Abstract

Two end-member models, i.e., vertically coherent deformation of the lithosphere and crustal flow that decouples the crust and mantle, have been proposed in the efforts to understand the lithospheric deformation in eastern and northeastern Tibet. To investigate the deformation pattern and the interactions of the plateau with surrounding blocks, we derived the anisotropy parameters, i.e., fast direction and delay time, by shear wave splitting (SWS) analyses on the teleseismic records from two nearly perpendicular linear arrays that cross the eastern margin of the Tibetan Plateau and the Sichuan Basin. We analyzed the features of SWS in this region and then delineate the pattern of deformation by comparing our results with GPS results, surface structures, crustal anisotropy and the direction of absolute plate motion (APM). Double-layer anisotropy was observed in the Western Qinling and Longmenshan belt. The fast directions for the upper layer in western Qinling are similar with the GPS observations, suggesting that the upper layer may stem from the crustal deformation. While those of the Longmenshan belt are different from the fast axis predicted by GPS data and the strike of faults, implying that there should be an anisotropic layer decoupling the upper crust and upper mantle. The fast directions for the lower layer in both areas are subparallel to the trend of mountains and active faults. In the Songpan-Ganzi block and the Yun-Gui Plateau, fast directions show the same feature and are consistent with the GPS observations and crustal anisotropy derived from receiver function analysis. These suggest that anisotropy can be associated with the current orogenesis and the lateral extrusion of the Tibetan Plateau. The NW-SE fast directions are different with that of APM in the Sichuan Basin. This would correspond to `fossil anisotropy' associated with the post-tectonic evolution. The regions of the Daba Mountains and the Qinling orogenic belt, especially the latter, show fast directions obviously varying with back azimuth. This feature may reflect a complex anisotropic structure there. The complex pattern of anisotropy suggests that individual tectonic blocks or units may be controlled by different lithospheric deformation, which in turn probably depends on the inherent structures and properties of their lithosphere.