Near-Bottom Geologic Studies of the East Pacific Rise Crest

A survey with near-bottom instruments was made over a 15 km by 25 km area of the East Pacific Rise crest at the mouth of the Gulf of California. The survey lines were positioned to an accuracy of about 10 m relative to three acoustic transponders on the sea floor. The abyssal hills that compose the rise crest are approximately 5 km wide, 200 m high, and are elongate parallel to the northeast trend of the rise. They have been formed by a combination of faulting and volcanism at the rise crest. In addition to producing relief, volcanism is also a means of smoothing relief in this region. A zone 22 km wide at the rise crest is essentially barren of sediment. Sediment is present on both sides of this bare zone and was measured to increase in thickness away from the rise crest on the western side. The sediment has been preferentially concentrated in local topographic lows by redistribution processes near the sea floor. The boundary between the Pacific and American plates at the rise crest is a very precise and narrow feature that lends itself to two definitions. The first is the center of intrusion which was about 100m wide during the last three magnetic field reversals and probably has been similarly as narrow for at least the past 1.2 m.y. The second definition of the plate boundary is a rift-defined zone of crustal extension at the axis of the rise crest. The plates behave rigidly beyond this zone that is presently about 5 km wide.