Impact Of Tectonic Activities And Climate Change On The Lacustrine Sediments In The Eastern Tibet During The Last Deglaciation

The eastern margin of the Tibet Plateau is the region with the strongest tectonic deformation and the most frequent seismic activity in China.Since the Ms 8.0Wenchuan Earthquake in 2008,a series of magnitude-7 earthquakes have caused serious casualties and economic losses in China.Most people live in river valleys and intermontane basins in the upper reaches of Min River,resulting in high local population density.Therefore,the geological disasters,such as earthquakes,landslids,debris flows,are more likely to cause serious casualties and economic losses.All of these make the upper Min River area become the current research hotspot.Lacustrine deposits in the region provide archives for revealing regional climate evolution and tectonic activities.Current studies mainly focus on the use of particle size,susceptibility,soft-sediment deformation structures and other indicators to reveal palaeoseismic history,but there are few records of climate changes.Since the last deglaciation period,the climate or tectonic information represented by the high-frequency fluctuated records of particle size and susceptibility indexes in lacustrine sediments and the main control mechanism behind them are still unclear.Furthermore,lacustrine deposits in the upper reaches of the Min River are now mostly distributed on both sides of the river.Is the provenance of lacustrine deposits supplied by the river?If yes,how much is it?The main contents of this thesis included the following three aspects:?1?Analysis of particle size and susceptibility of Min River sediments.On one hand,the influence of tectonic and climatic factors on river provenance supply is analyzed,and the response of river sediments to fault activity is discussed.On the other hand,the contribution of fluvial processes to lacustrine deposition is calculated quantitatively.?2?Through deep interpretation of the palynological records of the lake sedimentary section of Xinmocun,the sensitive indicators of regional climate change could be separated out,and regional climate evolution during the last deglaciation period was revealed.?3?Through detailed analysis of particle size,magnetic susceptibility,TIC,TOC,Lightness?L*?,p H and clay minerals of Diexi lacustrine deposits,the palaeoseismic events were revealed and the main control mechanism of high-frequency fluctuations of these records was discussed.Paleoseismic periods were calculated based on ¹⁴C and OSL ages.The preliminary results are as follows:1.The grain size distribution in the upper reaches of Min River is obviously segmented.Among them,Minjiangyuan-Songpan-Diexi segment mainly consists of fine particles background dust?2-20?m,52.9%-57.3%?and proximal dust?20-63?m,23.0%?.The significant increase in the local coarse particles?125-500?m,55.1%-63.3%?in the Diexi-Wenchuan-Dujiangyan segment mainly reflects the strong tectonic activity of the Maoxian-Wenchuan fault,while the activity of the Minjiang Fault is weak.The contribution of fluvial processes to lacustrine deposition is far less than 19.7%according to particle sizes,C-M diagrams,and End member modelling results.2.During the last deglaciation period?18.7-10.6 Ka?,the palynological records of lacustrine sediments in Xinmocun showed an open forest grassland vegetation landscape dominated by shrubs and herbs.The changes in the content of hygrophilous pollens,such as Cyperaceae,Poaceae,Polygonum,Typha and Myriophyllum,can be used as sensitive indicators of climate change.According to the change of hygrophilous pollen content,it can be divided into five stages:18.7-16.8 ka:The climate is getting wet;16.8-14.6 ka:The climate remains wet;14.6-12.9 ka:The wettest climatic conditions;12.9-11.6 ka:The climate is remarkably dry;11.6-10.6 ka:The climate got wet again and transitioned into the Holocene.In addition,through comparison of moist pollen content and global oxygen isotopes data,it was found that,during the B/A warm period?14.6-12.9 ka?,the increase of the longitudinal temperature gradient in the Northern Hemisphere drives the Indian monsoon to shrink southward,that leads to a gradual decrease of moist pollen in the Xinmocun section.3.The results of ¹⁴C and OSL ages showed that the lacustrine deposition of Diexi Museum was formed in the last deglaciation period?17.62-14.55ka?,with an average deposition rate of 6.5mm/a,and a sample resolution of 1.54 years.The sediments of Diexi section are characterized by aeolian sources based on the particle sizes,C-M diagrams,and End member modelling results.According to the paleoseismic identification mark of sudden increase and slow decrease of grain size,20environmental events that may be related to ancient earthquakes have been identified in the Diexi section.were recorded in this profile.In addition,three layers of soft sediment deformation structures may correspond to three paleoseismic events in Diexi lacustrine.4.The analysis of carbon content,lightness,clay minerals and p H value of the Diexi lacustrine deposition shows that,the high-frequency fluctuated records of particle size are caused by the frequent fluctuation of provenance caused by seismic activity,while the climate plays a minor role.In addition,component 20-63?m recorded three cycles of 26,53 and 161 years,first two cycles may correspond to medium-strong earthquakes,and the third cycle to large earthquakes.