A Preliminary Study Of Chronology And Seismic Event For The Lacustrine Sediments Atlixian In The Upper Reaches Of The Min River During The Late Pleistocene

The continuity of history of paleoearthquakes activity is the foundation and precondition for applying the research of active faults and paleosesmic studies to seismic hazard assessment. And how to ensure the continuity and integrity of paleoearthquake records remains the difficulty in this research field. Fortunately, well-preserved lacustrine sediments provide exceptionally important information of continental geological processes responding to tectonic and climatic influences, which is probably able to record a continuous history of paleoearthquakes. Such study opens a new perspective for extracting earthquake signals from lacustrine sediments. The upper reaches of the Min River run along the Longmen Shan fault zone with abrupt topographic change along the eastern margin of the Tibetan Plateau. This region is characterized by alpine valleys, active faults and frequent earthquakes. Because of the strong erosion and long-term man-made destruction, it is difficult to study the paleoearthquake sequence by trenches. Field investigations found a suite of well-exposed lacustrine sediments at Zagunao town, Lixian, in the upper reaches of the Minjiang River.Thesecontinuous lacustrine sediments are about 23 m thick, containing several types of deformational structures including liquefied convolute deformation, liquefied vein, crinkling, microfaults, flame structures, pseudonodules, ball-and-pillow structures that are interpreted as ancient seismic events. Therefore, this work combined microcosmic index including grain size, magnetic susceptibility and soft-sediment deformation structures to analyze these sediments, and using OSL dating to establish a chronological framework. The study objects includelate Pleistocene lacustrine sediments at Lixian County of the upper reaches Min River. The purpose is to determine the paleoearthquakes sequence along the upper reaches of Min River, revealing tectonic activity at the region and its surrounding areas. Furthermore, this work attempted to expand the use ofthe micro and macro indicators of seismic information identified and extracted in lauctrine sediments. The conclusions of this thesis are summarized below.(1) OSL dating indicates that the Lixian section of lacustrine sediments spans from 15.81 to 5.96 ka.(2) In total 28 layers of so ft-sediment deformation structures were identified, including liquefied convolute deformation, water-escape structures, flame structures, micro-faults, liquefied vein, crinkling, pseudonodules, ball structures and pillow structures. Based on the properties of deformation structures such as external morphological characters, size of deformation, spatial distribution, regional tectonic setting and lithology, the impact of fluvial, glacial and freeze-thaw action is excluded. Given that the study area is located in a tectonically active region, this study suggests thatthe most likely triggering mechanism of so ft-sediment deformation structures in the Lixian sectionis earthquakes.(3) Through the grain-size analysis, it is found that the grain-size distribution of Lixian County section has a highest-peak in the distribution curve of grain-size frequency, which is similar to those of the modern windblown sediment loess of the Fanjiaping section in Lanzhou.The Lixian lacustrine sediments can be divided into two groups:fine grains with typical peak values about 10-15 μm, and coarse grains with representative peak values at about 35-45 μm Accordingly, the Lixian lacustrine samples have similar features of grain-size distribution as those of typical loess from the Fanjiaping section in Lanzhou and Xinmocun section, indicating an eolian origin of the Lixian lacustrine sediments. The>20μm fraction was likely from the regional sources, and its high-resolution abundance varying with depth shows repeatedevents of abrupt coarsening and upward fining, likely indicating palaeoearthquake events. Large earthquakes in the study area have often caused widespread rockfalls and landslides, leading to exposure of fine-grained sediments that had been accumulating on mountain slopes and subsequent eolian transportation into the lake.(4) Changes in climate may also cause changes in grain size and SUS recorded within a section. To explore a possible climatic cause, this study compared the grain-size and SUS data with the stalagmite 5180 record of the Hulu Cave, Nanjing, the Guliya ice core δ18O record from the Tibetan Plateau. It was found that the data of this thesis have more frequent peaks than the climatic records. Therefore, it is concluded that the peaks in the data of this work cannot be attributed to regional or global climate change. This study identified 35 possible palaeoearthquakes with a mean recurrence interval of-0.28 ka. During 15.81-14.7 ka, there were 11 earthquakes with intervals of about 110 years.During 14.7-9.6 ka, there were 9 seismic events with intervals of about 570 years, and in 9.6-5.96 kal5 earthquakes might have occurred with an average interval of 240 years. Comparing with the Xinmocun section shows that 13 events are comparable within the age error range.(5) This thesis focuses on three aspects of regional rainfall, shape features of landslide slopes and the corresponding relationship between landslide areas.Compared with those in Nepal, both mean seasonal rainfall accumulation and mean daily rainfall for the past 30 years are too low to reach the threshold values triggering landslides in the upper reaches of the Min River. Secondly, landslides in the study area are usually absent of inner gorges (canyon topography) on the hillslope toes, which are confirmed in previous studies as typical features of landslides triggered by storms. Thirdly, wide distribution of the landslides in the study area supports the notion of earthquake-triggering because the landslides triggered by storms are commonly distributed locally. To sum up, this work suggests that tens of thousands of landslides in the upper reaches of the Min River were most likely triggered by earthquakes rather than storms. It also provides a theoretical support that lacustrine sediments in a tectonically activeregion have potential to record palaeoearthquake event. Thestudy area is characterized by alpine valleys, and frequent earthquakes. Earthquakesin the study area not only make fine-grained sediments become loose and easy to transport, but also trigger large number of new landslides providing large amount of dust for the study area. These grains were then transported to lakes by strong winds, showing the grain size coarsening. With the transportation of river and the restoration and rehabilitation of vegetation, the dust gradually reduced, and the particle size decreased slowly. Therefore, lacutrine sediments in a tectonically active region have the potential to continuously record earthquake history. At the same time, it also opens a new perspective for the study of the sources of Asian dust deposition.