Constraints on Subduction Geodynamics From Seismic Anisotropy

Abstract

[1] Much progress has been made over the past several decades in delineating the structure of subducting slabs, but several key aspects of their dynamics remain poorly constrained. Major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology, slab hydration and dehydration, mechanical coupling between slabs and the ambient mantle, the geometry of mantle flow above and beneath slabs, and the interactions between slabs and deep discontinuities such as the core-mantle boundary. Observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy: when mantle rocks are deformed, a preferred orientation of individual mineral crystals or materials such as partial melt often develops, resulting in the directional dependence of seismic wave speeds. Measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems. Here I review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge, slab, and sub-slab anisotropy. I also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations. I evaluate different models in light of constraints from seismology, geodynamics, and mineral physics. Finally, I discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress, particularly in frontier areas such as understanding slab dynamics in the deep mantle.

Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

“…Knowledge of the lattice preferred orientation (LPO) of olivine can be used to understand mantle flow and the seismic anisotropy of the upper mantle (Nicolas and Christensen 1987;Ben Ismail and Mainprice 1998;Long 2013). The water content of olivine is one of the most important factors affecting olivine LPO (Jung and Karato 2001a).…”

Lattice-Preferred orientations of olivine in subducting oceanic lithosphere derived from the observed seismic anisotropies in double seismic zones

“…These methods are mainly used to extract splitting parameters from seismograms as accurately as possible when the dt is much smaller than the characteristic period of the shear wave (e.g., Vecsey et al 2008;Wüstefeld et al 2009). Although the well-constrained estimates of dt can be obtained from seismogram records, the complex patterns of seismic anisotropy along a splitting ray path can result in very complicated shear wave (e.g., Silver and Savage 1994;Long 2013;Mohiuddin et al 2015). Additional complications arise as the splitting parameters are dependent on the frequency content of the shear wave (e.g., Wirth and Long 2010].…”

“…The DSZ in Juan de Fuca (JF1, JF2) is a warm subduction zone where a young plate is subducted and this DSZ satisfies the condition of type C LPO of olivine . Different types of LPO are often used to explain seismic anisotropy characteristics in subduction zones (e.g., Nakajima and Hasegawa 2004;Long andSilver 2008, 2009a, b). Statistical results in Sect.…”

“…Seismic anisotropy in subduction zones mainly describes the shear wave splitting characteristics of the mantle wedge and mantle beneath the slab. The shear wave splitting characteristics of the mantle wedge are relatively complex, while the fast wave polarization directions of the subslab mantle are mostly parallel to the trench strike (e.g., Long 2013). Long and Silver (2009a, b) compiled shear wave splitting data of 15 subduction zones from previously published literatures and found that only trench migration rate correlates well with the strength of subslab anisotropy in a Pacific hotspot reference frame.…”

“…Song and Kawakatsu (2012) found that fast wave polarization direction changes from trench-parallel under relatively steep subduction zones to trench-normal under shallow subduction zones. Long (2013) provided a detailed review of the subduction geodynamics from seismic anisotropy. Our study is more concerned about the seismic anisotropy within DSZs.…”