Seismic constraints on the processes and consequences of secondary igneous evolution of Pacific oceanic lithosphere

Abstract

This thesis examines the structure of Pacific oceanic lithosphere that has been modified by post-formation magmatism in order to better understand the processes of secondary magmatic evolution of the lithosphere, which can have global-scale implications for oceanic and atmospheric chemistry. In the western Pacific, widespread Cretaceous magmatism has modified oceanic lithosphere over hundreds of millions of square kilometers. Seismic models of the upper crust from within the Jurassic Quiet Zone and the crust and upper mantle near the Mariana Trench reveal crust that is locally thickened via focused extrusive volcanism and crust that is modestly but uniformly thickened over broad regions. These distinct modes of magmatic emplacement suggest the operation of both focused and diffuse modes of melt transport through the lithosphere. Analysis of seismic observations from Guaymas Basin, in the Gulf of California, endeavor to advance our understanding of sill-driven alteration of sediments, an important consequence of secondary magmatism. We show that seismically imaged physical disruption to sediments due to igneous sill intrusion can be related to changes in sediment physical properties that reflect alteration processes. We also show how sill thickness can be estimated, enabling alteration intensity to be related to sill thickness in a variety of settings.