Crustal structure and tectonic evolution: Central California

Abstract

Vertical incidence and wide angle seismic data suggest the central California transform margin is underlain at depth by a horizontal detachment surface above which strike-slip and compression occur. The boundary between the Pacific and North American plates is interpreted as a combination of horizontal and vertical surfaces distributed from the base of slope offshore eastward past the San Andreas fault rather than a single vertical boundary associated with a strike-slip fault extending through the lithosphere. Deep crustal reflection profiling shows the offshore margin is decoupled above landward dipping Pacific plate oceanic crust which underthrusts the margin. A 12 km thick wedge of material was accreted offshore during pre-Neogene convergence between North America and oceanic plates located westward. Neogene shortening offshore is minor, implying compression from oblique convergent motion between the plates is accommodated onshore. Structures in the Santa Maria basin exhibit lower to middle Miocene extension accompanied by strike-slip faulting followed by compression in the upper Miocene-lower Pliocene. Reflection and refraction profiles onshore also suggest shallow decoupling levels within the crust. Structural relief of anticlines along the western edge of the San Joaquin basin is attributed to thrusting in buried Cretaceous sediments. Thrust faults sole westward at 12 km depth, into a detachment surface. Crustal rocks beneath the decollement have velocities of 6.4-6.6 km/sec and extend eastward to the middle of the Central Valley where they shallow abruptly to 5 km depth. The Greenville fault near Livermore Valley, is a strike-slip fault only penetrating the upper 5 km of crust. West of the fault, Miocene sediment with high compressional velocity, 5.0 km/sec, are present to 5 km depth. Similar velocities have been associated with shallow Franciscan rocks calling into question the use of seismic velocities alone to distinguish between different Coast Range - Great Valley lithologies. The crustal structure and style of deformation observed at the western and eastern margins of the transform zone suggest that the entire zone is decoupled above depths ranging from 6-30 km, and that shallow subduction or underplating of oceanic crust is occurring beneath the margin.