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Browsing by Author "King, Scott D."

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    3D trench-parallel flow in the subduction region and correlation with seismic anisotropy direction
    Maiti, Tannistha (Virginia Tech, 2012-07-27)
    The motivation of this study is to understand the seismic anisotropy observations from various subduction regions of the world. In subduction zone backarcs both trench-parallel and trench-normal seismic anisotropy, or fast wave polarization direction of shear wave, are observed. In the mantle the general assumption is that seismic anisotropy is caused by Lattice Preferred Orientation (LPO) of olivine minerals and that the direction of anisotropy is an indicator of the direction of mantle flow. The complex pattern of seismic anisotropy observations suggests that the flow geometry in the vicinity of subduction zones differs at different subduction zones with some subduction zones having trench perpendicular flow, consistent with corner flow in the mantle wedge while other subduction zones have trench parallel flow, consistent with a mode of flow where material from the mantle wedge flows around the edges of the slab. It should be noted that the direction of LPO orientation can also be modified by the presence or absence of water, pressure, and temperature in the mantle and that it is possible that the difference in anisotropy observations reflects a difference in water content or thermal structure of back arcs. The aim of this study is to test whether the flow geometry of mantle in numerical subduction calculations can influence the direction of seismic anisotropy and if we parameters that control the pattern of flow can be identified. In this study we explicitly assume that seismic anisotropy occurs only due to plastic and dynamic re-crystallization of mantle mineral forming LPO. To approach the problem two different models are formulated. In one of the models the trench evolves self-consistently, with no prescribed artificial zones of weakness. The self-consistent model has a sticky-air layer at the top of the model domain that mimics a "free-surface." The other model has the same initial conditions but a trench-migration velocity boundary condition is imposed to the model. The mantle flow pattern for the self-consistent model is consistent with the 2D corner flow with no flow around the trench and no trench migration. However when the trench-migration velocity boundary condition is imposed, 3D flow around the mantle is observed. The stress field from these simulations are used to calculated instantaneous strain axis directions which correlate with LPO directions. The LPO orientations are measured from the models showing that the seismic-anisotropy direction is primarily trench-perpendicular for both models. Because the models have different flow patterns, the trench-perpendicular anisotropy alignment that is calculated for both the models is a bit puzzling. It could be that factors such as high temperature and non-linear rheology cause the LPO direction to align trench perpendicular in both the cases. It can also be possible that the 3D vertical flow is not strong enough to cause change in orientation of the LPO direction. From the present study it can be concluded that by looking at the LPO direction nature of mantle flow might not be predicted. This suggests that in addition to flow direction other factors such as the presence of water in mantle wedge, pressure, and high temperature due to viscous coupling modify the seismic anisotropy directions.
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    Analyzing Low Frequency Seismic Events at Cerberus Fossae as Long Period Volcanic Quakes
    Kedar, Sharon; Panning, Mark P.; Smrekar, Suzanne E.; Stahler, Simon C.; King, Scott D.; Golombek, Matthew P.; Manga, Michael; Julian, Bruce R.; Shiro, Brian; Perrin, Clement; Power, John A.; Michaut, Chloe; Ceylan, Savas; Giardini, Domenico; Lognonne, Philippe H.; Banerdt, William B. (2021-04)
    The InSight Mission began acquiring the first seismic data on Mars in early 2019 and has detected hundreds of events. The largest events recorded to date originate at Cerberus Fossae, a young volcanic region characterized by high volume, low viscosity lava flows. A handful of Low Frequency (LF) quakes that share key attributes of Long Period quakes recorded on Earth's volcanoes are also traced to Cerberus Fossae. This study explores whether a traditional volcanic source model that simulates the generation of tremor as pressurized fluid makes its way through a channel at depth, can explain these atypical LF events. We consider a wide range of physical parameters including fluid viscosity, the ratio of driving pressure to lithostatic pressure, aspect ratio of the channel, and the equilibrium channel opening. We find that the model can produce the observed seismic signature, with a combination of low-viscosity magma and high volume flux of similar to 10(4) - 10(5) m(3)/s that are within an order-of-magnitude agreement with Cerberus Fossae lava flow properties deduced from analysis of lava flow dimensions. It is impossible, however, at this stage to conclude whether or not this is a likely explanation for Mars, as the model results in fluxes that are extreme for Earth yet are just within bounds of what has been inferred for Cerberus Fossae. We therefore conclude that we cannot rule out active magma flow as the mechanism responsible for the atypical LF events that likely originate from Cerberus Fossae.
