Wakabayashi Progress in Earth and Planetary Science (2017) 4:18
DOI 10.1186/s40645-017-0132-y

REVIEW

Progress in Earth andPlanetary Science

Open Access

Structural context and variation of ocean
plate stratigraphy, Franciscan Complex,
California: insight into mélange origins andsubduction-accretion processes
John Wakabayashi

Abstract
The transfer (accretion) of materials from a subducting oceanic plate to a subduction-accretionary complex has
produced rock assemblages recording the history of the subducted oceanic plate from formation to arrival at the
trench. These rock assemblages, comprising oceanic igneous rocks progressively overlain by pelagic sedimentaryrocks (chert and/or limestone) and trench-fill clastic sedimentary rocks (mostly sandstone, shale/mudstone), havebeen called ocean plate stratigraphy (OPS). During accretion of OPS, megathrust slip is accommodated by imbricate
faults and penetrative strain, shortening the unit and leading to tectonic repetition of the OPS sequence, whereas OPS
accreted at different times are separated by non-accretionary megathrust horizons. The Franciscan subduction complex
of California accreted episodically over a period of over 150 million years and incorporated OPS units with a variety ofcharacteristics separated by non-accretionary megathrust horizons. Most Franciscan OPS comprises MORB
(mid-ocean-ridge basalt) progressively overlain by chert and trench-fill clastic sedimentary rocks that are composedof variable proportions of turbidites and siliciclastic and serpentinite-matrix olistostromes (sedimentary mélanges).
Volumetrically, the trench-fill component predominates in most Franciscan OPS, but some units have a significant
component of igneous and pelagic rocks. Ocean island basalt (OIB) overlain by limestone is less common than
MORB-chert assemblages, as are abyssal serpentinized peridotite slabs. The earliest accreted OPS comprises metabasiteof supra-subduction zone affinity imbricated with smaller amounts of metaultramafic rocks and metachert, but lacking
a clastic component. Most deformation of Franciscan OPS is localized along discrete faults rather than being
distributed in the form of penetrative strain. This deformation locally results in block-in-matrix tectonic mélanges, incontrast to the sedimentary mélanges making up part of the clastic OPS component. Such tectonic mélanges may
include blocks and matrix derived from the olistostromes. Franciscan subduction and OPS accretion initiatedin island arc crust at about 165–170 Ma, after which MORB and OIB were subducted and accreted following
a long (tens of mega-ampere) gap with little or no accretion. Following subduction initiation, a ridge crest
approached the trench but probably went dormant prior to its subduction (120–125 Ma), after which thesubducted oceanic crust became progressively older until about 95 Ma. From 95 Ma, the age of subductedoceanic crust decreased progressively until arrival of the Pacific-Farallon spreading center led to terminationof subduction and conversion to a transform plate boundary.
Keywords: Ocean plate stratigraphy, Subduction complex evolution, Subduction megathrust slip accommodation,
Tectonic and sedimentary mélanges

Correspondence: jwakabayashi@csufresno.edu
Department of Earth and Environmental Sciences, California State University
Fresno, 2576 E. San Ramon Avenue, Mail Stop ST-24, Fresno, CA 93740, USA© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

Introduction
Ocean plate stratigraphy (OPS) comprises assemblages oftrench-fill clastic sediments overlying pelagic sediments
and oceanic igneous rocks that make up the world’s subduction complexes (Wahrhafig 1984; Isozaki et al. 1990;
Isozaki and Blake 1994; Wakita and Metcalfe 2005; Kusky
et al. 2013; Wakita 2015). The specific OPS terminology
was first applied by Isozaki et al. 1990, although theconcept dates back at least as far as Wahrhaftig (1984).
OPS is interpreted to record initial submarine volcanism
at a mid-ocean ridge generating mid-ocean-ridge basalt(MORB) or at an off-axis location generating ocean island
basalt (OIB), followed by open ocean pelagic sedimentation as the ocean plate drifts and by deposition of trenchfill clastic sediments as that part of the subducting oceanic
plate reaches the trench (e.g., Chipping 1971; Wahrhaftig
1984; Isozaki et al. 1990; Osozawa 1994; Wakita andMetcalfe 2005; Kusky et al. 2013; Fig. 1).

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OPS is transferred from the subducting plate to thesubduction complex by the process of subductionaccretion during which the subduction megathrust cuts
into the subducting plate. During accretion of OPS, subduction megathrust slip is accommodated by imbricationand penetrative deformation of the OPS. During muchof the history of subduction zones, no accretion of any
sort takes place (e.g., von Huene and Scholl 1991) so that
accretion in any given subduction complex is expected to
be sporadic. Accretion is associated with an accretionarymode of megathrust slip, whereas non-accretionary megathrust horizons separate OPS accreted at different times
in a subduction complex (Wakabayashi 2016a). The
incorporation of OPS into a subduction complex
leads to intricate age relationships. The age of the igneous and the oldest pelagic components of a slice ofOPS may be much older than the clastic sedimentarycomponent that approximates the arrival time of theFig. 1 Cartoon showing OPS evolution and age patterns versus accretion. The schematic stratigraphic columns are age-scaled rather thanthickness-scaled. Columns a-d show how OPS arriving at the trench at the same time may vary along strike, in this case owing to subducting of fracturezones and crust formed off-axis. Obliquity of subduction relative to isochrons on the oceanic plate, which should be considered the general case, will
introduce another level of complexity. Columns a-e show that pelagic sections formed far apart may share a common age range for partof their pelagic section. This suggests caution in correlating incomplete OPS sections where the basal pelagic rocks directly overlying sea
floor volcanic rocks are not exposed. Columns f-j show the stacking of sporadically accreted OPS at one position along a trench. Compare to Franciscan
Complex columns shown in Fig. 5b, c

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

piece of ocean floor at a trench shortly before subductionaccretion (e.g., Hamilton 1969; Wahrhaftig 1984; Osozawa
1994; Wakita and Metcalfe 2005; Kusky et al. 2013) (Fig. 1).
Successive episodic subduction-accretion of OPS canbuild a tectonic stack in a subduction complex wherein
the clastic components young structurally downward
across non-accretionary megathrust horizons, but theocean crust age can have a less systematic age patternthat may trend both younger and older structurally
downward (columns f to j in the lower right of Fig. 1)(Wakabayashi 2015). Nearly all of the igneous oceanic
crust entering a subduction zone completely subducts
and only a tiny fraction transfers to the upper plate as apart of OPS. In some cases, accretion of trench sediments may take place, but without incorporation of anyof the underlying pelagic sedimentary and oceanic igneous rocks (Fig. 1, lower right “i” column).
Obliquity of plate convergence relative to isochrons of
the ocean crust as well as the subduction of oceanic
fracture zones, spreading ridges, and crust formed offaxis, such as seamounts, oceanic plateaus, can lead tocomplex along-strike patterns in the ages of oceanic
crust accreted at the same time along the length of a
subduction zone (Fig. 1; Osozawa 1994). Parts of thepelagic sections formed at different places and times on
the ocean floor may be age correlative, but their actual
sites of deposition may be far removed from one another(Fig. 1). Correlation of OPS sections lacking igneousrocks should not be made on the basis of similarity inage range of pelagic sections alone.
Whether OPS is or is not a synonym or subset of theterm “ophiolite” depends on the varied definitions ofophiolite (e.g., Moores 1982; Dilek and Furnes 2011;
Kusky et al. 2013). This has important implications forthe connection between OPS packages and tectonic
history. The largest ophiolite bodies, such as theTroodos, Semail, Bay of Islands, and Coast Range
ophiolites, are assemblages that were part of theupper plate above a subduction zone, rather than themuch smaller oceanic crust scraps transferred fromthe subducting plate to the upper plate as part of a
subduction complex (e.g. Moores 1982). Large upper
plate ophiolitic bodies are successively overlain bypelagic and clastic sedimentary rocks, so these havethe same components as OPS slices in subduction
complexes, but the tectonic-sedimentary history ofsuch units differs markedly.
In the California Cordillera, the largest bodies ofoceanic rocks (igneous rocks and overlying pelagic sedimentary rocks) are associated with the upper platesettings instead of subduction complexes. Figure 2 summarizes these relationships with various upper plate
ophiolitic igneous rock bodies designated with acronyms
vo, so, voc, soc, and bodies of island arc volcanic rocks

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designated with the acronym va. For example, the Coast
Range ophiolite (voc, soc) is the upper plate ophiolite
unit of the Coast Ranges, whereas “so” in the northwest
Klamath Mountains is part of the Josephine ophiolite,“so” in the eastern Klamath Mountains is part of the
Trinity ophiolite, and “va” along the northwestern
margin of the Sierra Nevada is the Smartville arc. Large
upper plate oceanic assemblages and subduction
complex OPS are easily distinguished from one anotherin coastal California, owing to the lack of postsubduction disruption, whereas this distinction is much
more difficult in many (most?) other orogenic belts(Wakabayashi 2015, 2016a). This paper will use the term
OPS solely for the components of subduction complexes
and will exclude the upper plate oceanic rocks, in contrast to Kusky et al. (2013; their Fig. 1) that included
units interpreted by many as upper plate, but consideredthem to have been derived from the downgoing plate.
The oceanic crustal components of Franciscan subduction complex crop out over much smaller areas than theupper plate oceanic fragments as shown by the size ofoutcrops labeled “vf, vfs, vrf, and va” on Fig. 2This paper presents new information on the variety of
OPS in the Franciscan Complex of the California Coast
Ranges and its relationship to Franciscan Complex tectonic history and subduction processes. Kusky et al. (2013)
reviewed OPS within the Franciscan Complex, but thispaper presents more detail bearing on a greater variety of
OPS types, and proposes different interpretations about
the relationship between mélanges and the formation anddeformation of OPS. Wakabayashi (2015) presented muchof the field-based framework of this paper, but all of theoutcrop photos and photomicrographs presented in thispaper are new. In addition, the connection between OPS,subduction process, and tectonic versus sedimentary
mélanges is more explicitly explained in this paper than in
Wakabayashi (2015).

