Investigations of Upper Mantle Structure using Broadband Seismology

by Wagner, Lara Suzanne.

This dissertation explores the uses for data collected at broadband seismic stations to investigate upper mantle structures. In the Barents Sea region, we use seismic waveform modeling on data collected from arrays in Norway and Finland to investigate the nature of the Hales discontinuity in this area. We find that the unusually high velocities required by the move-out of the diffracted first arrival requires a discontinuity below the Moho, which we believe is probably caused by a phase transition from spinel to garnet peridotite. In Chile and Argentina, we use data collected during the Chile Argentina Geophysical Experiment to perform a regional travel time tomography in order to investigate the nature of the mantle above this unusual subduction zone. The northern half of the study area (between 30° and 33°S) is characterized by the central Chilean flat slab segment, where the descending Nazca slab dives to 100 km depth and then flattens, traveling horizontally for hundreds of kilometers before resuming its descent into the mantle. The Nazca plate in the southern half of the study area has a relatively constant dip of about 30°. The southern half exhibits normal arc volcanism roughly above the 100 – 125 km 9 contours of the downgoing slab. The northern half has had no active volcanism in the past 2 Ma, and underwent an eastward displacement of arc volcanism beginning ~10 Ma. The northern half is also remarkable for the basement-cored uplifts of the Sierras Pampeanas. Our study of the upper mantle above the southern half indicates low P wave velocities, low S wave velocities, and high Vp/Vs ratios below the arc, consistent with partial melt. Above the flat slab segment we find low Vp, high Vs, and low Vp/Vs ratios. While the nature of the material responsible for these velocities cannot be uniquely determined, the velocities indicate it must be dry, cold, and depleted. In the transition from flat to normal subduction geometries, we find velocities consistent with frozen asthenosphere, which may have been displaced by the advancing flat slab during the Miocene. 10