HR: 11:35h
AN: T22C-06    [Abstracts]
TI: Tomographic Evidence for a Possible Link Between Mantle Lithosphere Delamination and North-south Trending Rifts in Southeast Tibet
AU: * Ren, Y
EM: ren@gso.uri.edu
AF: Graduate School of Oceanography at University of Rhode Island, South Ferry Road, Narragansett, RI 02882, United States
AU: Shen, Y
EM: yshen@gso.uri.edu
AF: Graduate School of Oceanography at University of Rhode Island, South Ferry Road, Narragansett, RI 02882, United States
AB: The Tibetan plateau is a complex tectonic region resulted from the India-Eurasia collision, which has deformed the southeastern Asia through a combination of thrust, extension and strikeslip faultings. The north-south and northwest-southeast trending rifts on the surface of the plateau, which are manifested by the east-west extension, are mainly distributed in south and central Tibetan plateau and their origins still remain enigmatic. Some previous studies attributed these structures to gravitational spreading of the over-thickened crust, caused by an abrupt rise of the plateau due to convective removal of the lower part of the mantle lithosphere. Other studies have suggested that the north-south rift zones in the Tibetan plateau are shallow features formed by the eastward motion of the shallow crust that are decoupled from the mantle lithosphere by a low-viscosity lower crust. In this study, we present evidence suggesting that these features are a surface manifestation of a coherent deformation throughout the entire crust and mantle lithosphere and that the mantle lithospheric delamination plays a key role in the process. We performed finite-frequency tomography to investigate the crustal and upper mantle structures beneath the region of the Eastern Himalayan Syntaxis in southeastern Tibet. Our dataset comes from the Namche Barwa seismic experiment, which deployed a 50 stations broadband array and a 20 element short period array during 2003-2004 in southeastern Tibet. We also used data from the Global Seismic Network (GSN) station LSA, and 5 broadband stations from the Bhutan experiment, which overlapped temporarily with the Namche Barwa seismic experiment in 2003. Using a multi-channel waveform crosscorrelation technique, we measured P and S differential travel times between different stations in three frequency ranges : 0.03-0.1 Hz, 0.1- 0.5 Hz and 0.5-2.0 Hz for P waves, and, 0.02-0.05 Hz, 0.05-0.1 Hz and 0.1-0.5 Hz for S waves. The measured P and S delay times are then utilized to invert for spatial variations in P and S-wave speed perturbations according to 3-D finite frequency kernel formulation. Our results show the presence of a low-velocity anomaly in the crust and upper mantle to ~ 300 km depth beneath a north-south trending rift zone in southeast Tibet. This low- velocity anomaly is situated above a tabular, high-dipping-angle, high-velocity anomaly that extends into the upper mantle transition zone. The Vp/Vs ratio of this high-velocity anomaly suggests that temperature variations are not the only cause and a highly melt-depleted mantle is required. These observations provide clear evidence for the delamination of a thickened Eurasian mantle lithosphere and its causal relationship to the formation of the north- south trending rift in southeastern Tibet.
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7218 Lithosphere (1236)
DE: 7270 Tomography (6982, 8180)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting