Mantle transition zone topography and structure beneath the central Tien Shan orogenic belt

In this study we calculated receiver functions (RFs) from teleseismic P waveforms recorded by stations of four seismic networks to determine the topography of the mantle transition zone (MTZ) beneath the central Tien Shan. We converted the RFs from the time domain to the depth domain and selected the depths of 410 and 660 km discontinuities after stacking the RFs in narrow raypath bins. To better determine the MTZ topography, we applied an updated RF method to invert the depth differences between the 410 and 660 km discontinuities in each RF for lateral depth variations of the two discontinuities. Using this approach, we detected several anomalies in the 410 and 660 km discontinuity depths beneath the central Tien Shan region. Extensive synthetic tests were carried out to confirm the main features of the result. Beneath the south and east of Lake Issyk-Kul, the 410 km discontinuity becomes shallower while the 660 km discontinuity becomes deeper, leading to a thicker MTZ with a lower temperature, possibly caused by pieces of thickened lithosphere dropping down to at least the bottom of the MTZ. Beneath the northwest of Lake Issyk-Kul, the 410 km discontinuity becomes deeper while the 660 km discontinuity becomes shallower, resulting in a thinner MTZ with a higher temperature, which may reflect a small-scale hot upwelling from the lower mantle.