Gravity Inversion Study Of Crustal Interface Based On Structural Constraints Of Seismic Velocity

The Moho surface usually characterizes the lithospheric structure and its changes,which not only records the deep processes of lithospheric formation and evolution,but also records the dynamics of the lithospheric superficial surface materials that constrain the deep processes.Therefore,obtaining accurate Moho surface depths in the study area can provide a basic overall crustal model for studying regional tectonics,seismic activity,etc.Moho surfaces are usually obtained by gravimetric and seismic methods,but both methods have their advantages and disadvantages.Seismic methods can obtain accurate crustal structures in the lower part of a profile or station,but their lateral resolution is weak and the distribution of results is heterogeneous,while gravity inversion calculations have a large lateral coverage and their vertical resolution is limited.Therefore,using highprecision local seismic data to constrain the gravity inversion model for a large area,combining the advantages of both methods,a more accurate crustal model can be obtained.In addition,the influence brought by the curvature of the Earth is taken into account in the calculation of the large area inversion.The extension and thinning of the lithosphere at continental margins and the rupture to form the oceanic lithosphere are accompanied by a rapid rise in ground temperature,and with the gradual decay of thermal perturbation,the lithosphere density changes laterally,producing thermal gravity anomalies.This effect is more obvious at continental margins and oceanic ridges,with a maximum value of-320 m Gal at active oceanic ridges(Indian mid-ocean ridge,Atlantic mid-ocean ridge),and it is necessary to correct for the gravity anomaly in the calculation of active continental margins.In this paper,we use the spatial domain gravity inversion method in the spherical coordinate system to invert the depth of Moho surface in the Sichuan-Yunnan region,and discuss the main scientific issues in the Sichuan-Yunnan region based on the inversion results.The following conclusions are drawn:(1)the regional Bouguer gravity anomaly in the Panzhihua area is due to the local uplift of the Moho surface in the western part of Panzhihua and the uneven mass density distribution caused by the depression below the Xiaojiang fault zone;(2)the depth of the Moho surface in the gravity data inversion in central and southern Yunnan is generally deeper than 40 km,but the seismic data inversion results are shallower than 40 km,and the gravity inversion and artificial seismic results in the(3)The distribution of strong earthquakes in Sichuan and Yunnan is related to the uneven distribution of density in the lower crust,and the deep tectonic movement triggers the rupture of the shallow strata,leading to strong earthquakes.In the inversion calculation of the Moho surface in the South China Sea region,the lithospheric thermal gravity anomaly generated by the active continental margin needs to be corrected,and the corrected regional Bouguer gravity anomaly is used to calculate the depth of the Moho surface and to delineate the extent of the ocean-land transition zone in the northern part of the South China Sea.The following conclusions are drawn:(1)The density change caused by the temperature anomaly is the direct cause of the lithospheric thermogravity anomaly,which varies in a "spindle shape" with the increase of the lithospheric depth(0-120 km),i.e.,the anomalies at the two ends of the lithosphere are small and the anomalies in the middle are large.(2)Based on the characteristics of Moho surface depth variation,and considering the location of high-speed anomalies in the lower crust,the thinning factor and the distribution of gravity anomaly gradient zones,the ocean-land transition zone in the northern part of the South China Sea is divided into a width of more than 200 km in the eastern part and about 70 km in the western part,which corresponds to a Moho surface depth of about 15-20 km.(3)From the results of Moho surface depth in the inverse performance of this paper,combined with the regional tectonic evolution history,the comprehensive analysis can confirm that the South China Sea region has experienced two phases of seafloor spreading,with three oceanic ridges remaining.