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

Abstract

AbstractThe 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.