Shear-Wave Birefringence and Current Configuration of Converging Lithosphere under Tibet

Shear-Wave Birefringence and Current Configuration of Converging Lithosphere under Tibet (a) to (c) Comparisons of three large-scale profiles of S-wave split time (dt in s) across Tibet. Open circles are isolated cases of birefringence south of the mantle suture. Measurements with error-bars (and shown in dark grey) are those analyzed by us; otherwise we plot values reported in the literature. The horizontal dotted-line represents the threshold for null birefringence. (d) A schematic cross-section showing the current configuration of the sub-continental lithospheric mantle, including a large-scale anomaly of high P-wave speed resting on top of the lower mantle and interpreted as the remnant of thickened (and subsequently detached due to Rayleigh-Taylor instability) lithospheric mantle (Chen and Tseng, 2007).
New data from west-central Tibet show that birefringence of S-waves has two pronounced increases in magnitude toward the hinterland. Null birefringence persists to about 75 km north of the Indus-Yarlung suture (IYS) between the Indian shield and the Lhasa terrane of southern Tibet. A second, rapid increase occurs about 100 km farther north of the Bangong-Nujiang sutures between the Lhasa terrane and the Qiangtang terrane in central Tibet. The latter feature is consistently observed along three long transects that collectively span a lateral (orogen-parallel) distance of about 600 km and is likely to mark the northern, leading edge of sub-horizontally advancing mantle lithosphere of the Indian shield (the “Greater India”) – an interpretation consistent with the latest results of finite-frequency tomography using both P- and S-wave travel-times, previous results of modeling gravity anomalies, and a host of other seismic observations. Similarly, complementary constraints indicate that the sudden onset of significant birefringence north of the IYS is likely to be the southern termination of Eurasian mantle lithosphere. Curiously, the shortest of three transects showed null birefringence through much of the Lhasa terrane, a pattern inconsistent with those of He isotopes and gravity.
</p><p>Chen, W.-P., M. Martin, T.-L. Tseng, R. L. Nowack, S.-H. Hung, and B.-S. Huang, Shear-wave birefringence and current configuration of converging lithosphere under Tibet, Earth Planet. Sci. Lett., 295(1-2), 297-304, doi:10.1016/j.epsl.2010.04.017, 2010
</p><p>Chen, W.-P., and T.-L. Tseng, Small 660-km seismic dis- continuity beneath Tibet implies resting ground for detached lithosphere, J. Geophys. Res., 112, B05309 (15 pp.), doi:10.1029/2006JB004607, 2007.
Acknowledgements: This work was supported by U.S. National Science Foundation grants EAR99-09362, EAR05-51995, EAR06-35419 (W.-P.C.), EAR EAR06- 35611(R.L.N.), U.S. Air Force contract FA8718- 08-C-002 (R.L.N. and W.-P.C.), National Science Council of Taiwan grants 96-2119-M-002-016 and 97-2745-M-002-011 (S.-H.H.), and Academia Sinica, Taiwan (B.-S.H.) which provided additional personnel (Wen-Tzong Liang, Chia-Lung Wu, John Lin, and Chun-Chi Liu). Seismic experiments were carried out jointly with Oregon State University (lead by John Nabelek), Chinese Academy of Geological Sciences (lead by Jiang Mei and Heping Su), Nepal Department of Mines and Geology (lead by Soma Sapkota and M. Pandey) and Peking University (lead by John Chen). We thank two anonymous reviewers for useful comments on the manuscript, F. Tilmann for discussions, and Zhaohui Yang (University of Illinois) for help with GIS databases.</p>


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