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. 1448 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/10/1448/3382983/i0016-7606-111-10-1448.pdf by guest on 01 October 2021 MIDDLE MIOCENE PALEOTEMPERATURE ANOMALIES WITHIN THE FRANCISCAN COMPLEX, NORTHERN CALIFORNIA 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 signif- icant amount of the long-term horizontal and/or vertical offset of King Range basement rocks since the time of peak heating, or does it repre- sent a relatively recent and minor structural ad- justment 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 dis- persed 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 reflect- ance (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.