2021 Vol. 27, No. 2

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2021, 27(2): 封二-封二.
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2021, 27(2): .
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2021, 27(2): 封三-封三.
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Chief Editor’s Address
2021, 27(2): 157-158.
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Geometrical imagery and kinematic dissipation of the late Cenozoic active faults in the West Qinling Belt: Implications for the growth of the Tibetan Plateau
ZHANG Yipeng, ZHENG Wenjun, YUAN Daoyang, WANG Weitao, ZHANG Peizhen
2021, 27(2): 159-177. doi: 10.12090/j.issn.1006-6616.2021.27.02.017
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The West Qinling Belt (WQB) situated in the central China continent, is an enormous structure on the crustal scale, which is not controlled only by the Tethyan tectonic domain but is more complex, involving additional tectonic domains. The composite WQB as the coordinate system, which underwent five major episodes of accretion and collision between discrete continental blocks, has distinct geological and geophysical structure, geomorphology and environment, characterized by complex structures, complex forming processes and mixed materials. Moderate-strong earthquakes occurred frequently in the WQB in recent years, attesting its tectonic activity. Numerous results from the studies related to active fault geological and geodesic observations gave us new insights into present-day crustal deformation characteristics and its dynamic mechanism and helped us in exploring the control effect of active tectonic system on significant earthquake events in the WQB. Two groups of faults striking in different direction (NWW-trending and NEE-trending) within the WQB have played significant roles in the tectonic deformation and the transference slip along the east end of the east Kunlun fault since the Quaternary. Recent results suggest that the < 2 mm/a slip rate at the tip of the east Kunlun fault is absorbed by low slip rate faults, crustal shortening, basin formation, mountain uplift and block rotation in the WQB. Whereas deformation in the shallow brittle crust does not occur on a major fault, deformation of a continuous medium at depth best describes the present-day tectonics of the WQB. Regionally, mantle magmatism, geophysical and geological data show that the actively deforming WQB crust is dominated by main mountain building contraction shortening strain in the upper crust, decoupled plastic deformation in the lower crust and extrusion of the mantle lithosphere below to the high-strain domains in the crust above, and such a transition zone (high and low velocity/resistivity anomalies) is relatively easy to accumulate stress, leading to occurrence of major earthquake in this area.
Spatiotemporal distribution and geodynamic mechanism of the nearly NS-trending rifts in the Tibetan Plateau
BIAN Shuang, YU Zhiquan, GONG Junfeng, YANG Rong, CHENG Xiaogan, LIN Xiubin, CHEN Hanlin
2021, 27(2): 178-194. doi: 10.12090/j.issn.1006-6616.2021.27.02.018
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A set of nearly NS-trending rifts developed in the Himalayan orogen and southern Tibet, which are the large-scale extensional structures formed under the continuous compression of the Indo-Eurasia continent, playing a significant role in revealing the post-collisional evolution of the Tibetan Plateau. Different predictions have been made on the spatiotemporal distribution of these rifts through the existing hypothesis models, constituting the key factors constraining the formation system of the rifts. In this study we synthesized previous studies on the initiation time of rifting, and further clarified the spatiotemporal trend. The analysis results showed that the rifting initiated progressively earlier westward, which is consistent with the evolution of post-collisional magmatism in the Lhasa Terrain. Moreover, combined with the geophysical observations, it is inferred that the geodynamic mechanism of the nearly NS-trending rifts accords closely with the hypothesis model concerning the eastward-propagating lateral detachment of the subducted Indian slab. The Indian slab detachment resulted in asynchronous gravitational potential energy gradients, which drove the lithosphere flow eastward and eventually caused the eastward development of the rifting.
