1 MA EAST PACIFIC RISE OCEANIC-CRUST AND UPPERMOST MANTLE EXPOSED BY RIFTING IN HESS DEEP (EQUATORIAL PACIFIC-OCEAN)

A series of dives with the French submersible Nautile has been conducted in the region of the Galapagos triple junction, at the tip of the Cocos-Nazca propagator. Most of the dives were devoted to the study of Hess Deep, a rift valley produced by rifting of oceanic crust created at the axis of the very fast-spreading East Pacific Rise. The dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra-rift ridge located to the north of the axis of Hess Deep. This shows that this situation is not specific of slow-spreading ridges. A nearly complete and unique log of young (1 Ma) crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered. We propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust. One model stresses symmetry and vertical tectonics through isostasy/diapirism of serpentinized mantle. Another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low-angle detachment faulting. In both models, which differ mainly in the structure at depth, amagmatic extension of East Pacific Rise crust by the Cocos-Nazca propagator tip results in uplift and denudation of lower crust and upper mantle.