Review
California coast ranges regional relationships

The Coast Ranges preserve the relationship between
paleoforearc basin deposits (Great Valley Group or
GVG) that rest in depositional contact on a remnant ofoceanic lithosphere (Coast Range Ophiolite or CRO)
tectonically overlying an exhumed subduction complex(Franciscan Complex) (Hamilton 1969; Dickinson 1970:
Ernst 1970; Bailey et al. 1970). These three megaunits
crop out together for more than 700 km along and
100 km across strike (Fig. 2). This outcrop length isgreater than 1000 km if strike-slip faulting is restored(e.g., Wakabayashi 2015). OPS, as defined herein isrestricted to the Franciscan Complex. In contrast to thematerials of the Franciscan Complex, that were transferred from the subducting plate to the upper plate, theWakabayashi Progress in Earth and Planetary Science (2017) 4:18

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Fig. 2 Map showing general geology of north-central California and special emphasis on the California Coast Ranges. Oceanic rocks are emphasizedon this map, and oceanic volcanic rocks associated with OPS are highlighted by red labels in the Franciscan subduction complex of the California
Coast Ranges. In some cases the actual size of the volcanic rock exposure is exaggerated because most of the actual exposures consist of imbricatedbasalt, chert, and siliciclastic rocks. The largest exposures of mafic-intermediate oceanic volcanic rocks and overlying pelagic sediments are associatedwith accreted island arc terranes (labeled “va”) or large ophiolitic sheets (some examples are labeled “voc”). Revised from Wakabayashi (2016b)

CRO and GVG were part of the upper plate of a convergent plate margin throughout their history (e.g., Hamilton
1969, Dickinson 1970; Ernst 1970; Bailey et al. 1970).
Part of the CRO, however, consists of mafic volcanic
rocks, successively overlain by chert and then thesiliclastic rocks of the GVG (Hopson et al. 1981; 2008).
Locally, the upper part of the CRO may be imbricated along with the basal part of the GVG so that
there are repeated slices of basalt-chert-siliclastic
rocks (Wakabayashi 2016b) that resemble the types

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

of field relationships seen in typical OPS in subduction complexes. There are widespread imbricate andblock-in-matrix relationships within the TehamaColusa mélange whose affinity as CRO or Franciscan(or neither) is unclear (Hopson and Pessagno 2005;
Shervais et al. 2011). In addition, large (up to
hundredths of meters) blocks of basalt, chert, andbasalt with overlying chert are present in olistostrome units of the basal GVG (Phipps 1984; 1992;
Wakabayashi 2016a). These units can be distinguished from Franciscan rocks on the basis of thelack of burial metamorphism (excluding some blocks
in mélanges/olistostromes) and their regional field
relationships.
General spatial-temporal relationships of Franciscan
Complex OPS

The Franciscan Complex accreted during a period of>150 Ma of continuous east-dipping subduction incoastal California from ca.165 to 12 Ma, with mostaccretion between ca. 120 and 32 Ma (e.g., McLaughlin
et al. 1982; Dumitru et al. 2010; 2015; Wakabayashi2015). Episodic accretion transferred material (OPS)from the downgoing plate to the subduction complex
to form a structural stack of accretionary slices/units
with different formational and accretion ages (Fig. 3);
Dumitru et al. (2016); Chapman et al. (2016). These
sheetlike accretionary units, bounded above and below
by narrow (<100 m thick) non-accretionary paleomegathrust horizons, have low-angle regional dips, but
dip steeply in many areas as a result of later folding(Wakabayashi 2015; 2016a) (Fig. 3). The accretionary
units extend up to tens of kilometers along strike andattain structural thicknesses of up to ~3 km. In thispaper, “accretionary units” and OPS units will be considered synonymous. The age of subduction-accretionof various accretionary units has been determined byages of metamorphism where metamorphism was sufficient to generate datable metamorphic minerals andby the depositional age of clastic sedimentary rocks
for rocks too low grade to yield metamorphic ages
(e.g., Wakabayashi 2015) (Fig. 3).
About a fourth of the exposed accreted rocks underwent high-pressure/low-temperature (HP-LT) lawsonitealbite, blueschist or higher grade metamorphism; nearly
all of the remainders are prehnite-pumpellyite or zeolite
grade (Blake et al. 1984, 1988; Ernst 1993; Terabayashi andMaruyama 1998; Ernst and McLaughlin 2012) (Fig. 2). A
small fraction (<<1%) of the Franciscan rocks are coarsegrained blueschist, amphibolite, garnet-amphibolite, andeclogite. Most of these rocks crop out as blocks-inmélange termed “high-grade blocks” (Coleman andLanphere 1971).These blocks are higher in metamorphic grade than surrounding matrix, whereas rare

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intact fault-bounded sheets of these rocks are found(“high-grade coherent sheets” of Wakabayashi et al.
2010; “csch/vfa” on Fig. 2).
Clastic sedimentary rocks make up most of the collective stack accreted OPS units, and the proportion of
intact igneous and pelagic varies between various
accreted units as well as in collective accreted stacks indifferent parts of the Franciscan (Wakabayashi 2015)(Figs. 3 and 4). This variability will be described further
in “Examples of the variety of OPS in the franciscancomplex” on specific types of OPS assemblages. The
clastic sedimentary rocks of the subduction complex
comprise turbidites and mélanges with non-native(exotic) blocks, such as those of higher metamorphic
grade than the matrix (Wakabayashi 2015). Some haveinterpreted these types of mélanges having resulted fromsedimentary (olistostromal) mixing of exotic blocks into
matrix (e.g., Aalto 1981; Cowan 1978; Macpherson et al.
1990; Platt 2015; Wakabayashi 2015) whereas others
have proposed that the introduction of blocks into
matrix resulted from tectonic strain as tectonic mélanges(e.g., Cloos 1984; Cloos and Shreve 1988; Gerya et al.
2002; Hsü 1968).
The origin of mélanges with exotic blocks is importantinterpreting their relationship to accreted OPS units andthe process of accretion and exhumation, so I will list
the main arguments in favor of sedimentary origins ofthese bodies presented in Wakabayashi (2015), as well as
presenting new evidence for the sedimentary origin of
mélanges with exotic blocks in “Olistostromes as part ofthe clastic component of OPS.” The sedimentary origin
of Franciscan mélanges with exotic blocks has been proposed based on the basis of multiple criteria, including(1) sedimentary structures or features in the least
deformed parts of the mélange from outcrop to submillimeter scale, (2) interbedding and interfingering ofmélanges with ordinary bedded turbidites, and (3) blocks
that originated at shallow levels of the upper plate. New
data will be presented in support of points 1 and 2 in“Olistostromes as part of the clastic component of OPS.”In addition to the evidence for sedimentary origin of
mélanges with exotic blocks, geochronologic evidence
indicates episodic accretion rather than continual mixing
in a progressively widening subduction channel spanningthe full thickness of the subduction complex, as envisaged in some numerical models (e.g., Gerya et al. 2002).
Collectively, the evidence for origin of blocks with exotic
blocks, episodic accretion within a subduction complex,
and narrow (<100 m thick) zones between accreted units
suggests that the accommodation of subduction megathrust slip is more localized than the prediction ofnumerical models (Wakabayashi 2015; Wakabayashi andRowe 2015). The analysis of the internal deformation of
OPS gives additional insight into subduction slip

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

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Fig. 3 Schematic diagram showing tectonic stacks of accretionary units in different parts of the Franciscan Complex. This shows the variability of theFranciscan accreted stack at different locations keyed to transects shown on Fig. 2. Units accreted at different times are separated by non-accretionary
megathrust contacts shown in bold red (solid or dashed). Internal imbrication of each accreted unit is not depicted but the total structural thickness isestimated. The age of the pelagic sections of oceanic sections associated with accreted units is given in green but this information is not available forall units. The age of the oldest pelagic rocks in each unit approximates the age of the igneous oceanic crust. Accretion ages are also
given based on different types of data, color coded with acronyms, updated from Wakabayashi (2015)

localization; new observations will be presented in“Imbrication and deformation of OPS and distinguishing
25 tectonic from sedimentary melanges.”
The interpreted origin of the mélanges with exotic blocks
as olistostromes suggests that they should be consideredpart of the trench-fill (clastic) component of larger scale
OPS packages, whereas tectonic mélanges lack exotic
blocks and formed by progressive deformation of OPS(Fig. 4) (Wakabayashi 2015). The proportion of mélange
within the clastic units of the Franciscan varies fromaccreted unit to unit and spatially within each accreted unit.
Trace element and radiogenic isotopic data suggests thatthe high-grade coherent rocks and blocks-in-mélange havenascent island-arc protoliths (Wakabayashi et al. 2010),
whereas lower grade coherent sheets of Franciscan metavolcanic rocks display ocean island basalt (OIB), or midocean ridge (MORB), geochemical affinity (Shervais 1990;
Ghatak et al. 2012). Metaigneous blocks-in-mélange,
excluding the high-grade blocks, include those of OIB,
MORB, and arc affinity (Macpherson et al. 1990).

