New Insights Into the Driving Mechanism of the Mid‐Eocene (43 Ma) Tectonic Transition in the Western Pacific Margin—A Case Study of Stress Field Modeling in Pingbei

Abstract The East China Sea Shelf Basin (ECSSB), an important part of the western Pacific margin, is located in a key area of subduction and coupling of the western Pacific/East Asian continent. There are many unknowns regarding the basin structure and formation mechanism. There are three different viewpoints on the driving mechanism of the tectonic transition at 43 Ma: the India–Eurasia Plate hard collision, the subduction direction change in the western Pacific Plate, and the joint action of the western Pacific Plate and Philippine Sea Plate movements. To address this controversy, we use the SHELLS modeling method to simulate the tectonic stress field of the Pingbei area (PBA) under the constraints of paleofaults, paleotopography, paleoheat flow, petrophysical parameters, and different tectonic boundary conditions during two critical periods: before and after 43 Ma. The modeling results that most closely match the observations are as follows: at 43 Ma, the boundary extension direction changed from NW‒SE to NNW‒SSE, and the extension rate changed from 0.16 ∼ 0.52 mm/a to 0.15 ∼ 0.24 mm/a. Based on analysis of the coupling relationship between stress field simulation results and the formation of peripheral plates in the region, we believe that the driving mechanism of the 43 Ma tectonic transition in the ECSSB and western Pacific/Southeast Asia region is mainly due to changes in the retreat direction of the western Pacific Plate and the extension direction and velocity of the study area, combined with the northward movement of the Philippine Sea Plate.