The terrain in southeastern Tibet is steep and the valleys are crisscrossed. Since the Quaternary, glacial ice and debris have blocked the course of the Yarlung Tsangpo River and its tributary river valleys to form giant dammed lakes, and the huge flood deposits formed by the dammed lake outburst floods are often associated with moraines, ice water deposits, lacustrine deposits, aeolian sand or other running water sediments to form complex river valley accumulation landforms. Different types of sediments in alpine and canyon areas are similar in morphology, structure and fabric, and are difficult to distinguish. Grain size and morphological characteristics are the most important structural characteristics of sediment, and the distribution rules are controlled by many factors such as sedimentary environment, physical properties of detrital material, transporting medium and transporting mode, etc., which is an important proxy index for restoring paleoclimate and inverting paleoenvironment. However, the relevant research on identifying sediment types in alpine valley area of southeast Tibet by grain size and morphology index is still in the exploratory stage. In order to understand the particle size characteristics and spatial differentiation laws of outburst flood sediments and the micromorphological characteristics of particle surfaces, we collected 33 samples of Holocene flood retention sediments preserved along the river within about 350km from the outlet of the Jiacha Gorge in the middle reaches of the Yarlung Tsangpo River to Pai Town, and measured them with Malvern 3000 laser diffraction particle size meter and Zeiss Signma scanning electron microscope, combined with digital geomorphology(DEM)data extracted river channel width and steepness coefficient. The features of spatial distribution law of particle size are analyzed, and the following understanding is obtained. The particle size of outburst flood retention deposits is characterized on the whole by fine-silty sand(2.57~5.18Φ)with poor sorting, positive skew and narrow peak state. Two end element models are obtained: The main peak of EM1 terminal element is 3.16Φ, with an average percentage content of 42.7%, which may represent the alluvial characteristics of higher energy of outburst floods in alpine valley areas, and the main peak of EM2 terminal elements is 2.06Φ with an average percentage content of 55.6%, which can be used to indicate the accumulation process of the outburst flood lag deposits. Affected by the width of the river, the EM1 content has a tendency to increase downstream, while EM2 has the opposite trend. The surface microstructure of quartz particles in the outburst flood lag deposits is mainly characterized by mechanical scratches, shell-like fractures, upturn cleavage and cleavage steps, with low structural maturity, mostly angular shape, and rare denudation pores of chemical origin. As a typical representative of climbing sand dunes in the valley area of the semi-humid monsoon area, the genesis of the dunes is of great guiding significance for revealing the source of sand dunes in the valley area of the alpine valley area, identifying paleoflood deposit and aeolian deposit, distinguishing aeolian deposit and paleoflood slackwater deposits on both sides of the riverbank, and windbreak and sand fixation engineering in the Yarlung Tsangpo River. By comparing the particle size and surface micromorphology characteristics of the known outburst flood deposits of the Yarlung Tsangpo River, we believe that the sand source of the Fozhang dunes is mainly from the outburst flood deposits and was transformed later by wind forces.

The Xijiang Fault is an important NW-trending fault with a length of~200km, located in the western part of the Pearl River Delta. A M4 3/4 earthquake occurred at the northern end of the fault(Sihui)in 1445 and a magnitude 5 earthquake occurred at the southern end of the fault(Modaomen Waters)in 1905. Heshan is the boundary between the southern and the northern segments of this fault. The southern segment which is called Heshan-Modaomen segment is mainly hidden faults. The activity of Heshan-Modaomen segment remains controversial due to the lack of systematic studies for the deep and shallow exploration, which affects the assessment and prevention of earthquake disaster risk. In this paper, we concentrate particularly on the distribution and activity of Heshan-Modaomen segment using seismic geological surveys, shallow seismic exploration, joint borehole profile detection, and Quaternary geochronology.
Field geological surveys show that the fault zone is prominently normal sinistral strike-slip faults, striking about N310°~330°W, with a width of 10~20m. Most of them dip northeast at angles of 60°~80°. Observations on typical outcrop show that cataclasite, breccias and siliceous rocks are developed on the faults. Fault planes often have smooth and polished surfaces and no fault geomorphology has been developed along the fault zone. The overlying eluvial weathered soil materials have not been disturbed or cut. We carried out shallow cross-fault sounding of 7 profiles in the hidden section of the fault zone using longitudinal wave reflection method of multifold coverage observation system. As a result, we obtained the reflection time sections of the target stratum and the main structure. A total of 13 breaking points to be investigated were explained. We also performed cross-fault drilling at the location of the seismic data interpretation profile and catalogued drilling cores. ¹⁴C and OSL samples were collected systematically. The ¹⁴C dating was performed by the BETA Laboratory in the United States and 16 valid age data were obtained. OSL dating was performed by the OSL Laboratory of China University of Geosciences(Wuhan)and 6 age data were obtained.
This paper presents the study results of two representative cross-fault profiles. The shallow exploration survey line XJ1 and the row drill profile P1 are located in the southern section of the fault where six boreholes are arranged. We find the existence of bedrock faults on the joint borehole profile. The grooves developed thereupon are filled with the late Pleistocene paleochannel deposits with no obvious faults observed. The overlying Holocene strata are horizontal and continuous, without cutting and disturbance. Combined with the stratigraphic age, we infer that the fault has been inactive for at least about 11 000 years. The shallow exploration survey line XJ2 and row drill profile P3 are located in the northern section of the fault, where a total of seven boreholes are arranged. The borehole sections reveal the existence of fault crushed zone in the underlying bedrock(Cambrian hornstone). The tectonites are mainly fault breccias and cataclastic rocks with chlorite alteration. Groove landforms are formed along the fault zone with strong erosion at the later stage, and filling and accumulation occurred since the Holocene transgression with no fracture cutting or stratum disturbance. According to the landform, the occurrence of faults and the development of transverse active faults, the Heshan-Modaomen segment of Xijiang Fault can be further divided into two segments with the boundary of Zhupai Island. Both of them have been inactive since the Holocene.