International Geology Review, Vol. 46, 2004, p. 1103–1118.
Copyright © 2004 by V. H. Winston & Son, Inc. All rights reserved.
Contrasting Settings of Serpentinite Bodies, San Francisco
Bay Area, California: Derivation from the Subducting Plate
vs. Mantle Hanging Wall?
JOHN WAKABAYASHI1
Geologic Consultant, 1329 Sheridan Lane, Hayward, California 94544
Abstract
Although the best-known outcrops of serpentinite in the California Coast Ranges are part of
the Coast Range ophiolite (CRO), significant serpentinite bodies crop out within the Franciscan
subduction complex, and others have ambiguous affinity. These serpentinites provide evidence of
subduction-accretion processes.
The Hunters Point shear zone (HPSZ), an intra-Franciscan structural horizon that consists of
both a regionally extensive serpentinite body and shale matrix mélange, extends tens of km along
strike, and has a structural thickness of 1 to 1.5 km. The HPSZ exhibits little mixing of serpentinite
and shale. The HPSZ consists of a structurally high shale matrix mélange with mostly sandstone
blocks, an intermediate zone composed of a serpentinite sheet (or sheets) containing metagabbro
lenses, and a structurally low shale matrix mélange including a variety of different block types, but
lacking serpentinite and gabbro. These field relations suggest that the serpentinite in the HPSZ
became part of the subduction complex by offscraping of the remnants of a mantle core complex from
the downgoing plate, rather than having originated from the upper mantle hanging wall of the subduction zone.
In contrast, serpentinite exposed within a km east of the Hayward fault in the southern Hayward
Hills is intermixed with shale on scales of tens of meters to centimeters. This serpentinite is present
in shear zones, ranging from 50 m to several centimeters in structural thickness; these shear zones
cut sandstones of the Great Valley Group (GVG), forearc basin deposits that locally overlie the Coast
Range ophiolite in depositional contact. The shear zones also include blocks of of basalt, blueschist,
amphibolite, and gabbro. Serpentinite bodies in the southern Hayward Hills may have been derived
from the mantle hanging wall of the subduction zone relatively early in the history of the subduction
zone before much tectonic underplating occurred; after substantial underplating, the mantle hanging
wall would have been largely blocked from contributing material to the subduction channel. The
serpentinite and shale matrix were exhumed to approximately the same crustal level as the CRO and
GVG, before a final stage of faulting mixed pieces of both into the mélange and emplaced the
mélange into shear zones cutting GVG rocks.
Introduction
SERPENTINITE OCCURS in a variety of tectonic settings in California, as summarized by Coleman
(2000). Of these tectonic settings, perhaps the best
known are those associated with major ophiolite
units such as the Coast Range ophiolite (CRO),
Josephine ophiolite, Trinity ophiolite, and others
(Coleman, 2000). Although described in detail by
Coleman, serpentinites that occur within subduction
complexes have not received as much attention as
those associated with major ophiolite sheets. In this
paper, I describe reconnaissance-level field rela1Email:
wako@tdl.com
0020-6814/04/774/1103-16 $25.00
tionships of two serpentinite occurrences, one from
within the Franciscan subduction complex and
another of somewhat ambiguous affinity. Field relationships of these serpentinites may illuminate some
details about the relationship between subduction
processes and the incorporation of serpentinites into
subduction complexes or related tectonic settings.
Before describing the field occurrences, I summarize the tectonic setting of the Franciscan Complex
and Coast Range ophiolite in California.
The Franciscan Complex, an assemblage of
variably deformed rocks with widespread high-pressure/low-temperature metamorphism (including
blueschist-facies metamorphism), makes up the
1103
1104
JOHN WAKABAYASHI
FIG. 1. Distribution of Franciscan Complex, Coast Range ophiolite/Great Valley Group and other basement rocks of
central and northern California. Modified from Wakabayashi (1999a).
