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

Interpretation of seismic reflection data and restoration of depositional g eometries of Cretaceous forearc basin strata in the northwest Great Valley of California provide important controls on structural reconstructions of t he western margin of the Sacramento Valley and northern Coast Ranges. Monoc linal eastward dips of Great Valley Group strata and fault systems striking northwest-southeast, which are features proposed as evidence for a west-di pping blind Great Valley-Franciscan sole thrust and related backthrusts, in stead 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 separate s east-dipping Aptian and younger Great Valley Group strata from beds of lo wer Great Valley Group and Coast Range ophiolite that were deformed and ero sionally or structurally truncated by mid-Cretaceous time, Dip divergence i maged 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 hypothe sis, however, is incompatible with borehole, potential-field, and seismic-r efraction data that characterize the Sacramento Valley basement as ophiolit ic, Seaward-dipping reflections in the ophiolitic basement of the Sacrament o Valley are analogous to layering developed in the oceanic crust of volcan ic rifted margins or generated along midocean ridges. Thus, late-stage tect onic mechanisms are not required to interpret a forearc that owes much of i ts present-day bedding architecture to processes coeval with deposition, Thickening of the Great Valley Group stratigraphic section (Valanginian-Tur onian) in the hanging walls of the Paskenta, Elder Creek, and Cold Fork fau lt 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 a nd portray the Paskenta and related faults as half-graben bounding faults t hat accommodated significant northwestward tectonic transport of hanging-wa ll rocks. It is significant that these faults sole into the Coast Range fau lt, an enigmatic forearc structure that juxtaposes rocks of the Franciscan Complex (blueschists) with rocks of the Coast Range ophiolite and Great Val ley Group that have sustained only zeolite-grade metamorphism, Discovery of Jurassic-Cretaceous crustal-scale extension in the Great Valley forearc su ggests that a significant part of Coast Range fault-related attenuation dev eloped early in the history of the subduction complex.