Annotated record of the detailed examination of Mn deposits at the mouth of the Gulf of California during an IPOD Site Survey prior to DSDP Leg 65

During an IPOD site survey at the mouth of the Gulf of California, an axial valley about 50 km long and 600 meters deep was found on the East Pacific Rise (EPR) south of the Tamayo Fracture Zone. Magnetic anomalies along the axis of the rise change character from the axial valley to normal sections of the EPR, and off-axis anomalies tend to meander slightly with age, probably indicating that the axial injection zone wanders slightly with time. Along the Baja California margin the magnetic lineations cut across the bathymetric expression of the margin, suggesting that the original rifting of Baja from the mainland occurred along a roughly linear zone. A decrease of heat flow and an increase in sediment thickness and water depth with age from 0 to 3.5 m.y. are consistent with concepts of plate tectonics. Seismic refraction data across and adjacent to the axis of spreading suggest the presence of low-velocity material under the axis, possibly the result of a magma chamber, and a crust which thickens rapidly away from the axis.