Two-Phase Model of Damage, Shear Localization and Plate Boundary Formation

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Bercovici, DavidEAR-0105269 The investigator proposes to study the process of lithospheric shear localization (focussing of deformation in solid materials due to nonlinear weakening mechanisms) and plate boundary formation through his newly developed two-phase damage theory. The theory states that a damaged material (i.e., with microcracks and voids) is, in its simplest form, a two phase material (a matrix phase representing solid host, and a fluid phase representing void-filling material such as water or air). It also states that the energy going into making microcracks is the surface energy of the crack wall, which in the two phase model is the surface energy of the interface between phases. Preliminary results show sharp localized weak zones under intense shear, and distributed damage with extreme shear (suggestive of processes such as crack-branching instabilities). The theory, however, is still in a very basic form and has only been tested for simple one-dimensional geometries. Thus, the proposed project entails further development and testing to 1) apply it to more mantle/lithosphere-relevant geometries and conditions; and 2) include further pertinent physics. The model will be applied to cases of uniaxial compression; compressive folding and tensile necking; gravity-driven flows (as in slope and continent stability); source-sink driven flows (to examine the excitation of toroidal motion from purely poloidal flow); and convectively driven motion. The new physics to be studied and incorporated includes healing and annealing processes; inter-phase reactions (e.g., hydration); anisotropy (to account for pore/microcrack alignment); thermomechanical effects (e.g., fluid thermal expansion which affects pore pressure, thermoviscous behavior of matrix material, and temperature sensitivity of interface surface tension); and partitioning of deformational work between irreversible dissipative heating and semi-reversible storage as interface surface energy. This project will further our understanding of the fundamental processes of lithospheric and other shear localization mechanisms, as well as the origin of tectonic plates and plate boundaries.