Middle Miocene Paleotemperature Anomalies Within the Franciscan Complex of Northern California: Thermo-Tectonic Responses Near the Mendocino Triple Junction

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Middle Miocene paleotemperature anomalies within the Franciscan Complex of northern California: Thermo-tectonic responses near the Mendocino triple junction

Michael B. Underwood* Kevin L. Shelton } Department of Geological Sciences, University of Missouri,Columbia, Missouri 65211 Robert J. McLaughlin U.S. Geological Survey, 345 Middlefield Road, M.S. 975, Menlo Park, California 94025 Matthew M. Laughland Mobil Research and Development, P.O. Box 650232, Dallas, Texas 75265-0232 Richard M. Solomon Geotechnology Inc., 2258 Grissom Drive, St. Louis, Missouri 63146

ABSTRACT ature metalliferous fluids in the King Range ter, 1989). The first direct interaction between the terrane could have advected either from a site Franciscan-Cascadia subduction front and seg- This study documents three localities in the of ridge-trench interaction north of the Men- ments of the East Pacific Rise (Pacific-Farallon Franciscan accretionary complex of northern docino fracture zone or from a “slabless win- boundary) evidently occurred ca. 30 Ma. That col- California, now adjacent to the San Andreas dow” in the wake of the northward migrating lision was followed by development of a dextral fault, that were overprinted thermally between Mendocino triple junction. transform boundary, with the Mendocino triple 13.9 and 12.2 Ma: Point Delgada–Shelter Cove A separate paradox involves the amount of junction to the north (transform-transform-trench) (King Range terrane); Bolinas Ridge (San Quaternary offset of Franciscan basement and the Rivera triple junction to the south (trans- Bruno Mountain terrane); and Mount San rocks near Shelter Cove by on-land faults that form-ridge-trench). Subsequent migration of the Bruno (San Bruno Mountain terrane). Vein as- some regard as the main active trace of the two triple junctions resulted in progressive length- semblages of quartz, carbonate, sulfide miner- San Andreas plate boundary. Contouring of ening of the San Andreas fault system (Fig. 1A). als, and adularia were precipitated locally in vitrinite reflectance values to the north of an Lithosphere-scale mechanical and thermal re- highly fractured wall rock. Vitrinite reflect- area affected by A.D. 1906 surface rupture sponses to migration of the Mendocino triple

ance (Rm) values and illite crystallinity de- indicates that the maximum dextral offset junction have been discussed thoroughly by oth- crease away from the zones of metalliferous within the interior of the King Range terrane ers (e.g., Dickinson and Snyder, 1979a, 1979b; veins, where peak wall-rock temperatures, as is only 2.5 km. If this fault extends inland, and Furlong, 1984; Furlong et al., 1989). One of the

determined from Rm, were as high as 315 °C. if it has been accommodating most of the most important concepts in this regard is the de- The δ18O values of quartz and calcite indicate strike-slip component of San Andreas offset at velopment of a “slabless window” (Dickinson that two separate types of fluid contributed to a rate of 3–4 cm/yr, then its activity began only and Snyder, 1979a; Zandt and Furlong, 1982). vein precipitation. Higher δ18O fluids pro- 83–62 ka. This interpretation would also mean This phenomenon is thought to occur in the wake duced widespread quartz and calcite veins that that a longer term trace of the San Andreas of a migrating triple junction as attenuated lithos- are typical of the regional paleothermal fault must be nearby, either offshore or along phere, in what had been the overriding plate edge, regime. The widespread veins are by-products the northeast boundary of the King Range is displaced by strike-slip motion. As this dis- of heat conduction and diffuse fluid flow dur- terrane. An offshore fault trace would be con- placement progresses, hot asthenosphere rises up ing zeolite and prehnite-pumpellyite–grade sistent with peak heating of King Range strata to unusually shallow depths beneath the attenu- metamorphism, and we interpret their paleo- north of the Mendocino triple junction. Con- ated plate (i.e., 20–30 km). Crustal-scale mani- fluids to have evolved through dehydration re- versely, shifting the fault to the east would be festations of these adjustments can be imaged actions and/or extensive isotopic exchange compatible with a slabless window heat source using a variety of geophysical techniques (e.g., with accreted Franciscan rocks. Lower δ18O and long-distance northward translation of Griscom and Jachens, 1989; Benz et al., 1992; fluids, in contrast, evolved from relatively high the King Range terrane after peak heating. Verdonck and Zandt, 1994; Beaudoin et al., 1996). temperature exchange between seawater (or Other responses to the unusually shallow asthen- meteoric water) and basaltic and/or sedimen- INTRODUCTION osphere include anomalies in near-surface heat tary host rocks; focused flow of those fluids re- flow (Lachenbruch and Sass, 1980), anomalous sulted in local deposition of the metalliferous One of the most significant episodes in the dy- volcanism (Johnson and O’Neil, 1984; Fox et al., veins. Heat sources for the three paleothermal namic geologic history of coastal California has 1985; Cole and Basu, 1995; Dickinson, 1997), anomalies remain uncertain and may have been the transformation from subduction (Kula- and hydrothermal activity (Lambert and Epstein, been unrelated to one another. Higher temper- Farallon and North American plate convergence) 1992; Donnelly-Nolan et al., 1993). to transform motion between the Pacific and North Perturbations in the thermal structure of sub- *E-mail: [email protected]. American plates (Engebretson et al., 1985; Atwa- duction zones also can be caused by collision of

GSA Bulletin; October 1999; v. 111; no. 10; p. 1448–1467; 18 figures.

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which is shown on most maps to extend into the King Range terrane from a coastline intersection near Shelter Cove (e.g., Curray and Nason, 1967; Brown and Wolfe, 1972; Kelsey and Carver, 1988). Is this active fault responsible for a significant amount of the long-term horizontal and/or vertical offset of King Range basement rocks since the time of peak heating, or does it represent a relatively recent and minor structural adjustment of the triple-junction region?

METHODS FOR EVALUATING PALEOTEMPERATURE AND PALEOFLUIDS

Vitrinite Reflectance

Vitrinite reflectance is one common method of measuring thermal maturity in sedimentary rocks (Dow, 1977). Our measurements utilized dispersed organic particles that were concentrated Figure 1. (A) Regional plate tectonic framework of the Mendocino triple junction and vicinity. from mudstones, shales, and argillites via acid (B) Enlarged index map of northern and central California showing geographic locations of the maceration (Laughland and Underwood, 1993). King Range, Bolinas Ridge, Mount San Bruno, Clear Lake, and major faults of the San Andreas We calculated values of mean vitrinite reflectance (R ) after measuring (in oil) ~50 individual dextral system. m randomly oriented particles per specimen. Sev- eral approaches allow one to convert these values mid-ocean ridge segments with the trench, or a of each peak heating event. Such restorations for to estimates of absolute paleotemperature (Mid- close approach (DeLong et al., 1979; Dumitru, northern California are complicated by uncertain- dleton, 1982; Morrow and Issler, 1993). Among 1991a). Examples of present-day ridge-trench ties in the distances of dextral offset along all of the time-independent methods, the Barker (1988) interaction include the intersection between the the subsidiary faults within the San Andreas sys- regression, where temperature T (°C) = 148 +

Chile Rise and the Chile Trench (Cande et al., tem (Fox et al., 1985; McLaughlin et al., 1996). 104[ln(Rm)] yields the lowest temperature esti- 1987) and the eastern edge of the Solomon Sea, The King Range terrane (Fig. 2) plays a partic- mate for a given value of Rm. Models based on where the Woodlark Rift intersects the New ularly important role in Cordilleran geologic his- chemical kinetics (e.g., Sweeney and Burnham, Britain Trench (Crook and Taylor, 1994). Inferred tory because its present-day location is just south 1990) match the curve of Barker (1988) quite ancient analogues include southern Alaska (Sis- of the Mendocino triple junction (McLaughlin et closely, provided the effective duration of heating son and Pavlis, 1993) and the Shimanto Belt of al., 1982, 1994; Clarke, 1992). Given this unique is limited to 1 m.y. or less. Thus, for the purpose Japan (Underwood et al., 1992; Sakaguchi, 1996). locale, scientists need to ascertain whether the of first-order estimates of peak wall-rock temper- Criteria to discriminate the geologic effects of thermo-tectonic evolution of the King Range is ature, unbiased by speculative reconstructions of ridge-trench collision from those of gradual triple- exceptional in some respect or, conversely, em- burial history, we prefer the equation of Barker junction migration are not obvious, but the most blematic of trench-transform-transform triple (1988). The possibility of liquid-dominated hy- defensible interpretations are based on orderly, junctions. Several unresolved questions need to drothermal alteration could not be eliminated a regional-scale time-space shifts in thermo-tec- be answered within this context. Hydrothermal priori in our study, so we also employed Barker’s tonic anomalies (Sisson et al., 1994). As a work- circulation within the King Range terrane re- (1983) equation for hydrothermal systems, where

ing hypothesis for northern California, one might sulted in unusual types of vein mineralization T (°C) = 122 + 146[ln(Rm)]. Scatter about both predict a series of thermal overprints near the San during middle Miocene time (McLaughlin et al., regression curves is roughly ±30 °C for a given

Andreas fault; some might be more subtle, per- 1985). Farther south along the San Andreas fault value of Rm, and we round estimates of paleo- haps, than anomalous volcanic centers, but all zone, Franciscan rocks at Bolinas Ridge and temperature to the nearest 5 °C. should shift successively northward in partner- Mount San Bruno (Fig. 1B) likewise display evi- ship with the migrating Mendocino triple junc- dence of unusual heating and vein mineralization X-ray Diffraction tion. One way to substantiate such a pattern is during middle to late Miocene time. Were the hy- through analyses of thermal alteration products in drothermal fluids at all three sites related geneti- Another widely used method for characteriz- basement rocks of the Coast Ranges, including cally? Were all three successions affected by a ing levels of diagenesis and incipient metamor- such parameters as organic-matter alteration, single, localized slabless window overprint, and phism in sedimentary rocks is the measurement clay-mineral diagenesis, pressure-temperature then dispersed, or were the heating events dis- of illite crystallinity index (CI) by X-ray diffrac- (P-T) conditions of vein-producing fluids, geo- connected? If the events were synchronous, but tion (Kisch, 1990; Robinson et al., 1990). This in- chemical sources of vein minerals, and timing of separated spatially, can the anomalies be attrib- dex is based on the width of the ~10 Å illite peak peak heating or hydrothermal vein precipitation. uted to similar ridge-trench processes at three dif- (001 reflection) at half height, using oriented To be successful in this regard, the geologic do- ferent locations north and/or south of the triple <2 µm size fractions and ethylene glycol satura- mains under investigation must be restored to junction? A related debate involves the north- tion (Underwood et al., 1993). Unlike vitrinite re- their correct paleolatitudinal positions at the time ernmost onland trace of the San Andreas fault, flectance, illite crystal growth is affected by

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fluid salinity during vein formation. Calibration

was accomplished using pure water and pure CO2 standards. Fluid-inclusion homogenization tem-

peratures (Th) and last ice-melting temperatures (Tm) have standard errors of ±1.0 °C and ±0.2 °C, respectively. Salinity estimates (equivalent wt% NaCl) for simple water-rich fluid inclusions are based on freezing-point depression in the system

H2O-NaCl (Potter et al., 1978). Many calcite veins in Franciscan wall rocks are sheared with fi- brous habits. We regard quartz as superior to calcite as a host mineral of inclusions because inclusion stretching is less likely during post- emplacement deformation. New data reported

herein include 98 Th and 66 Tm values from 13 samples of quartz and calcite from the King Range study area (Solomon, 1994). These data

complement 97 Th and 64 Tm values determined by McLaughlin et al. (1985) for quartz, sulfide, and carbonate vein minerals at Point Delgada. All of the fluid inclusions observed and measured in this study are primary two-phase aqueous inclusions without daughter minerals. They vary from <5 to 25 µm in length, and vapor bubbles consti- tute ~5–20 vol% of each. The inclusions homog- enize to the liquid phase and contain no detectable

CO2, as determined by crushing under oil.

Stable Isotopes

Stable isotopes help elucidate the origin and history of hydrothermal fluids, their dissolved constituents, and fluid-rock interactions in many types of settings (e.g., Burkhard and Kerrich, Figure 2. Simplified geologic map of the King Range and vicinity, northern California, show- 1988; Bebout and Barton, 1989; Kirschner et al., ing tectonostratigraphic terranes of the Franciscan Complex and Miocene–Pliocene strata of 1995; O’Hara et al., 1997). We measured the the Wildcat Group. Localities and ages for paleontologic samples and radiometrically dated oxygen and carbon isotope compositions of vein- rocks are from McLaughlin et al. (1994). filling quartz and calcite and the sulfur isotope compositions of hydrothermal sulfides. The techniques of isotopic analysis of C and O in calcite many external and internal variables, including et al., 1993). According to Guidotti and Sassi and O in quartz followed those of McCrea (1950) maximum temperature, heating time, protolith (1986), empirical boundaries for three metamor- and Clayton and Mayeda (1963), respectively.

geochemistry, composition and migration history phic facies series are as follows: bo < 9.000 Å = Sulfide samples were analyzed for their sulfur of pore fluid, and rock deformation (Kisch, 1987; low-pressure facies series; 9.000 Å < bo < 9.040 Å isotope values after they were oxidized to SO2 by Robinson et al., 1990), so caution is warranted = intermediate-pressure facies series; and bo > heating to 1200 °C under vacuum with excess when interpreting CI data as an absolute geother- 9.040 Å = high-pressure facies series. At lower cupric oxide in the presence of metallic copper mometer. Two thresholds of thermal alteration levels of thermal maturity (i.e., <300 °C), interpre- (Grinenko, 1962). Data are reported in conven- have been defined by CI values (Blenkinsop, tations can be problematic if there is interference tional delta (δ) notation as per mil (‰) deviations 1988): diagenesis to anchizone (transition into among multiple populations of authigenic illite relative to the Peedee belemnite standard (PDB) lowermost greenschist facies) = 0.42 ∆°2θ and and detrital white mica, but meaningful compar- for carbon; the Vienna mean ocean water stan- anchizone to epizone (lowermost greenschist isons have been made among many lower rank dard (SMOW) for oxygen; and the Cañon Diablo ∆ θ facies) = 0.25 °2 . Comparable Rm values for successions (e.g., Padan et al., 1982; Cloos, 1983). troilite standard (CDT) for sulfur. Standard error these thresholds are 2.6% and 4.5%, respectively for each analysis is ~± 0.1‰, using the Univer- (Underwood et al., 1993). Fluid Inclusions sity of Missouri’s automated Finnigan MAT We also used X-ray diffraction on random pow- Delta E mass spectrometer. Calculations of the

der mounts to calculate the bo lattice dimension of All of the microthermometric measurements equilibrium isotopic compositions of paleowaters illite-mica as a measure of metamorphism. Di- reported herein were made on doubly polished utilized the equations of O’Neil et al. (1969) for

mensions of the bo unit cell were calculated from thin sections of quartz and calcite veins using a calcite and Clayton et al. (1972) for quartz. When- the corrected d value of the dioctahedral (060) Fluid Inc. gas-flow heating and freezing stage. ever possible, temperatures utilized in the calcu- peak, using the quartz (211) reflection (d value The purpose of these measurements was to deter- lations are based on fluid-inclusion homogeniza- = 1.5418 Å) as an internal standard (Underwood mine estimates of minimum fluid temperature and tion data; otherwise, we assumed that the veins

1450 Geological Society of America Bulletin, October 1999

were precipitated at temperatures approximately volcanic rocks are surrounded by a penetratively eral high-angle tension faults, where vein min- equal to the maximum temperature of the enclos- sheared mudstone-matrix melange with phacoids erals include adularia and base-metal sulfides

ing or nearby wall rock (derived from Rm data), of sandstone, conglomerate, chert, greenschist fa- (McLaughlin et al., 1985). following Barker (1988). cies metaigneous rocks, limestone, and rare glau- The King Peak subterrane is composed mostly cophane schist. Zones of relatively coherent, but of argillite and thinly bedded sandstone and silt- K-Ar Dating complexly folded sandstone turbidites and sandstone turbidites, with minor chert, siliceous mudstone-rich debris-flow deposits are structurally in- stone, limestone, and basalt. Complex fold geom- Methods used for K-Ar age determinations terleaved with slivers of melange. The epiclastic etries typify the structural style, and three phases were described by Russell et al. (1989). Adularia sedimentary rocks, including the pelitic melange of folding have been mapped (Beutner et al., was segregated from other vein minerals and wall- matrix, remain undated. The sandstone and large 1980). Penetrative shear fabrics occur locally and rock materials using heavy liquids, hand-picking, slabs of volcanic rocks are metamorphosed to are especially common near the subterrane’s mar- and magnetic separation, then treated with HF and prehnite-pumpellyite grade (McLaughlin et al., gins. Laumontite is common near the eastern

HNO3. Potassium was analyzed by flame photom- 1994), but the absolute timing of this metamor- margin, occurring as a vein mineral and as dis- etry using a lithium metaborate fusion technique, phism is unknown. Veins of calcite and quartz are seminated cement in sandstone. Calcite veins are and argon analyses were completed by isotope common throughout the subterrane. In addition, ubiquitous, but quartz veins are more abundant dilution mass spectrometry. Constants used in the hydrothermal alteration is pronounced along sev- near the contact between the Point Delgada and age calculations are as recommended by Steiger and Jager (1977), and precision is given at the ±1σ level. Because of uncertainties regarding argon systematics for adularia, we view the K-Ar dates as representative of vein cooling.

KING RANGE STUDY AREA

Geologic Background

The King Range terrane (Fig. 2) represents the westernmost and youngest tectono-stratigraphic component of the Franciscan Complex in northern California (Blake et al., 1985). The composite King Range terrane includes two subterranes, Point Delgada and King Peak; these rocks structurally overlie Eocene melange of the adjacent Franciscan Coastal terrane to the east along a southwest-dipping thrust fault (Fig. 2). The western edge of the terrane is cryptic because of cover by the Pacific Ocean. The terrane, however, appears to be juxtaposed against rocks of the Vizcaino block along offshore segments of the San Andreas fault zone (McCulloch, 1987, 1988). The Vizcaino block is a structurally complicated composite terrane, and its lithostratigra- phy and crustal structure remain poorly documented (McLaughlin et al., 1994; Beaudoin et al., 1996). Fragments of the King Range terrane, for example, could be imbricated with the eastern edge of the Vizcaino block. The Point Delgada subterrane is exposed along seacliffs and wave-cut benches beneath landslide deposits and marine terraces that surround the vil- lage of Shelter Cove (Fig. 3). Substantial portions of the subterrane consist of coherent volcanic rocks (pillow basalt, basalt breccia, and tuff) that are intruded locally by diabase dikes. Geochemi- cal data from the basaltic rocks indicate a mid- ocean ridge to off-ridge (seamount or plateau) Figure 3. Detailed map of Shelter Cove and vicinity (see Fig. 2 for regional location), showing origin (McLaughlin et al., 1994). Lenses of sili- sample localities for vitrinite reflectance (open dots) and apatite fission-track analyses (FT, closed ceous mudstone within the volcanic succession dots). Numbers in boxes are values of mean random vitrinite reflectance (Rm, %) from Laugh- contain Late Cretaceous (Coniacian to Campa- land (1991). Numbers in ovals are fission-track cooling ages (Ma) from Dumitru (1991b). PDS— nian) radiolaria; these microfossils provide the Point Delgada subterrane of the composite King Range terrane. Terrane boundaries are from only depositional ages for the subterrane. The Blake et al. (1985). Position of the on-land trace of the San Andreas(?) fault is from Brown (1995).

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King Peak subterranes. The depositional age of features near Shelter Cove have uncovered new 2.8% (Fig. 3). The highest values from King Peak the King Peak subterrane is middle Miocene age, evidence for strike-slip Quaternary faulting along strata overlap the lowest values from Point Del- based on radiolaria, planktonic foraminifers, and the 1906 surface ruptures (Prentice et al., 1999). gada strata, and there are no sharp discontinuities

benthonic foraminifers (McLaughlin et al., 1982, These workers also traced possible Quaternary in Rm values at the subterrane boundary. Thermal 1994). Blocks of basalt are similar geochemically faults diagonally into the interior of the King maturity decreases in all directions away from to both oceanic-island tholeiites and oceanic-is- Range, through a series of lineaments, scarps, the Point Delgada headlands, in a relatively land alkali basalts, so they probably formed dur- and shutter ridges. The long-term significance of smooth concentric pattern (Fig. 5). This regional- ing seamount volcanism (McLaughlin et al., this young fault, and its fundamental role as the scale spatial pattern, moreover, cuts obliquely 1994). Basalt sampled from a shear zone along actual Pacific–North America plate boundary, across the fabrics of most folds and faults that the northern margin of the subterrane (Fig. 2) remain open to debate (McLaughlin et al., 1994; Beutner et al. (1980) mapped within the subter- yielded a whole-rock K-Ar age of 16.2 ± 0.5 Ma Brown, 1995). rane. The only mapped structures that are aligned (McLaughlin et al., 1982). Deposition of the subparallel to the isoreflectance contours are pelagic and hemipelagic sediments, therefore, Vitrinite Reflectance some second-generation folds in the northwest

must have occurred shortly after the basaltic mag- part of the King Peak subterrane. The lowest Rm mas erupted and cooled. The data of Laughland (1991) show that the values from the King Peak subterrane

The composite King Range terrane is an in- Point Delgada subterrane ranges in Rm from (0.8%–1.3%, equivalent to 120–170 °C) occur in triguing part of the Franciscan Complex because 2.3% to 4.9% (Figs. 3 and 4). These values corre- rocks near the sheared eastern terrane boundary of its young age and local occurrences of quartz- spond to paleotemperatures of 230–315 °C (sed- (Fig. 5) and overlap with the highest values from sulfide vein mineralization. Sulfide vein minerals imentary-burial curve of Barker, 1988) or the adjacent Coastal terrane (Underwood et al., include pyrite, sphalerite, argentiferous galena, 240–350 °C (hydrothermal-alteration curve of 1988; Laughland, 1991).

and chalcopyrite. These vein fillings occur in as- Barker, 1983). The highest Rm values (4.0%– sociation with at least two generations of quartz 4.9%) cluster near the boundary between the X-ray Diffraction and carbonate minerals (calcite, dolomite, anker- Point Delgada subterrane and King Peak subter- ite, rhodochrosite, and siderite), plus fluorite, rane at Telegraph Creek, where impressive metal- Contours of illite crystallinity index for rocks adularia, sericite, and kaolinite. Fluid-inclusion liferous veins are concentrated (McLaughlin et al., of the composite King Range terrane mimic the homogenization temperatures (McLaughlin et 1985). There is a second location of higher values pattern displayed by vitrinite-reflectance data al., 1985), uncorrected for pressure effects, indi- (3.1%) near the center of the headlands (Fig. 3). (Fig. 6). Illite is more crystalline near Point Del- cate that some of the early quartz veins precipi- Values from Point Delgada strata outside of these gada and becomes increasingly less crystalline tated at temperatures hotter than 270 °C. Previous localized zones of metalliferous veining range toward the Coastal terrane boundary. Regression

analyses of vitrinite reflectance showed that the from 2.3% to 2.8%. analysis of Rm-CI data pairs yields a reference entire King Range terrane may have been affected Rm values from pelitic rocks of the King Peak curve that passes near the established boundaries by a thermal overprint, with paleotemperatures subterrane near Shelter Cove range from 1.3% to of the anchizone (Fig. 7A). Thus, for the most decreasing in all directions away from the locations of quartz-sulfide alteration on the Point Del- gada headlands (Underwood et al., 1988). Cogenetic quartz-sulfide vein systems, with identical orientations and mineral assemblages, occur on both sides of a northwest-striking high- angle fault that marks the boundary between the Point Delgada and King Peak subterranes (Fig. 3). Mineralization at that site, near the mouth of Tele- graph Creek, occurred during middle Miocene time based on K-Ar dating of two adularia specimens (13.9 ± 0.4 and 13.6 ± 0.4 Ma) that were extracted from carbonate-quartz veins. The veins also contain chalcopyrite, argentiferous galena, sphalerite, and pyrite (McLaughlin et al., 1985). These radiometric dates are only slightly younger than the middle Miocene biostratigraphic age of the King Peak strata, and they establish the minimum age of subterrane amalgamation. North of Point Arena (Fig. 1), the main trace of the San Andreas fault zone has been mapped almost entirely in the offshore realm of the continental shelf (McCulloch, 1987). The 1906 San Francisco earthquake event, however, triggered geomorphic expressions of surface faulting as far north as Shelter Cove. Descriptions of the

original field interpretations were published by Figure 4. Histograms of mean random vitrinite reflectance values (Rm) from the Point Del- Brown (1995), and recent trenching investiga- gada subterrane, King Peak subterrane, Coastal terrane, Bolinas Ridge, and Mount San Bruno.

tions and detailed analyses of geomorphologic Paleotemperature scale (°C) is based on the Rm –°C regression analysis of Barker (1988).

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intermediate-pressure conditions. Similar bo values have been obtained from the Helvetic zone of the Swiss Alps and the Caledonides (Padan et al.,

1982). Elsewhere in California, bo values of illitic material from the base of the Great Valley Group vary between 9.010 and 9.025 Å, whereas values from low-temperature melange matrix of the Franciscan Central terrane, and from low-temperature, high-pressure metapelites of the Fran- ciscan Eastern belt and Diablo Range, generally are between 9.030 and 9.050 Å (Cloos, 1983). Based on these comparisons, we see no evidence to suggest that metamorphic pressures were unusually high during burial of the composite King Range terrane.

Veins

A complete understanding of the regional and local thermal history of the composite King Range terrane cannot be achieved without con- sidering the role of fluid migration and vein precipitation. Veins containing calcite, quartz, and quartz-calcite intergrowths occur throughout the King Range, although quartz is more common near Point Delgada. The typical veins vary from <5 mm to a few centimeters in width. We made a concerted effort to sample and measure both common and unusual vein orientations. Most veins are aligned at high angles to thin interbeds of sandstone but terminate or change orientation Figure 5. Isoreflectance map of the composite King Range terrane, northern California, mod- as they pass through interbeds of argillite; con- jugate sets of through-going vein-filled frac- ified from Laughland (1991). Open circles indicate sample localities. Vitrinite reflectance (Rm) data from immediately adjacent rocks of the Coastal terrane (solid dots; values in brackets) are tures are relatively rare. Small veinlets of calcite more erratic and not contoured (see Underwood et al., 1988, for additional data). within the King Peak subterrane are all but im- possible to measure because they form web-like patterns within the wall rocks and display no part, thermal alteration of phyllosilicates emu- (1994) also found that CI values for specimens of sense of systematic planar orientation. In con- lates changes displayed by organic constituents wall rock in direct contact with quartz and calcite trast, robust quartz-sulfide-carbonate veins with- of the same rocks. veins are lower than expected (more crystalline) in the Point Delgada subterrane are 1–15 cm in

Two types of anomalous Rm-CI pairs are su- over the entire range of Rm data (Fig. 7C). This width and strike consistently northeast-south- perimposed on the regional pattern. Rock speci- type of response is probably caused by millime- west (McLaughlin et al., 1985). Related zones of mens without veins were extracted from larger ter-scale gradients in the availability of cations alteration extend as much as 60 m into sedimen- scale zones of hydrothermal alteration, such as the (especially potassium) away from the boundaries tary wall rock but less than 5 m into volcanic contact between the Point Delgada and King Peak of fractured fluid conduits. With increasing dis- wall rock.

subterranes along Telegraph Creek; CI values for tance from a fluid-filled fracture, conduction of The Th values for primary fluid inclusions vary those rocks are higher than expected (i.e., less heat into a lithified pelitic rock should be impeded, from 131 to 252 °C in regional calcite veins of

crystalline) for a given value of Rm (Fig. 7B). but less so than the transfer of cations through the the King Peak subterrane (mean 184 °C); calcu- These unequal responses of vitrinite and clay rock’s diminished intergranular porosity. Con- lated salinities from their companion Tm values minerals to heat conduction through nonveined versely, with appropriate fluid chemistries and range from 3.4 to 7.6 equiv. wt% NaCl

wall rocks have been reported elsewhere where flow rates, illite growth should be enhanced where (Solomon, 1994). The maximum Th values for in- unusually rapid heating by fluid-dominated hy- wall rocks sustain contact with migrating fluids. dividual King Peak samples are similar to paleo-

drothermal circulation or magmatic intrusions has Calculated bo lattice dimensions for illite-mica temperatures derived from Rm data for nearby affected the rocks (e.g., Smart and Clayton, 1985; from the Point Delgada subterrane are consistent host rocks. The Th values for fluid inclusions in Velde and Lanson, 1993). The anomalies, on a with relatively small amounts of phengite substi- early-stage quartz and calcite within the Point scale of several centimeters, can be attributed to tution and are within the field established by Delgada subterrane (McLaughlin et al., 1985; the slower reaction rates of illite crystallization as Guidotti and Sassi (1986) for the intermediate- Solomon, 1994) range from 209 to 287 °C (mean

compared to rates of change in vitrinite at the pressure facies series (Fig. 8). One value from the = 253 °C), whereas Th values for late-stage quartz same temperature (Barker et al., 1986). Illite crys- King Peak data falls on the high-pressure facies veins are slightly lower, from 194 to 266 °C tal growth, moreover, can be more erratic because series boundary of 9.040 Å, but the remainder of (mean = 226 °C). Fluid salinities for the quartz- of variations in pore-fluid chemistry. Solomon King Peak subterrane data are also indicative of calcite veins range from 1.4 to 4.6 equiv. wt%

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NaCl. The Th values for fluorite veins in the Point Delgada subterrane average 231 °C, and their cor- responding fluid salinities are near 7.0 equiv. wt% NaCl. Fluid pressures at the time of vein precipitation remain unknown in detail, but McLaughlin et al. (1985) inferred that the Point Delgada vein system formed at depths greater than 400 m. Even if

uncorrected for pressure effects, most of the Th values from veins in Point Delgada subterrane are only slightly lower than the temperature esti-

mates of 240–315 °C from Rm data (Fig. 4). These differences of 20–30 °C between Th values (which represent fluid-temperature minima) and

wall-rock temperatures estimated from Rm indicate that the pressure corrections to the Th values are modest. Fluid pressures at the time of quartz- sulfide vein formation probably were only 200– 400 bar (following Potter, 1977). We suggest, therefore, that metalliferous veining closely post- dated peak heating of the sedimentary host rocks. Assuming hydrostatic fluid-pressure conditions at the time of fluid entrapment, the maximum depths of vein formation would have been 2–4 km. If pressure conditions approached lithostatic values during hydrofracturing and fluid flow, then vein formation may have been as shallow as 0.8–1.5 km, assuming bulk rock densities of ~2.65 gm/cm3.