Whereas the ages of clastic rocks in each accreted unit
young structurally downward, the age of the oceanic
crust subducted (and the age of this crust at time of subduction) shows a more complex relationship (Fig. 5a)(Wakabayashi 2015). Some of this complex age pattern
resulted from periodic subduction of off-axis-generated
oceanic crust, whereas the early (in subduction history)
increase in the age of subducted oceanic crust has been
proposed to have resulted from subduction of a dormant
spreading ridge (Wakabayashi 2015). The subducted(dormant) ridge proposal conflicts with plate reconstructions that have not modeled a ridge crest near the trenchat this time (Seton et al. 2012). The (dormant) ridge subduction proposal was based on geochronologic data as
well as the temporal variation in peak metamorphism in
the Franciscan (Wakabayashi 2015). Some of the data
used to propose the (dormant) ridge subduction model
was not available at the time Seton et al. (2012))
performed their analysis of paleoplate geometry. Data
supporting the early increase in age of subducted oceanic

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

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Fig. 4 Cross-sectional cartoon showing accretion of OPS and the connection between this process and the rock assemblages found in subduction
complexes, including different types of mélanges. This schematic also illustrates that olistostromes of serpentinite and/or siliciclastic matrix comprisepart of the clastic part of OPS

crust during Franciscan history was first presented
Murchey and Blake (1993), who proposed that this age
pattern resulted from subduction of an active spreading
ridge at ca. 170, followed by conversion to a transformmargin until initiation of new subduction at ca. 150.
Franciscan OPS sections have an apparent age gap of>10 Ma between the youngest pelagic rocks and theclastic cover, except for the OPS section of the Marin
Headlands (Sliter and McGann 1992; Sliter 1984;
Murchey and Jones 1984) (Fig. 5b). The age gap reflects
the absence of the youngest part of the pelagic record rather than the oldest part of the clastic record because in
several cases, the ages for the clastic section are maximum
depositional ages. It is not clear whether the apparent age
gaps are a product of the lack of original pelagic deposition during those times, excision of a deposited record byfaulting and/or erosion, or incomplete biostratigraphic
sampling.
Age-scaled depictions of OPS columns do not represent the relative proportions of the different OPS components in a subduction complex because the different
components formed at different rates. For this reason, it
is useful to consider the thickness-scaled columns ofFig. 5c for a graphical representation that better reflects
the relative proportion of the various OPS components
in the subduction complex.

The age relationships on Fig. 5 are a composite of theentire Franciscan Complex, and they do not necessarilyreflect the subduction-accretion history at any specificreach of the trench. The accretionary unit stacks in Fig. 3
are arranged in approximate along-strike relative position
after restoration of post-subduction dextral faulting. This
figure illustrates the considerable along-strike variation in
the Franciscan Complex, including along-strike variationin the age of oceanic crust subducted at approximately thesame time. Age data is too sparse, however, to enable amore detailed analysis of the subducted plate history
based on significant along-strike age variation as done by
Osozawa (1994) for Japan.
For a number of the accretionary slices of the Franciscanshown in Fig. 3, ages of their assumed oceanic crustal basehave not been determined because of the lack of identified
pelagic-igneous imbricates, whereas the age of incorporation has been estimated from depositional ages of clastic
sedimentary rocks. The schematic OPS accretionary
stacks in Fig. 5b, c includes multiple accretionary units ofFig. 3 that lack pelagic-igneous imbricates and associatedocean crust age information.
In “Examples of the variety of OPS in the Franciscan
Complex” that follows, Franciscan OPS assemblages will
be categorized according to their lithologic assemblages.
Following these descriptions, “Olistostromes as part ofWakabayashi Progress in Earth and Planetary Science (2017) 4:18

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a

b

c

Fig. 5 a Subduction age versus ocean crust age for the Franciscan Complex. The green dashed line depicts the estimated age of subducted oceanic
crust generated at an oceanic spreading center. For this reason, off-axis-generated oceanic crust plots younger than the green dashed line for any given
age of subduction. Note that this diagram is a composite for the entire Franciscan Complex and does not address the potential for along-strike variationin the age of oceanic crust subducted. Existing age information (shown in Fig. 3) is insufficient at present to evaluate along-strike variation inage of subducted oceanic crust. Part a revised from Wakabayashi (2015). b Diagram showing the composite tectonic stacking of OPS that resulted fromsporadic Franciscan subduction; the age ranges are from “a” and Fig. 3. As above, this is a composite of the Franciscan Complex and this does not
represent a single reach along the subduction zone, and thus, it does not include all of the different accreted units. In addition, there are
multiple accreted packages shown for which intact basalt and overlying pelagic sedimentary rocks have not been identified or dated. In thisdiagram, each accreted OPS package is separated by a non-accretionary paleomegathrust and the column height and vertical position is related to age.
Accordingly, each column corresponds to an accreted unit between non-accretion megathrusts within a given accreted column stack on Fig. 3 rather
than corresponding to an entire column on Fig. 3. The accreted OPS is imbricated by various faults that accommodated accretionary paleomegathrustzones (Fig. 4), but these are not shown on this figure. c Same OPS units as “b” but scaled to thickness within major imbricates rather than to age. This is
not the total thickness of each accreted unit because each accreted unit is thickened as a result of imbricate fault repetitionWakabayashi Progress in Earth and Planetary Science (2017) 4:18

the clastic component of OPS” will review publishedwork and present new data suggesting that olistostromes
make up a significant component of the clastic part of
OPS packages. “Imbrication and deformation of OPS
and distinguishing tectonic from sedimentary melanges”
will summarize the imbrication and deformation ofFranciscan OPS, and “Franciscan OPS and tectonic
history” will summarize the history of OPS accretion in
the context of Franciscan tectonic history.

Examples of the variety of OPS in the Franciscan
Complex
Basalt-chert-clastic

The most common type of OPS in the Franciscan
Complex consists of basalt overlain by chert and thenclastic sedimentary rocks. The basalt of most suchpackages exhibits mid-ocean ridge (MORB) geochemical affinity, based on comparatively sparse sampling(Shervais 1990; Ghatak et al. 2012). An exception is the
Nicasio Reservoir terrane (Murchey and Jones 1984;
Blake et al. 1984) that has mafic rocks of OIB affinity(Ghatak et al. 2012).
Basalt-chert-clastic OPS packages range from prehnitepumpellyite facies for units such as the Marin Headlands
and Nicasio Reservoir terranes (Wahrhaftig 1984;
Blake et al. 1984) to epidote blueschist facies for thehighest grade parts of the South Fork Mountain schist(Worrall 1981). The proportion of basalt and chert toclastic sedimentary rocks varies from basalt and chert-richPage 9 of 23

packages of the Marin Headlands terrane (Fig. 6) to themore common types that are dominated by clastic
sedimentary rocks, such as the Sunol Regional Wilderness(Fig. 7), or the Pacheco Pass areas of the northern Diablo
Range (e.g., Ernst et al. 2009). The structurally higher
parts of Tiburon Peninsula of the San Francisco Bay
region include a blueschist facies OPS unit of the basaltchert-clastic type with a proportion of clastic rocks greaterthan Marin Headlands but less than the Diablo Range
examples noted above (Bero 2014). Part of this unit
(primarily the clastic component) is shown in thegeologic map on Fig. 8, and the significance of these
exposures will be discussed further in “Olistostromes
as part of the clastic component of OPS.”The thickest stratigraphic section of chert, about
80 m, is associated with the Marin Headlands terrane,
but chert horizons are thickened by thrust faults, so thatstructural thicknesses locally reach 300 m (Murchey
1984; Wahrhaftig 1984) (Fig. 6). The thickest chertsection also records the longest depositional historyof any pelagic section in the OPS of the Franciscan
Complex, from Pliensbachian to Cenomanian (Murchey1984) (Fig. 5). This is an age range of ca. 191–94 Ma,
based on the 2016–12 International Commission on
Stratigraphy (ICS) Time Scale (Cohen et al. 2013,
updated). In addition, the Marin Headlands terrane
was one of the first global localities where the fault
repetition of the pelagic biostratigraphy was recognized, and the basalt-pelagic-clastic succession and its

Fig. 6 Down-dip projection of the geology of the Marin Headlands region of the Franciscan Complex adapted from Wahrhafig (1984). Location
on Fig. 2. This view illustrates the imbricate fault repetitions of prehnite-pumpellyite facies OPS and the true structural-stratigraphic thicknesses(from the down-dip projection) of the imbricates. This is the best example of OPS in the Franciscan Complex, but it has a higher proportion of
volcanic rocks (basalt) and pelagic rocks (chert), than other OPS packages in the Franciscan

