units are overlain unconformably by
Upper Paleocene to Eocene formations. This
contrasts with the relatively more continuous
Maastrichtian to Paleocene deposition within
the Salinian block (see previous).
Units analyzed here from San Miguel Island,
the Santa Monica Mountains, and the Simi
Hills exhibit strong sedimentologic affinities with, and probably were located close to,
the Peninsular Ranges prior to Neogene rotation (Bartling and Abbott, 1983; Bottjer and
Link, 1984; Link et al., 1984; Alderson, 1988;
Fritsche et al., 2001). The sequences from San
Miguel Island, the Santa Monica Mountains,
and the Simi Hills, however, appear to have
been deposited somewhat farther offshore than
coeval units of the Peninsular Ranges (Santa
490
Ana Mountains; Fritsche et al., 2001; Saul and
Alderson, 2001). In analogous fashion, the
western Transverse Ranges represent an even
more distal setting (westerly in palinspastic coordinates; e.g., Fig. 2) within the forearc basin
(Dibblee, 1950, 1966, 1991; MacKinnon, 1978;
Vedder et al., 1983, 1998; Thompson, 1988;
Dickinson, 1995). Sedimentation in the western Transverse Ranges began in latest Jurassic
and/or earliest Cretaceous time. The base of the
section is depositional upon ophiolite, which in
turn sits structurally above the Franciscan Complex. This relationship is similar to that of the
Nacimiento block.
Some paleomagnetic data have been taken to
indicate large-scale northward translation of the
Peninsular Ranges prior to slip on the San Andreas system. However, we accept the alternate
view that the Peninsular Ranges are essentially
in place other than for dextral translation along
the San Andreas system and possibly sinistral
slip on the Nacimiento fault (Dickinson and
Butler, 1998).
METHODS
Zircon U-Pb Ages
Based on the regional scope of the study, we
considered it most useful to maximize the number of samples analyzed rather than the density
of age data collected per sample. Our study is
thus best viewed as an exit poll, with more comprehensive work required to identify all minor
populations and answer detailed provenance
questions related to individual samples. For
the Pelona-Orocopia-Rand schists, our data set
includes 1405 zircon ages for 55 samples of
quartzo-feldspathic schist (Table DR1 [see footnote 1]) reflecting nine to 55 ages per sample. Of
these, 850 ages, representing 40 samples, come
from our previous work (Jacobson et al., 2000;
Barth et al., 2003; Grove et al., 2003). We also
incorporated results from Grove et al. (2008) for
the last accreted elements of the Catalina Schist.
The latter data set includes 305 zircon ages from
21 samples. For the forearc units, we determined
2983 detrital zircon ages from 100 sandstones of
Cenomanian to middle Eocene age (Tables DR2
and DR3 [see footnote 1]). The number of ages
per sample ranged from nine to 64. Sixty-eight
of the forearc samples were collected specifically for this study (the stratigraphic context of
these samples is described in the GSA Data Repository item [see footnote 1]). The remaining
forearc samples were collected as part of a more
focused study of the Peninsular Ranges by one
of us (M. Grove).
Zircons were extracted from ~1 kg samples
using standard density and magnetic tech-
niques. Most zircons from both the schists
and forearc units were analyzed by secondary
ionization mass spectrometry (SIMS) using
the Cameca IMS 1270 ion microprobe at the
University of California, Los Angeles. Procedures followed those described in Grove
et al. (2003). Ion microprobe analysis is time
intensive, and because of our desire to sample
a broad geographic and stratigraphic range,
we determined a median of only 15–16 SIMS
zircon U-Pb ages per sample. Since age distributions for individual samples are not well
constrained, we focus upon pooled results
from groups of samples.
During the latter part of the study, zircon
analyses were conducted at the University of
Arizona using the laser ablation–multicollector–
inductively coupled plasma–mass spectrometer (LA-ICP-MS) approach carried out with a
193 nm excimer laser and a GVI Isoprobe mass
spectrometer. Procedures followed those of
Gehrels et al. (2008). The LA-ICP-MS technique is highly efficient, which generally led to
larger data sets than with the SIMS (although
some large data sets were collected even with
the SIMS and some small ones with the LAICP-MS). This creates a problem when pooling
data, because samples with a large number of
analyses will be weighted more heavily than
those for which fewer ages were determined.
To avoid this complication, we utilized a subsampling procedure in which ages from individual samples with a large number of results
were sorted in numeric order. We then selected
the oldest and youngest ages and every second,
third, etc., age as needed to reduce the number
of analyses to a more appropriate level. This approach yielded probability plots for individual
“weeded” samples that were virtually indistinguishable from those obtained for the same
samples using all analyses.
Analytical results generated specifically for
this study are included in the GSA Data Repository item (Table DR4 [see footnote 1]). Additional results can be found in Barth et al. (2003)
and Grove et al. (2003), or were provided by one
of us (M. Grove).
Sandstone Petrology
Detrital framework modes were estimated using the Gazzi-Dickinson method (e.g., Ingersoll
et al., 1984) for 59 of the 68 forearc samples
collected specifically for this study (i.e., the
samples from the Peninsular Ranges were not
included in this part of the study). All samples
were stained for potassium feldspar and plagioclase. Three hundred to 400 grains were counted
per sample depending on grain size and size of
the thin section billet.
Geological Society of America Bulletin, March/April 2011
Late Cretaceous–early Cenozoic tectonic evolution of southern California
10
RESULTS
Mio
20
Pelona-Orocopia-Rand and
Catalina Schists
30
Protolith Age
One of the most critical parameters to determine for any given schist body is the depositional age of the sandstone protolith, which
must lie within a time window bounded by the
youngest reliable detrital zircon age and oldest
reliable metamorphic age (Fig. 3). Grove et al.
(2003) referred to this time span as the “cycling
interval,” because it encompasses erosion in
the source area, transport of that material to the
site of deposition, underthrusting beneath the
arc, accretion to the overriding plate, and initial
stages of exhumation. As observed by Grove
et al. (2003), the cycling interval of the PelonaOrocopia-Rand schists becomes younger from
northwest to southeast (Fig. 3). Also notable,
the duration of the cycling interval increases
to the southeast, although this may be at least
partly an artifact in the data. For example, we
obtained no hornblende 40Ar/39Ar ages from
the easternmost schist bodies (Fig. 3) and
thus may not have captured the oldest cooling ages in this region. In addition, igneous
bodies younger than ca. 70 Ma are not common in the inferred provenance areas (Barth
et al., 2008a), making it difficult to constrain
the maximum depositional age of sediments
younger than latest Cretaceous. For example,
in delineating the cycling interval, we ignored
three early Cenozoic ages from the Orocopia
Schist due to their status as outliers (Fig. 3).
However, it should be kept in mind that any or
all of these ages could be significant. Despite
the uncertainties, the schist protoliths clearly
range from Turonian to at least as young as
middle Paleocene, representing a time span of
30 m.y. or greater.
For purposes of comparison to the forearc
units, we divided the schists into four groups
based on depositional age and geographic
location (Figs. 3 and 4). This contrasts with
a threefold division utilized by Grove et al.
(2003). Our fourth category includes the
Catalina Schist, which was not considered by
Grove et al. (2003), and the Pelona-OrocopiaRand schist of the San Emigdio Mountains,
which Grove et al. (2003) grouped with the
Rand schists.
40
Age (Ma)
Aside from the fact that we did not previously consider the Catalina Schist, our results
are little changed from those obtained by Grove
et al. (2003). We thus emphasize only those
points most relevant for comparison with the
forearc data.
Olig
Eoc
Metamorphic
bio
mus
hbd
Detrital
zircon
al
interv
50
60
70
80
90
Pal
Maast
Cycling
Slip on Nac flt
Camp
Sant
Con
Tur
Cen
100
Age/
Epoch
SC SE
Cat-San Emig
PR RA
SS
MP SP
Rand
BR
Pelona
EF
OR CH TR CD NR
Orocopia
Figure 3. Comparison of 40Ar/ 39Ar cooling ages and zircon U-Pb ages ≤100 Ma for the
Pelona-Orocopia-Rand-Catalina schists plotted by area (see Figs. 1, 2, and 4 for locations).