Stable Isotope Data

We determined δ34S values of sulfide minerals from one 15-cm-wide quartz vein in the Point Delgada subterrane that contains massive lay- Figure 6. Zonation of illite crystallinity values (∆°2θ) from the composite King Range terrane ered sulfides; the values for specific minerals and margins of the adjacent Coastal terrane, northern California (modified from Solomon, are: pyrite, 2.4‰; sphalerite, 0.1‰; galena, 1994). Bold line represents terrane boundary. Solid dots indicate sample localities. Boundaries –3.3‰. The sphalerite-galena pair yields an ap- for the anchizone (transition to greenschist facies metamorphism) are from Blenkinsop (1988). parent sulfur-isotope equilibrium temperature of 210 ± 35 °C (Ohmoto and Rye, 1979). This temperature is in agreement with the paragenetic and 22‰, whereas the δ18O values from Point creases away from Point Delgada (Fig. 5). Thus, constraints of McLaughlin et al. (1985), who Delgada quartz veins range more broadly, from we interpret the coincidence of increasing vein- suggested that sulfide deposition occurred be- 9.0‰ to 18.2‰ (Fig. 10). mineral δ18O values and decreasing wall-rock pa- δ18 tween early-stage (mean Th = 253 °C) and late- Spatial variations in the O values of both leotemperatures to indicate that spatial variations stage (mean Th = 226 °C) quartz deposition. quartz and calcite veins are easier to recognize in fluid temperature were responsible for the gra- Carbon and oxygen isotope compositions of when data are projected onto a coastline-parallel dient of oxygen isotope compositions. quartz and calcite veins show significant differ- profile (Fig. 11). All of the lower δ18O values (i.e., At any given distance from Point Delgada, ences between the Point Delgada and King Peak <15‰) come from sites that include metalliferous δ18O values of quartz tend to be higher than δ18O subterranes. The δ18O values for calcite veins veins; those are the same sites that produced the values of nearby calcite (Fig. 11). This trend may

from the King Peak subterrane cluster between highest Rm and Th values. Outside of the metallif- reflect an approach toward isotopic equilibrium 15‰ and 20‰, and most of the companion δ13C erous vein sets, quartz and calcite display trends of with isotopically similar fluids. In addition, the values are between +2‰ and –8‰ (Fig. 9). In increasing δ18O with distance from Point Delgada. difference between δ18O values of quartz and cal- contrast, calcite veins from the Point Delgada Calcite veins collected across the contacts with cite (δ18O) appears to increase with distance from subterrane display a much wider range of δ18O older rocks of the Coastal terrane tend to show in- Point Delgada. Although quartz and calcite were values (8.9‰–20.6‰) and seem to cluster into creased scatter but overall shifts toward lower δ18O not coprecipitated in the measured vein sets, we two groups of data. One group overlaps results values (Fig. 11). Theoretically, spatial changes of suspect that nearby veins formed from a fluid of from the King Peak subterrane; the other, which this type could be caused by either temperature ef- nearly constant oxygen isotope composition. If coincides spatially with sites of metalliferous fects or changes in fluid isotopic compositions. In this speculation is correct, then the increasing δ18 δ18 veining, shows a pronounced shift toward lower the King Range, the trends of increasing O val- Oquartz-calcite values would also be a result of δ18O values. In a similar fashion, δ18O values for ues mimic the spatial patterns of paleothermal in- progressive decreases in temperatures, from

King Peak quartz veins vary between only 18‰ dicators (Rm and CI) that show progressive de- about 300 °C within 1 km of Point Delgada to

1454 Geological Society of America Bulletin, October 1999

Figure 8. Histogram of illite-mica bo values for pelitic rocks of Point Delgada subterrane, King Peak subterrane, Bolinas Ridge, and Mount San Bruno. Boundaries for low-pressure (P), intermediate-pressure, and high- pressure facies series are from Guidotti and Sassi (1986).

Andreas fault (Fig. 1B). Most of the rocks within this terrane consist of well-bedded, biotite- and K-feldspar–bearing turbidites (Blake et al., 1984). No fossils have been discovered within the terrane. Lithologic similarities between these sandstones and those of the Novato Quarry terrane provide tenuous indications of a Campanian depositional age (Wakabayashi, 1992), but extensive potassic hydrothermal alteration tends to obscure the original sandstone compositions. The San Bruno Mountain terrane is sandwiched by shear zones between quartzo-feldspathic sandstones and shales of the Marin Headlands terrane (structurally above and to the northeast) and melange of the Central terrane (Blake et al., 1984; Waka- bayashi, 1992). Hydrothermal mineralization occurs at two localities near Mount San Bruno. Adularia at both sites yields K-Ar cooling ages of 13.5 ± 0.1 Ma (Russell et al., 1989). At Brisbane quarry (Fig. 12), rocks of the San Bruno Moun- Figure 7. Correlations between values of mean random vitrinite reflectance (%) and illite crys- tain terrane contain abundant quartz-carbonate tallinity index (∆°2θ) from Franciscan pelitic rocks. Reference box for the anchizone (incipient veins with epidote, chlorite, dolomite, pyrite, greenschist facies metamorphism) is from Underwood et al. (1993). (A) Regression curve for non- and chalcopyrite, plus minor galena and sphal- veined samples from the composite King Range terrane; r-value is correlation coefficient. (B) Re- erite, and a well-exposed (but undated) diabase sults from zones of metalliferous veining at Point Delgada, Bolinas Ridge, and Mount San Bruno; dike cuts the Franciscan sandstones. The sec- values that plot well above the King Range reference curve (i.e., hydrothermal “suppression”) ond locality occurs within lithic sandstones of are probably due to differences between maturation rates of organic matter and illite crystal the Central terrane immediately south of the growth under transient conditions of hydrothermal heating. (C) Comparison of values from ad- terrane boundary (Fig. 12); mineralization jacent subsamples of shale with and without vein mineralization; pelitic material with pervasive there is mostly in the form of quartz-adularia veins displays a consistent increase in illite crystallinity (i.e., lower Cl value). veinlets with traces of cinnabar. The two dated sites of mineralization are separated by the <200 °C at a distance of ~18 km (following SAN BRUNO MOUNTAIN Hillside fault, which is a steeply dipping, north- O’Neil et al., 1969; Clayton et al., 1972). These west-trending structure thought to have a com- paleofluid temperatures are in reasonable agree- Geologic Background ponent of strike-slip offset (Wakabayashi, ment with wall-rock temperatures determined 1992). The adularia K-Ar ages, however, indi-

from Rm values, and this concurrence supports The type locality of the San Bruno Mountain cate that the San Bruno Mountain and Central our contention that veins precipitated near peak terrane is found immediately south of the city of terranes were juxtaposed along the Hillside wall-rock temperatures. San Francisco on the northeast side of the San fault before middle Miocene time.

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Paleotemperature Data

Nine shales from San Bruno Mountain were

analyzed for vitrinite reflectance, and the Rm results range from 1.8% to 3.9% (Fig. 12). Figure 4 shows that these values correspond to paleotemperatures of 210 to 320 °C (following Barker, 1983) and 210 to 290 °C (following Barker,

1988). The lowest Rm values are basically the same as those from adjacent melange matrix of the Franciscan Central terrane, which indicates that the Hillside fault has not caused significant displacement of the paleotemperature structure

since peak heating. We regard Rm values above 2.0% as unusual, compared to nearby Franciscan paleotemperatures. Most data from blueschist facies terranes of the Diablo Range, for example, range from 1.6% to 2.0% (Dalla Torre et al., 1996); the Central terrane north of the San Fran- cisco Bay area is typically between 0.7% and

1.7% Rm (Blake et al., 1988). Illite crystallinity indices for shales near Mount San Bruno range from 0.63 to 0.39 ∆°2θ

(Fig. 12). For the most part, Rm-CI data pairs plot close to the reference curve established for the King Range terrane (Fig. 7B); two samples show

CI suppression relative to Rm. Mica bo values are 8.998 and 9.001 Å for samples from the zone of higher thermal maturity (Fig. 12); these results indicate minimal phengite substitution and low- pressure incipient metamorphism. The remaining Figure 9. (A) Plot of δ18O (standard mean ocean water [SMOW], ‰) and δ13C (Peedee belem- data (9.008–9.031 Å) are consistent with the in- nite [PDB], ‰) for calcite veins from the Point Delgada subterrane, King Peak subterrane, Boli- termediate-pressure facies series (Fig. 8). nas Ridge, and Mount San Bruno. Note shift toward lower δ13C and δ18O within the local met- Only two calcite veins were analyzed from alliferous vein systems at all three sites. (B) Comparative plot of δ18O (SMOW, ‰) and δ13C Brisbane quarry, but crosscutting field relations (PDB, ‰) for calcite veins from the active hydrothermal system at Middle Valley, Juan de Fuca indicate that one is late-stage calcite, which evi- Ridge (Früh-Green et al., 1994), and from Franciscan-hosted rocks in The Geysers hydrother- dently formed under retrograde conditions with mal system, California (Lambert and Epstein, 1992). Data from The Geysers are consistent with respect to the phase of peak heating. This late- at least two episodes of fluid-rock interaction: the older Franciscan metamorphic event and re- stage calcite yielded a δ13C value of –16.1‰ and cent hydrothermal activity associated with Clear Lake magmatism. a δ18O value of 24.6‰. The δ13C and δ18O values for the earlier vein are –8.4 ‰ and 15.4‰, respectively, similar to values documented in the tiary strata above the basement have been subdi- rane is composed mostly of K-feldspar–bearing King Peak subterrane (Fig. 9). The δ18O values vided into two lithogenetic sequences; the lower graywacke of unknown age; the K-feldspar may of quartz veins range from 10.9‰ to 16.7‰ sequence includes the Miocene Laird Sandstone be due largely to hydrothermal alteration. For the (Fig. 12), and one adularia specimen yielded a and Monterey Formation, and the upper sequence most part, bedding in the San Bruno Mountain δ18O value of 10.2‰. There are no meaningful includes the Santa Margarita Sandstone, Santa turbidites is intact but disrupted locally by high- differences in the isotopic signatures of veins that Cruz Mudstone, and strata correlated with the angle faults and shear zones. we collected from opposite sides of the Hillside Purisima Formation (Clark et al., 1984). One of the most notable geologic aspects of fault (Fig. 12). Immediately east of the San Andreas fault the Bolinas region is a zone of hydrothermal min- zone, Franciscan rocks near Bolinas (Fig. 13) eralization that occurs within rocks of the San BOLINAS RIDGE have been assigned to four terranes of the Central Bruno Mountain terrane. The McKinnan Gulch belt (Blake et al., 1984). The Central terrane is a copper prospect is located ~3.5 km east of the Geologic Background polymict, mudstone-matrix melange that con- San Andreas fault (Fig. 13). Epithermal quartz- tains a wide variety of tectonic blocks and fossils carbonate veins include a base-metal sulfide as- South of Point Arena (Fig. 1B), the San Andreas of Tithonian to Valanginian age. The Nicasio semblage of argentiferous galena, sphalerite, transform boundary passes through Tomales Bay Reservoir terrane (Tithonian to Valanginian) con- pyrite, and chalcopyrite. The sulfides occur as and Bolinas Bay and separates basement rocks of sists largely of pillow basalt, overlain succes- disseminated euhedral to subhedral crystals and the Salinian terrane from the Franciscan Com- sively by radiolarian chert and graywacke. The as massive crystal aggregates. Principal gangue plex. Salinian basement in this region is com- Marin Headlands terrane (Tithonian to Ceno- minerals in vein assemblages are calcite and posed mostly of middle Cretaceous granitic rocks manian) contains chert and basalt overlain by quartz, accompanied by minor chlorite, dolomite, and minor metamorphic rocks (Ross, 1978). Ter- lithic graywacke. The San Bruno Mountain ter- and adularia. Narrow bands of hydrothermally al-

