Tectonic evolution of the Jurassic–Cretaceous Great Valley forearc, California: Implications for the Franciscan thrust-wedge hypothesis

Abstract

Interpretation of seismic reflection data and restoration of depositional geometries of Cretaceous forearc basin strata in the northwest Great Valley of California provide important controls on structural reconstructions of the western margin of the Sacramento Valley and northern Coast Ranges. Monoclinal eastward dips of Great Valley Group strata and fault systems striking northwest-southeast, which are features proposed as evidence for a west-dipping blind Great Valley–Franciscan sole thrust and related backthrusts, instead are expressions of bedding geometry that resulted from folding of the Paskenta and related synsedimentary normal faults, depositional onlap, and a major structural-stratigraphic discontinuity. The discontinuity separates east-dipping Aptian and younger Great Valley Group strata from beds of lower Great Valley Group and Coast Range ophiolite that were deformed and erosionally or structurally truncated by mid-Cretaceous time. Dip divergence imaged between the supracrop and subcrop of the discontinuity is not unique to the ancient Great Valley forearc, but is also observed in modern forearc basins. Advocates of the Franciscan thrust-wedge model have also proposed that west-dipping, shingled patterns of seismic events imaged beneath the Sacramento Valley are imbricate thrust slices of the Great Valley Group. This hypothesis, however, is incompatible with borehole, potential-field, and seismic-refraction data that characterize the Sacramento Valley basement as ophiolitic. Seaward-dipping reflections in the ophiolitic basement of the Sacramento Valley are analogous to layering developed in the oceanic crust of volcanic rifted margins or generated along midocean ridges. Thus, late-stage tectonic mechanisms are not required to interpret a forearc that owes much of its present-day bedding architecture to processes coeval with deposition. Thickening of the Great Valley Group stratigraphic section (Valanginian–Turonian) in the hanging walls of the Paskenta, Elder Creek, and Cold Fork fault zones, combined with attenuation or complete omission of preextensional units (including the Coast Range ophiolite) and geometric evidence based on seismic reconstructions, suggest that these faults are Jurassic–Cretaceous normal faults that developed in a submarine setting. Down-structure views of the Great Valley outcrop belt simplify otherwise complex map relations and portray the Paskenta and related faults as half-graben bounding faults that accommodated significant northwestward tectonic transport of hanging-wall rocks. It is significant that these faults sole into the Coast Range fault, an enigmatic forearc structure that juxtaposes rocks of the Franciscan Complex (blueschists) with rocks of the Coast Range ophiolite and Great Valley Group that have sustained only zeolite-grade metamorphism. Discovery of Jurassic–Cretaceous crustal-scale extension in the Great Valley forearc suggests that a significant part of Coast Range fault-related attenuation developed early in the history of the subduction complex.