Two-plane wave tomography and lithospheric structure beneath eastern Tibet

Abstract

This dissertation focuses on two specific research topics: First, I study the lithospheric structure beneath E. Tibetan Plateau, using new data from INDEPTH-IV (INternational Deep Profiling of Tibet and the Himalayas) seismic experiment. I show new shear wave tomographic maps, in order to better constrain the existing geodynamic models regarding the growth and dynamics of the region. In the second part, I investigate seismic resolution capabilities of the two-plane wave method using synthetic seismograms. Further, I modify and apply this approach to Love waves. My shear wave tomography results indicate that subhorizontal underthrusting of the Indian plate beneath E. Tibet does not extend much further north of Bangong-Nijuang Suture. Moreover, my tomographic images clearly show that the Indian lithosphere is laterally torn into at least two fragments. The westernmost fragment is detached from the rest of the Indian plate, and vertically sinking into the astenosphere. Azimuthal fast directions are consistent at depth beneath the central part of the study region, suggesting vertically coherent deformation of crust and uppermost mantle. Further, I observe low velocity zones along the northern and southern branches of the Kunlun fault, and I attribute these zones to strain heating due to ductile deformation. Moreover, these low velocity zones below the Kunlun Shan are continuous at depth, providing evidence against a present southward continental subduction or underthrusting. One of the most important limitations of seismic tomography studies is spatial resolution of applied methods. To determine the resolution of my tomographic models, I calculate synthetic seismograms for ideally isotropic checkerboard earth models, and apply two-plane wave tomography. My results prove that SV anomalies smaller than the dominant wavelength of surface waves can be recovered down to ?200 km, wheretraditional approaches break down. For Love waves, a modified version of the approach is required, incorporating two perpendicular components for two plane wave solutions along x and y directions of the local coordinate system. Moreover, the method exhibits adequate resolution for SH waves down to ?100 km. Smearing is more evident in SH results due to shallower depth sensitivity and longer wavelengths of Love waves than Rayleigh waves.