Upper Crustal Weak Zone in Central Tibet: An Implication From Three-Dimensional Seismic Velocity and Attenuation Tomography Results

A series of conjugate strike-slip faults is the most prominent geologic feature in central Tibet and is considered to accommodate east-west extension and coeval north-south contraction. The development mechanism of the conjugate strike-slip fault system is under debate because of unclear crustal physical properties and compositional variations. P and S wave arrivals from 414 local earthquakes recorded by the temporary Seismic Array Integrated Detection for a Window of Indian Continental Head array and the permanent China National Seismic Network were used for the velocity tomography, with additional P and S wave arrivals from 12 shots of the International Deep Profiling of Tibet and the Himalaya III reflection/refraction profile. The local earthquakes were simultaneously relocated with the updated velocity models. We also inverted for a three-dimensional upper crustal Qp model with the same earthquake data set. The Vp structure near the surface shows that low-Vp anomalies generally correspond to sedimentary basins and high-Vp anomalies are related to exhumed metamorphic blocks in the study area. Relatively low Vp/Vs ratios in the upper crust indicate widely distributed quartz-rich rocks. The low-Vp zone from 0- to 10-km depth (resolving depth limit) is spatially correlated with the Bangong-Nujiang suture, possibly reflecting the compositional difference along the ophiolitic melange belt accompanied by twin volcanic arcs from a double-sided subduction. This interpretation is supported by relatively heterogeneous Qp values. This low-velocity zone also implies relatively uniform stress and continuous deformation in the upper crust of central Tibet. The relatively weak materials in at least the upper crust would result in strain concentration and help the development of the conjugate strike-slip fault system along the Bangong-Nujiang suture.