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    Ceres internal structure from geophysical constraints
    King, Scott D.; Castillo-Rogez, Julie C.; Toplis, M. J.; Bland, Michael T.; Raymond, Carol A.; Russell, Christopher T. (2018-09)
    Thermal evolution modeling has yielded a variety of interior structures for Ceres, ranging from a modestly differentiated interior to more advanced evolution with a dry silicate core, a hydrated silicate mantle, and a volatile-rich crust. Here we compute the mass and hydrostatic flattening from more than one hundred billion three-layer density models for Ceres and describe the characteristics of the population of density structures that are consistent with the Dawn observations. We show that the mass and hydrostatic flattening constraints from Ceres indicate the presence of a high-density core with greater than a 1 sigma probability, but provide little constraint on the density, allowing for core compositions that range from hydrous and/or anhydrous silicates to a mixture of metal and silicates. The crustal densities are consistent with surface observations of salts, water ice, carbonates, and ammoniated clays, which indicate hydrothermal alteration, partial fractionation, and the possible settling of heavy sulfide and metallic particles, which provide a potential process for increasing mass with depth.
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    Ceres' Broad-Scale Surface Geomorphology Largely Due To Asymmetric Internal Convection
    King, Scott D.; Bland, Michael T.; Marchi, Simone; Raymond, Carol A.; Russell, Christopher T.; Scully, Jennifer E. C.; Sizemore, Hanna G. (American Geophysical Union, 2022-06)
    While we now know much about the volatile-rich world of Ceres from the Dawn mission, the deep interior remains something of an enigma, shrouded by a crust composed of water ice, carbonates, phyllosilicates, salts and clathrate hydrates. While smaller than most active moons or planets, Ceres has many features commonly associated with active, icy bodies including: hydrothermal, cryovolcanic, and tectonic features. Yet on active icy moons tidal heating is a significant component of the thermal budget; it is unclear whether radiogenic heating alone would be sufficient to supply the heat necessary for Ceres' interior to undergo solid-state convection. Here we show that transient asymmetric convection develops as the temperature within the body rises from heat generated by the decay of long-lived radionuclides (e.g., U, Th, K). The onset of transient asymmetric convection may reconcile a number of puzzling features on Ceres including: the missing large craters, Hanami Planum-the region of thickened crust, the gravity and crustal thickness, and the lithospheric stress state represented by the Samhain Catenae. Hemispheric-scale instabilities may also be important in the evolution of small bodies with small cores throughout the solar system, including the small icy moons of Saturn and Uranus as well as Kuiper belt objects.
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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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    Composition and structure of the shallow subsurface of Ceres revealed by crater morphology
    Bland, Michael T.; Raymond, Carol A.; Schenk, Paul M.; Fu, Roger R.; Kneissl, Thomas; Pasckert, Jan Hendrik; Hiesinger, Harry; Preusker, Frank; Park, Ryan S.; Marchi, Simone; King, Scott D.; Castillo-Rogez, Julie C.; Russell, Christopher T. (Nature Publishing Group, 2016-07-01)
    Before NASA's Dawn mission, the dwarf planet Ceres was widely believed to contain a substantial ice-rich layer below its rocky surface. The existence of such a layer has significant implications for Ceres's formation, evolution, and astrobiological potential. Ceres is warmer than icy worlds in the outer Solar System and, if its shallow subsurface is ice-rich, large impact craters are expected to be erased by viscous flow on short geologic timescales. Here we use digital terrain models derived from Dawn Framing Camera images to show that most of Ceres's largest craters are several kilometres deep, and are therefore inconsistent with the existence of an ice-rich subsurface. We further show from numerical simulations that the absence of viscous relaxation over billion-year timescales implies a subsurface viscosity that is at least one thousand times greater than that of pure water ice. We conclude that Ceres's shallow subsurface is no more than 30% to 40% ice by volume, with a mixture of rock, salts and/or clathrates accounting for the other 60% to 70%. However, several anomalously shallow craters are consistent with limited viscous relaxation and may indicate spatial variations in subsurface ice content.