Discovery of active faults in the southern Beishan area, NW China: Implications for regional tectonics
YUN Long, ZHANG Jin, WANG Ju, ZHAO Zhitao, BAO Yintu, ZHUANG Haiyang, CHEN Su, ZHANG Jingjia, ZHANG Jia, ZHAO Heng, ZHANG Beihang
2021, 27(2): 195-207. doi: 10.12090/j.issn.1006-6616.2021.27.02.019
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On the north side of the Hexi Corridor, two active faults, which belong to two different fault systems, have been discovered on the southern margin of Beishan, namely the Jiujing-Bantan fault and the Ebomiao fault. The NE-trending (40°~50°) Jiujing-Bantan fault with a NW-trending dip angle of 60°~70° is ~28 km long and ~55 km away from Yumen City. It consists of 4 branches and presents a complex Y-shaped distribution, controlling the development of the two late Pleistocene basins on its west side. The nearly EW-trending Ebomiao fault with a NW-trending dip angle of 60°~80° is ~18 km long and ~50 km away from Jinta County. Base on results from the satellite image interpretation, offset geomorphological survey, trench excavation and optical luminescence dating, we discovered a series of ridges, gullies and terraces offset by the Jiujing-Bantan fault which has been active since ~20 ka ago mostly with normal left-lateral strike-slips. The northward thrusting of the Ebomiao fault formed a clear linear scarp and offset the gullies with left-lateral strike-slips. This fault has been active since ~30 ka ago, mainly with reverse left-lateral strike-slips. The neotectonic activities of these two faults evidences that the long-range strain transmission from the northern margin of the Tibetan Plateau has entered into the southern margin of the Beishan orogenic belt since the late Cenozoic.
Estimation of displacements along strike-slip fault on a million-year timescale: A case study of the AltynTagh fault system
HUANG Feipeng, ZHANG Huiping, XIONG Jianguo, ZHAO Xudong
2021, 27(2): 208-217. doi: 10.12090/j.issn.1006-6616.2021.27.02.020
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Fault slip rate matters not only as one of the important parameters for quantitative study of Cenozoic tectonics but also a key element in geodynamic research. However, most studies have focused on the long-term (>Ma) cumulative displacement of geological mass, short-time (since the late Quaternary) dislocation of geomorphic units as well as annual-decadal geodetic observations, and few people studied the fault displacement on a timescale in between, leaving a gap in understanding the evolutionary history of fault on a million-year timescale. Since the strike-slip fault breaks the system of alluvial fans and their catchment basins, causing the spatially uneven distribution of residual offset alluvial fans along the fault direction, we proposed three methods to determine the large-scale cumulative displacement of strike-slip fault based on the offset alluvial fans. The first method commonly presents a correspondence between the alluvial-fan area and catchment basin area as Af=γAc(Af is the alluvial-fan area, Ac is the catchment basin area, γ is a constant 0.5±0.35), which leads us to determine the strike-slip displacement of the offset basin by identifying whether the correspondence is unusual. The second method helps us to get the strike-slip displacement by distinguishing geomorphic units with the same lithological mineral components distributed at both sides of the fault. The third is to measure the strike-slip displacement by comparing the residual geomorphic unit with the corresponding stream outlet. In this study, we applied the above three methods to study the strike-slip displacement of the AltynTagh fault system on a million-year timescale, and to estimate the formation age of these geomorphic units based on the exiting fault slip-rate. It is further verified that the estimation methods proposed in this paper can provide a new angle and technical solution to accurately determine the evolution history of the strike-slip fault on a million-year scale.
Discussion of the seismogenic structure of the 1901 M 6¾ Nyemo earthquake
HA Guanghao, WU Zhonghai
2021, 27(2): 218-229. doi: 10.12090/j.issn.1006-6616.2021.27.02.021
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According to historical records, an M 6¾ earthquake occurred in Nyemo, Tibet on April 21, 1901 without any reports on its seismogenic structure yet. To sort out its seismogenic structure helps to understand the earthquake recurrence pattern of the Nyemo graben group (NGG) and to scientifically evaluate the future earthquake risk of surrounding areas. According to our results from the remote sensing interpretation and geological survey, a 30 km-long fault with a trend of nearly NW-NNW at the western boundary of the Panggang graben (PG), one of the NGG, is divided into north segment and south segment by the Penggang River. The north segment is characterized by significant linear features, with fault scarps developed, offsetting multistage river terraces and moraines. The recovered displacements show that the vertical offsets of Terrace T0, T1 and T2 are ~1.0 m, ~2.6 m and ~5.0 m, respectively. The south segment with obvious linear features develops along the Xiaqing River and extends to the north Nyemo County, with fault scarps developed, offsetting multi-stage alluvial fans. We found the offset Terrance T2 profile in northern Nyemo, indicating the extension of the fault. Through the magnitude recovery, we calculated the moment magnitude of the latest earthquake as about MW6.8 along the PG, which is consistent with the Nyemo earthquake. Inferred from the results of the satellite image interpretation, geological survey and magnitude recovery, the PG is likely the seismogenic structure of the Nyemo earthquake. Also based on historical seismic records, we consider that each graben in the NGG could independently generate moderate-strong earthquakes, and the earthquake recurrence pattern and the seismicity difference between the NGG and the north and south segments of the Yadong-Gulu rift need further study.
