| The work presented in this dissertation addresses mid-ocean ridge processes which govern the formation of oceanic lithosphere, including: the nature of mantle flow and melt delivery to the oceanic crust, the storage of melt within the crust, crustal formation, and the penetration of seawater into the crust for hydrothermal cooling of the magmatic system. This study was made possible through the development of new marine seismic techniques, including tomographic methods that were expanded to include secondary seismic arrivals (Chapter II), seismic anisotropy (Chapters II–IV), new analysis methods (Chapter II), and parallel computation (Appendix A). The results, when combined with those of previous reports, yield an improved view of axial magmatic and hydrothermal systems, one that relates the physical characteristics of these systems to the ascent and aggregation of melt at mantle depths and its subsequent delivery to the crust.; Using P wave data collected by a seismic experiment near 9°30′N on the East Pacific Rise, Chapters II–IV present tomographic images of the crust and shallowmost mantle seismic structure. The data set analyzed here has allowed, for the first time, the determination of the three-dimensional seismic structure of the lower crust and shallow mantle. The seismic structure of the rise is characterized by a low velocity volume (LVV; ΔVp < −0.2) that extends from 1.4 km depth below the seafloor into the mantle. The cross-axis width of the LVV is narrow in the lower crust (4–8 km) and broad in the mantle (18 km). Along the rise, the LVV varies in magnitude with the lowest velocities located between minor rise-axis discontinuities at 9°28N and 9°35′ N. Shallow crustal anisotropy is 4% from 0.5–1 km depth below the sea floor, 2% from 1–2 km depth, and 0% below; the fast direction of P wave propagation is along the rise axis. Mantle anisotropy is 6–8%; the fast direction of P wave propagation is across the rise.; The results indicate that (1) seismically-detectable, rise-parallel cracks form on or near the rise axis and penetrate to ∼2 km, (2) off-axis heat removal is relatively efficient throughout the crust near the rise and inefficient at Moho and mantle depths, (3) melt accumulates in two axial reservoirs, one near the top of the magmatic system and one near the Moho transition zone, (4) along-axis variations in the estimated melt content in the crust are similar to that in the mantle, implying that the mantle midway between the 9°28′ and 9°35′ discontinuities is presently delivering greater amounts of melt to the lower crust than to regions immediately to the north or south, and (5) mantle flow is uniformly diverging from the spreading axis; no evidence for diapiric mantle flow was found in the data.; Chapters II–IV were/will be published elsewhere and include co-authored material. |