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    Compressible Convection and Subduction: Kinematic and Dynamic Modeling
    Lee, Changyeol (Virginia Tech, 2010-10-05)
    Subduction is a dynamic and time-dependent process which requires time-dependent models for its study. In addition, due to the very high pressures within the Earth's interior, an evaluation of the role of compressibility in subduction studies should be undertaken. However, most subduction studies have been conducted by using kinematic, steady-state, and/or incompressible mantle convection models; these simplifications may miss important elements of the subduction process. In this dissertation, I evaluate the effects of time-dependence and compressibility on the evolution of subduction by using 2-D Cartesian numerical models. The effect of compressibility on the thermal and flow structures of subduction zones is evaluated by using kinematically prescribed slab and steady-state models. The effect of compressibility is primarily expressed as an additional heat source created by viscous dissipation. The heat results in thinner thermal boundary layer on the subducting slab and increases slab temperatures. With that exception, the effect of compressibility is relatively small compared with, for example, the effect of the mantle rheology on the thermal and flow structures of the mantle wedge. Plate reconstruction models show that the convergence rate and age of the incoming plate to trench vary with time, which poses a problem for steady-state subduction models. Thus, I consider the time-dependent convergence rate and age of the incoming plate in the kinematic-dynamic subduction models in order to understand the localization of high-Mg# andesites in the western Aleutians. The results show that the localization of high-Mg# andesites is a consequence of the time-dependent convergence rate and slab age along the Aleutian arc. The influence of mantle and slab parameters as well as compressibility on the slab dynamics is evaluated by using 2-D dynamic subduction models. The results demonstrate that periodic slab buckling in the mantle results in periodic convergence rate and dip of the subducting slab; time-dependence is a natural expression of subduction. The effect of compressibility on the slab dynamics is not significant. The periodic convergence rate and dip of the subducting slab explain time-dependent seafloor spreading at the mid-ocean ridge, convergence rate of the oceanic plate at trench and arc-normal migration of arc volcanoes.
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    Contemporary Ionospheric Scintillation Studies: Statistics, 2D Analytical and 3D Numerical Inversion
    Conroy, James Patrick (Virginia Tech, 2022-08-31)
    The propagation of radiowaves through ionospheric irregularities can lead to random amplitude and phase fluctuations of the signal, otherwise known as scintillation, which can severely impact the performance of Global Navigation Satellite System (GNSS) and communication systems. Research into high latitude scintillation, through statistical analysis and inverse modeling, was completed to provide insight into the temporal and spatial distribution, and irregularity parameters, which can ultimately support the development of impact mitigation techniques, and deepen our understanding of the underlying physics. The work in this dissertation focused on the statistical analysis of Global Positioning System (GPS) scintillation data, data inversion, two-dimensional (2D) and three-dimensional (3D) scintillation modeling. The statistical analysis revealed distinct trends in the distribution of scintillation, while demonstrating that for GPS signals, phase scintillation occurs most frequently and can be treated as stochastic Total Electron Content (TEC); findings which have significant implications for impact mitigation. For the first of two inversion studies, scintillation data associated with a series of Polar Cap Patches (PCPs), which are common large-scale high latitude structures, was inverted to gain insight into the composition of the underlying irregularities. The results of this study suggest that the irregularities can be modeled as rods interbedded with sheets, which is knowledge that is crucial for the anchoring of models used to develop system mitigation techniques. The final study presents the results of modeling and inversion work to identify the conditions under which a 2D analytic version of the 3D numerical Satellite-beacon Ionospheric-scintillation global model of the upper atmosphere (SIGMA) model can be used to perform modeling in high latitude regions. During the study, it was found that the analytic model tends to diverge for electron density variance times irregularity layer thickness values exceeding 2, matched reasonably well for correlation length to thickness ratios up to 0.2, and was incompatible when ratios approached 0.35. An elevation angle limitation was also identified for the 2D model, and inflated values for the electron density variance were observed overall, which are thought to result from the weak scatter limits of the analytic model. These inflated values were particularly acute in the auroral zone during elevated conditions and suggest that the analytic model used in the study is not well suited for modeling the highly elongated irregularities associated with auroral precipitation.
