| Quantitative information that is used to infer climate changes and tectonic movement was derived from the Quaternary sedimentary records and surface topography along the Chianti Anning River, to demonstrate a long-term active characteristic of the Anning River basin and boundary faults of the Sichuan-Yunnan rhombic block, which may provide some basic data for strong earthquake prediction and the study of dynamic process of the boundary faults.The Anning river valley is the second valley plain only inferior to the Chengdu plain in Sichuan, It is situated in the world-famous Panxi big rift valley and the complex geologic structure and the special climate characteristic. Meanwhile it is a rift valley where the Anning River fault zone runs through. Predecessors have done much hard work and have made great achievements in this area, particularly in the Quaternary geology, active tectonics, and seismic activity characteristics. Since the late Quaternary the fault activity is strong and there is much elaboration on the active tectonic characteristics of the Anning River valley in western Sichuan. One of the most important research results is the quantitative data of the Anning River fault zone in Late Quaternary, while there have been some meaningful data such as the height and the age of the terrace and platform in the Anning River valley, which has laid a good foundation for the research of tectonic activity and evolution of this area since the Late Quaternary.However, the research on stratigraphy and geomorphic sequence since the late Quaternary has not yet been comprehensive and systematic in the area, It lacks knowledge of the sediments and the formation of micro-topography, which inevitably leads to the deviation of understanding displacement and slip rate data and other quantitative data of the boundary faults with different stages since the late Quaternary, Therefore, it has further affected understanding the tectonic evolution of this area.Active tectonics research is based primarily on the times of geomorphic surfaces and the displacement of the tectonic activity determined by the landscape of the field micro-topography of structure, using the dislocation time of surface topography to estimate the rate of displacement (including slip rate, uplift rate, etc.). It is clear that this technical approach is feasible in the absence of other influence factors. But in fact, because the Quaternary stratum distribution is unstable, the lothological change is enormous; the valley edge is a steep pitch and the determination of the absolute age of deposit formations is difficult and inaccurate, It is very hard to identify an obvious lithologic maker unit on the both sides of fault zone, at the same time the current dating techniques are also difficult to accurately access the formation age of geomorphic surfaces, such as the terrace, platforms and alluvial fans, all those factors lead to the results of much uncertainty. Existing research results show that different people in the same area have a large deviation in results, which indicates this method in the study of the rate of tectonic activity has some uncertainties.For a further understanding of fault activity and the terrace formation in the Anning River valley in the Late Quaternary, this work takes the late Quaternary Period stratum and river terrace’s research as a breakthrough point, this purpose is to correctly understand overall evolution characteristic of the Anning River area and the faults in Quaternary time, the Quaternary climate change characteristic and the major affect factors of the fault movement, focusing on analysis of regional climate change and the causes of river terraces since late Pleistocene. After a thorough evaluation of previous studies, this work establishes a unified landform stratum sequence of this area,analyzes the origin and the time of the Quaternary landform unit and builds the foundation of the quota parameters for the Anning River fault。At last the reasonable fault movement parameters are derived.On the basis of previous work, with 973 research projects, field observations, and laboratory sample analyses, this thesis attempts to address much better the above-mentioned questions of the Anning River valley by means of multi-disciplinary methods including structural geology, Quaternary geology, geomorphology, stratigraphy, sedimentology, paleontology, pale-climatology, and geo-chronology. The following conclusions are derived:⑴38kaB.P～21ka.B.P, the terrace of III is formed. During terrace development, the regional vegetation was dominated by pine forest. This indicates a temperate wet or slightly wet climate; The Guliya ice core reflected the Qinghai-Tibet Plain MIS3 section warm degree has reached the interglacial period the degree.⑵21kaB.P～7.2ka.B.P.The terrace of II is formed. During terrace development, the pollen records indicate that temperature and humidity in the deposition period were rarer than in the first terrace and in the last interglacial period.⑶7.2ka.B.P to present is the post-glacial period, since the formation of terrace I, A change to coniferous forest occurred and was dominant during terrace deposition, indicating a wet or slightly wet climate.1) The comprehensive study of the valley terraces dating and pollen analysis and recovery pale climatic and pale environmental since the late PleistoceneThe pollen samples and dating samples were collected on the terraces I-III section of the Anning River and its tributaries Maniao River I-II terrace profile in the Mianning Lugu township.the research shows that: during the deposition of these terraces, both can be seen with a high degree of pine and fir tree needles and other gymnosperms can be seen located in different terrains. There are birch, oak, walnut and elm and other broad-leaved angiosperms, There are Artemisia, Chenopodium, Poaceae and Ephedra and other shrubs and herbaceous plants, indicating that these bands to profile in the vicinity of and around the