Abstract

The 400-km-long offset of the East Pacific Rise spreading axis between the Quebrada Fracture Zone (lat 3.5° S) and the Gofar Fracture Zone (lat 4.6° S) has been surveyed using the long-range sidescan sonar GLORIA. The offset is accomplished in three distinct fracture zones, each of which contains up to four actively slipping transform faults separated by short spreading centers. Transform spacings range from 5 to 16 km. Each transform is associated with its own transverse valley, but at Quebrada Fracture Zone, and to some extent at the other two, these valleys merge into one large, slightly oblique, fracture-zone valley. The morphological trends of fracture-zone valleys are therefore not necessarily reliable for estimating directions of relative plate movement. Multitransform fracture zones appear to be much commoner on the fast-spreading East Pacific Rise than on slower spreading ridges. The “tectonic spreading fabric,” which comprises closely spaced normal faults formed near the spreading axes and striking subparallel to them, is modified to a sigmoidal pattern between the closely spaced transforms. Even midway between transforms, these faults remain oblique to the spreading direction. The degree of obliquity increases as the transform separation decreases, showing progressive rotation of the stress ellipse as transforms are approached.

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