HR: 0800h
AN: T31B-0473    [Abstracts]
TI: Three-dimensional velocity structure of the crust beneath the central Tien Shan, Kyrgyzstan: Implications for large- and small-scale mountain buildings
AU: * Omuralieva, A
EM: aika@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, 6-6 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 9808578, Japan
AU: Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, 6-6 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 9808578, Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, 6-6 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 9808578, Japan
AB: A temporary broadband seismic network was installed during 1997–2000 in the central part of the Kyrgyz Tien Shan. Three-dimensional P- and S-wave velocity structure of the crust in the central Tien Shan was obtained by applying a tomographic method to arrival-time data from shallow local earthquakes recorded by the seismic network of the Tien Shan. The central part of the Tien Shan is upthrusted between the Kazakh Shield to the north and the Tarim Basin to the south and represents a crust-scale ramp with main thrusts at its edges. This active region is under north-south compression due to the India-Eurasia collision. Accurate and precise tomographic imaging sheds light upon dynamics of the mountain building, interaction of these tectonic blocks in this complicated region. The study area selected is an area enclosed by 39.5-43.5°N and 73.5- 80.5°E. The used arrival-time data from about 1500 local earthquakes recorded by the network, which consists of 42 stations of KNET, GHENGIS, KazNet and Geoscope spanned over the central Kyrgyz Tien Shan. The tomography method by Zhao et al. (1992) has been used in this study. The spacing between grid nodes is about 30-50 km in horizontal direction and 5-15 km in vertical direction. The total number of grid nodes is about 1500. Obtained tomographic images at depths of 0-5 km show a good correlation with subsurface geology. Low-velocity (low-V) zones are distributed beneath the basins, whereas high-velocity (high-V) zones exist beneath the mountain ranges at the surface layer. The low-V zones beneath the basins are probably due to the Cenozoic deposition accumulated with a thickness more than 4 km. Paleozoic crystalline basement rocks may be responsible for the high-V zones in the mountain belts. South-dipping and north-dipping low-V zones within the crust can be seen on both north and south sides of the central Tien Shan, respectively. These zones spatially correspond to the north boundary fault and the south boundary fault, where the Kazakh Shield and the Tarim Basin underthrust the central Tien Shan, respectively. These low-V zones probably originate from a prominent low-V zone explicitly imaged in the lower crust beneath a high-elevated region, where a south-dipping mantle upwelling flow reaches the Moho. Fluids conveyed through the upwelling flow might have penetrated to the lower crust and caused the north-dipping low-V anomaly. Then laterally intruded fluids and/or melt seen as a low-V zone within the low- to the mid-crust might have caused the blurred south-dipping anomaly on the opposite side. This observation confirms a crust-scale ramp model of the entire Tien Shan by means of the seismic tomography. High-V zones distributed beneath basins and low-V zones beneath encompassing mountains at depths of about 20-50 km that gradually replaced by low-V and high-V regions at the shallow depths, respectively, are found in the most parts of the Tien Shan. It seems that along with north-south compressional stress due to the India-Eurasian collision, the small-scale convection or mantle upwelling is an additional driving force of the mountain building in this region which might have played a key role in the basin-mountain development within the Tien Shan.
DE: 1038 Mantle processes (3621)
DE: 7218 Lithosphere (1236)
DE: 8107 Continental neotectonics (8002)
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting