In the summer of 1998 as part of the INDEPTH III project, wide-angle seismic data were recorded from large shots along a 400 km long NNE-SSW profile crossing the Banggong-Nujiang suture (BNS) at about 89.5°E in central Tibet. Analysis of these data suggests that i) crustal thickness in the vicinity of the Banggong-Nujiang suture is 65+/-5 km and there is no evidence at 89.5°E for a step of 20 km in the Moho at the BNS and ii) a 25-30 km thick high velocity (6.5-7.2 km/s) lower crustal layer exists. In addition the present P-wave model contains no crustal low velocity layer. Combining the results from the INDEPTH III wide-angle profile with other seismic results allows a Moho cross-section to be constructed across Tibet. This cross-section shows that crustal thickness tends to decrease from south to north with values of 70-80 km south of the middle of the Lhasa block, 60-70 km in the northern part of the Lhasa block and the Changtang terrane and less than 60 km north of the Jinsha river suture. As at the same time the average topography is uniform or if anything increases to the north (at least to the Jinsha suture) and as the plateau is in isostatic equilibrium, the northwards decrease in crustal thickness implies that either the average crustal density decreases northwards and/or the mantle density decreases northwards. A northwards decrease in mantle density could support the hypothesis that the mantle under northern Tibet is hotter than under southern Tibet.
- Wide-angle experiment - June - August 1998
- Dr. J. Mechie (GFZ Potsdam)
- Prof. Dr. R. Kind (GFZ Potsdam)
- Prof. Dr. L. Ratschbacher (Uni. Wuerzburg, now Uni. Freiberg)
- Prof. Dr. R. Meissner (Uni. Kiel)
- Chinese Academy of Geological Sciences, Ministry of Land and Resources, Beijing, China
- Stanford University, Syracuse University, Cornell University, New Mexico State University
Methods & Equipment
- wide-angle reflection/refraction - 30 Refteks from GIPP, 30 Refteks from IRIS-PASSCAL
- Zhou, B., Liang, X., Lin, G., Tian, X., Zhu, G., Mechie, J. and Teng, J., 2019. Upper crustal weak zone in central Tibet: an implication from three-dimensional seismic velocity and attenuation tomography results. J. Geophys. Res., 124: 4654-4672, doi:10.1029/2018JB016653.
- Shi, D., Zhao, W., Klemperer, S.L., Wu, Z., Mechie, J., Shi, J., Xue, G. and Su, H., 2016. West-east transition from underplating to steep subduction in the India-Tibet collision zone revealed by receiver-function profiles. Earth Planet. Sci. Lett., 452: 171-177, doi:10.1016/j.epsl.2016.07.051.
- Mechie, J., Sobolev, S.V., Ratschbacher, L., Babeyko, A.Y., Bock, G., Jones, A.G., Nelson, K.D., Solon, K.D., Brown, L.D., Zhao, W., 2004. Precise temperature estimation in the Tibetan crust fom seismic detection of the alpha-beta quartz transition. Geology, 32: 601-604, doi:10.1130/G20367.1.
- Shi, D., Zhao, W., Brown, L., Nelson, D., Zhao, X., Kind, R., Ni, J., Xiong, J., Mechie, J., Guo, J., Klemperer, S. and Hearn, T., 2004. Detection of southward intracontinental subduction of Tibetan lithosphere along the Bangong-Nujiang suture by P-to-S converted waves. Geology, 32: 209-212.
- Haines, S.S., Klemperer, S.L., Brown, L., Jingru, G., Mechie, J., Meissner, R., Ross, A. and Wenjin, Z., 2003. INDEPTH III seismic data: From surface observations to deep crustal processes in Tibet. Tectonics, 22(1), 1001, doi:10.1029/2001TC001305.
- Kind, R., Yuan, X., Saul, J., Nelson, D., Sobolev, S.V., Mechie, J., Zhao, W., Kosarev, G., Ni, J., Achauer, U. and Jiang, M., 2002. Seismic images of crust and upper mantle beneath Tibet: Evidence for Eurasian plate subduction. Science, 298: 1219-1221.
- Zhao, W., Mechie, J., Brown, L.D., Guo, J., Haines, S., Hearn, T., Klemperer, S.L., Ma, Y.S., Meissner, R., Nelson, K.D., Ni, J.F., Pananont, P., Rapine, R., Ross, A., and Saul, J., 2001. Crustal structure of central Tibet as derived from project INDEPTH wide-angle seismic data. Geophys. J. Int., 145: 486-498.
- Huang, W.-C., Ni, J.F., Tilmann, F., Nelson, K.D., Guo, J., Zhao, W., Mechie, J., Kind, R., Saul, J., Rapine, R. and Hearn, T.M., 2000. Seismic polarization anisotropy beneath the central Tibetan Plateau. J. Geophys. Res., 105: 27979-27989.