Kinematics and dynamics of the Namche Barwa Syntaxis, eastern Himalaya: Constraints from deformation, fabrics and geochronology

Field observations, deformation and fabric analyses, and precise age data acquired by zircon SHRIMP, LA-ICP-MS U-Pb and ⁴⁰Ar-³⁹Ar dating methods have yielded new constraints on the kinematics and dynamics of the Namche Barwa Syntaxis (NBS) which is the eastern corner of the Himalaya. A two-stage model has been established to explain the formation and evolution of the NBS. The northward indentation of the Indian plate beneath the Lhasa terrane began at 55-40 Ma, and crustal materials at this corner were subducted to depths > 70 km where they experienced HP (UHP?) metamorphism. Since 40 Ma, large-scale, right-lateral strike-slip along the Sagaing fault has accommodated the rapid northward movement of the eastern Indian plate corner with respect to the Indochina block. This caused significant and progressive bending of the Indus-Yarlung suture zone (IYSZ) such that it became the Dongjiu-Milin left-lateral, strike-slip, shear zone (DMSZ) in the west and the Aniqiao-Motuo right-lateral, strike-slip, shear zone (AMSZ) in the east. Both zones underwent strong mylonitization. Meanwhile, the HP (UHP?) metamorphic rocks were rapidly exhumed, first into the deep crust at 22-18 Ma and then to the shallow crust to form an antiformal dome at 6-2 Ma. Our model provides new insight into the processes of post-collisional crustal thickening related to the formation of the Himalayan orogenic belt.