largest outcrop area of any rock unit in the California Coast Ranges (Fig. 1). The Franciscan formed as
a subduction complex associated with east-dipping
subduction at the western North American plate
margin from the Late Jurassic through the Miocene,
an interval of over 140 million years (e.g., Hamilton,
1969; Ernst, 1970; Page, 1981). Franciscan rocks
consist predominantly of shales and sandstones with
subordinate basaltic volcanic rocks, chert, serpentinite, and minor limestone. Most Franciscan rock
units were tectonically scraped off the downgoing
plate, originating either as trench sediments (sandstones and shales), with a component of olistostrome
blocks of varied lithologies from the upper plate
(MacPherson et al., 1990), or as the upper part of the
downgoing oceanic crust (pelagic and volcanic
CONTRASTING SETTINGS OF SERPENTINITE
1105
FIG. 2. Map showing Franciscan Complex and Coast Range ophiolite exposures in the central San Francisco Bay
area. Modified from Wakabayashi (1999b).
rocks; e.g., Hamilton, 1969; Dickinson, 1970). The
Coast Range ophiolite structurally overlies the
Franciscan, and is in turn depositionally overlain by
well-bedded sandstones and shales of the Great
Valley Group (GVG) that are coeval with the Franciscan (e.g., Dickinson, 1970). The CRO and GVG
lack high P/T metamorphism and the degree of
deformation exhibited by the Franciscan. The CRO
crops out in mostly sheetlike remnants or in a nearly
continuous belt along the eastern margin of the
northern Coast Ranges (Fig. 1). CRO lithologies
include serpentinite, gabbro, quartz diorite,
diabase, basalt, and silicic volcanic rocks; remnants
range in structural thickness from a few hundred
meters to 5 km (Hopson et al., 1981).
Hunters Point Shear Zone
The Franciscan Complex of the San Francisco
Bay area comprises a stack of coherent or nearly
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JOHN WAKABAYASHI
FIG. 3. Map showing distribution of serpentinite in the Hunters Point shear zone. Note that “hu” marks uncertain
lithology and includes serpentinite. Adapted from Schlocker (1974), Bonilla (1971), and Wahrhaftig (1984).
coherent thrust nappes separated by mélange zones
(Blake et al., 1984; Wakabayashi, 1992; Fig. 2).
Mélange zones separating coherent nappes are
predominantly shale matrix mélanges, although
variable amounts of serpentinite are present in most
(Blake et al., 1984; Wakabayashi, 1992).
One Franciscan structural horizon, the Hunters
Point shear zone (HPSZ) in San Francisco, contains
especially abundant serpentinite (Schlocker, 1974;
Wahrhaftig, 1984; Figs. 2 and 3). The HPSZ forms a
NW-trending belt 12 km long and up to 3 km wide,
with a structural thickness of 1 to 1.5 km. The serpentinite of the HPSZ has a prominent magnetic
anomaly associated with it, which suggests that the
HPSZ extends an additional 20 km to the southeast
of Hunters Point beneath San Francisco Bay
(Jachens and Roberts, 1993; Fig. 3). The serpentinite consists primarily of serpentinized harzburgite
(Fig. 4). Replacement of primary harzburgite miner-
als by lizardite and chrysotile ranges from 100% to
about 60%. Serpentinite outcrop textures ranges
from sheared, foliated serpentinite to massive,
blocky serpentinite, with local sheared zones (Figs.
5 and 6).
Small (centimeters in size) lenses of nearly fresh
clinopyroxenite occur locally, as well as lenses of metagabbro that range up to a few tens of meters in size.
The metagabbro has undergone amphibolite-facies
metamorphism; most consists of brownish green hornblende, and plagioclase has been altered to finegrained mats of secondary or retrograde metamorphic
minerals, including pumpellyite (Fig. 7). The hornblende has a preferred orientation that defines a prominent foliation within each metagabbro lens. Pyroxene
is rare in the metagabbro. No high-pressure/low-temperature metamorphic minerals—such as sodic
amphibole, sodic pyroxene, or lawsonite—are present
in thin sections of the metagabbro.
CONTRASTING SETTINGS OF SERPENTINITE
1107
FIG. 4. Photomicrograph of serpentinized harzburgite from the Potrero Hill area of the Hunters Point shear zone.
Some unserpentinized orthopyroxene (opx) and olivine (ol) are present. Mesh-textured serpentine is evident. Field of
view is about 1.5 mm. Cross-polarized light.
