Deep crustal reflection profiling offshore southern central California

Abstract

Deep crustal reflection profiling offshore southern central California shows that the offshore Santa Maria Basin and adjacent shelf and slope are underlain by oceanic crust which underthrusts the margin. On common‐midpoint (CMP) sections the top of the oceanic crust appears as a strong coherent reflection occurring at 6 s two‐way time (twt). The reflection can be traced continuously more than 50 km from the deep ocean basin eastward beneath the continental shelf. Depth conversion places the top of oceanic crust at 6–16 km depth with a dip of 8° toward the coast. Beneath the Santa Lucia High the crustal reflection breaks up into several east dipping segments. Above the oceanic crust an accretionary wedge increases in thickness from 3 km at the base of the slope to over 12 km beneath the Santa Lucia High. The Neogene sedimentary section above the accretionary prism exhibits relatively little deformation, implying that most of the accretion occurred during pre‐Neogene convergence between the North American plate and oceanic plates located to the west. The offshore Santa Maria Basin exhibits at least two periods of Neogene deformation: an early extensional phase, accompanied by strike‐slip faulting, occurred in the lower to middle Miocene followed by compression and basin inversion in the upper Miocene‐lower Pliocene. The development of structures in the basin began in the northwest and is progressively younger to the southeast. Faults identified in the offshore sedimentary column offset acoustic basement but are not imaged in the Franciscan basement material below 2–3 s twt and do not extend to, or offset, the deep reflections. We suggest that the oceanic crust served as a detachment surface above which the shallow deformation occurred. Neogene sediments in the offshore basins show only 1–3 km of shortening. This implies that shortening, due to Neogene oblique plate motion between the Pacific and North American plates, extends east of the offshore Santa Maria Basin and is distributed across the entire transform margin.