The density structure associated with oceanic crustal rifting at the Hess Deep: a seafloor and sea-surface gravity study

Seafloor and sea-surface gravity measurements provide new constraints on the lithospheric structure of the Hess Deep at the triple junction between the Pacific, Cocos and Nazca plates. As the Cocos-Nazca ridge propagates westward, breaking into young oceanic crust created at the East Pacific Rise, rocks attributed to the base of the crust or to the upper mantle are exposed on the seafloor at Hess Deep, the tip of the propagator. Seafloor gravity data were collected along two north-south profiles: the western profile runs through the deepest part of the Hess Deep valley, and the eastern profile runs over the tip of the Cocos-Nazca neovolcanic ridge. Shipboard gravity track lines provide coverage of the rift valley from the northern to the southern rim. Bouguer gravity anomalies are negative over the deepest part of the basin and positive north of it at the intra-rift ridge where gabbros and hartzburgites have been drilled. Seafloor and sea-surface gravity data are modeled together in a 2D stochastic inversion and a 3D ideal body inversion. When interpreted in the light of geologic data, gravity results favor an asymmetric emplacement mode for the deep-origin rocks, the alteration of the mantle material playing a key role in the gravity signal. In addition, new magma is emplaced from the east forming a neovolcanic ridge over strongly serpentinized peridotite; the density distribution shows that positive buoyancy cannot be the driving force for magma ascent through the upper crust.