Zircon ages are from Barth et al. (2003), Grove et al. (2003, 2008), and this study. Argon
ages are from Jacobson (1990), Jacobson et al. (2002, 2007), Barth et al. (2003), and Grove
et al. (2003, 2008). The Pelona-Orocopia-Rand schists are organized by the conventional
northwest-southeast grouping, i.e., Rand Schists in the northwest Mojave Desert and environs, Pelona Schists in the central Transverse Ranges, and Orocopia Schists in southeastern California–southwestern Arizona. The exception is the Rand Schist of the San Emigdio
Mountains, which we group with the Catalina Schist. Within the Pelona and Orocopia
Schist groups, individual areas are ordered from northwest on the left to southeast on the
right. Northwest-southeast position for the Rand Schists is not clear owing to potential rotation of the Sierran tail; these bodies were instead ordered from left to right by depositional and metamorphic age. Diagonally ruled box indicates time of most likely movement
on the Nacimiento fault. Yellow band indicates the cycling interval, which, as discussed in
the text, is not well constrained in the southeast. Also note that 40Ar/ 39Ar and zircon ages
overlap for the Catalina Schist (the two types of ages have been separated slightly along
the x-axis to clarify this relation). The low grade of metamorphism for this unit (Grove
et al., 2008) suggests that the 40Ar/ 39Ar ages may be influenced by excess radiogenic argon
or lack of complete recrystallization of detrital mica during metamorphism. Abbreviations
for schist localities are same as in Figure 1. Other abbreviations are: bio—biotite; Camp—
Campanian; Cen—Cenomanian; Eoc—Eocene; hbd—hornblende; Maast—Maastrichtian;
Mio—Miocene; mus—muscovite; Olig—Oligocene; Pal—Paleocene; Sant—Santonian;
Tur—Turonian.
Variation in Detrital Zircon Populations as a
Function of Depositional Age and Location
Pie diagrams of detrital zircon ages for individual ranges and/or adjacent ranges are plotted in Figure 4 using the palinspastic base of
Figure 2. Probability density functions for the
four regional groupings are illustrated in Figures
5B–5E. As pointed out by Grove et al. (2003),
detrital zircon patterns in the Pelona-OrocopiaRand schists vary systematically from northwest to southeast, and thus with depositional
age of the protolith. The northwestern schists,
which have protolith ages of Turonian to Campanian, are dominated by Early to middle Cre-
taceous zircons, with lesser, but still significant,
proportions of Proterozoic and Jurassic ages.
In contrast, the central to southeastern schists,
which have depositional ages of Maastrichtian
and younger, are characterized by progressively
decreasing abundances of Early to middle Cretaceous detrital zircons and increasing numbers of Proterozoic and latest Cretaceous–early
Cenozoic ages. As concluded by Grove et al.
(2003), the patterns imply a shift from outboard
to inboard sources with time (e.g., Fig. 2 and
following discussion).
Compared to the Pelona-Orocopia-Rand
schists, the zircon age distribution within
Geological Society of America Bulletin, March/April 2011
491
Jacobson et al.
San Emigdio
Mts. (3/82)
Gf
Sierra
Nevada
NV
AZ
Rand Mts.
(5/166)
Hf
SG
SA
f
Portal Ridge
(1/37)
Blue Ridge
(2/70)
Rf
Sierra de
Salinas (5/138)
CA
Orocopia
Orocopia Mts.
(10/253)
Mt. Pinos
(5/125)
Nf
Nf
Trigo Mts.
(4/92)
Neversweat
Ridge (2/66)
f
EH
Rand
Mojave
Desert
SG
SG
f
H
f
100 km
Nf
l
Pe
a
55–70 Ma
70–85 Ma
85–100 Ma
100–135 Ma
135–300 Ma
>300 Ma
on
Detrital zircon ages
?
Sierra Pelona
(7/199)
East Fork
(4/110)
Catalina
Schist
(21/305)
N
SE Chocolate Castle Dome
Mts. (3/79)
Mts. (4/77)
Peninsular
Ranges
Figure 4. Pie diagrams of detrital zircon ages from the Pelona-Orocopia-Rand-Catalina
schists plotted on the same palinspastic base as in Figure 2. The Nacimiento fault is indicated
by a heavier line weight than the other faults. Present-day outcrops of Pelona-OrocopiaRand schists are shown in black. The Catalina Schist is inferred to restore beneath the
northern Peninsular Ranges (Crouch and Suppe, 1993). Mesozoic magmatic rocks and associated wall rocks of the Sierra Nevada and Peninsular Ranges batholiths are shown by the
plus pattern. Arc and wall rocks of the Mojave Desert and Salinian block are omitted for
clarity. Dashed blue outlines delineate three of the four schist groupings used for plotting
zircon results in Figure 5 (see also Fig. 3). The fourth group includes the spatially separated Catalina Schist and schist of the San Emigdio Mountains (see text). Paired numbers
in parentheses indicate number of samples and number of zircon ages.
the Catalina Schist is geographically distinct
(Fig. 4); i.e., despite its southerly location, the
Catalina Schist is dominated by detrital zircons
of Early Cretaceous age and, as noted previously,
appears most similar to the Pelona-OrocopiaRand schist of the San Emigdio Mountains. As
we will discuss later, the origin of this pattern is
ambiguous owing to the uncertain nature of slip
on the Nacimiento fault.
Forearc Sedimentary Units
Age Divisions of Forearc Units
Samples from the forearc units were divided
into four age groups: (1) Cenomanian-Turonian,
(2) Campanian–early Maastrichtian, (3) middle
Maastrichtian–Paleocene, and (4) early to
middle Eocene. These subdivisions were guided
by structural and stratigraphic breaks in the
sedimentary record (see the descriptions of individual sampling areas in the GSA Data Re-
492
pository item [see footnote 1]) and correspond
closely in age to the four schist groups, particularly with respect to the three older age divisions (Fig. 3). However, the youngest forearc
age group (early to middle Eocene) may have
an average age somewhat younger than that of
the youngest schist group (Orocopia Schist);
i.e., as discussed already, the minimum age of
the Orocopia Schists is not well defined and
conceivably could be no younger than middle
Paleocene (Fig. 3). Despite this uncertainty, it is
clear that the schists and forearc units analyzed
here represent at least 30 m.y. of overlapping
depositional history during a critically important period in the tectonic evolution of southern
California.
Variation in Detrital Zircon Populations as a
Function of Depositional Age and Location
Probability plots for the forearc age groups
are illustrated in Figures 5F–5J alongside the
matching divisions for the schists. The overall
similarity between the two sample sets is impressive. By analogy with the schists, the variation from oldest to youngest parts of the forearc
section is considered to reflect a transition from
outboard to inboard source areas.
Pie diagrams of the zircon age distributions
plotted on a geographic base (Fig. 6) confirm
the temporal progression from marginal to
inboard sources and reveal significant alongstrike variations in provenance (see also Figs.
DR2–DR4 [see footnote 1]). Material derived
from east of the main Sierran–Peninsular
Ranges arc first appeared at the continental
margin in a few localities in the central part of
the study area during Campanian–early Maastrichtian time (Fig. 6B; Cambria and Pfeiffer
slabs, Santa Ynez Mountains, Santa Monica
Mountains, Simi Hills). With time, detritus
originating from inboard sources became the
dominant component in the central part of
the area and was also delivered in significant
quantities both to the northwest and southeast
(Figs. 6C and 6D).
It is important to keep in mind that Figure 6
does not take into account displacement on the
Nacimiento fault. Because movement appears
to have been over by the late Paleocene, the
Eocene reconstruction (Fig. 6D) should be independent of this event. However, the preceding
time frames presumably require varying degrees
of correction, depending on the exact nature and
age of the Nacimiento fault. If this structure is
dominantly a thrust (Page, 1970, 1981; Hall
1991; Saleeby, 2003), then displacement should
have been largely normal to the length of the
margin. In this case, the along-strike variations
evident in Figure 6 would not be significantly
altered. On the other hand, for 500–600 km of
sinistral slip (Dickinson, 1983; Dickinson et al.,
2005), the Nacimiento block would restore approximately outboard of the Diablo Range of
central California, without the Salinian block
intervening. Further implications of the sinistral
case are considered later herein.
Figure 6 reveals an impressive similarity in
the Cenomanian-Turonian to Eocene patterns
from San Miguel Island with those of equivalent
age from the vicinity of San Diego and northern
Baja California. This is consistent with models involving large-scale rotation of the western Transverse Ranges from an initial position
alongside the Peninsular Ranges.
One striking anomaly in our results pertains
to a single sample of Paleocene age from northern Baja California (Fig. 6C). Work by one of
us (M. Grove) suggests that the distinctive character of this sample reflects a highly restricted
source region in the vicinity of the Sierra El
Mayor of eastern Baja California.
Geological Society of America Bulletin, March/April 2011
Late Cretaceous–early Cenozoic tectonic evolution of southern California
POR-Catalina Schists
Cumulative prob.
