Lithospheric effective elastic thickness and its anisotropy in the northeast Qinghai-Tibet plateau

Adjacent to Qiangtang terrane, Tarim basin, Sichuan basin and North China block, the northeast Qinghai-Tibet plateau is one of the major tectonic provinces in mainland China, which is a combinative region with subduction, collision and intracontinental gathering and plays an important role in the deformation and evolution process of continental China. The lithospheric effective elastic thickness (T-e) and its correspondent mechanical anisotropy are tightly related to the rheologic properties and the mechanic structures. Thus, studying on these characteristics in the northeast Qinghai-Tibet plateau will provide important information for understanding the rheologic structure and the dynamic process of continental lithosphere. In this work, Fan wavelet analysis on coherence between Bouguer anomaly and topography is applied to obtain a more detailed two dimensional lithospheric T-e structure as well as its anisotropic properties in the Songpan-Garze terrane. The results show that the effective elastic thickness of the lithosphere varies significantly from 5 km to 100 km. In general, in the east margin of the northeast Qinghai-Tibet plateau, the Songpan-Garze terrane is a weak tectonic zone with a low T-e (5 km < T-e < 40 km) and strong anisotropy. Low T-e anomalies are also distributed in the marginal zones of the tectonic blocks, such as the fault zones and the suture zones. In Longmenshan mountain the strength is extremely weak, where the T-e only ranges 5 similar to 020 km, and the anisotropic structure can be divided into north and south segments. T-e in the Zoige basin is a little higher than surrounding area, which implies that the Zoige basin is still capable of strong rigidity. The distribution of T-e shows that the Qaidam basin is a rigid block, the magnitude of T-e ranges 50 similar to 90 km, with a well-defined edge which is underlain by Paleozoic crust. Therefore, we believe that the Qaidam basin should be underlain by fragments of old cratonic blocks. Compared with the geological surveys and the seismological studies, our results show that the weak mechanical axes are perpendicular to the main tectonic boundaries. By comparing our results with SKS and the seismic azimuthal anisotropy axes, we suggest that the source of our observed anisotropy in Alxa massif is likely to be vertically coherent "frozen" deformation of the lithosphere due to alignment of olivine crystals. In Qaidam basin, the T-e anisotropy therein most likely reflects the fossil strain field from past tectonic events.. Anisotropy in Songpan-Garze terrane is maybe developed by lateral flow.