Institute for Geophysics Theses and Dissertations

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This collection is updated each semester. For the most complete record of theses and dissertations, please see the ETD Collection.

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    Andean deformation and basin evolution during changes in subduction zone geometry (29–33°S)
    (2023-05-01) Mackaman-Lofland, Chelsea; Horton, Brian K., 1970-; Ketcham, Richard A; Stockli, Daniel F; Constenius, Kurt N; Folguera, Andres; Fosdick, Julie C
    Understanding the surface, crustal, and tectonic processes controlling deformation and crustal evolution along subduction margins remains an outstanding challenge in Earth science, with implications for characterizing Earth systems, resource potential, and natural hazard assessment. The southern Central Andes of Chile and Argentina define type examples of deformation, arc magmatism, and basin evolution during long-lived subduction, and provide unique opportunities to study the overriding plate response to changing plate margin conditions and tectonic regimes. This dissertation presents new geo- and thermochronological assessments of sediment provenance, basin accumulation, and deformational timing at ~29–33°S that provide critical insight into retroarc deformational patterns and subsidence mechanisms over the past ~200 Myr. Mesozoic to mid-Cenozoic deposits recorded the unroofing of basin margins and sediment contributions from the Andean magmatic arc during Late Triassic to Early Cretaceous extension, thermal subsidence, and possible slab rollback, followed by initial shortening and sediment derivation from western sources during the Late Cretaceous. Newly dated volcanic and sedimentary deposits in the retroarc hinterland provide evidence for a late Paleogene episode of intra-arc and proximal retroarc extension preceding Negoene growth of the modern Andes. Geochronological and thermochronological data for thrust sheets and Neogene foreland basin fill indicate Andean shortening initiated with the inversion of the intra-arc basin system, followed by sequential eastward migration of the fold-thrust belt–foreland basin system along a regionally connected, hybrid thin- and thick-skinned decollement. Finally, new structural interpretations and flexural thermo-kinematic model results quantify the influence of inherited structures, precursor stratigraphic architectures, thrust belt critical taper, and non-flexural geodynamic processes during Neogene construction of the modern Andes
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    A mixed forward/inverse modeling framework for earthquake deformation problems
    (2023-08-11) Puel, Simone; Becker, Thorsten W.; Lavier, Luc; Ghattas, Omar; Hesse, Marc A; Johnson, Kaj M
    Subduction is responsible for the most powerful earthquakes and dangerous volcanic eruptions, resulting in significant human casualties and economic losses. However, the prediction of these natural events remains challenging due to an incomplete understanding of the underlying physics that govern these phenomena. Key questions persist regarding stress accumulation and dissipation, rock behavior under extreme pressure and temperature, and the influence of fluids and melt in these processes. Recent advancements in space geodesy and seismic networks have enabled the measurement of seismic responses and surface displacements, revealing the complex dynamics of subduction zones. To enhance our comprehension of these processes, computational modeling that integrates various types of observations and constraints is crucial. This can be achieved through forward modeling, where model parameters are adjusted to better fit the observations, or through inverse modeling, which extracts critical parameters and the underlying physical mechanisms directly from the data. However, a comprehensive numerical physics-based modeling framework that combines both forward and inverse capabilities, using adjoints, within a unified infrastructure is currently lacking. The objective of this dissertation is to address this gap by developing an open-source, flexible, transparent, and easily extendable framework capable of handling multi-physics coupled problems. This framework will also drive the advancement of innovative techniques for analyzing earthquake systems. It introduces a novel implementation of fault discontinuity within the finite-element model, an improved fault slip inversion method that does not require Green’s function computations, and a novel approach to infer material structure solely from surface displacement data, eliminating the need for seismic velocity analysis. Furthermore, by incorporating these approaches, it offers a novel joint inversion of surface geodetic data, facilitating the simultaneous recovery of subduction zone structure and coseismic slip distribution. This provides valuable insights into the interplay between heterogeneous material structure and fault processes. As a demonstration, the framework successfully recovers the coseismic slip distribution and subduction zone structure by inverting the coseismic surface displacements recorded during the 2011 M9 Tohoku-oki earthquake in Japan. The results reveal weaker material beneath several volcanoes in the same region where local coseismic subsidence was reported during the earthquake. Accounting for heterogeneity in fault slip inversions is crucial for accurately matching the surface displacement data, as suggested by previous studies. Overall, the proposed framework represents a significant advancement in subduction zone modeling, providing a comprehensive tool for understanding and analyzing these complex phenomena, thereby paving the way for improved hazard assessment and risk mitigation strategies.