Numerical simulation of the present seismic risk of the HaiyuanLiupanshan fault zone
JIANG Fengyun, JI Lingyun, ZHAO Qiang
2021, 27(2): 230-240. doi: 10.12090/j.issn.1006-6616.2021.27.02.022
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In this study, the orthotropic theory-based characterization of fault deformation behavior was made, with the ratio of the shear modulus parallel to fault plane to the shear modulus of surrounding media as the inversion parameters and the present-day crust horizontal movement velocity field observed by GPS near the Haiyuan-Liupanshan fault as the constrain. We built a 3D finite element model, using the genetic algorithm, to estimate the shear moduls distribution parallel to the Haiyuan-Liupanshan fault plane. The inversion results show that the shear modulus parallel to the Liupanshan fault plane is close to that of the surrounding media, and the seismic activities are sparsely distributed along the fault plane, reflecting a small deformation difference on both sides of the Liupanshan near the fault, which is similar to the situation of the Longmenshan fault before the Wenchuan Earthquake. The fault zone may be in a state of strong locking. The shear modulus parallel to the fault plane of the Hanyuan fault in the narrow sense is much smaller than that of the surrounding media, all below 0.4, and the shear modulus within 0~5 km is larger than that of the deep, which may reflect that the entire fault has still been in post-earthquake adjustment since the Haiyuan 8.5-magnitude earthquake in 1920. The shear modulus of the Jinqianghe, Maomaoshan and Laohushan faults in the western section is relatively low at the shallow section of the faults (0~5 km), while the shear modulus of 5~20 km is relatively high. Combined with the fault surface seismic activity distribution characteristics, it is considered that creep slips may exist in the shallow sections of the Jingqianghe and Maomaoshan faults; however, there is strain energy accumulation in the depth of 5~20 km, which has the background of strong earthquake. Seismic activity of the Laohushan fault is relatively intensive from the surface to the deep, and there may be creep through, with a small probability of strong earthquake.
Numerical simulation of coseismic and postseismic deformation through a node-splitting algorithm: A case study of the Wenchuan earthquake
SUN Yunqiang, LUO Gang, HUANG Luyuan
2021, 27(2): 241-253. doi: 10.12090/j.issn.1006-6616.2021.27.02.023
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The coseismic and postseismic effects are crucial elements in analyzing fault interactions and the regional seismic risk. In this paper, we developed a three-dimensional viscoelastic finite element code to simulate the coseismic and postseismic deformation. We calculated the coseismic and postseismic deformations caused by the strike-slip fault with a conceptual model, and compared the results with the analytic and semi-analytic solutions so as to verify the reliability of the code. Meanwhile, we also analyzed the influence of different parameters on the coseismic and postseismic deformation, uncovering a significant effect of the earth's lateral heterogeneity on the coseismic displacement. And the viscosity of the middle-lower crust and upper mantle plays a major role in controlling the postseismic displacement. At last, we used the three-dimensional viscoelastic model to calculate the changes of the coseismic and postseismic Coulomb stress caused by the 2008 MW7.9 Wenchuan earthquake, and analyzed the subsequent effect on the 2013 MW6.6 Lushan earthquake and the 2017 MW6.5 Jiuzhaigou earthquake. The calculation results show that the Coulomb stress changes caused by the Wenchuan earthquake is positive near the hypocenters of the Lushan earthquake (0.013 MPa) and the Jiuzhaigou earthquake (0.009 MPa), indicating the Wenchuan earthquake might have triggered both the Lushan earthquake and Jiuzhaigou earthquake.
