Rigid widths of active block boundary faults and crustal layered anisotropy in the intersection of faults Honghe and Xiaojiang in the SE margin of the Tibetan Plateau

Abstract

SUMMARYThe distribution of block boundary faults, both horizontally and vertically, has significant implications for crustal movements and structural variations. In the SE margin of the Tibetan Plateau, the south part of the Sichuan–Yunnan block is surrounded by the first-order block boundaries of the Honghe faults (HHF) trending NW–SE and the Xiaojiang faults (XJF) trending N–S. Crustal anisotropy provides valuable insights into the ongoing deformation and evolution. This study investigates the upper-crustal anisotropy at 26 stations, using local seismic waveforms from the temporary linear Honghe–Xiaojiang seismic broad-band Array (i.e. the HX Array) in 2017–2019 and regional permanent stations in 2012–2019. The PFS (i.e. polarization of fast S wave) shows an abrupt change in the Yimen region, which is the middle of the HX Array survey line between two large faults. The NW–SE dominant PFS orientations across the HHF agree well with the strike of faults and the direction of regional principal compressive stress, illustrating cracks or minerals aligned with the HHF, influenced by regional stress. The NE–SW dominant PFS orientations crossing the XJF differ from the faults strike or regional stress orientation, suggesting alignment with microcracks caused by the in situ principal compressive stress. The comparison between the upper-crustal and whole-crustal anisotropy demonstrates a noticeable PFS consistency across the two large faults, the HHF and the XJF. This observation implies that these two large faults may tectonically control a wide zone, with wide rigid widths of approximately 150 and 200 km, respectively. This study presents a new pattern indicating the control of deep physical properties on the boundary faults and uncovering the layered deformation in the upper and lower crust.