FIG. 5. A. Massive, blocky, serpentinized harzburgite, Hunters Point shear zone along Innes Avenue, Hunters Point,
San Francisco. View to southeast. The location of this outcrop is shown on Figure. 3. B. Close-up view of massive serpentinized harzburgite in the right-hand part of photo A.
1108
JOHN WAKABAYASHI
FIG. 6. View of outcrop of sheared serpentinite (sp) with lenses of metagabbro (gb), southern base of Potrero Hill,
Hunters Point shear zone, north of Cesar Chavez Street west of Connecticut Street. View to the north. The foliation in the
serpentinite dips gently to the right (northeast). The location of this outcrop is shown on Figure 3.
FIG. 7. Photomicrograph of metagabbro from a lens in serpentinite in the Potrero Hill area of the Hunters Point shear
zone. This rock is composed of greenish-brown hornblende (hb) and what appears to have formerly been plagioclase (pl)
that was largely replaced by albite and pumpellyite. Field of view is about 3 mm. Plane light.
Whether or not the serpentinite associated with
the HPSZ forms a continuous sheet or comprises a
number of separate blocks is unclear because of the
lack of exposure along much of its strike length (Fig.
3). The largest exposures of serpentinite, in the
southeast part of the exposed HPSZ (Fig. 3) appear
to be part of a continuous body, because there are
many extensive exposures (Figs. 5 and 6 show parts
of such exposures) in this area with no other rock
types. Collectively these outcrops are part of a
serpentinite body that extends at least 6 km in strike
length by 2 km in map width, with a structural thick-
CONTRASTING SETTINGS OF SERPENTINITE
1109
FIG. 8. Within the shale matrix mélange, structurally lowest part of the Hunters Point shear zone at Baker Beach, San
Francisco. Some of the shale matrix is shown here between two metabasalt blocks. The location of this outcrop is shown
on Figure 3.
ness of about 1 km. Sea cliff exposures at Fort Point
at the northwestern end of the HPSZ also form a
continuous outcrop of serpentinite, rather than a
number of blocks separated by other lithologies. In
both Fort Point and the Hunters Point–Potrero Hill
exposures, foliation in the serpentinite strikes northwest and dips at shallow angles (generally 15–30°)
to the northeast (Fig. 6). Exposures are scarce in the
central part of the HPSZ. Whether or not the Fort
Point serpentinite exposures form the western (on
land) end of an unbroken slab that includes the
exposures at Potrero Hill and Hunters Point is not
clear, inasmuch as much of the intervening rock has
been mapped as undifferentiated serpentinite,
shale, and other lithologies (Schlocker, 1974) and
few of these urban outcrops are visible or accessible
today. These serpentinite outcrops do not appear to
be part of a serpentinite-matrix mélange. The only
non-serpentinite rocks found within serpentinite
bodies are the rare clinopyroxenites and metagabbro
lenses. “Exotic” lithologies such as sandstones,
shales, cherts, or volcanic rocks do not occur as
blocks within the serpentinite.
Shale matrix mélange crops out on the margins of
the HPSZ. The best exposures of the shale matrix
mélange are at Fort Point/Baker Beach and at Hunt-
ers Point, where the mélange forms the southern
margin of the HPSZ. At those localities, the shale
matrix mélange is structurally beneath the serpentinite and consists of sheared shale containing
blocks of sandstone, chert, and basalt (Fig. 8). At
both localities, the shale matrix mélange foliation is
subparallel to the foliation in the serpentinite, striking NE and dipping NE. At Fort Point/Baker Beach,
the shale matrix mélange crops out along the base of
the sea cliff, where much of it is obscured by landslides, whereas intact serpentinite crops out at the
top of the cliff (Wahrhaftig, 1984). In the Hunters
Point–Potrero Hill area, the shale matrix mélange
crops out along the southwestern base of the hills
that are composed of serpentinite (Bonilla, 1971;
Schlocker, 1974).