Using historical aerial images to accurately locate the urban "invisible" active faults: A case study of the Shuiyu fault of the Datong Basin in Shanxi province
GUO Fei, REN Junjie, GUO Hui, SU Qiang, REN Jianguo, YAN Xiaobing
2021, 27(2): 254-266. doi: 10.12090/j.issn.1006-6616.2021.27.02.024
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Active fault is a major risk source of urban earthquake disaster. Accurately identifying the spatial geometry distribution features of active fault is the basis for urban seismic risk migration. However, due to the large-scale and deep urban renewal by human activities and urbanization, the traces of active faults are obscure and invisible on the surface, which makes it hard to identify the surface geometry of this kind of active faults. Though the northern Shuiyu fault in the central part of the Datong Basin is bounded by the eastern edge of the Mapu Mountain and characterized by linear displaced landforms, the southern section of the Shuiyu fault crossing the Yudong District of Datong City is covered by dense buildings and roads, becoming an "invisible" fault. In this study, based on the 1965 historical aerial photos of the Datong region in the 1960's and the 1∶10000 topographic maps, we reconstructed the original DEM and DOM of this region using the aerial photo stereopair and stereoscopic photography. These original data revealed the geometry distribution characteristics of the Shuiyu fault. Our results show that the piedmont fault scarps are marked by prominent linear features along the northern Shuiyu fault. We can accurately locate the fault by the images. Based on the previous Keyhole satellite images, DOM and DEM data, the geometric distribution of the fault in the southern Shuiyu fault can be accurately determined. Topographic profiles extracted from the original DEM show that the vertical offset on terrace T3 along the Shuiyu fault is about 19 meters. On this basis, the fault natural exposures and shallow seismic reflection data demonstrate that it is feasible to accurately locate the "invisible" active fault in the city by using historical aerial photos and stereoscopic photography. This study provides not only an important basis for the seismic hazard assessment in the Datong region but a useful technique for the detection of "invisible" active faults beneath a city.
Quaternary activity of the Luodian-Zhoupu buried fault in the Shanghai region: Integrated exploration and research
ZHANG Hao, SHI Gang, WU Hong, SHAO Lei, SONG Chunhua, YU Fei
2021, 27(2): 267-279. doi: 10.12090/j.issn.1006-6616.2021.27.02.025
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The Luodian-Zhoupu fault is an important NW-trending buried active fault that runs through the downtown area of Shanghai City. Its Quaternary activity significantly affects the urban geological safety. We applied a set of integrated geophysical methods to carry out high-precision gravity, shallow seismic and controlled source audio magneto telluric (CSAMT) explorations in the study area, and studied the distribution pattern and activity level of the Luodian-Zhoupu fault, combining with joint borehole profile and optically stimulated luminescence chronology. The exploration results reveal good ductility of the fault in southern Shanghai, and the fault trace is well reflected in the profiles of gravity, CSAMT and shallow artificial seismicity. Gravity area measurements show that the fault appears as a group of nearly parallel fault tracks in southern Shanghai. The buried depths of the shallowest breakpoints researched by the shallow seismic exploration and joint borehole profile method are in good agreement (~200 m). TCSAMT profile displays obvious deep fault features. The research outcomes from the profiles with geophysical exploration and joint borehole indicate no obvious trace of the fault breaking into the overlying Middle Pleistocene, and it is believed that the latest active age of the fault is the Early Pleistocene referring to the result of optically stimulated luminescence dating. Since the Luodian-Zhoupu fault is closely related to regional historical seismic events, and the regional stress state is currently NW-compressive, with the fault prone to tensile and torsional activities, its activity and seismic risk characteristics require continuous attention.
