Petrofabrics and Seismic Properties of Himalayan Amphibolites: Implications for a Thick Anisotropic Deep Crust Beneath Southern Tibet.

The bulk composition of overthickened Tibetan deep crust has been generally believed to be mafic granulite with eclogite at the lowermost crust. However, a granulitic/eclogitic deep crust is in contradiction to geological and geophysical observations in southern Tibet. Here we present petrofabrics and seismic properties of amphibolites from exhumed crustal part of the Indian plate in the eastern Himalayan syntaxis. Our results show strong fabrics of amphibole, nearly random fabrics of plagioclase and strong seismic anisotropies of amphibolites (AVp = 5.6–12.0% and Max. AVs = 5.1–7.7%). Comparing to a deep crust composed of nearly isotropic mafic granulite and weakly anisotropic eclogite, a thick amphibolitic layer in the middle‐lower crust would better account for the strong shear wave splitting (0.2–0.5 s in delay times or 4–15% in S wave anisotropy), the suture boundary parallel faster shear wave polarization, and the widespread postcollisional adakite‐like potassic rocks in southern Tibet. Plain Language Summary: We study the petrofabrics and seismic anisotropies of exposed lower crust of the subducting Indian plate. Our results show that a thick layer of amphibolites in the middle‐lower crust would agree better with the geological and geophysical observations than a deep crust entirely made of granulite and eclogite underneath southern Tibet. Key Points: Himalayan amphibolites are characterized by strong fabrics of amphibole and nearly random fabrics of plagioclaseHimalayan amphibolites show strong whole‐rock P wave and S wave seismic anisotropies mostly resulting from their amphibole componentsA thick amphibolite layer can account for the strong S wave splitting and the suture boundary parallel Vs1 polarization in southern Tibet [ABSTRACT FROM AUTHOR]