Abstract:
Shear zones are the most ubiquitous features observed in planetary surfaces. They appear as a jagged network of faults at the observable brittle surface of planets and, in geological exposures of deeper rocks, they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of “mylonitized”, reduced grain sizes behind. The overall size of the entire shear network rarely exceeds kilometer scale at depth. Although mylonitic shear zones are only visible to the observer, when uplifted and exposed at the surface, they govern the mechanical behavior of the strongest part of the lithosphere below 10–15 km depth. Mylonitic shear zones dissect plates, thus allowing plate tectonics to develop on the Earth. We review the basic multiscale physics underlying mylonitic, ductile shear zone nucleation, growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization. We also discuss a framework for analytic and numerical modeling including the effects of thermal–mechanical couplings, thermal-elasticity, the influence of water and void-volatile feedback. The physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium. Positive feedback can amplify strength heterogeneities by cascading through different scales. We define basic, intrinsic length scales of strength heterogeneity such as those associated with plasticity, grain size, fluid-inclusion and thermal diffusion length scale.