Spatial characteristics of quantitative geomorphic indices in the Taihang Mountains, north China: Implications for tectonic geomorphology
GUAN Xue, PANG Lichen, JIANG Yutong, LYU Honghua, ZHENG Xiangmin
2021, 27(2): 280-293. doi: 10.12090/j.issn.1006-6616.2021.27.02.026
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Tectonic geomorphology focuses on the coupling relationship between tectonics and surface processes, as well as their influence on topographic evolution. Spatial analysis techniques based on digital elevation model (DEM) and geographic information system (GIS) have gradually become one of the quantitative methods of tectonic geomorphologic study. The Taihang Mountains is located at the boundary of China's second and third topographic steps, and has important tectonics and geomorphological significance. However, at present, there are still few studies on the relationship between topographic evolution status of the Taihang Mountains and geological tectonics. Based on the ASTER GDEM data (30 m×30 m) and GIS spatial analysis method, we selected the elevation, slope, hypsometric integral, stream length-gradient index SL and Hack Profile of river slopes in the mainstream of the Taihang Mountains for analysis. Combining with the lithology and tectonics, we discussed the regional landform evolution status of the Taihang Mountains. The results show that, among the 11 rivers analyzed, the HC of 7 rivers is S-shaped, the HI between 0.35 and 0.60, indicating that the geomorphy is in the prime of evolution, while that of the other 4 rivers is concave, HI less than 0.35, indicating strong erosion and destruction effect in the old age. The Hack profiles of 7 rivers are up-convex with relatively high K value, demonstrating the relatively active regional neotectonic activity, while that of the other 4 rivers are approximately straight with low K value, demonstrating the weak regional neotectonic activity. On the whole, the average HI of the Taihang Mountains is 0.36, and the HC is nearly concave S-shaped. The geomorphological evolution of this area is generally in the transitional stage from the "prime age" to the "old age". The neotectonic activity (fault activity) of the Taihang Mountains shows difference in space, with strong activity in the east and relatively weak activity in the west.
River capture and divide migration of the Zhuozishan area in the northwestern margin of the Ordos Block
LIN Lingling, LI Xuemei, ZHANG Huiping, MA Zifa
2021, 27(2): 294-303. doi: 10.12090/j.issn.1006-6616.2021.27.02.027
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Divide is a dynamic feature of a landscape that routinely migrates, either through progressive or discrete river capture, in some cases even leading to the complete reorganization of river networks. Most of the existing geomorphological or drainage evolution studies focus on the single river capture, and few report on the overall movement and the geomorphologic adjustment of divide.Recently, it was proposed in a digital study of fluvial geomorphology that the chi (χ) value difference on both sides of the drainage divide could be used to explain the large-scale river capture and demonstrate the reorganization of drainage when describing the dynamic migration process of divide. In this paper, the chi (χ) values were calculated using the 12.5 m DEM data. The chi (χ) values were higher in the east and lower in the west, indicating an eastward migration. The lithological erosion resistance is the main factor controlling the eastward migration of the divides in Zhuozishan under the condition that there are no obvious differences in the tectonic uplift and precipitation conditions on the east and west sides. When the Cambrian and Ordovician limestones with stronger erosion resistance lie in the west wing of the Zhuozishan anticline, the other weak sedimentary clastic rocks located in its lower part, the river in the west wing would be through the core of the anticline, laterally capturing the rivers on the east of the anticline.
Study on borehole provenance tracing and fluvial sediment diffusion in the Bohai Sea: Double constraints from detrital zircon U-Pb age and in-situ geochemical element of apatite grains
LIN Xu, LIU Jing, WU Zhonghai, LI Changan, LIU Haijin
2021, 27(2): 304-316. doi: 10.12090/j.issn.1006-6616.2021.27.02.028
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Bohai Bay Basin, located in the North China Craton, are surrounded by the Yanshan Mountains to the north, the Taihang Mountains to the west, the Jiaodong and Liaodong peninsulas to the south and east, respectively. The Yellow River, combined with the Liaohe, Luanhe and Haihe rivers, produces a huge amount of detrital sediments to the Bohai Bay Basin each year. However, it has not been clear whether the sediments have been transported to the Bohai Sea and the gulf of Jiaodong Peninsula. In addition, there is no definite result suggesting that these sediments are related to the provenance in the gulf of Jiaodong Peninsula. Zircon and apatite are common accessory minerals in the river sediments. The zircon U-Pb age and the in-situ geochemical data of apatite grains show significant differences in different regions, making them the ideal minerals for the provenance study. In this case, we used the published detrital zircon U-Pb ages from the major inflow rivers of the Bohai Bay Basin, combining with the drilling cores zircon U-Pb ages and integrating with the multi-dimensional identification (MDS) of kolmogorov-smirnov statistical method, to systematically identify the potential source areas in the Liaodong Bay, Bohai Central Basin and Laizhou Bay. The results show that the detrital sediments in the Liaodong Bay mainly came from the Liaohe River, while that in the Bohai Central Basin and Laizhou Bay from the Yellow River. The Yellow River plays a major role in the material composition of the central and western parts of the Bohai Sea. At the same time, we carried out the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on the detrital apatite grains from the lower reaches of the Yellow River (n=70), and the Weihai Bay (n=120) and Yintan Bay (n=60) from the Jiaodong Peninsula. The results show that there is no provenance relationship between them. Combined with the in-situ geochemical analysis results of detrital K-feldspar in this area, it further indicates that there is no provenance relationship between the Yellow River and the gulf of Jiaodong Peninsula. Therefore, the combination of in-situ geochemical analysis of detrital zircon and apatite helps to accurately determine the provenance relationship.
