MARINE GEOPHYSICAL TRANSECT OF THE CONTINENTAL BORDERLAND AND ADJACENT PACIFIC SEAFLOOR, CALIFORNIA AND MEXICO

Geophysical cruise Nathaniel B. Palmer 02-07 traversed from Port Hueneme, CA to New Zealand in November 2002. We collected gravity, swath bathymetry, echo sounder, and magnetics data on the transit, as part of a Caltech marine geophysics class. The project was sponsored by NSF-OPP. Our focus in the continental borderland was to study some of the major faults and basins using primarily bathymetry and gravity. The ship has a new Simrad EM120 multibeam system with 191 beams in a swath up to 150 degrees wide, which returned bathymetry, amplitude, and sidescan with a swath width up to 7 times the water depth; and an ODEC Bathy2000 3.5 kHz wide-beam (30 degree aperture) echo sounder with 100 m subbottom penetration. The Bathy2000 subbottom files were stored in SEGY format, and retrieved with seismic data processing software to reveal the surficial strata and /or rocks. Our track headed roughly S from Port Hueneme across the Santa Cruz Island fault and the fault S of San Nicolas island, and then SSE in the Nicolas forearc terrain of the borderland. Here, we see some NNW-striking faults in an en-echelon pattern with left-stepping segments on a scale of approximately 5 km along strike. We followed a track along the E side of the Velero Basin to 31 N, 118 W, where we turned and went SW across the Patton Escarpment to the Pacific oceanic lithosphere. We find some evidence for near-surface offset on some faults, visible in the subbottom penetration data. Gravity anomalies were generally correlated with large-scale structural highs and lows. The most negative gravity anomaly we recorded in this region was in the basin south of San Nicolas island. Swath bathymetry west of the borderland revealed structures due to Pacific-Farallon and Pacific- microplate spreading including abyssal hill fabric and pseudofault traces. Further analysis of our data set may help to constrain the degree of recent activity, and/or modification by sedimentary processes, of these borderland faults.