Serpentinite blocks are rare or absent in most of
the shale matrix mélange exposures, although some
fault-bounded slivers of serpentinite occur locally
within the shale matrix mélange (Fig. 3). Most
blocks in the shale matrix mélange are prehnitepumpellyite grade, with the exception of several
amphibolite and blueschist blocks found on the
beach south of Fort Point, for which the structural
affinity is more complex. The metamorphic blocks
appear to have slid to the beach from a horizon at or
1110
JOHN WAKABAYASHI
FIG. 9. Border between amphibolite border and serpentinite, Hunters Point shear zone, Baker Beach. The border
zone consisting of mixed layers of metashale, metamafic and meta-ultramafic material narrows from nearly a meter wide
at the base of the block to about 30 cm wide near its top. The location of this outcrop is shown on Figure 3.
near the contact between serpentinite and shale
mélange. One amphibolite block exhibits a piece of
serpentinite attached to its southern margin (Fig.
9). The border zone, up to about 80 cm wide,
between this attached piece of serpentinite and the
amphibolite block is highly sheared and recrystallized with metamafic, meta-ultramafic, and
metashale materials mixed at mm to cm scales
(Figs. 10 and 11). Fine-grained actinolite/tremolite
is common in this border zone, and occurs in felted,
folded, and brecciated mats. The attached piece of
serpentinite consists of serpentinized harzburgite
and is composed primarily of lizardite. The amphibolite block has been variably overprinted by blueschist-facies assemblages, similar to other
metamorphic blocks studied at the beach (Wakabayashi, 1990).
Sandstone and shale of the Alcatraz terrane
structurally overlies the HPSZ and bounds the shear
zone on its northeastern margin. Owing to small-
scale folding and erosion, some klippe of the
Alcatraz terrane overlie the northeastern part of the
HPSZ, and shale matrix mélange appears to be
developed structurally above the serpentinite and
below the Alcatraz terrane (Schlocker, 1974; Fig. 3).
The most common blocks in the mélange above the
serpentinite are sandstone, and exotic blocks such
as chert and basalt are less common than the
mélange structurally below the serpentinite.
In summary, the HPSZ comprises three structural subhorizons (Fig. 3): a thin structurally low
shale matrix mélange including common exotic
blocks; a thick structurally intermediate horizon
consisting of a large sheet or several large sheets of
serpentinite, and an intermediate thickness structurally high shale matrix mélange or broken formation that contains fewer exotic blocks than the
structurally low mélange. The HPSZ strikes NW and
dips NE, and is bounded above by the Alcatraz
terrane and below by the Marin Headlands terrane,
CONTRASTING SETTINGS OF SERPENTINITE
1111
FIG. 10. Close-up view of part of the border zone between the amphibolite block and serpentinite shown in Figure 9.
Orientation of figure corresponds with that of Figure 9.
both of which are metamorphosed to prehnitepumpellyite grade.
Serpentinite in the Southern Hayward Hills
In contrast to the serpentinite of the HPSZ, serpentinite exposed in the southern Hayward Hills
(Figs. 2 and 12) appears to form both blocks and
matrix in mélange zones. Whether this serpentinite
is associated with the Franciscan, the CRO, or the
GVG is problematic. Serpentinite crops out within
an extensive belt of CRO serpentinite, gabbro, and
silicic volcanic rocks that parallels the Hayward
fault (Fig. 2). Within this belt, Franciscan rocks are
exposed only in small, isolated outcrops south of
Oakland, whereas GVG rocks form extensive exposures in depositional contact above CRO rocks, or
are faulted against them.
At the study locality, serpentinite is present in
shear zones, ranging from 50 m to several centimeters in structural thickness, which cut sandstones of
the GVG (Figs. 12 and 13). A geotechnical investigation for a proposed housing development provided
an opportunity to examine details of the site geology
that would not be possible from surface exposures
alone. The investigation involved the excavation of
numerous test pits and drilling of many exploratory
borings, all of which contributed to the geologic data
shown in Figure 12, and afforded excellent opportunities to study the matrix of the mélange/shear
zones. The shear/mélange zones large enough to
show on Figure 12 are up to at least 50 m thick, and
have a serpentinite and shale matrix in which the
two rock types are interleaved and mixed at scales
down to centimeters (Fig. 13). One shear zone, only
centimeters thick, was identified in a test pit exposure (location on southern part of Fig. 12); it cut
Great Valley Group sandstones, and this shear zone
consists of both sheared shale and serpentinite. The
largest piece of serpentinite without other lithologies mixed in it is probably on the order of 10
meters in maximum dimension; serpentinite is
present as both blocks and granular layers within
the mélange. Much of the serpentinite appears to be
derived from harzburgite, and it displays highertemperature metamorphic minerals than are typical
1112
JOHN WAKABAYASHI
FIG. 11. Photomicrograph of part of the border zone between the serpentinite and amphibolite block shown in Figures
9 and 10. Fine-grained, folded mats of actinolite (am) and quartz-rich zones (q) are shown along with a clast of clinopyroxenite with chromian spinel (Cr). Field of view is about 2 mm. Plane light.
for most CRO or Franciscan serpentinite bodies.
The serpentinite includes metamorphic minerals
such as tremolite, talc, and antigorite (Fig. 14),
although lizardite and chrysotile are also present in
many samples.
A number of block types occur in the mélange
zones, including chert, basalt, fine- and coarsegrained blueschist, amphibolite, actinolite schist,
gabbro, and sandstone. Gabbro comprises the largest blocks within the mélange; the largest gabbro
block at the study site is up to 20 m in long dimension. Unlike the metagabbro of the HPSZ, this
gabbro has well-preserved pyroxenes and lacks
metamorphic hornblende. Apparent igneous plagioclase is replaced by very fine grained secondary
minerals, and rare actinolite occurs mostly as overgrowths on pyroxene. The metamorphic blocks and
the chert blocks in the mélanges are Franciscan in
origin. In contrast, the gabbro appears to be of CRO
affinity, based on the lack of high-pressure metamorphism and rarity of gabbro in the Franciscan.
The basalt lacks high-pressure metamorphic minerals, but without geochemical data or clear-cut field
relations it is difficult to determine whether these
rocks are low-grade Franciscan or CRO basalts.
Some sandstone blocks within the mélanges are
GVG sandstones similar to those that bound the
shear zones (lithic-rich sandstones lacking metamorphic minerals), whereas blocks of unusual
volcanic-clast-rich sandstone (consisting of about
80% lithic clasts, most of which are volcanic) found
in the eastern shear zone shown on Figure 12 are
Franciscan, based on a well-developed cleavage and
metamorphic pumpellyite. In summary, the block
population of the mélange appears to have a mixed
Franciscan, CRO, and GVG source.
The serpentinite and shale matrix mélanges cut
GVG rocks, strike NW, and dip NE (Fig. 12). This
orientation appears to be subparallel to the bedding
in the Great Valley Group sandstones that bound the
shear zones.
In addition to the locality described here in
southern Hayward, I have also observed similar
occurrences (not shown on published maps) of Franciscan metamorphic rocks in shear zones (matrix not
well exposed, but containing serpentinite in at least
one locality) cutting GVG and/or CRO rocks at two
other localities about 4 and 15 km northwest along
the Hayward fault, respectively.
Discussion: Speculation on the Origin
of the Serpentinite Bodies
The southern Hayward Hills and HPSZ serpentinites clearly have drastically different local and
regional field relationships. In order to speculate on
the origins of the serpentinite at the two localities, I
first review some of the proposed sources for serpentinites within an accretionary complex and forearc
setting.
A variety of tectonic origins have been proposed
for serpentinites in the California Coast Ranges
CONTRASTING SETTINGS OF SERPENTINITE
1113
FIG. 12. Geologic map of serpentinite and shale matrix mélange zones in part of the southern Hayward Hills. “Western sheet,” “central sheet,” and “eastern sheet” denote the three slabs of Great Valley Group sandstone that are separated by mélange zones and are the source of samples for the corresponding photomicrographs shown in Figure 16.
(Fig. 15). Perhaps the most commonly proposed tectonic setting from which serpentinites in the Coast
Ranges are derived is the basal ultramafic part of
the Coast Range ophiolite (e.g., Bailey et al., 1970;
Hopson et al. 1981; Page, 1981). Such serpentinite
bodies are present both along the eastern margins of
the Coast Ranges and as outliers that have been displaced to positions farther west in the Coast Ranges
by dip-slip (thrust or normal) faulting and/or strikeslip faulting (e.g., McLaughlin et al., 1988). Serpentinites derived from the CRO structurally overlie
rocks of the Franciscan Complex, in contrast to
those serpentinites structurally interleaved with it.