Using EH4 audio-magnetotelluric sounder to detect the gigantic Qiaojia paleo-landslide and its structural characteristics
LI Zhong, WU Zhonghai, WANG Jinming, ZHANG Xiaobing, FENG Zhen, HU Mengmeng
2021, 27(2): 317-325. doi: 10.12090/j.issn.1006-6616.2021.27.02.029
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In this study, we used the EH4 audio-magnetotelluric sounding, combining with the drilling data, to detect and define the spatial electrical properties and structural characteristics of the Qiaojia paleo-landslide. Based on the detection results, the average depth of the main sliding surface is considered to be 250 m and six faults (F1-F6) were identified, among which the Xiaojiang fault zone (F1) is the main active fault. Meanwhile, it is inferred from the sounding results and drill date that the terrace along the Jinsha River fall into four layers from the surface (Qh) to the deep, namely the sandy clay layer, gravel layer, broken limestone layer, and complete limestone bedrock. The broken limestone layer and the complete limestone bedrock are in the bedrock outcrop area from the surface to the deep. The comprehensive analysis shows that the paleo-landslide is temporarily stable at present, but along with large earthquake activity in the north section of Xiaojiang fault zone in the future, whether the paleo-landslide is likely to revive or similar paleo-landslides will reoccur in this region needs further study. Our study proved the effectiveness of the EH4 audio-magnetotelluric method in detecting gigantic paleo-landslide.
Geological environment changes during the late Pleistocene-Holocene on the E'mei tableland in the northern Yuncheng basin, Shanxi Province: Implications for the distribution of human settlements
QIU Duwei, GONG Wangbin, YAN Jiyuan, ZHAO Yuanfang
2021, 27(2): 326-338. doi: 10.12090/j.issn.1006-6616.2021.27.02.030
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Neotectonic movements and the changing natural environment have profoundly influenced human settlements on their formation and distribution. The Gushan uplift, Niandi low-lying area and Sanguan highland developed from north to south in the central section of the E'mei tableland; however, human settlements nowadays distribute on the peripheral slopes around the low-lying area rather than right in the Niandi low-lying area. Our field investigation reveals that the Malan loess is overlied by a set of alluvial-diluvial deposits, lacustrine deposits, and fluvial deposits. This paper mainly presents field evidence and chronological analyses of OSL and 14C for the typical sections of different geomorphic sites to discuss the geological environment evolution of the E'mei tableland during the late Pleistocene-Holocene and its influence on the migration of human settlements. Our field investigation and analysis results show that the formation age of the bottom of the late Pleistocene lacustrine deposits above the loess in the Niandi low-lying area is about 17 ka B.P., indicating a depression had developed in the middle section of the E'mei tableland during this period with water converging in, forming a lake afterwards. Combined with the regional structural data, it is speculated that the formation of this depression was caused by the fault activity on the north side of the Sanguan highland. The location of 5000-year-old ruins of Jingcun Village and Yuanjiazhuang Village shows that the lake may have expanded to the piedmont of Gushan in the middle Holocene. Along with increasing dryness of climate and the shrinkage of lake, human settlements kept migrating to the low-lying lands. Nowadays, villages around the periphery of the Niandi low-lying area carries on the distribution pattern of living by water in the middle-late Holocene, which indicates that the distribution and evolution of human settlements are closely connected with the changes of natural environment.