Institute of Geophysics
Stuttgart University
Germany
Email:
wolle@geophys.uni-stuttgart.de
poster/oral: poster
| The structure of the mantle beneath East Asia down to 800 km depth is investigated using full waveforms of seismic shear and surface waves with epicentral distances of less than 40 degrees. In contrast to previous waveform inversions, we use (1) exact 3D waveform sensitivity kernels which correctly reflect off-path sensitivity and the existence of Fresnel zones. We apply (2) an accurate 3D forward modelling technique based on a coupled-mode, multiple forward scattering approach allowing us (3) to iterate the inversion procedure through several 3D-models and (4) to evaluate the true misfit between the data and the synthetics for the 3D-model. Average lateral resolution of the model is 400 km with a still smaller resolution length in the upper 250 km of the mantle beneath the West Pacific subduction zones and Tibet. Major features of the model are high-velocity slabs along the West Pacific subduction zones stagnating at the 660 km discontinuity, a strong, elongated low-velocity zone between 80 and 250 km depth in the West Pacific backarc regions, a plume-like low-velocity feature beneath the Baikal rift zone extending into the transition zone, a low-velocity region under the Tien Shan with connection into the transition zone and thick crust under Tibet reaching its maximum thickness of about 80 km south of 35 N. The northern part of the Tibet plateau is underlain by a mantle-wide zone of average-to-slow velocities which is encircled by fast Indian and Asian lithosphere at 100-250 km depth. Neither lithosphere descends into the mantle ruling out upper-mantle wide subduction beneath Tibet. Instead, copmparison with convection modelling suggests that the lithospheric mantle under northern Tibet has been convectively removed and replaced by hot, upwelling asthenosphere. |