Other serpentinites associated with the upper
plate of the forearc system appear to occur as sedimentary deposits that depositionally overlie the
CRO (Phipps, 1984). Such deposits locally make up
part of the basal GVG, and Fryer et al. (2000)
suggested that they represent deposits from forearc
serpentinite mud volcanoes. The recognition of
sedimentary serpentinite deposits in the GVG
also raises the possibility that some Franciscan
1114
JOHN WAKABAYASHI
FIG. 13. Core from one of the serpentinite and shale matrix mélange zones in the Hayward study area. The serpentinite and shale are mixed at scales down to centimeters (inch and centimeter metal scale). A few examples of representative clasts and layers are labeled.
FIG. 14. Photomicrograph of serpentinite from one of the shear zones in the Hayward Hills. The minerals in this view
are intergrown talc, tremolite, and antigorite. Field of view is about 1.5 mm. Cross-polarized light.
serpentinites also may have a sedimentary or olistostromal origin (Phipps, 1984).
Coleman (2000) suggested that some serpentinites within the Franciscan Complex, particularly
the larger bodies, originated from the downgoing
plate, probably as the offscraped and underplated
remnants of mantle core complexes formed near
spreading ridge-transform intersections (e.g.,
Tucholke et al., 1998; Karson, 1999). Other intraFranciscan serpentinites may have been derived by
CONTRASTING SETTINGS OF SERPENTINITE
1115
FIG. 15. Diagrams showing proposed tectonic origins of serpentinite in the California Coast Ranges. Possible sources
of serpentinite in trench-forearc regions are indicated in bold type, with Franciscan Complex as an example.
plucking from the mantle of the upper plate of the
subduction zone (e.g., Cloos, 1984).
Hunters Point shear zone serpentinites
The HPSZ is a structural horizon within the
Franciscan, and it is not associated with the CRO or
the basal GVG. In addition, the serpentinite is
either massive or occurs as a sheared but not disaggregated rock mass, in contrast to the textures of
sedimentary serpentinite, so did not originate as
sedimentary debris shed into a trench. It is difficult
to envision the tectonic plucking of serpentinite
bodies as large as those in the HPSZ from the deep
mantle hanging wall of a subduction zone. Moreover,
such a mechanism would suggest a deep source for
the serpentinites, because many Franciscan units,
including blueschist-facies units still awaiting
exhumation, already existed structurally above the
HPSZ (between the subduction channel and the
mantle hanging wall) at the time of the accretion of
the HPSZ at approximately 100 Ma (Wakabayashi,
1992, 1999a). If the serpentinite was derived from
such a deep source, metagabbro pods within the serpentinite should have experienced high-P/low-T
metamorphism, but they lack such metamorphism.
The amphibolite metamorphism of the metagabbro
pods may have occurred near a spreading ridge.
Accordingly, I suggest that the HPSZ serpentinites
were scraped off the downgoing plate, having originated as mantle core complexes, as suggested by
Coleman (2000) for Franciscan serpentinites he
termed Franciscan peridotite wedges. The mantle
material appears to have been offscraped as discrete
blocks or slabs, rather than becoming tectonically
mixed with the shale matrix mélange, because
exotic blocks do not occur within the serpentinite,
1116
JOHN WAKABAYASHI
with the possible exception of those at the border
with the shale matrix mélange. Thus the HPSZ
consists of structurally distinct shale matrix
mélange and serpentinite horizons, rather than
being a serpentinite and shale matrix mélange.
The serpentinite attached to the amphibolite
block at Baker Beach appears to have undergone a
different history than the larger pieces of serpentinite within the HPSZ. Although the serpentinite
attached to the block no longer contains higher temperature meta-ultramafic minerals, the contact zone
between the serpentinite and the amphibolite block
has shale that is recrystallized to a degree much
greater than the rest of the exposed mélange matrix.
It is likely that the attached serpentinite once had
higher-temperature meta-ultramafic minerals that
were erased by retrogression to lizardite, and that
the serpentinite was attached to the amphibolite
block during at least part of its metamorphic evolution from amphibolite- to blueschist-facies metamorphic conditions. Speculation on how the block
became incorporated into the mélange is beyond the
scope of this paper.
