Doctoral training on internal Earth

Les Houches, 15-26 octobre 2018

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Seismic imaging of upper mantle and plates

Monday 14 September 2015

- Lectures : Benoit Tauzin (Lyon), Thomas Bodin (Lyon), Stéphane Rondenay (Bergen)

  • sismo1 (1h): Seismic imaging of upper mantle and plates part 1: Fundamentals of seismology (lecture 1; B. Tauzin)
  • sismo2 (1h): Seismic imaging of upper mantle and plates part 2: Major contributions (lecture 2; B. Tauzin)
  • sismo3 (1h30): Upper mantle imaging with array recordings of scattered body-waves (lecture 3; B. Tauzin)
  • sismo4 (1h30): Bayesian approaches to geophysical inverse problems, applications to seismic imaging (lecture 4; T. Bodin)
  • sismo5 (1h30): Elastic scattered-wave migration and inversion (lecture 5; S. Rondenay)
  • sismo6 (1h30): Structure and deformation from SKS splitting and surface-waves (lecture 6; B. Tauzin)

Seminar: Stephane Rondenay (Bergen, Norway)
Seismic imaging of subduction zones: Eclogitization, serpentinization, and joint analysis with magnetotelluric data

Subduction zones transport water into the Earth’s interior. The subsequent release of this water through dehydration reactions may trigger intraslab earthquakes and arc volcanism, regulate slip on the plate interface, control plate buoyancy, and regulate the long-term budget of water on the planet’s surface. This presentation provides an overview of how high-resolution seismic imaging can be used, in conjunction with other geophysical and geological methods, to investigate these hydrous reactions and the associated water distribution in subduction systems. Our multidisciplinary approach provides direct insight into the depth range over which the downgoing plate undergoes dehydration, the degree of hydration-serpentinization of the mantle wedge, and the source of partial melt that feeds volcanic arcs. Applications to Cascadia, Alaska, and Greece are presented. We find that the depth at which the oceanic crust transforms from hydrated metabasalts into eclogite is highly dependent on the age of the subducted slab, with the youngest crust in Cascadia undergoing eclogitization at 40 km depth while older ones in Alaska and Greece eclogitizing at depths greater than 100 km, as predicted by thermal models. The degree of serpentinization of the mantle wedge can be assessed though the magnitude and the polarity of the overriding plate’s Moho. Finally, we show that the joint analysis of seismic and magnetotelluric results can clearly image the source of partial melt in the warm, circulating portion of the mantle wedge.