Crustal And Upper Mantle Structure Of The Ordos Block And The NE Margin Of The Tibetan Plateau

The northeastern margin of the Tibetan Plateau, which is adjacent to the stable Ordos Block, is the frontier of the whole plateau expanding into the interior of the continent. The information of deep structure beneath the whole region is key to revealing the mechanism for the uplift of the plateau or the thickening of the crust, the interaction between the plateau and the neighboring stable block, and the deep environment for the occurrence of large intraplate earthquakes. Using the data from the temporary broadband stations deployed by Nanjing university and surrounding permanent stations, my doctoral thesis have obtained3D high resolution crustal structure and1D upper mantle structure of the Ordos block and the NE margin of the Tibetan Plateau through ambient noise tomography and traditional surface wave dispersion inversion, respectively.The Ordos Block shows low velocity at the upper crust, corresponding to the thick Mesozoic sedimentary. The lower crust and upper mantle are characterized by high wave speed, with upper mantle velocity as high as4.74km/s and nonexistence of asthenosphere above200km depth. These features are commonly related to stable craton. To the south, the Weihe rift is imaged as abnormally low velocity for the upper crust because of the very thick Cenozoic sedimentary. Velocity low also appears in the middle and lower crust of this region, agreeing roughly with the high heat flow. Obvious low velocity anomaly is present in the upper mantle, probably indicating subsolidus grain-boundary weakening effect under high temperatures near solidus instead of partial melt in the asthenosphere. The lithosphere is only about100km thick, below which the asthenosphere velocity is as low as4.16km/s. Combined with previous studies, our results suggests that the Weihe rift probably is a passive rift triggered by regional stress field caused by the Indo-Asia collision. To the south of the Weihe rift, the upper crust is present as high velocity belt in the East Qinling, which is correlated with the igneous rock and metamorphic rock formed during orogenics. The velocity of the lithospheric mantle is about4.5km/s, extending down to140km depth. The asthenosphere velocity is about4.2km/s.Compared to the Ordos Block, the Qiliang-Qinling orogenic belt located at the NE margin of the Tibetan Plateau has relatively low-velocity middle and lower crust, with velocities increasing from3.4km/s to3.6km/s. There is no obvious low velocity layer, suggesting that the middle/lower crustal flow has not entered into the Qiliang-Qinling orogenic belt. Due to its low velocity and thus low strength structure, the NE margin of the Tibetan Plateau has formed the broad and gentle plateau margin with long-wavelength and low topographic gradient, contrary to the steep margin west of the high velocity Sichuan Basin. Radial anisotropy of the crust varies with depth, suggesting different deformation styles. Vertically oriented anisotropic fabric (VSV> VSH) in the upper crust probably indicates that the crustal thickening has largely occurred at the top of crust. Horizontally oriented anisotropic fabric (VSV<VSH) of the middle and lower crust shows that the maximum strain axis is horizontal, probably resulting from the deep effects of the large strike-slip faults. As the boundary between the Qiliang-Qinling orogenic belt and the Ordos and Ala shan Block, the Haiyuan fault is at least a crust-scale fault.The Haiyuan arcuate tectonic region (HATR) located at the southwestern margin of the Ordos Block has a thin lithosphere of only100km and a high velocity mantle lid. The notable low-velocity asthenosphere is present here, with the lowest velocities of-4.2km/s found at depths of150to170km. The asthenosphere beneath the East Qinling and the HATR and the Weihe rift show similar shear wave speed, suggesting that the lateral flow of the asthenospheric mantle driven by the Indo-Asia collision may have focused beneath these zones. The weak lithosphere beneath the Weihe rift may has been strongly modified by the hot asthenospheric mantle flow through thermo-chemical erosion.Large earthquakes in the study region are mainly located in the zones featured by low-velocity crust and thin lithosphere, which are bordered by high-velocity stable structure. The substantial variations in deep structure and contrasting rheological properties are considered as possible mechanisms for the contrasting seismicity between the study regions. Strong anisotropy resulting from previous deformation appears almost exactly where the lithosphere has been much less affected. In contrast, weak and random anisotropy is found in the northern Weihe rift, where the mantle fabric may have been severely disrupted by the significant lithospheric modification.