Fault contacts in the northeastern Diablo Range, California, between the partially mélanged late Mesozoic Franciscan Complex and the broadly coeval, less deformed sedimentary rocks of the Great Valley sequence have been called, by definition, the Tesla-Ortigalita fault. “Coast Range thrust” is the name applied by Bailey and others (1970a) to the fault of regional extent that originally separated subducted oceanic crust and sedimentary rock of the Franciscan Complex from structurally overlying ophiolite plus shelf-slope facies sedimentary rock of the Great Valley sequence. The two faults are not equivalent. High-angle Neogene segments of the Tesla-Ortigalita fault truncate older fault surfaces of the Coast Range thrust-fault system.
Franciscan metamorphic rocks contain low-to high-pressure, low-temperature mineral parageneses characterized by the phases pumpellyite, prehnite, aragonite, lawsonite, glaucophane, and jadeitic pyroxene. Metamorphism has been variously ascribed to metastable re-crystallization, metasomatism by fluids generated in and around serpentinite, structural burial produced by subduction of an oceanic lithospheric plate, and burial plus tectonic overpressures generated beneath the Coast Range thrust fault. Suppe (1970) and Ernst (1971a) argued against metastable recrystallization and metasomatism on the basis of available field and laboratory evidence.
If metamorphism is related to serpentinization, a metamorphic aureole should surround ultramafic bodies undergoing present-day ser-pentinization. Metamorphism resulting from tectonic overpressures generated beneath the Coast Range thrust fault will be revealed by an increase in metamorphic grade toward the thrust, whereas structural burial would result in an increase in metamorphic grade with structural depth. Distribution of mineral parageneses in the northeastern Diablo Range, revealed by analyses of more than 300 thin sections of metaclastic rocks, shows no spatial relation between highest grade rocks and either exposed segments of the Coast Range thrust fault or the margins of ultramafic masses undergoing present-day serpentinization. Thus the available evidence fails to support the metastable recrystallization, metasomatic, and tectonic overpressure concepts. Only the hypothesis of structural burial is not negated by the observed relations.