Modeling Three-Dimensional Upper Mantle Seismic Anisotropy with Higher Mode Surface Waves
This dissertation presents a new 3-D global upper mantle model of elastic anisotropy obtained from surface wave seismic tomography. This research contributes to our understanding of deep Earth structure. The two main results are the following: (1) Our work unravels the presence of azimuthal seismic anisotropy in the mantle transition zone, to greater depths than previously found, thereby challenging common views of mantle deformation mechanisms. It also reveals a striking correlation between changes in seismic anisotropy where upper mantle phase transitions occur, which provides new constraints on the style of mantle convection; (2) We confirm the dominantly thermal nature of the oceanic lithosphere-asthenosphere boundary (LAB), and show that the Gutenberg discontinuity associated with vertical changes in azimuthal anisotropy inside the lithosphere, implying that this interface is not equivalent to the LAB, contrary to what is commonly assumed. The origin of the Gutenberg discontinuity is a result of frozen-in lithospheric structures, regional compositional variations of the mantle, or dynamically perturbed LAB.