1456 Geological Society of America Bulletin, October 1999

metamorphism. Calculated values of bo lattice dimensions range from 9.011 to 9.028 Å within the intermediate-pressure facies series (Fig. 8). The δ13C values of calcite veins from Bolinas Ridge are –11.5‰ to –14.4‰, and their δ18O values are 10.5‰ to 11.2‰ (Fig. 12). The δ18O values of vein quartz are 9.4‰ to 13.3‰. These values are similar to results from metalliferous veins at Point Delgada (Fig. 9A). The δ34S values of vein-hosted sulfide minerals from the McKinnan copper prospect are: pyrite, 1.5‰; sphalerite, 1.0‰. Pyrite from one altered wall-rock specimen yielded a δ34S value of 7.0‰ (Fig. 13).

DISCUSSION

Are the Franciscan Paleotemperature Anomalies Related Genetically?

Franciscan rocks near Point Delgada, Bolinas Ridge, and Mount San Bruno were altered by heat conduction and fluid flow along localized fracture systems during middle Miocene time. The Point Delgada event dissipated ca. 13.9– 13.6 Ma and affected strata within the King Peak subterrane (middle Miocene), as well as the Point Delgada subterrane (Upper Cretaceous). Thermal overprints at Bolinas Ridge and Mount San Bruno dissipated ca. 12.2 Ma and 13.5 Ma, respectively. Conservative estimates of peak paleotemperature for wall rocks from the three affected areas are 280 to 310 °C. In all three study areas, the locations of metalliferous veins coincide with wall rocks that contain lower CI values (higher crys-

tallinity) and the highest Rm values. Focused fluid flow resulted in precipitation of quartz-carbonate, adularia, and sulfide deposits. Fluid-inclusion homogenization temperatures (uncorrected for Figure 10. Detailed map of Shelter Cove and vicinity (see Fig. 2 for regional location) show- pressure) from quartz and calcite at the sites of ing sample localities for veins analyzed by Solomon (1994) (open dots) and McLaughlin et al. metalliferous veining are lower by only 20 to (1985) (closed dots with numbered station designations). Numbers in ovals are δ18O values (stan- 30 °C than the peak wall-rock temperatures. Col- dard mean ocean water [SMOW], ‰) of calcite. Numbers in boxes are δ18O values (SMOW, ‰) lectively, these lab data and field observations of, quartz. Two K/Ar ages (Ma) from adularia (McLaughlin et al., 1985). Bold numbers without bolster the idea of close temporal and spatial con- boxes are average values of fluid-inclusion homogenization temperature (°C) from quartz veins. nections among peak heating of wall rocks, fluid See Figure 3 for additional keys. flow, and vein precipitation. Except for Miocene strata within the King Peak subterrane, the hydrothermal events occurred long after the Fran- tered sandstone are aligned parallel to high-angle, 290–300 °C based on Barker (1983). This degree ciscan strata were deposited and deformed. north- to northwest-striking faults. The cooling of thermal maturity is significantly higher than Given their similarities in age, temperature, and age of this mineralization is 12.2 ± 0.1 Ma, based what has been documented to the north within composition, we believe that all three anomalies on K-Ar dating of adularia in vein fillings (Russell the Franciscan Coastal and Central belts (Under- are related genetically. As discussed in the fol- et al., 1989; McLaughlin et al., 1996). wood et al., 1988; Blake et al., 1988), or within lowing, however, the specific sources of heat and the Diablo Range (Dalla Torre et al., 1996). paleofluid are by no means clear. Paleotemperature Data CI values for illitic minerals range from 0.68 to 0.42 ∆°2θ (Fig. 13). Conditions of inorganic Identification of Sulfur Sources Only four shale samples from Bolinas Ridge alteration, therefore, extend from moderate dia- were analyzed for vitrinite reflectance, which genesis to the lower boundary of anchimetamor- The δ34S values of sulfide minerals from the

precludes mapping of spatial gradients. Values phism. Rm-CI data pairs plot above the Francis- Point Delgada veins yield an apparent sulfur iso- are between 3.2% and 3.5% (Fig. 13) and yield can reference curve established for the King tope equilibrium temperature of 210 ± 35 °C paleotemperature estimates of 270–280 °C using Range (Fig. 7B), which indicates that crystalliza- (Ohmoto and Rye, 1979). Values from Bolinas the Barker (1988) regression model (Fig. 4), or tion of illite lagged behind increases in organic Ridge are similar. Sulfide deposition evidently

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occurred after the highest temperatures were im- types of fluid reservoirs: seawater; meteoric water; rocks at moderate temperatures (<300 °C) and

parted on pelitic host rocks (based on Rm and Th metamorphic water from dehydration reactions; high water-to-rock conditions. If meteoric water δ34 data). Calculated S values of H2S in the hy- and connate brine. Values near 0‰, or shifted to circulates through a hydrothermal system under drothermal fluid are –0.5‰ to –0.7‰ (Ohmoto slightly positive values, should occur if seawater similar water-to-rock conditions, then similar or and Rye, 1979), indicating a dominantly igneous interacts with host sedimentary and/or basaltic more negative δ18O values should result. In gen- (basaltic) source of sulfur (Sakai et al., 1984). As a possible analogue, some sulfides from the Mid- dle Valley segment of Juan de Fuca Ridge have significantly higher δ34S values (to 9.8‰), indicating mixing in the hydrothermal fluids of basalt- derived sulfide with reduced seawater sulfate (Goodfellow and Franklin, 1993; Zierenberg, 1994). However, at locations that are separated from the effects of seawater sulfate reduction, sediment-hosted massive sulfides in Middle Valley have δ34S values similar to our Franciscan data, between –1.2‰ and 4.7‰ (Zierenberg, 1994). Thus, paleofluids responsible for quartz-sulfide veins in the Franciscan rocks probably interacted with a basaltic reservoir, although not necessarily within a mid-ocean ridge basalt–type setting.

Oxygen Isotope Compositions of Vein-Forming Waters

As a further test of the possibility that vein sys- Figure 11. Values of δ18O (standard mean ocean water [SMOW], ‰) for quartz and calcite tems near Point Delgada, Bolinas Ridge, and veins, plotted as a function of distance from the Point Delgada headlands. All data have been Mount San Bruno shared similar hydro-geochem- projected onto a transect line oriented N38°W. Coastal calcite data are from veins in the Fran- ical origins, we calculated δ18O values for vein- ciscan Coastal terrane. PDS—Point Delgada subterrane of the composite King Range terrane; forming waters (Fig. 14). Interpretation of these KPS—King Peak subterrane of the composite King Range terrane. Note segregation of values results becomes complicated, however, because from metalliferous veins and crude trends of increasing δ18O with distance northwest of Point of the hypothetical involvement of four generic Delgada. Modified from Solomon (1994).

Figure 12. Detailed map of Mount San Bruno and its highly urbanized vicinity. Information within boxes includes sample identification number,

values of mean random vitrinite reflectance (Rm, ∆ θ %), illite crystallinity index (CI, °2 ) illite-mica bo lattice spacing (Å), paleotemperature estimates δ34 from Rm (T, °C), S values of sulfide minerals (CDT, ‰), δ13C values of calcite (Peedee belemnite [PDB], ‰), δ18O values of quartz, calcite, and adularia (standard mean ocean water [SMOW], ‰), and K/Ar ages of adularia (Ma). Franciscan tectonostratigraphic terranes are from Blake et al. (1984).

