Three-dimensional rheological structure of the lithosphere in the Ordos block and its adjacent area

The 3-D rheological structure of the lithosphere in the Ordos block and its adjacent area (32 degrees-43 degrees N, 102 degrees-116 degrees E) is obtained using iterative and averaging methods, based on the seismic velocity structure, thermal structure and composition of the lithosphere. The thermal structure is obtained from the newly published surface heat flow data sets. The brittle fracture mechanism is taken into account when the rheological structure is calculated. The strain rate obtained from GPS observation in the area under study is used to calculate the strength and the viscosity of the lithosphere. The results show that there are low-viscosity layers at the bottom of the upper crust, the lower crust and the bottom of the lithosphere both under the Ordos block and its adjacent regions, and the viscosity varies horizontally. The viscosity in grabens surrounding the Ordos block is one to two orders of magnitude lower than that in the Ordos block. The results also show that the rheological structure coincides with the major geological structure, below the major fault systems the viscosity is relatively low. The earthquakes with Ms >= 4.0 occurred along the large active faults. Most of the hypocentres of the earthquakes are in the upper and middle crust with high viscosity. The maximum depth of the earthquake is above the viscosity contour line of 10(22) Pa s. Numerical studies show that the active geological structures must be taken into account when the 3-D rheological structure is used for geodynamics study.