Oceanic crustal structure from two-dimensional active source traveltime tomography

This thesis presents an analysis of crustal thickness and velocity structure at widely differing sites along the global mid-ocean ridge system imaged using two-dimensional active source traveltime tomography. Oceanic crust is formed from partial melting of upper mantle ascending beneath divergent plate margins. Changes in seismic velocity reflect variations in crustal density and elasticity, while crustal thickness measurements derived from velocity modeling are a proxy for melt production and mantle temperature. This thesis uses the results of seismic methods to examine how crustal accretion is governed by spreading rate and axial thermal structure, while also considering the role of upper mantle dynamics.;The ultrafast-spreading segment of the East Pacific Rise south of the Garrett transform fault is marked by a cross-grain pattern of gravity lineations and intraplate volcanic ridge systems. Velocity models show these ridges are supported by an isostatic root, plate flexure, and a dynamic mantle component, the latter possibly arising as viscous fingering develops in low-density asthenospheric return flow toward the mid-ocean ridge. The intermediate-spreading Southeast Indian Ridge (SEIR) systematically varies in axial depth and ridge morphology at a constant spreading rate between 100°-116°E. Crustal thickness progressively decreases from west-to-east, implying that cooling of Indian Ocean mantle reduces melt production toward the Australian-Antarctic Discordance (AAD). Dramatic changes in axial depth and ridge morphology are observed across the transform boundary on the opposite side of the AAD, paired with a rapid eastward increase in crustal thickness. Isotopic studies show the area overlies a westward-migrating boundary between Indian and Pacific Ocean mantle domains. The AAD thus represents the cool, melt-deficient edge of Indian Ocean mantle that is gradually being displaced by more robust Pacific mantle, potentially making the AAD the simplest global example of a mantle boundary.