This project seeks to constrain the evolution of mantle structure since Paleozoic, associated with the assembly and breakup of Pangea, by using geological observations of intraplate magmatism, long-term sea level changes, and true polar wander (TPW) in the same time period. The present-day Earth's mantle is predominated by spherical harmonic degree-2 structure that is characterized by the two antipodal, major seismically slow anomalies under Africa and central Pacific (i.e., the African and Pacific superplumes) and circum-Pacific seismically fast anomalies. This degree-2 long-wavelength mantle structure is associated with the plate motion history. Observations of supercontinent cycles (i.e, assembly and breakup of Pangea and Rodinia) for the last 1 Ga and geodynamic considerations suggest that the mantle may have been predominated by even longer-wavelength structure possibly at degree 1 during supercontinent assembly. Because mantle convective structure controls heat transfer and chemical mixing in the mantle, and geology and tectonics at the Earth's surface, it is important to understand the time evolution of Earth's mantle structure.
3-D spherical models of compressible, thermochemical convection with realistic mantle rheology and plate motion history constrained by the paleogeography (i.e., Pangea assembly and breakup) will be formulated to test the following two hypotheses regarding intraplate magmatism: 1) The reduced level of intraplate magmatism before Pangea assembly and ~100 Ma after, results from the convergence of continental plates (e.g., Gondwana and Laurassia) that cool the mantle in the African hemisphere, while the subsequent enhanced magmatism associated with Pangea breakup is caused by formation of the African superplume structure as a result of circum-Pangea subduction induced upwelling return-flow below Pangea. The implication is that the mantle in the African hemisphere may have been relatively cold before the African superplume structure is formed ~100 Ma after Pangea assembly. 2) The correlation between eruption sites of intraplate magmatism and the boundaries of the African and Pacific seismic anomalies near the core-mantle boundary (CMB) is characteristic of thermochemical convection in which the dense component near the CMB is only moderately denser than the ambient mantle, but inconsistent with purely thermal convection or layered mantle convection with a flat chemical interface. While the proposed convection models reproduce the general features of mantle structure, surface dynamic topography, and the geoid (i.e., rotational pole position) for the present-day Earth, the observations of long-term TPW and sea level changes which are controlled by long-wavelength dynamic topography and geoid, pose constraints on global mantle structure evolution, particularly that in the Pacific hemisphere during Pangea time.
This four-year project with the final year as an no-cost extension investigated time-dependent mantle structure evolution and vertical motion history of continental cratons since the early Paleozoic associated with supercontinent Pangea assembly and breakup. Paleogeographic reconstruction has been used to build a proxy of plate motion model that reflects the assebly and breakup of Pangea. The plate motion model was used as time-dependent boundary conditions in 3-dimensional spherical models of mantle convection to predict the time-evolution of mantle structures. Different model parameters were tested as a sensitivity study to draw robust conclusions. Model calculations showed that the present-day Earth's mantle structure with two major hot upwelling systems in African and Pacific hemispheres (i.e., degree-2 structure) may have come to formation only in the last 200 Million years, and that the mantle in Paleozoic may have had only one major upwelling system in the Paleo-Pacific (i.e., largely degree-1 structure). This scenario of time evolution of mantle structure may provide a simple explanation of large-scale mantle magmatism since the Paleozoic. The models predicted vertical motions of continental cratons (e.g., the Slave craton in North America) since Paleozoic that agree well with burial and unroofing histories inferred from geothermochronology studies. The models also determined core-mantle boundary heat flux evolution that may have implications for geomagnetic polarity reversals. This project has led to publication of 12 peer-reviewed research articles.