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

imbrication was linked to oceanic plate movement andsubduction-accretion (Wahrhaftig 1984).
Chert horizons of tens of meters of structural thickness and less are typical. Volcanic sections attain structural thicknesses of up to about 500 m (Worrall 1981;
Wahrhaftig 1984). Clastic sections reach thicknesses of
up to about 1 km or more (e.g., Worrall 1981; Ernst et al.
2009; Prohoroff et al. 2012).
Basalt-limestone-clastic

A much less common type of OPS in the Franciscan
Complex consists of basalt overlain by limestone overlain by clastic sedimentary rocks. The basalt of suchpackages exhibit ocean island basalt (OIB) geochemicalaffinity (Ghatak et al. 2012). Limestone is rare in theFranciscan Complex and much less abundant than chert

Page 10 of 23(Sliter 1984). Limestone horizons are commonly tens ofmeters in thickness, and the thickest limestone horizon (130 m) is associated with the Permanente terrane(Sliter 1984). Owing to the faulting and deformationof the rocks, it is not clear whether this 130 m thickness
was thickened relative to its original stratigraphic thickness by internal faults, but it probably was, by analogy towell-constrained chert exposures of the Marin Headlands
as well as ubiquitous occurrence of imbricate faults in all
OPS lithologies as will be reviewed in “Imbrication anddeformation of OPS and distinguishing tectonic from sedimentary melanges.” Much of the limestone section of thePermanente terrane is interbedded with chert (Sliter 1984;
Murchey and Jones 1984). The longest duration of limestone deposition is recorded in the Permanente terrane
ranges from early Albian to Turonian (Sliter 1984) (Fig. 5)

Fig. 7 Geologic map of part of Sunol Regional Wilderness in the northern Diablo Range. Location on Fig. 2. This is an example of OPS imbricatesthat are much richer in clastic rocks. Here, the OPS imbricates are metamorphosed to blueschist facies. The clastic rocks have a significant olistostromalcomponent and include serpentinite matrix as well as siliciclastic matrix olistostromes, and these olistostromal units apparently interfinger with ordinary
turbidite units. Repetitions of OPS in the most clastic rich parts of this region (southern part of map) are difficult to identify owing to the difficulty in
distinguishing intact slices of basalt and chert from olistostrome blocks. In the more volcanic-rich area to the north, two of the imbricates are difficult
to locate owing to the small amount of clastic and/or pelagic rocks associated with the fault repetition. Updated from Wakabayashi (2016b)

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

Page 11 of 23

Fig. 8 Geologic map of the Ring Mountain area, San Francisco Bay area. Location on Fig. 2. This locality includes an example of sedimentary
serpentinite mélange that is part of a clastic component of an accreted OPS unit, most likely of the basalt-chert-clastic type (Bero 2014). Some ofthe high-grade metamorphic blocks within the mélange may preserve metabasalt-metachert-metaultramafic OPS imbricates internally, preserving
at a smaller scale the relationships seen in map view in the Willow Spring slab locality of Fig. 9 (see Fig. 10s). Updated from Wakabayashi (2015)

or ca. 110–90 Ma (Cohen et al. 2013, updated). The
Permanente terrane has a significant clastic component,
but it is less than most Franciscan units excluding theMarin Headlands. This clastic component is primarily
siliciclastic, including siliciclastic-matrix mélange, but it
may include sedimentary serpentinite mélange as well
(Wakabayashi 2012).
Other limestone-bearing OPS units of the Franciscan include the well-known Laytonville limestone (Alvarez et al.
1980; Tarduno et al. 1990) that has a pelagic age rangethat overlaps with the Permanente terrane, as well as
younger limestone units, such as the Parkhurst limestone(Sliter 1984). The far-traveled nature of limestone-bearing
OPS has been shown by paleomagnetic studies, and the
Laytonville limestone remains an enigma owing to theanomalously rapid plate motions indicated by the
combination of paleomagnetic and biostratigraphic
data (Alvarez et al. 1980; Tarduno et al. 1990). A new
estimate for an older accretion time of the limestone
based on new data from associated clastic rocks

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

would require even higher apparent plate motions(Wakabayashi 2015).

Serpentinite-basalt-chert (with no siliciclastic rocks)

The structurally highest horizons of the Franciscan
Complex include rare sheets of coarse-grained metamorphic rocks ranging up to several kilometers in long
dimension and several hundred meters in thickness. Theseslabs have been called “high-grade coherent sheets” todistinguish them from high-grade blocks-in-mélangethat have the same metamorphic mineral assemblages,
lithologies, geochenmical affinity, and metamorphic
ages (Wakabayashi et al. 2010).
The Willow Spring slab of the southern Diablo Range
is one of the largest such slabs and serves as a good
example of the characteristics of such units. It consistsof an imbricate stack of primarily mafic schists ranging
from medium to coarse-grained lawsonite blueschist at
structurally lowest level to eclogite at the structurally
highest level (Wakabayashi and Dumitru 2007) (Fig. 9).
This stack of dominantly metabasite sheets structurally
overlies a structural stack of metasandstones. The Willow
Spring slab covers a map area of about 2 km long by 1 km
wide with a structural thickness of several hundred meters. Several similar, but smaller, intact imbricated metamorphic slabs crop out in the southern Diablo Ranges,
and these include slabs of predominantly amphibolite andgarnet-amphibolite (Wakabayashi and Dumitru 2007) andanother high-grade slab has been identified at Goat
Mountain in the eastern part of the northern Coast
Ranges (Ernst et al. 1970). In addition, part of theCazadero-Ward Creek area in the western part of thenorthern Coast Ranges, several kilometers in long dimension, shows similar characteristics (Wakabayashi andDumitru 2007; Wakabayashi et al. 2010).
Metabasite predominates in these high-grade rocks andhas nascent island-arc affinity (Wakabayashi et al. 2010).
Metachert horizons are spatially associated with antigorite
schist bodies so that the inferred ocean floor stratigraphy
was one where serpentinized mantle rocks were overlain
by basalt and chert successively. Metaclastic rocks areabsent in this package.
High-grade blocks in mélange have similar lithologic
associations on a smaller (≤200 m) scale. They are composed predominantly of metabasite with small metachert
layers and lack metaclastic components. Whereas they
are commonly at least partly encased by actinolitephengite-chlorite “rinds” derived from metasomatized
ultramafic rock (e.g., Coleman and Lanphere 1971;
Ghatak et al. 2012; Sorensen et al. 1997), some also
include metaultramafic layers within them as described in“Imbrication and deformation of OPS and distinguishingtectonic from sedimentary melanges.”

Page 12 of 23

Serpentinite-clastic

Serpentinized peridotite lacking block-in-matrix character(“coherent”) is also found within the Franciscan Complex(Wakabayashi 2004; Prohoroff et al. 2012; Wakabayashi
2015; Wakabayashi, in press). These serpentinite bodies
range in structural thickness up to 1.5 km and extend for
up to 30 km along strike, preserve primary peridotite textures in large domains, and lack non-serpentinite blocks
other than rare gabbro (Wakabayashi 2004; 2012; 2015; inpress). The serpentinized ultramafic rocks vary in texture
from massive to areas with massive blocks surrounded byzones with brittle foliation, to pervasively foliated rocks.
The dominant serpentine mineral in these rocks is lizardite (Wakabayashi 2004; in press). Prohoroff et al. (2012)
proposed that clastic sedimentary rocks depositionallyoverlie the larger bodies in Marin County (“sfs” label
directly above “Marin Headlands vfr” label on Fig. 2),
but the contact relationships are disputed (Raymondand Bero 2015). Basalt and/or pelagic sedimentaryrocks do not appear to depositionally overlie theseserpentinite bodies. The parent peridotite has abyssal
affinities, based on whole rock and mineral geochemistry (Barnes et al. 2013), which distinguishes it fromthe upper plate (Coast Range ophiolite) serpentinizedperidotite, which has both arc and abyssal affinity(e.g., Choi et al. 2008; Jean et al. 2010).
Metaclastic rocks are imbricated with and may
depositionally overlie serpentinized peridotite, exposedin the northwestern Diablo Range along Kincaid Road
(Fig. 10i, j). The serpentinite makes up an exposure about
5.5 km long by up to 800 m wide (Wentworth et al. 1999).
Most of this body is massive serpentinized harzburgite
with local foliated zones. A schistose metasandstone thathas been correlated to the Skaggs Springs schist of
Sonoma County (Wakabayashi 1999) is locally imbricatedwith the serpentinite (Fig. 10k). At least one of thecontacts of metasandstone on serpentinite may be depositional based on the presence of small blocks of silicacarbonate rock (Fig. 10j), a hydrothermal replacement ofserpentinite also referred to as listvenite (e.g., Halls andZhao 1995; Smith et al. 2008).
Serpentinized peridotite capping Tiburon Peninsula,
including the Ring Mountain locality (Fig. 8), has beeninterpreted as being abyssal in origin and thus of intraFranciscan affinity (Barnes et al. 2013), but clastic rocks
have not been found in depositional contact atop thisserpentinite unit (Bero 2014). This serpentininizedharzburgite sheet tectonically overlies serpentinite
matrix mélange interpreted to be sedimentary origin
by Wakabayashi (2012; 2015; additional evidence forsedimentary origin presented in “Sedimentary origins
of mélanges with exotic blocks: interbedding withsiliciclastic turbidites and internal sedimentaryfeatures”).

