The Crustal Structure Along the Geoscience Transect from Altay to Altyn Tagh

Based upon the P- and S-wave data acquired along the geoscience transect from Altay to Altyn Tagh in Northwest China, the crustal structures of velocities and Poisson's ratio were established. The crustal velocity structure features an obvious three-layer structure with velocities of 6.0~6.3km/s, 6.3~6.6km/s and 6.9~7.0km/s from surface to depth, respectively. The crustal thickness along the entire profile is nearly uniform about 50km with the thickest crust (56km) beneath the Altay and the thinnest (46km) beneath the Junggar basin. The velocities under Moho are 7.7 to 7.8km/s between the Tianshan and the Junggar basin, and 7.9 to 8.0km/s below the Altay Mountains and eastern margin of the Tarim basin. The southern half of the profile, including the eastern Tianshan Mountains and eastern margin of the Tarim basin, shows low P-wave velocities and σ =0.25 to a depth of 30km, which suggests a quartz-rich, granitic upper crustal composition. The northern half of the profile below the Altay Mountains and Junggar Accretional Belt has a higher Poisson's ratio of σ = 0.26 ~ 0.27 to a depth of 30km, indicative of an intermediate crustal composition. The entire profile is underlain by a 15 to 30km thick high-velocity (6.9~7.0km/s, σ = 0.26 ~0.28) lower crustal layer that we interpret to have a bulk composition of mafic granulite. At the southern end of the profile a 5-km-thick mid-crustal low-velocity layer (Vp=5.9km/s, σ =0.25) underlies the Tianshan and the region to the south, and may be indicative of granitic intrusive in Late Paleozoic.