100
50
100
150
200
250
300
600
Forearc Sedimentary Units
900 1200 1500 1800 2100 50
100
A
80
80
100
150
200
250
300
600
900 1200 1500 1800 2100
F
Kuu
PE
60
60
OR
40
Orocopia (OR)
Pelona (PE)
Rand (RA)
San Emig.-Catalina (SE)
RA
SE
20
Eoc
40
20
0
Eocene (Eoc)
Mid Maast-Pal (KP)
Camp-early Maast (Kuu)
Cen-Tur (Klu)
KP
Klu
Relative probability
0
Orocopia (23/478)
B
Eocene (28/827)
G
Pelona (18/504)
C
Mid Maast-Pal (30/582)
H
Rand (11/341)
D
S. Emigdio-Catalina (24/387)
Camp-early Maast (32/853)
Cen-Tur (10/333)
E
Vert exag = 2
for ages>300 Ma
50
100
150
200
250
300
600
900 1200 1500 1800 2100 50
Detrital zircon age (Ma)
100
150
200
250
300
Vert exag = 2
for ages>300 Ma
600
I
J
900 1200 1500 1800 2100
Detrital zircon age (Ma)
Figure 5. Detrital zircon age distributions for the Pelona-Orocopia-Rand-Catalina schists (left column) and forearc sedimentary units (right
column). Zircons older than 2.1 Ga comprise <0.5% of the total and are not included. Sample groupings are explained in the text. Forearc
and schist groups of similar depositional age are plotted next to each other and indicated by the same fill pattern. Note x-axis scale break at
300 Ma. Vertical scales also differ to the left and right of the x-axis break such that equal areas represent equal probability throughout the
graph, except for plots E and J, for which ages older than 300 Ma are exaggerated by a factor of 2. Black lines in plots G–J indicate schist
probabilities from plots B–E, respectively. They are raised above the x-axis so as not to obscure the plots for the forearc units. Abbreviations for geologic time periods are as in Figure 3. Paired numbers in parentheses indicate number of samples and number of zircon ages.
Note that some analyses were excluded from these plots in order to avoid overweighting samples with a large number of analyses (see text
and explanation for Fig. 6).
Sandstone Petrology
Summary modal data for the 59 sandstones
analyzed petrographically are presented in
Table 1. More complete results, including statistical values, are provided in the GSA Data
Repository item (Tables DR5 and DR6 [see
footnote 1]). The Data Repository item also
includes QtFL and QmKP ternary diagrams of
the results, along with plots of various compositional parameters shown in geographic coordinates using the same format as Figure 6 (Figs.
DR5–DR9 [see footnote 1]).
Most (56) of the samples are Campanian to
middle Eocene. This group is rich in quartz
and feldspar, with low to modest contents
of lithic grains (Table 1). Volcanic fragments comprise less than 30% of total lithics.
Plagioclase is more abundant than K-feldspar,
although generally not by much. Total lithic
contents and P/F values show a weak tendency to decrease with decreasing depositional age (Table 1; see also Figs. DR5–DR8
and Table DR6 [see footnote 1]). Other than
this, the samples exhibit no systematic trends
in composition with time.
In contrast to the Campanian to Eocene
samples, three of Cenomanian-Turonian age
from the San Rafael Mountains and Atascadero area are marked by high total lithics,
Lv/Lt, and P/F.
COMPARATIVE PROVENANCE
Detrital Zircons
Sources of Individual Zircon Populations
The most distinctive finding of this study is
the remarkable parallelism in evolution of detrital zircon suites within the Pelona-OrocopiaRand-Catalina schists and spatially associated
forearc basin. As already noted, the evolving
patterns suggest a transition from outboard to
inboard source areas with time, compatible with
previous interpretations based on sandstone
petrology and conglomerate clasts (Gilbert and
Dickinson, 1970; Lee-Wong and Howell, 1977;
Geological Society of America Bulletin, March/April 2011
493
SA
f
Nf
Rf
SGHf
SGf
Nf
Geological Society of America Bulletin, March/April 2011
Northern Baja
(2/88)
A
Northern Baja
(2/158)
San Diego
(4/169)
Figure 6.
B
Northern Baja
(1/39)
C
Santa Monica
Mts-Simi Hills
(4/66)
San Diego
(2/93)
Nf
San Diego
(1/58)
Santa Ana
Mts.(2/82)
SGHf
S. Miguel Is.
(2/64)
SGf
Santa Monica
Mts.-Simi Hills
(4/85)
EHf
S. Miguel Is.
(2/69)
Nf
?
SGf
Santa Ana
Mts. (2/56)
Nf
?
S. Miguel Is.
(1/53)
D
Orocopia
Mts. (2/129)
Santa Monica
Mts.-Simi Hills
(3/65)
San Diego
(4/212)
?
Northern Baja
(3/146)
Santa Ana
Mts. (2/184)
Santa Ynez
Mts. (5/74)
SGHf
Santa Ana
Mts. (2/61)
EHf
Pine Mt.
Block (2/26)
N
Sierra Madre
Mts. (3/39)
f
SG
55–70 Ma
70–85 Ma
85–100 Ma
100–135 Ma
135–300 Ma
>300 Ma
Rf
Santa Ynez
Mts. (2/65)
San Gabriel
Block (5/73)
Cajon
Pass
(1/13)
Gf
Early-Mid
Eocene
Indians Ranch
(2/50)
Ben Lomond
Mt. (1/28)
SGHf
?
Upper McCoy
Tur-Camp
Barth et al.
(4/119)
Nf
La Panza
Range (4/50)
f
San Rafael
Mts. (3/55)
Atascadero
(4/55)
NW Santa
Lucia Range
(3/35)
Mid MaastPaleocene
Nf
Detrital zircon ages
SGHf
Indians
Ranch
(2/40)
Pt. Lobos
(2/29)
Pt. San
Pedro (4/62)
Rf
Point Sur Pfeiffer
Slab
(3/43)
Rf
Pigeon Point
(3/39)
f
EHf
San Rafael
Mts. (2/31)
SGHf
Cambria
(3/64)
Del Puerto
Cyn (1/36)
CampEarly Maast
300 km
SA
Atascadero
(1/26)
Mojave
Desert
Coalinga
DeGraaffSurpless et al.
(2/112)
120 km
Rf
f
100 km
Coalinga
DeGraaffSurpless et al.
(3/166)
Cen-Tur
Si
N err
e
va a
da
494
SA
SA
Nf
Pe
R ni
n
a
ng sula
es
r
120 km
Jacobson et al.
Late Cretaceous–early Cenozoic tectonic evolution of southern California
Figure 6. Pie diagrams of detrital zircon ages from the forearc sedimentary units plotted on
the same palinspastic base as in Figures 2 and 4. Present-day outcrops of Pelona-OrocopiaRand schists (black) and the Sierra Nevada and Peninsular Ranges batholiths (pluses) are
included for reference. See Figures 1 and 2 for distributions of the forearc units. The four
panels correspond to the four age groups plotted in Figure 5. Note that the left and right
boundaries are positioned in slightly different locations in each panel, the average of which
is indicated in Figure 2. Individual sampling localities are described in the GSA Data Repository item (see text footnote 1). Results from the Great Valley Group of the Coalinga area
(panels A and B) are from DeGraaff-Surpless et al. (2002). Those for the McCoy Mountains
Formation (panel B) are from Barth et al. (2004). Paired numbers in parentheses indicate
number of samples and number of zircon ages. Note that the total number of analyses represented in this figure is larger than that shown in Figures 5G–5J. This relates to the fact
some analyses were excluded from the plots of Figure 5 in order to avoid overweighting
samples with an exceptionally large number of zircon results (see text). Fault abbreviations
are as in Figure 1.
TABLE 1. AVERAGE DETRITAL MODES OF SELECTED SANDSTONES
Qt
F
L
M
n
(%)
(%)
(%)
(%)
Lv/Lt
Sample group
This study
Early–mid-Eocene
Mid-Maastrichtian–Paleocene
Campanian–early Maastrichtian
Cenomanian–Turonian
14
23
19
3
36
41
36
25
52
47
46
35
12
12
18
40
3
7
7
2
0.24
0.28
0.23
0.50
P/F
0.59
0.56
0.67
0.90
Great Valley Group (Ingersoll, 1983)
Rumsey (Santonian–Maastrichtian)
34
40
36
24
10
0.60
0.55
Grabast (Cenomanian–Turonian)
17
30
30
41
7
0.37
0.68
Stony Creek (Tithonian–Barremian)
33
25
22
53
2
0.60
0.92
Note: n—number of samples; Qt—monocrystalline and polycrystalline quartz relative to total QtFL; F—feldspar
relative to total QtFL; L—lithic fragments relative to total QtFL; M—framework mica relative to total framework
grains; Lv/Lt—fraction of volcanic lithic fragments relative to total lithic fragments; P/F—fraction of plagioclase
relative to total feldspar; 300 to 400 grains were counted per sample (mainly the latter); see the GSA Data
Repository item (see text footnote 1) for more complete results, including standard deviations and standard errors.
Kies and Abbott, 1983; Bachman and Abbott,
1988; Abbott and Smith, 1989; Seiders and Cox,
1992; Grove, 1993; Dickinson, 1995; Dickinson
et al., 2005). Here, we evaluate in more detail
the potential source areas for individual zircon
age populations (Fig. 5).
Cretaceous–Early Cenozoic. Early to early
Late Cretaceous grains characteristic of the
older depositional units are thought to have
a source in the western to central Sierran–
Peninsular Ranges arc (Fig. 2; Silver et al., 1979;
Saleeby and Sharp, 1980; Stern et al., 1981;
Chen and Moore, 1982). In contrast, latest Cretaceous grains that typify the younger units were
presumably derived from widespread Laramide
plutons of the Mojave Desert, southwestern
Arizona, and northwestern Sonora (Fig. 2;
Barth et al., 1995, 2001a, 2004, 2008a;
McDowell et al., 2001; Wells and Hoisch,
2008). Laramide intrusive rocks younger than
ca. 70 Ma have not been reported from southern
California, although igneous rocks with ages of
ca. 70–50 Ma occur locally in southern Arizona
and northwestern Sonora (Haxel et al., 1984;
Spencer et al., 1995) and are relatively common
in north-central Sonora (González-León et al.,
2000; McDowell et al., 2001). Grains within the
latter age range form a distinctive component of
some of the younger samples of both the schists
and forearc units (Figs. 3–6).
