Tehama-Colusa Serpentinite Mélange: A Remnant of Franciscan Jurassic Oceanic Lithosphere, Northern California

The Coast Range ultramafic belt that borders the Sacramento Valley is largely a serpentinitematrix melange, called the Tehama-Colusa serpentinite melange (TCSM) after the counties it spans. It is bordered on the west across the Coast Range fault by exhumed high-P/T metamorphic rocks of the Franciscan subduction complex, and on the east, across the steep Stony Creek fault, by the Jura-Cretaceous terriginous clastic strata of the Great Valley Group (GVG). Remnants of the Coast Range ophiolite (CRO) that lie stratigraphically beneath the basal GVG are exposed only at the northern end of the Tehama-Colusa serpentinite melange belt. The serpentinite protolith was peridotite tectonite (harzburgite > dunite), comprising oceanic upper mantle. Non-native melange blocks in the serpentinite are chiefly basaltic submarine lava and sparse radiolarian ribbon chert (Middle and early Late Jurassic), plus rare plutonic rocks and slaty argillite. The serpentinite melange protolith was basaltic oceanic crust above uppermost...