Nature of the upper crust beneath central Tibet

A wide-angle reflection/refraction seismic line obtained through Sino-French joint research near the Bangung-Nujiang Suture in Tibet 1] yielded crustal velocity estimates for both P and S waves. AV_p/V_s ratio of 1.65 was derived fo waves propagating in the upper ( < 25 km) crust. According to the high-pressure, high-temperature velocity measurements of Kern 2] and others, such a ratio implies the presence of quartz-rich rocks, e.g., granites and granitic gneisses, in the upper Tibetan crust. Because of the high geothermal gradient in Tibet ( > 50°C/km near the surface), a shallow low-velocity zone is expected to form in a granitic upper crust for both P and S waves. A two-dimensional ray-tracing study indicates that the observed first P and S waves 1] can be interpreted as diving waves above a low-velocity zone where a positive gradient exists. The low-velocity zone commences at a depth of 10 km and extends possibly to 20 km or deeper.