Upper Crustal Collapse Reconstructed the Topography and Remodeled the Fault System of the Chuandian Fragment in the Southeastern Edge of the Tibetan Plateau, Evidenced by Anisotropy of Magnetic Susceptibility Data Sets

Abstract

AbstractThe steps of the southeastward stepwise‐descending topography of the Chuandian Fragment (CDF) in the southeastern edge of the Tibetan Plateau coincide with N‐S and NE‐SW trending faults that changed from thrusting to normal activity, indicating that since the Middle‐Late Miocene the CDF maintained an E‐W oriented extensional tectonic environment. However, the anisotropy of magnetic susceptibility (AMS) data sets presented herein, from the Cretaceous and Cenozoic sedimentary strata of the southeastern Tibetan Plateau and its edge, show that since ∼28.0 Ma the CDF remained within a stabilized compressional stress field, oriented NEE‐SWW. A comprehensive analysis of the AMS results, topography, fault systems, and geophysical observations indicate that the northward‐advancing eastern Himalaya syntaxis impeded the southeastward‐flowing viscous lower crust and correspondingly decreased its flux, which could have reduced the thickness of the lower crustal channel beneath the CDF. The original low‐gradient topography of the CDF formed by the viscous lower crustal flow could no longer be supported by the thinned lower crustal channel, and thus gravity drove the collapse of the uplifted CDF. This process reconstructed the topography and remodeled the preexisting fault systems of the CDF, resulting in the coexistence of the E‐W oriented extensional tectonic environment and the stabilized NEE‐SWW oriented compressional stress field of the western part of the CDF, since the Middle‐Late Miocene. Thus, the change in the status of the lower crustal channel related to block interactions provided a major geodynamic setting for shaping the topography of the southeastern edge of the Tibetan Plateau.