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    Dike-Driven Hydrothermal Processes on Mars and Sill Emplacement on Europa
    Craft, Kathleen Liana (Virginia Tech, 2013-11-07)
    Evidence of hydrothermal and tectonic activity is found throughout our solar system. Here I investigated hydrothermal and fracturing processes on three planetary bodies: Earth, Mars and Europa. For the first project, we set up a dike-driven hydrothermal system and calculated heat and water flow using boundary layer theory. Water flow rates and volumes were then compared to the requirements for surface feature formation. Results found that the water volumes produced were adequate to form Athabasca Valles, except the flow rates were low. Episodic flood releases could enable the higher flow rates if water was first collected in aquifers, possibly stored beneath ice. On the icy moon Europa, I modeled a proposed sill emplacement mechanism using a finite element code and found that water could flow up through an approximately 10 km thick ice shell without freezing. The analysis also found that shallow cracks in the ice combined with deep cracks cause a stress direction change that helps the fracture turn and propagate more horizontally. However, the sill lifetime is less than the time a study by Dombard et al. [2013] calculated to be necessary for the formation of flexure fractures along margins of double ridges. Replenishment processes will be explored in future work to help extend sill lifetime. The last investigation calculated dike induced permeability changes in the crust on Earth and Mars and related the changes to water and heat flow rates and water volumes. Comparisons were made to event plume heat and elevated fluid temperatures observed at mid-ocean ridges. Heat values determined by the models agreed well with the 10^14 to 10^17 J expected. For the Martian model, water flow rates and volumes were compared to formation requirements for the valley system Athabasca Valles. Results found that flow rates would be adequate in the high permeability damage zone adjacent to the dike. However, the lowered permeability outside the damage zone would restrict replenishment flow and could cause the need for water storage and periodic release between flood events as the volume within the damage zone is not adequate for the valley formation.
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    Dome formation on Ceres by solid-state flow analogous to terrestrial salt tectonics
    Bland, M. T.; Buczkowski, D. L.; Sizemore, H. G.; Ermakov, A., I.; King, Scott D.; Sori, M. M.; Raymond, C. A.; Castillo-Rogez, Julie C.; Russell, C. T. (2019-10)
    The dwarf planet Ceres's outer crust is a complex, heterogeneous mixture of ice, clathrates, salts and silicates. Numerous large domes on Ceres's surface indicate a degree of geological activity. These domes have been attributed to cryovolcanism, but that is difficult to reconcile with Ceres's small size and lack of long-lived heat sources. Here we alternatively propose that Ceres's domes form by solid-state flow within the compositionally heterogeneous crust, a mechanism directly analogous to salt tectonics on Earth. We use numerical simulations to illustrate that differential loading of a crust with compositional heterogeneity on a scale of tens of kilometres can produce dome-like features of scale similar to those observed. The mechanism requires the presence of low-viscosity and low-density, possibly ice-rich, material in the upper 1-10 km of the subsurface. Such substantial regional heterogeneity in Ceres's crustal composition is consistent with observations from the National Aeronautics and Space Administration's Dawn mission. We conclude that deformation analogous to that in terrestrial salt tectonics is a viable alternative explanation for the observed surface morphologies, and is consistent with Ceres being both cold and geologically active.
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    Double-sided subduction with contrasting polarities beneath the Pamir-Hindu Kush: Evidence from focal mechanism solutions and stress field inversion
    Yang, Yu; Zeng, Zuoxun; King, Scott D.; Shuang, Xiao (China University of Geosciences, 2022-07)
    The Pamir-Hindu Kush region at the western end of the Himalayan-Tibet orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate conti-nental collision linked to two intra-continental subduction zones with different polarities. The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir. Here, we collect seismic catalogue with 3988 earthquake events to compute seismicity images and waveform data from 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region. Our results define two distinct seismic zones: a steep one beneath the Hindu Kush and a broad one beneath the Pamir. Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting, whereas the Pamir is domi-nated by strike-slip stress regime with shallow and intermediate-depth earthquakes. The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region. We inter-pret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate, and the Pamir zone shows southward sub-duction of the Eurasian plate. A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates. ?? 2022 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).
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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.
