DOI: 10.25549/usctheses-c40-315776
title: Trench migration, slab bending, and mantle flow at subduction zones

description: The subduction of dense and stiff tectonic plates into the mantle exerts a fundamental control on plate velocities and flow in the upper mantle. On Earth, a broad spectrum of subduction styles are apparent, both in terms of slab morphology (e.g., slab dip angles) and slab kinematics (e.g., trench migration rates). The development of a dynamic framework to explain the origin of various types of slab kinematics is required both to understand the temporal evolution of the integrated plate tectonic system, and to use plate kinematic observations to infer the mechanical properties of subduction systems (e.g., lithospheric plate rheology). We use dynamically evolving numerical models of regional subduction to isolate the first order physical processes governing the subducting slab dynamics, particularly the lateral migration of slabs, and the bending of plates during subduction. We focus particularly on the role of the overriding plate, and mantle rheology, on dictating the slab kinematics observed: two features often neglected in dynamically evolving subduction models. Chapter 1 explores the control that the mechanical properties of the overriding plate can exert on subducting slab evolution, and introduces the general numerical modeling strategy adopted throughout. We find that the structure of the overriding plate exerts a strong control on subducting slab evolution, and therefore suggest that the wide range of overriding plate styles on Earth potentially induces a significant amount of the observed variability in slab rollback rates and slab morphologies. In chapter 2, we utilize global estimates of subducting plate curvature, in conjuction with 2-D numerical models, to investigate subducting plate rheology. We conclude that plastic weakening in the subducting plate, in conjunction with overriding plate structure, is required to generate the slab curvature systematics on Earth. In chapter 3, we turn our attention from lithospheric to asthenospheric rheology, in order to address the role of a power-law mantle viscosity on rates of trench migration. A power-law mantle rheology reduces the mantle viscosity directly adjacent to the plates, where mantle shear stress is greatest, an effect which reduces the relative importance of trench retreat. In addition to the inclusion of an overriding plate, this provides a mechanism for reducing the high average retreat rates, relative to Earth, often observed in subduction modeling studies. In chapter 4, we extend our subduction models to move beyond passive overriding plates, and consider far-field forces transmitted from proximal subduction zones. We substantiate the suggestion that slab interactions at “double slab” systems can induce non-standard plate kinematics, particularly advancing trench motion and rapid plate convergence. In addition, we lay out the subduction configurations within which conventional “single slab” behavior, or a variant of it, is exhibited, and conversely where such single slab physics is not applicable. Developing an understanding of the components of single slab subduction dynamics that are applicable is an important step towards a physics-based parameterization of such complex subduction systems in plate tectonic reconstructions, where geological constraints are often sparse.