Deformation Temperature, Kinematics, and Internal Strain during Emplacement of Greater Himalayan Rocks in North-Central and Northeastern Bhutan

Abstract:

Across the Himalayan orogen, high-grade Greater Himalayan (GH) rocks are juxtaposed between low-grade metasedimentary rocks, and the deformation mechanisms that led to their emplacement are debated. In this study, geologic mapping and microstructural analysis are utilized to understand the deformation conditions and exhumation path of GH rocks in northern Bhutan. Three north-south transects of GH rocks were mapped, corresponding to structural thicknesses of 11-15 km, and schist, paragneiss, and quartz veins were the main lithologies sampled and analyzed.

Quantitative temperature estimates (~430°C, 545°C, 595°C, and 630°C) from quartz crystallographic preferred-orientation plot opening angles, collected from an electron backscatter diffraction detector on a scanning electron microscope, combined with semi-quantitative deformation temperature estimates obtained from cataloguing quartz recrystallization mechanisms in thin-section, show that GH rocks were initially deformed at temperatures between ~500-750°C, and were later overprinted by a lower-temperature (~400–500°C) recrystallization event. The higher-temperature event is interpreted to be associated with ca. 22–15 Ma displacement on the Main Central thrust, at or near peak metamorphic conditions. The lower-temperature overprint is interpreted to have occurred at a shallower point along the deformation path as GH rocks were exhumed, structurally-elevated, and passively translated southward, concurrent with ca. 18-10 Ma duplexing of Lesser Himalayan rocks.

During the multiple stages of deformation, the GH rocks experienced a combination of pure- and simple-shear components of strain, as indicated by multiple outcrops that exhibit both top-to-the-north and top-to-the-south kinematic indicators. In addition, mean kinematic vorticity numbers, obtained from the oblique quartz shape-preferred orientation method, ranged from 0.61-0.94 (22-57% pure shear) during the higher-temperature event and 0.18-0.58 (60-88% pure shear) during the lower-temperature overprint. A significant pure-shear component observed on all three transects indicates that emplacement of GH rocks in Bhutan was accompanied by layer-normal flattening strain.