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    Exploring the relationship between crustal permeability and hydrothermal venting at mid-ocean ridges using numerical models
    Singh, Shreya (Virginia Tech, 2015-06-16)
    Hydrothermal systems associated with oceanic spreading centers account for a quarter of Earth's total heat flux and one third of the heat flux through the ocean floor. Circulation of seawater through these systems alters both the crust and the circulating fluid, impacting global geochemical cycles. The warm vent fluids rich in nutrients support a wide variety of unique biological communities. Thus, understanding hydrothermal processes at oceanic spreading centers is important to provide insight into thermal and biogeochemical processes. In this dissertation I present the results of numerical modeling efforts for mid-ocean ridge hydrothermal systems. In the three manuscripts presented, permeability emerges as a key controlling factor for hydrothermal venting. In the first manuscript, I use 2-D numerical models to find that the distribution of permeability in the crust controls fluid velocity as well as the amount of mixing between hot hydrothermal fluids and cold seawater. This, in turn, effects the temperature and composition of fluids emerging on the surface. For the second manuscript, I construct single-pass 1-D models to show that a sudden increase in permeability caused due to magmatic or seismic events in the seafloor causes a sharp rise in the fluid output of the system. This, in conjunction with steep thermal gradients close to the surface, results in a rapid increase of venting temperatures. In the third manuscript, I develop a particle tracking model to study fluid trajectories in the subsurface. The results show that permeability distribution in the subsurface governs fluid paths and consequently, the residence time of fluids in the crust. Based on the work presented in this document, I conclude that permeability distribution, both local and field scale, exerts a major control on hydrothermal circulation in the subsurface and on the temperature and composition of venting fluids on the surface.
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    Exploring two-phase hydrothermal circulation at a seafloor pressure of 25 MPa: Application for EPR 9°50′N
    Han, Liang (Virginia Tech, 2011-10-21)
    We present 2-D numerical simulations of two phase flow in seafloor hydrothermal systems using the finite control volume numerical scheme FISHES. The FISHES code solves the coupled non-linear equations for mass, momentum, energy, and salt conservation in a NaCl-H2O fluid to model the seafloor hydrothermal processes. These simulations use homogeneous box geometries at a fixed seafloor pressure of 25 MPa with constant bottom temperature boundary conditions that represent a sub-axial magma chamber to explore the effects of permeability, maximum bottom temperature and system depth on the evolution of vent fluid temperature and salinity, and heat output. We also study the temporal and spatial variability in hydrothermal circulation. The two-phase simulation results show that permeability plays an important role in plume structure and heat output of hydrothermal systems, but it has little effect on vent fluid temperature and salinity, given the same bottom temperature. For some permeability values, multiple plumes can vent at the seafloor above the simulated magma chamber. Temporal variability of vent fluid temperature and salinity and the complexity of phase separation suggest that pressure and temperature conditions at the top of the axial magma chamber cannot be easily inferred from vent fluid temperature and salinity alone. Vapor and brine derived fluids can vent at the seafloor simultaneously, even from neighboring locations that are fed by the same plume.
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    First observations of core-transiting seismic phases on Mars
    Irving, J. C. E.; Lekic, V.; Durán, C.; Drilleau, M.; Kim, D.; Rivoldini, A.; Khan, A.; Samuel, H.; Antonangeli, D.; Bruce Banerdt, W.; Beghein, C.; Bozdagk, E.; Ceylan, S.; Charalambous, C.; Clinton, J.; Davis, P.; Garcia, R.; Giardini, D.; Catherine Horleston, A.; Huang, Q.; Hurst, K. J.; Kawamura, T.; King, Scott D.; Knapmeyer, M.; Li, J.; Lognonné, P.; Maguire, Ross; Panning, M. P.; Plesa, A. C.; Schimmel, M.; Schmerr, N. C.; Stählerc, S. C.; Stutzmann, E.; Xu, Z. (Proceedings of the National Academy of Sciences, 2023-05-02)
    We present the first observations of seismic waves propagating through the core of Mars. These observations, made using seismic data collected by the InSight geophysical mission, have allowed us to construct the first seismically constrained models for the elastic properties of Mars core. We observe core-Transiting seismic phase SKS from two farside seismic events detected on Mars and measure the travel times of SKS relative to mantle traversing body waves. SKS travels through the core as a compressional wave, providing information about bulk modulus and density. We perform probabilistic inversions using the core-sensitive relative travel times together with gross geophysical data and travel times from other, more proximal, seismic events to seek the equation of state parameters that best describe the liquid iron-Alloy core. Our inversions provide constraints on the velocities in Mars core and are used to develop the first seismically based estimates of its composition. We show that models informed by our SKS data favor a somewhat smaller (median core radius = 1,780 to 1,810 km) and denser (core density = 6.2 to 6.3 g/cm3) core compared to previous estimates, with a P-wave velocity of 4.9 to 5.0 km/s at the core mantle boundary, with the composition and structure of the mantle as a dominant source of uncertainty. We infer from our models that Mars core contains a median of 20 to 22 wt% light alloying elements when we consider sulfur, oxygen, carbon, and hydrogen. These data can be used to inform models of planetary accretion, composition, and evolution.
