Seismic characterization of mantle flow in subduction systems: Can we resolve a hydrated mantle wedge?

Abstract

This study provides new constraints on the resolvability of mantle flow in subduction zone settings as inferred by observations of seismic anisotropy. We are motivated by the broad range of shear wave splitting observations in subduction systems that suggest complex flow geometries, changes in the deformation state of mantle minerals, or a combination of both. While shear wave splitting fast polarization directions are typically interpreted as a proxy for flow or maximum finite extension, experimental studies suggest that olivine slip systems change under higher stress and hydration states, conditions likely appropriate for subduction systems. In this study, we predict shear wave splitting as a result of mantle silicate lattice-preferred orientation development in steady-state two-dimensional mantle flow models using a textural development theory that incorporates the combined effects of intracrystalline slip and dynamic recrystallization. We utilize the resulting textures to predict shear wave splitting for populations of seismic raypaths traversing the model within the subduction zone mantle wedge. The results of our study demonstrate that combined observations of variations in fast polarization directions and splitting times make it possible to resolve a shift from anhydrous to hydrous mantle insubduction zone settings provided very good sampling of the mantle wedge. Our models are generally consistent with observed splitting variations for several subduction zones with dense data sampling, including Tonga, Japan, and Kamchatka. The implications of our work suggest that, provided adequate data sampling, shear wave splitting measurements can provide the necessary information to evaluate potential competing effects between variations in mantle flow direction and changes in the stress and hydration states of subduction zone mantle wedges.

Introduction

As part of the convective system that mobilizes Earth's mantle, subduction zones are one of the key expressions of plate tectonics. Convergent margins represent an important setting to study the processes of arc volcanism, seismicity, and deformation, which are influenced by thermal, rheological, and compositional factors. In particular, understanding the relationship between deformation and resulting fabric in the mantle wedge between subducting and overriding lithospheric plates is essential to improving our understanding of the subduction factory. Significant progress in constraining mantle wedge processes has been made from seismological observations, mineral physics experiments, and geodynamic models [1], [2], [3], [4], [5], but linking results from these fields remains a significant challenge. Of particular importance is providing an improved understanding of the influence of mantle hydration and its effects on inferring mantle wedge deformation using seismic observations [6], [7].

Subduction zones exhibit a broad range of mantle seismic anisotropy observations which are inferred to express dominant deformational processes in this tectonic setting. A particularly well-studied manifestation of seismic anisotropy is shear wave splitting (SWS). Observations of SWS are comprised of a fast polarization direction, which reflects the orientation of fabric, and a splitting time, which reflects the organization and strength of the fabric. In many cases, a range of fast polarization directions has been documented within the same subduction system, such as in Tonga (e.g., [8], [9], [10]), Japan (e.g., [11], [12]), Kamchatka (e.g., [13], [14]), and South America (e.g., [15], [16], [17]) (Fig. 1). These sometimes discrepant observations hint at the complex array of dynamic processes that occur near subduction zones.

Seismic anisotropy in the mantle is generally assumed to be the result of lattice-preferred orientation (LPO) of mantle minerals such as olivine. For systems with little or no hydration, LPO of olivine a-axes (the seismically fastest axis for shear waves) are assumed to be aligned with the direction of flow in the dislocation creep regime (e.g., [18], [19]). Conversely, experimental studies suggest that olivine slip systems change under higher stresses and hydration states [4]. Since some subduction zones are likely regions of high stress and hydration (e.g., [20]), the development of olivine LPO in these regions may be significantly different [4], [21], [22].

An extensive range of models for subduction zone flow and deformation have been developed (e.g., [23], [24], [25], [26]). While important advances have been made to connect deformation in flow models with seismic anisotropy observations (e.g., [9], [27]), more detailed links between geodynamical models and observed seismic anisotropy for subduction zones can yield significantly improved insight into SWS observations. Additionally, understanding the role mantle hydration plays in seismic anisotropy development due to deformation can improve interpretations of shear wave splitting parameters. Here we provide important new connections between subduction zone flow models and the seismic manifestation of flow for a range of hydration states in the mantle.

In this study, we examine the connections between dynamic flow models and seismic anisotropy to interpret deformation in subduction systems. We assume that the bulk of observed seismic anisotropy in subduction zones likely results from fabric in the mantle wedge and focus on wedge deformation as imaged by seismic anisotropy observations. To provide intuition regarding how a shift from anhydrous to hydrous olivine is manifested in shear wave splitting observations, we examine simple shear models with a first order transition in olivine hydration state. We then apply this approach to determine the patterns of shear wave splitting measurements that would develop from a hydrated wedge by examining a range of models with anhydrous and hydrated mantle wedge regions. Finally, we relate our results to subduction zones around the Pacific Rim to evaluate SWS observations in these regions.