A Multichannel Seismic Investigation of the EPR at 9'N and 13'N

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The lack of detailed geophysical constraints on the structure of mid-ocean ridges has severely limited efforts to develop a comprehensive understanding of the processes forming the oceanic crust. The generation of new oceanic crust and lithosphere at accretionary plate boundaries is fundamental to plate tectonic theory. Current geological models for the crustal generation process include the existence of shallow crustal magma chamgers. Despite the growing support for the models, data defining both the existence and geometry of axial magma chambers remain controversial. This project is a multichannel seismic investigation between two well-studied segments of the East Pacific Rise at 9 degrees north and 13 degrees north. The volcanic petrology has been examined through a series of dredging and ALVIN dive programs. The tectonic and structural fabric has been examined with side scan sonar and multibeam bathymetry. Existing seismic refraction data indicate the presence of an axial low velocity zone, presumably associated with a magma chamber beneath the rise crest. The objective of the present award is to complete the analysis and interpretation of this unique data set. Specific problems to be addressed include: 1) The physical properties of the material below the magma chamber reflector; 2) the shape of the magma chamber walls and floor; 3) the development of ocean crustal structure during its first few million years; 4) the effect of basement morphology of recorded seismic reflections; and 5) variations in magma chamber characteristics along the rise axis.