Effects of 410- and 660-km seismic discontinuities on the subduction of Pacific Plate and slab geometry beneath the Changbaishan volcanic province

The origin of the Changbaishan volcanic province (CBSVP) has been considered to mainly relate to the Pacific plate subduction. Therefore, it is necessary to dynamically reconstruct the Pacific plate subduction process towards and beneath the CBSVP since the Cenozoic in order to investigate its origin. This reconstruction requires consideration of the effects of Clapeyron slopes and the thicknesses of the 410-km and 660-km discontinuities in the upper mantle. However, due to their wide distribution ranges, it remains an urgent issue to determine how to select them reasonably in the reconstruction. Accordingly, the main purpose of this study is to provide a reference for the selection of geodynamic model parameters for the reasonable reconstruction of the dynamics and structure of the subduction of the Pacific plate towards and under the CBSVP since the Cenozoic based on the modeling on the effects of the 410-km and 660-km Clapeyron slopes and thicknesses. In this study, we set up a series of data-assimilation three-dimensional geodynamic models with varying thicknesses and Clapeyron slopes of the discontinuities and carry out numerical modeling. The results indicate that all models predict similar slab dynamics and geometries, to the first order, during the subduction of the Pacific plate towards and beneath the CBSVP since Cenozoic; when the absolute value of the Clapeyron slope for the 660-km discontinuity (g₆₆₀) is smaller than 4 MPa/K, small g₄₁₀ helps the Pacific slab to migrate westward in the mantle while large g₄₁₀ is useful for the sinking of slab. If |g₆₆₀| ³4 MPa/K, the effects of g₄₁₀ becomes very weak and could be negligible; g₆₆₀ has significant effects on the westward movement and sinking of the Pacific slab. As |g₆₆₀| gets larger, the sinking depth of the Pacific slab becomes monotonously shallower, while the amount of westward movement first increases and then decreases. The main part of the Pacific slab is located in the mantle transition zone only if |g₆₆₀| ³4 MPa/K; the thicknesses of both discontinuities have minor effects on the dynamic subduction process and structure of the Pacific slab.