Three-Dimensional Density Structure and Multi-Scale Horizontal Stress of the Qilian Orogenic Belt Based on the EIGEN-6C4 Gravity Model

Abstract The specific mechanisms and extent of crustal deformation and thickening in the Qilian Orogenic Belt (QLOB) remain subjects of considerable debate. To enhance research on these issues across the entire region, the EIGEN-6C4 satellite gravity model was utilized in this study. We derived residual and regional gravity data by employing a wavelet multi-scale decomposition technique, following the removal of noise and interference. Then the detailed three-dimensional density structure was obtained by the rapid imaging inversion. Furthermore, we calculated the horizontal stress in both the upper and lower crust and determined the fluctuation characteristics of Moho. Based on these calculations, integrating the geological and geophysical information, we analyzed the distribution of density structures, horizontal stress, and Moho fluctuations in the QLOB. The results indicate significant regional variations in tectonic characteristics between the eastern and western sections of the belt, with the approximately 98° north-south-trending Erlashan fault serving as the boundary. In the west, the crust thickening with Moho deepening to ~68–72 km near Hala Lake is primarily the result of plate collision and compression under the background of north-south dual-directional compression, with intense compression leading to pronounced decoupling features. As compression persists, fluids or partially molten materials within the crust are extruded eastward along strike-slip fault zones or softer regions. Conversely, in the east, crustal thickening is predominantly attributed to fluid activity or partial melting within the crust. Both the upper and lower crusts are characterized by eastward extrusion of material and exhibit relatively weaker decoupling characteristics.