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

Fig. 9 (See legend on next page.)

Page 13 of 23

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

Page 14 of 23(See figure on previous page.)
Fig. 9 Geologic map of the Willow Spring slab, southern Diablo Range. Location on Fig. 2. These (dark green, purple, and magenta colors) areamong the highest grade coherent (non-mélange) rocks in the Franciscan Complex and are completely recrystallized and schistose. The Willow
Springs slab is an example of OPS that is predominantly metabasite, with minor ultramafic schist and metachert, but no metaclastic rocks. Theserocks tectonically overlie intact lower grade units including a blueschist facies unit with abundant metavolcanics (mov) and metachert (s) in addition tosiliciclastic (mostly sandstone, ms2). The reconnaissance mapping done here did not completely define the extent of metachert and metaultramafic
horizons in the high-grade rocks, but they certainly extend further along strike than shown. Adapted from Wakabayashi (2015)

Basalt-limestone-chert-clastic or basalt-chert-limestoneclastic

As noted earlier, limestone is rare in the Franciscan
Complex and most of it is part of basalt-limestoneclastic sequences. A very small amount of limestone ispresent in sequences where basalt is successively overlain by chert and limestone. An example from an apparent coherent imbricated assemblage of blueschist facies
rocks with this assemblage crops out along the Skaggs
Road in Sonoma County (Fig. 10g). This unit may
extend over a kilometer along strike based on my fieldreconnaissance; published data on this unit is otherwise
lacking. The limestone appears limited to a thickness of
20 cm or less, whereas metchert, metabasalt, and metaclastic rocks may reach thicknesses of 10 m, several hundred meters, and about 1 km, respectively. Even rarer in
the Franciscan is OPS composed of basalt overlain
successively by limestone, chert, and clastic rocks. To
my knowledge, these assemblages have not been previously reported in the literature on the Franciscan, but
minor exposures are found as small (<20 m in size) blocksin-mélange at Sunol Regional Wilderness (Fig. 10h).

Olistostromes as part of the clastic component ofOPS
Clastic components of Franciscan OPS units are composed of variable proportions of turbidite and mélangewith non-native (exotic) blocks, including those ofhigher metamorphic grade than the matrix. This type of
mélange has been suggested to have formed by submarine
sliding (as an olistostrome) prior to subsequent deformation as summarized previously (Macpherson et al. 1990;
Wakabayashi 2015). The olistostromal horizons have siliciclastic or serpentinite matrix, and these matrix types may
grade into and interfinger with each other as proposed by
Wakabayashi (2015) and schematically shown in Fig. 4.
New evidence for gradation between siliciclastic and clastic
serpentinite rocks will be presented in “Interfingering, interbedding, and gradation between siliciclastic rocks and clastic serpentinite.” Interbedding of siliciclastic olistostromal
horizons and turbidites was proposed by Wakabayashi(2015), and additional information will be presented in“Sedimentary origins of mélanges with exotic blocks: interbedding with siliciclastic turbidites and internal sedimentary features,” along with photographs showing sedimentarytextures in siliciclastic and serpentinite matrix.

Sedimentary origins of mélanges with exotic blocks:
interbedding with siliciclastic turbidites and internal
sedimentary features

The interbedding and interfingering of sedimentary
mélange and turbidites (bedded sandstone and shale) issuggested by 100 m to kilometer scale field relationships
at Sunol Regional Wilderness, where the sedimentary
mélange horizons include those of siliciclastic andserpentinite matrix (Fig. 7). Outcrop scale relationships
from meter to 100 m scale show interbedding of siliciclastic turbidites and olistostromal horizons at many
Franciscan localities (Wakabayashi 2015).
The clastic part of the Marin Headlands unit containsmélange horizons and beachcliff exposures locally show
the matrix of these mélanges to be mud-rich conglomerate, pebbly sandstone, and clean sandstone (Fig. 10a, d,and e). These block-in-matrix horizons grade into or areinterbedded with block-free sandstone, shale, andconglomerate (Fig. 10a). The blocks in the mélange are
predominantly sandstone, chert, and basalt of prehnitepumpellyite grade, but detailed studies have not been
done to determine whether these rocks are the same as
the intact Marin Headlands imbricates that have similar
general character. Serpentinite blocks to tens of metersin size are present in some of the mélange zones of the
Marin Headlands are (Wahrhaftig 1984). Some serpentinite blocks are found in the colluvial and landslidematerial draping part the outcrop shown in Fig. 10a, andthe position of these blocks suggests derivation from theexposed block-in-matrix horizons. Serpentinite clasts arefound in the pebbly sandstone matrix of Fig. 10d, andthe tremolite-bearing metamorphic assemblage of someof these clasts (Fig. 11a) records a higher temperature
metamorphism than the prehnite-pumpellyite metamorphic assemblages of the host rock and the otherblocks observed.
Along Panoche Road in the southern Diablo Range,
blocks of up to tens of meters in size, including serpentinite and high-grade (amphibolite, garnet-amphibolite,
eclogite) rocks, as well as lower-grade (blueschist facies)
blocks of a variety of lithologies, are embedded in blueschist facies siliciclastic conglomerate/sedimentary matrix(Wakabayashi 2015). The matrix contains centimeter- andmillimeter-sized clasts that reflect the same lithologic
range as the larger blocks. These mélange horizons areinterbedded with block-free sandstone and pebblyWakabayashi Progress in Earth and Planetary Science (2017) 4:18

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Page 15 of 23

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Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

Page 16 of 23(See figure on previous page.)
Fig. 10 Field photographs of OPS in the Franciscan Complex. In these photos, all contacts (dashed lines) of apparent “inverted” OPS stratigraphy(such as basalt over chert or sandstone, or chert over sandstone) and “omitted” OPS (clastics over basalt when pelagic rocks are present in thefull sequence) are faulted. Some of the contacts in “normal” OPS stratigraphic order are depositional whereas others are faults. Dashed lines drawn
within the same lithology represent faults. a Marin Headlands, prehnite-pumpellyite facies OPS imbricates at the western side of Black Sand Beach,
Bonita Cove (Fig. 6), where basalt and chert (itself internally imbricated) is thrust over siliciclastic rocks. Whereas the siliciclastic section is primarily
bedded with local broken formation, sedimentary mélange/conglomerate horizons are present. “b” and “c” show outcrops within the Rodeo Coveshear zone of the Marin Headlands (Fig. 6), a locality described in detail in Meneghini and Moore (2007). At the scale of Fig. 6, the Rodeo Coveshear zone is one of the faults repeating the OPS section and this imbrication is seen on a smaller scale within the shear zone itself as shown in
the outcrop photos. The imbrication locally leads to block-in-matrix relationships such as the chert-in-sandstone seen in the “ch/ss” parts of “b”,
the basalt in basalt matrix part (labeled as “bs”), and the basalt and chert in shale matrix (“ch/sh/bs”) part of “c”. “d” and “e” show close ups of
sedimentary block-matrix horizons in the outcrops of (“a”). d Pebbly mud-rich sandstone/conglomerate. This conglomerate grades into cleaner
sandstone with blocks shown in (“e”), and this is interbedded with deformed shale with blocks of other lithologies. These sorts of textures andrelationships are common in siliciclastic matrix mélange as seen in “k-n” as well as Figs. 16, 17, 18, and 20 of Wakabayashi 2015). f Siliciclastic-chert-basalt
OPS from blueschist facies rocks, Sunol Regional Wilderness. The preserved parts of chert and siliciclastic sections in the imbricates of mainly metavolcanic
exposures are very thin in this area (commonly <10 m and too small to show on the Fig. 7 map). g OPS stack of sandstone-limestonechert-basalt in blueschist facies rocks along the Skaggs Road, Sonoma County (location on Fig. 2). This type of OPS stack is very rare in
the Franciscan. h Block-in-sandstone matrix, Sunol Regional Wilderness showing OPS of sandstone overlying chert overlying limestone thatoverlies basalt. Coherent imbricates of this type of OPS have not been found to date in the Franciscan. i Skaggs Spring schist (completely
recrystallized quartz-rich glaucophane-lawsonite-phengite ± jadeite schist) imbricated with intact serpentinized peridotite. Close up in “j” shows silica
carbonate rock (hydrothermally replaced serpentinite, also called listvenite) blocks along one of the contacts that places Skaggs Springs Schist on
serpentinite. This may represent a depositional contact. Note that there is a lot of surficial material draped over the outcrop in this photo.
k Photo showing interbedded sandstone and pebbly sandstone and shale matrix sedimentary breccia/conglomerate, blueschist facies rocks along
Panoche Road (location on Fig. 2). This shows the interbedding of ordinary turbidites and sedimentary mélange matrix. These horizons locally haveblocks of tens of meters in size (compare with different photos in Fig. 18 of Wakabayashi 2015). l Interbedding of sedimentary breccia/conglomerateand sandstones and gradational depositional contact between sedimentary breccia/conglomerate (sedimentary mélange matrix), blueschist facies
rocks, Sunol Regional Wilderness (location on Fig. 7). The elevation difference between the bottom of the photo and top is about 15 m. m, n Sunol
Regional Wilderness exposures showing local deformational overprint on conglomerate/sedimentary breccia that makes up the siliciclastic mélange
matrix in this area. In “m,” note the pencil on the center of the upper photo border for scale. In addition to the locally developed deformational fabric,
note the late brittle fault in (“m”). The most deformed zones in these outcrops are typical of what has been interpreted as “tectonic” mélange matrix in
the Franciscan Complex. o–r Sedimentary serpentinite mélange, Ring Mountain (locations on Fig. 8) o metamafic block in sedimentary serpentinite
matrix. The block is an omphacite-chlorite schist on the left side of the photo (chlorite replaces most of the omphacite and there is late growth oflawsonite), and this grades to or is bordered by nearly monomineralic chlorite schist on the right. The matrix is serpentinite conglomerate and sandstone
with many exotic clasts ranging from <0.1 mm to cm scale. p Close up of part of “o” showing texture of serpentinite conglomerate/pebbly sandstone.
q Close of a separate outcrop of sandy/pebbly serpentinite matrix with many rounded grains, including a coarse blueschist clast. r Mafic
block in serpentinite matrix, showing internal faulting and folding. The block is garnet amphibolite (grt-am) nearly completely overprinted
by blueschist facies assemblages (bsch) in most areas. The block is internally imbricated and tightly folded resulting in repetitions of mafic(bsch/grt-am) and metaultramafic (UM) schist. The latter is composed of tremolite/actinolite + phengite + chlorite. Progressive deformation has createdlocal block-in-matrix textures in which mafic blocks (bsch) are enclosed in metaultramafic schist (UM). Most of these block-in-matrix features are too
small to be seen in this photo. Similar features are present in other blocks in this mélange. They apparently predate the incorporation into the clastic
serpentinite matrix which is not part of the imbricates. The features preserved in these blocks may preserve an ocean plate stratigraphic relationshipsimilar to that shown at map scale in Fig. 9 (Willow Spring Slab). s Pebbly serpentinite sandstone matrix from Sunol Regional Wilderness (location on Fig. 7)
showing well-preserved sedimentary texture. This type of material lacks erosional resistance and tends to be poorly exposed, except on the steepest slopes or
artificial excavations. t, u Examples of imbrication of siliciclastic (sandstone and shale) (locations on Fig. 2). Without good exposures, these structures are much
more difficult to identify in these rocks in contrast to rock assemblages with volcanic and/or pelagic sedimentary rocks. “t” shows exposures of theprehnite-pumpellyite facies Novato Quarry terrane along Nave Drive in Marin County and “u” shows lawsonite-albite facies rocks that may becorrelative to the Burnt Hills terrane along Panoche Road of the southern Diablo Range. For a higher resolution image, please see Additional file 1