vegetation, also similar to the present with obvious zonation in vertical direction.The plant spores and pollen are more desirable to the recovery of organic indicators of environmental change. It had great significance using palynological indicators to restore the Anning River since the late Pleistocene climate evolution of the environment for the Quaternary stratigraphic subdivision and regional paleoclimate contrast. Based on detailed investigation, the Anning river terraces are divided. The results show that I-class terraces of Anning river bands formed during the Holocene (Q4), when climate belonged to wet or light wet. I-class terraces deposited during the Pleistocene (Q3) or the early Holocene, when temperature and humidity were lower than I–class terraces at that time, and meanwhile vegetation and climate occurred more than two times with obvious changes and fluctuations during the course of sedimentary terraces. III-class terraces deposited during the Late Pleistocene when climate was wet or light wet. At the same time, the terrace accumulated when temperature and humidity were higher than II-class, yet lower than I-class terraces.2) Structural features and formation ages of terraces I–III in the Anning River ValleyThe Anning River valley has 5–6 terraces of different ages. Terrace I is an aggradation terrace, and its height is 7.9-9 m. Terrace II is a fill-cut terrace near Lugu County, with a height of 30.3-38.8 m. In most other areas this is an accumulation-fill terrace. The base is visible at some localities, with clay and fine sand lithology. Terrace III is the base terrace, comprised of early Pleistocene latchstring deposits. The height of the terrace is 81.3-96.8 m; the profile can be divided into three layers: the upper layer is alluvial flats. The middle layer is river bed accumulations derived from ice accretion with low psephicity. The layers are interbedded with fine-grained and coarse-grained layers, which represent unstable hydrodynamic conditions in the early accumulation period, and the alternating layers reflect the climate changes. The lower layer is the Xigeda group’s lacustrine, which is the terrace base. Based on research on river terraces in Central Europe, Starkel identified four lines of evidence that terraces were formed by climate processes:(1) elevation differences between the various terraces should be smaller,(2) Accumulation of gravel layers, usually with several dual structures, (3) Mixed alluvium and gravel layers, and (4) the bottom of the terraces is often low-angle slopes. Anning River terraces do not have these characteristics, as seen in the field investigation, suggesting that climate fluctuations are not the main cause of terrace formation. According to this comprehensive analysis, terrace I of the Anning River was formed between 4.1–10.3 ka, terrace II was formed 16–26 ka, and terrace III was formed 30–46 ka. Terrace IV was formed 550kaB.P.3) Preliminary study on causes of Anning River valley terrace formationAccording to research, it can be concluded that formation of the Anning River terraces is related to both tectonic uplift and the climatic fluctuations. During the glacial period, the resistance of sediment transported into the river system combined with decreasing stream power might exceed the driving energy produced by increasing river gradient. This was caused by tectonic uplift and finally resulted in river accumulation. In the transition glacial to interglacial, river sediment accumulation is reduced because of increasing runoff and stream power. The effect of tectonic uplift is manifested only when the driving energy surpasses the resistance energy, and then the river channels incise. For example, terrace II accumulated during the Last Glacial Maximum. In the transition from glacial to interglacial, the increase in stream power resulted in river incision to form terraces I and II. The terrace structure also suggests that the influence of tectonic movement on terrace III (base terrace) was larger than that when terrace I and II were formed.4) Establishment of the sequence of the landscape or stratigraphy and the evolution of geomorphology, tectonics, climate since late Quaternary in Anning River ValleyAccording the field investigation, the samples dating and regional contrast, the sequence of the landscape or stratigraphy was established in this area, as basic data to Quantitative research of the active tectonics. At last, the evolution of geomorphology, tectonics, and climate since late Quaternary in the Anning River Valley is:5) Quantitative research of kinematic parameters of the Anning River faults since late QuaternarySince 20~25 ka.B.P, on the north section of the Anning river fault: the ratio of the vertical slip rate to horizontal slip rate is 4:1, the average of vertical slip rate is 0.9~1.4mm/a and the horizontal slip rate is 3.8~4.2mm/a.The transition section of the Anning River fault is in the northwest of the Mianning County. There are two branch faults and the east branch is in the Dahaizi-Ganhaizi vallge which produced the Xichang earthquake of Ms7.5 in 1536 according to the research of palaeo-quakes.On the south section of the Anning River fault zone; the vertical slip is 1.8~0.73 mm/a, the horizontal slip rate is 5-7mm/a and the uplift rate is 1.9 mm/a. The ratio of the horizontal slip rate to vertical slip rate is 3:1. This result shows that the slip rate is different in the results of the study on fault miro-landscape, because the uplift rate is determined using the overall uplift of the river terraces of the eastern plate of the fault, not the landscape of a small amount of dislocation cell. The quantitative calculation results show the fact of east wall uplift of the Anning River fault, especially in the southern section. The different vertical slip rates and slip ratios existing between the north and south sections also imply difference in the movement feature and forms. |
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