California Sea Grant College Program

The Hess Deep rift valley, at approximately 2° 14' N, 101° 33' W, displays exposures of young, lower crustal and upper mantle rocks formed at the nearby, fast­ spreading East Pacific Rise. A seismic refraction experiment was conducted across the Hess Deep rift valley to provide p-wave travel times between sea floor explosives and Ocean Bottom Seismometers. These travel time data were processed with an iterative, damped least-squares, inverse method to produce a velocity model of the subsurface structure. The resulting velocity contrasts were interpreted as lithologies originating at different depths and/or alteration of the preexisting rock units. Petrologic and bathymet­ric data from previous studies were used, along with the seismic interpretation, to pro­duce a geologic model. The model supports low-angle detachment faulting with serpentinization of peridotite as the preferred mechanism for creating the distribution and exposure of lower crustal and upper mantle rocks within the Hess Deep.

In addition to the geologic information gained from this study, linearity limita­tions of the tomographic inversion have been shown to be dependent on topography. Topography on the scale of the ray paths has been shown to effectively increase or decrease the velocity gradient, as does the Earth-flattening approximation. Valleys decrease the apparent velocity gradient; whereas, the converse is true for hills. If the velocity gradient is already weak (ie. at depths > 500 m in oceanic crust), then further decrease in gradient beneath a valley produces an environment where ray paths are highly sensitive to model change. Consequently, to avoid violating the linearity assump­tion, model changes beneath a valley must be smaller than for a hill or flat topography.