Seismic characterization of mantle flow in subduction systems: Can we resolve a hydrated mantle wedge?
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.
Section snippets
Causes of seismic anisotropy in subduction systems
There are several possible causes of seismic anisotropy. Shape-preferred orientation (SPO) can result from cracks in the crust, melt-filled cracks, or lenses in the mantle (e.g., [2]). SPO may exist in areas beneath mid-ocean ridges and subduction zones in the uppermost mantle, as well as in the lowermost mantle (e.g., [28]) LPO results from mineral alignment during strain (e.g., [4], [6], [19]), and is inferred to be the dominant cause of upper mantle anisotropy (e.g., [2], [8], [29], [30],
Predicting shear wave splitting from mantle flow models
In this section, we present the methodology for linking models of mantle flow with predictions of SWS patterns for subduction zone settings following the general approach of Fischer et al. [9], Kaminski and Ribe [5], Fouch et al. [35], and Hall et al. [27]. In its original form, this methodology consists of utilizing the velocity field from mantle flow models to predict LPO development and resulting SWS. These previous studies mapped LPO for mantle mineralogies by either a) orienting
Results of LPO and shear wave splitting modeling
Here we present our LPO development and predicted SWS results for flow models using anhydrous, hydrous, and anhydrous to hydrous transition slip systems. To examine anhydrous and hydrous mantle olivine LPO development, we used the critically resolved shear stresses from Kaminski and Ribe [5], [6] for anhydrous olivine LPO development models and from Jung and Karato [4] for hydrous olivine models. We note that the simple flow fields used in our modeling are not viscosity-dependent; we,
Shear wave splitting and mantle fabric in hydrated subduction systems
The results of our LPO modeling and SWS predictions place important constraints on our understanding of the links between flow dynamics and seismic observations used to infer flow. While the underlying models in this work are relatively simple and certainly do not fully represent all details of Earth dynamics, they provide an important set of guidelines in understanding LPO development and its relationship with SWS, particularly in subduction zone environments where hydrated mantle phases
Conclusions
This study demonstrates that changes in LPO geometry due to transitions from anhydrous and hydrous olivine rheologies can be resolved with shear wave splitting provided appropriate sampling coverage of the mantle wedge. In simple shear models, complete reorientation of olivine aggregates due to a transition between anhydrous and hydrous olivine rheologies requires strains on the order of 370–450%. Continued LPO development can be observed in these models via increases in predicted splitting
Acknowledgements
We express our gratitude to Karen Fischer for the predicted shear wave splitting code and David Abt and Karen Fischer for an early version of the shear wave splitting compilation presented in Fig. 1. Thanks also to Megan Anderson, Lara Wagner, George Zandt and Susan Beck for fruitful discussions. We also thank Jules Browaeys for help with the LPO plotting and Mark Stevens for computer support. Martha Savage and an anonymous reviewer provided helpful comments that strengthened the clarity and
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