sandstone at scales as small as centimeters, and the mélange matrix is commonly more mud rich than the interbedded sandstones as reflected by the darker gray color(Fig. 10k; compare with Fig. 18b, f of Wakabayashi 2015).
The blueschist facies rocks of Sunol Regional Wilderness of the northern Diablo Range include units of siliciclastic and serpentinite matrix mélange with high-grade
blocks that include eclogite, garnet-amphibolite, andamphibolite to tens of meters in size, as well as a varietyof lower grade (blueschist facies) lithologies ranging toblock sizes of several hundred meters (Wakabayashi 2015)(Fig. 7). Several outcrops show interbedding of siliciclastic

matrix, composed of conglomerate and sedimentary breccia, with block-free sandstone and shale (Fig. 10l). Thismatrix has centimeter and millimeter size clasts with thesame lithologic range as the larger blocks (Wakabayashi2015). Nearly undeformed conglomerate and sedimentary
breccia is variably overprinted by later deformation, sothat it grades into strongly deformed mélange matrix(Fig. 10m, n). The derivation of sheared matrix from
conglomerate and sedimentary breccia has been shownfor multiple Franciscan localities by Wakabayashi(2015, Figs. 16–18, 20). The inhomogeneous distributionof the later deformation results blocks of less-Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

aPage 17 of 23bc trem-phen-chl

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qtz
jd

flattened serpentinite clast
gln

qtz
gln

jd
jd

gln

mm

gln

gln
truncation at
detrital grainboundary

mm

qtz

qtz

Fig. 11 (See legend on next page.)

jdjd
gln
qtz
jd

qtz

jd

lws

neoblastic gln

gln

hb

clastic serpentinite clast

u

jd

jd

gln

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

Page 18 of 23(See figure on previous page.)
Fig. 11 Photomicrographs of clastic parts of OPS of the Franciscan Complex with emphasis on the clastic/olistostromal horizons. Mineral abbreviations
are chl chlorite, gln glaucophane, hb hornblende, jd jadeitic clinopyroxene, lws lawsonite, phen phengite, qtz quartz, trem tremolite/actinolite.
a Metaultramafic schist clast in the pebbly sandstone from Marin Headlands shown in Fig. 10d. The metamorphic assemblage in this clast is
higher grade than the prehnite-pumpellyite facies host rock. Note that this clast exhibits an internal fabric that predates deposition, and thisfabric appears to be the result of progressive deformation of serpentinized peridotite, rather than a deformational overprint on metaultramafic
sandstone of the sort shown in “e” and (“i”). Plane light. b Cross-polarized light view of (“a”). c Imbricate and block-in-matrix zone in metaultramafic
schist from the block in Fig. 10r, showing alternating blueschist layers and lenses with abundant sodic amphibole (blue in this plane light view) andpale tremolite/actinolite (trem)-phengite (phen)-chlorite (chl) schist. Note the contrast in textures between the metaultramafic schist and the serpentinite/metaultramafic sandstones shown in “d, e, g, and h,” as well as the clast in (“q”). d Serpentinite sandstone matrix from the outcrop in Fig. 10o at Ring
Mountain. The clasts in this view are primarily antigorite schist (lighter colored) or tremolite schist (darker). Although some of the rounding of the grains is
a result of the folding of these grains (two examples shown with arrows) and development of locally curved shears, there are many rounded grainboundaries that abruptly truncate the internal grain fabric without drag; these are original detrital grain boundaries (examples shown byarrows). Plane-polarized light. e Cross-polarized light view of (“d”). f Rutile-bearing hornblendite clast (long dimension about 5 cm) in thesame serpentinite matrix sampled about 1 m away from the sample in (“d”). The internal fabric in this clast is truncated along therounded grain boundary with the enclosing serpentinite sandstone/conglomerate (dark) without drag features (most apparent in the upper
left hand part of the view). Hornblende (hb) locally has small glaucophane (gln) rims, and there is some late growth of chlorite (chl). Thisis a cm-scale analog of the larger high-grade blocks found in this mélange. Plane-polarized light. g Serpentinite matrix from the same locality showing
hornblende (hb) grains, some of which are rounded and rimmed with glaucophane (gln). Note the truncation of the hornblende and rim against therounded grain boundary, best seen at the left-rounded termination of the hb grain in the upper left corner of the photo. Other grains in thisview include tremolite/actinolite, and chlorite-tremolite-antiorite schist. Plane-polarized light. h Serpentinite matrix from serpentinite mélange, Sunol
Regional Wilderness (sample location on Fig. 7) showing abrupt truncation of internal grain fabric along grain boundaries, similar to that seen at the Ring
Mountain locality. Clasts in this view are primarily antigorite-chlorite schist with some grains of tremolite/actinolite, hornblende, and chlorite.
Plane-polarized light. i Chlorite-rich sandstone from Sunol Regional Wilderness (Fig. 7). This sandstone is medium green in outcrop and resembles clastic
serpentinite in outcrop appearance. Much of the chlorite rich zones (chl) have fine greenish phengite, and polycrystalline growths of chlorite form
textures similar to antigorite schist. Chrome spinel is locally present in the chlorite-rich zones. The chlorite-rich zones probably originated as serpentinite
clasts. Other clasts or minerals include jadeitic clinopyroxene (jd), lawsonite (lws), quartz (qtz), and glaucophane. Some of the metamorphic mineral
grains are part of polymineralic clots (jd, lws or lws, qtz or jd, lws, qtz) and the polymetamorphic clots, and many of the metamorphic mineral
grains have rounded grain boundaries that truncate their internal fabric, suggesting a detrital origin. Many of these grains also exhibit late neoblastic
overgrowths with sharp terminations. Plane-polarized light. j Cross-polarized light view of (“i”). k Serpentinite clast with chrome spinel in mostly siliciclastic
conglomerate, Sunol Regional Wilderness. Long dimension of the clast is about 10 mm. Such serpentinite clasts make up 5–10% of the clasts in somehorizons in this conglomerate. Plane-polarized light. l Cross-polarized view of (“k”). m Sandstone that makes up most of non-mélange siliciclastic outcrops
at Sunol Regional Wilderness. This sandstone has much less chlorite than “i,” but some chlorite (chl) clasts have the appearance of serpentinite(best viewed in the cross-polarized light view of “n”) such as the clast labeled that is directly below the shale chip in the center of the photo.
This sample has neoblastic lawsonite (lws), jadeitic clinopyroxene (jd), and glaucophen (gln), but relict albite is still present as seen by the albite twinningseen in the cross-polarized light view of “n” (good example along the upper left edge of photo). o–u El Cerrito quarry area, eastern San Francisco Bay area
(general location on Fig. 2). Two serpentinite clasts from prehnite-pumpellyite siliciclastic conglomerate, matrix of the Hillside mélange
unit. This conglomerate/pebbly sandstone contains up to 15% serpentinite clasts. This photo shows a lizardite-rich and an antigorite-richclast, both of which appear to have been derived from intact peridotite (they are not themselves clastic). Plane-polarized light. p Cross-polarized lightview of (“o”). q Serpentinite sandstone clast from the same thin section showing the internal rounded grains that truncate fabric within them. Compare
to the serpentinite matrix samples shown in “d” and (“h”) The grains are primarily antigorite schist. Plane-polarized light. r Cross-polarized light view of(“q”). s Clast with jadeite-chlorite ± lawsonite, along with another serpentinite clast in the same thin section. t View of blueschist facies metasandstone
unit of the upper El Cerrito quarry. This particular sample consists of more than 50% detrital blueschist clasts, most of which have hornblende relics.
Much of the blue amphibole growth is truncated on the detrital grain boundaries indicating that these were blueschist-overprinted amphibolite
clasts (equivalent of the high-grade blocks) at the time of deposition. There is also some late neoblastic glaucophane growth along one of themargins of this clast. u Same unit as “t” collected about 10 m away showing serpentinite clast with chrome spinel. For a higher resolution image,please see Additional file 2

