Crustal structure along the strike of the offshore Santa Maria basin, California

Abstract

The geology of the central California offshore region contains the record of the Early Miocene transition of the western North American continental margin from a site of subduction to one of transform motion. The fundamental elements of crustal structure and composition within the margin are defined by the seismic reflection image on the marine profile, PG&E-2. The profile shows that the crust in the offshore region is composed of three layers: (a) 1–2 km of sediments in the offshore Santa Maria basin; (b) 10–12 km of Franciscan basement rocks; (c) a 7–10-km thickness of oceanic crust. Seismic sequence analysis provides a foundation for interpreting the evolution of the offshore Santa Maria basin. Results indicate that the basin fill is predominantly Miocene to Pliocene in age and that subsidence began in Early Miocene times. A Late Miocene to Early Pliocene shortening event led to faulting and folding of basin strata and to uplift of basement blocks. The Franciscan Complex, an accretionary wedge assemblage which floors the basin, reveals little internal seismic structure. However, reflections from the Franciscan-oceanic crust boundary at 12–15 km depth supply evidence for the existence of an antiform with 2–3 km relief within the oceanic crustal layer. This antiform probably formed during Early Miocene shortening of the oceanic crust. Alternatively, the antiform could represent primary oceanic-crustal topography such as seamounts or a fracture zone.