Low electrical resistivity associated with plunging of the Nazca flat slab beneath Argentina

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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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Figure 1: Location map showing main physiographic features, contours of the depth in kilometres to the Wadati–Benioff earthquake zone1 and the MT sites.
Figure 2: Inversions of MT data for electrical structure beneath central Argentina.


  1. 1

    Cahill, T. & Isacks, B. Seismicity and shape of the subducted Nazca Plate. J. Geophys. Res. 97, 17503–17529 (1992)

  2. 2

    Ulmer, P. Partial melting in the mantle wedge – the role of H2O in the genesis of mantle-derived “arc-related” magmas. Phys. Earth Planet. Inter. 127, 215–232 (2000)

  3. 3

    Cloos, M. Lithospheric buoyancy and collisional orogenesis: subduction of oceanic plateaus, continental margins, island arcs, spreading ridges and seamounts. Geol. Soc. Am. Bull. 105, 714–737 (1993)

  4. 4

    McGeary, S., Nur, A. & Ben-Avraham, Z. Spatial gaps in arc volcanism: the effect of collision or subduction of oceanic plateaus. Tectonophysics 119, 195–221 (1985)

  5. 5

    Kay, S. M. & Mpodozis, C. Magmatism as a probe of Neogene shallowing of the Nazca Plate beneath the modern Chilean flat-slab. J. S. Am. Earth Sci. 15, 39–57 (2002)

  6. 6

    Ramos, V. A., Cristallini, E. O. & Pérez, D. J. The Pampean flat-slab of the central Andes. J. S. Am. Earth Sci. 15, 59–78 (2002)

  7. 7

    Bercovici, D. & Karato, S. Whole mantle convection and the transition-zone water filter. Nature 425, 39–44 (2003)

  8. 8

    Jordan, T. & Allmendinger, R. Sierras Pampeanas of Argentina: a modern analogue of Rocky Mountain foreland deformation. Am. J. Sci. 286, 737–764 (1986)

  9. 9

    Rogers, J. W. A history of continents in the past three billion years. J. Geol. 104, 91–107 (1996)

  10. 10

    Vozoff, K. in Electromagnetic Methods in Applied Geophysics Vol. 2 (ed. Nabighian, M. N.) 641–711 (Society of Exploration Geophysicists, Tulsa, 1991)

  11. 11

    Jones, A. G. in Continental Lower Crust (eds Fountain, D. M., Arculus, R. & Key, R. W.) 81–143 (Elsevier, Amsterdam, 1992)

  12. 12

    Jones, A. G. Imaging the continental upper mantle using electromagnetic methods. Lithos 48, 57–80 (1999)

  13. 13

    Schilling, F. R., Partzsch, G. M., Brasse, H. & Schwarz, G. Partial melting below the magmatic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data. Phys. Earth Planet. Inter. 103, 17–31 (1997)

  14. 14

    Williams, Q. & Hemley, R. J. Hydrogen in the deep Earth. Annu. Rev. Earth Planet. Sci. 29, 365–418 (2001)

  15. 15

    Xu, Y., Shankland, T. J. & Poe, B. T. Laboratory-based electrical conductivity in the Earth's mantle. J. Geophys. Res. 105, 27865–27875 (2000)

  16. 16

    Peyronneau, J. & Poirier, J. P. Electrical conductivity of the Earth's lower mantle. Nature 342, 537–539 (1989)

  17. 17

    Carter, N. L. & Tsenn, M. C. Flow properties of continental lithosphere. Tectonophysics 136, 27–63 (1987)

  18. 18

    Chebli, G. A., Mozetic, M. E., Rosello, E. A. & Bühler, M. in Geología Argentina (ed. Caminos, R.) 627–644 (Inst. de Geología y Recursos Minerales, Buenos Aires, 1999)

  19. 19

    Chave, A. D. & Smith, J. T. On electric and magnetic galvanic distortion tensor decompositions. J. Geophys. Res. 99, 4669–4682 (1994)

  20. 20

    Rodi, W. & Mackie, R. Non-linear conjugate gradient algorithm for 2-D magnetotelluric inversion. Geophysics 66, 174–178 (2001)

  21. 21

    Wannamaker, P. E. in Three-dimensional Electromagnetics (eds Oristaglio, M. & Spies, B.) 511–527 (Society of Exploration Geophysicists, Tulsa, 1999)

  22. 22

    Pearson, D. G. et al. The characterization and origin of graphite in cratonic lithospheric mantle: a petrological carbon isotope and Raman spectroscopic study. Contrib. Mineral. Petrol. 115, 449–466 (1994)

  23. 23

    Roberts, J. J. & Tyburczy, J. A. Partial-melt electrical conductivity: Influence of melt composition. J. Geophys. Res. 104, 7055–7065 (1999)

  24. 24

    Park, S. & Ducea, M. Can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts? J. Geophys. Res. 108, 2270 (2003)

  25. 25

    Norabuena, E. O., Dixon, T. H., Stein, S. & Harrison, C. G. A. Decelerating Nazca-South America and Nazca-Pacific plate motions. Geophys. Res. Lett. 26, 3402–3404 (1999)

  26. 26

    Wiley, P. J. Magmas and volatile components. Am. Mineral. 64, 469–500 (1979)

  27. 27

    Bureau, H. & Keppler, H. Complete miscibility between silicate melts and hydrous fluids in the upper mantle: experimental evidence and geochemical implications. Earth Planet. Sci. Lett. 165, 187–196 (1999)

  28. 28

    James, D. E. & Sacks, S. in Geology of Ore Deposits of the Central Andes (ed. Skinner, B. J.) 1–25 (Special Pub. 7, Society of Economic Geologists, Littleton, Colorado, 1999)

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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.

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Correspondence to John R. Booker.

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Supplementary information

Supplementary Information 1

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)

Supplementary Information 2

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)

Supplementary Information 3

The minimum structure models as a function of data misfit. (PDF 344 kb)

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