HR: 10:40h
AN: T52A-02    [Abstracts]
TI: Project Hi-CLIMB: A Synoptic View of the Himalayan Collision Zone and Southern Tibet
AU: * Nábělek, J L
EM: nabelek@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AU: Vergne, J
EM: vergne@geologie.ens.fr
AF: Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond, Paris, 75231 France
AU: Hetenyi, G
EM: hetenyi@geologie.ens.fr
AF: Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond, Paris, 75231 France
AB: Project Hi-CLIMB is a broadband seismic experiment whose goal is to produce a high-resolution continuous profile across the Himalaya and southern Tibet. The centerpiece of the project is a closely spaced, linear array of broadband seismographs, extending from the Ganga lowland, across the Himalayas, and onto the central Tibetan plateau. A complementary array of sparsely spaced stations flanks the linear array. Over 270 sites were occupied during the experiment. The principal institutions involved in the field operations were the Oregon State U. and U. of Illinois (USA), Dept. of Mines and Geology (Nepal), Chinese Academy of Geol. Sci. and Peking U. (China) and the Inst. of Earth Sci. (Taiwan). The major funding for this project was provided by the NSF, Continental Dynamics program. We focus on the receiver function images from the main profile. We observe clear Moho and the upper-mantle discontinuities. The Moho, which in southern Nepal is at 45 km depth (relative to sea level), dips at a gentle angle under the Himalaya. Crossing the Himalaya, its depth rapidly increases, reaching the of 70 km near the Yarlung River. We have succeeded in imagining the Main Himalayan Trust (MHT) as it descends northward at a shallow depth from its surface expression, the Main Frontal Thrust in southern Nepal. In Nepal along the profile, MHT is expressed by a pronounced seismic low velocity zone, which we believe indicates a presence of trapped aqueous fluids in the fault zone, thus lowering the strength of the megathrust. The low velocity associated with the MHT disappears for a short distance north but reappears again as the MHT increases its dip under S. Tibet. We believe the characteristics of the low velocity associated with the MHT in S. Tibet indicate a presence of partial melt due to an increase in depth and frictional heating. A low-velocity wedge above the MHT suggests an accumulation of the melt. This could be an ongoing process of generation of the Himalayan granites. The Tibetan data reveal Indian crust tucking under the "Asian" crust and sliding under it all the way to the Banggong-Nujiang suture where its lower portion peals off and subducts steeply under the Qiangtang terrene. Under the Lhasa terrene, where we observe fully doubled-up Indian and Asian crust, the relative motion appears to be taken up along a midcrustal low-viscosity/low-velocity zone. The lower crust is high velocity, dense and strong, thus enabling its subduction north of the BNS. The strength of the lower crust seems to be inherited from the lower Indian crust, which is high velocity already under Nepal and undergoes further densification by eclogitisation as it slides to greater depths under Tibet. North of the BNS the high-velocity lower crust is absent. The lower crust north of the BNS may be formed by a northward transfer of unsubducted viscous quartz-rich midcrustal material from the Lhasa terrene.
DE: 7218 Lithosphere (1236)
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8108 Continental tectonics: compressional
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: Fall Meeting 2005