HR: 08:00h
AN: NG31A-01 INVITED     [Abstracts]
TI: A Grained Continuum Theory of Damage and Coarsening
AU: * Bercovici, D
EM: david.bercovici@yale.edu
AF: Yale University, Dept. Geology and Geophysics PO Box 208109, New Haven, CT 06520-8109 United States
AU: Ricard, Y
EM: ricard@ens-lyon.fr
AF: ENS-Lyon/CNRS/Univ-Lyon1, Universite Claude Bernard Bat Geode, 2, rue Raphael Dubois, Villeurbanne, 69622 France
AB: Formation of tectonic plate boundaries from mantle and lithosphere dynamics involves shear localization during deformation as well as remnant weak zones after deformation ceases. The requisite state variable that delineates the weak zone could be, for example, increased temperature or water content, although a well documented feature of lithospheric weak zones is reduced grainsize (i.e., mylonites). Grainsize reduction is typically attributed to dynamic recrystallization although the models arising from this hypothesis are problematic: grainsize reduction occurs during dislocation creep while the rheological effect of grainsize occurs in diffusion creep and these creep mechanisms exist in different stress regimes. Moreover, the grain-growth ("healing") laws employed for these models assume static grain-growth or coarsening, although the setting itself is far from static or equilibrium. Here we present a new grained-continuum theory for simultaneous and competing coarsening and grainsize reduction through a "damage" mechanics and nonequilibrium thermodynamics approach. The theory contains coupled grainsize/statistical and continuum/macroscopic components. The grainsize/statistical element of the theory prescribes the evolution of the grainsize distribution through space and time, and a kinetic phenomenological (i.e., nonequilibrium thermodynamic) law for how grainzize changes depending on free energy differences between grains, including both grain-boundary surface energy (which controls coarsening) and the contribution of deformational work to these free energies (which controls damage). The continuum level of the theory considers standard mass, momentum and energy conservation on the statistically averaged grained continuum; however, the continuum treatment of energy conservation and entropy-source positivity provide the phenomenological law for the statistical grain-growth law. A fundamental thermodynamic requirement arising from the theory is that deformational work must always cause large grains to shrink and small ones to grow, causing the grainsize distribution to propagate to smaller grainsizes and thus to mean grainsize reduction. The theory also captures the essential static-coarsening predictions of self-similar grainsize distributions from Lifshitz-Slyosov and Hillert theories. However, with the inclusion of nonstatic/nonequilibrium conditions such as damage and deformational work, the theory also predicts a range of self-similar and non-self-similar (even singular) grainsize distribution evolutions involving either coarsening or grainsize reduction and shear localization.
DE: 4485 Self-organization
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8150 Plate boundary: general (3040)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Nonlinear Geophysics [NG]
MN: Fall Meeting 2005