Subduction of the Chile Ridge: Upper Mantle Structure and Flow

Schematic 3-d block diagram of upper mantle flow in the vicinity of the slab window delineated by the travel time inversions. Map on top of block shows relief and locations of Chile Ridge structures before subduction and after. Bottom of block shows coastline and Chile Ridge structures over color-coded slab plates: Nazca plate in dark yellow, Antarctic plate in blue-green. Same colors for portions of slab visible in the block diagram itself. Red arrow parallels shear wave splitting fast trends and upper mantle flow in the vicinity of the slab window opening. View from the SW, looking NE. Note the northern, shallower portion of the slab window is not visible from this viewpoint.
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We deployed 39 broadband seismometers in southern Chile from Dec. 2004 to Feb. 2007 to determine lithosphere and upper mantle structure, including possible slab windows, in the vicinity of the subducting Chile Ridge. Body-wave travel-time tomography clearly shows the existence of a long-hypothesized slab window, a gap between the subducted Nazca and Antarctic litho- spheres. P-wave velocities in the slab window are distinctly slow relative to surrounding asthenospheric mantle. Thus, the gap between slabs visible in the imaging appears to be filled by unusually warm asthenosphere, consistent with subduction of the Chile Ridge. Shear-wave splitting in the Chile Ridge subduction region is very strong (mean delay time nearly ~3 s) and highly variable. North of the slab window, splitting fast directions are mostly trench parallel, but, in the region of the slab gap, splitting fast trends appear to fan from NW-SE trends to the north, through ENE-WSW trends toward the middle of the slab window, to NE-SW trends south of the slab window. We interpret these results as indicating flow of asthenospheric upper mantle into the slab window.
</p><p>References
</p><p>Russo, R. M., J.C. VanDecar, D. Comte, V.I. Mocanu, A. Gallego, and R.E. Murdie, Subduction of the Chile Ridge: Upper Mantle Structure and Flow, GSA Today, 20 (9), 4-10, doi: 10.1130/GSATG61A.1, 2010.
</p><p>Acknowledgements: Supported by U.S. National Science Foundation grant EAR-0126244 and CONICYT grant no. 1050367 from the govern- ment of Chile.</p>

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