HR: 0800h
AN: T41A-1286    [Abstracts]
TI: Very Small 660-km Discontinuity beneath Tibet: Evidence for Detached Lithosphere?
AU: * Tseng, T
EM: tseng1@uiuc.edu
AF: Department of Geology, University of Illinois, 1301 W Green St., Urbana, IL 61801
AU: Chen, W
EM: wpchen@uiuc.edu
AF: Department of Geology, University of Illinois, 1301 W Green St., Urbana, IL 61801
AB: The fate of mantle lithosphere during continent-continent collision is a key question in continental dynamics. Unfortunately there were few tight constraints on mantle processes beneath the Himalayan-Tibetan orogen - the most prominent, active collision zone on the Earth. We address this issue with high-resolution seismic results concerning the nature of the 660-km discontinuity beneath central Tibet. Furthermore, we obtain similar constraints beneath the Indian shield which provide a crucial baseline for comparison with the mantle transition zone (TZ) under Tibet. Using data recorded by several permanent and temporary broadband arrays, we construct a sequence of large-aperture (over 1,000 km) seismic profiles that sample the TZ beneath Tibet and India. By modeling both the timing and amplitude of triplicate waveforms, our results are particularly sensitive to seismic anomalies near the TZ. It turns out that the contrast in P-wave speeds across the 660-km discontinuity (Δ VP660) is remarkably small beneath central Tibet: only 3.1± 0.5% at a depth of 670 ± 10 km. The contrast is only about half of the global average (cf. 5.6% in the iasp91 model) which also seem to characterize regions to the north of Tibet. More important, the corresponding value beneath stable India is also about 1/3 larger (about 4.2% in Δ VP660) than that under central Tibet. In other words, even though the Tibetan plateau is the result of collision between India and Eurasia, Δ VP660 beneath Tibet resembles neither end-member. It follows that a small Δ VP660, most likely due to a thermal anomaly of high VP in the bottom of the TZ, must have been allochthonous in origin, due to impounding of either subducted oceanic lithosphere or convective removal of thickened continental lithosphere. Since major episodes of subduction ceased about 50 Ma ago, recent sinking of once thickened Asian lithosphere, now resting on the 660-km discontinuity, seems to the most plausible cause of a small Δ VP660 under central Tibet.
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8100 TECTONOPHYSICS
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005