Jurassic. Grains of Jurassic age are present in
modest abundances throughout the sample suite
(Figs. 4–6). Those in the oldest units occur in
association with abundant Early to middle Cretaceous ages (Figs. 5E and 5J). This same correlation is also evident within the type Great Valley
Group of central California (Figs. 6A and 6B),
as represented by one sample of Campanian age
that we collected from Del Puerto Canyon in
the northern Diablo Range and five of Cenomanian to Campanian age from the Coalinga area
of the southern Diablo Range from the work of
DeGraaff-Surpless et al. (2002). This pattern
is consistent with a source in the western flank
of the central to northern Sierra Nevada, where
the Cretaceous and Jurassic arcs overlap (Stern
et al., 1981; Chen and Moore, 1982; Irwin and
Wooden, 2001; DeGraaff-Surpless et al., 2002).
In contrast, Jurassic grains in younger samples
tend to be associated with high proportions of
Proterozoic and latest Cretaceous ages and low
abundances of Early to middle Cretaceous ages
(Fig. 5). The latter assemblage likely reflects a
source in southeastern California, southwestern
Arizona, and northwestern Sonora, where the
Jurassic arc lies inboard of the Early to middle
Cretaceous arc (Fig. 2; Tosdal et al., 1989; Barth
et al., 2008b).
Permian–Triassic. Grains of Permian and
Triassic age comprise a small but distinctive
component of both the schists and forearc units
with depositional ages of middle Maastrichtian
and younger (Fig. 5). Triassic plutons are minor in abundance but widely distributed within
the inboard side of the Cordilleran arc (Barth
et al., 1997; Barth and Wooden, 2006). Permian
igneous rocks are less common, but have been
reported from the northwestern Mojave Desert
and vicinity (Martin and Walker, 1995, and references therein). This distribution is consistent
with the fact that Permian (and Triassic) grains
are most common in samples from the northwestern part of the study area (Figs. DR3 and
DR4 [see footnote 1]).
Pre-Permian. Both the schists and forearc
units provide evidence for two distinct associations of pre-Permian ages. That present within
the San Emigdio–Catalina schists and Cenomanian-Turonian forearc units is characterized
by a comparatively wide spread of ages from
Paleozoic to Paleoproterozoic (Figs. 5E and
5J). This distribution is similar to that exhibited
by pre-Mesozoic detrital zircons within Lower
Mesozoic sedimentary framework rocks of the
western belt of the Peninsular Ranges batholith (Fig. 7C; Grove et al., 2008). Such grains
are considered to have a wide range of ultimate
sources within North America (e.g., Dickinson
and Gehrels, 2003, 2009). Also noteworthy, this
assemblage occurs in units marked by a strong
concentration of Early to middle Cretaceous detrital zircons (Figs. 5E and 5J), which likewise
point to a source in the western Sierra Nevada
(see previous). Hence, the lower Upper Cretaceous deposits can be explained entirely by erosion of the western flank of the Cordilleran arc,
including its older wall rocks.
The second pre-Permian association is characterized by a main peak centered at ca. 1.7 Ga
and variably developed peaks at ca. 1.4 and
1.2 Ga. These three peaks are broadly equivalent to the Yavapai-Mazatzal, anorogenic, and
Grenville populations, respectively, identified
by Dickinson and Gehrels (2009) within Jurassic eolianites and related sequences of the Colorado Plateau region. However, for the relatively
outboard setting considered here, we assume
that the ca. 1.7 Ga peak includes a substantial
contribution from Mojave sources in addition to
those from the Yavapai and Mazatzal terranes.
The size of the anorogenic peak is consistent
with the proportion of intrusive rocks of this age
Geological Society of America Bulletin, March/April 2011
495
Jacobson et al.
Cumulative prob.
100
50
100
150
200
250
300
600
900 1200 1500 1800 2100 2400 2700 3000
F-POR-C
80
60
A
MC
40
20
ERG
PR
0
Forearc-POR-Catalina (F-POR-C) (172/4305)
B
creasing average age of the Cretaceous–early
Cenozoic population and increasing abundance
of Proterozoic grains with decreasing depositional age is somewhat more systematic within
the forearc sequence than the schists (Fig. 5).
This is probably due to the fact that the forearc
samples represent a greater geographic range,
array of depositional environments, and total
number of samples and analyses than the
schists; i.e., the forearc groups likely reflect a
more complete averaging of the margin than
the schists.
Relative probability
Sandstone Petrology
50
Lower Mesozoic wall rocks Peninsular Ranges (PR) (4/433)
C
Upper McCoy Mountains
Formation (MC) (4/119)
D
Jurassic ergs (ERG) (10/890)
E
100
150
200
250
300
600
900 1200 1500 1800 2100 2400 2700 3000
Detrital zircon age (Ma)
Figure 7. Detrital zircon age distributions for various sedimentary sequences in California and adjacent areas of the southwestern United States. (A) Cumulative probabilities for
sample groups plotted in B–E. (B) Combined data for the Pelona-Orocopia-Rand-Catalina
schists and forearc units (this paper). (C) Lower Mesozoic sedimentary framework rocks of
the western belt of the Peninsular Ranges batholith (Morgan et al., 2005; Grove et al., 2008).
(D) Upper McCoy Mountains Formation in the McCoy Mountains (Barth et al., 2004).
(E) Colorado Plateau eolianites (Dickinson and Gehrels, 2003, 2009). See Figure 5 for explanation of scale break at 300 Ma. Paired numbers in parentheses indicate number of samples
and number of zircon ages.
within the Mojave-Yavapai-Mazatzal basement
of the Southwest United States (Karlstrom et al.,
1987; Anderson and Bender, 1989; Wooden and
Miller, 1990; Barth et al., 2000, 2001a, 2001b;
Anderson and Silver, 2005; Farmer et al., 2005)
and with predicted zircon “fertility” of the inferred source rocks (Dickinson, 2008). In contrast, whereas zircons of Grenville age are not
abundant in either the schists or forearc units,
they nonetheless appear to be overrepresented
compared to the known distribution of Grenville
rocks in the southwestern United States and
northwestern Sonora (Barth et al., 1995, 2001b;
Anderson and Silver, 2005; Farmer et al., 2005).
On the other hand, detrital zircons of Grenville
age, apparently derived from the Appalachian
496
orogen, are common in Neoproterozoic to Cambrian cratonal and miogeoclinal sections of this
region (Gehrels, 2000; Stewart et al., 2001;
Gehrels et al., 2002; Barth et al., 2009). This implies that zircons of Grenville age in the schists
and forearc units are recycled. The cratonal and
miogeoclinal sequences also include substantial
populations of anorogenic and Mojave-YavapaiMazatzal ages; consequently, some of these
grains may be recycled, as well.
Contrasts between the Schists and
Forearc Units
Despite the striking similarities between the
schists and forearc units, there are some differences. For example, the overall pattern of de-
Results from the sandstone modal analysis
are consistent with the detrital zircon data but
not as diagnostic. For example, the high total
lithics, Lv/Lt, and P/F for the CenomanianTuronian forearc units (Table 1; see also fig. 13
in Dickinson, 1995) confirm a source in the
western side of the Sierran–Peninsular Ranges
arc (cf. Ingersoll, 1983), as concluded based
on the zircon data. For the younger samples,
however, detrital zircon and sand compositions are not strongly coupled. Specifically, the
Campanian to Eocene units exhibit a dramatic
shift in detrital zircon populations with depositional age reflecting progressively more inboard
source regions (Figs. 5G–5I). In contrast, sand
compositions for these same units show only
subtle variations with age (Table 1). The detrital
zircons thus provide a reliable indication of
ages of rocks within the source areas, whereas
modal mineralogy reflects only their lithology.
The latter is apparently relatively uniform both
spatially and temporally within the eastern side
of the arc and southwestern part of the craton.
The sandstones analyzed here are broadly
similar in detrital modes to those from the type
Great Valley Group of central California east
of the San Andreas fault (Table 1; Ingersoll,
1983). Overall, our samples tend to be somewhat lower in lithics and P/F than the Great
Valley rocks, suggesting a more inboard and
deeply denuded source area (cf. Dickinson,
1985; Marsaglia and Ingersoll, 1992; Ingersoll
and Eastmond, 2007). This probably reflects
the younger average age of our samples and
their proximity to the strongly disrupted segment of the arc at the latitude of the Mojave
Desert (see also Dickinson, 1995).
