STRAIN AND CIRCULATION IN THE MANTLE WEDGE IN RESPONSE TO SLAB MIGRATION : NEW INSIGHS FROM ANALOG MODELING

In plate convergence tectonic settings, shear-wave splitting seems to represent the preferred axis of mantle wedge deformation fabric. Fast polarization directions can be parallel, perpendicular, or oblique to the trench azimuth. This study is based on the hypothesis that olivine is the mineral responsible for the mantle anisotropic character and that the mantle shear direction is represented by the preferred orientation of the [100] axis of olivine within the anisotropic layer [Mainprice, 1998], rather than [001] axis for a dry mantle [Karato, 2001]. Many numerical and analogical approaches have attempted to predict the variability of fast polarization directions in the mantle wedge as a function of subduction parameters. In this study, we are studying the effect of trench advance or retreat on mantle flow. Using an approach based on plate kinematics to predict mantle wedge deformation, we conducted a series of analogical experiments in Roma 3 University.

In these experiments, the downgoing lithosphere and mantle are modeled by a silicon plate and glucose syrup, respectively [Funiciello et al., 2005]. Material choices are dimensioned to natural properties [Weijermars et al., 1986]. Our conceptual models, based on the results obtained from analog models, point out the following principle : (i) deformation of the mantle wedge is a function of trench motion ; and (ii) shear due to the viscosity contrast at the slab-mantle interface also contributes to the mantle wedge anisotropy. Model results are in good agreement with shear-waves splitting data derived from N-W Pacific S-waves.