A Two-Ship Multichannel Seismic Investigation of the East Pacific Rise South of the Garret Fracture Zone

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This is a two-ship multichannel seismic field program that will provide the first detailed geophysical constraints on the crustal structure along the East Pacific Rise south of the Garrett fracture zone. Three types of data will be collected: (1) twenty closely-space, single-ship multichannel seismic reflection profiles along the East Pacific Rise; (2) twenty- five two-ship expanding spread profiles along and parallel to the rise axis in this same area; and (3) a 700-km-long multichanel seismic reflection profile along the East Pacific Rise from the Garrett fracture zone to the large overlapping spreading center at 20.5S. These data will be used to test the spreading rate dependence of magma chamber size predicted by many ridge crest thermal models, examine the relationship between crustal magma chambers andalong-axis changes in morphology, segmentation and basalt geochemistry, and to resolve better how the first-order seismic structure of oceanic crust, including Moho and seismic layer 3, developed during the first few hundred thousand years after crustal formation. The section of the East Pacific Rise south of the Garrett fracture zone is to be a major focus of the RIDGE initiative.