The Crustal and Upper-Mantle Structures beneath the Northeastern Margin of Tibet

Teleseismic receiver functions are used to estimate the crustal thickness, V-P and V-P/V-S (or Poisson's ratio, delta) beneath the northeastern margin of the Tibetan Plateau. Surface-wave dispersion measurements are combined with teleseismic receiver functions to improve estimates of model parameters. A migration method based on a local velocity model is used to image the upper-mantle discontinuities. According to the location of pierce points of the direct converted phases on the observed receiver functions, we estimate the topography of upper-mantle discontinuities. The thicker crust in the Altyn Tagh fault-Hexi Corridor basin may indicate the superposition of crusts during the collision between the Indian and the Eurasian plates. Beneath Tianshui and its neighboring regions in the eastern margin of Tibet, high VP and high V-P/V-S values close to uplifts of magma and hot springs might result from the upwelling of mantle materials induced by the collision. The depressed 410-km discontinuity might be related to the warm temperature below the northeastern margin of Tibet. We speculate that the mantle flow beneath the eastern margin of Tibet may be turning downward and subsiding into the mantle transition zone.