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    Geodynamic investigation of a Cretaceous superplume in the Pacific ocean
    Xue, Jing; King, Scott D. (Elsevier, 2016-08-01)
    The similarity in both age and geochemistry of the Ontong-Java, Hikurangi, and Manihiki plateaus suggests that they formed as a single superplateau from a unique mantle source. We investigate the necessity of a thermal superplume to form the Great Ontong-Java plateau at about 120 Ma using 3D spherical models of convection with imposed plate reconstruction models. The numerical simulations show that the giant plateau which formed as a result of melting due to the interaction of a plume head and the lithosphere would have been divided into smaller plateaus by spreading ridges, and end up at the present locations of Ontong-Java, Manihiki, and Hikurangi plateaus as well as a fragment in the western Caribbean. By comparing temperature and melt fraction between models with and without an initial thermal superplume, we propose that a Cretaceous superplume in Pacific at 120 Ma is required to form large igneous plateaus.
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    A Geodynamic Investigation of Continental Rifting and Mantle Rheology: Madagascar and East African Rift case studies
    Rajaonarison, Tahiry A. (Virginia Tech, 2021-02-18)
    Continental rifting is an important geodynamic process during which the Earth's outer-most rigid shell undergoes continuous stretching resulting in continental break-up and theformation of new oceanic basins. The East African Rift System, which has two continentalsegments comprising largely of the East African Rift (EAR) to the West and the easternmostsegment Madagascar, is the largest narrow rift on Earth. However, the driving mechanismsof continental rifting remain poorly understood due to a lack of numerical infrastructure tosimulate rifting, the lack of knowledge of the underlying mantle dynamics, and poor knowl-edge of mantle rheology. Here, we use state-of-art computational modeling of the upper660 km of the Earth to: 1) provide a better understanding of mantle flow patterns and themantle rheology beneath Madagascar, 2) to elucidate the main driving forces of observedpresent-day∼E-W opening in the EAR, and 3) to investigate the role of multiple plumesor a superplume in driving surface deformation in the EAR. In chapter 1, we simulate EdgeDriven convection (EDC), constrained by a lithospheric thickness model beneath Madagas-car. The mantle flow associated with the EDC is used to calculate induced olivine aggregates'Lattice Preferred Orientation (LPO), known as seismic anisotropy. The predicted LPO isthen used to calculate synthetic seismic anisotropy, which were compared with observationsacross the island. Through a series of comparisons, we found that asthenospheric flow result-ing from undulations in lithospheric thickness variations is the dominant source of the seismicanisotropy, but fossilized structures from an ancient shear zone may play a role in southern Madagascar. Our results suggest that the rheological conditions needed for the formationof seismic anisotropy, dislocation creep, dominates the upper asthenosphere beneath Mada-gascar and likely other continental regions. In chapter 2, we use a 3D numerical model ofthe lithosphere-asthenosphere system to simulate instantaneous lithospheric deformation inthe EAR and surroundings. We test the hypothesis that the∼E-W extension of the EAR isdriven by large scale forces arising from topography and internal density gradients, known aslithospheric buoyancy forces. We calculate surface deformation solely driven by lithosphericbuoyancy forces and compare them with surface velocity observations. The lithosphericbuoyancy forces are implemented by imposing observed topography at the model surfaceand lateral density variations in the crust and mantle down to a compensation depth of 100km. Our results indicate that the large-scale∼E-W extension across East Africa is driven bylithospheric buoyancy forces, but not along-rift surface motions in deforming zones. In chap-ter 3, we test the hypothesis that the anomalous northward rift-parallel deformation observedin the deforming zones of the EAR is driven by viscous coupling between the lithosphereand deep upwelling mantle material, known as a superplume, flowing northward. We testtwo end-member plume models including a multiple plumes model simulated using high res-olution shear wave tomography-derived thermal anomaly and a superplume model (Africansuperplume) simulated by imposing a northward mantle-wind on the multiple plumes model.Our results suggest that the horizontal tractions from northward mantle flow associated withthe African Superplume is needed to explain observations of rift-parallel surface motions indeforming zones from GNSS/GPS data and northward oriented seismic anisotropy beneaththe EAR. Overall, this work yields a better understanding of the geodynamics of Africa.
