Crustal density structures and isostasy beneath the Western North China craton, Trans-North China Orogen, and surrounding regions

To determine the lateral and vertical variations in crustal structure and their influence on the seismicity of the Western North China Craton, the Trans-North China Orogen, and the surrounding regions, the wavelet multi-scale structures, Moho depth, crustal density structures, and isostatic state are modelled using Bouguer gravity anomaly data, topography, and earthquake focal mechanisms. We obtained homogeneous crustal densities and deviations of <1 km between the crustal thicknesses estimated from the isostatic model and those inverted from the Bouguer gravity anomalies in the Ordos Block, the Inner Mongolia Suture Zone, the Sichuan Basin, and the Jizhong Depression. These results provide new evidence for relatively simple and stable continental crustal structures, and indicate that these regions will remain stable in both the vertical and lateral directions. The Hetao Graben, Yinchuan Graben, Weihe Basin, and Shanxi graben system have heterogeneous crustal densities and are isostatically over-compensated. In contrast, the crust beneath the Yinshan Uplift, Lvliang Uplift, and northern and central Taihang Uplift is thin and under-compensated. The heterogeneous crustal densities and non-isostatic state beneath the Tibetan Plateau and Qinling Central China Orogen indicate that these two blocks are unstable in the vertical and lateral directions. Although Cenozoic deformation of the North China Craton is thought to be driven by lithospheric stresses related to the India-Eurasia collision and Pacific slab retreat in South East Asia, we suggest that gravitational potential energy created by the heterogeneous crustal structure modulates these first-order forces. The results of this study could constrain the causes of seismicity in systems surrounding the Ordos Block.