Is rapid exhumation of the High Himalaya driven by ramp overthrusting, the formation of mid- crustal duplex, or out-of-sequence thrusting associated with channel flow?

HR: 16:15h
AN: T34C-02 INVITED [Abstracts]
TI: Is rapid exhumation of the High Himalaya driven by ramp overthrusting, the formation of mid- crustal duplex, or out-of-sequence thrusting associated with channel flow?
AU: Herman, F
EM: frederic@erdw.ethz.ch
AF: California Institute of Technology, Tectonics Observatory, Pasadena, CA 91125, United States
AU: * Avouac, J
EM: avouac@gps.caltech.edu
AF: California Institute of Technology, Tectonics Observatory, Pasadena, CA 91125, United States
AU: Bollinger, L
EM: laurent.bollinger@cea.fr
AF: Laboratorie de Detection et de Geophysique, CEA, Bruyeres-le-Chatel, 91680, France
AU: Maheo, G
EM: gweltaz.maheo@univ-lyon1.fr
AF: California Institute of Technology, Tectonics Observatory, Pasadena, CA 91125, United States
AU: Farley, K
EM: farley@gps.caltech.edu
AF: California Institute of Technology, Tectonics Observatory, Pasadena, CA 91125, United States
AU: Harrison, M
EM: tmh@oro.ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Drive East, Los Angeles, CA 90095, United States
AB: A number of studies of river incision and exhumation across the Nepal Himalaya indicate that uplift rates must increase abruptly from the Lesser Himalaya to the High Himalaya. Locally high uplift rates could relate to: 1- thrusting over a mid-crustal ramp; 2- mid-crustal duplex growth; or 3- out-of-sequence thrusting along the front of the High Himalaya. Ramp-overthrusting can be a viable mechanism only over a short period of time since, considered alone, this mechanism fails to account for the growth of the orogenic wedge. The same problem arises with out-of-sequence thrusting, if considered alone. Thus an additional mechanism is needed to account for the transfer of material from the Indian crust to the Himalayan wedge. The structure of the Himalayan wedge as well as the pattern of active deformation across the range suggests that underplating, through the development of a duplex system at mid-crustal depth, has been the dominant mechanism of accretion over the last ~15Myr. In an attempt at determining plausible kinematic models over this period, we have applied a formal inverse approach based on the Neighbourhood Algorithm. The forward models consider the possibility of either thrusting localized along a single major thrust fault (the MHT) with non-uniform underplating due to duplexing, or out-of sequence thrusting in addition to thrusting along the MHT and with uniform underplating rate. The models are computed using a thermokinematic FEM model (PECUBE), and tested against thermochronological, thermometric and thermobarometric data from central Nepal compiled from the literature and complemented with new (U-Th)/He data. The formal inversion approach allows definition of best fitting values of model parameters and their uncertainties. In addition to the geometric parameters, model variables include overthrusting rates; radiogenic heat production in the High Himalayan Crystalline (HHC) sequence; the timing of enhanced rock uplift /exhumation rates corresponding to the formation of the duplex or out-of-sequence thrust re-activation. A model with out-of-sequence thrusting can provide a satisfactory fit to the data (with a minimum reduced χ² of 1.41) but requires a large overthrusting rate of 11 +- 1mm/yr and implies a total convergence rate >30 mm/yr. The duplex model, with a minimum reduced χ² of 1.01, is more consistent with observation. According to this model, the 20 mm/yr convergence results from an overthrusting rate of 5 +- 1 mm/yr and an underthrusting rate of 15 +- 1 mm/yr. Modern uplift rates are estimated to increase from about 0.8+-0.2 mm/yr in the Lesser Himalaya to 3.5 +- 0.5 mm/yr at the front of the high range, 95 +- 5 km from the MFT. The effective friction coefficient is estimated to be 0.09 +- 0.01 and the radiogenic heat production of HHC units is estimated 2.2 +- 0.1 μW/m³. The mid-crustal duplex initiated at 10 +-2 Ma, leading to an increase of uplift rate at front of the High Himalaya from ~0.8 to 3.5 mm/yr. Thus the analyzed dataset appears more consistent with a duplex model relative to out-of-sequence-thrusting. Given the excellent fit of the duplex model to the geologic, petrologic and geophysical constraints, there appears little basis to support a channel flow-type model in a Himalayan context over the last 15Myr.
DE: 1140 Thermochronology
DE: 8102 Continental contractional orogenic belts and inversion tectonics
SC: Tectonophysics [T]
MN: 2007 Fall Meeting