HR: 17:15h
AN: T44B-06    [Abstracts]
TI: A theoretical model of grainsize evolution during deformation
AU: * Ricard, Y
EM: ricard@ens-lyon.fr
AF: Laboratoire de Sciences de la Terre, ENS Lyon, 46 allée d'Italie, Lyon, 69007, France
AU: Bercovici, D
EM: david.bercovici@yale.edu
AF: Yale University, Department of Geology and Geophysics, P.O. Box 208109, New Haven, CT 06520-8, United States
AU: Rozel, A
EM: antoine.rozel@univ-lyon1.fr
AF: Laboratoire de Sciences de la Terre, ENS Lyon, 46 allée d'Italie, Lyon, 69007, France
AB: Lithospheric shear localization, as occurs in the formation of tectonic plate boundaries, is often associated with diminished grainsize (e.g., mylonites). Grainsize reduction is typically attributed to dynamic recrystallization; however, theoretical models of shear-localization arising from this hypothesis are problematic since (1) they require the simultaneous action of two exclusive creep mechanisms (diffusion and dislocation creep), and (2) the grain-growth ("healing") laws employed by these models are derived from static grain-growth or coarsening theory, although the shear-localization setting itself is far from static equilibrium. We present a new first-principles grained-continuum theory which accounts for both coarsening and damage-induced grainsize reduction. Damage per se is the generic process for generation of microcracks, defects, dislocations (including recrystallization), subgrains, nucleii and cataclastic breakdown of grains. The theory contains coupled statistical grain-scale and continuum macroscopic components. The grain-scale element of the theory prescribes both the evolution of the grainsize distribution, and a phenomenological grain-growth law derived from non-equilibrium thermodynamics; grain-growth thus incorporates the free energy differences between grains, including both grain-boundary surface energy (which controls coarsening) and the contribution of deformational work to these free energiesconservation and positivity of entropy production provide the phenomenological law for the statistical grain-growth law. We identify four potential mechanisms that affect the distribution of grainsize; two of them conserve the number of grains but change their relative masses and two of them change the number of grains by sticking them together or breaking them. In the limit of static equilibrium, only the two mechanisms that increase the average grainsize are allowed by the second law of thermodynamics. The first one is a diffusive mass transport from small grains to large grains that captures the essential component of normal grain-growth theories. The second one is the aggregation of grains. With the inclusion of nonstatic/nonequilibrium conditions, the theory predicts two mechanisms for which the thermodynamic requirement of entropy positivity always imposes large grains to shrink and small ones to grow. A first damage mechanism opposite to diffusion, tends to homogenize the distribution of grainsize around its initial average grainsize. A second damage mechanism favors the creation of small grains by division of larger grains. This second mechanism reduces the average grainsize with time. The competition between the two coarsening mechanisms, the damage-induced grain homogenization and the damage-induced grain reduction controls the general evolution of the grainsize population. Under uniform and constant shear, and assuming than grains cannot stick together, the average grainsize is related to the energy dissipated during the deformation (in agreement with Austin and Evans, 2007). The prediction of this theory fits satisfactorily with the available observations and we will discuss the implications of this model for large scale geodynamics.
DE: 8160 Rheology: general (1236, 8032)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting