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

Abstract

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.

Deformation microstructures and seismic anisotropy of Yeoncheon amphibolites in the Imjingang Belt, South Korea

“…Amphibole is one of the representative minerals in the middle to lower crust that has a marked influence on the rheological characteristics of the continental crust (Aspiroz et al, 2007;Christensen and Mooney, 1995;Gao et al, 1999;Tatham et al, 2008). As amphibole is also an elastically anisotropic mineral (Brown and Abramson, 2016;Ko and Jung, 2015), LPO of amphibole may contribute notably to the seismic anisotropy of the middle to lower crust (Cao et al, 2010;Kim and Jung, 2019;Ko and Jung, 2015;Liu and Cao, 2023;Tatham et al, 2008;Wang et al, 2023). Recent experimental studies have classified LPOs of amphibole into four types (type-I, II, III, and IV) and reported that temperature, differential stress, and shear strain during deformation may govern the LPO of amphibole (Kim and Jung, 2019;Ko and Jung, 2015).…”