Deep reflection surveying in central Tibet: lower-crustal layering and crustal flow

In the summer of 1998, a series of multichannel, deep seismic reflection tests were conducted in central Tibet around 89°E as part of the third phase of Project INDEPTH, a multidisciplinary study of the structure of the core of the Tibetan Plateau. Short seismic reflection sections from four locations spanning the Jurassic Banggong–Nujiang Suture show several features: thin (<2 km), sedimentary cover sequences; an unreflective upper crust down to approximately 25 km depth; strongly reflective lower crust down to 65 km depth (22 s traveltime) and a distinct change in seismic character from layered reflectivity to unreflective that we interpret as marking the reflection Moho. These profiles are unlike those in parts of the Yadong–Gulu Rift to the south where few deep reflections exist and reflection profiles are dominated by high-amplitude reflection bright-spots at 6 s which have been interpreted as fluid bodies, either water or magma. Analyses of seismic amplitude decay on individual shot records confirm source-generated energy at traveltimes beyond 22 s, thus indicating that the change in reflection character at the Moho is not the result of a loss of signal penetration. The depths to the top and bottom of the lower-crustal reflectivity match both a mid-crustal velocity increase and the Moho determined for this area from refraction profiling. Lower-crustal reflectivity is consistent with current ductile deformation as suggested by models for middle and lower-crustal flow in Tibet. However, Cretaceous-age magmatic intrusions may explain at least part of the observed reflectivity.