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    2021, Volume 28 Issue 2
    25 March 2021
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     A review on the dating techniques for mountain hazards-induced sediments
    LAI Zhongping, YANG Anna, CONG Lu, LIU Weiming, WANG Hao
    2021, 28(2): 1-18. 
    DOI: 10.13745/j.esf.sf.2020.9.7

    Abstract ( 38 )   PDF (5741KB) ( 81 )  
    Chronology of mountain hazard events is the foundation for understanding hazard mechanisms and making hazard predictions. On the long timescale, the conventional dating methods include OSL, 14C, cosmogenic nuclides, tephrochronometry, etc.; on the short timescale, dendrochronology is normally used, supplemented by lichenometry. The selection of a specific technique depends on the availability of dating materials in the field. In order to enhance the reliability of dating results, cross-checking by different techniques is essential whenever dating materials are available. The field sampling should meet the age determination task for hazard events as well as the chronological requirements, thus researchers carrying out hazard and dating studies should collaborate routinely in the field during sample collection. We summarize here the routines and key points of related dating methods in order to provide some instructions for laboratory work and field trips.  In particular, we emphasize the role of OSL and 14C techniques in dating mountain hazard sediments, as they are the most widely used dating methods at present. We suggest replicating dating results for critical sediment layers as an effective and practical way to  assess the impact of OSL partial bleaching: If replication provides similar ages within error, the partial bleaching could be minor or negligible. OSL is ideal for dating dammed lake deposits which is the common recorder of mountain hazards. For 14C dating, the saturation age is mostly between 2535 ka BP regardless of the dating materials, indicating that age falling into this range or beyond should be taken with caution. 
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    Geochronological analysis of fossil landslides and improvement of dating accuracy
    BAI Shibiao, CUI Peng, GE Yonggang, WANG Hao
    2021, 28(2): 19-34. 
    DOI: 10.13745/j.esf.sf.2020.9.14

    Abstract ( 30 )   PDF (1882KB) ( 76 )  
    It is extremely useful to identify the ages and recurrence interval of giant fossil landslides for determining the conditions under which landslides occur and assessing landslide risk. It also helps to understand the Quaternary geological history of high mountainous areas. This paper reviews the current progress and achievements in landslide dating using geochronological methods. We introduce the principles and applicability of terrestrial cosmogenic nuclides (TCN), optically stimulated luminescence (OSL), and 14C, and discuss their advantages and limitations in landslide studies. We focus on the primary datable landslide materials and geomorphological features, such as the main scarp, sliding surface, slide shoulders, and landslide lakes. Furthermore, aiming at solving current problems in fossil landslide dating, some methods for improving dating accuracy are proposed.
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    A preliminary study on the development of dammed paleolakes in the Yarlung Tsangpo River basin, southeastern Tibet
    WANG Ping, WANG Huiying, HU Gang, QIN Jintang, LI Cuiping
    2021, 28(2): 35-45. 
    DOI: 10.13745/j.esf.sf.2020.9.18

    Abstract ( 20 )   PDF (13310KB) ( 249 )  
    Lacustrine sediments of dammed paleolakes are widely distributed along the valley of the Yarlung Tsangpo River basin across the southeastern Tibetan Plateau. The development of paleolakes is closely related to tectonics, climate change, and surface processes. Through extensive geological investigation, we identified more than 10 dammed paleolakes in the Yarlung Tsangpo River basin and, combined with previous research, established the preliminary stratigraphic chronological framework of these paleolakes through sedimentological, geomorphological and geochronological studies. The remaining lacustrine sediments occurred mostly in the Last Glacial Maximum and the early Holocene, lasting from several to ten thousand years. We summarize the research results on the Dazhuka, Gega and Yigong paleolakes with a focus on their basic developmental characteristics, and discuss the significance, current challenges and research directions for studying dammed paleolakes influenced by tectonics-climate-erosion interactions. We suggest that the extreme climatic change and natural disasters, triggered by the glacial (debris flow) river damming and outburst flooding events since the Last Glacial Period, have had an significant impact on the  Yarlung Tsangpo River landform and paleogeographic environment.
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    Research progress on landslides and dammed lakes in the upper reaches of the Yellow River, northeastern Tibetan Plateau.
    YIN Zhiqiang, WEI Gang, QIN Xiaoguang, LI Wenjuan, ZHAO Wuji
    2021, 28(2): 46-57. 
    DOI: 10.13745/j.esf.sf.2020.9.4

