UBC Theses and Dissertations
Ductile extrusion, underplating, and out-of-sequence thrusting within the Himalayan metamorphic core, Kanchenjunga, Nepal Ambrose, Tyler Kurtis
The Himalayan Metamorphic Core (HMC) is a package of greenschist to granulite grade metamorphic rocks that was buried to midcrustal levels and subsequently exhumed during Himalayan orogenesis. Until recently, most kinematic models for burial and exhumation have focused on the two fault systems that bound the HMC: the South Tibetan detachment system above and the Main Central thrust below. Increasing chronologic and thermobarometric data from across the HMC indicate that a significant amount of horizontal shortening and vertical thickening was accommodated along structures within the HMC. These structures have only a cryptic surface expression; they are often recognized in P-T-t(-D) paths only after fieldwork has been completed. Although such P-T-t(-D) discontinuities have been identified along the length of the Himalaya, little is yet known about how they developed and their overall importance to the evolution of the orogen. 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 (ca. 41-21 Ma) and a later history of underplating that drove metamorphism in the footwall material (ca. 31-12 Ma). These structures were repeated by early Miocene (ca. 20-18 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 that 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 elucidating the kinematics of the orogen and convergence accommodation processes in general.
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