Southern Hayward Hills serpentinite
Unlike serpentinites of the HPSZ, serpentinites
from the southern Hayward Hills locality defy a simple structural categorization. They occur in shear
zones that cut Great Valley Group sandstones, part
of the “upper plate” structurally above the Franciscan Complex. Yet the shear zones contain blocks
of Franciscan rocks with high-P/low-T metamorphism in addition to blocks of apparent CRO and
GVG affinity. Serpentinite is intermixed with shale
down to centimeter scales, in contrast to the large
sheets and blocks of serpentinite in the HPSZ. Serpentinite at the southern Hayward Hills locality also
exhibits higher-temperature metamorphic minerals
than are commonly present in serpentinites in the
California Coast Ranges. Such higher-temperature
metamorphic minerals, and the occurrence of Franciscan metamorphic rocks (including amphibolites
with blueschist overprints) suggest that the serpentinite in these shear zones may have been derived
from the deep mantle hanging wall of the subduction
zone, with considerable mixing between serpentinite and the shale of the subduction channel (e.g.,
Cloos, 1984). The mechanism of eventual emplacement of the shale and serpentinite matrix mélange
into shear zones cutting the GVG is difficult to envision. Metamorphism of the Franciscan blocks in the
shear zone, as well as the metamorphic mineralogy
of the serpentinites, suggests considerable tectonic
transport (tens of km relative vertical movement)
relative to the GVG on either side of the shear zones.
Yet each shear zone does not have significant vertical separation across it because the GVG sandstones on the hanging and footwalls of the shear
zones are unmetamorphosed, and all of the tectonic
slices of GVG rocks are petrographically identical
(Fig. 16). A simple explanation for such field relations might be diapiric emplacement of serpentinite
and shale mixtures upward along shear zones or
faults in the GVG (e.g., Dickinson, 1966; Lockwood,
1972), but Phipps (1984) has shown that the density
contrast between such serpentinite and the rocks of
the GVG is too small (if it existed at all) to drive
such diapirism. An alternative explanation may be
that the serpentinite and shale originated as sedimentary flows, similar to the GVG basal serpentinites, but the shale and serpentinite matrix of the
shear zones at the southern Hayward locality is
much more coherent in appearance than sedimentary serpentinites of the Coast Ranges. In addition,
the higher-temperature meta-ultramafic minerals
found in the southern Hayward Hills locality seem
incompatible with a sedimentary serpentinite origin.
I propose the following speculative scenario for
the origin of these enigmatic serpentinites in the
southern Hayward Hills. They were derived from the
deep mantle hanging wall above the subduction
zone, with the serpentinite having been incorporated
and mixed at small scales into the shale matrix of
the mélange subduction channel. The subduction
channel may have been particularly rich in serpentinite during the early history of subduction because
most of the mantle hanging wall had not been
covered yet (blocked from providing material to the
channel) by tectonically underplated material.
Exhumation by return flow in the channel brought
the mantle material and associated high-P metamorphic rocks to relatively shallow levels in the crust,
equivalent to those of the Coast Range ophiolite and
Great Valley Group. Following this exhumation, a
second stage of deformation imbricated the GVG
and associated CRO and resulted in the juxtaposition of the shale and serpentinite mélanges with
GVG and CRO rocks, as well as the inclusion of
CRO and GVG blocks in the mélange. Such faulting, whether originally dip-slip, strike-slip, or
oblique-slip, lacked large-scale (several km or
more) offset because there is no lithologic contrast
between the two sides of each shear zone.
CONTRASTING SETTINGS OF SERPENTINITE
1117
ing projects funded by Northgate Environmental
Management, Inc. and BGC. This paper was
improved by helpful reviews by R. Erickson, W. G.
Ernst, and T. Kato.
REFERENCES
FIG. 16. Photomicrographs of Great Valley Group sandstone from the south Hayward Hills locality. These three samples are taken from different fault-bounded sheets of
sandstone shown on Figure 12. A. From western sheet. B.
From central sheet. C. From eastern sheet. These sandstones
have not been metamorphosed and are nearly identical petrographically, indicating little displacement across the shear
zones that separate them.
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