Structure and topography of the Siqueiros transform fault system: evidence for the development of intra-transform spreading centers

The Siqueiros Transform Fault system, which offsets the East Pacific Rise between 8 degree 20'N-8 degree 30'N, has been mapped with the Sea MARC II sonar system and is found to consist of four intra-transform spreading centers and five strike-slip faults. The bathymetric and side-looking sonar data define the total width of the transform domain to be similar to 20km. The transform domain includes prominent topographic features that are related to either seafloor spreading processes at the short spreading centers or shearing along the bounding faults. The spreading axes and the seafloor on the flanks of each small spreading center comprise morphological and structural features which suggest that the two western spreading centers are older than the eastern spreading centers. Structural data for the Clipperton, Orozco and Siqueiros transforms, indicate that the relative plate motion geometry of the Pacific-Cocos plate boundary has been stable for the past similar to 1.5 Ma. Because the seafloor spreading fabric on the flanks of the western spreading centers is similar to 500,000 years old and parallels the present EPR abyssal hill trend (350 degree ) it is concluded that a small change in plate motion was not the cause for intra-transform spreading center development is Siqueiros.