Seismic structure at mid-ocean ridges

Abstract

The seismic velocity structure of mid-ocean ridges is controlled by crustal thickness, cracking or porosity, temperature, melt, and crystal orientation. Low-velocity regions in the mantle that are caused by high temperatures and partial melt indicate that melt is generated in a broad region beneath spreading centers. Asymmetries in the velocity structure show that upwelling and melting beneath ridges are strongly influenced by global mantle circulation and plate motions. Crustal low-velocity regions are concentrated very near the ridge axis, so melt must migrate both vertically and horizontally to the ridge axis from the broad melt production region. The narrowness of the low-velocity zone in the crust requires that hydrothermal circulation must rapidly cool the crust. Along-axis variations, particularly at slow-spreading ridges, suggest that melt is preferentially delivered to the center of ridge segments.