Subducted Oceanic Asthenosphere and Upper Mantle Flow beneath the Juan de Fuca Slab

Subducted Oceanic Asthenosphere and Upper Mantle Flow beneath the Juan de Fuca Slab Block diagram showing Juan de Fuca slab geometry (green meshed surface), and top-of-slab depth contours (heavy blue dashed lines, bottom). Source side S splitting measurements are compatible with presence of two anisotropic layers beneath the Juan de Fuca slab: a layer immediately beneath the slab which is characterized by trench-parallel fast shear azimuths, and a deeper layer with fast shear trends parallel to the motion of the Juan de Fuca plate with respect to assumed fixed hotspots (Gripp and Gordon, 2003). Orange arrows schematically show the anisotropy and possibly upper mantle flow directions in these two layers.
<p>
Many studies have shown that typical oceanic lithosphere is underlain by a well developed asthenosphere characterized by slow seismic velocities from ~100-250 km depth. However, the fate of the oceanic asthenosphere at subduction zones is poorly understood. I show here using shear wave splitting of S waves emanating from earthquakes in the Juan de Fuca slab that upper mantle asthenospheric anisotropy beneath the slab is consistent with the presence of two distinct subducted asthenospheric layers, one with fast shear trends parallel to the subduction trench, and a second, deeper layer with fast upper mantle fabrics parallel to the motion of the Juan de Fuca plate with respect to the deeper mantle. The consistent orientation of unsubducted Pacific asthenospheric anisotropy in the direction of current plate motion implies that the trench-parallel sub-slab anisotropy develops when the lithosphere subducts.
</p><p>References
</p><p>Russo, R. M., Subducted oceanic asthenosphere and upper mantle flow beneath the Juan de Fuca slab, Lithosphere, 1, 195-205, 2009.</p>

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