Date of Award
Doctor of Philosophy (PhD)
Understanding the Earth's evolution is a fundamental goal of geophysics. The mantle plays the key role in understanding the Earth's evolution. The convective planform of the mantle influences the energy exchange of the core on the core-mantle boundary (CMB) and hence the geodynamo process, determines the heat release and hence the thermal evolution of the Earth, and shapes the long wavelength topography on the surface of the Earth. Given the observationally constrained mantle viscosity structure, and realistic convective vigor and internal heating rate, the numerical modeling of mantle convection shows that the mobile-lid mantle convection is characterized by either a spherical harmonic degree-1 planform with a major upwelling in one hemisphere and a major downwelling in the other hemisphere when continents are absent, or a degree-2 planform with two antipodal major upwellings when a supercontinent is present. The Earth's mantle evolves from one to the other of these two modes due to modulation of continents, causing the cyclic processes of assembly and breakup of supercontinents. However, to constrain the realistically temporal evolution of mantle convection, other observations such as the time-dependent plate motion and geological records are needed. I reconstruct a proxy model for global plate motion for the last 450 Myr. Using the proxy plate motion model as time dependent boundary conditions, I reproduce well the basic features of the present-day mantle structure including the African and Pacific superplumes and chemical piles, and a predominantly degree 2 structure throughout the lower mantle. I further demonstrate that the mantle in the African hemisphere around the Pangea time is predominated by cold downwellings resulting from the convergence between Gondwana and Laurussia, consistent with the 1-2-1 cyclic model from the numerical modeling of mantle convection. Based on the evolution of the three-dimensional mantle structures, I reconstruct tempo-spatial evolutions of the surface and CMB heat fluxes, and the dynamic topography since the Paleozoic. My result shows that the surface heat flux increases by ~16% from 200 to 120 Ma ago as a result of Pangea breakup and the equatorial CMB heat flux has two minima that coincide with the Kiaman (316-262 Ma) and Cretaceous (118-83 Ma) Superchrons, respectively, and may be responsible for the Superchrons. My results of the dynamic topography show that the Slave Craton subsided when the major downwelling occupied the mantle beneath North America, while Sao Francisco Craton, Kaapvaal Craton, and Yilgarn Craton were supported by the large scale upwellings in the mantle beneath the very south of Pangea around 330 Ma during Pangea formation. After Pangea formed, Slave Craton started to uplift as the major downwelling heated up with time and and were controlled by the subductions close to it. Sao Francisco Craton and Kaapvaal Craton kept uplifting due to the returning African Superplume. My reconstructed dynamic topography history compares well with the vertical motion history of Slave Craton indicated by the thermochronometry study.
Zhang, Nan, "A Model for the Large-scale Evolution of Mantle Structure, Surface and Core-mantle Boundary Heat Fluxes and Surface Vertical Motions since the Early Paleozoic" (2011). Physics Graduate Theses & Dissertations. 33.