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

Abstract

The Coast Range ultramafic belt that borders the Sacramento Valley is largely a serpentinitematrix mélange, called the Tehama-Colusa serpentinite mélange (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 mélange belt. The serpentinite protolith was peridotite tectonite (harzburgite > dunite), comprising oceanic upper mantle. Non-native mélange 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 mélange protolith was basaltic oceanic crust above uppermost mantle, overlain by radiolarian chert and thin argillite (deep-sea clay?). An open-ocean setting, far from a continent margin or volcanic arc, is inferred from the absence of terriginous clastic or volcaniclastic sediments. The TCSM wraps around the Stonyford volcanic complex, a basaltic seamount that grew atop the basaltic oceanic crust during the late Middle and Late Jurassic. A widespread deepsea tectonic event in the Late Jurassic disrupted the proto-TCSM upper mantle and crust. Pervasive hydration turned peridotite into serpentinite, which invaded and mixed with the basalt/chert oceanic crust, creating mélange. Possible causes are discussed. The TCSM basaltic lavas and radiolarian chert closely resemble Franciscan oceanic crustal (not seamount) rocks, and are within the Franciscan basalt/chert age span; they are unlike the CRO igneous assemblage and its Late Jurassic tuffaceous chert. The TCSM was a segment of Franciscan oceanic lithosphere that records only Jurassic open-ocean depositional and tectonic history. It escaped latest Jurassic-Cretaceous continent-margin and trench sedimentation, subduction, subduction accretion, high-P/T metamorphism, and exhumation of deeply subducted rocks that characterize the Franciscan Complex farther west in the Coast Ranges. The TCSM and CRO terranes are remnants of two different mid Jurassic paleoequatorial oceanic plates, separated now by the Stony Creek-Beehive Flat fault system. TCSM evolution involves four successive stages of Mesozoic tectonic history: (1) Late Jurassic, when the CRO oceanic plate was being drawn NNE toward the Great Valley subduction zone in front of the oceanic Nevadan arc, trailed by TCSM (eastern Franciscan) lithosphere; (2) the latest Jurassic-earliest Cretaceous, when (a) N-S dextral transform faulting replaced earlier dextral oblique subduction, bringing the Jurassic TCSM/Franciscan oceanic plate northward alongside the unsubducted CRO plate remnant, and (b) terriginous sediments from the new Nevadan orogen spread progressively seaward across the CRO and adjacent TCSM/Franciscan Jurassic ocean floor; (3) resumption of eastward subduction in the Early Cretaceous, from a new trench lying farther west, caused (a) disruption, mélanging, and metamorphism of the Tithonian-Valanginian clastic strata and underlying Jurassic (Franciscan) oceanic crust, and (b) underthrusting, uplift, and eastward tilting of the unbroken TCSM/CRO oceanic basement, forming a new forearc ridge; and (4) a post-Valanginian Cretaceous era when Franciscan subduction operated west of the TCSM/CRO submarine ridge, and GVG forearc basin sedimentation progressed behind the ridge on the east. The Stony Creek-Beehive Flat composite fault sytem between the TCSM and CRO was a dextral transform fault during stage 2 (Tithonian-Valanginian) and a W-vergent reverse fault during stage 3 (Valanginian), during underthrusting and tilting of TCSM/CRO basement. The extensional Coast Range fault, bounding TCSM on the west, brought up deeply subducted Franciscan Cretaceous high-P/T metamorphic rocks in the late Cretaceous (Jayko et al., 1987).