Geophysical and Geochemical Constraints on the Evolution of Oceanic Lithosphere From Formation to Subduction

Gregory Horning, Ph.D., 2017
Juan Pablo Canales, Co-Advisor
Robert Sohn, Co-Advisor

This thesis investigates the evolution of the oceanic lithosphere. Alteration of the oceanic lithosphere begins at the ridge through hydrothermal flow, continues off axis through low temperature circulation, and may occur approaching subduction zones. In Chapter 2 I use a dataset of thousands of microearthquakes recorded at the Rainbow massif on the Mid-Atlantic Ridge to characterize the processes that are responsible for the hydrothermal discharge. I conclude that the diffuse zone of low-magnitude seismicity hosted in ultramafic rock and located above seismic reflectors interpreted to be magmatic sills suggests that serpentinization may play a role in microearthquake generation. In Chapter 3 I find that the Juan de Fuca plate is lightly hydrated at upper crustal levels except in regions affected by propagator wakes where hydration of lower crust and upper mantle is evident. Finally, in Chapter 4 I examine samples of cretaceous age serpentinite sampled just before subduction at the Puerto Rico Trench. I show that these upper mantle rocks were completely serpentinized under static conditions at the Mid-Atlantic Ridge. I conclude that complete hydration of the upper mantle is not the end point in the evolution of oceanic lithosphere as it spreads from the axis to subduction