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Browsing by Author "Liu, Shangxin"

Now showing 1 - 4 of 4
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    A comparison of 3-D spherical shell thermal convection results at low to moderate Rayleigh number using ASPECT (version 2.2.0) and CitcomS (version 3.3.1)
    Euen, Grant T.; Liu, Shangxin; Gassmoller, Rene; Heister, Timo; King, Scott D. (Copernicus, 2023-06-09)
    Due to the increasing availability of high-performance computing over the past few decades, numerical models have become an important tool for research in geodynamics. Several generations of mantle convection software have been developed, but due to their differing methods and increasing complexity it is important to evaluate the accuracy of each new model generation to ensure published geodynamic research is reliable and reproducible. Here we explore the accuracy of the open-source, finite-element codes ASPECT and CitcomS as a function of mesh spacing using low to moderate-Rayleigh-number models in steady-state thermal convection. ASPECT (Advanced Solver for Problems in Earth's ConvecTion) is a new-generation mantle convection code that enables modeling global mantle convection with realistic parameters and complicated physical processes using adaptive mesh refinement . We compare the ASPECT results with calculations from the finite-element code CitcomS , which has a long history of use in the geodynamics community. We find that the globally averaged quantities, i.e., root-mean-square (rms) velocity, mean temperature, and Nusselt number at the top and bottom of the shell, agree to within 1% (and often much better) for calculations with sufficient mesh resolution. We also show that there is excellent agreement of the time evolution of both the rms velocity and the Nusselt numbers between the two codes for otherwise identical parameters. Based on our results, we are optimistic that similar agreement would be achieved for calculations performed at the convective vigor expected for Earth, Venus, and Mars.
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    Dynamics of the North American Plate: Large-Scale Driving Mechanism From Far-Field Slabs and the Interpretation of Shallow Negative Seismic Anomalies
    Liu, Shangxin; King, Scott D. (American Geophysical Union, 2022-02-17)
    With a small fraction of marginal subduction zones, the driving mechanism for the North American plate motion is in debate. We construct global mantle flow models simultaneously constrained by geoid and plate motions to investigate the driving forces for the North American plate motion. By comparing the model with only near-field subducting slabs and that with global subducting slabs, we find that the contribution to the motion of the North American plate from the near-field Aleutian, central American, and Caribbean slabs is small. In contrast, other far-field slabs, primarily the major segments around western Pacific subduction margins, provide the dominant large-scale driving forces for the North American plate motion. The coupling between far-field slabs and the North American plate suggests a new form of active plate interactions within the global self-organizing plate tectonic system. We further evaluate the extremely slow seismic velocity anomalies associated with the shallow partial melt around the southwestern North America. Interpreting these negative seismic shear-velocity anomalies as purely thermal origin generates considerably excessive resistance to the North American plate motion. A significantly reduced velocity-to-density scaling for these negative seismic shear-velocity anomalies must be incorporated into the construction of the buoyancy field to predict the North American plate motion. We also examine the importance of lower mantle buoyancy including the ancient descending Kula-Farallon plates and the active upwelling below the Pacific margin of the North American plate. Lower mantle buoyancy primarily affects the amplitudes, as opposed to the patterns of both North American and global plate motions.
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    Dynamics of the North American Plate: Numerical Development, Mantle Flow Modeling, and Receiver Function Analysis
    Liu, Shangxin (Virginia Tech, 2021-06-15)
    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.
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    Evaluating Models for Lithospheric Loss and Intraplate Volcanism Beneath the Central Appalachian Mountains
    Long, Maureen D.; Wagner, Lara S.; King, Scott D.; Evans, Rob L.; Mazza, Sarah E.; Byrnes, Joseph S.; Johnson, Elizabeth A.; Kirby, Eric; Bezada, Maximiliano J.; Gazel, Esteban; Miller, Scott R.; Aragon, John C.; Liu, Shangxin (2021-10)
    The eastern margin of North America has been shaped by a series of tectonic events including the Paleozoic Appalachian Orogeny and the breakup of Pangea during the Mesozoic. For the past similar to 200 Ma, eastern North America has been a passive continental margin; however, there is evidence in the Central Appalachian Mountains for post-rifting modification of lithospheric structure. This evidence includes two co-located pulses of magmatism that post-date the rifting event (at 152 and 47 Ma) along with low seismic velocities, high seismic attenuation, and high electrical conductivity in the upper mantle. Here, we synthesize and evaluate constraints on the lithospheric evolution of the Central Appalachian Mountains. These include tomographic imaging of seismic velocities, seismic and electrical conductivity imaging along the Mid-Atlantic Geophysical Integrative Collaboration array, gravity and heat flow measurements, geochemical and petrological examination of Jurassic and Eocene magmatic rocks, and estimates of erosion rates from geomorphological data. We discuss and evaluate a set of possible mechanisms for lithospheric loss and intraplate volcanism beneath the region. Taken together, recent observations provide compelling evidence for lithospheric loss beneath the Central Appalachians; while they cannot uniquely identify the processes associated with this loss, they narrow the range of plausible models, with important implications for our understanding of intraplate volcanism and the evolution of continental lithosphere. Our preferred models invoke a combination of (perhaps episodic) lithospheric loss via Rayleigh-Taylor instabilities and subsequent small-scale mantle flow in combination with shear-driven upwelling that maintains the region of thin lithosphere and causes partial melting in the asthenosphere.