Comparison to Other Depositional Systems
Upper McCoy Mountains Formation
The McCoy Mountains Formation of southeastern California and southwestern Arizona
(Fig. 1) is a weakly metamorphosed Upper Jurassic to Upper Cretaceous fluvial sequence with
Geological Society of America Bulletin, March/April 2011
Late Cretaceous–early Cenozoic tectonic evolution of southern California
a stratigraphic thickness of over 7 km (Harding
and Coney, 1985; Tosdal and Stone, 1994; Barth
et al., 2004; Spencer et al., 2005). The Upper
Cretaceous part of the formation was deposited
on the foreland side of the Sierran–Peninsular
Ranges arc (Barth et al., 2004) and provides an
important reference section compared to the coeval forearc units analyzed in this study.
Detrital zircon ages (Barth et al., 2004) for
four samples of upper McCoy Mountains Formation with depositional ages younger than
Turonian but older than middle Campanian are
plotted in Figures 6B and 7D (based on these
depositional ages, the McCoy data could also
have been included in Fig. 6A, but were omitted for reasons of space). Results for the upper
McCoy samples contrast sharply with those
from contemporaneous parts of both the forearc basin and Pelona-Orocopia-Rand-Catalina
schists. In particular, the McCoy Mountains
Formation is dominated by Proterozoic detritus
(Figs. 6B and 7D), whereas coeval forearc units
and Pelona-Orocopia-Rand-Catalina schists
were derived overwhelmingly from Cretaceous
sources (Figs. 5D, 5E, 5I, 5J, 6A, and 6B). This
confirms that the arc served as a topographic
barrier between forearc and retroarc basins at
this time. The forearc basin was apparently supplied with detritus derived largely from the west
side of the arc. In contrast, the retroarc basin received sediment from both the east flank of the
arc and nearby cratonal areas to the north and
east of the basin that were experiencing foreland shortening and uplift (see also Harding and
Coney, 1985; Barth et al., 2004).
Colorado Plateau Sequences
Recent studies of Permian and Jurassic eolianites from the Colorado Plateau (Dickinson
and Gehrels, 2003, 2009) provide important
constraints on the eastern limit of the source
area for the Pelona-Orocopia-Rand-Catalina
schists and forearc units. The Colorado Plateau
sequences exhibit some peaks in common with
the units analyzed here (e.g., in the range of
ca. 1.8–1.0 Ga; Figs. 5, 7B, and 7E). However,
the relative sizes of those peaks differ greatly between the two groups. The Colorado Plateau sequences also include substantial proportions of
late Neoproterozoic and early to middle Paleozoic ages. Grains of the latter ages are notably
rare in the schists and forearc units, particularly
in the youngest parts, which are those derived
from the most easterly source areas. This indicates the persistence of a topographic divide
between coastal and interior regions, even following the removal of the western belt of the arc
and inner forearc basin. This is consistent with
longstanding inferences of a topographic high,
the Mogollon Highlands, in southwestern Ari-
zona during the Paleogene (Young and McKee,
1978; Peirce et al., 1979; Potochnik and Faulds,
1998; Spencer et al., 2008).
TECTONIC INTERPRETATIONS
The Pelona-Orocopia-Rand-Catalina schists
and Nacimiento fault represent first-order tectonic elements in the Late Cretaceous–early
Cenozoic evolution of the Southwest United
States. While it has long been suspected that these
two features are in some way tied together, the
exact nature of this relationship has been difficult
to discern (Page, 1982; Hall, 1991; Barth and
Schneiderman, 1996; Barth et al., 2003; Saleeby,
2003). One point that is now evident, however,
is that underthrusting of the Pelona-OrocopiaRand-Catalina schists began significantly earlier
than movement on the Nacimiento fault (>90 Ma
versus <75 Ma, respectively). Furthermore, slip
on the Nacimiento fault was probably over by
the late Paleocene, whereas emplacement of
the schists may have extended into the Eocene
(Fig. 3). Because of this timing relationship, we
focus the following discussion around the evolution of the schists, and treat the Nacimiento fault
and truncation of arc and forearc as one element
within that broader context.
Preferred Model for Underplating of the
Pelona-Orocopia-Rand-Catalina Schists
Our preferred model for the origin of the
Pelona-Orocopia-Rand-Catalina schists is based
on the common, although not universal, view
that these units represent a subduction complex
(see references cited in the Introduction). This
interpretation is compatible with the turbidite
sandstone-basalt-chert protoliths and the relatively high-P metamorphism. It is also supported by the extended time frame (>30 m.y.) of
emplacement of the schists combined with the
short cycling interval (Fig. 3). In fact, we find
it difficult to imagine a tectonic setting other
than an Andean-style subduction zone that could
produce this pattern of ongoing deposition followed almost immediately by underplating and
progressive exhumation.
Prior to initiation of the Pelona-OrocopiaRand-Catalina event in the early Late Cretaceous, subduction geometry was presumably
similar to that commonly invoked to explain the
Franciscan–Great Valley–Sierra Nevada triad of
central California east of the San Andreas fault
(Fig. 8A; for details of the Late Jurassic–Early
Cretaceous behavior of the California convergent margin, see Dumitru et al., 2010). By
ca. 95–90 Ma, however, the Farallon plate had
apparently transitioned, at least in southern California, to a shallow mode of subduction, perhaps
related to the presence of an aseismic ridge or
oceanic plateau (Livaccari et al., 1981; Henderson et al., 1984; Barth and Schneiderman,
1996; Saleeby, 2003). This geometry would
have favored subduction erosion of the overriding North American plate (cf. von Huene and
Scholl, 1991; Clift and Vannucchi, 2004; Scholl
and von Huene, 2007), inboard migration of
the axis of arc magmatism, and underplating
of Pelona-Orocopia-Rand-Catalina schist, although continued accretion within the outboard
Franciscan wedge is not precluded. Figure 8B
depicts the inferred geometry for the early phase
of this event, prior to inception of Nacimiento
slip at ca. 75–68 Ma. This panel is relevant to the
emplacement of the Catalina and San Emigdio
Schists and most, if not all, of the Rand Schists
(Fig. 3). Note that the western part of the arc
would still have been in place at this time, consistent with detrital zircon results for the Catalina–
San Emigdio Schists (Fig. 5E) and zircon and
sandstone petrologic data for CenomanianTuronian forearc units (Fig. 5J; Table 1). For the
most part, the Campanian–early Maastrichtian
forearc units (Fig. 5I) suggest a similar paleogeography. The mode of the Cretaceous detrital
zircon age peak in the latter group is somewhat
younger than for the Cenomanian-Turonian units
(100 Ma versus 110 Ma, respectively). However,
this could be the result of eastward retreat of the
arc front, as described by Linn et al. (1992) for
central California, without necessarily requiring any major structural reorganization of the
forearc basin and arc. On the other hand, some
Campanian–early Maastrichtian sedimentary
sequences (Cambria and Pfeiffer slabs, Santa
Ynez Mountains, Santa Monica Mountains,
Simi Hills; Fig. 6B; Fig. DR10 [see footnote 1])
and the coeval Rand Schists (Fig. 5D; Fig. DR10
[see footnote 1]) include a fraction of sediment
that appears to have been derived from sources
east of the main axis of the Sierran–Peninsular
Ranges arc. It is not clear whether this denotes
local erosional breaching of the arc edifice prior
to the initiation of slip on the Nacimiento fault or
the earliest stages of fault movement.
A second transition in the nature of the margin coincided with the removal of the western
belt of the arc and associated inner forearc basin beginning in late Campanian or early Maastrichtian time. Considering the uncertain nature
of slip on the Nacimiento fault, we present two
alternatives for this time period, one involving
thrusting (Fig. 8C), and the other strike slip
(Fig. 8D). The thrust option (equivalent to the
Sur thrust of Hall, 1991) has been supported
by numerous workers (Page, 1970, 1981; Yin,
2002; Barth et al., 2003; Saleeby, 2003; Kidder
and Ducea, 2006; Ducea et al., 2009) but in our
opinion is at odds with the fact that movement
Geological Society of America Bulletin, March/April 2011
497
Jacobson et al.
A ca. 100 Ma
Forearc basin
Franciscan
Future
Colorado
Plateau
Upper McCoy
Mountains Fm.
Pre-Cret. crust
Top o
f
Fa
Continental
lithosphere
ral
100 km
lon
sla
b
B ca. 80 Ma
Franciscan
Upper McCoy
Mountains Fm.
Forearc crust: Jur. and
E. Cret?
Future
Colorado
Plateau
Pre-Cret. crust
Top
of
Farallon
Continental
lithosphere
sla
b
Rand/Catalina Schist
C ca. 50 Ma—thrust option
Rand/Catalina
Schist
Franciscan
Forearc basin
Mogollon
Highlands
Upper McCoy
Mountains Fm.
NF
Future
Colorado
Plateau
Pre-Cret. crust
Top
Pelona Schist
of
Orocopia Schist
Peraluminous
crustal melts
Fa
ra
llo
n
D ca. 50 Ma—strike-slip option
Rand/Catalina
Schist
Franciscan
Forearc basin
slab
Mogollon
Highlands
Upper McCoy
Mountains Fm.
