Dislocation structure of the crust-mantle boundary and low-velocity body within the crust beneath the Dabie Shan collision orogen

P5; From April of 2001 to March of 2002, a passive seismic array experiment was carried out in the Dabie Shan and its adjacent region. In this experiment, totally 34 broadband seismic stations were deployed along a profile across the Dabie Shan orogen and North-China platform. This profile is about 500 km long from Cuilin (34°40′N, 114°49′E), Henan Province, to Dajipu (30°20′N, 115°03′E), Hubei Province. The space between stations is about 3-8 km in the Dabie Shan orogenic belt and about 15-20 km in other area. The receiver function profile and S-wave velocity structure of the crust and upper mantle down to 100 km depth along the profile are investigated in terms of the receiver function techniques (Liu et al., 1996, 2000). Our results show that the crust beneath the Dabie Shan orogen has an obviously asymmetric blocked structure in the direction perpendicular to the mountain strike. The maximal crustal thickness reaches to 42 km. The crust-mantle boundary has a dislocation structure correlated to the crustal blocks and the largest offset reaches to 8 km. In the kernel of the orogen exists a low-velocity body inside the crust, which is separated into two parts corresponding to the South Dabie and North Dabie on the surface, respectively. Probably a vertical divergent movement between both took place in history. The crust below this low-velocity area has a positive gradient velocity structure with the depth, and the upper mantle down to the depth of 70 km has the lower S-wave velocity than its both sides. Beneath the Dabie Shan, however, a high-velocity anomaly exists in the upper mantle below 70 km.