Transcurrent tectonic system and deep seismogenic mechanism in the southeastern Tibetan Plateau: A view from gravity and magnetic anomalies

Abstract

The southeastern Tibetan Plateau margin (SETPM) is characterized by a series of curved strike-slip faults around the eastern Himalayan Syntaxis. Influenced by the southeastward movement of materials originated from the SETPM during the Cenozoic, the Sichuan-Yunnan Block underwent intense crustal deformation, accompanying frequent seismic activity and episodic deformation, and forming the present transcurrent tectonic system of the region. Herein, we compiled gravity, magnetism, Moho discontinuity, and Curie surface distribution to analyze the transcurrent tectonic system that significantly influenced the deep seismogenic mechanism of the SETPM. Meanwhile, we combined the seismic velocity structure and sedimentation data to illustrate the tectono-sedimentary characteristics and seismogenic environment of the SETPM. Generally, the deep seismogenic environment in the study area is closely related to the distribution of material density in the middle-lower crust and upper mantle, and the significant difference in the distribution of transverse versus vertical medium density on both sides of the active fault systems provide a deep tectonic background for major earthquakes. The lateral extrusion of the Qinghai-Tibet Plateau materials was blocked by rigid blocks in the Sichuan Basin, thus accumulating in the northern part of the Sichuan-Yunnan Block. There may be partial melt and lower crustal channel flow beneath the SETPM where active fault zones may provide channels for the lower crustal flow to move southeastward. The SETPM may have experienced intense crustal shortening and thickening accompanying block extrusion during the Eocene-Oligocene, causing partial melting and regional metamorphism. With the continuous northward wedging or indentation of the Indian Plate, the main fault systems in the SETPM and its adjacent areas have experienced a kinematic transformation during the mid-late Miocene, when the deformation pattern changed from early block extrusion to late diffusive deformation. The ongoing convergence of the India-Eurasia collision and the deep subduction of the Indo-Burmese arc provide driving force for the deep asthenospheric flow and crustal deformation. Meanwhile, the rollback and back-arc spreading of the West Pacific Ocean also provided free spaces for the eastward extrusion of the Tibetan Plateau.