Future
Colorado
Plateau
NF
Pre-Cret. crust
Top
Pelona Schist
SW
of
Orocopia Schist
Peraluminous
crustal melts
Fa
ra
llo
n
slab
NE
Figure 8. Tectonic model for underplating of the Pelona-Orocopia-Rand-Catalina schists
and development of the Nacimiento fault. (A) Geometry prior to the onset of flat subduction
and emplacement of the Pelona-Orocopia-Rand-Catalina schists. (B) Early phase of flat
subduction preceding initiation of slip on the Nacimiento fault. (C–D) Relations following
cessation of slip along the Nacimiento fault (NF), assuming thrusting and sinistral strike
slip, respectively. For simplicity, slip on the Nacimiento fault is shown in both C and D as
postdating emplacement of the Pelona Schist but predating emplacement of the Orocopia
Schist. In actuality, movement on the Nacimiento fault is likely to have overlapped with the
underthrusting of either or both of those schist groups. Inset in C shows an enlargement of
our interpretation of relations adjacent to the Nacimiento fault system in the case of thrusting. The North American craton is held fixed in all panels. Fill patterns for the igneous rocks
correspond to the color scheme of Figure 2 with the addition that yellow represents ages of
ca. 70–50 Ma.
498
on the Nacimiento fault must have occurred
during emplacement of the Pelona-OrocopiaRand schists (Fig. 3). The missing parts of the
arc and forearc basin represent a substantial
volume of material. Whereas much of this mass
could have been completely subducted, it seems
reasonable to expect that at least some fraction
would have been interleaved with the schists
(Fig. 8C). However, as pointed out by Dickinson et al. (2005, p. 14), the Pelona-OrocopiaRand schists bear little resemblance to the
omitted units. The schists consist predominantly of coherent meta-sandstone with small
amounts of meta-basalt, meta-chert, marble,
and ultramafic rock. In contrast, the inferred
missing units include parts of the Franciscan
mélange, Coast Range ophiolite, uppermost Jurassic through Cretaceous Great Valley Group,
and western belt of the Sierran arc, including
its pre–Late Jurassic metamorphic framework
rocks. The only component of this assemblage
that reasonably matches the Pelona-OrocopiaRand-Catalina schists is the Upper Cretaceous
part of the Great Valley Group. However, it
is not clear how this part of the missing section could have been incorporated within the
schists while completely excluding the remainder. Mafic rocks of the Coast Range ophiolite
bear superficial compositional resemblance to
metabasites within the schists, but they differ in
detail. Specifically, the Coast Range ophiolite
exhibits an arc signature (Shervais and Kimbrough, 1985; Giaramita et al., 1998; Shervais et al., 2005), whereas mafic rocks in the
Pelona-Orocopia-Rand-Catalina schists were
derived overwhelmingly from ocean-floor basalts (Haxel et al., 1987, 2002; Dawson and
Jacobson, 1989; Moran, 1993). Consequently,
we conclude that the Pelona-Orocopia-RandCatalina schists can be explained entirely as
subducted trench materials and off-scraped
fragments of oceanic crust from the Farallon
plate (Figs. 8B and 8D) without any admixture
of materials excised from along the Nacimiento
fault. Nonetheless, we cannot rule out the possibility that fragments of the missing terranes are
hidden within unexposed parts of the PelonaOrocopia-Rand-Catalina schists (Fig. 8C).
In contrast to thrust models, strike-slip models
for the Nacimiento fault imply that the excised
parts of the arc and forearc basin remained at the
surface but were translated out of the plane of
margin-normal cross sections at the latitude of
the Mojave Desert (Fig. 8D). Omission of units
by strike-slip motion along the Nacimiento fault
can be reconciled with either dextral or sinistral
movement, as long as the fault cuts in the appropriate sense across the strike of the margin.
Nonetheless, for reasons discussed by Dickinson et al. (2005), we favor the sinistral option.
Geological Society of America Bulletin, March/April 2011
Late Cretaceous–early Cenozoic tectonic evolution of southern California
In this case, the missing units would have been
transported southward and would be represented by the Peninsular Ranges batholith, the
Cretaceous forearc sequence along its western
margin, and the more distal, rotated units of the
Channel Islands and western Transverse Ranges
(see following).
The sinistral slip model for the Nacimiento
fault avoids some of the issues of the thrust
model, but it may be problematic in other ways.
Correction for dextral slip on the San Andreas
fault (310–320 km; Matthews, 1976) and Rinconada fault (45 km; Graham, 1978) and sinistral
slip on the Nacimiento fault (500–600 km; Dickinson et al., 2005) would restore the main body
of the Nacimiento block ~150–250 km to the
north of its present location and place it directly
against the Franciscan Complex and Great Valley Group east of the San Andreas fault (see following and Dickinson, 1983). This is consistent
with the work of Gilbert and Dickinson (1970),
who demonstrated a parallelism in temporal
evolution of petrofacies between forearc units
of the Nacimiento block and the Great Valley
Group east of the Salinian block. On the other
hand, Long and Wakabayashi (2009) argued
that accretionary rocks of these two regions do
not make a good match. In particular, distinctive sequences within the Franciscan Complex
east of the San Andreas fault, such as the Marin
Headlands and Permanente terranes and Skaggs
Springs Schist (Wakabayashi, 1999), have not
been recognized within the Nacimiento block
(Wakabayashi, 2010, personal commun.). Considering the extreme spatial variability characteristic of accretionary wedges, these relations
do not, in our view, preclude the sinistral slip
model. Nonetheless, they do suggest the need
for further investigations.
A second potential problem with the sinistral
slip model relates to longstanding views that
relative motion between the Farallon and North
American plates was approximately head-on to
dextral during the Late Cretaceous–Paleocene
(Engebretson et al., 1985; Stock and Molnar,
1988). Dickinson et al. (2005, p. 15), however,
suggested that this interpretation may need to be
revised in light of recent advances in the understanding of hotspot reference frames. In addition, we propose that sinistral slip may have
been an expression of “escape” or “extrusion”
tectonics (cf. Tapponnier et al., 1982; Burke
and Şengör, 1986) related to subduction of the
aseismic ridge or plateau previously called upon
to explain the anomalous nature of the southern
California margin (Livaccari et al., 1981; Henderson et al., 1984; Barth and Schneiderman,
1996; Saleeby, 2003). Westward translation of
the Wrangell terrane of southeast Alaska along
the Denali fault due to subduction of the oceanic
Yakutat block may serve as a modern analog
(Eberhart-Phillips et al., 2006; Fuis et al., 2008).
Irrespective of whether the Nacimiento fault
operated in a thrust or strike-slip sense, movement on this structure appears to correlate
broadly in time with the provenance shift exhibited by the Pelona-Orocopia-Rand-Catalina
schists and forearc units. This is not surprising
considering that excision of the western arc and
inner forearc basin resulted in narrowing of the
margin by 150 km or more, thus decreasing
the transport distance from the former retroarc
(cratonal) region to the coastline (Figs. 8C and
8D). This process can account for the influx
of cratonal detritus to the continental margin
at the latitude of the Mojave Desert beginning
in the latest Cretaceous (Fig. 6). To the north
and south of the Mojave, material from inboard sources comprises a smaller fraction of
the total sediment volume and first appeared
during the latest Paleocene to early Eocene
(Figs. 6C and 6D). Regional drainages in the
latter areas probably developed across the full
width of the Sierran–Peninsular Ranges batholith following arc extinction associated with the
Laramide event (Kies and Abbott, 1983; Abbott
and Smith, 1989).
Telescoping of the arc and forearc basin along
the Nacimiento fault helps explain the location
of the southeastern (Orocopia) schists within the
cratonal province of southwestern Arizona, i.e.,
inboard of the primary axis of Cretaceous magmatism. Without the removal of the western belt
of the arc and inner forearc basin, emplacement
of schist this far from the trench requires an extreme degree of flat subduction. In fact, Haxel
et al. (2002, p. 124) used this line of reasoning
to argue against correlating the Orocopia Schists
with the Franciscan Complex and Catalina
Schist. However, because the Orocopia Schists
postdate much or even all of the truncation of
the forearc region (Fig. 3), the required transport
distance is not exceptional (Figs. 8C and 8D). In
particular, all flat-slab configurations illustrated
in Figure 8 fall well within the range of geometries inferred from the modern record (e.g.,
fig. 5 in Gutscher et al., 2000).
