Structure and topography of the Siqueiros transform fault system: Evidence for the development of intra-transform spreading centers |
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| Authors: | Daniel J Fornari David G Gallo Margo H Edwards John A Madsen Michael R Perfit Alexander N Shor |
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| Institution: | (1) Lamont-Doherty Geological Observatory of Columbia University, 10964 Palisades, NY, USA;(2) Center for Marine Exploration, Woods Hole Oceanographic Institution, 02543 Woods Hole, MA, USA;(3) Department of Geology, University of Delaware, 19716 Newark, DE, USA;(4) Department of Geology, University of Florida, 32611 Gainesville, FL, USA;(5) Hawaii Institute of Geophysics, University of Hawaii, 96822 Honolulu, HI, USA |
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| Abstract: | The Siqueiros transform fault system, which offsets the East Pacific Rise between 8°20 N–8°30N, 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  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 1.5 Ma. Because the seafloor spreading fabric on the flanks of the western spreading centers is 500 000 years old and parallels the present EPR abyssal hill trend (350°) we conclude that a small change in plate motion was not the cause for intra-transform spreading center development in Siqueiros. We suggest that the impetus for the development of intra-transform spreading centers along the Siqueiros transform system was provided by the interaction of small melt anomalies in the mantle (SMAM) with deepseated, throughgoing lithospheric fractures within the shear zone. Initially, eruption sites may have been preferentially located along strike-slip faults and/or along cross-faults that eventually developed into pull-apart basins. Spreading centers C and D in the eastern portion of Siqueiros are in this initial pull-apart stage. Continued intrusion and volcanism along a short ridge within a pull-apart basin may lead to the formation of a stable, small intra-transform spreading center that creates a narrow swath of ridge-parallel structures within the transform domain. The morphology and structure of the axes and flanks of spreading centers A and B in the western and central portion of Siqueiros reflect this type of evolution and suggest that magmatism associated with these intra-transform spreading centers has been active for the past 0.5–1.0 Ma. |
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| Keywords: | transform Sea MARC II side-looking sonar intratransform spreading centers East Pacific Rise |
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