Crustal structure and tectonic boundary characteristics in South China: Constraints from joint tomography of ambient noise and gravity

Understanding the fine crustal structure of the South China plate and characterizing the crustal material properties and boundary characteristics of different tectonic blocks is critical for investigating the deep dynamic genesis of the South China metallogenic belt and the formation and evolution of the South China plate. Results of previous investigations suggest that the tectonic attribution of the Jiangnan orogenic belt is key to understanding the formation and evolution of South China plate. Three issues need to be addressed: (1) The difference between the crustal material properties of the Jiangnan orogenic belt and those of the Yangtze and Cathysia blocks, especially in the southern section of the Jiangnan orogenic belt, (2) the location of the tectonic boundary of the southern segment of the Jiangnan orogenic belt, which is required for clarifying its southern extension, and (3) the dynamics of the collisional orogeny in the southern segment of the Jiangnan orogenic belt (i.e., the Indosinian, Yangtze and Cathaysian blocks). The multi-solution of geophysical inversion is one of the important factors affecting the reliability of tomographic imaging. The problem of multi-solution of inversion will not only reduce the accuracy of single data imaging, but also there are often large deviations or even contradictions in the results of separate imaging between different data, which causes serious problems to investigate the above scientific problems. For seismic imaging, one of the important ways to effectively reduce the problem of multiple solutions of geophysical inversion is joint tomography based on different types of data. With the development of new methods and the in-depth analysis of multiple types of data, there has been a gradual improvement in the resolution of the crustal fine structure. Therefore, we collected waveform data from seismic stations in South China and used ambient noise surface wave and Bouguer gravity anomaly to invert the crustal S-wave velocity structure of the South China plate. The results show that the Bouguer gravity anomaly obtained by the joint inversion model fits well with the observed gravity data, which confirms the reliability of the new method. Our study reveals low S-wave velocity in the upper crust of the main sedimentary basin, with crustal thickness thinning from west to east. The crustal structure of the southern Jiangnan orogen is characterized by a north-east (NE) zonal distribution of high to low velocity, along with "mushroom" shaped middle crust uplifts between the north-south (NS) gravity gradient belt and the Jiangshan-Shaoxing-Pingxiang line. In the lower crust, at depths of 21-30 km, the lateral variation of velocity no longer correlates with shallow tectonic features. The velocity distribution in the entire region shows a trend of higher velocities in the east and lower velocities in the west, a feature that becomes more prominent with increasing depth. At 30-40 km depth, the NE boundary of velocity variation approximately coincides with the NS Bouguer gravity anomaly gradient zone. Our model defines the tectonic boundaries of the Jiangnan orogen with the Yangtze and Cathaysia blocks along the Jiujiang-Changde-Jishou-Tongren-Baise and the Jiangshan-Shaoxing-Pingxiang-Yongzhou-Liuzhou-Qinzhou line, respectively. A dynamic model of regional tectonic evolution inferred from this tomographic image suggests that the Yangtze block continuously provides the driving force for the north-west (NW) thrust nappe of the west wing of the Jiangnan orogen. The crustal structure model obtained by joint inversion provides essential constraints for exploring the deep dynamic process and regional mineralization of the collision and splice of the Indochina, Yangtze, and Cathaysia blocks.