Three-dimensional seismic structure of the Mid-Atlantic Ridge (35°N) : evidence for focused melt supply and lower crustal dike injection

Abstract

We gathered seismic refraction and wide-angle reflection data from several active source experiments that occurred along the Mid-Atlantic Ridge near 35°N and constructed three-dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness. The tomographic images reveal anomalously thick crust (8–9 km) and a low-velocity “bull's-eye”, from 4 to 10 km depth, beneath the center of the ridge segment. The velocity anomaly is indicative of high temperatures and a small amount of melt (up to 5%) and likely represents the current magma plumbing system for melts ascending from the mantle. In addition, at the segment center, seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction. Our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust. Our results also indicate that the segment ends are colder, receive a reduced magma supply, and undergo significantly greater tectonic stretching than the segment center.