deformed mélange surrounded by more strongly deformed zones (Wakabayashi 2015).
It is more difficult to demonstrate sedimentaryfeatures of serpentinite mélanges than siliciclastic
matrix mélanges because apparent rounding of grainsin the former may take place by two different tectonic mechanisms: (1) the formation of curved shear
surfaces and (2) the tight folding early-formed fabrics
that result in rounded apparent clast borders formed
from detached fold hinges. For this reason I present
outcrop and petrographic thin section photos showing
what I interpret as sedimentary textures with goal ofpresenting more convincing relationships than thoseshown in my earlier papers (Wakabayashi, 2012; 2015).
In addition, recent work allows a direct comparison
between the textures observed in these Franciscan
serpentinite mélanges to the GVG serpentinite deposits
that are indisputably sedimentary in origin (e.g., Moiseyev
1970; Lockwood 1971; Phipps 1984; (Wakabayashi 2016b;
Wakabayashi, in press).
Franciscan clastic serpentinite mélanges have comparatively fine-grained matrix that crops out poorly, andwhen it does, it forms outcrops that appear powdery
from a distance owing to the lack of large pieces of moreintact rock (Wakabayashi, in press). This contrasts withthe blockier appearance of intact (not mélange) Franciscan

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

serpentinized peridotite slabs or the ultramafic sections of the CRO, but is identical to the appearanceof the GVG sedimentary serpentinites (Wakabayashi
in press). Exposures of Franciscan serpentinite matrix
range from serpentinite sandstone to serpentinite conglomerate/sedimentary breccia (Fig. 10o–s; see also
Fig. 6a, b, Fig. 9 of Wakabayashi 2012, and Fig. 21c–f
of Wakabayashi 2015). These textures are identical tothose displayed by the sedimentary serpentinite of theGreat Valley Group, but differ from the Franciscan
serpentinized peridotite and Coast Range ophiolite
serpentinitinized peridotite slabs (Fig. 6 of Wakabayashi,in press.
At thin section scale, certain grain boundary geometries are compatible with original detrital grain boundaries but not with formation by fracturing or shearing.
Internal fabric within clasts is abruptly truncatedwithout any drag features along rounded grain boundaries, including those grain boundaries oriented athigh angles to the foliation or cleavage direction in
the rock (Fig. 11d–h). There are no deformed tails associated with these clasts. The clasts include (non-ultramafic)
metamorphic rock fragments and metamorphic mineral
grains (Fig. 11f, g). Deformation has variably overprinted
the sedimentary textures leaving lenses of relatively
undeformed domains (millimeter to tens of meter scales)
between strongly deformed zones (millimeter scale examples in Fig. 11d, e, g, and h). These textures differ markedly
by textures that appear formed by progressive deformation,such as those shown in Fig. 11a–c that lack the grainboundary relationships, whether it is between serpentinite
domains, or between non-ultramafic and ultramafic
material.
The matrix at the Ring Mountain locality includes millimeter- to centimeter-scale clasts of minerals that have thesame characteristics of the larger high-grade blocks in the
mélange. For example, the clasts and mineral grains amphibolite grade metamorphism overprinted by blueschist
facies metamorphism (Fig. 11f, g) as is common in thehigh-grade blocks of the Franciscan, including those of
the Ring Mountain locality (e.g., Wakabayashi 1990).
Abrupt truncation of sodic amphibole in these clasts andmineral grains by detrital grain boundaries suggests theblueschist overprint of the earlier higher temperature
metamorphism took place prior to incorporation(sedimentation) (Fig. 11g).
The serpentinite mélange at Ring Mountain and at
Sunol Regional Wilderness also contains lower grade
metamorphic blocks, such as metachert lacking macroscopic metamorphic minerals, metavolcanic rocks withpreserved igneous textures and incipient growth of finegrained (mostly <0.1 mm) blueschist facies mineralssuch as lawsonite and glaucophane, and metasandstonethat displays neoblastic growth of jadeitic clinopyroxene,

Page 19 of 23

lawsonite, and glaucophane (Wakabayashi 2015). At
Sunol Regional Wilderness, the (non-serpentinite) block
types in serpentinite matrix mélange are the same as
those in the adjacent siliciclastic matrix mélange.
Interfingering, interbedding, and gradation between
siliciclastic rocks and clastic serpentinite

Interbedding and interfingering of siliciclastic rocks anclastic serpentinite is suggested by map relationships at
100 m and larger scales, as well as the comparatively
common occurrence of blocks of serpentinite and hydrothermally altered serpentinite (silica-carbonate rock or
listvenite) in siliciclastic matrix mélange, as well as similar(non-serpentinite) block populations in clastic serpentinite
and siliciclastic mélanges at Sunol Regional Wilderness(Fig. 7). However, outcrops demonstrating depositional
relationships between clastic serpentinite and siliciclastic
horizons have not been found in the Franciscan Complex.
This contrasts to the Great Valley Group where depositional contacts of serpentinite over siliciclastic rocks andgradation between clastic serpentinite and siliciclastic
rocks has been shown (Wakabayashi 2016b).
Petrographic inspection provides evidence for mixed siliciclastic and clastic serpentinite deposition in Franciscan
rocks. Serpentinite clasts are more common in horizonsof siliciclastic mélange matrix (Fig. 11a, b, k, l, and o–r)than “ordinary” sandstones (Fig. 11m, n ). Serpentinite
clasts may make up at least 15% of the rock (Fig. 11o), andthe detrital serpentinite consists of intact serpentinite (nointernal detrital grain boundaries, Fig. 11a, b, k, l, o, and p)
and redeposited serpentinite sandstone (Fig. 11q, r). The
host rocks have high-pressure metamorphic detrital clasts
that are higher grade than the post-sedimentary burial
metamorphism including clasts equivalent to Franciscan
high-grade blocks (Fig. 11s, t). Whereas serpentinite or
metaultramafic mineralogy is preserved in some detrital
serpentinite clasts, in other cases, the original mineralogy
has been altered, commonly to chlorite, or chromium-richwhite mica (fuchsite), or a combination of the two(Fig. 11i, j). The serpentinite origin of such detritalgrains may be ascertained in some cases by the presenceof chrome spinel or the pseudomorphing of serpentinite
textures by polycrystalline chlorite clots. Sandstones richin chlorite clasts of the type noted above have an unusual
green color in outcrop and resemble serpentinite.
Most Franciscan siliciclastic sandstones have small
amounts of detrital serpentinite, whether inferred by
textures in chlorite clasts (Fig. 11m, n) or directly verified
by presence of preserved serpentinite. Larger amounts of
detrital serpentinite are found in siliciclastic mélange
matrix, and in limited horizons of pebbly sandstone within
otherwise block-free sandstone, such as one pebbly sandstone from El Cerrito Quarry that is locally so rich in blueschist clasts (Fig. 11t) as to have a blue color in outcrop.

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

In summary, the common occurrence of detrital
serpentinite clasts in siliciclastic rocks (both siliciclastic
mélange matrix and bedded sandstones), a similar rangeof block types in siliciclastic and serpentinite mélanges
in contact with one another, map-scale contact relationships, and serpentinite blocks in siliciclastic matrix
mélange, suggest that interbedding, interfingering, andgradation between siliciclastic and clastic serpentinite
horizons is common in the Franciscan.

