Evolution Of Debris Flow In Yuanmou Hailuo And Its Tectonic Significance

Yuanmou County is located in the northwest of Yunnan Province.The Hailuo Daqing in the study area is located in Dongshan mountain,Yuanmou County.It belongs to the middle-high mountain landform with deep valleys and steep slopes.There are more than 20 debris flow gullies along the Dongshan Mountain.Hailuo Daqing is one of the typical debris flow ditches.Previous studies on the effect evaluation of debris flow control projects along Dongshan have been carried out,but no corresponding studies have been carried out on the evolution of debris flow gullies,the characteristics of deposits and the relationship between debris flow gullies and new structures.Therefore,it is necessary to systematically study debris flow along Dongshan,so as to provide a reference for the study of debris flow along Dongshan.On the basis of previous studies,this paper further analyses the formation conditions of debris flow,the characteristics of debris flow deposits,the evolution stages of debris flow and its relationship with neotectonic movement.Through field investigations,particle experiments,spore and pollen experiments and chronological studies,the comprehensive evaluation of debris flow evolution has been made.The main achievements are as follows:(1)Based on the study of the source area,circulation area and deposits area of Hailuo Daqing,it is considered that the deposits of Hailuo Daqing gully mouth and terraces are paleodebris flow.According to their present distribution,the deposits can be divided into stages I to V.The main sources of the debris flow are upstream collapses,landslides and erosion materials of early deposits.The gully is a debris flow gully with many outbreaks.Modern valleys have the conditions to develop large-scale debris flow.The Lately debris flow outbreaked in September 2016.(2)According to the analysis of debris flow accumulation particles,it is found that the change of the inhomogeneity coefficient and the curvature coefficient of the particles indicates that the debris flow accumulation has a poor overall sorting property.The average separation coefficient was 2.39.The skewness changes from positive skewness to negative skewness,indicating that the composition of debris flow material changes from early medium coarse particles to late fine particles.Kurtosis type were 3 peakwise narrow(>1.11),1 medium(0.91 ~ 1.11),4 wide and very wide(<0.90).The overall sedimentary environment is turbidity facies.(3)Through spore and pollen study,the author thinks that the pollen zone in the study area is divided into two zones: V1 and V2.Pinus,Tsuga-Gramineae and Betula are the main combinations in V1 zone,Pinus and Tsuga are the main representatives in the top of V1 zone,indicating that the overall temperature was low at that time,which is consistent with the results of chronological study.Gramineae,Betula and other Gramineae and herbaceous plants are the main ones at the bottom of V1 zone,revealing farming and cultivation.Activities have occurred here,and a small amount of medium carbon debris also indicates the local burning of crops.In this zone,the climate has gradually changed from early cold to late warm.In V2 zone,Pinus-Gramineae and Cannabis assemblages are dominant,and the emergence of drought-tolerant plants such as Cannabis indicates that the climate in the study area has changed from wet and cold to dry environment in 2200.The carbon debris in the zone is mainly micro-debris,which indicates that there are large fires in and around the zone in the past 100 years.(4)The hronology studies show that the development time of debris flow in the late stage of stage I is 27.7+1.5Ka B.P.,in the early stage of stage II and in the third stage is 27.7+1.5~2.2+0.1KaB.P.,and in the late stage is 2.2+0.1Ka B.P.The development time of the later sub-debris flow is 2.2+0.1 Ka B.P.This period is located in the last glacial maximum stage and MIS2 stage of the Quaternary glacial period.Comprehensive analysis shows that there are no Quaternary glacial relics in Yuanmou area.Based on the analysis of the controlling factors of debris flow genesis,the chronology of debris flow and the characteristics of debris flow deposits,it is concluded that the development and evolution of Hailuo Daqing paleodebris flow in Hailuo is closely related to neotectonic movement,and is also a long-range response to the uplift and Southeast expansion of the Qinghai-Tibet Plateau.This paper can provide a reference for the evolution of Dongshan series debris flow and provide a theoretical basis for debris flow prevention and control.