| Collision of Indian subcontinent and Eurasian plates has been building the Orogenic zone of Himalaya and Tibetan Plateau since 50-55 Ma. Crustal thickening in Tibet occurs due to the collision, and results in uplift of Himalaya-Tibet Plateau substantially. It changes lithospheric tectonic framework in Asia continent, and it also impacts deeply on climate change and environment in East Asia. Altyn Tagh Fault, as a major boundary controlling active fault in north rim of Tibet, is been more considered due to its kinematic since 1970 s.Different viewpoints about magnitude of slip rate along Altyn Tagh Fault extracted two end-members. One end-member model is wholesale lateral extrusion model of the lithosphere in which eastward translation of Tibetan crust occurs via coeval movement along major strike-slip faults, e.g., Karakoram and Altyn Tagh faults. Another end-member model,approximating the Eurasia continents as a viscous thin sheet indented by the rigid Indian Plate,predicts progressively northward propagation of crustal thickening and plateau uplift, with limited lateral lithosphere extrusion. Since approximate 10-15 Ma, N-S trending grabens and rifts that are arrayed on the southern and central Tibetan Plateau, provides evidence for Negoene/Qauternary E-W extension of Tibetan Plateau. It demonstrates widespread normal faulting, and therefore crustal thinning, of the Tibetan Plateau. In the northern rim of the Tibetan Plateau, How does the crustal strain be distributed, revealed by crustal deformation field across the central segment Altyn Tagh Fault measured by GPS? Is there uniform between tectonic deformation in a geological time scale and Present-day GPS measurements?Above all questions are more important to understand lithospheric deformation of the Tibetan Plateau.This study was separated as two parts. Part I is about continuous GPS measurement across the central segment of Altyn Tagh fault with 9 GPS sites. The GPS monuments are designed by referring to PBO, and we produce GPS antenna bases with same technology, which is from PBO. We installed 7 GPS sites in July 2011 and 2 sites in April 2013. These GPS sites are under unattended operation because there are located in Altyn Tagh Mountain natural preservation zone. Up to July 2015, we obtained 4-years GPS datasets in this local GPS network. Perfect recording data are obtained in majority of GPS sites in continuous style.Unfortunately, two sites was destroyed by wild animals where one of them is close to northern margin of Jingyuhu and another is located in the southern margin of Kumu kuli Baisn. We guess the southmost one was destroyed by wild brown bear in Tibet, according to the field scene when we arrived there in July 2015. Thus, we collected our data with some COMONC II sites in the vicinity of our study region, e.g., three GPS sites which are located at Qiemo, Tazhong and Ruoqiang counties, respectively, in Xingjiang Uygur Autonomous Region, and three GPS sites where they are at Mangya, Lenghu, and Golmud counties in Qinghai Province. In August 2015, we combined them through aligning to ITRF2008 and Tarim reference framework, respectively. For this study, continuous data from GPS measurements is most precious to deduce a high precise 3D GPS velocity filed for central segment of Altyn Tagh Fault, North Altyn Fault and Qimen Tagh Fault, in north rim of Tibetan Plateau.We devise a 3D linear elastic fault-block model constrained by 6 fault patches using back-slip theory in this study, which includes the eastern, central and western segments of Altyn Tagh Fault, the western and eastern segment of North Altyn Fault, Qimen Tagh Fault using 3D crustal deformation across central segments of Altyn Tagh Fault. We invert 3components of each block, e.g., Tarim block, North Altyn block, Qaidam block and Qimen Tagh block, respectively, and depth and dip angle of North Altyn Fault. As the results, we derive that the uplift rate is up to 1.32±0.2 mm/yr from Tarim block to North Altyn Block,and the uplift rate is 0.73±0.3 mm/yr from Qaidam block to North Altyn Block. It reveals thatNorth Altyn Block is prominently uplifted as a whole. The sinistral slip rate of Altyn Tagh Fault is about 8.21±0.6 mm/yr, and shortening rate is 0.66±0.6 mm/yr across fault. The fault-strike and fault-normal components of Qimentagh fault is 0.53±0.6 mm/yr and 1.53±0.6mm/yr, respectively. North Atyn Fualt has a sinistral slip rate, 0.87±0.6 mm/yr, and convergent slip rate, 0.69±0.6 mm/yr. We use a simple Okada fault-patch for each fault segment because GPS sites are scarce across north rim of the Tibetan Plateau in this study.For GPS data from the paper published by He et al., 2013, there exists asymmetric deformation across western segment of the Altyn Tagh fault. A best dislocation offset of 13 km of surface fault is calculated through the best fitting solution, with a strike-slip rate of9.0mm/yr, a locking depth of 14.5km. We doubt this result, because their assumption implies dip angle of fault surface is almost to ~45o and fault