Imbrication and deformation of OPS anddistinguishing tectonic from sedimentarymelangesInternal imbrication of OPS is easily seen where theigneous and pelagic components of OPS are common,such in the Marin Headlands unit (Fig. 10a–c). Internal
imbrication of the clastic and volcanic units is difficult
to evaluate in most cases owing to the lack of obvious
marker horizons. In the metavolcanic-rich domains ofnorthern Sunol Regional Wilderness, imbrication isshown by siliciclastic and/or chert horizons, commonlyless than 10 m in thickness (Fig. 10f ). Owing to the thinness of these non-volcanic layers, they have not beenfound outside of one continuous set of roadcut exposures. In mélange rich exposures, it is difficult to distinguish throughgoing imbricate faults from a series ofisolated blocks such as in the central part of the Sunol
Regional Wilderness map of Fig. 7. With high-quality
exposures, however, internal imbrication is observed tobe ubiquitous in clastic Franciscan rocks (examples inFig. 10t, u, as well as the spatially variable deformationin mélange matrix of Fig. 10m, n). The magnitude ofpenetrative strain in most rocks is small, so most of thedeformation and accordingly accommodation of accretionary megathrust slip takes place along discrete faults
or narrow (less than tens of meters thick) fault zones(Fig. 10a–c, m, n, t, and u).
Fault zones that repeat the OPS section, such as the
Rodeo Cove shear zone of the Marin Headlands, locally exhibit block-in-matrix textures (Meneghini andMoore 2007) (Fig. 10a, b). In such cases, none of theblocks are exotic and all are of the same metamorphic
grade, unless this late imbrication and deformation involves a clastic section that has olistostromal horizons
that have exotic blocks (schematically shown in Fig. 4).
Such deformation can result in blocks of sedimentary
mélange within a tectonic mélange, as illustrated by
the deformational overprint shown in Fig. 10m, n(see also Wakabayashi 2015, Fig. 18c, d).
Examples of Franciscan tectonic serpentinite mélange,
developed by the progressive deformation of metaultramafic and metabasite/metachert may be preserved in somehigh-grade blocks in sedimentary mélange, as shown at
outcrop (Fig. 10r) and thin section scale (Fig. 11c, d). In

Page 20 of 23such examples, all of the blocks in the metaserpentinite
matrix are of the same metamorphic grade and compatible with the metamorphic mineralogy of the matrix. Such
relationships are similar to serpentinite mélanges found in
the Alps and Iran that have been shown to have developedby progressive deformation of ultramafic-bearing OPS(e.g., Angiboust et al. 2012; 2013; Balestro et al. 2015;
Vitale Brovarone et al. 2014), but differ markedly from thesedimentary serpentinite mélanges described in this paperand by Wakabayashi (2012; 2015; in press). Intact equivalents of such tectonic serpentinite mélanges have not beenfound in Franciscan rocks, but imbricated high-grade slabs
such as the structurally high Willow Spring slab, whose
imbricates include serpentinite, may locally have suchserpentinite-matrix mélanges (Fig. 9).

Franciscan OPS and tectonic history
The Franciscan OPS exposures exhibit a large range inlithologic combinations, reflecting a correspondingly
large range in character of oceanic crust subducted during the >150 Ma of the assembly of this accretionary
complex. The lack of post-subduction disruption in theFranciscan results in good preservation of the variationin OPS packages and their relationship to the architecture of the subduction complex, and hence theirrelationship to the tectonic history. In addition, the lackof post-subduction disruption facilitates distinction between Franciscan subduction complex rocks (transferredfrom lower to upper plate of subduction system) androcks that were always part of the upper plate (CRO and
GVG).
The earliest accreted OPS units of the Franciscan are
high-grade coherent sheets (“Basalt-limestone-chertclastic or basalt-chert-limestone-clastic”) and related
blocks-in-mélange that underwent high P-high T metamorphism at about 150–170 Ma (Ross and Sharp 1988;
Anczkiewicz et al. 2004; Wakabayashi and Dumitru
2007; Ukar et al. 2012; Mulcahy et al. 2014; Cooper et al.
2011). These rocks may have formed during or shortly
after subduction that initiated at ca.165–170 Ma withinyoung oceanic arc lithosphere, in an environment lacking clastic sediment input (Wakabayashi et al. 2010).
Following subduction initiation and early accretion ofisland arc crust, subsequent oceanic igneous rocks subducted and accreted were of MORB affinity with lesser
amounts of OIB. A significant gap in accretion of tens of
million years followed the initial accretion of the highgrade material (Dumitru et al. 2010; Wakabayashi 2015).
Following subduction initiation, the age progression ofaccreted oceanic crust, as well as the progression ofmetamorphism, suggests that a ridge crest approached
the trench (120–125 Ma) but probably shut off prior to
subduction (Wakabayashi 2015) (Fig. 5a). After this nearridge-trench encounter, the age of subducted oceanic

Wakabayashi Progress in Earth and Planetary Science (2017) 4:18

crust became progressively older until about 95 Ma(subduction accretion of the Marin Headlands terrane),after which the age of the subducted oceanic crust
became progressively younger until termination of subduction and conversion to a transform plate boundary.
In contrast to the age of the subducted oceanic crust,the age of clastic sedimentary rocks in each OPS package was progressively younger with each successively
accreted unit.
These OPS packages had significant components oftrench fill overlying the pelagic sedimentary rocks. This
trench fill included both turbidites as well as siliciclastic and serpentinite matrix olistostromes. Some of theoceanic crust that subducted had serpentinized peridotite exposed on the sea floor (e.g., Coleman 2000;
Wakabayashi 2004), resulting in OPS composed ofabyssal serpentinized peridotite and overlying clastic
trench fill.
In contrast to the Franciscan OPS, imbricates of theupper CRO and basal GVG were formed by shorteningof the upper plate rather than by transfer from thesubducting plate to the upper plate. Some of thisshortening may have taken place synchronous withFranciscan subduction, whereas some may have taken place
after subduction termination and conversion to a transform
plate boundary (Phipps 1984; 1992; Wakabayashi and
Unruh 1995). This imbrication did not result in burial
metamorphism or penetrative deformation in these rocks,
in contrast to Franciscan OPS.

Conclusions
OPS in the California Cordillera consists both of oceanic
assemblages that were never part of a subducting plate(upper plate assemblages) and were not incorporated
into subduction-accretion complexes as well as materials
transferred from the subducting plate to the upper plate
to form subduction complexes such as the Franciscan.
Whereas the largest oceanic crustal remnants in the
California Cordillera are of the upper plate type, the subduction complex materials reflect the OPS concept as
commonly applied (e.g., Wahrhaftig 1984; Isozaki et al.
1990; Isozaki and Blake 1994; Wakita and Metcalfe
2005; Kusky et al. 2013).
Franciscan OPS varies considerably in characteristics.
This variation reflects both the variety of ocean crust
subducted during the ca. 150 million years of Franciscan
subduction as well as a variety of processes of subductionaccretion and trench sedimentation. The variation in
OPS and its relationship to tectonic process is more
complex than generally considered in studies of thesetypes of rocks.
The clastic component of OPS includes olistrostromal(sedimentary mélange) horizons that are interbedded withand interfinger with bedded turbidites. The olistostromal

Page 21 of 23

component includes units of both siliciclastic and serpentinite matrix, and the different types of matrix may beinterbedded, interfinger, or grade into one another.
Non-accretionary megathrust horizons separate OPS
accreted at different times, whereas megathrust slip
during accretion was accommodated by internal deformation of OPS, chiefly imbrication of OPS along discrete
faults. Such deformation results in local block-in-matrix
horizons (tectonic mélanges) that differ from the olistostromal horizons noted above in the lack of exotic blocks.
However, because the deformed OPS packages include
olistostromal horizons, pieces of the olistostromes that
include exotic blocks may be incorporated as blocks in
tectonic mélanges.

Additional files
Additional file 1: Higher resolution image of Figure 10. (JPG 9779 kb)
Additional file 2: Higher resolution image of Figure 11. (JPG 3932 kb)
Abbreviations
CRO: Coast range ophiolite; GVG: Great valley group; MORB: Mid-ocean-ridge
basalt; OIB: Ocean island basalt; OPS: Ocean plate stratigraphy
Acknowledgements
Parts of this research were supported by the National Science Foundation
grant EAR-0635767. I thank the detailed and constructive reviews that led tosignificant improvements in the paper.
Funding
Not applicable.
Authors’ contributions
All works were conducted by the (single) author.
Authors’ information
Wakabayashi has conducted research on subduction complexes for over
30 years with emphasis on the Franciscan Complex of California, startingwith his Ph. D. dissertation research with Eldridge Moores that began in theearly 1980s. His first exposure to ocean plate stratigraphy came from geologic
mapping of the Marin Headlands as an undergraduate student (1980) under
the guidance of Clyde Wahrhaftig who several years later authored awell-known paper on the Marin Headlands in the context of the ocean
plate stratigraphy concept (Wahrhaftig 1984 cited in the manuscript).
Recent work by Wakabayashi in the Franciscan Complex has included manystudies based on geologic mapping, with much new information published in
Wakabayashi (2015, cited in manuscript). Much of the primary field observations
presented in this paper are derived from Wakabayashi (2015) but there are newobservations presented in this paper as well, in addition to review of work andconcepts not previously presented.
Competing interests
The author declares that he has no competing interest.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Received: 24 November 2016 Accepted: 6 June 2017

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