Understanding lithospheric and mantle dynamics using geodynamic models with data-assimilation

Cao, Zebin

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https://hdl.handle.net/2142/121464 Copy

Description

Title

Understanding lithospheric and mantle dynamics using geodynamic models with data-assimilation

Author(s)

Cao, Zebin

Issue Date

2023-07-11

Director of Research (if dissertation) or Advisor (if thesis)

Liu, Lijun

Doctoral Committee Chair(s)

Liu, Lijun

Committee Member(s)

• Lundstrom, Craig C
• Bass, Jay D
• Gregg, Patricia

Department of Study

Earth Sci & Environmental Chng

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Geodynamics
• Tectonics
• Lithosphere
• Mantle
• Numerical simulation
• Data-assimilation

Abstract

Surface tectonics and geological events are ultimately linked to the deep processes in the lithosphere and the convecting mantle. Geodynamic modeling is a natural approach to investigating such links. However, numerical simulations usually suffer simplified model assumptions. For example, many previous works treat the lithosphere and the convecting mantle separately. As a result, the important role of lithosphere-asthenosphere interaction may be underestimated. Besides, geodynamic simulations usually assume uniform lithospheric density and/or thickness. This assumption is inappropriate for the continental lithosphere, which has a spatially variable density and effective viscosity profile. Therefore, we created a new data-oriented geodynamic modeling approach that simultaneously computes the lithospheric and mantle dynamics with realistic structures in one physical framework. This modeling approach allows us to quantify the relative contributions of different dynamic components in driving surface tectonics and the importance of their interaction as well. In this dissertation, we apply this modeling approach to understanding the lithospheric and mantle dynamics beneath the conterminous U.S. and Hawaii. With the fully coupled lithosphere-mantle models, we first evaluate the relative roles of different mechanisms in generating crustal stress over the conterminous U.S. Geophysical observation reveals that the tectonically active western U.S. (WUS) has a very different crustal stress pattern than the stable central and eastern U.S. Through systematic simulations, we found that a continental lithosphere with spatially varying thickness and a larger-than-ambient density is critical in reproducing the observed crustal stress pattern. Additionally, the force applied by mantle convection dominates the crustal stress over the conterminous U.S. We further show that within the WUS, lithosphere-asthenosphere interaction represents a key driving force for contemporary tectonics. The WUS presents a spectacular crustal deformation pattern with a clockwise rotation in crustal motion and clustered intraplate earthquakes and volcanism. Our simulation shows that the interaction between the lithosphere and the convecting mantle primarily drives this unique crustal deformation pattern. The shallow eastward mantle flow beneath the WUS is blocked by the abruptly thickened lithosphere east of the Basin & Range, a process that generates prominent stress within the lithosphere. The blocking effect locally increases the crustal strain rate, which forms the intraplate seismic belt and drives differential crustal motion within the WUS. Moreover, we show that the formation of WUS intraplate volcanism requires not only hot asthenospheric upwelling but trans-lithospheric extension zones. Importantly, we demonstrate that a denser-than-ambient continental lithospheric mantle (CLM) with a layered density structure plays a key role in defining the supercontinent cycle, which is characterized by the consumption of old subduction zones along active margins during supercontinent formation and initiation of new subduction zones along passive margins during its separation. However, the mechanism of such subduction initiation (SI) remains elusive. We show that the gravitationally unstable dense lower CLM could delaminate under dynamic perturbations like plumes. After the delamination, the remaining lithosphere consisting of the intact continental crust and upper CLM is more buoyant than the adjacent oceanic plate, so it will facilitate SI at the nearby passive margin. Our numerical simulations also show that the compositional density of the CLM primarily controls the success and timing of subduction initiation at the passive margin. Further, the emerging oceanic slab applies considerable extensional stress to the overriding plate during the incipient subduction, as may lead to the breakup of a supercontinent. The modeled geodynamic process is consistent with multiple independent geological and geophysical observations over supercontinent cycles, particularly during the breakup of Pangea. Interactions between different mantle plumes are also crucial for surface tectonics. For example, the Hawaii-Emperor seamount chain showed two sub-parallel geographical and geochemical trends, Loa and Kea, since ~5 Ma. Numerous studies try to explain the double volcanic chain using models involving a single plume. However, these models fail to explain the dramatically increased eruption rate of the Hawaiian hotspot since ~5 Ma and the nearly simultaneous southward bending of the volcanic chain. Therefore, we propose a new plume-plume interaction model, where the compositionally depleted Kea trend represents the original Hawaiian plume tail, and the relatively enriched Loa trend represents an emerging plume head southeast of the Hawaiian plume. Our geodynamic simulation shows that the interaction between the existing Hawaiian plume tail and the emerging Loa plume head is responsible for the southward bending of the volcanic chain. Additionally, we show that the arrival of the new plume head dramatically increases the eruption rate along the hotspot track.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Zebin Cao

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