HR: 1340h
AN: S13B-1063    [Abstracts]
TI: Probing the Mantle Transition Zone beneath Tibet Using Triplicate Seismic Waveforms
AU: * Tseng, T E
EM: tseng1@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W. Green Street, Urbana, IL 61801 United States
AU: Chen, W
EM: wpchen@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W. Green Street, Urbana, IL 61801 United States
AB: Tibet, the world­Ęs largest and highest plateau, is a key for understanding how the entire continental lithosphere responds to active collision. Many studies in the past 30 years focused on the intriguing double-thickness of Tibetan crust. However, equally important is the role of mantle lithosphere during continent collision, as density differences in the mantle ultimately drives tectonism. Nearly all tectonic models of Tibet invoke removal and sinking of the mantle lithosphere (of either India or Eurasia), including processes such as delamination, subduction, or convective instability. Dense, removed mantle lithosphere would have continued its descent and probably rests somewhere near the transition zone of the mantle where significant increases in density occur across major seismic discontinuities near depths of 410 and 660 km. As such, current state of the mantle transition zone beneath Tibet offers important constraints on mantle dynamics. Results from the mantle, in turn, have the potential of constraining how the crust achieves a double-thickness in Tibet. We construct a sequence of seismic profiles that sample the mantle beneath Tibet by combining broadband data from several permanent and temporary seismic arrays. The profiles comprise of triplicate waveforms that are particularly sensitive to anomalies near the mantle transition zone such as the size and topography of the 410- and the 660-km discontinuities. The data coverage is particularly favorable beneath the vast Qiangtang terrane of northern Tibet, with profiles extending over an aperture of more than 1,000 km, sampling both the 410- and the 660-km discontinuities. Preliminary results suggest that there are lateral (east-west) variations in the size of the 660-km discontinuity, on the order of about 1% in {\it P}-wave speed, over distances of only about 200 km.
DE: 7218 Lithosphere and upper mantle
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
SC: Seismology [S]
MN: 2004 AGU Fall Meeting