1458 Geological Society of America Bulletin, October 1999

teoric water), the oxygen isotope composition of which was modified through interaction with basaltic and/or sedimentary rocks at somewhat higher temperatures (probably >300 °C) and higher water-to-rock ratios. This second component of paleofluid was responsible for deposition of localized hydrothermal quartz-carbonate-sulfide veins that occur where wall-rock paleotemperatures are highest (Fig. 14, B and C). Signifi- cant heat transfer by advection may have occurred within these fractures. δ18 Calculated Owater values, by themselves, do not permit us to determine unique fluid origins for each Franciscan vein, but they are important because they document the presence of an isotopically distinct fluid that was restricted to the highest temperature vein systems. Did the lower δ18O waters evolve during marine hydrothermal activity near a mid-ocean ridge or by continental fluid-rock interaction near the slab window of the Mendocino triple junction? Neither case can be refuted by the data, and comparisons among data from different types of active hydrothermal systems fail to provide a definitive match. For example, calcite veins from Middle Valley of the δ18 Juan de Fuca Ridge yield Owater values that range from –0.7‰ to 7.7‰ relative to SMOW (Fig. 14D), with an average value of 3.5‰ (Früh- Green et al., 1994). Those veins definitely precipitated from moderately evolved seawater, rather than meteoric or metamorphic water. Per- Figure 13. Detailed map of Bolinas Ridge and vicinity showing sample localities. Information haps some of the Franciscan paleofluids migrated within boxes includes sample identification number, values of mean random vitrinite reflectance from a similar source. (R , %), illite crystallinity index (CI, ∆°2θ) illite-mica b lattice spacing (Å), paleotemperature m o A second empirical comparison involves fluids estimates from R (T, °C), δ34S values of sulfide minerals (CDT, ‰), δ13C values of calcite m from active continental hot springs. Existing sys- (Peedee belemnite [PDB], ‰), δ18O values of quartz and calcite (standard mean ocean water tems of this type within the California Coast [SMOW], ‰), and K/Ar ages of adularia (Ma). Mineralization is most prominent at McKinnan Ranges are rather complicated, with abundant ev- Gulch Mine. Fault traces within San Andreas fault zone are from Brown and Wolfe (1972). idence for mixing among multiple fluid sources (Peters, 1991, 1993). Veins in the Knoxville and Sulphur Creek gold-mercury districts, as exam- δ18 eral, higher Owater values (to ~12‰) will result values to be an indication of fundamentally dif- ples, are hosted mostly within the Great Valley if dehydration reactions occur within accreted ferent types of fluids with distinct histories of Group and associated ophiolitic rocks, with sub- δ18 rocks or if isotopic equilibration occurs between fluid-rock interaction. The first group ( Owater sidiary alteration of nearby Franciscan rocks. pore water and deep-seated rocks at elevated tem- values >5‰–6‰) is consistent with fluids gen- Peters (1991) showed that waters emanating from peratures (300 to 400 °C) and low water-to-rock erated through dehydration reactions and/or the active gold-bearing hot springs are moderately conditions. Most whole-rock δ18O values of Fran- extensive isotope exchange with deep-seated saline (3 to 4 wt% NaCl) and have δ18O values ciscan graywackes and metagraywackes range rocks at relatively low metamorphic tempera- from 2.1‰ to 8.5‰; most values are greater than from 10‰–15‰, whereas Franciscan shales are tures (<300 °C) under lower water-to-rock ratio 5.0‰ (Fig. 14D). Cooler spring waters and mete- 11‰–13‰ (Magaritz and Taylor, 1976). Rock- conditions. Those rock-dominated fluids proba- oric spring waters, in contrast, have δ18O values of dominated Franciscan paleowaters should ap- bly led to precipitation of widespread quartz and –1.1‰ to –9.0‰. The isotopic and chemical proach the δ18O values of those host rocks. As a calcite veins, typical of the regional paleother- compositions of the warm hot-spring waters seem final possibility, a connate brine also might result mal regime. Veins of this type are commonplace to reflect variable amounts of mixing between in high δ18O signatures. However, the relatively throughout the Franciscan Complex (Magaritz meteoric waters and highly exchanged waters that low salinities of fluid inclusions in the King Peak and Taylor, 1976; Lambert and Epstein, 1992) migrated, along with thermogenic hydrocarbons, subterrane (3.4–7.6 equiv. wt% NaCl) and Point and are particularly abundant in the King Peak from a deeper sedimentary or metasedimentary Delgada subterrane (1.5–4.6 equiv. wt% NaCl) subterrane (Fig. 14A). We regard these vein- source (Peters, 1991). Peters (1993) argued that appear to preclude this possibility. filled fractures as products of diffuse terrane- the high-temperature end member of the apparent On frequency diagrams (Fig. 14), calculated wide heat conduction and fluid flow during re- mixing trend is connate water that was trapped in δ18 Owater values for the Franciscan vein minerals gional metamorphism. the Cretaceous Great Valley Group. seem to define two modes within a continuum. We believe that the second group of values A third logical analogue for Miocene activity δ18 δ18 We interpret this crude segregation of Owater ( Owater <5‰–6‰) represents seawater (or me- within the Franciscan Complex is the active

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Geysers–Clear Lake geothermal area (Donnelly- Nolan et al., 1981; Thompson et al., 1992), which lies within the Central belt of the Franciscan Complex. Introduction of basaltic magma into the crust beneath the Clear Lake area has been attributed to east-southeast extension that accompanied northward propagation of the San An- dreas transform system, and its slabless window, within the past 3 m.y. (Liu and Furlong, 1992). Donnelly-Nolan et al. (1993) postulated that the vapor-dominated geothermal field at the Geysers has evolved from a preexisting hydrothermal system within fractured Franciscan metamorphic rocks; instead of tapping connate fluids, however, the active system is dominated by highly evolved meteoric waters that reacted with Franciscan strata. Donnelly-Nolan et al. (1993) also described multiple fluid components within the Clear Lake regional system (Fig. 14D): (1) cold δ18 and warm, dilute meteoric waters ( Owater =~ –9‰ to –5‰); (2) chloride-rich, evolved connate δ18 waters ( Owater = ~+5‰) emanating from rocks of the Great Valley sequence rocks; (3) moder- δ18 ately saline, evolved meteoric waters ( Owa- ter = ~+3‰ to +6‰) emanating from Franciscan rocks; and (4) mixtures of these fluids. Among these choices, fluid type 3 appears to give us the best match for the moderately saline, lower δ18O waters that were responsible for precipitation of localized metalliferous veins of Miocene age at Point Delgada, Bolinas Ridge, and Mount San Bruno. Figure 14. Calculated δ18O values of vein-precipitating waters for quartz and calcite veins. Identification of Carbon Sources Paleotemperatures used in most calculations were derived from values of mean random vitrinite reflectance. (A) Calculated δ18O values for the King Peak subterrane (KPS). (B) Cal- Paired δ13C and δ18O values (Fig. 9A) allow water culated δ18O values for the Point Delgada subterrane (PDS). Temperature estimates for us to discriminate among various generations water quartz veins include both vitrinite reflectance data (R ) and fluid-inclusion homogenization of calcite veins and provide additional criteria m temperatures (T ). (C) Calculated δ18O values for Bolinas Ridge (BR) and Mount San for identifying fluid sources. Isotope composi- h water Bruno (MSB). (D) Calculated δ18O values of calcite-precipitating waters in Middle Valley, Juan tions of aqueous carbonic species and carbon- de Fuca Ridge, Ocean Drilling Program (ODP) Site 857 (Früh-Green et al., 1994), with temper- ates depend on both carbon sources (CaCO , 3 atures based on in situ borehole measurements. Also shown for comparison are measured values organic carbon, or CH ) and temperature. Cal- 4 of waters extracted from gold springs in northern California (Peters, 1991) and waters from the cite data from the King Peak subterrane define Clear Lake hydrothermal field (Donnelly-Nolan et al., 1993). a cluster (δ18O = 17‰ to 20‰; δ13C = 0‰ to –7‰) that is representative of vein precipitation during typical episodes of Franciscan metamorphism (Magaritz and Taylor, 1976). (Lambert and Epstein, 1992). Most of the Gey- marine carbonates, typically occurs when pore Most of that carbonate probably was derived sers calcite veins formed during earlier (Meso- fluids incorporate carbonate derived from the from marine microfossils, together with minor zoic) episodes of Franciscan basement metamor- thermal decomposition of organic matter. As an amounts of marine organic matter. When com- phism; their compositions are δ18O = 12‰ to example of this type, Figure 9B shows that δ13C pared to data from the King Peak subterrane, 20‰ and δ13C = +2‰ to –15‰ (Fig. 9B), simi- values of –10‰ to –20‰ are characteristic of results from the Point Delgada subterrane, lar to the King Peak subterrane results (Fig. 9A). sediment-hosted calcite cements and calcite nod- Bolinas Ridge, and Mount San Bruno collec- Crosscutting veins associated with the younger ules from active hydrothermal vents in the Mid- tively display more scatter (Fig. 9A); there is Pliocene-Pleistocene hydrothermal system dis- dle Valley segment of Juan de Fuca Ridge (Früh- also a marked shift toward more negative δ13C play pronounced shifts toward lower δ18O (0‰ Green et al., 1994). As a final possibility, values (–10‰ to –15‰), especially in metallif- to 10‰) and δ13C values (–8‰ to –15‰). Fluids methane-derived carbonates show extreme de- erous veins. responsible for the younger veins had a meteoric grees of 13C depletion, with typical δ13C values The two fields of data shown in Figure 9A are origin, and their maximum paleotemperatures as low as –30‰ to –60‰ (Ritger et al., 1987; similar to results from the Geysers vapor-domi- evidently reached 260 to 320 °C (Lambert and Sample and Reid, 1998). There is no evidence for nated field, where at least two generations of Epstein, 1992). this type of carbon source in the Franciscan cal- fluid flow have affected Franciscan host rocks Moderate depletion of 13C, relative to normal cite veins. In conclusion, we surmise that the

