RHEOLOGICAL PREDICTABILITY IN THE VISCOUS REGIME

The spatial and temporal distribution of mechanical properties within the lithosphere affects a wide range of Earth processes and patterns, including seismicity, post-seismic relaxation, and topography. Particularly when exploring general characteristics of orogen-scale features, the approximations we make for elastic properties, the transition to viscous behavior, viscous flow laws, and similar, can be sufficient for the question under consideration, at least within a desired level of uncertainty. But other times a more precise answer to the question is hampered by inexact knowledge of the spatial distribution of material properties. Highlighting four topics – the relative strengths of mafic and felsic rocks across metamorphic grade, weakening mechanisms in shear zones, the role of phase morphology on rock strength, and the distribution of polycyclic vs. monocyclic units – this presentation explores the possibilities and challenges in attempting to describe the rheology of the viscous portion of the continental crust at the km scale. The bulk relationship between stress and strain rate for a given rock depends primarily on temperature, microstructure, and stress regime. The least well known component is microstructure: both the constitutive laws for individual phases and homogenizing those individual laws and the interactions between phases into a bulk constitutive law. At present, factors such as anisotropy, mass transfer, and spatially and temporally variable deformation mechanisms preclude robust homogenization schemes for natural rocks. However, the complex nature of the microscale interactions does not preclude some degree of predictability at larger scales. Combining our work with that of others, we demonstrate that mafic and felsic rocks exhibit widely variable relative strengths depending primarily on metamorphic grade, mass transfer is essential in strain-related weakening, and lithologic inheritance can strongly influence orogenic rheology.