Seismic imaging of the mantle transition zone

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.Cataloged from PDF version of thesis.Includes bibliographical references.In this thesis, we developed a generalized Radon transform of SS precursors for large-scale, high-resolution seismo-stratigraphy of the upper mantle transition zone. The generalized Radon transform (GRT) is based on the single scattering approximation and maps singularities (reflections) in broad-band data into singularities (reflectors/scatters) in the medium. It is able to detect and characterize mantle discontinuities at a lateral resolution of several hundred kilometers. Synthetic tests with realistic source-receiver distributions demonstrate that the GRT is able to detect and image deep mantle interfaces at correct depths, even in the presence of noise, depth phases, phase conversions, and multiples generated by reverberation within the transition zone. We apply the GRT to ~1,600,000 broadband seismograms to delineate transition zone interfaces beneath distinct tectonic units, including a cross-section in the northwest Pacific Ocean that is far away from known down- and up-wellings, the volcanic islands of Hawaii, and the northwest Pacific subduction system. We account for smooth 3D mantle heterogeneity using first-order perturbation theory and independently derived global tomography models. Through integration with mineral physics data, the GRT seismic sections can put important constraints on the mantle temperature and mineralogy of the transition zone. Our GRT imaging results beneath the Central Pacific (including the Hawaii hotspot) reveal a more complicated mantle convection picture than a thin narrow vertical mantle "plume" passing through the transition zone. We found an 800- to 2000-kilometer-wide thermal anomaly (with a maximum temperature increase of -300 to 400 kelvin) deep in the transition zone west of Hawaii, by explaining the 410 and 660 km discontinuity topographies with olivine and garnet transitions in a pyrolitic mantle. According to our geodynamical modeling study of mantle upwellings, this might suggest that the hot materials feeding the Hawaii volcanoes do not rise from the lower mantle directly through a narrow vertical plume but may accumulate near the base of the transition zone before being entrained in flow toward Hawaii. In the GRT images of the subduction system, we found a deepened 660 km discontinuity in the slab that penetrates directly into the lower mantle according to tomography results. In another cross-section, where tomography results show that the slab is stagnant above the top of the lower mantle, we found broadening of the 660 km discontinuity signals at both edges of the slab. No corresponding uplift of the 410 km discontinuity is found. However, deepening of the 410 km discontinuity is observed beneath the continental side of the subduction system in both cross-sections, indicating hot anomalies at 410 km depth at the continental side if only the thermal effect is playing a role.by Qin Cao.Ph.D