Experimental Evidence for a Weak Calcic-Amphibole-Rich Deep Crust in Orogens

Amphibole-rich rocks constitute significant components of the mid- to lower continental crust, particularly in active orogens characterized with thick and hot crusts. Nevertheless, experimental data on their viscosity remain scarce. We conducted axial compression deformation experiments on synthetic amphibolites under temperature and pressure conditions resembling deep sections of overthickened crust. A novel flow law for a calcic-amphibole-rich rock (80% amphibole +20% garnet) in the dislocation creep regime is derived from these experiments. Contrary to common assumptions, our results reveal that calcic-amphibolite is 1-2 orders of magnitude weaker than plagioclase-rich amphibolite, granulite, or gabbro. A calcic-amphibole-rich, low viscosity deep crust may not only support the "channel flow" model proposed for the Tibetan Plateau but also explain the observed high crustal seismic anisotropy in the region.