DEEP CRUSTAL STRUCTURE AND TECTONICS IN THE OFFSHORE SOUTHERN SANTA-MARIA BASIN, CALIFORNIA

Reprocessed deep-penetration seismic reflection profiles across the offshore southern Santa Maria Basin suggest a complex history of tectonic deformation. The top of subducted oceanic crust, imaged at the north end of EDGE line RU-10, is disrupted and eventually disappears to the south. Farther south, a strong lower crustal reflection ramps up and may reflect a possible north-dipping low-angle detachment, or a change in crustal geometry associated with the subducted Morro Fracture Zone. Above a time-transgressive unconformity, Miocene and younger sediments onlap to the south and water bottom deepens, suggesting a major regional tilt reversal in post-Miocene time. Below the unconformity, basement is displaced by largely northwest-striking normal faults that we believe form tilted crustal blocks; it is possible that Cretaceous and Paleogene sections are preserved in resulting half-grabens. Some of these normal faults, including splays off the outer Santa Lucia Bank fault system, were reactivated as minor reverse faults in post-Miocene time. There is no evidence that requires any post-17.5 Ma tectonostratigraphic terrane boundary through this area; rather, much of the structure of the southern Santa Maria Basin appears to be continuous with the rotated Western Transverse Ranges and northern Channel Islands.