CRUSTAL STRUCTURE OF THE OFFSHORE SOUTHERN SANTA-MARIA BASIN AND TRANSVERSE RANGES, SOUTHERN-CALIFORNIA, FROM DEEP SEISMIC-REFLECTION DATA

Seismic reflection data from the 1986 EDGE/Pacific Gas and Electric ex periment together with refraction and gravity data are used to constru ct an integrated north-south geophysical cross section along the offsh ore Santa Maria Basin and across the offshore extension of the Transve rse Ranges. Thrust faults offsetting acoustic basement and folding Mio cene sediments root in a midcrustal detachment at 12-13 km depth. The detachment coincides with an increase in reflectivity seen on the seis mic reflection section and the maximum depth of earthquakes. A high-ve locity oceanic crustal layer at the base of the crust (17-22 km) and M oho (almost-equal-to 25 km) is offset by a low-angle thrust fault with an apparent dip almost-equal-to 10-degrees to the north offshore Poin t Arguello. Gravity models indicate crustal thickening and a change in the depth to Moho from 25 to 30 km in this region. However, the contr ibution of elastic flexure from loading of the Transverse Ranges is co nsidered to be minor in this part of the borderland. The cross section constrains Neogene shortening to less than 3% based on faulting and f olding of the sedimentary section. Shortening of the upper crust is ba lanced by a combination of ductile flow and subduction of the oceanic crust beneath the midcrustal detachment.