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    Numerical modeling of viscoplastic mantle convection with damage rheology to investigate dynamics of plate tectonics
    (2023-08-11) Heilman, Erin; Becker, Thorsten W.; Faccenna, Claudio; Lavier, Luc; Hesse, Marc; Dannberg, Juliane
    Mantle plumes are typically considered secondary features of mantle convection, yet their surface effects over Earth's evolution may have been significant. We use 2-D convection models to show that mantle plumes can in fact cause the termination of a subduction zone. This extreme case of plume-slab interaction is found when the slab is readily weakened, e.g. by damage-type rheology, and the subducting slab is young. We posit that this mechanism may be relevant particularly for the early Earth, and more generally, plume "talk back" to subduction zones may make plate tectonics more episodic in certain cases. When these models are carried out in a 3-D geometry, we see the same plume-slab terminations take place and can observe the effect of lateral extent on the dynamics. We examine the dynamics of these terminations through their geometry, frequency, and effect on the surface. By varying the proportion of internal heating, we show the effect of mantle temperature on the efficacy of plume-slab terminations and draw parallels to the evolution of the Earth's mantle temperature. A subdued version of these plume-slab interactions may remain relevant for past and modern subduction zones. Such core-mantle boundary – surface interactions may be behind some of the complexity of tomographically imaged mantle structure, e.g., for South America. Continuing the exploration of our damage rheology, we investigate spreading ridges, which are another feature integral to plate tectonics. We carry out 3-D internally heated mantle convection modeling to produce discrete spreading ridges and transform faults in a freely convecting model. The inclusion of damage in these models allows for transform faults to develop more easily than in previous modeling attempts. We vary the strength of the damage in its weakening and healing proportions to understand the effect on the dynamics and lifespan of the transform faults. These transform faults match well with observations from Earth, and as a result these models are a stepping stone to a new class of global mantle convection modeling.
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    Mechanisms of lithospheric failure during late continental rifting and early subduction
    (2021-08-13) Shuck, Brandon Douglas; Van Avendonk, Harm J. A.; Gulick, Sean P. S.; Bangs, Nathan L.B.; Becker, Thorsten W.; Lavier, Luc L.; Shillington, Donna J.
    Two fundamental components of plate tectonics are the separation of continents, leading to new ocean basins, and the initiation of subduction zones, which facilitate recycling of the Earth's outer shell into its interior. In order for continental rifting and subduction initiation to succeed, tectonic driving forces must overcome resisting forces and strength of the lithosphere. If achieved, the lithosphere undergoes failure and a new plate boundary is established, wherein subsequent strain is localized along a narrow weak zone, such as a subduction zone megathrust or seafloor spreading center. Though these processes are conceptually straightforward, many aspects remain elusive. In particular, intact lithospheric strength is thought to be far greater than available tectonic forces, yet observationally continental breakup and subduction initiation occur frequently throughout Earth's history. The goal of this dissertation is to further investigate this force paradox by exploring the weakening mechanisms that assist lithospheric failure during late continental rifting and early subduction. Active-source seismic data are used to image geologic processes and the tectonic evolution along two study areas - the Eastern North American Margin and the Puysegur Margin, New Zealand. Along the Eastern North American Margin, I show that new mafic crust was emplaced above a thinning subcontinental mantle lithosphere that resisted breakup despite abundant magmatism. I propose a new model in which continental crust separated before the lithosphere and complete breakup was not achieved for ~25 Myrs after the arrival of melts. I then image mantle dynamics near the lithosphere-asthenosphere boundary during final stages of rifting and show that rupture was enabled by highly organized crystallographic textures that focused melt and deformation into a narrow weak zone. At the Puysegur Margin, I argue that subduction initiation was aided by previous phases of continental rifting and strike-slip. Rifting stretched continental crust of Zealandia and later dextral strike-slip translated thin and dense oceanic crust from farther south and juxtaposed it with thick continental crust at a collisional restraining bend. Ideal conditions ensued, where buoyancy contrasts and pre-existing fault zones weakened the lithosphere and facilitated subduction nucleation. Since initial underthrusting, subduction initiation became more efficient as the trench propagated southward over time. I conclude with a novel 4D model where subduction initiation is resisted at the site of nucleation but followed by mechanically easier and faster initiation and lateral propagation as the plate boundary develops along-strike. Inherited lithospheric heterogeneities and weak zones are the dominant mechanism allowing the plate tectonic cycle to persist on Earth.
