Investigations of magmatic underplating beneath the northwestern Basin and Range Province, Nevada, seismic data acquisition and tectonic problems of the Columbia Plateau, Washington, and the nature of the Mohorovicic Discontinuity worldwide

In this dissertation, I present three studies that rely principally upon controlled-source seismic methods to infer how portions of the earth work.; In the first study, near-vertical and wide-angle seismic reflection data are used to delineate a magma body at the base of the crust beneath Buena Vista Valley, northwestern Nevada. The seismic response of the magma body is characterized by high-amplitude, near-vertical P-wave reflections and a comparably strong P-to-S converted phase. It is inferred that the magma body is relatively small (thickness {dollar}<{dollar} 200 meters; length {dollar}<{dollar} 1.8 km) and has a melt fraction greater than approximately 50%. It is also shown that the seismic properties of magma at high temperature and pressure can be accurately calculated using the third-order Birch-Murnaghan equation of state and the partial molar properties of component oxides.; In the next study, it is found that the traditional working model for the Mohorovicic Discontinuity (Moho) is inadequate in most geologic provinces. New geological and geophysical evidence suggests that the Moho can be a transition zone up to several kilometers thick that exhibits considerable structural and petrological complexity, and strong lateral variability in character as well as depth. Instead of requiring wholesale rejection of the traditional working model for the Moho, these new insights build upon it, and lead to an understanding of the processes that form and modify this fundamental boundary through geologic time.; In the final study, a method is formulated for obtaining clear seismic images on the Columbia Plateau, Washington, where large impedance contrasts in surface flood basalts severely degrade the seismic wavefield. This degradation is mitigated using a large-explosive-source, wide-recording-aperture shooting technique. Faults in the basalt, over 2.5 km of relief on the base of basalt, and nearly 6 km of relief on the sediment-basement interface have been imaged using a simultaneous travel-time inversion for both velocity and interface position. The geological implications of these results are considerable and, among other things, suggest that wildcat wells were not optimally located during a recent hydrocarbon exploration "boom" and that the interpretation of critical well information was in error.