Lithospheric Weak-material Dynamical Models Beneath The Tibetan Plateau

Tibetan Plateau (TP) is an ideal region for studying lithospheric evolution and crustal motion mechanisms. The TP's uplift and its significant impacts on surrounding regions since Cenozoic have garnered many geoscientists' attention. To investigate the TP's dynamic mechanisms has great scientific significance for understanding ecosystem evolvement rules and human civilization developments.Compared with observations from modern geodetic techniques, terrestrial gravity data can provide supplementary useful information. For instance, it is capable of detecting the mass transportation beneath the Earth surface which is beyond the sensibility of the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR);the terrestrial gravity data are more sensitive to shallow-Earth mass motions than satellite gravity observations; the terrestrial gravimetry technique is able to observe the dynamical mass transfer signature in depth that cannot be revealed by seismic tomography. In general, the strength of tomographically imaged low velocity zones (LVZ) is several orders of magnitude less than that of normal lithosphere. Therefore, the lithospheric LVZ of the TP should deform more easily and produce prominent gravity changes when undergoing the India-Eurasia collision. This study focuses on deriving the gravity change caused by the lithospheric LVZ deformation, finding the coherence between the gravity-change signals and the TP's lithospheric structure, and combining multiple data/models to propose TP's regional dynamic models.There are no similar studies before, and I build the fundamental theories systematically. The theoretical work are summarized as follows:(1) The scheme of combining multiple data for proposing and analyzing the TP's regional dynamic models was put forward. The method of deriving LVZ-caused gravity change was explored. To achieve this goal, diverse data/models were collected and were used to correct unconcerned gravity-change components from the total gravity-change signal.(2) Among various gravity-change components, how to calculate the gravity change caused by crustal deformation was compherihensively investigated. A formula that links the prameters of crustal dilatational strain and terrestrial gravity change was derived. The case study for the TP was also conducted.(3) The TP was divided into 22 sub-regions. Their crustal strain-caused gravity change, dilatational strain and vertical velocity were estimated for analyzing the TP's regional dynamic models.Several TP's regional dynamic models were proposed as follows:(1) There is a large-scale and inverted triangular LVZ beneath the northern TP, where weak materials are probably squeezed by N-S trending India-Eurasia collision, followed by upwelling and then spreading along Moho when they are hindered by crust.(2) A positive gravity change signal bulge southward in the southern TP, consistent well with the N-S trending tensional faults. This result supports a slab tear explanation because of the differential northward advancements of the Indian Plate.(3) The negative gravity change in the northern Qilian Orogen is generated by crustal thickening where the LVZ at the lower crust is protruded into the mantle.(4) Three positive gravity change signals are distributed in the southeastern TP, corresponding well with the two LVZ belts in the mid-lower crust north to ?26°N and one LVZ beneath Moho south to ?26°N. The two positive gravity change belts are thought to be generated by the Songpan-Ganzi Terrane which moves southward, squeezes and thickens the two LVZ belts located in between blocks; the positive gravity change south to ?26°N may be caused by Airy's isostatic mechanism.(5) The southwestern and the northeastern ends of the Longmen Shan Fault Zone are undergoing significant rise and subsidence, respectively, which serves as a distinct geodynamical feature for this region. The increasing Gravitational Potential Energy per unit area (GPE) contrast between the two ends may greatly facilitate the crustal mass motions from the southwest end to the northeast end and thus causes most of the large earthquakes therein, including the recent two ones:the 2008 Mw 7.9 Wenchuan earthquake and the 2013 Mw 7.0 Lushan earthquake.