Microstructural analysis of Greater Himalayan rocks in northern Bhutan

Across the Himalayan fold-thrust belt, high-grade metamorphic rocks of the Greater Himalayan (GH) zone are juxtaposed between low-grade metasedimentary rocks structurally above and below. In Bhutan, the highergrade GH rocks lie structurally over lower-grade Lesser Himalayan rocks and are separated by the Main Central Thrust. However, many aspects of the deformation path, deformation conditions, and the emplacement mechanism that led to the exhumation of GH rocks are poorly understood. In this study geologic mapping and quantitative microstructural analysis are utilized to gain insight into the deformation history of GH rocks in Bhutan, and to test the applicability of end-member emplacement models. Microstructural datasets include characterization of kinematic indicators, determination of deformation temperatures through analysis of quartz deformation microstructures and quartz crystal-preferred orientation (CPO) data, and classification of strain and shear type using CPO and kinematic vorticity data. Semi-quantitative deformation–temperature estimates obtained from cataloguing quartz-recrystallization mechanisms, combined with quantitative temperature estimates from CPO plot opening angles, suggest that GH rocks were deformed at temperatures of ca. 500 to 750°C at both structurally-lower and higher levels, and were later overprinted by a lower-temperature recrystallization event around that occurred at conditions of ca. 400–500°C. The highertemperature recrystallization event is interpreted to be associated with earlier slip (~22–15 Ma) along the Main Central Thrust, at or near peak metamorphic temperature conditions. The lower-temperature overprint is interpreted to have occurred at a higher point along the pressure–temperature–deformation path as GH rocks were passively translated and structurally elevated southward, concurrent with duplexing of Lesser Himalayan rocks (~18–10 Ma). Internal deformation within structurally-lower and higher GH rocks consisted of components of both coaxial (pure shear) and non-coaxial (simple shear) strain, as indicated by localities with opposite shear-sense kinematics, type-I crossgirdle CPO patterns and kinematic vorticity, which suggests that exhumation of GH rocks was accompanied by significant flattening in north-central and northeast Bhutan