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

Abstract

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.

Development of Amphibole Crystal Preferred Orientations (CPOs) and Their Effects on Seismic Anisotropy in Deformed Amphibolites

“…Model CPOs were used further to explore the effect of amphibole CPOs on seismic anisotropy. CPOs were modeled using the Bingham Distribution of Quaternions, which can represent the ideal CPO types in amphibole (type I, type III and type IV) based on the relationship among the shape parameters (λ 1 , λ 2 , λ 3 , λ 4 ) (Kunze & Schaeben, 2005; Biedermanna et al., 2018; Li et al., 2020). It can also represent the transitional cases from type I to type III and type I to type IV.…”

Microstructure and seismic properties of amphibole-rich rocks from the deep crust in southern Tibet

“…These studies propose that the observed crustal anisotropy mainly originates from CPOs formed during ductile deformation of the middle-lower crust (Agius and Lebedev, 2017;Chen et al, 2016;Shapiro et al, 2004). Amphibole, because of its abundance in the deep crust and high intrinsic seismic anisotropies, has often been proposed to explain strong crustal anisotropies (Ko and Jung, 2015;Li et al, 2020;Tatham et al, 2008). However, observations of naturally and experimentally deformed amphibolites show that most of the processes cited above may produce strong amphibole CPOs (Elyaszadeh et al, 2018;Getsinger and Hirth, 2014;Imon et al, 2004;Ko and Jung, 2015;Skrotzki, 1992;Stokes et al, 2012;Tommasi et al, 2017).…”

Crustal melting in orogenic belts revealed by eclogite thermal properties

“…In the Himalaya-Tibet orogenic belt, due to the differences in heat flow and heat production (Supplementary Fig. 5 ), the temperature at Moho discontinuity (~65 km) 48 , 49 reaches to 1340 K, and the lithospheric thickness is close to 150 km (Fig. 3b ).…”

“…( 8 )–( 10 ), the geotherm profiles are calculated from the surface downward using a depth increment of 10 m in each step (Δ z = 10 m). Following this approach, the crustal structures, composition, and heat production in the Sulu UHPM belt 50 , 51 and Himalaya-Tibet orogenic belt 48 , 49 , 52 , 84 , 85 are shown in Supplementary Fig. 5 , and the temperature profiles are given in Fig.…”