中甸-大具断裂南东段位于哈巴和玉龙雪山北麓, 属于川西北次级块体西南边界, 断裂总体走向310°~320°, 是一条重要的边界断裂。了解该断裂的活动性质、 活动时代和滑动速率等对分析川西北次级块体运动, 研究该断裂与玉龙雪山东麓断裂的交切关系等问题具有重要意义。文中基于 1:5万活动断层地质填图, 对断裂沿线地层地貌、 陡坎地貌、 地表破裂、 典型断层剖面以及河流阶地等进行了详细的研究。研究表明: 1)中甸-大具断裂南东段按几何结构、 断错地貌表现、 断裂活动性可分为马家村—大具次级段和大具—大东次级段。 2)通过野外地质调查发现, 马家村—大具次级段断错了全新世冲洪积扇, 形成了地表破裂, 为全新世活动段; 而大具—大东次级段虽然也断错了晚更新—全新世地层, 但其断错规模及滑动速率均较小, 由此认为其全新世以来活动较弱。3)通过分析断裂沿线断层陡坎、 水平位错及地表破裂等地质地貌问题, 认为马家村—大具次级段的活动性质为右旋走滑兼正断, 其晚更新世以来的垂直滑动速率为0.4~0.8mm/a, 水平滑动速率为1.5~2.4mm/a; 大具—大东次级段以右旋走滑为主、 正断为辅, 其晚更新世晚期以来的垂直滑动速率为0.1mm/a。4)在大具盆地内发现的NW向地表破裂带的形成时代很年轻, 不排除是1966年中甸6.4级地震或1996年丽江7.0级地震造成的地表破裂。

The southeast section of Zhongdian-Daju Fault is located in the northern part of Haba and Yulong Snow Mountain, belonging to the southwestern boundary of the secondary block in northwestern Sichuan, an important boundary fault striking 310°~320° on the whole. The nature of the fault, the age of its activity and the slip rate are of great significance for the analysis of the secondary block movement in the northwestern Sichuan and the intersection relationship with the eastern piedmont fault of Yulong Mountains.
Based on the 1:5 million-scale active fault geological mapping, this paper studies in detail the stratigraphic landform, scarp landform, surface rupture, typical fault profile and river terrace along the fault. Based on the research results, we divide the southeastern section of Zhongdian-Daju Fault into two sub-segments, the Majiacun-Daju sub-segment and the Daju-Dadong sub-segment, according to the geometric structure, fault landforms and fault activity.
(1)Fault scarp: In the Majiacun-Daju sub-segment, the fault parallelly controls the two sides of the Haba fault depression. It cuts the late Pleistocene moraine deposits, forming a fault scarp of about 4.5km long and(14±2)m high. The continuity of the scarp is very good, and it is also very obvious in the remote sensing image. In the Daju-Dadong sub-segment, a scarp with a height of about 2m is formed, and an optical luminescence dating sample is collected from the upper part of the gravel layer on the second-order terrace to obtain an age of(22±2.2)ka.
(2)Horizontal dislocation: In the Majiacun-Daju sub-segment, through the analysis of the development of outwash fans in the area and the measurement and induction of the gully dislocations, it is considered that there are at least three stages of outwash fans developed in the area and there may be four phases of faulting. That is, the earliest-stage outwash fan and gully are horizontally dislocated about 1km; the second-stage outwash fan and gully are horizontally dislocated about 47m, and the vertical dislocation is about(14±2)m; the gully in the third stage outwash fan is horizontally dislocated twice, the first dislocation formed a beheaded gully with a dislocation of 22m, and the second formed a beheaded gully with a dislocation of 8.5m. It is further proved that the fault has strong activity since the Holocene in the Majiacun to Daju area. In the Daju-Dadong sub-segment, there are no obvious horizontal dislocations in the alluvial deposits since the Holocene. Only 3~4 gullies are found to be offset right-laterally in the ridges east of Wenhe Village, with the maximum dislocation of 210m, which may be the older phase dislocation.
(3)Surface rupture: In the northwest direction of Dabazi Village on the T3 terrace in the basin between Majiacun and Daju, an earthquake surface rupture zone is found, extending in the NW direction. The rupture zone left clear traces on the about 1m-thick, hard T₃ terrace surface formed by calcification of sand gravels, and the overburden either upwarps and bulges, or ruptures, generates ground fissures, or forms small pull-apart “depressions” locally. However, the rupture zone is not large in size, about 350m long, 60m wide at the widest point, and 0.3~1.5m high. It is partially en-echelon or obliquely arranged, dominated by compressive ruptures. Through observation, the possibility of artificial transformation is ruled out for these upwarping bulges, ruptures or ground fissures. The fault section is found in the southeast direction of the rupture zone. The slickensides at the section show that the fault is dominated by right-lateral strike-slip with a small amount of thrust. In the eastern sub-segment, only intermittently distributed surface ruptures are found in the northern part of the village, and the scale is small.
In summary, through the field geological survey, it is found that the Majiacun-Daju sub-segment is a Holocene active segment. Though the Daju-Dadong sub-segment also offset the late Pleistocene to Holocene strata, it is considered that its Holocene activity is weak in terms of either the dislocation amount or the slip rate of this segment.
By analyzing the geological and geomorphological evidences, such as fault scarps, horizontal dislocation and surface ruptures along the fault, it is considered that the Majiacun-Daju sub-segment is a right-lateral strike-slip fault with a normal faulting component, and its vertical slip rate since the late Pleistocene is(0.4~0.8)mm/a, the horizontal slip rate is 1.5~2.4mm/a. The Daju-Dadong sub-segment is dominated by right-lateral strike-slip with a normal faulting component, and its vertical slip rate since the late Late Cenozoic is 0.1mm/a.
The formation of the NW-trending surface rupture zone found in the Daju Basin is very young, where there are only two major earthquakes, namely, the M_S6.4 1966 Zhongdian earthquake and the 1996 Lijiang M_S7.0 earthquake, and both earthquakes produced NW-oriented surface rupture zones. Therefore, it cannot be ruled out that the rupture zone is a product of the 1966 Zhongdian M_S6.4 earthquake or the 1996 Lijiang M_S7.0 earthquake.