Modeling anisotropy and plate‐driven flow in the Tonga subduction zone back arc

Abstract

Abstract. The goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions, the Tonga subduction zone in particular, can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate. We calculated two-dimensional mantle flow models using known Tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures. Shear wave splitting was predicted for the anisotropy due to lattice preferred orientation (LPO) of olivine and orthopyroxene in the flow model finite strain fields. The predicted shear wave splitting provides a good match to the fast directions (parallel to the azimuth of subducting plate motion) and splitting times (0.5-1.5 s) observed in Tonga, both for models where LPO anisotropy develops everywhere above 410 km and for models where LPO anisotropy is confined to regions of relatively high stress. If LPO anisotropy does develop over the entire upper 410 km of the mantle, the strength of anistropy induced by a given amount of shear strain must be relatively weak (-4% for shear strains of 1.5, with a maximum value of-6% for very large strains). The splitting observations are comparably fit by a wide range of different viscosity models. Anisotropy due to melt-filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting.

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

“…These observations of trench-parallel fast directions contradict the predictions of simple cornerflow models for flow in the mantle wedge, in which viscous coupling between the downgoing slab and the overlying wedge material induces flow that is parallel to the convergence direction (e.g., Fischer et al, 2000). However, new laboratory results have shown that when olivine aggregates are deformed under high-stress, lowtemperature, water-rich conditions, the fast axes of individual olivine crystals tend to align 90 • from the flow direction (Jung and Karato, 2001).…”

“…Some contribution to the observed signal from the crust is likely in this splitting dataset, but anisotropy in the crust cannot explain the observed δt values up to nearly 2 s. The crustal thickness in the Ryukyu arc, ∼35-40 km (Taira, 2001), is insufficient to explain such large splitting, and observed crustal splitting times in Japan average about 0.2 s (Kaneshima, 1990). It has been demonstrated that both lithospheric and asthenospheric contributions to anisotropy are important in many regions (e.g., Fouch et al, 2000;Simons et al, 2002;Simons and van der Hilst, 2003;Fischer et al, 2005;Waite et al, 2005), although splitting measurements in subduction zone settings are nearly always interpreted in terms of flow in the asthenosphere (e.g., Fischer et al, 1998Fischer et al, , 2000Smith et al, 2001;Anderson et al, 2004). The lithosphere beneath the Ryukyu arc stations is likely to be thin, due to thermal erosion associated with mantle wedge flow (e.g., Conder et al, 2002), and it is unlikely that the lithosphere is thick enough to explain the large split times.…”

“…The contribution of aligned melt to anisotropy in the mantle has been examined by Kendall (1994) for the case of a midocean ridge, by Kendall and Silver (1998) and Moore et al (2004) for the D region, and by Fischer et al (2000) for a subduction zone. Fischer et al (2000) considered a model in which melt-filled cracks align 20-30 • from the maximum deviatoric compressive stress, following the experimental results of Zimmerman et al (1999), and concluded that melt-filled cracks could result in trench-parallel splitting.…”

“…The contribution of aligned melt to anisotropy in the mantle has been examined by Kendall (1994) for the case of a midocean ridge, by Kendall and Silver (1998) and Moore et al (2004) for the D region, and by Fischer et al (2000) for a subduction zone. Fischer et al (2000) considered a model in which melt-filled cracks align 20-30 • from the maximum deviatoric compressive stress, following the experimental results of Zimmerman et al (1999), and concluded that melt-filled cracks could result in trench-parallel splitting. However, melt production in subduction zones is likely restricted to a thin column of vertical melt transport (see, for example, Gaetani and Grove, 2003) and therefore anisotropy due to aligned melt structures would have to be concentrated in a small zone directly beneath the stations to explain both the teleseismic and local splitting for Ryukyu.…”

Upper mantle anisotropy beneath Japan from shear wave splitting

“…Knowing the distribution of seismic anisotropy can help address these issues (e.g., . The presence of anisotropy in the upper mantle beneath subduction zones has been well established (e.g., Ando et al, 1983;Fischer and Yang, 1994;Hiramatsu and Ando, 1996;Fouch and Fischer, 1996;Iidaka and Obara, 1997;Fischer et al, 1998Fischer et al, , 2000. A diverse range of splitting behavior has been associated with different subduction zones.…”