Deformation at Oceanic Plate Boundaries: Insights from Geophysical Observations

Abstract

Studies of deformation along submarine plate boundaries are constrained by the difficulties associated with conducting seismic and geodetic measurements on the seafloor. This dissertation advances three geophysical methods (tomography, geodesy, and ambient noise interferometry) to investigate how deformation is accommodated within strain cycles across the principal tectonic systems of the Wilson Cycle: rifting, seafloor spreading, and subduction. At Orca Volcano in the Bransfield Basin, a new tomographic workflow incorporating secondary arrivals through a magma chamber. The tomographic models revealed a transition in rifting style controlled by variations in mantle hydration linked to a tear in the subducting Phoenix slab. At Axial Seamount on the Juan de Fuca Ridge, three years of horizontal acoustic ranging across the caldera demonstrated that inter-eruptive extension is primarily accommodated by volumetric inflation from two pressure sources at different depths, providing new constraints on magma storage geometry and the localization of eruptions. In the Cascadia subduction zone, a decade of ambient noise interferometry with novel denoising methods uncovered distinct regional variations in shallow megathrust dynamics, including evidence for slow slip on protothrusts and fluid migration. Across these three settings, two themes emerged: fluids exert a first-order control on crustal deformation at every stage of the plate tectonic cycle, and slab tears on different depths along the subducting plate influence crustal deformation in distinct ways. Together, these studies demonstrate the value of diverse geophysical methods for resolving deformation processes that remain largely hidden beneath the oceans.