Beneath much of the Andes, oceanic lithosphere descends eastward into the mantle at an angle of about 30° (ref. 1). A partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material2. This wedge is the ultimate source for magma erupted at the active volcanoes that characterize the Andean margin. But between 28° and 33° S the subducted Nazca plate appears to be anomalously buoyant3,4, as it levels out at about 100 km depth and extends nearly horizontally under the continent1,5,6. Above this ‘flat slab’, volcanic activity in the main Andean Cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge5,6. But it is unknown where slab volatiles go once this happens, and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward. Here we present results from a magnetotelluric profile in central Argentina, from which we infer enhanced electrical conductivity along the eastern side of the plunging slab, indicative of the presence of partial melt. This conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth, or that melt is supplied from the 410 km discontinuity, consistent with the transition-zone ‘water-filter’ model of Bercovici and Karato7.
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This research would not have been possible without the help of our field technician, G. Giordinengo of INGEIS and of B. Narod, whose new generation of MT instruments were used to collect the data and who solved critical instrument problems in the field. We also thank M. Lopez, S. Kay, D. James, S. Constable and S.-I. Karato for their insightful comments.
The authors declare that they have no competing financial interests.
A brief synopsis of analysis of magnetotelluric data for model dimensionality and strike and its application to the data used in the main paper. (PDF 977 kb)
Plots of the measured data and computed responses of the inversion in pseudosection form (site location versus period). Period is a proxy for depth. (PDF 235 kb)
The minimum structure models as a function of data misfit. (PDF 344 kb)
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