SEISMIC STUDY OF GULF OF CALIFORNIA RIFTING AND MAGMATISM

A crustal-scale, active-source seismic experiment was conducted in the Gulf of California in the fall of 2002. This experiment, funded through NSF MARGINS, aimed to image crustal structure across conjugate margins of four major basins throughout the gulf with the goals of determining the modes of extension, the influence of sedimentation and magmatism on breakup, and other features leading to a better understanding of the rifting process. Here we present an overview of the experiment, which was substantially altered at sea due to concerns for marine-mammal safety, and present some preliminary findings. The experiment involved two ships, the R/V Maurice Ewing and the R/V New Horizon. The Ewing provided the acoustic source and acquired multi-channel seismic (MCS) data using a 6-km-long streamer, and the New Horizon tended to 206 deployments of ocean bottom seismometers (OBSs). MCS and OBS data were acquired along three flow line transects across Guaymas Basin, Alarcon Basin, and between Puerto Vallarta and Cabo San Lucas. A fourth, two-part, "coast-perpendicular" transect extended from the Pacific margin across the Baja Peninsula through Bahia de La Paz and, on the Mexican mainland, across the margin south of Mazatlan and up into the Sierras. Each of these transects was instrumented with OBSs spaced 10-15 km apart and similarly spaced seismometers on land recording the offshore shots to ~100 km inland. Some initial findings include the suggestion of substantial magmatism in an early failed rift of the Alarcon Basin, fundamental asymmetry in Guaymas Basin extension and a "curious" rifted margin on the mainland side, and the existence of active subduction beneath the margin at the Tres Marias islands.