1460 Geological Society of America Bulletin, October 1999

metalliferous fluids of Miocene age probably proximity. In addition, accreted Miocene rocks tively high basement temperatures certainly ap- tapped a carbon source that included abundant of Franciscan affinity (False Cape terrane) have pear to affect the thermal structure of the overrid- organic matter, perhaps disseminated in nearby been documented along the coastline north of ing accretionary wedge (Hyndman et al., 1993). A marine shales. the Mendocino fracture zone (Aalto et al., 1995); combination of young subducting lithosphere and broadly coeval King Range and False Cape thick sediment during Miocene time, therefore, Paleogeographic Positions of Overprints strata could represent parts of the same Miocene may have caused unusually high geothermal gra- accretionary prism. If true, then large-scale slip dients over regions larger than the composite Several models have been proposed to account is not necessary to explain their present geo- King Range terrane. for time-space patterns of paleotemperature anom- graphic positions. In theory, susceptibility of accreted Franciscan alies within the Coast Ranges of northern Cali- The likelihood of ridge-trench magmatic inter- rocks to localized ridge-trench heating, either fornia; all have strengths and weaknesses. One action north of the Mendocino triple junction is through advection of magma or focused flow of unresolved problem, as discussed in detail by supported indirectly by analyses of anomalous hydrothermal fluid, should have increased where McLaughlin et al. (1994), is the unconstrained volcanic rocks that are distributed along the strike segments of the fragmented, mechanically unsta- geographic position of the King Range compos- length of the California Coast Ranges (Cole and ble ridge drew closer to the accretionary prism. ite terrane at the time of peak heating and vein Basu, 1995). Those Nd-Sr isotopic data have Analysis of magnetic anomalies indicates that the precipitation. One scenario (Fig. 15A) requires been interpreted to indicate that magma mixing Pacific-Farallon ridge was more linear ca. 15 Ma little latitudinal translation of the terrane (relative occurred as a depleted mantle-derived basaltic than at present (Wilson, 1988). Small ridge off- to its present-day position) and a heat source end member (analogous to sources for mid- sets (of about 20 km) evidently did develop be- rooted in the subducting Farallon plate north of ocean-ridge basalts) migrated through a crustal tween 14 Ma and 12 Ma, however, and the birth the Mendocino triple junction. Recent passage of reservoir that was enriched in incompatible ele- of three new propagators disrupted the geometric the triple junction through 40°N latitude may ments. In addition, rather than defining a trend of stability of the ridge system between 15 and have shifted the terrane modestly to the north, as gradual migration of a single magmatic point 12 Ma. That time interval overlaps the ages of hy- indicated by seismicity patterns (e.g., Castillo source, paleogeographic restorations of the vol- drothermal activity documented in our study. It is and Ellsworth, 1993). A second line of reasoning canic centers define discrete provinces of anom- possible that spreading-ridge instability also led invokes large-scale translation of the terrane from alous near-trench magmatism, with two promi- to the eruption of off-axis seamounts. Hydrother- a paleolatitude of about 36°N, close to the triple nent clusters of ages at 26–22 Ma and 19–16 Ma mal loci might have shifted episodically along junction’s position at 13–14 Ma (see also Mc- (Cole and Basu, 1995; Dickinson, 1997). The the Miocene subduction margin in a series of Laughlin et al., 1982). According to this recon- trend of northwest-younging volcanism, as pre- northward jumps to successive fracture-zone off- struction (Fig. 15B), King Range rocks were de- dicted for steady triple-junction migration, im- sets of the Pacific-Farallon spreading system. En- posited and accreted along the western margin proves with examples that are younger than hanced hydrothermal activity might have been of North American just north of the triple junc- 15 Ma (Fox et al., 1985; Dickinson, 1997). especially vigorous near the unstable southern tip tion, overprinted by effects of the slabless win- To be viewed as viable for Point Delgada and of the subduction front (i.e., immediately north of dow shortly thereafter, then translated to the related thermal anomalies, the hypothesis of the triple junction). Regardless of the exact de- north as part of the Pacific plate. To refute or ridge-trench collision requires some type of phys- tails, we believe that igneous basement could confirm either scenario has important ramifica- ical mechanism to transfer heat and/or fluid from have ruptured in a number of locations during its tions for the interpretation of middle Miocene the subducting plate into the overriding plate. The subduction. Fluids near 300 °C within the down- paleothermal anomalies. active Cascadia subduction zone, north of the going basaltic reservoirs could have been liber- Mendocino fracture zone, provides some clues as ated and transferred rapidly into the overriding Heating by Ridge-Trench Interaction to how this might happen. The youngest litho- accretionary prism, perhaps advecting into older sphere now being subducted (chron 3, 4.2 Ma) is Franciscan rocks well landward of the Miocene Fluid-rock interactions and perturbations in located just south of the Blanco fracture zone subduction front (Fig. 15A). heat transfer through the Franciscan Complex (Wilson, 1993). Subducting lithosphere immedi- may have been triggered by fragmentation of the ately north of the Blanco fracture zone (chron 4A) Heating by a Slabless Window Farallon plate, the sporadic development of is 8.8 to 9.4 Ma. Mean values of near-surface heat short-lived ridge-trench-transform triple junc- flow (± standard deviations) for chron 3 litho- The slabless-window hypothesis (Dickinson tions, and multiple ridge-trench collisions. Plate sphere range from 118 ± 12 to 160 ± 17 mW/m2 and Snyder, 1979a; Zandt and Furlong, 1982) reconstructions show that, at the time of middle (Moran and Lister, 1987). Because of downward has been promoted as the cause of many geo- Miocene subduction, oceanic lithosphere enter- circulation of cold seawater, most calculated heat- logic anomalies in the Coast Ranges of Califor- ing the trench at the paleolatitude of San Fran- flow values are substantially lower than what nia. This concept is appealing because it permits cisco Bay was probably no older than 9 m.y. models of pure conductive decay would predict a relatively simple plumbing connection to a shal- (Engebretson et al., 1985; Wilson, 1988). Geo- (e.g., Lister, 1977), even at distances of 125 km low magmatic heat source, hydrothermal circula- logic evidence from the King Range also sup- from a ridge crest. Sediment cover in Cascadia tion of meteoric and/or metamorphic fluids, and ports the idea of subduction of youthful oceanic basin is thick (Carlson and Nelson, 1987), which the emplacement of anomalous vein systems in lithosphere. The amount of time that elapsed be- markedly inhibits downward flow of cold sea- much older parts of the accretionary complex. tween crystallization of oceanic basalt at 16 Ma water; the insulating blankets of low-permeability Perhaps the active hydrothermal systems in the and deposition of middle Miocene microfossils sediment also inhibit venting of hydrothermal Clear Lake region (Lambert and Epstein, 1992; was brief. Quartz-sulfide-adularia mineraliza- fluids from fractured igneous crust (Davis et al., Donnelly-Nolan et al., 1993) represent the most tion at 13.9–13.6 Ma, moreover, must have oc- 1997). Between chrons 3 and 4A, the calculated logical present-day analogue for the middle Mio- curred shortly after the King Range sediments values of sediment-corrected temperature at the cene events at Point Delgada, Bolinas Ridge, and were deposited and deformed. All of these ob- top of igneous basement range from 109 to Mount San Bruno. servations support the contention of ridge-trench 198 °C (Moran and Lister, 1987), and these rela- One problem with the slabless-window inter-

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Figure 15. Two conceptual models for the King Range terrane (KRT) and hydrothermal overprints ca. 13 Ma (from McLaughlin et al., 1994). Interpretation A requires little or no latitudinal translation of the King Range terrane from a position of accretion near 40°N. Hydrothermal circulation, in this case, was caused by near- trench magmatism and/or fluid flow from the subducting Farallon plate (FAR) into the Franciscan-Cascadia accretionary prism. Inter- pretation B shows initial KRT accretion near the present latitude of Mount San Bruno. Hydrothermal activity, in this case, was part of a more extensive system associated with the Mendocino triple junction and its northward-migrating slabless window. The second explanation requires post 13 Ma northward translation of the KRT, followed by accretion back to the North American plate (NAM), as transpres- sive motion occurred between the Pacific (PAC) and North American plates near the propagating tip of the San Andreas transform. ERB—Eel River basin; MFZ—Mendocino fracture zone; VIZ— Vizcaino structural block.

1462 Geological Society of America Bulletin, October 1999

continuous segments of reverse and strike-slip faults, including a rupture trace of the 1906 earthquake. Recent trenching and detailed analyses of geomorphologic features near Shelter Cove have uncovered new evidence for Quaternary strike- slip faults that appear to extend diagonally into the interior of the King Range through a series of lineaments, scarps, and shutter ridges (Prentice et al., 1999). Brown (1995) showed the fault to bend offshore north of Telegraph Hill (Fig. 10), whereas Prentice et al. (1999) proposed that it may continue inland for at least 9 km to the vicinity of Saddle Mountain. It is important to consider the geometry and cumulative slip history of this fault strand, as well as its inferred role as the long-term Pacific–North America plate boundary, but the context of such considerations must include the thermo-tectonic history of Franciscan basement on both sides. Data presented herein do not support the con- Figure 16. Reconstructed position of the San Francisco Bay block at 8 Ma (McLaughlin et al., tention that basement rocks within the interior of 1996). Faults within east San Francisco Bay region (ESFBR) system include the San Andreas, the King Range terrane have undergone substan- Rogers Creek, Tolay-Bloomfield (TBF), Hayward (HF), and Calaveras (CA). Other faults tial distances of horizontal displacement along the shown are San Gregorio–Hosgri, Pilarcitos (PF), and Bartlett Springs. Sites of middle Miocene San Andreas(?) fault. For example, the similarity volcanism and hydrothermal activity are shown with solid dots (ages in Ma). Restoration as- of isotope values for quartz-carbonate veins on sumes a single northward-younging of thermal activity beginning ca. 18 Ma. This trend was dis- opposite sides of the fault argues against juxta- rupted after ca. 8 Ma when the Mendocino fracture zone jumped eastward and propagated up- position of chemically unrelated vein systems ward through the North American plate, thereby initiating slip along ESFBR fault system. (Fig. 10). Faults can also cause offsets of isoreflectance contours if displacements postdate peak heating (Laughland and Underwood, 1993; Orange and Underwood, 1996), but sharp discon- pretation is the poor match between time-space gration of the heat source would have averaged tinuities of this type do not exist in the King data from the sites of middle Miocene anomalies ~3.5 cm/yr based on the differences in age and Range. Instead, the relatively smooth changes in and the rate of northward migration for the in- distance between those two anomalies. A rate of illite crystallinity (Fig. 6) and vitrinite reflectance ferred heat source. Radiometric dating of hydro- 3.5 cm/yr is equal to the current rate of triple- within the interior of the terrane (Figs. 3 and 5) are thermal products and volcanic rocks from through- junction migration (Argus and Gordon, 1991) but consistent with gentle tilting and differential ero- out the San Francisco Bay region (Fig. 16) shows roughly one-half the average rate for the time in- sion of the paleothermal structure. The maximum that Bolinas Ridge and San Bruno Mountain are terval between 15 and 10 Ma (Dickinson and distance of horizontal offset can be constrained by

at the north end of a generally northward-young- Snyder, 1979a). recontouring Rm values near the mapped fault ing age progression between the San Andreas and The slabless-window heat source carries with it trace (Fig. 17). This reiteration permits no more Calaveras-Hayward fault zones (McLaughlin et additional complications because it demands dex- than ~2.5 km of total horizontal displacement al., 1996). We reiterate here that K-Ar ages for tral displacement of the King Range terrane after since peak heating. The only way to accommo- adularia at Point Delgada, Bolinas Ridge, and culmination of hydrothermal activity. The pre- date more displacement would be to bend the Mount San Bruno are similar (13.9–13.6, 13.5, sent-day distance from Point Delgada to Mount fault parallel to isoreflectance lines and move it and 12.2 Ma). One might conclude, therefore, that San Bruno is ~310 km. For the King Range to offshore just north of Telegraph Creek. all three hydrothermal overprints occurred in move that far, the San Andreas transform bound- Brown (1995) used the apatite fission-track proximity during a single protracted phase of ac- ary must have been located to the east of the ter- data of Dumitru (1991b) to support his contention tivity (Fig. 15B). This type of occurrence, how- rane, thereby transferring those elements of the that major amounts of differential vertical and/or ever, would not be expected with steady triple- accretionary complex from the North American horizontal slip have occurred along the San An- junction migration. plate to the northeast corner of the Pacific plate dreas(?) fault near Shelter Cove. Those fission- Near synchroneity of hydrothermal events (Fig. 15B). After the terrane reached its present track ages were obtained from reset detrital ap- would be easier to explain if the composite King latitude, the San Andreas boundary of the triple atite grains and provide the time of wall-rock Range terrane had been overprinted thermally junction jumped west of the King Range terrane. cooling through an isotherm of ~110 to 135 °C. near the latitude of Mount San Bruno (Fig. 16), As discussed in the following, these requirements Six of the cooling ages are between 13.4 ± 2.6 Ma thereby reducing the number of anomalous sites also carry with them ramifications for interpreta- and 9.5 ± 1.6 Ma (Fig. 3); those dates establish a to two. A single magmatic heat source rooted in a tion of neotectonic activity near Shelter Cove. close temporal match with the K-Ar ages (13.9– migrating or widening zone of shallow astheno- 13.5 Ma) for adularia in the adjacent metalliferous sphere then could be shifted modestly from the San Andreas(?) Fault near Shelter Cove veins (McLaughlin et al., 1985). Track-length dis- San Bruno region (13.9–13.5 Ma) to Bolinas Ridge tributions, moreover, indicate rapid cooling of (12.2 Ma). If this shift did occur, either gradually There is little doubt that crustal blocks near wall rocks near the metalliferous veins (Dumitru, or as an abrupt jump, the rate of northward mi- Shelter Cove have been offset along several dis- 1991b). The close correspondence among maxi-

