Graduation date: 1974

In April of 1974, Oregon State University conducted a geophysical survey of the Orozco fracture zone, a Left-lateral transform fault which offsets the East Pacific Rise off the coast of Mexico near 15°N, 105°W. Magnetic, gravity, bathymetric, and seismic reflection data were collected during a four day period. This survey is combined with previous surveys by Oregon State University and other institutions to provide a geophysical interpretation of the Orozco fracture zone and the surrounding area and to develop a tectonic history of the northern Cocos plate. The Orozco fracture zone is characterized by a typical zone of seismicity and an offset in the magnetic anomaly pattern. There is, however, a conspicuous absence of a well defined topographic trough. This appears to be a result of the small age offset of the ridge crest, a reorientation of the fracture zone trend, and a possible southward migration of the fracture zone down the ridge axis. Three crustal and subcrustal cross sections over the Orozco fracture zone are constructed from the gravity data. One, across the active portion between the ridge offset, shows the active troughs to be underlain by a broad, low-density root extending two kilometers into the mantle. Two gravity cross sections across the East Pacific Rise show a thinning of oceanic layer 3 of nearly 2 kilometers at the rise crest and a corresponding 0.5 kilometer thickening of layer 2. A large magnetic anomaly of over 1300 gammas is found at the intersection of the Orozco fracture zone and the East Pacific Rise. A comparison with a very similar observation at the intersection of the Juan de Fuca ridge and the Blanco fracture zone in the northeast Pacific suggests that the East Pacific Rise is "leaking" into the fracture zone in this area. Many features have been observed on the northern Cocos plate which cannot be accounted for by present Pacific-Cocos motion the northeast strike of the eastern extension of the Orozco fracture zone, an apparent fanning of magnetic anomalies, and the northeast strike, as well as the origin, of the Tehuantepec ridge. Several possible schemes are examined to explain these observations and all but one are completely eliminated. The proposed explanation supposes a reorientation of the spreading center after a large change in the Pacific- Cocos pole of rotation resulting in the Zed pattern described by Menard and Atwater (1968). / Graduation date: 1976

Graduation date: 1998 / Best scan available for figures. Original is a black and white photocopy.