HR: 0830h
AN: T41C-0232    [PDF]
TI: The Anisotropic and Rheological Structure of the Oceanic Upper Mantle From a Simple Model of Plate Shear
AU: * Podolefsky, N S
EM: noah.podolefsky@colorado.edu
AF: University of Colorado, Campus Box 390, Boulder, CO 80309
AU: Zhong, S
EM: szhong@spice.colorado.edu
AF: University of Colorado, Campus Box 390, Boulder, CO 80309
AU: McNamara, A K
EM: allen.mcnamara@colorado.edu
AF: University of Colorado, Campus Box 390, Boulder, CO 80309
AB: We have developed a channel flow model that dynamically couples plate motion and mantle stress with a composite rheology (diffusion creep and dislocation creep) to study rheological and anisotropic structures of the oceanic upper mantle. A semi-analytic approach is used to solve for mantle stress and viscosity, allowing fast calculations and exploration of a wide range of rheological parameters. Mantle stress in our model is due to shearing by a moving plate. By comparing mantle stress to a transition stress for dislocation creep, we identify regions where either diffusion creep or dislocation creep is active. Deformation by dislocation creep results in mineral fabric that may be responsible for observed seismic anisotropy. Our study suggests that there is an important relation between plate motion, seismic anisotropy, mantle viscosity, and transition stress. Using laboratory results for rheological parameters, we find that dislocation creep exists only in a layer at certain depths in the upper mantle. For plate velocity of 10 cm/yr, asthenospheric viscosity of $10^{19}$ Pa-s, and asthenospheric transition stress of 0.1 MPa, our model predicts a ~200 km thick dislocation creep layer, which is broadly consistent with the observations of seismic anisotropy. For plate velocity of 10 cm/yr and asthenospheric transition stress of 0.1 MPa, the asthenospheric viscosity needs to be greater than $5x10^{18}$ Pa-s to produce any dislocation creep deformation, and the asthenospheric viscosity needs to be larger for slower plate motion or larger transition stress. Slower plate motion leads to a thinner dislocation creep layer, which may partially explain the observed asymmetry in anisotropic structure in the East Pacific Rise.
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting