Thrust fault zones around the Tibetan Plateau (TP) record the tectonic evolution between the Plateau and its surrounding terranes, which is helpful for understanding the uplift mechanism and deformational processes of the TP. The Longmenshan fault (LMSF) is the tectonic boundary (TB) between the Yangtze terrane (YT) and Songpan-Garze terrane (SGT), while the TB of its western segment, either the Lijiang-Xiaojinhe fault (LXF) or Jinhe-Qinghe fault (JQF), is controversial. Therefore, we conducted magnetotelluric (MT) imaging, surface structure surveys, and petrologic analysis to further determine the deep–shallow structural relationship of the western LMSF segment. Resistors R1 and R2 revealed by MT imaging may have originated from different magmatism. Among them, R2 may have originated from plume underplating, which is consistent with previous studies, while R1 may have originated from the residue of episodic mafic magma intrusion along the JQF over a broader period. Based on regional geophysics, surface structural patterns and petrologic mineralogy, it is suggested that the JQF may have deformed deep into the lower crust or upper mantle, accommodating the southeast expansion of the TP by thrusting and acting as the TB between the YT and SGT before ∼15 Ma. After ∼15 Ma, due to the activation of the large-scale strike-slip faults, the LXF gradually replaced the JQF to dominate the structural deformation of the western LMSF segment. Our results indicate that the above tectonic transition might be associated with the geodynamic process from centralized deformation to diffuse deformation within the southeast TP during the late Cenozoic.

中文翻译：

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青藏高原东南缘龙门山断裂西段新生代构造转型：地质和地球物理资料的启示

青藏高原周边逆冲断裂带记录了青藏高原与周围地体之间的构造演化过程，有助于了解青藏高原的隆升机制和变形过程。龙门山断裂带（LMSF）是扬子地体（YT）与松潘-甘孜地体（SGT）之间的构造边界（TB），而其西段的TB则是丽江-小金河断裂带（LXF）或金河-清河断裂带。断层（JQF），是有争议的。因此，我们进行了大地电磁（MT）成像、表面结构调查和岩石学分析，以进一步确定LMSF西段的深浅结构关系。MT成像揭示的电阻R1和R2可能源自不同的岩浆作用。其中，R2可能起源于地幔柱底侵，这与之前的研究一致，而R1可能起源于更广泛时期沿JQF的幕式镁铁质岩浆侵入的残留物。根据区域地球物理、地表构造格局和岩石矿物学，认为JQF可能已变形深入下地壳或上地幔，通过逆冲作用容纳了青藏高原的东南扩展，并充当了YT和SGT之间的TB。 〜15马。～15 Ma后，由于大规模走滑断裂的激活，LXF逐渐取代JQF主导LMSF西段的构造变形。研究结果表明，上述构造转变可能与晚新生代高原东南部从集中变形到扩散变形的地球动力学过程有关。