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    Seismic traveltime inversion in three-dimensional heterogeneous media
    (1990) Finn, Christopher Jude, 1960-; Backus, Milo M., 1932-
    The measured traveltimes of specular reflection events are inverted to obtain a low spatial frequency, three-dimensional model of the reflector geometry and the compressional wave propagation speed. B-spline functions are used to describe the shapes of the interfaces and the lateral variations in velocity. The inversion is performed by optimizing a maximum likelihood criterion using a Newton based iteration. Model updates are obtained by iterative forward modeling and solution of the linearized equation set derived from the maximum likelihood criterion. In the forward problem, the ray tracing equations are solved as a two point boundary value problem with appropriate internal boundary conditions at velocity discontinuities. Analytic expressions for the Frechet derivatives necessary to obtain the model updates are given. Conventional methods are compared to the traveltime inversion technique using synthetic examples. For a relatively simple earth model containing only moderate lateral velocity variations hyperbolic moveout analysis followed by a Dix inversion produces a biased estimate of the velocity and depth. This is a consequence of the simplifying assumptions of the method. In this case, the more general traveltime analysis provides a better result. This is also true for a more complex earth model containing lateral velocity variations and interfaces with large dips and curvatures where the conventional methods fail badly. Picked traveltimes are used as the data in the inversion although the use of the data semblance or the stack power along the predicted traveltime trajectory is also explored. These criterion are shown to be more nonlinear than the least-squares data residual measure. Thus, it is difficult to converge to a global minimum using these criterion and more accurate initial guesses are necessary. An application of the traveltime inversion technique to a 3D marine data set is presented. In this application the effects of the seismic source and the recording system on the measured traveltimes are estimated. The time delay between the first break and the main pulse of the minimum phase source wavelet and the effect of the ghost reflections from the free surface are compensated for in the prediction of the measured traveltimes
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    Hydrate-bearing sands in the Terrebonne Basin record the transition from ponded deposition to bypass in the deep-water Gulf of Mexico
    (2022-12-02) Varona, Gabrielle Marie; Flemings, Peter Barry, 1960-; Covault, Jacob A; Cook, Anne E
    Herein, I show how seismic stratigraphy can be used to describe the transition from ponded deposition to bypass within a gas hydrate system in deep-water Gulf of Mexico. In Walker Ridge Block 313 (WR 313), the Green and Orange sands are gas hydrate bearing sheet sands incised by a paleo channel system within the southwestern lobe of the Terrebonne Basin. I discuss two intervals that characterize the deposition of the sheet sands in conjunction with the channel. The Green interval captures the first appearances of coarse-grained material which I classify as the Green sand. The Orange interval encompasses channelized deposits and corresponding muddy levee deposits and is capped by the Orange sand. Within WR 313, the channel is oriented NW-SE and flowed towards the SE where salt related uplift took place. During the Green interval, salt movement influenced the pattern of deposition upstream and incision downstream. During the Orange interval, the channel aggraded, encountered the sheet deposition of the Orange sand, and then shut off. Well log data from two different wells support my interpretation of the Green and Orange sands as sheet sands due to their thickness patterns ~2000 m away from the channel axis. The WR 225 001 (WR 225-1) well penetrates the Green and Orange deposits upstream and the WR 313 H001 (WR 313-H) well encounters these deposits downstream. The gamma ray and resistivity logs from the WR 225-1 well record two coarsening upward signatures several meters apart which are interpreted as the Green sand (74 meters thick) and the Orange sand (22 meters thick). I correlate these gamma ray signatures with thinner packages of the Green sand (35 meters) and the Orange sand (12 meters) in the WR 313-H well on the downstream end. Due to the thickness of the sands away from the channel and the corresponding seismic character, I interpret that the Green and Orange sands record the last episodes of high energy deposition that interact with a submarine channel system.
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    Terrestrial constraints on the distribution of brine in Europa’s ice shell
    (2022-09-26) Wolfenbarger, Natalie Soheila; Blankenship, Donald D.; Soderlund, Krista Marie, 1982-; Hesse, Marc; Goudge, Timothy A; Heimbach, Patrick; Moore, William B
    Jupiter’s ice-covered moon Europa is considered a prime target in the search for habitable worlds within our Solar System. Freezing at the ice-ocean interface (accretion) entrains material from the ocean, including salts and potential biosignatures. In this work, we first examine how accreted ice on Earth can be used to estimate the bulk composition of Europa’s ice shell through a review of published ice core data, sampling low temperature gradient environments. We find that ice forming beneath ice shelves can serve as a valid analog for ice accreting beneath the ice shells of Europa and Enceladus and that the mechanism of ice accretion (frazil vs. congelation) will serve as the primary factor governing the amount of salt entrained in the ice shell from the ocean. We then introduce a framework to model the brine, salt, and ice volume fraction for a given composition as a function of bulk salinity and temperature by translating the output of the aqueous geochemistry software to polynomial functions of temperature. Using this framework, we build models for two endmember (NaCl and MgSO₄) and two multi-ion “analog” endmember (chloride-dominated and sulfate-dominated) ice shell compositions. Motivated by the observation that a percolation threshold could ultimately govern the stable bulk salinity of a congelation ice shell, we use these models to study the efficiency of salt entrainment as a function of ocean salinity, composition, and the effective critical porosity, a new parameter introduced in this work. We find that the efficiency of salt entrainment in a congelation ice shell is minimally influenced by composition and ocean salinity and is ultimately governed by the effective critical porosity. Finally, we study the habitability of ice shell brine pockets and find that they are not geochemically prohibitive to life as we know it. Furthermore, we suggest that brine volume fraction, as a proxy for nutrient transport and recycling, may be a critical factor governing the habitability of Europa's ice shell and use it to define different classes of potential habitats within Europa’s ice shell.
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    Geophysical insights into the crater subsurface at the Chicxulub and Ries impact craters
    (2022-12-02) McCall, Naoma; Gulick, Sean P. S.; Christeson, Gail; Hesse, Marc; Ketcham, Richard; Poelchau, Michael
    Large impact events have shaped the evolution of life on Earth both during early Earth history and the K-Pg extinction event. They transport huge volumes of rock distances of kilometers in timescales ranging from seconds to minutes. Impacts are found on every rocky and icy planetary body and are the dominant form of crustal resurfacing on planets without plate tectonics. As impact cratering is an important geologic process, but one that is rarely observed in real time, our understanding relies on modeling and existing craters. On Earth, impact craters are often eroded, tectonically deformed, or buried in sediment. This dissertation uses drill core and seismic data at two craters largely obscured but preserved by sedimentation: the Chicxulub impact crater, Mexico and the Nördlingen Ries impact crater, Germany. My research ground-truthed impact cratering models and strengthened theories of faulting and acoustic fluidization at Chicxulub. At Ries, the data acquired, processed, and interpreted during this PhD shows that Ries is a transitional crater that exhibits neither a central peak nor peak ring and that the suevite deposition at the crater was emplaced via ground-hugging flows. I measured the permeability of peak ring rocks from the International Ocean Discovery Program (IODP) Expedition 364 core, an important parameter for determining the duration of the post impact hydrothermal system at Chicxulub, I found that the permeabilities were likely responsible for an isolated and heterogeneous post-impact hydrothermal system at Chicxulub.
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    Controls on terminus change of marine terminating glaciers in Greenland over the last 40+ years
    (2022-11-17) Goliber, Sophie Ann; Catania, Ginny A.; Goudge, Timothy; Greenbaum, Jamin; Heimbach, Patrick; Spikes, Kyle
    Since the 1980s, the Greenland ice sheet has been losing ice mass at an increased rate. Our current understanding of the complex physical processes that control dynamic mass loss is incomplete and, therefore, leads to a wide range of possible future contributions to sea level. Ice dynamics, or changes due to changes in ice flux, is dominated by the behavior of fast-moving outlet glaciers in Greenland. These glaciers are changing through melting of the terminus face and/or calving of icebergs; the combination of these processes and ice motion determines the position of a glacier terminus. In understanding how and why outlet glacier termini change over time compared to external forcing and internal glacier dynamics, we are able to move toward a better understanding of marine-terminating glaciers. In this dissertation, I use terminus traces to observe how and why marine-terminating glaciers change in order to better understand the mechanisms behind these complex heterogeneous changes in Greenland. I develop the largest database of manually-traced marine-terminating glacier terminus data for use in scientific and machine learning applications. These data have been collected, cleaned, assigned with appropriate metadata, including image scenes, and compiled so that they can be easily accessed by scientists. Then I use the location of the termini to identify features in the bed topography that inhibit the retreat of glaciers following the onset of ocean warming and widespread glacier retreat in the late 1990s. I find that the slope and lateral dimensions of bed features exhibit the strongest correlation to retreat and that the shape of the bed features allows different styles of terminus retreat, which may be indicative of how different ablation mechanisms are distributed across termini. Finally, I produce a time series of terminus morphological properties for four glaciers in western Greenland to identify the characteristics that are indicative of calving processes with the goal of categorizing glaciers by calving style. I find that a concave shape and low sinuosity are present at glaciers that calve via buoyant flexure, while the opposite is true at glaciers that are dominated by melt-induced calving via serac failure. I also find that glaciers do not persistently fit into single calving styles and may change over time. By studying how the terminus changes over time compared to external forcing and internal glacier dynamics, we are able to move toward a better understanding of marine-terminating glaciers.
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    Autoencoders for seismic model upscaling and facies identification
    (2022-08-12) DeFabry, Cameron Mark; Sen, Mrinal K.; Cardenas, Meinhard B; Spikes, Kyle T
    The research presented here focuses on the resolution enhancement of inverted seismic volumes and geological facies identification. In the first section I utilize a regularized neural network in the form of an autoencoder to improve the resolution of seismic models which were inverted for compressional wave impedance. In the second section, I focus on the utilization of autoencoders to classify spatially small geologic facies with inverted seismic models and a computed facies map. In section one I focus on the processing and inversion of seismic data from the 3D Penobscot field on the Scotian shelf to produce models for compressional wave impedance. The dataset is inverted using two different approaches, first using a deterministic method, and secondly using a stochastic method. The stochastically derived models have higher frequency content than the deterministic models and are used as the target for the task of resolution enhancement. An autoencoder is trained to recreate the stochastic models with a set of randomly chosen starting weights. During training the network attempts to create a feature map that correlates the low-resolution deterministic model to the high-resolution stochastic model by using the deterministic models as input data and the stochastic models as target data. Once training is complete the network is given the deterministic model as an input and asked to predict an output with the convolution filters learned by recreating the stochastic models. The result is a model of higher resolution than the original deterministic model, but lower resolution than the original stochastic model. In section two I characterize a seismic volume from the Marco Polo field collected in the Gulf of Mexico and then classify five distinct facies, a shale member, an oil sand member, a gas sand member, and discrete brine sand members corresponding to the sand units. The brine sand members were simulated through fluid substitution and then have their probabilistic properties derived through a rock physics template. Bayesian classification is used to create an initial facies map with the brine sands predicted with rock physics templates, and the remaining units predicted directly from the distributions inherent to the well log. Geologic units less than 600 meters in any direction were specifically targeted by converting the standard facies map into a binary facies map. This binary facies map was used as an input in an autoencoder along with two seismic volumes inverted for compressional wave impedance and Vp/Vs. The result is a trained network that can take inverted model inputs and produce a probabilistic output predicting the location of a given facies. Additionally, when provided with smoothed inputs, the autoencoder can produce outputs of a similar resolution to that of the original data, with a loss of performance noted in the probabilities displayed. This result, along with the result from section one are used to justify the claim that autoencoders can be effectively used for the tasks of seismic model upscaling and facies identification without the direct use of well log data as a network input. Convolutional layers provide a way of processing these data in a manner often seen in image recognition and enhancement problems. These networks are limited to the data they were trained on, so additional training would be required for use on separate datasets. The utilized methods though, should maintain their efficacy provided the appropriate training has taken place.