Three-dimensional seismic velocity structure of the crust beneath the central Tien Shan, Kyrgyzstan: Implications for large- and small-scale mountain building

The central Tien Shan, Kyrgyzstan is located approximately 2000 km from the main collisional suture between India and Eurasia and is sandwiched between the Kazakh Shield to the north and the Tarim Basin to the south. A three-dimensional (3-D) seismic velocity structure of the crust beneath the central Tien Shan was estimated to a depth of 50 km by means of seismic tomography using arrival-time data of P- and S-waves from local earthquakes recorded by a temporary seismic network of the Tien Shan during 1998–2000. Velocity structure images show a good correlation with the subsurface geology at shallow depths. A low-velocity zone is distributed beneath the basins probably due to the Cenozoic deposition, whereas a high-velocity zone is visible beneath the mountain ranges likely due to Paleozoic crystalline basement rocks at the surface layer. Results obtained at greater depths revealed the following features: (1) A low-velocity zone is clearly observed in the middle to lower crust, and is probably caused by melt and/or fluids conveyed through the flow of mantle upwelling; (2) Both north-dipping and south-dipping low-velocity zones were imaged at locations where the Kazakh Shield and the Tarim Basin underthrust beneath the Tien Shan, respectively, and are probably caused by mantle materials somehow being intruded into the crust. (3) The lateral variation of high- and low-velocity zones seen in the middle to lower crust beneath basins and surrounding mountain ranges, respectively, seems to be caused by melt and/or fluids vertically penetrated and/or laterally spread. All these findings suggest that in-process northward shortening and crustal thickening owing to collision, and mantle convective motion, i.e. upwelling and, as a consequence, partial melting are “work-horses” in the mountain building beneath the Tien Shan.