55 Seismic structure of the oceanic crust and passive continental margins

Abstract

The crustal structure varies systematically with a small number of parameters such as the age of the lithosphere, the spreading rate at which the crust is formed, its position with respect to offsets in the ridge axis, and the proximity of thermal or chemical perturbations such as mantle plumes at the time of crustal formation or later in its history. Numerical modeling of the processes governing the formation and evolution of the oceanic crust allow some of these correlations to be tied to the physics of these processes, so that they can be used as predictive tools. The basic P -wave velocity structure of the oceanic crust was established in the early days of marine geophysics, before the discovery of sea-floor spreading. Oceanic crust was thinner than continental crust. Few data are available on S -wave velocities for the oceanic crust and uppermost mantle, because marine seismic sources are commonly within the water column and can only generate P waves. S waves are only generated if fortuitous mode conversions occur.