Present-day stress field in SE margin of Tibetan plateau by comprehensive updated earthquake focal mechanisms and its tectonic implications

Abstract

The Sichuan-Yunnan region of the SE and E margin of the Tibetan Plateau, situated at the transitional nexus between the seismically-active and intensely-deformed Tibetan Plateau and the tectonically stable Yangtze block with comparatively low seismicity, has experienced substantial tectonic transformations during the Quaternary. Given the pronounced seismicity, there is an escalating imperative for an accurate and refined distribution of the stress field in the region. To unravel the contemporary stress state within major active blocks and along active faults in the study area, the tectonic stress field is calculated by comprehensive updated earthquake focal mechanisms catalogue. The tectonic stress field in Sichuan-Yunnan region exhibits significant lateral variations in direction and stress pattern. The directions of the maximum and minimum principal stress in the north are nearly E-W compressions and N-S tensions, respectively. Conversely, in the south, there is a discernible clockwise rotation trend from east to west. Localized normal faulting stress regimes are observed in the middle section of Xianshuihe fault and SW side of Litang fault. The extensional environment of the former may be attributed to the tectonic activities such as block translation, clockwise rotation and the strike variation of the Xianshuihe fault from NW-SW to NNW-SSE. The latter may be related to the extensional structures, rift basins and the normal fault movements in the crust formed by the detachment of the plates and delamination of the mountain roots at the end of Triassic. We also found that the stress field under the large faults, such as Longmenshan fault, Red River fault, Xiaojiang fault and Lijiang-Xiaojinhe fault, show segmented variation. The findings yield invaluable insights into the intricate dynamics of tectonic deformation along the SE and E margin of the Tibetan Plateau.