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

This study documents three localities in the Franciscan accretionary comple x of northern California, now adjacent to the San Andreas fault, that were overprinted thermally between 13.9 and 12.2 ha: Point Delgada-Shelter Cove (King Range terrane); Bolinas Ridge (San Bruno Mountain terrane); and Mount San Bruno (San Bruno Mountain terrane), Vein assemblages of quartz, carbon ate, sulfide minerals, and adularia were precipitated locally in highly fra ctured wall rock. Vitrinite reflectance (R-m) values and illite crystallini ty decrease away from the zones of metalliferous veins, where peak wall-roc k temperatures, as determined from R-m were as high as 315 degrees C, The d elta(18)O values of quartz and calcite indicate that two separate types of fluid contributed to vein precipitation. Higher delta(18)O fluids produced widespread quartz and calcite veins that are typical of the regional paleot hermal regime. The widespread veins are by-products of heat conduction and diffuse fluid now during zeolite and prehnite-pumpellyite-grade metamorphis m, and we interpret their paleo-fluids to have evolved through dehydration reactions and/or extensive isotopic exchange with accreted Franciscan rocks . Lower delta(18)O fluids, in contrast, evolved from relatively high temper ature exchange between seawater (or meteoric water) and basaltic and/or sed imentary host reeks; focused flow of those fluids resulted in local deposit ion of the metalliferous veins, Heat sources for the three paleothermal ano malies remain uncertain and may have been unrelated to one another. Higher temperature metalliferous fluids in the King Range terrane could have advec ted either from a site of ridge-trench interaction north of the Mendocino f racture zone or from a "slabless window" in the wake of the northward migra ting Mendocino triple junction. A separate paradox involves the amount of Quaternary offset of Franciscan b asement rocks near Shelter Cove by on-land faults that some regard as the m ain active trace of the San Andreas plate boundary. Contouring of vitrinite reflectance values to the north of an area affected by A.D. 1906 surface r upture indicates that the maximum dextral offset within the interior of the King Range terrane is only 2.5 km. If this fault extends inland, and if it has been accommodating most of the strike-slip component of San Andreas of fset at a rate of 3-4 cm/yr, then its activity began only 83-62 ka, This in terpretation would also mean that a longer term trace of the San Andreas fa ult must be nearby, either offshore or along the northeast boundary of the King Range terrane. An offshore fault trace would be consistent with peak h eating of King Range strata north of the Mendocino triple junction. Convers ely, shifting the fault to the east would be compatible with a slabless win dow heat source and long-distance northward translation of the Ring Range t errane after peak heating.