Seismic and Magnetic Constraints on the Structure of Upper Oceanic Crust at Fast and Slow Spreading Ridges

This thesis examines the magnetic and seismic structure of the upper crust at two contrasting ridges to better understand the roles of volcanic, tectonic and hydrothermal processes in the near-ridge environment. A direct inversion technique is developed and applied to near-bottom magnetic data from the MAR. We conclude that volcanic emplacement is focused and propose a cyclic faulting model to explain the oscillatory cross-axis magnetization pattern. Seismic analyses of the magma sill along the EPR reveal it to be thin and partially molten, which has important implications for melt availability and transport. A genetic algorithm-based inversion is developed and applied to multichannel seismic data from the EPR and MAR to compare in detail their upper crustal velocity structure. While final extrusive thickness is comparable at all spreading ridges, the style of thickening may vary. Both ridges show evidence for a narrow zone of volcanic emplacement. Vigorous hydrothermalism at the EPR may rapidly increase basement velocities relative to the MAR. Rapid modification of the extrusivedike transition at both ridges indicates that hydrothermalism is enhanced in this interval. Along-axis transport of lavas may thicken the extrusive pile at slow spreading segment ends.