Abstract
In order to investigate the crust and upper mantle structure of velocity and anisotropy for Dabieshan orogenic belt, a wide-angle reflection/refraction survey from Anyi, Jiangxi province to Zhuangmu, Anhui province, PR China, was undertaken in 1994. We reconstructed initial images of P- and S-wave velocities and two anisotropy factors with Anderson’s notation by using multi-parameter velocity analysis technique in 2D anisotropic media, and fitted the observed traveltimes for P- and S- reflecting waves to that calculated by forward modelling, and produced the final models of P- and S-wave velocity and two Anderson’s anisotropy factors. The structures of two anisotropy factors are very different from the structures of P- and S-wave velocities. The structures of P- and S-wave velocities show the characteristic of layering and blocking, and the structures of Anderson’s anisotropy factors displays the characteristics of ‘chimney’ structure. The thickness of the crust in the studied area varies in the 34–41 km range, with a mountain root beneath the Dabieshan orogenic belt. The average P-wave (and S-wave) velocity in the upper, middle and lower crust are 5.6–6.0 km/s (about 3.4 km/s), 6.2–6.3 km/s (about 3.6 km/s), and 6.4–6.8 km/s (about 4.0 km/s), respectively. The Poisson ratio varies from 0.24 to 0.29. With the polarization analysis to the phase Sg, we find that the fast polarization directions for the Yangtze block, Dabie block and northern China block are about N77°E, N18°W and N53°E, respectively. The seismic anisotropy factors are up to about 5%. The zone with strong anisotropy shows the ‘chimney’ structure, which indicates force direction endured by materials flowed upward. We inferred that the subduction of the Yangtze plate and the mantle flow of materials have played important role in exhuming the ultra-high pressure metamorphic rocks of the Dabieshan orogenic belt.
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