Numerical simulation of structural deformation characteristics of the Moho offset on the northern margin of the Tibetan Plateau

The double-pulse teleseismic P-wave imaging of the northern margin of the Tibetan Plateau indicates that there is a 15 ~ 20 km Moho offset within a range of less than 5 km at the East Kunlun-Qaidam boundary. However, the dynamic structural characteristics and influence factors of the Moho offset in this area remains unclear. Based on seismic wave imaging results, this study established a two-dimensional viscoelastic finite element model, quantitatively analyzed the velocity fields and strain ratio fields of the profile under gravity drive, and discussed the effects of steep slope angle, surface elevation, viscosity coefficient and upper mantle density on the temporal variation of Moho offset. The results suggest that the velocity field of the profile gradually decreases over time with a clockwise rotation trend. Moreover, the rotation center of the velocity field gradually shifts from the middle of the steep slope to below the deep corner of the steep slope with time. The main strain rate field of the profile rotates clockwise as a whole, and the values of the main strain rate and maximum shear strain rate are concentrated near the corners of the Moho offset. The Moho offset gradually flattens over time, with a more significant slowing trend before 3 Myrs, and the slowing trend decreased significantly after 5 Myrs. The research explained that the magnitude of the upper mantle viscosity coefficient is the main factor affecting the flattening of Moho offset over time, and a lower upper mantle viscosity coefficient can promote the flattening of Moho offset. In addition, the smaller steep slope angle, larger upper mantle density and high topographic relief between the Songpan-Garzê Block and Qaidam Basin all significantly promote the Moho offset tend to flatten out. At the same time, this study also exhibited that there is mass transfer under the East Kunlun-Qaidam block, which is of great significance for understanding the dynamic mechanism of the northward expansion of the Tibetan Plateau and the dynamic characteristics of the Moho offset.