With only approximately one quarter of plate margins composed of subduction zones, North American plate is an unique continental plate featured with a western active continental margin atop widespread slow seismic velocity anomalies in the asthenosphere, an eastern passive continental margin covering several localized regions of slow seismic velocity, and a strong central cratonic root (Laurentia). The coexistence of the prominent thermal and compositional structures beneath the North American plate complicates the construction of numerical models needed to investigate the dynamics of the whole plate. Recently, a new generation mantle convection code, ASPECT (Advanced Solver for Problems in Earth ConvecTion) equipped with fully adaptive mesh refinement (AMR) technology opens up the potential to build a multi-scale global mantle flow model with a local high-resolution focus beneath the North America plate. Given the immature state of this new code for mantle flow modeling in 3-D spherical shell geometry at the beginning of my doctoral study, I first developed a new geoid algorithm for the 3-D spherical AMR numerical modeling based on ASPECT. Then I systematically benchmarked the velocity, dynamic topography, and geoid solutions from ASPECT through analytical kernel approach in the uniform mesh. I further verified the accuracy of the AMR mantle flow computation in the 3-D spherical shell geometry. Based on the improved ASPECT code, I construct global mantle flow models to investigate the driving forces for the North American plate motion. I focus on the comparison between the effects of near-field slabs (Aleutian, central American, and Caribbean slabs) and far-field slabs (primarily those around western Pacific subduction margins) and find that the far-field slabs provide the dominant driving forces for the North American plate. I further identified that interpreting the extremely slow seismic anomalies associated with the partial melt in the uppermost mantle around southwestern U.S. as purely thermal in origin results in considerably excessive resistance to North American plate motion. My numerical experiments prove that a significantly reduced velocity-to-density scaling (0.05 or smaller in our models) from the original thermal scaling coefficients (0.25 in our models) for these negative seismic shear-velocity anomalies must be incorporated into the construction of the buoyancy field to predict North American plate motion. I also examine the role of the lower mantle buoyancy including the ancient descending Kula-Farallon plates and the active upwelling below the Pacific margin of North American plate. Lower mantle buoyancy primarily affects the amplitudes, as opposed to the patterns of both North American and global plate motions. Another part of this dissertation reports the receiver function analysis along a recent dense seismic array across the eastern U.S from the western border of Ohio to the Atlantic coast of Virginia. 3D stacking yields shallowing trends of 410-km and 660-km discontinuities and thinning transition zone thickness from the inland to the coast. These results are hard to reconcile with any of the three existing hypotheses regarding the vertical mantle flow patterns beneath the eastern U.S., including edge-driven convection excited by the craton edge, hydrous upwelling from the dehydration of the deep Farallon slab, and the sinking of the delaminated or dripped mantle lithospheric block below the central West Virginia/Virginia border. A hydro-thermal upwelling beneath the eastern U.S. coastal plain due to hydrated transition zone and the neighboring passive hot upwelling induced by the descending Farallon slab in the lower mantle is consistent with the results from 3D stacking. The hydro-thermal upwelling hypothesis is also able to reconcile the shallower tectonic processes and deeper mantle dynamics below the eastern U.S. through its dehydration melting atop 410-km discontinuity. Overall, this dissertation documents the technical details on the improvements of the ASPECT code in mantle flow modeling and provides new insights into the dynamics and evolution of the North American continent. / Doctor of Philosophy / Chapter 1 details the motivation of the study in this dissertation, which covers three topics in the disciplines of geodynamics and seismology. Recently, the new computational tools of geodynamic modeling into the Earth's interior have been extensively developed. One of the cutting-edge technical advances is adaptive mesh refinement (AMR), which enables the construction of mantle flow models in highly variable resolution within the domain. However, the accuracy of the results from these multi-scale models needs to be verified. In addition, the algorithms of the geoid (equipotential surface of the gravity on the Earth) in spherical harmonic domain needs to be updated in accordance with AMR mantle flow computation. Chapter 2 documents a geoid algorithms in spherical harmonic domain working with AMR mantle flow simulation. This geoid algorithm is developed based on a a new generation mantle convection code, ASPECT (Advanced Solver for Problems in Earth ConvecTion). The numerical results including velocity, topography, and geoid from ASPECT are systematically benchmarked in both the uniform mesh and the adaptive variable mesh. The AMR simulation of ASPECT is able to achieve nearly the same high accuracy as that from the highly resolved uniform mesh. Chapter 3 systematically investigates the driving forces for the North American plate motion. Through the mantle flow modeling based on well-developed ASPECT code, I find that the remote subducting slabs (primarily those around western Pacific subduction margins), as opposed to the nearby marginal slabs (Aleutian, central American, and Caribbean slabs), provide the dominant driving forces for the motion of the North American plate. I further confirm that a reasonable estimation on the positive buoyancy from the extremely slow seismic velocity anomalies associated with partial melt in the uppermost mantle around southwestern U.S. is necessary to predict North American plate motion. Lower mantle buoyancy primarily affects the amplitudes, as opposed to the patterns of both North American and global plate motions. Chapter 4 reports the results of a seismic survey on the transition zone (the mantle region between ~410-km and ~660-km depths) structures below the eastern U.S. Our results can be explained by a hydro-thermal upwelling beneath the eastern U.S. coastal plain due to hydrated transition zone and the neighboring passive hot upwelling induced by the descending Farallon slab (an ancient oceanic plate subducting below the North American plate) in the lower mantle. Chapter 5 concisely summarizes the major findings of the above three topics in this study.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/112811 |
| Date | 15 June 2021 |
| Creators | Liu, Shangxin |
| Contributors | Geosciences, King, Scott D., Caddick, Mark J., Stamps, D. Sarah, Chapman, Martin C. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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