Magnetostratigraphic Evidence From the Gengma Basin for the Fast Response of the Crustal Rotation of the Northern Part of the Shan‐Thai Block to the India‐Eurasia Convergence

The Cenozoic tectonic evolution of the southeastern edge of the Tibetan Plateau is interpreted in terms of two alterative dynamic mechanisms: lateral extrusion of coherent lithospheric blocks and viscous lower crustal flow. To contribute to this debate, we conducted a magnetostratigraphic study of the Early Miocene sedimentary strata of the Gengma Basin, in the northern part of the Shan‐Thai Block (STB). The results show that variations in the deposition rate were synchronous with the rate of clockwise rotation during ∼18.05–15.93 Ma, indicating that the crustal clockwise rotation of the northern STB was the driver of the tectonic transformation of the NE‐SW trending strike‐slip system within the STB by at least ∼18.05 Ma. The rapid decrease in the clockwise rotational rate of the Gengma Basin was also synchronous with the sharp decrease in the convergence rate between the Indian Plate (IDP) and Eurasia at ∼17.23 Ma, which is similar to that occurred in the interior of the Tibetan Plateau during the Paleocene and Eocene, demonstrating the fast response of the southeastern edge of the Tibetan Plateau to the convergence of the IDP and Eurasia, in the form of the continuous upper crustal ductile deformation of the Tibetan Plateau and its periphery. The lateral flow of the viscous lower crust beneath the southeastern edge of the Tibetan Plateau may be the predominant driver of the tectonic evolution of the southeastern edge of the Tibetan Plateau since the Early Miocene. Plain Language Summary The classic tectonic models of the lateral extrusion of coherent lithospheric blocks and viscous lower crustal flow could be responsible for the different crustal kinematics and deformation characteristics of the southeastern edge of the Tibetan Plateau during the Cenozoic. Our magnetostratigraphic results for the northern Shan‐Thai Block (STB) show that the sediment deposition rate of the Gengma Basin changed synchronously with the rate of clockwise rotation during ∼18.05–15.93 Ma, indicating that crustal clockwise rotational movement caused the Nantinghe strike‐slip fault within the northern STB to transform from right‐lateral strike‐slip to left‐lateral strike‐slip, since at least ∼18.05 Ma. The sediment deposition rate and the clockwise rotation rate of the basin changed synchronously with the rate of convergence of the Indian Plate (IDP) and Eurasia at ∼17.23 Ma. Even the Gengma Basin far from the Tibetan Plateau, the upper crustal material responds rapidly to the c