|Abstract or Summary
- 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).