Combined Normal-Incidence and Wide-Angle Reflection Studies of the Lower Crust and Moho

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The nature of the lower crust, Moho, and upper mantle is the major uncertainty in the genesis of continental lithosphere, and recent studies have indicated major differences in these among different tectonic provinces. A series of normal incidence and wide-angle recording experiments involving discontinuous expanding spread profiles around common depth points. The normal-incidence data will determine the detailed layering and the wide-angle data will resolve the velocity structure and whether it is sharp or gradational. S-wave data will be recorded to make combined P-and S-wave interpretations for fluid content and lithology. Areas to be studied include the Basin and Range where strong subhorizontal lower-crustal and Moho reflections are found, an accreted terrain in NE Washington where many arcuate lower crustal reflections and some clear discontinuous Moho reflections occur, Archean-Proterozoic transitional crust (overprinted during the Laramide?) in SE Wyoming with sporadic lower crustal and questionable Moho reflections, Proterozoic crust in Kansas with abundant arcuate lower crustal reflection pattern and no Moho reflections. These areas provide widely varying tectonic regimes, ages, and differences in reflection response that must indicate different structure. Completed pilot studies show that lower crustal velocity structure can be resolved in NE Nevada to address the question of the "double Moho reflection," and that a Moho reflection is seen at 80 km range in SE Wyoming but not observed at normal incidence. Processing will be based on a combination of standard reflection processing and new approaches for improving S/N, tau-P analysis, and migration. These studies will provide a relatively accurate and detailed velocity profile and also mean crustal velocity which can be related to tectonic history and continental growth.