Date of Award
Fall 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Geology and Geophysics
First Advisor
Brandon, Mark
Abstract
Subduction zones are a fundamental part of plate tectonics yet changes in the morphology and behavior of subducting slabs with time and their relations to surface expression and deformation in the upper plate remain uncertain. The west margins of North and South America have been shaped by long periods of subduction, providing a valuable opportunity to explore the long-term evolution of subduction zones. This thesis presents three studies on subduction zones along the western margins of North and South America. The first study focuses on the Central Andes above the subducting Nazca plate in northern Bolivia and southern Peru. The following two studies focus on the subduction along the west margin of North America, specifically along Coastal California, and its most recent tectonic history since the arrival of the Farallon-Pacific ridge crest to the coast.After the introduction in Chapter 1, Chapter 2 focuses on two locations along the west margin of South America and examines the relationship between the morphology of the subducting slab and the flow field of the subduction system, as imaged by seismic anisotropy. This study analysis azimuthal variations in radial and transverse components of receiver functions to constrain seismic anisotropy within and above the subducting slab beneath Peru and Bolivia. The northern location overlies the Peruvian flat slab, and the southern one overlies the normally dipping slab beneath Bolivia. Our results show evidence for seismic anisotropy that varies across multiple layers and orientations. These results suggest a complex deformation regime beneath the Andes. In particular, the identification of depth-dependent seismic anisotropy within the overriding plate crust implies a change in deformation geometry, dominant mineralogy, and/or rheology with depth, shedding light on the nature of deep crustal deformation during orogenesis. Chapter 3 focuses on an alternative interpretation of the tectonic history of Coastal California. We propose that the arrival of the Farallon-Pacific ridge at central California initiated a transition to a slow oblique subduction, with the San Andreas fault system located above the subduction and accommodating right-lateral motion across the plate boundary. This is in contrast to the widely-held view that the transition marks the cession of subduction, the formation of a transform boundary, and the development of a slab window. Our interpretation is motivated by extensive seismic data showing that California is widely underlain by young oceanic lithosphere. In this study, we present a new kinematic reconstruction of the oceanic plates and microplates beneath North America, assuming that oceanic-like lithosphere continues to form within the divergence after the subduction of a ridge, that microplates develop due to divergence processes and are bound by two active ridges, and that slabs and subducted ridges move with the surrounding mantle flow field. Our reconstruction predicts the production and subduction of about ~6000 km of slab since ~40 Ma underneath North America. This analysis also predicts an average convergence rate of ~5 mm/a between the Pacific and the Sierra- Nevada-Great-Valley block since the transition to a slow oblique subduction. Finally, we propose that the ‘Coast Range Orogeny’, may simply reflect the recent emergence of a forearc high above sea level, the onset of erosion, and the resulting localized shortening and deformation of a subduction wedge controlled by its critical taper geometry. Chapter 4 compares our kinematic reconstruction from Chapter 3 relative to observational data, such as the MIT16 tomography model for the western US by Burdick et al. (2017) and the current NAVDAT compilation of Cenozoic magmatism (Glazner, 2022) across western US and Mexico. For a better comparison with seismic tomography, we add an estimated depth component to our kinematic reconstruction based on a simplified sinking rate approach where sinking rates are a function of the radial depth and time. Overall, the inferred slab depth is well correlated with the time since subduction or formation and the tomographic imaging seems supportive of our notion that the subduction beneath North America is more coherent and continuous than previously thought. While our slab model mostly aligns with the fast seismic anomalies, there are gaps and irregular thicknesses along the slab’s trajectory. These can be attributed to the young and slow subduction process, resulting in dispersed and low-amplitude temperature or density contrasts. Lastly, our slow oblique subduction hypothesis represented by our plates’ reconstruction also seems consistent with the Western North American volcanic database. The well-known northward propagating magmatism along the west coast of California that was commonly viewed as evidence for the slab-window idea seems highly consistent with our predicted divergence across the Mendocino fracture zone. Our model relates plates’ kinematic reconstruction, magmatism, and the subduction history imaged in seismic tomography.
Recommended Citation
Bar, Neta, "Seismic and kinematic studies of subduction zones along the western margins of North and South America" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1207.
https://elischolar.library.yale.edu/gsas_dissertations/1207
