Mantle Flow in Subduction Systems from the Global Pattern of Shear Wave Splitting above and below Subducting Slabs

Mantle Flow in Subduction Systems from the Global Pattern of Shear Wave Splitting above and below Subducting Slabs Sketch of the model proposed by Long and Silver (2008) for mantle flow in subduction zones controlled by trench migration.
<p>The character of the mantle flow field in subduction zone regions remains poorly understood, despite its importance for our understanding of subduction dynamics. Observations of seismic anisotropy, which manifests itself in shear wave splitting, can shed light on the geometry of mantle flow in subduction zones, but placing constraints on anisotropy in various parts of the subduction system (including the overriding plate, the mantle wedge, the subducting slab, and the sub-slab mantle) remains challenging from an observational point of view. In order to identify dynamic processes that make first-order contributions to the pattern of mantle flow in subduction zones, we analyze a global compilation of shear wave splitting measurements for a variety of ray paths, including SK(K)S and teleseismic S phases as well as local S and source-side splitting from slab earthquakes. We have compiled shear wave splitting measurements from subduction zones globally to produce estimates of average shear wave splitting parameters – and their spatial variation – for the mantle wedge and the sub-wedge region for individual subduction segments [Long and Silver, 2008]. These estimates are then compared to other parameters that describe subduction. The sub-wedge splitting signal is relatively simple and is dominated by trench-parallel fast directions in most subduction zones worldwide (with a few notable exceptions). Average sub-wedge delay times correlate with the absolute value of trench migration velocities in a Pacific hotspot reference frame, which supports a model in which sub-slab flow is usually trench-parallel and is controlled by trench migration [Long and Silver, 2009]. Shear wave splitting patterns in the mantle wedge are substantially more complicated, with large variations in local S delay times and complicated spatial patterns that often feature sharp transitions between trench-parallel and trench-perpendicular fast directions. We find a relationship between average wedge delay times and the ratio of the trench migration velocity and the convergence velocity. This supports a model in which a trench-parallel flow field (induced by trench migration) interacts with a 2-D corner flow field (induced by downdip motion of the slab) in the mantle wedge, with the relative influence of these flows being governed by the relative magnitude of convergence velocity and trench migration velocity in a Pacific hotspot reference frame.
</p><p>Long, M. D., and T. W. Becker (2010), Mantle dynamics and seismic anisotropy, Earth Planet. Sci. Lett., in press.
Long, M. D., and P. G. Silver (2009), Mantle flow in subduction systems: The sub-slab flow field and implications for mantle dynamics. J. Geophys. Res., 114, B10312.
</p><p>Acknowledgements: This work was supported by the Carnegie Institution of Washington and by NSF grant EAR-0911286.</p>


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