The Sichuan-Yunnan block is one of the most active regions in the world in terms of tectonic and seismic activities. The frequent large earthquakes occurring on the boundary faults, whereas the activity and seismicity of the intra-block faults have not attracted enough attention. At the same time, these intra-block faults also have the potential to trigger destructive earthquakes. In addition, intra-block fault activity causes cascading response of boundary faults, and intra-block faults can accommodate kinematic inconsistency between boundary fault motions, which affects regional crustal deformation patterns. Therefore, it is important to understand intra-block fault activity and associated surface-rupturing earthquake features for regional seismic hazards assessment and tectonic activity analysis.

During an active fault investigation in the Liangshan area of the Sichuan-Yunnan block, a distinct lithological boundary named Caimashui fault was found. The NE-striking Caimashui fault is a intra-block fault within the Sichuan-Yunnan block, with a total length of about 80 km. Located 12 km to the southeast of Huidong County, the fault generally strikes NE. It lies on the west side of the northern segment of the Zemuhe fault zone and Xiaojiang fault zone, runs roughly parallel to the Ninghui fault, and represents a relatively continuous and well developed structure. However, due to the complex geological conditions and poor accessibility, the formation age, active characteristics and slip rate of the Caimashui fault are still unknown. In this paper, we conduct comprehensive analyses including remote sensing interpretation, field investigation, unmanned aerial vehicle （UAV） topographic mapping, trench excavation and Quaternary dating, to clarify the geometric distribution of the Caimashui fault and quantitatively investigate its Late Quaternary active characteristics.

Field geological investigation shows that the activity of the Caimashui fault varies significantly on both sides of Haiba township. In the northeastern segment on the east side of Haiba township, the Caimashui fault is an Early-Middle Pleistocene fault with a length of 24 km, while in the southwestern segment extending from Haiba township to Xinfa township, the Caimashui fault is a Holocene fault with a length of 58 km. Typical dextral strike-slip landforms are well developed along the southeastern segment of the Caimashui fault, including displaced river terraces, drainage systems, mountain ridges, and alluvial fans, which collectively indicate that the Caimashui fault has been dominated by dextral strike-slip since the Late Quaternary.

Based on the high-resolution UAV mapping and dating results, the dextral strike-slip rate of the Caimashui fault can be quantitatively constrained. Field investigations show that, to the northwest of Xiaochacun, the Caimashui fault has produced obvious synchronized dextral displacement of ridges and gullies, and the morphology of the fault scarps inside the fault trough is clear, with faults scarps distributed in the form of geese. There is no trace of artificial modification, and the fault scarp ponds possess favorable depositional conditions and are rich in organic materials, allowing complete preservation of paleoearthquake records. Therefore we excavated the Xiaochacun trench perpendicularly to the fault scarp. The trench exposure reveals that the Caimashui fault has experienced at least three paleoearthquake events since 30 ka: event E₁ （29002—20312 BC）, event E₂ （9774—6919 BC）, and event E₃ （2730—1526 BC）. The fault has the potential to generate earthquakes with M≥7.0. In the trench, the stratigraphic unit U₁ is a black peat layer deposited within an abandoned plug pond. Using the age obtained from the top of stratigraphic unit U₁ and the gully dextral dislocation of 22.8 m, the average dextral slip rate of the Caimashui fault since the Late Pleistocene is estimated to be （0.85±0.01） mm/a, although this value may be slightly higher than the actual long-term slip rate. The study of the active tectonics around the Sichuan-Yunnan block shows that the left-lateral strike-slip rate of the Xiaojiang fault zone is much greater than that of the Xigeda-Yuanmou fault, resulting in westward extrusion and counterclockwise rotation of the central Yunnan micro-block. To accommodate such rotational motion, intra-block regions develop compressional deformation at the block margin. In addition, the left-lateral shear deformation of the block causes the intra-block faults to undergo dextral slip and compressional motions, accommodating regional strain partitioning.