Wide-angle seismic profiling of southern Tibet

Three-component, wide-angle seismic data was acquired along the route of Project INDEPTH (International Deep Profiling of Tibet and the Himalayas) to constrain the crustal structure and composition of southern Tibet. The INDEPTH seismic reflection profile in the southern Tethyan Himalaya imaged the top (Main Himalayan Thrust or MHT) of the underthrusting Indian continental crust. I used the profile's first-breaks and the wide-angle data to construct a seismic-velocity model down to the MHT. The depth of young extensional basins along the profile is constrained to be {dollar}leq{dollar}2 km, yielding an estimate of 1.5% east-west extension in the Himalayas. A reflector dipping 12.5{dollar}spcirc{dollar}NNE to a depth of 22 km, is interpreted to suggest that the South Tibetan Detachment System is deeply-rooted. The MHT has a constant dip of 7.5{dollar}spcirc{dollar}NNE to 40 km depth about 70 km south of the Indus-Yarlung suture, suggesting that Indian crust underthrusts the surface expression of the suture but does not form the lower crust of central Tibet.; Project INDEPTH imaged a band of bright-spots c. 15 km depth along 150 km of the northern Yadong-Gulu rift. I show that the bright-spot reflections are diffractions from short segments each extending 1-2 km laterally; their waveform is a composite of diffractions and near surface reverberations; and at the resolution of the wide-angle data each represents a single first-order reflector, rather than a thin-layer. Travel-time modeling of P- and P-to-S-converted reflections from the bright-spots yields average P- and S-velocities of 5.3 {dollar}pm{dollar} 0.2 km{dollar}cdot{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} and 3.2 {dollar}pm{dollar} 0.2 km{dollar}cdot{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} respectively for the upper-crust. Merging reflections from multiple Tibetan bright-spots allows me to analyze their amplitude variation with offset (AVO), constraining P- and S-velocities to 3.0 {dollar}pm{dollar} 0.5 km{dollar}cdot{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} and 1.6 {dollar}pm{dollar} 0.5 km{dollar}cdot{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} respectively. These velocities imply that the Tibetan bright-spots represent concentrations of free aqueous fluids in the middle crust, placing an upper temperature limit of 650{dollar}spcirc{dollar} at 15 km depth.