Contrasting Settings of Serpentinite Bodies, San Francisco Bay Area, California: Derivation from the Subducting Plate vs. Mantle Hanging Wall?

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1 International Geology Review, Vol. 46, 2004, p 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 WAKABAYASHI 1 Geologic Consultant, 1329 Sheridan Lane, Hayward, California 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 1 wako@tdl.com 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 relationships 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 /04/774/ $

2 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

3 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

4 1106 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 minerals 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.

5 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.

6 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-

7 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 ) 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 Hunters 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

8 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 smallscale 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,

9 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

10 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

11 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

12 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 intra- Franciscan serpentinites may have been derived by

13 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,

14 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.

15 CONTRASTING SETTINGS OF SERPENTINITE 1117 I thank Berlogar Geotechnical Consultants, Inc. (BGC), for permission to examine and photograph their drill core. My field study of the southern Hayward Hills serpentinite locality was part of consulting 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. Acknowledgments Bailey, E. H., Blake, M. C., Jr., and Jones, D. L., 1970, Onland Mesozoic ocean crust in California Coast Ranges: United States Geological Survey Professional Paper 700-C, p Blake, M. C., Jr., Howell, D. G., and Jayko, A. S., 1984, Tectonostratigraphic terranes of the San Francisco Bay Region in Blake, M. C., Jr., ed., Franciscan geology of Northern California: Pacific Section Society of Economic Paleontologists and Mineralogists, v. 43, p Bonilla, M. G., 1971, Preliminary geologic map of the San Francisco South quadrangle and part of the Hunters Point quadrangle: U. S. Geological Survey Miscellaneous Field Studies Map MF-574. Cloos, M., 1984, Flow mélanges and the structural evolution of accretionary wedges, in Raymond, L., ed., Mélanges: Their nature, origin, and significance: Geological Society of America Special Paper 198, p Coleman, R. G., 2000, Prospecting for ophiolites along the California continental margin, in Dilek, Y. D., Moores, E. M., Elthon, D., and Nicolas, A., eds., Ophiolites and oceanic crust: New insights from field studies and the Ocean Drilling Program: Geological Society of America Special Paper 349, p Dickinson, W. R., 1966, Table Mountain serpentinite extrusion in California Coast Ranges: Geological Society of America Bulletin, v. 77, p Dickinson, W. R., 1970, Relations of andesites, granites, and derivative sandstones to arc-trench tectonics: Reviews of Geophysics and Space Physics, v. 8, p Ernst, W. G., 1970, Tectonic contact between the Franciscan mélange and the Great Valley Sequence, crustal expression of a Late Mesozoic Benioff Zone: Journal of Geophysical Research, v. 75, p Fryer, P., Lockwood, J. P., Becker, N., Phipps, S., and Todd, C. S., 2000, Significance of serpentine mud volcanism in convergent margins, in Dilek, Y., Moores, E. M., Elthon, D., and Nicolas, A., eds., Ophiolites and oceanic crust: New insights from field studies and the Ocean Drilling Program: Geological Society of America Special Paper 349, p Hamilton, W. B., 1969, Mesozoic California and underflow of the Pacific mantle: Geological Society of America Bulletin, v. 80, p Hopson, C. A., Mattinson, J. M., and Pessagno, E. A., Jr., 1981, The Coast Range ophiolite, western California, in Ernst, W. G., ed. The geotectonic development of

16 1118 JOHN WAKABAYASHI California: Englewood Cliffs, NJ, Prentice-Hall, Rubey Volume 1, p Jachens, R. C., and Roberts, C. W., 1993, Aeromagnetic map of the San Francisco Bay Area: U.S. Geological Survey, Geophysical Investigations Map GP-1007, scale 1: 286,500. Karson, J. A., 1999, Geological investigation of a lineated massif at the Kane transformation fault: Implications for oceanic core complexes, in White, R. S., Hardman, R. F. P., Watts, A. B., and Whitmarsh, R. B., eds., Response of the Earth s lithosphere to extension: Philosophical Transactions, Royal Society of London: Mathematical, Physical, and Engineering Sciences, v. 357, p Lockwood, J. P., 1972, Possible mechanisms for the emplacement of alpine peridotites: Geological Society of America Memoir 132, p Macpherson, G. J., Phipps, S. P., and Grossman, J. N., 1990, Diverse sources for igneous blocks in Franciscan mélanges, California Coast Ranges: Journal of Geology, v. 98, p McLaughlin, R. J., Blake, M. C., Jr., Griscom, A., Blome, C. D., and Murchey, B., 1988, Tectonics of formation, translation and dispersal of the Coast Range ophiolite of California: Tectonics, v. 7, p Page, B. M., 1981, The southern Coast Ranges, in Ernst, W. G., ed., The geotectonic development of California: Englewood Cliffs, NJ, Prentice-Hall, Rubey Volume 1, p Phipps, S. P., 1984, Ophiolitic olistostromes in the basal Great Valley sequence, Napa County, northern California Coast Ranges, in Raymond, L., ed., Mélanges: Their nature, origin, and significance: Geological Society of America Special Paper 198, p Schlocker, J., 1974, Geology of the San Francisco North quadrangle, California: U. S. Geological Survey Professional Paper 782, 109 p. Tucholke, B. E., Lin, J., and Kleinrock, M. C., 1998, Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid-Atlantic Ridge: Journal of Geophysical Research, v. 103, p Wagner, D. L., Bortugno, E. J., and McJunkin, R. D., 1990, Geologic map of the San Francisco San Jose quadrangle: California Division of Mines and Geology Regional Geologic Map Series, Map 5A, scale: 1:250,000. Wahrhaftig, C., 1984, A streetcar to subduction: Washington, DC, American Geophysical Union, 76 p. Wakabayashi, J., 1990, Counterclockwise P-T-t paths from amphibolites, Franciscan Complex, California: Relics from the early stages of subduction zone metamorphism: Journal of Geology, v. 98, p Wakabayashi, J., 1992, Nappes, tectonics of oblique plate convergence, and metamorphic evolution related to 140 million years of continuous subduction, Franciscan Complex, California: Journal of Geology, v. 100, p Wakabayashi, J., 1999a, Subduction and the rock record: Concepts developed in the Franciscan Complex, California, in Sloan, D., Moores, E. M., and Stout, D., eds., Classic Cordilleran concepts: A view from California: Geological Society of America Special Paper 338, p Wakabayashi, J., 1999b, Distribution of displacement on, and evolution of, a young transform fault system: The northern San Andreas fault system, California: Tectonics, v. 18, p