HR: 11:55h
AN: T32A-07 INVITED     [Abstracts]
TI: Strain and anisotropy in the deep upper mantle and transition zone
AU: * Tommasi, A
EM: deia@dstu.univ-montp2.fr
AF: Lab. Tectonophysique, CNRS/Universite de Montpellier II, Montpellier, 34095 France
AU: Mainprice, D
EM: david@dstu.univ-montp2.fr
AF: Lab. Tectonophysique, CNRS/Universite de Montpellier II, Montpellier, 34095 France
AU: Couvy, H
EM: Helene.Couvy@uni-bayreuth.de
AF: Lab. Structure et Propri‚t‚s de l'Etat Solide, CNRS/Universite de Lille, Villeneuve d'Ascq, 59655 France
AU: Couvy, H
EM: Helene.Couvy@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universit„t Bayreuth, Bayreuth, 95447 Germany
AU: Cordier, P
EM: Patrick.Cordier@univ-lille1.fr
AF: Lab. Structure et Propri‚t‚s de l'Etat Solide, CNRS/Universite de Lille, Villeneuve d'Ascq, 59655 France
AU: Thoraval, C
EM: thoraval@dstu.univ-montp2.fr
AF: Lab. Tectonophysique, CNRS/Universite de Montpellier II, Montpellier, 34095 France
AB: Seismic anisotropy has proven to be a powerful tool to image the uppermost mantle deformation. Recent experimental data on flow mechanisms of mantle minerals under high-pressure conditions allow now to relate seismic anisotropy observations and deformation deeper in the mantle. We use forward models based on high-pressure experimental data on olivine, wadsleyite, and ringwoodite to predict the seismic anisotropy produced by plastic strain in the deep upper mantle and in the transition zone. For the upper mantle below 250 km, polycrystal plasticity simulations with dominant [001] slip in olivine produce CPO that result in an extremely low seismic anisotropy characterized by fast directions roughly normal to the flow direction. Transition at high pressure from dominant [100] to [001] glide in olivine may explain the variation with depth in P and S waves anisotropy patterns even if the entire upper mantle deforms coherently with a dominant horizontal shearing component. The present results challenge previous interpretations of the weak seismic anisotropy in upper mantle below 250 km as resulting of deformation by diffusion creep or of poor coherence of the deformation at seismic length-scales. For the transition zone, forward models using experimentally determined [100] and 1/2[111] slip systems for wadsleyite and [110] slip for ringwoodite predict a weak seismic anisotropy for a polycrystal of pyrolitic composition at upper transition zone conditions: ~2% for P and ~1% for S-waves, and a roughly isotropic behavior in the lower transition zone. Analysis of global observations of seismic anisotropy in the transition zone in the light of these models supports dominant horizontal flow also in the uppermost transition zone, suggesting coherent flow of the upper 520 km of the mantle.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8160 Rheology--general
DE: 5120 Plasticity, diffusion, and creep
DE: 7218 Lithosphere and upper mantle
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting