HR: 08:30h
AN: T31F-03 INVITED     [Abstracts]
TI: Seismic Evidence for Water Atop the Mediterranean Transition Zone
AU: * van der Meijde, M
EM: vandermeijde@itc.nl
AF: International Institute for Geo-information Science and Earth Observation (ITC), P.O. Box 6, Enschede, 7500 AA Netherlands
AU: Marone, F
EM: federica@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, University of California, 215 McCone Hall, Berkeley, CA 94720 United States
AU: van der Lee, S
EM: suzan@earth.northwestern.edu
AF: Dept. of Geological Sciences, Northwestern University, 306 Locy Hall, 1850 Campus Drive, Evanston, 60208-2150 United States
AB: We present seismic evidence for up to 1000 wt ppm of water near depths of 400 km below the Mediterranean region. This water has possibly been carried there by subducting oceanic plates, which release most of the subducted water at depths shallower than 150 km into the mantle wedge overlying the plate but could carry small amounts to as deep as 400 km or below. The water causes the olivine to wadsleyite transition at 410 km to broaden to a 20-30 km thick interval (Wood, 1995). This broadening is detected by receiver functions that we constructed from seismograms recorded by permanent stations in the Mediterranean region as well as seismograms from our temporary MIDSEA array, which samples new locations in and south of the Mediterranean Sea. The receiver functions highlight S waves that converted from P waves at upper-mantle discontinuities. The S waves that converted at the 410-km discontinuity decrease significantly in amplitude with increasingly high frequency content of the receiver functions. The most likely explanation for this observation is a 410-km discontinuity that is thicker than the global average and reaches thicknesses of 20 to $>$30 km below multiple Mediterranean locations. Discontinuity relief and split discontinuities are less likely explanations for these observations than increased water content of olivine, because of their lateral incoherency. The Mediterranean transition zone is densely populated with subducted lithospheric fragments (Marone et al., 2004), which could have supplied the water.
UR: http://www.sg.geophys.ethz.ch/midsea/
DE: 9335 Europe
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting