滇西南地区孟连断裂晚第四纪新活动特征初步研究

Preliminary Study on the Late Quaternary Activity Characteristics of the Menglian Fault in Southwest Yunnan

摘要:

孟连断裂位于云南省西南部与缅甸交界地带,是川滇菱形块体南部一条规模较大的活动断裂带,总体呈NEE向延伸,长约90 km,走向N70°E,倾向NW,倾角50°~60°,断裂晚第四纪活动较强烈,以左旋走滑为主,兼具有倾滑特征。通过卫星影像解译和野外调查发现,其断错地貌主要以线性断层崖为主,高度不等,其次为断层谷地、断层沟槽和断层垭口地貌,冲沟及阶地的水平位移多在几十米至几百米之间。在孟连县城西侧开挖的大型探槽中,揭露出多条断层,通过分析剖面和¹⁴C测年结果认为,孟连断裂晚第四纪发生过4次古地震事件,除最早一次年代较久远以外,其他3次均发生在全新世中晚期以来,最近一次古地震事件的年代为(1860±30)~(1090±30)a B.P.。

Abstract:

Because it is located in an area of extensive collision between the Indian Plate and the Eurasian Plate, the Sichuan-Yunnan rhombic block in southwest Yunnan at the southeastern margin of the Tibetan Plateau is one of the regions with the highest concentration and frequency of strong earthquakes in China. Due to compression of this block in the SE direction, the region developed two groups of active transverse faults in NE and NNW directions. Both groups are involved in the process of rupture and the occurrence of earthquakes of above M7.0. In this area, the main feature of the late Quaternary activity of these faults is strike-slip. The NW-trending faults have right-lateral strike-slip characteristics, whereas the NE-trending faults have left-lateral strike-slip characteristics. The Menglian fault is located in the boundary zone between southwest Yunnan and Myanmar. It is a large active fault in the south of the Sichuan-Yunnan rhombic block and its late Quaternary activity is intense. This fault has the main characteristics of left-lateral strike-slip with dip-slip. However, there are few research data on this fault and the extent of study has been low.There are no records of earthquakes on the Menglian fault. However, in the southwest of the fault, a M_S7.3 earthquake occurred in the territory of Myanmar on July 12, 1995, which may be related to its activity. The linear structure of the fault in satellite images and landforms is very clear. The stream and water system are obviously offset to the left side along the fault. According to scanning electron microscopy results of the characteristics of the surface of quartz fragments in gravel from a fault gouge and research into active faults, Weixian et al.(2002) believed that the Menglian fault was formed before the Pliocene, displayed a high frequency of activity in the late Pleistocene, and has been active since then.Through an investigation of the active characteristics of the Menglian fault in the late Quaternary, this study aims to determine the structural transformation and obtain a mechanical interpretation of the active faults in different directions and natures in the study area. According to the interpretation of satellite images and field investigation, we found that the fault is 90 km long and has a trend of N70°E. The main landform due to fault dislocation is a linear fault scarp of unequal height, whereas the others are fault valleys, fault trenches, and fault passes. The horizontal displacement of gullies and terraces is of the order of tens of meters to several hundred meters. A large trench excavated in the west of Menglian County revealed many faults. Through analysis of the fault profile and ¹⁴C dating results, we found that four paleoearthquake events have occurred on the Menglian fault. Except for the earliest events, the three that followed all occurred in the middle and late Holocene. The latest paleoearthquake occurred between(1860±30) and(1090±30) a B.P.