Isostasy And Density Structure Of The Curst In Longmen Shan And Liupan Shan

The isostatic state, the strength of lithosphere, and the mechanism of uplift provide the significant reference for studying the dynamic mechanism in the study area. This research studied density structure of the crust, the isostatic state, the strength of the lithosphere and the uplifting mechanism, which based on the gravity/GPS joint measurements, EGM2008 gravity model and Global Topography V18.1 data. Simultaneously, the two-dimensional inversion method, Airy equilibrium theory and free-air gravity anomaly admittance method was adopted in this research. This study mainly included as follows: 1. The gravity admittance method and the reliability verificationThe gravity admittance method used the linear correlation of spectral domain between gravity anomaly and topography to inverse the strength of the lithosphere and the initial loading ratio of the equilibrium adjustment. In order to verify the reliability of the method, the random Free-Air Gravity Anomalies(FGAs) and topography data, satisfied with the flexure model, were used to inverse the strength of the lithosphere(effective elastic thickness T) and the load ratio F. The results show as follows:When the initial loading of the equilibrium adjustment is from the surface totally and T is less than 20 km, the inversion results restores finely. However, the restores of T is over estimated, when the preset of T is over 20 km. When initial loading comes from one interface totally, the restores of T will be fine.When the initial loading comes from the different interface, the restores of the loading between upper and lower crust are underestimated. The restores are good, while the initial loading comes from one interface only. 2. The density structure of the crust, the isostatic state, the strength of lithosphere and the uplifting mechanism in Longmen Shan area.The gravity/GPS joint data from 2009 to 2013 were reduced into Bouger Gravity Anomalies(BGAs). The BGAs reduced from east to west. The various range of the BGAs in the western Longmen Shan(LMS) area is from-450 m Gal(10 ms) to-400 m Gal, and the range is from-240 to-150 m Gal in the eastern LMS area.The isostasy and Moho depth were acquired by using the data described above and Airy equilibrium theory, then the difference of the isostatic and Moho depth represents the isostatic anomaly. The distribution of the isostatic anomalies in LMS area shows that the most areas are under the positive state of isostatic anomaly. More specifically, the biggest positive anomalies distribute in Xiaojin and Kangding area, the negative anomalies distribute in the area of LMS faults, and the lowest anomalies attend in Yaan area.The measurements and EGM2008 model data were merged by using the Least Square Collocation(LSC), and the FGAs and BGAs field were updated in the LMS area. The density structure of the crust was inversed by using the BGAs, and the new distribution of the isostatic anomalies was acquired by considering the density structure of the crust. The distribution shows that the isostatic positive anomalies distribute in the north of the Sichuan basin, Bayan Har block, the east of Songpan plateau and Sichuan-Yunan diamond block, and the negative anomalies distribute in the area between Xianshuihe faults and LMS faults, the Guanxian-Anxian fault and the south of Songpan plateau area. The most of the earthquakes(Ms > 5) distribute in the gradient zone of the isostatic anomalies, and the correlation between the earthquakes and the gradient of isostatic anomalies shows more apparent with considering the density structure of the crust.The strength of the lithosphere and the uplifting mechanism in LMS area were acquired by using the merged data described above and the FGAs admittance method. Furthermore, the results show that the T of the west of the LMS area is 6 km, and the east of the LMS area is over 30 km. The loading of the flexure comes from the surface of the crust totally, and this result indicates that the uplifting mechanism of the LMS area is due to the surface loading. 3. The density structure of the crust, the isostatic state, the strength of lithosphere and the uplifting mechanism in Liupan Shan area.The data of the survey line across the Liupan Shan(LPS) fault were collected by using gravity/GPS joint method in 2014, and the length of the survey line is about 200 km. The BGAs of the line show that the BGAs in the east end is about-190 m Gal, and-250 m Gal in the west end. The BGAs reduce from east to west.Using the isostatic analysis method as same as LMS area, the LPS is in the positive state of isostatic anomaly. The density structure of the crust was acquired by using BGAs of the line survey, and the result with considering the density structure shows that the LPS is also in the positive state.The EGM2008 FGAs and Global Topography V18.1 data was adopted to inverse the strength of the lithosphere and initial loading of LPS area by using FGAs admittance method. The results show that the T is 5 km, the initial loading comes from surface mainly. Furthermore, the results indicate that the surface loading is the uplifting mechanism of the LPS area.The isostatic anomalies of elastic plate model in LPS area was calculated by the results of admittance method. The discrepancies between isostatic anomalies of elastic plate model and Airy model represent the capacity of the lithosphere, and the biggest gradient of the discrepancies is in the location of LPS fault.