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mum wall-rock paleotemperatures (>300 °C), metalliferous veins, and Miocene cooling ages leads us to conclude that they are all related genetically. In contrast, fission-track ages for all but one of the sandstones analyzed from the King Peak subterrane are much younger, ranging from 2.0 ± 0.6 to 0.3 ± 0.3 Ma (Fig. 3). One specific sample locality (Dumitru’s station #9, with an age of 12.0 ± 1.4 Ma) becomes vital to Brown’s (1995) argument that an oblique- slip fault (up on the east and right lateral) places older, thermally stable rocks to the west against recently uplifted rocks to the east. That site is located roughly 300 m to the east of the Point Del- gada subterrane–King Peak subterrane boundary, but west of the line of 1906 ruptures attributed to the San Andreas fault (Fig. 3). It is possible, we believe, to account for this result in other ways, with minimal amounts of Quaternary fault offset. One alternative explanation is that Dumitru’s locality #9 lies within a window of Point Delgada subterrane rocks, structurally below the King Peak subterrane; this would segregate all of the earlier (Miocene) cooling ages within the Point Delgada subterrane. A second possibility is that all of the Miocene cooling ages are associated with decay of a single hydrothermal system that penetrated rocks of both Point Delgada and King Peak affinity near the subterrane boundary. McLaughlin et al. (1985) demonstrated clearly that rocks of both subterranes were intruded by kindred systems of metalliferous veins. The east- ernmost limit of the veins remains to be documented, but it seems plausible to suggest that the

high-temperature fluids affected King Peak Figure 17. Contour map of vitrinite reflectance values (Rm) from the composite King Range rocks as far as 300 m from the edge of the Point terrane, northern California. Open circles indicate sample localities. Dashed line through the in- Delgada subterrane. Once hydrothermal circula- terior of the terrane is the San Andreas(?) fault, as mapped by Prentice et al. (1999). This itera- tion ended, wall-rock temperatures decayed tion shows that the maximum distance of dextral offset of isoreflectance contours is ~2.5 km. rapidly and probably dropped below the partial annealing zone with little or no Miocene uplift. If cooling of the hydrothermal system was followed by late Pliocene uplift and tilting of the amalga- able; thicknesses of eroded overburden also must geomorphic analyses of coastal streams and ma- mated King Peak composite terrane, then the increase from the interior toward the coast. Thus, rine terraces show that Quaternary rates of uplift combined effects of uplift and erosion would have not only are the Point Delgada strata (and King decrease over more than 150 km of coastline as a transferred most of the King Peak subterrane Peak rocks in nearby strike-parallel coastline ex- function of distance from the present triple junc- through the annealing zone for apatite after 2 Ma. posures) positioned closer to pivot points of long- tion (Merritts and Bull, 1989; Merritts and Vin- Structural and geophysical data show that term terrane-wide tilting, they also have been cent, 1989; Carver et al., 1994; Merritts, 1996). King Range strata were obducted eastward over subjected to greater amounts of cumulative de- Rates are highest near the center of the King Peak the Coastal terrane (McLaughlin et al., 1982); nudation since peak heating. subterrane, rather than Point Delgada. According this vergence direction means that long-term up- The rate of uplift at Point Delgada since middle to Merritts (1996), King Range deformation is dis- lift rates since peak heating probably have been Wisconsin time has been relatively high, at least tributed broadly along a restraining bend at the ter-

higher toward the east. Furthermore, Rm values 1 mm/yr (McLaughlin et al., 1983). We find it du- mination of the San Andreas fault. Dumitru for outcrop samples decrease consistently with bious, however, to attribute the neotectonic activ- (1991b) suggested that differential uplift of the outcrop elevation (Laughland, 1991). This three- ity solely to movement on the San Andreas fault. King Range is being caused by space problems dimensional pattern demonstrates that the paleo- Farther north, for example, coastal uplift rates among various crustal blocks moving around the isothermal surfaces within the King Range ter- landward of the Cascadia subduction zone are Mendocino triple junction. Thermal and mechani- rane dip toward the coast. The highest elevation greater than 2 mm/yr from latitude 40°N to nearly cal responses to an active slab-window might also within the King Range is ~1250 m. Unless geo- 44°N (Mitchell et al., 1994). Consequently, the contribute to differential uplift (Merritts and Bull, thermal gradients changed significantly from southwest edge of the Gorda plate probably pro- 1989; McLaughlin et al., 1994). Collectively, these west to east, differential denudation and exposure vides an additional component of uplift within the studies show that rapid uplift must be complicated of the paleothermal structure have been consider- triple junction region. South of Cape Mendocino, by interactions among three plates within the triple

1464 Geological Society of America Bulletin, October 1999

should be a focal point for present-day earthquake-hazard assessment; however, the influence of this fault on the long-term evolution of Fran- ciscan basement seems to have been minor.

CONCLUSIONS

Franciscan rocks in the King Range, Bolinas Ridge, and Mount San Bruno study areas were overprinted thermally during middle Miocene time. Regional-scale veining and wall-rock heating were products of conduction and diffuse fluid flow. This ubiquitous component of the δ18 fluid budget ( Owater > 5‰–6‰) could have originated as seawater, metamorphic water, evolved meteoric water, or some combination thereof; the fluids probably evolved through dehydration reactions and/or extensive isotope exchange with deep-seated rocks at relatively low metamorphic temperatures (<300 °C) under low Figure 18. Schematic cross section of the Shelter Cove region, oriented from southwest to water-to-rock ratio conditions. In contrast, local- northeast. A corner of the subducting Gorda plate projects beneath the northeast edge of the ized metalliferous vein systems formed at higher King Range terrane, and the possibility of asthenospheric upwelling is indicated immediately temperatures from an isotopically distinct paleofluid reservoir (δ18O = –1‰ to +6‰). The south of the Mendocino triple junction. Mafic crust of the Pacific plate underlies the Vizcaino water block and is truncated by a major offshore structure that we interpret to be the main trace of fluid likely originated as seawater (or meteoric the San Andreas fault (SAF). The composite King Range terrane occupies a position within an water), the oxygen isotope composition of which active flower structure. Surface rupture occurred within the flower structure during 1906, but was modified through interaction with basaltic that fault trace has had minimal effect on the long-term slip history of the King Range terrane. and/or sedimentary rocks at elevated tempera- PDS—Point Delgada subterrane. tures (>300 °C) and high water-to-rock ratios. Heat could have been supplied from either ridge- trench interaction north of the Mendocino frac- junction region, not just the San Andreas fault. than what has been predicted by plate boundary ture zone or a slabless window in the wake of the A slabless window might have provided the reconstructions, which typically bring the north- northward-migrating Mendocino triple junction. source of heat for the King Range paleotempera- ern tip of the San Andreas to latitude 40°N some- Both scenarios have theoretical strengths and ture anomaly, but if this interpretation is correct, time between 2 and 1 Ma (Engebretson et al., empirical weaknesses. We recognize no more then another paradox is created. If heating from a 1985). Thus, it appears as though the 1906 fault than 2.5 km of dextral offset of isoreflectance slabless window occurred during middle Mio- trace at Shelter Cove represents a relatively re- contours through the interior of the King Range cene time, it must have occurred well south of cent adjustment. terrane. Accordingly, the mapped 1906 trace of present-day Shelter Cove; if so, then a separate We believe that some of the contractional de- the San Andreas(?) fault at Shelter Cove proba- proto-San Andreas fault, not the 1906 Shelter formation that has affected the Shelter Cove re- bly represents a relatively recent adjustment of Cove strand, must have been responsible for gion must be accommodated by faults along the the crust to triple-junction tectonics, with mini- moving the composite King Range terrane to its east side of the King Range terrane (Fig. 18), mal impact on the long-term slip history of Fran- present geographic position. Accordingly, the possibly including east-vergent blind thrusts that ciscan basement within the King Range. long-term (Pliocene-Pleistocene) trace of the extend beneath the King Range. Modeling of plate boundary must have been located some- gravity and aeromagnetic data (e.g., Griscom ACKNOWLEDGMENTS where other than Shelter Cove. Possibilities in- and Jachens, 1995) indicates that mafic base- clude the shear-zone boundary between the King ment of the Pacific plate is truncated offshore, Funding to Underwood and Shelton was pro- Peak subterrane and Coastal terrane and the cur- just west of the King Range, by a major fault that vided by National Science Foundation grant EAR- rently submerged continental shelf. dips steeply to the northeast (Fig. 18). The 8803538. T. Pallesen and D. Howell assisted in the If the King Range had been altered thermally downdip projection of the 1906 Shelter Cove field, and M. Breting helped with sample prepara- by ridge-trench interaction at or near its present rupture zone appears to root in or toward this tion. We thank C. Prentice, D. Merritts, R. Zieren- latitude, then significant displacement of Francis- fundamental offshore structure, as does the berg, J. Martin, and S. Roeske for their thoughtful can basement by the San Andreas fault system southwest-dipping boundary between the King and constructive reviews of the manuscript. would be neither required nor expected. We have Range subterrane and the Coastal terrane. Con- demonstrated that the offset of isoreflectance sequently, we regard the offshore structure as the REFERENCES CITED contours within the interior of the King Range to- most likely long-term carrier of dextral motion tals no more than 2.5 km (Fig. 17). If the average along the Pacific–North American plate bound- Aalto, K. R., McLaughlin, R. J., Carver, G. A., Barron, J. A., Sliter, W. V., and McDougall, K., 1995, Uplifted Neogene dextral slip rate since the fault’s arrival has been ary, the composite King Range terrane occupy- margin, southernmost Cascadia-Mendocino triple junc- 3–4 cm/yr (Argus and Gordon, 1991), then the ing a position within an active flower structure tion region, California: Tectonics, v. 14, p. 1104–1116. initiation time must have been only 83–62 ka. (Fig. 18). The 1906 rupture zone at Shelter Cove Argus, D. F., and Gordon, R. G., 1991, Current Sierra Nevada– North America motion from very long baseline inter- This time window is significantly more recent is also within the flower structure and certainly ferometry: Implications for the kinematics of the western

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