Paper No. 91-7
Presentation Time: 9:40 AM
DUCTILE EXTRUSION, UNDERPLATING, AND OUT-OF-SEQUENCE THRUSTING WITHIN THE HIMALAYAN METAMORPHIC CORE , KANCHENJUNGA, NEPAL
AMBROSE, Tyler K.1, LARSON, Kyle P.2, GUILMETTE, Carl3, COTTLE, John M.4, LEDERER, Graham5 and BUCKINGHAM, Heather1, (1)Earth and Environmental Sciences, University of British Columbia, Okanagan, Kelowna, BC V1V 1V7, Canada, (2)Earth, Environmental and Geographic Sciences, University of British Columbia, Okanagan, 3247 University Way, Kelowna, BC V1V 1V7, Canada, (3)Département de géologie et de génie géologique, Université Laval, 1065 avenue de la Médecine, Quebec, QC G1V 0A6, Canada, (4)Dept of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106-9630, (5)Department of Earth Science, University of California, Santa Barbara, CA 93106
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 the bound the HMC; the South Tibetan detachment system above and the Main Central thrust below. An increasing number of chronologic and thermobarometric data from across the HMC, however, 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 orogen, 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 (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.
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