On the effect of damage and healing from a two-phase damage theory perspective in generating narrow plate boundaries

The generation of plate tectonics from mantle convection requires shear localization in order to form narrow, weak zones that separate the broad, strong plate interiors. Two-phase damage theory provides a theoretical framework to describe the failure and weakening that leads to shear localization. Two-phase damage theory allows for the development of damage to be manifested in two distinct ways: void generation associated with dilation of the matrix and increasing the fineness of the mixture (e.g. grain size reduction). This work will examine the application of two-phase damage theory to the problem of generating plate-like behavior from mantle convection, as well as investigating the influence of the rheology in simpler flow systems (i.e. simple shear). Our objective is to determine how successful the different manifestations of damage are at producing plate-like behavior in a more sophisticated simulation than previously examined. Our results suggest that divergent and convergent plate boundaries undergo localization in different physical regimes. Convergent plate boundaries are most sensitive to grain size reducing mechanisms, while divergent plate boundaries respond significantly to the interplay of advection and healing. Our results also suggest that allowing for both forms of damage (void and fineness) to work in tandem enables another regime of localization for both convergent and divergent plate boundaries. A series of scaling analyses have been developed to categorize and comprehend the range of behaviors seen in the numerical results.