The effects of hydration on low-angle subduction systems

Petersen, Sarah Elizabeth (2021) The effects of hydration on low-angle subduction systems. Doctoral thesis, Northern Arizona University.

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Abstract

Low-angle subduction has a profound impact on subduction zone behavior. However, the mechanisms that initiate, drive, and sustain flat-slab subduction are debated. Within all subduction zone systems, metamorphic dehydration reactions within the down-going slab create conditions that allow for seismicity and melting of the asthenospheric wedge, leading to arc volcanism. In flat-slab systems, the low angle of the subducting slab cuts off corner-flow in the asthenospheric wedge, leading to a colder thermal regime inboard of the trench relative to typical subduction, and effectively shutting off arc volcanism. This allows for the stability of H2O-bound minerals well inboard of the trench. The implications of this include increased slab buoyancy, hydration of the overriding lower continental lithosphere, and a delay in slab eclogitization processes. This dissertation presents new modeling results that assess the role of water in flat-slab subduction. Thermal modeling of the Alaskan flat-slab subduction system and stable mineral assemblage calculations provide insight into the effects of hydration on slab density. Results show that an anhydrous slab is not sufficiently buoyant to maintain a low-angle subduction geometry, and would require dynamic forces (i.e. asthenospheric upwelling, slab suction) to explain the observed subduction angle. A moderate amount of hydration (1-1.5% chemically bound water) reduces slab density by 0.5-0.8%, and is sufficient to produce a buoyant slab that extends to 300-400 km from the trench. By using the thermal modeling and mineral assemblage calculations in Alaska to estimate seismic velocities, a comparison can be made between our models and observational seismic data. Seismic velocities are sensitive to temperature, pressure, composition, hydration state, and the presence of absence of melt. The non-uniqueness of seismic data makes isolating any one of these factors challenging using only seismic observations. To overcome this, we constrain pressure, temperature, and composition states using our models in to better isolate the effects of hydration. Results indicate that approximately 3% chemically-bound H2O is present within the subducting Alaskan flat-slab. Finally, we calculate density models of the Colorado Plateau and adjacent Basin and Range Province and Great Basin Province in the Southwestern region of the United States. Flat-slab subduction associated with Laramide tectonics is thought to have hydrated the lower crust of the Colorado Plateau lowering its density. We quantify the amount of uplift that can be associated with isostatic support in this study. A better understanding of the role that water plays during flat-slab subduction can provide more informed interpretations of geological and geophysical data associated with regions that are current or former flat-slab systems. By using forward modeling approaches along with observational data, we can better constrain and interpret geological observations. Altogether, this dissertation presents new methods and insights into the role of water in flat-slab subduction systems, with clear implications for future research, data analysis, and data interpretation of regions affected by flat-slab tectonics.

Item Type:	Thesis (Doctoral)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Alaska; flat-slab subduction; hydration; subduction zone; water; Volcanism; Asthenospheric upwelling;
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	11 Feb 2022 21:41
Last Modified:	26 Aug 2022 08:30
URI:	https://openknowledge.nau.edu/id/eprint/5691

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