abstract: Seismic observations have revealed two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath Pacific and Africa. One hypothesis for the origin of LLSVPs is that they are caused by accumulation of subducted oceanic crust on the core-mantle boundary (CMB). Here, I perform high resolution geodynamical calculations to test this hypothesis. The result shows that it is difficult for a thin (~ 6 km) subducted oceanic crust to accumulate on the CMB, and the major part of it is viscously stirred into the surrounding mantle. Another hypothesis for the origin of LLSVPs is that they are caused by thermochemical piles of more-primitive material which is remnant of Earth's early differentiation. In such case, a significant part of the subducted oceanic crust would enter the more-primitive reservoir, while other parts are either directly entrained into mantle plumes forming on top of the more-primitive reservoir or stirred into the background mantle. As a result, mantle plumes entrain a variable combination of compositional components including more-primitive material, old oceanic crust which first enters the more-primitive reservoir and is later entrained into mantle plumes with the more-primitive material, young oceanic crust which is directly entrained into mantle plumes without contacting the more-primitive reservoir, and depleted background mantle material. The result reconciles geochemical observation of multiple compositional components and varying ages of oceanic crust in the source of ocean-island basalts. Seismic studies have detected ultra-low velocity zones (ULVZs) in some localized regions on the CMB. Here, I present 3D thermochemical calculations to show that the distribution of ULVZs provides important information about their origin. ULVZs with a distinct composition tend to be located at the edges of LLSVPs, while ULVZs solely caused by partial melting tend to be located inboard from the edges of LLSVPs. This indicates that ULVZs at the edges of LLSVPs are best explained by distinct compositional heterogeneity, while ULVZs located insider of LLSVPs are better explained by partial melting. The results provide additional constraints for the origin of ULVZs. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2015

abstract: This research investigates Earth structure in the core-mantle boundary (CMB) region, where the solid rocky mantle meets the molten iron alloy core. At long wavelengths, the lower mantle is characterized by two nearly antipodal large low shear velocity provinces (LLSVPs), one beneath the Pacific Ocean the other beneath Africa and the southern Atlantic Ocean. However, fine-scale LLSVP structure as well as its relationship with plate tectonics, mantle convection, hotspot volcanism, and Earth's outer core remains poorly understood. The recent dramatic increase in seismic data coverage due to the EarthScope experiment presents an unprecedented opportunity to utilize large concentrated datasets of seismic data to improve resolution of lowermost mantle structures. I developed an algorithm that identifies anomalously broadened seismic waveforms to locate sharp contrasts in shear velocity properties across the margins of the LLSVP beneath the Pacific. The result suggests that a nearly vertical mantle plume underlies Hawaii that originates from a peak of a chemically distinct reservoir at the base of the mantle, some 600-900 km above the CMB. Additionally, acute horizontal Vs variations across and within the northern margin of the LLSVP beneath the central Pacific Ocean are inferred from forward modeling of differential travel times between S (and Sdiff) and SKS, and also between ScS and S. I developed a new approach to expand the geographic detection of ultra-low velocity zones (ULVZs) with a new ScS stacking approach that simultaneously utilizes the pre- and post-cursor wavefield.. Strong lateral variations in ULVZ thicknesses and properties are found across the LLSVP margins, where ULVZs are thicker and stronger within the LLSVP than outside of it, consistent with convection model predictions. Differential travel times, amplitude ratios, and waveshapes of core waves SKKS and SKS are used to investigate CMB topography and outermost core velocity structure. 1D and 2D wavefield simulations suggest that the complicated geographic distribution of observed SKKS waveform anomalies might be a result of CMB topography and a higher velocity outermost core. These combined analyses depict a lowermost mantle that is rich in fine-scale structural complexity, which advances our understanding of its integral role in mantle circulation, mixing, and evolution. / Dissertation/Thesis / Ph.D. Geological Sciences 2012