Source-Side Shear Wave Splitting and Upper Mantle Flow in the Chile Ridge Subduction Region

Source-Side Shear Wave Splitting and Upper Mantle Flow in the Chile Ridge Subduction Region Tectonic and relief map of study region. Chile Ridge spreading centers (red) and transform-fracture zones (black) shown. Black triangles=volcanoes; FZ=fault zone. Stars mark epicenters of six study earthquakes, relocated by Russo et al. (2010) colored same as source-side splitting measurements (bars; see delay time scale, upper right). Dotted circles are 200 km radius about source events; splits plotted in azimuth of takeoff. Dashed lines mark the boundary of the slab window, defined as the “0.5% velocity perturbation con- tour at 50, 100, and 200 km depths from the tomography of VanDecar et al. (2007). Note that the northern slab window boundary is well resolved; the southern boundary is not.
The actively spreading Chile Ridge has been subducting beneath Patagonian Chile since the Middle Miocene. After subduction, continued separation of the faster Nazca plate from the slow Antarctic plate has opened up a gap—a slab window—between the subducted oceanic lithospheres beneath South America. We examined the form of the asthenospheric mantle flow in the vicinity of this slab window using S waves from six isolated, unusual 2007 earthquakes that occurred in the generally low-seismicity region just north of the ridge subduction region. The S waves from these earthquakes were recorded at distant seismic stations, but were split into fast and slow orthogonally polarized waves at upper mantle depths during their passage through the slab window and environs. We isolated the directions of fast split shear waves near the slab window by correcting for upper mantle seismic anisotropy at the distant stations. The results show that the generally trench-parallel upper mantle flow beneath the Nazca plate rotates to an ENE trend in the neighborhood of the slab gap, consistent with upper mantle flow from west to east through the slab window.
</p><p>Russo, R. M., A. Gallego, D. Comte, V. Mocanu, R.E. Murdie, and J.C. VanDecar, Source-side shear wave splitting and upper mantle flow in the Chile Ridge subduction region, Geology, 38, 707-710, doi: 10.1130/GE30920.1, 2010.
</p><p>Acknowledgements: This work was supported by U.S. National Science Foundation grant EAR-0126244 and CONICYT-CHILE grant 1050367.</p>


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