Abstract
The rarity of earthquakes despite strong deformation in the deep crust of mountain belts produced by continental collisions, such as the Himalayas, implies that the deep continental crust is rather weak, easily dissipating imposed stresses by ductile deformation. The major components of the orogenic lower crust are plagioclase, amphibole, pyroxene, and garnet. Experimental data on the ductile deformation of coarse-grained amphibole-rich rocks was until now missing. We present new deformation experiments, from which we derive a flow law for an amphibolitic aggregate (80% amphibole +20% garnet). These data show high volumes of amphibole should result in high ductility and, hence, low strength in the deep crust, which should be considered when modeling the mechanical behavior of the lower crust during continental collisional. An amphibole-rich deep crust also accounts for the high crustal seismic anisotropy in southern Tibet.