Abstract: The Sudian Fault extends in nearly NS direction and crosses the border between China and Myanmar, with a length of about 100km. Historically, many earthquakes have occurred along the fault. However, restricted by traffic, climate and other factors, there has been little research on the late Quaternary activity of the fault for a long time. On the basis of results of field geological and geomorphological investigation, trenching and geochronology, the movement characteristics of the fault in late Quaternary, the latest active age and sliding rate are analyzed in this paper. The Neotectonic activity of the Sudian Fault is obvious. Beaded Quaternary basins in areas of Sudian, Mengdian, Huangcaoba and Longzhong have developed along the fault. Many boiling springs and gas springs are distributed linearly in the area of Humeng in the south section of the fault. The fault controls the Lama River and Zhanda River obviously. Fault landforms are mainly characterized by clear fault scarps, straight linear ridges and fault valleys. Mengdian pull apart basin is developed in the middle segment of Sudian Fault. In the Zuojiapo area of the western margin of the basin, there is a clear linear ridge about 1.7km long and a parallel fault valley which is close to the west side of the linear ridge. Trench excavation was carried out in this fault valley(24.97°N, 97.93°E). Zuojiapo trench reveals that three faults have developed in Quaternary deposits. At the position of 2~3m(from west to east)on the S wall of the trench, a fault dislocated all the strata(unit②~unit⑥)below the modern loam layer(unit①). These strata are obviously offset and some of them are cut off. The ¹⁴C age of the displaced unit ④(tested by BETA laboratory, USA)is(7 680±30)a, two ¹⁴C ages of the displaced unit ③are(6 970±30)a and(5 860±30)a, and the ¹⁴C age of the displaced unit ②is(1 260±30)a. The fault developed at 21m in the east section of S-wall of the trench has offset the lower bedrock(unit⑧), the middle gravel layer(unit⑤and unit⑦), the upper dark gray gravelly clay layer(unit④)and the peat interlayer(unit④'). In the peat interlayer(unit④'), there is obvious structural deflection deformation, and its ¹⁴C age is(350±30)a. There is another fault developed at 26~27m in the east section of S-wall of this trench, which cuts off the light yellow and light gray gravelly clay(unit ②), gray black gravelly clay(unit③), gray white sandy gravel(unit⑤), yellow gravelly silty clay(unit ⑥), yellow clay gravel(unit ⑦)and hornblende schist and quartz schist of Gaoligongshan group(unit ⑧). The fault shows obvious normal fault property, and the maximum offset is 1.3m. A 10cm wide schistosity zone is developed and gravels are arranged along the fault plane. The ¹⁴C ages of the faulted upper stratum(unit ③)are(1 100±30)a and(870±30)a. The N-wall also reveals the existence of faults, corresponding to the S-wall of the trench. These faults and dislocated strata fully indicate that the fault was active during the Holocene. According to field investigation, the Sudian Fault is mainly characterized by horizontal dextral strike-slip movement. For example, in Mengnong tea field, obvious synchronous dextral displacement occurred in three gullies along the fault. From south to north, the displacements of the three gullies are 40m, 42m and 45m, respectively. Shutter ridge landform is developed at the gully mouth. In the lower part of the northernmost gully, there is a pluvial fan, and the ¹⁴C age of the bottom of the pluvial fan is(13 560±40)a, which is less than the formation age of the gully, but roughly represents the formation age of the gully, indicating that the Sudian Fault is mainly characterized by horizontal dextral strike-slip movement. In Sudian area, the Mengga River is right-laterally offset 1 050~1 100m by the fault. At 1.7km north of Sudian, a diluvial fan is right-laterally offset 18~22m. There are fault scarps with a height of 1~1.5m developed on the alluvial fan, Quaternary faults and bedrock fault scarps with a height of about 8m developed on its extension line. The three points of the scarps, Quaternary faults and bedrock scarps are in a straight line, which absolutely shows the reliability of the dislocation of the alluvial fan. An organic carbon sample is obtained 1.8m below the alluvial fan, and its ¹⁴C test age is (6 210±30)a. This age should be close to the formation age of the pluvial fan, indicating that the fault underwent obvious horizontal dextral strike-slip movement during the Holocene. In the Sadung Basin, Myanmar, a river is offset about 380m right-laterally, forming a hairpin bending landform. Due to the continuous collision between the Indian plate and the Eurasian plate, the Indosinian block in the southeastern margin of the Tibet Plateau around the Eastern Himalayan Syntaxis escaped southerly, and the western Yunnan became the most intense part of the south extrusion. During the southerly escapement of the Indosinian block, the right-lateral strike-slip movement of Sudian Fault and other faults striking near SN plays a role in adjusting and absorbing the block strain. Under the action of current NNE tectonic stress field, the intersection of the dextral strike-slip Sudian Fault striking NS and the sinistral strike-slip Dayingjiang Fault striking NE is the key part of tectonic stress concentration, which will be the seismic risk area to be focused in the future. The research result of late Quaternary activity of the fault is of great practical significance for the correct understanding and reasonable assessment of the medium to long-term strong earthquake risk in this area, and for the mitigation and prevention of the earthquake disaster in the border area.

Key words: Sudian Fault, Holocene, dextral strike-slip movement, tectonic significance

摘要： 苏典断裂呈近SN向延伸, 跨越中缅边境, 长约100km, 历史上沿断裂多有地震发生。 但受交通、 气候等因素制约, 长期以来有关断裂的晚第四纪活动性几乎没有研究。 据野外地质地貌调查、 探槽和年代学测试结果, 文中对此断裂的晚第四纪运动特征、 最新活动时代进行分析。 苏典断裂的新构造活动明显, 沿断裂发育有苏典、 勐典、 黄草坝、 陇中等串珠状第四纪盆地, 南段户勐一带有多个沸泉和气泉呈线状分布, 断裂对腊马河、 盏达河具有明显的控制作用, 地貌上主要表现为清晰的断层陡坎、 平直的线性山脊和断层谷地等地貌。 左家坡探槽揭示出3条断层, 断错了几乎所有的河流相、 洪积相砾石层、 砂土层和黏土层等堆积物, 地层错距明显, 部分层位被截切。 在探槽内共获得11个被断错地层的¹⁴C年龄数据, 年龄结果为(7 680±30)~(350±30)a, 充分表明该断裂在全新世有明显活动。 晚第四纪苏典断裂表现出右旋走滑运动。 勐弄茶厂一带沿断裂发育的3条冲沟发生明显的同步右旋位错, 位错量分别为40m、 42m和45m。 苏典一带勐嘎河被断裂右旋位错1 050~1 100m。 苏典北1.7km处一洪积扇被右旋位错18~22m。 缅甸撒当盆地内一河流被断裂右旋位错约380m并形成发卡型地貌。 受印度板块与欧亚板块持续碰撞的影响, 东喜马拉雅构造结外围的青藏高原东南缘的印支块体发生S向逃逸, 滇西地区成为青藏高原向S挤出最强烈的部位, 在印支块体S向运动过程中, 近SN向苏典等断裂的右旋走滑运动起着调节、 吸纳块体应变的作用。 在NNE构造应力场作用下, 近SN向右旋走滑的苏典断裂与NE向左旋走滑的大盈江断裂的交会部位是构造应力易于集中的关键部位, 该区是未来需要重点关注的地震危险区。

关键词: 苏典断裂, 全新世, 右旋走滑运动, 构造意义