The subduction model for the PelonaOrocopia-Rand schists is typically framed in
the context of the Laramide orogeny. However,
it is worth noting that emplacement of the oldest Pelona-Orocopia-Rand-Catalina schists at
ca. 95–90 Ma significantly predates the ca. 80 Ma
extinction of the main Sierran arc and ca. 75 Ma
onset of the primary pulse of Laramide tectonism
in the Rocky Mountain foreland (Dickinson
et al., 1988; DeCelles, 2004). This could
reflect the time lag between impingement of
the flat slab at the continental margin and its
propagation to a position beneath the foreland
(Lawton, 2008). Alternatively, it might be an indication of tectonic erosion even during normal
subduction (cf. von Huene and Scholl, 1991;
Clift and Vannucchi, 2004; Scholl and von
Huene, 2007). Additional evidence of modest
pre-Laramide subduction erosion may be indicated by the 2.7 km/m.y. eastward migration
of arc magmatism within the Sierra Nevada between 120 Ma and 90 Ma (Stern et al., 1981;
Chen and Moore, 1982). Furthermore, Dumitru
et al. (2010) argued for subduction erosion, or
at least nonaccretion, in northern California and
central California east of the Salinian block prior
to ca. 123 Ma. Hence, shallow subduction during Laramide time may simply have enhanced
existing erosive processes. A corollary is that
rocks like the Pelona-Orocopia-Rand-Catalina
schists could be relatively commonplace beneath the outboard edges of Andean-style arcs.
Possible analogs of the Pelona-Orocopia-RandCatalina schists in this sense might be the Condrey Mountain Schist of the Klamath Mountains
(Brown and Blake, 1987), the Swakane Gneiss
of the North Cascades, Washington (Matzel
et al., 2004), and the Qiangtang metamorphic
belt of northern Tibet (Kapp et al., 2000).
Alternative Models for Underplating of the
Pelona-Orocopia-Rand-Catalina Schists
Forearc Model
In the previous section, we interpreted the
Pelona-Orocopia-Rand-Catalina schists as a
subduction complex formed along the contact between the Farallon and North American
plates. Alternatively, Barth and Schneiderman
(1996, their fig. 5) proposed that the schists were
derived from the Great Valley forearc basin by
thrusting beneath the Sierran arc along a fault
sympathetic to the subduction boundary, yet
contained entirely within the North American
plate. As argued already herein, however, we
consider the rock types of the Pelona-OrocopiaRand schists to be incompatible with derivation
from the forearc basin. Additional difficulties
posed by the forearc model are discussed by
Haxel et al. (2002) and Grove et al. (2003).
Backarc Model
This category includes several variants, all
of which involve suturing of an intraoceanic
continental fragment to North America (Haxel
and Dillon, 1978; Ehlig, 1981; Vedder et al.,
1983; Haxel et al., 2002). One major obstacle
for such models is the lack of compelling evidence for a suture zone (Burchfiel and Davis,
1981; Crowell, 1981; but see counterargument
by Haxel et al., 2002). In addition, backarc
models imply that the sandstone protoliths of
the Pelona-Orocopia-Rand schists and coeval
Geological Society of America Bulletin, March/April 2011
499
Jacobson et al.
Ra
50
70
na-O
Ma
(N
pia
.S
vs
E
op
n
tio
s)
A
Ma
90
Ra
nd
70
W
(N
Ma
Pelo
na-O
roco
pia
vs
.S
)
ns
tio
op
Continental
margin
E
500
roco
W
Continental
margin
ina
tal
Progressive Subduction Erosion
Grove et al. (2003) proposed that buzz-saw
removal of the base of North America could
allow younger schists to “leap-frog” beneath
Pelo
Ca
One of the most distinctive features of the
Pelona-Orocopia-Rand schists is the >30 m.y.
decrease in depositional and emplacement ages
from northwest to southeast (Fig. 3). Because of
the relatively linear distribution of the schist outcrops, it is not clear whether this age variation
relates more to distance inboard from the continental edge (Fig. 9A) or to position along strike
of the margin (Fig. 9B). Consideration of the
Catalina Schist provides some additional constraints but does not fully solve the problem. Assuming no sinistral slip on Nacimiento fault, the
Catalina Schist would initially have been located
outboard of the Pelona and Orocopia Schists
(southeast option in Figs. 9A and 9B). In view of
the ca. 95–90 Ma age of the Catalina Schist, this
reconstruction is consistent with the isochrons of
Figure 9A, but not those of Figure 9B. In other
words, the southeast option for the Catalina
Schist requires at least some component of decreasing age from outboard to inboard within the
schist terrane. Alternatively, for the case of 500–
600 km of sinistral strike slip on the Nacimiento
fault, the Catalina Schist would restore to the
northwest end of the Pelona-Orocopia-Rand
schist belt. This location is consistent with either
decreasing age outboard-inboard (Fig. 9A) or
northwest-southeast (Fig. 9B). These uncertainties require that we consider several mechanisms
to explain the observed variation of age within
the schist terrane.
Ma
Ma
90
ina
tal
Northwest-Southeast Decreasing Age of the
Pelona-Orocopia-Rand Schists
would migrate in the same direction, leading to
progressively younger packets of accreted material away from the trench. We refer to the predicted geometry as a “tectonic onlap” based on
its resemblance to an upside-down sedimentary
onlap (clearest in Fig. 8D).
nd
Ca
forearc sequences would have been deposited
on opposite sides (east and west, respectively)
of the intraoceanic terrane. However, it seems
highly unlikely that the detrital zircon age patterns of the schists and forearc units would be
so strongly correlated over >30 m.y. (Fig. 5) if
their protoliths had been deposited in isolated
basins. The McCoy data, in particular, imply
that a backarc basin would be highly enriched in
cratonal detritus compared to the forearc region
(Figs. 6B and 7D). Consequently, we interpret
the zircon results to indicate that the forearc
units and schists represent the proximal (forearc
basin) and distal (trench) facies, respectively,
of a single depositional system on the outboard
side of the Sierran–Peninsular Ranges arc. Between the previously identified weaknesses
(e.g., Burchfiel and Davis, 1981; Crowell, 1981)
and zircon data, we see little justification for retaining the backarc models.
50
Ma
100 km
B
Figure 9. End-member possibilities for age
variation within a sheet of Pelona-OrocopiaRand schist underplated beneath North
American crust. Northwest and southeast
locations of the Catalina Schist reflect the
cases of 500–600 and 0 km of sinistral slip
on the Nacimiento fault, respectively. The
figure is highly schematic and does not take
into account the narrowing of the margin
that would have resulted from displacement
on the Nacimiento fault, whether by thrusting or strike slip.
older schists to be underplated at increasingly
greater distances inboard (Fig. 8). In this case,
age contours would parallel the strike of the
margin (Fig. 9A), and the young age of the
southeastern schists would be a consequence
of their position relatively far inboard. We envision this process involving transfer, due to
buoyancy, of some fraction of trench materials from the subducting (Farallon) to overriding (North American) plate above the hinge at
which the subducting plate descended toward
the mantle. With time, such subcreted materials could become quite thick, leading to tectonic
and/or erosional denudation of the overlying
plate (cf. Platt, 1986; Yin, 2002). In this fashion,
underplated materials would be driven upward,
consistent with thermochronologic evidence
for exhumation of the Pelona-Orocopia-Rand
schists concurrent with subduction (Jacobson
et al., 2002, 2007). This would allow permanent
incorporation of some trench materials within
the overriding plate. As the flat slab continued
to erode inboard, the locus of underplating
Migrating Aseismic Ridge
As indicated already, Barth and Schneiderman (1996) proposed that the Pelona-OrocopiaRand schists were derived by thrusting of the
arc over the forearc basin. They attributed this
event to subduction of an aseismic ridge (cf.
Livaccari et al., 1981; Henderson et al., 1984)
with its long dimension oriented slightly more
northerly than the convergence vector between
the Farallon and North American plates. In this
case, the intersection point between ridge and
trench would have migrated southward with
time, presumably causing the thrust between arc
and forearc basin to propagate in the same direction. As discussed previously, we consider that
the schists were derived from trench materials
rather than from the underthrust forearc basin.
Nonetheless, the geometric argument of ridge
migration seems applicable to either situation.
A similar interpretation was also proposed by
Dickinson et al. (1988, p. 1036) to explain north
to south migration of Laramide deformation in
the foreland region.
Sinistral Slip on the Nacimiento Fault
The model of a migrating ridge-trench intersection seems particularly effective in explaining the northwest-southeast decrease
in age of the schists, assuming sinistral slip
on the Nacimiento fault. This is illustrated in
Figure 10, where panel A represents the paleogeography prior to fault slip. At this time, the
northwestern schist localities would have been
situated relatively close to the trench, requiring only modest flat subduction to emplace
trench materials beneath the former arc (Figs.
