DUCTILE EXTRUSION, UNDERPLATING, AND OUT-OF-SEQUENCE THRUSTING WITHIN THE HIMALAYAN METAMORPHIC CORE , KANCHENJUNGA, NEPAL
The Kanchenjunga region of far north eastern Nepal exposes a thick section of garnet + biotite to migmatitic, K-feldspar + sillimanite grade paragneisses of the HMC. Pseudosection modelling and in-situ laser ablation split- stream U-Th/Pb monazite petrochronology methods were applied to anatectic paragneisses from this area to identify and elucidate cryptic structures within the HMC. The resulting P-T-t paths confirm previously reported discontinuities and reveal the presence of others previously unidentified. Our data outline a series of thrust sense discontinuities that record an early protracted history of ductile extrusion (c. 42-21 Ma) and a later history of underplating that drove metamorphism in the footwall material (c. 31-12 Ma). These structures were repeated by early Miocene (c. 20 Ma) out-of-sequence thrusting coincident with the previously mapped High Himal thrust. The resulting kinematic model for the evolution of the HMC in the Kanchenjunga area demonstrates the HMC is significantly more complex than a thick package of homoclinal rocks bound between two faults. Understanding the internal structure of the HMC is critical to elucidate the kinematics of the orogen and convergence accommodation processes in general.