TEHAMA-COLUSA SERPENTINITE MELANGE: A REMNANT OF FRANCISCAN JURASSIC OCEANIC LITHOSPHERE, NORTHERN CALIFORNIA

The serpentinite belt that borders the west side of Sacramento Valley is mostly serpentinite-matrix melange (Tehama-Colusa serp. melange; TCSM) comprising mid to late Jurassic oceanic crust (radiolarian chert on basalt) and mantle (peridotite), disrupted and serpentinized in the late Jurassic along an oceanic fracture zone. Long considered part of the Coast Range ophiolite (CRO), the TCSM is now recognized as a remnant of Franciscan Jurassic oceanic lithosphere. But it predates late Jurassic and Cretaceous continent-margin and trench terriginous clastic sedimentation, and it escaped the subduction, tectonic accretion, high P/T metamorphism, and exhumation of subducted rocks that characterize the Franciscan Complex farther west in the Coast Ranges.

Franciscan Jurassic oceanic lithosphere (paleoequatorial) was first emplaced against CRO Jurassic oceanic lithosphere (also paleoequatorial but reached northern latitudes earlier) by dextral transform faulting during northward transport in the latest Jurassic and earliest Cretaceous. The original transform contact is obscured by early Cretaceous thrusting, late Cretaceous exhumation of the Franciscan subduction complex along extensional high-angle faults (Coast Range fault of Jayko et al., 1987), and by Cenozoic strike-slip faulting. Early Cretaceous uplift and eastward tilting of the joined TCSM and CRO basement terranes (from underplating) accompanied the inception of outboard Franciscan subduction. This uplift and tilting of TCSM/CRO created the forearc ridge that separated Franciscan subduction on the west from Great Valley forearc basin sedimentation on the east during the remaining Cretaceous.