HR: 11:55h
AN: T52A-07    [Abstracts]
TI: Seismicity and 3-D Velocity Structure of the Himalayan Collision Zone: Lateral Variations in Lithospheric Structure
AU: * Monsalve, G
EM: monsalve@colorado.edu
AF: University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309 United States
AU: Sheehan, A
EM: afs@cires.colorado.edu
AF: University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309 United States
AU: Rowe, C
EM: char@lanl.gov
AF: Los Alamos National Laboratory, EES-11 M.S. D408, Los Alamos, NM 87545 United States
AB: Using P and S arrival time data from the 29-station Himalayan Nepal Tibet Seismic Experiment (HIMNT), we performed local earthquake tomography for eastern Nepal and southern Tibet. Our results show that local earthquakes beneath the network reach depths up to 100 km below sea level, and over a hundred seismic events recorded during the one year deployment locate in the upper mantle. These subcrustal earthquakes suggest that in some areas beneath the Himalayan collision zone, the upper mantle deforms by brittle processes. There are no clear gaps in the vertical distribution of the seismicity. Our tomographic images show dramatic crustal thickening from an average of 45 km beneath southern Nepal to 75 km beneath southern Tibet. The presence of earthquakes around Moho depths and high P wave velocities (up to 8.7 km/s) in the upper mantle beneath the High Himalayas and southern Tibet suggest a relatively cold upper mantle. Most of the lower crustal and subcrustal earthquakes beneath the High Himalaya and southern Tibet are concentrated east of longitude 86.5E, where the lower crust has relatively low P wave velocities (between 6.2 and 6.5 km/s). West of that longitude, earthquakes deeper than 50 km are rare, and P wave velocities in the lower crust are as high as 7.5 km/s. We interpret these high velocities in the Tibetan lower crust as the result of metamorphic reactions causing partial transformation into eclogite of materials in the lower Indian crust. The Vp/Vs ratio is anomalously low (an average of 1.66) in the upper crust of southern Tibet (Tethyan Himalaya), which may correspond to a very silicic basement of the sedimentary sequence seen at the surface.
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: Fall Meeting 2005