CRUSTAL SHEAR ZONES AND THE VISCOSITY OF THE LOWER CRUST (Invited Presentation)

The viscous behavior of the lower crust provides rate-dependent coupling between the brittle upper crust and mantle flow. However, the wide range of crustal compositions limits the efficacy of using flow laws for individual minerals to uniquely constrain viscosity. In addition, a wealth of geologic and geophysical observations demonstrate that strain localization is common in many tectonic environments. In this presentation, we will first illustrate how rheological mixing laws can be used to constrain the influence of composition on crustal viscosity (Shinevar et al., 2015). Then, motivated by microstructural observations in natural rocks, we will introduce how these same mixing laws can provide new constraints on the scale, and rheological impact, of strain localization in the lower crust and the lithosphere. For the latter problem, we leverage microstructural observations from natural shear zones that indicate the texture of the polyphase rock evolves such that the effective viscosity of polyphase regions deforming by diffusion creep is comparable to that of the single-phase regions deforming by dislocation creep (Mehl and Hirth, 2008; Getsinger et al., 2013; Homburg et al., 2010; Warren and Hirth, 2006; Skemer et al., 2010).