The Hess Deep, in the eastern equatorial Pacific, is a rift valley providing an exposure of young crust produced by fast- spreading on the East Pacific Rise. In the walls of the rift valley a section of normal ocean crust is available for sampling and study. The presence of lower crustal rocks in the basal wall and rift valley floor suggests that drilling within the Hess Deep will provide rocks from the plutonic layers of the crust and shallow mantle. A proposal for drilling these deep crustal rocks has been highly ranked by JOIDES. The present award will support a detailed geophysical study to constrain the crustal structure in the region proposed for drilling. Specifically the project will undertake sea-floor seismic and gravity experiments to elucidate velocity and density structure in the region surrounding the proposed drilling locations. These measurements will help to test differing models for exposure of deep crustal layers in Hess Deep by serpentine diapirism or by low-angle detachment faulting. Additionally, a short survey will be done of the west flank of the East Pacific Rise on crust which is conjugate to Hess Deep to define the history of spreading when the Hess Deep crust was formed.
