Studies of Crust and Upper Mantle Structure, Mantle Flow, and Geodynamics of the Chile Ridge Subduction

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Subduction of active oceanic spreading ridges has been an important process in Cenozoic tectonics of the Pacific Basin. The Chile Ridge, the divergent Nazca-Antarctic plate boundary, is currently subducting beneath continental South America along the Chile Trench at ~46.5S, forming a plate triple junction in the vicinity of the Taitao Peninsula. Chile Ridge spreading rate and morphology, and the history of ridge subduction, are well-known: at least three southern Chile Ridge segments have subducted beneath South America 14, 6 and 3 m.y. ago. The current seismicity of the southern portion of subducting Nazca lithosphere defines the dip of the Nazca slab north of the Nazca-Antarctica-South America triple junction, albeit poorly. However, south of the triple junction seismicity diminishes markedly and Antarctic plate slab dip is not known. Nonetheless, given spreading and convergence histories, and knowledge of the motion of South America with respect to the deep mantle, the morphology of the subducted Nazca and Antarctic labs and their associated spreading ridge can be predicted. Thus, the loci and expected shapes of gaps formed between actively spreading subducted portions of the Nazca and Antarctic slabs - slab windows can be determined.Implicit in the slab window idea is the assumption that spreading between the two sides of the subducted ridge continues after subduction, but that no new lithosphere is formed after subduction, leading to formation of a progressively widening gap and asthenospheric mantle flow between the two edges of the former ridge. Preliminary results from studies of shear wave splitting and seismic attenuation in the Chile Ridge subduction region indicate that the upper mantle flow field beneath South America may be highly variable. Deployment of some 50 passive-source seismic stations in the Chile Margin triple junction area offers the prospect of direct observation of a slab window and the possibility that the form and function of the mantle flow field in this geodynamically unique tectonic regime can be determined. Studies of crust and upper mantle structure, mantle anisotropy, seismic attenuation, and tectonics of the Nazca-Antarctica-South America triple junction region are designed to yield the structure of slab gaps and mantle flow through those gaps. Targeted tests of mantle responses to the ridge subduction include establishment of the presence and thickness of mantle asthenosphere beneath the study region, determination of potentially heterogeneous crustal and lithospheric structure due to slab window magmatic processes and enhanced tectonic erosion, observations of mantle flow directions in the vicinity of the gaps, and estimation of seismic attenuation and associated inferences concerning mantle temperature fields.