An investigation of the crustal structure of the Clipperton transform fault area using 3D seismic tomography

This dissertation deals with the geology, and in particular the internal structure, of the Clipperton transform at the East Pacific Rise. A major question is whether the transform offsets disrupt the mantle upwelling and crustal accretion processes at the adjacent spreading segments. A more specific problem with the geological interpretation of the morphological expression of the Clipperton transform fault is the very narrow but elevated band of terrain associated with the strike of this fault zone. While the East Pacific Rise is known to be approximately isostatically compensated, the rugged bathymetry of the Clipperton transform domain would require very large density heterogeneities at depth. These problems have been the motivation for the CLASSIC marine geophysical experiment of 1994. A three-dimensional seismic refraction data set was collected using an airgun source and ocean bottom receivers. The seismic traveltimes from these refraction data are used to determine the seismic velocity structure of the crust and upper mantle around the Clipperton transform. For this purpose an iterative nonlinear tomography scheme has been developed. Ray paths associated with crustal and mantle refractions (Pg and Pn) and Moho reflections (PmP) are traced between the shot receiver pairs using a hybrid raytracing technique. First, a crude estimate is made of the rays using the shortest path approximation in a roughly discretized seismic velocity model. Next, these preliminary ray paths are optimized with a local conjugate gradient ray bending technique. A new seismic velocity model can be constructed by linearized inversions of the traveltime residuals with respect to a reference model. We find that the Clipperton transform is characterized by normal crustal thickness of 5.8 km, which supports an essentially two-dimensional mantle upwelling pattern at the northern East Pacific Rise. The shear zone of the Clipperton transform exhibits seismic velocities 1.0 to 1.5 km/s lower at all crustal depths than form normal oceanic crust, probably due to a higher porosity. The seismic velocity structure of the upper crust correlates strongly with the morphology of the neighboring rise axis, which shows the sensitivity of processes like the formation of a magma chamber and dike intrusion in a variable tectonic setting like the Clipperton transform fault area.