HESS-DEEP RIFT-VALLEY STRUCTURE FROM SEISMIC TOMOGRAPHY

We present results from a seismic refraction experiment conducted acro ss the Hess Deep rift valley in the equatorial east Pacific. P wave tr avel times between seafloor explosions and ocean bottom seismographs a re analyzed using an iterative stochastic inverse method to produce a velocity model of the subsurface structure. The resulting velocity mod el differs from typical young, fast spreading, East Pacific Rise crust by approximately +/-1 km/s with slow velocities beneath the valley of the deep and a fast region forming the intrarift ridge. We interpret these velocity contrasts as lithologies originating at different depth s and/or alteration of the preexisting rock units. We use our seismic model, along with petrologic and bathymetric data from previous studie s, to produce a structural model. The model supports low-angle detachm ent faulting with serpentinization of peridotite as the preferred mech anism for creating the distribution and exposure of lower crustal and upper mantle rocks within Hess Deep.