8B and 10A). At the same time, the future sites
of the Pelona and Orocopia Schists would have
resided far inboard, in a region of arc magmatism (Fig. 10A; Mattinson, 1990; Barth et al.,
1995, 1997, 2001a, 2008a; Kidder et al., 2003;
Barbeau et al., 2005). With commencement
of Nacimiento slip, the latter regions would
have been drawn toward the continental margin (Fig. 9B) in the same way that slip on a
low-angle normal fault will translate rocks of
the lower plate closer to the surface. As a consequence, progressively more southeastward
(inboard) regions would have been transferred
into the zone of underplating near the continental edge. In this scenario, the schists need
not define more than a relatively narrow band
(becoming younger to the southeast) beneath
Geological Society of America Bulletin, March/April 2011
Late Cretaceous–early Cenozoic tectonic evolution of southern California
Nevada
Nevada
plate
ra
CA
AZ
Nf
Nf
Ranges
CA
MEX
NV
SAf
ge
rid
lk
nb
lar
lk
ob
ent
AZ
Mojave
block
inia
Sal
CA
Gf
i
cim
Na
plate
ic
sm
NV
SAf
Peninsu
lk
nb
inia
Sal
Mojave
block
Sie
r
ra
Sie
r
a
F
Gf
Nf
CS
N
ei
As
Farallon
100 km
Nf
ra
ll
o
n
–North
Farallon
a
c
ri
e
Am
B ca. 60 Ma
Diablo
range
Nacimiento blk
A ca. 75 Ma
POR-Catalina Schists
Prot. basement w/55–85 Ma plutons
Early Cret. batholith (100–135 Ma)
A
lar
Peninsu
Late Cret. batholith (85–100 Ma)
CS
c
mi
s
sei
ge
rid
Foothills belt
CA
MEX
es
ng
100 km
Accretionary complex
Ra
N
Forearc basin
Figure 10. Origin of Nacimiento fault by sinistral strike slip (modified from Dickinson
[1983] using more recent estimates for age of faulting). Faulting is assumed to have been
driven by subduction of an aseismic ridge. Ridge scale is based on analogy to the Yakutat
block (Fuis et al., 2008) and ridges on the Cocos and Nazca plates (Gutscher et al., 2000).
Present-day outcrops of Pelona-Orocopia-Rand-Catalina schists are shown in black. Only
the northwesternmost schist bodies would have been in place at the time represented by
panel A. Outlined area labeled “Nacimiento Blk” is based on present-day outcrop of Franciscan Complex between the Nacimiento and San Gregorio–Hosgri faults southeast of Point
Sur (Fig. 1). Panel A indicates inferred location of this body prior to sinistral slip on the
Nacimiento fault. Panel B shows its location outboard of the Salinian block immediately
following Nacimiento fault slip in the early Cenozoic, which closely matches the present-day
spatial relationship (Fig. 1). Note that the reconstruction of panel B is a simplified equivalent of that in Figure 2. Heavy arrow shows Farallon–North America relative plate motion.
CS—Catalina Schist; Nf—future (panel A) and actual (panel B) traces of Nacimiento fault;
Gf—future trace of Garlock fault; SAf—future trace of San Andreas fault; other abbreviations are as in Figure 1.
the outer edge of the continent, in contrast to
the sheet-like geometry typically envisioned
(e.g., Fig. 9). This could explain the relatively
linear nature of the schist exposures, which
seems fortuitous if the distribution at depth is
more laterally persistent. In the sinistral slip
model, extraregional detritus is supplied to the
coast via a gradually enlarging “window” between the southern Sierra Nevada and northern
Peninsular Ranges (Fig. 10B). Sinistral slip on
the Nacimiento fault would also translate the
Nacimiento block from an initial location op-
posite the Diablo Range to the current position
adjacent to the Salinian block (Fig. 10; see previous discussion).
Marine Sequences of the Salinian Block
The Maastrichtian to Paleogene marine transgression of the Salinian block and adjacent areas
has long been viewed as an important tectonic
signal. Clast distributions in conglomerates indicate a number of localized basins, at least during
early phases of deposition (Grove, 1993). Some
workers have interpreted this “borderland-style”
geometry as direct evidence that strike-slip
faulting, whether in a dextral or sinistral sense,
played an important role in emplacement of the
Salinian block (Nilsen and Clarke, 1975; Howell and Vedder, 1978; Vedder et al., 1983; Dickinson et al., 2005). However, whereas strike-slip
faulting can generate local topographic highs
and lows, it is not clear that this process, by
itself, can account for the substantial net subsidence implied by the regional transgression
of the Salinian block. Saleeby (2003, Fig. 4C)
attributed this event to extension within the upper plate of the Vincent–Chocolate Mountains
thrust system. Alternatively, or in addition, we
observe that forearc subsidence is a common
feature in regions of inferred subduction erosion
as an isostatic response to removal of material
from the base of the overriding plate (Clift and
Vannucchi, 2004). By analogy, we propose that
marine transgression of the Salinian block may
have been related, at least in part, to tectonic
erosion of North American mantle lithosphere
and lowermost crust during underplating of the
Pelona-Orocopia-Rand-Catalina schists.
This interpretation implies that the age of the
transgression in any given area should correlate
with the time of local schist underplating. At
least to a first approximation, this appears to be
the case for the Salinian block and central Transverse Ranges. The correlation does not hold true
in the San Emigdio Mountains, where the schist
is early Late Cretaceous in age, but the base of
the marine section in the upper plate is Eocene
(Nilsen, 1987b). However, because this region
is at the northwesternmost end of the schist belt,
it is conceivable that subsidence was limited by
the flexural rigidity of a full thickness of lithosphere beneath the adjacent Sierra Nevada.
CONCLUSIONS
Similar detrital zircon patterns exhibited by
the Pelona-Orocopia-Rand-Catalina schists and
coeval sedimentary units of the forearc basin
provide evidence that these two sequences comprise parts of a single depositional system on the
outboard side of the Sierran–Peninsular Ranges
arc. The >30 m.y. time frame for emplacement
of the schists and the short cycling interval are
most consistent with a subduction origin. The
sandstone protoliths of the schists are best interpreted as trench sediments complementary to
the forearc basin sequence.
Emplacement of the schists directly beneath
the Cordilleran magmatic arc requires the removal of North American mantle lithosphere
and lowermost crust. The initial stages of this
underplating may have occurred at the end of a
period of modest subduction erosion associated
Geological Society of America Bulletin, March/April 2011
501
Jacobson et al.
with slow, eastward migration of Sierran magmatism during the middle Cretaceous. Later
schist emplacement, however, may have been
expedited by flat subduction associated with the
classic Laramide orogeny of the Rocky Mountain foreland.
Initial underplating of the schist (ca. 95–
75 Ma) was probably not associated with any
major changes in the paleogeography of the
arc and forearc basin. Sediment supplied to
the forearc basin and trench during this stage
was derived by erosion of the western flank of
the Sierran–Peninsular Ranges arc and associated wall rocks. The arc formed a continuous
topographic barrier that separated the forearc
region from an area of retroarc sedimentation
that included the McCoy Mountains Formation.
Beginning as early as ca. 75 Ma, but no later
than ca. 68 Ma, slip on the Nacimiento fault led
to progressive removal of the western belt of the
arc and eastern to central portions of the forearc
basin. As a result of the truncation event, basement terranes formerly within the eastern flank of
the arc or adjoining craton were relocated much
closer to the continental margin, i.e., in a position where they could more easily shed sediment
to the continental margin. However, the apparent
absence of detritus in either the schists or forearc
units derived from the region of the present-day
Colorado Plateau implies the continued presence
of a high-standing mountain range (Mogollon
Highlands) relatively close to the coast.
Displacement on the Nacimiento fault most
likely involved either thrusting or sinistral strike
slip, as opposed to dextral strike slip. The thrust
option has been most heavily favored, but it is
difficult to reconcile with the lack of exposure
of the missing complements of the Salinian and
Nacimiento blocks in windows through southern California arc and cratonal crust. The model
of sinistral strike slip raises questions regarding
the correlation of accretionary rocks between
the Nacimiento block and central California east
of the San Andreas fault. It is also not obviously
consistent with current interpretations of relative plate motions during the Laramide orogeny,
but this could be explained by escape tectonics
related to collision of an aseismic ridge with the
Farallon subduction zone.
ACKNOWLEDGMENTS
Our work was supported by National Science Foundation grants EAR-0106123, EAR-0106881, EAR0408580, and EAR-0408730. The ion microprobe
facility at the University of California–Los Angeles
(UCLA) and the LaserChron Center at the University of Arizona are partly supported by grants from
the Instrumentation and Facilities Program, Division
of Earth Sciences, National Science Foundation. Ana
Vućić is thanked for providing substantial support
in sample preparation and analysis at UCLA. Scott
502
Johnston, Alex Pullen, and Victor Valencia provided
critical assistance with use of the inductively coupled
plasma mass spectrometer (ICP-MS) at the University
of Arizona. We are deeply indebted to many workers who assisted our sampling efforts by suggesting
localities, providing maps, and/or accompanying us
in the field. These include Pat Abbott, Bill Bartling,
Alan Chapman, Mark Cloos, Ivan Colburn, Brett Cox,
Gene Fritsche, Steve Graham, Karen Grove, Clarence
Hall, Gordon Haxel, Dave Howell, Ray Ingersoll,
Dave Kimbrough, Marilyn Kooser, and Jason Saleeby.
Our understanding of the problems discussed herein
has also benefited from discussions with Kim Bishop,
Bill Dickinson, Mihai Ducea, and Steve Kidder. Ray
Ingersoll provided helpful comments on an early draft
of the manuscript. We also thank Luke Beranek, Bill
Dickinson, Cees van Staal, and John Wakabayashi for
their important insights during the Geological Society
of America review process.
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