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    A Geodynamic Investigation of Magma-Poor Rifting Processes and Melt Generation: A Case Study of the Malawi Rift and Rungwe Volcanic Province, East Africa
    Njinju, Emmanuel A. (Virginia Tech, 2021-01-12)
    Our understanding of how magma-poor rifts accommodate strain remains limited largely due to sparse geophysical observations from these rift systems. To better understand magma-poor rifting processes, chapter 1 of this dissertation is focused on investigating the lithosphere-asthenosphere interactions beneath the Malawi Rift, a segment of the magma-poor Western Branch of the East African Rift (EAR). Chapter 2 and 3 are focused on investigating the sources of melt beneath the Rungwe Volcanic Province (RVP), an anomalous volcanic center located at the northern tip of the Malawi Rift. In chapter 1, we use the lithospheric structure of the Malawi Rift derived from the World Gravity Model 2012 to constrain three-dimensional (3D) numerical models of lithosphere-asthenosphere interactions, which indicate ~3 cm/yr asthenospheric upwelling beneath the thin lithosphere (115-125 km) of the northern Malawi Rift and the RVP from lithospheric modulated convection (LMC) that is decoupling from surface motions. We suggest that the asthenospheric upwelling may generate decompression melts which weakens the lithosphere thereby enabling extension. The source of asthenospheric melt for the RVP is still contentious. Some studies suggest the asthenospheric melt beneath the RVP arises from thermal perturbations in the upper mantle associated with plume head materials, while others propose decompression melting from upwelling asthenosphere due to LMC where the lithosphere is thin. Chapter 2 of this dissertation is focused on testing the hypothesis that asthenospheric melt feeding the RVP can be generated from LMC using realistic constraints on the mantle potential temperature (Tp). We develop a 3D thermomechanical model of LMC beneath the RVP and the entire Malawi Rift that incorporates melt generation. We find decompression melt associated with LMC upwelling (~3 cm/yr) occurs at a maximum depth of ~150 km localized beneath the RVP. Studies of volcanic rock samples from the RVP indicate plume signatures which are enigmatic since the RVP is highly localized, unlike the large igneous provinces in the Eastern Branch of the EAR. In chapter 3, we test the hypothesis that the melt beneath the RVP is generated from plume materials. We investigate melt generation from plume-lithosphere interactions (PLI) beneath the RVP by developing a 3D seismic tomography-based convection (TBC) model beneath the RVP. The seismic constraints indicate excess temperatures of ~250 K in the sublithospheric mantle beneath the RVP suggesting the presence of a plume. We find a relatively fast upwelling (~10 cm/yr) beneath the RVP which we interpret as a rising plume. The TBC upwelling generates decompression melt (~0.25 %) at a maximum depth of ~200 km beneath the RVP where the lithosphere is thinnest (~100 km). Our results demonstrate that an excess heat source from may be plume materials is necessary for melt generation in the sublithospheric mantle beneath the RVP because passive asthenospheric upwelling of ambient mantle will require a higher than normal Tp to generate melt. Studies of volcanic rock samples from the RVP indicate plume signatures which are enigmatic since the RVP is highly localized, unlike the large igneous provinces in the Eastern Branch of the EAR. In chapter 3, we test the hypothesis that the melt beneath the RVP is generated from plume materials. We investigate melt generation from plume-lithosphere interactions (PLI) beneath the RVP by developing a 3D seismic tomography-based convection (TBC) model beneath the RVP. The seismic constraints indicate excess temperatures of ≈ 250K in the sublithospheric mantle beneath the RVP suggesting the presence of a plume. We find a relatively fast upwelling (≈10 cm/yr) beneath the RVP which we interpret as a rising plume. The TBC upwelling generates decompression melt (≈0.25 %) at a maximum depth of ≈200 km beneath the RVP where the lithosphere is thinnest (≈100 km). Our results demonstrate that an excess heat source from may be plume materials is necessary for melt generation in the sublithospheric mantle beneath the RVP because passive asthenospheric upwelling of ambient mantle will require a higher than normal Tp to generate melt.