surface turn downward straightly at depth of 14.5km. As noted above, we cannot accept this assumption. In our study, we analyze that the difference of crustal medium shear modulus between northern Tibetan Plateau and Tarim with an assumption, which the stress remain unchanged near the fault, and strain occurs changed and shear modulus would be changed reciprocally. This approach can provide a perfect solution to solve asymmetric situation in two sides of Altyn Tagh Fault. We find the non-linear relation between slip-rate of fault and shear modulus. After inversion, a shear modulus ratio, ~1.5, between Tarim basin and Northern Tibet, is obtained confirmedly. We consider relation between S wave velocities and shear modulus in crustal medium, accounting for formula of S wave velocities. Another ratio about S wave velocity is deduced, as ~ 1.22. A3 D Vsv model of crust and upper mantle is shown in paper by Yang et al., 2012. A low-velocity zone in middle crust from 20 to 40 km is detected. If we suggest the value of Vsv at 20-30 km depths is 3.25, the corresponding value of Vs is about 3.32 at the south side of Altyn Tagh fault, Tibetan Plateau. If applied the ratio, ~1.22, we can deduce the value of Vs in Tarim is 4.05, which is similar to the observed value at the same depth. Under consideration of extreme situation, we collect the value of Vs, 3.3 in Tibet and 3.9 in Tarim,respectively. There would a value, ~1.18. These two numbers are almost close to each other.Thus, we obtain a comparably semi-quantitative relationship between GPS measurement in geodetic surveying and S wave velocity in Seismology through the ratio of shear modulus between Tibet and Tairm.Part II described about the crustal thinning in Tibet. GPS measurements from sites within the Tibetan Plateau show not only east-southeast–west-northwest extension, but more importantly crustal thinning throughout the interior of the plateau. Vertical(thinning) strain rates of 8.9±0.8 nanostrain a-1 and 7.4±1.2 nanostrain a-1 in the northern and southern parts of the plateau, and 12.0±3.2 nanostrain a-1 in the southwestern part, suggest no measureable difference across the plateau. Extension rates also are similar: 21.9±0.4 nanostrain a-1 oriented N114.3±0.7°E in northern Tibet, 16.9±0.2 nanostrain a-1 oriented N92.6±1.2°E in southern Tibet, and 22.2±1.8 nanostrain a-1 oriented N74±3°E in southwestern Tibet. If crustal thinning began at 10-15 Ma and the current rate applied to that period, the crust should have thinned by5.5-8.5 km. If isostatic equilibrium applied, the mean elevation of the plateau would have dropped ~1 km. The similar rates for northern, southern, and southwestern Tibet suggest that the processes dictating crustal extension, normal faulting, and crustal thinning in the three regions differ little.In conclusion, we summary the result combined with two parts in the thesis, while there is a great difference between interior and marginal regions in Tibetan Plateau. Crustal thinning occurs in interior of Plateau and different level crustal thickening and uplift occurs in the marginal regions of the plateau. The vertical strain rate, nanostraina-1 ~5-20, in the northern margin of Tibetan Plateau, if the gravity equilibrium effect is considered, the corresponding vertical uplift rate is about 0.04~0.14mm/yr. But for the local region, such as the northern Altyn Tagh block, the crustal lower basement is flexural supported by crustal rigid plate insouth margin of the Tarim Basin, due to the absence of gravity balance, crustal uplift rate can reach up to 1 mm / yr. There has different levels of vertical deformation in Himalayan region,crustal vertical strain rate is about ~10-80 nanostraina-1. The mountains basement is supported by India subcontinent plate, without the assumption of gravity equilibrium for uplift rate estimation using vertical strain rate. But under the constraints by the GPS and the leveling surveying data and the slab model of the Indian-Eruasian collision, the mountain uplift rate reached up to ~7mm/yr. And in the Qilian Mountains area, GPS strain rate presumes that vertical strain rate is about ~20-40 nanostraina-1, under the application of Airy isostasy theory,the average surface uplift rate for 0.15~0.3 mm/yr in a geological time scale, and due to the thrust structure and fold zone of deformation, there may still exist in the elastic deformation in the middle and lower crust, can achieve complete gravity equilibrium and actual geological uplift rate may be higher than the estimates from GPS velocity field. In this thesis,quantitative estimation of 3D deformation field and deformation rate in different regions of Tibet Plateau is shown, which is an important modification of the "continuous deformation and crustal thickening" model. Our results do not support the "block movement and eastward escape" model, and that there are virtually no Tarim block southward subduction in north-south bilateral subduction model of Tibetan plateau. |
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