Tectonic Relationship between Upper and Lower Lithosphere in Guangxi Area: A Comprehension from Lithospheric Density and Magnetic Structures

Guangxi is located in the convergent region among South China block, Indo-Chinese block and West Pacific plate. Due to its special tectonic position, there have been disputed on several basic geological problems in Guangxi region, such as the ascription and boundaries of tectonic units. Because the lithosphere has been reconstructed by the plate tectonic movements since the Cenozoic, the crust and upper mantle beneath Guangxi area have obvious differences in seismic wave velocity and temperature structures. It employs satellite gravity and magnetic data as well as regional gravity and aeromagnetic survey data to investigate the relationship between lithospheric density and magnetic susceptibility structure and upper crust structure in Guangxi area. Results suggest that the crustal density and upper-crust susceptibility structures coincide with the surface geological structures, and the density structures of the lower crust and the upper mantle are discontinuous beneath Guangxi area. Susceptibility structure implies that different extents of mid-lower crust decoupling had caused in different parts in Guangxi area. As a result, the scale and scope of mantle-derived materials into the upper crust were limited under the background of large-scale lithospheric reconstruction since Mesozoic. It may be the main reason why the upper mantle structure in Guangxi area is not corresponding to that of the crust. The mid-lower crust decoupling results in that the magmatic activity in Guangxi area is obviously weaker than that in the surrounding area since Cretaceous, especially in the northern region of the Pingxiang-Dali fault zone.