    Abstract ( 17 )   PDF (5777KB) ( 55 )  
    As ecological protection and high quality development  of the Yellow River Basin become part of a national strategy, prevention and control of landslide disasters in this region has become the fundamental problem that  needs to be resolved urgently. Large variation in terrain elevation has historically caused frequent earthquakes and heavy rainfalls, which resulted in a wide distribution of highly  hazardous ancient landslides and landslide-dammed lakes. In recent years, this region has become a hotspot for studying the development and evolutionary mechanisms of landslides and the effect of landslide-dammed lake outbursts. In this contribution, we summed up seven major research progress in the region based on a comprehensive review of our own study results over the past 20 years, including landslide surveys and evaluations, test analysis reports, and disaster prevention and mitigation outcomes, combined with published studies by other researchers. The areas of progress include investigation and risk assessment of ancient landslides, temporo-spatial distribution of landslides and its controlling factors, duration and damage of typical ancient landslide-dammed lakes, utilization of ancient landslide deposits, and landslide disaster prevention and mitigation. We also identified four scientific problems that merit additional attention in future researches on the ancient landslides, dammed-lake deposits, river terraces, and effect of dammed lake outburst in the region. This contribution provides a valuable reference for revealing the primary controlling factors of landslide development and dammed lake formation during the geological history of the upper Yellow River, exploring the dynamic mechanisms of landslide development and landslide response to earthquakes and rain falls, and expanding the role of Quaternary geology in applied research on the evolution of ancient landslides.
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    Influence of natural dams on the river profile of the Yalong River Basin
    LIU Weiming, ZHOU Liqin, CHEN Xiaoqing, ZHOU Zhen, Wolfgang SCHWANGHART, HU Xudong, LI Xuemei, ZHANG Xiaogang
    2021, 28(2): 58-70. 
    DOI: 10.13745/j.esf.sf.2020.9.1

    Abstract ( 15 )   PDF (10539KB) ( 126 )  
    Natural dams are known to impose pronounced and long-lasting perturbations on the river profiles in mountaineous areas. In some cases, the steepened profiles caused by natural dams are easily confused with the migrating knickpoints associated with relative base-level fall linked to tectonism. However, the degree to which natural dams may compromise geomorphometry-based tectonics has not been systematically investigated. In this research, we studied the Yalong River Basin on the eastern edge of the Tibetan Plateau. Our investigation of the relation between river dams and the river longitudinal profile relies on the interpretation and analysis of remote sensing imagery and digital elevation models (DEM). Through such analysis and field work, we identified 34 natural dams in the Yalong River Basin. We extracted the river longitudinal profile from 30 m SRTM DEM and calculated the locations of convex knickpoints, the channel steepness index, and other geomorphic parameters using TopoToolbox. Our analysis reveals  18 natural dams with locations coinciding with  knickpoints that are over 30 m high. Among them, eight have a significant influence on the river longitudinal profile as they can form knickpoints with heights exceeding 100 m. Moreover, we show that landslide dams have bigger influence on the river profile than mudslide dams. Further analysis of the Yalong River and its tributary Liqiu River shows that the river steepness index attains higher values at the dams. Excluding the influence of variable lithology and fault activity on the river profile, there are several knickpoints along the river that can still be attributed to dams. Therefore, we conclude that to accurately and reliably extract and analyze information on regional tectonics from DEMs, the potential occurrence of river knickpoints due to dams must be taken into consideration.
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    Recent research on the Diexi paleo-landslide: dam and lacustrine deposits upstream of the Minjiang River, Sichuan, China.
    FAN Xuanmei, DAI Lanxin, ZHONG Yujin, LI Jingjuan, WANG Lansheng
    2021, 28(2): 71-84. 
    DOI: 10.13745/j.esf.sf.2020.9.2

    Abstract ( 33 )   PDF (15493KB) ( 46 )  
    A paleo-lacustrine deposit, more than 200 m thick, named the “Diexi paleo-landslide-dammed lake deposit” was found upstream of the Minjiang River in Sichuan Province. The paleo-lake was formed around 30 ka BP and the dam started to fail around 15 ka BP. Therefore, the paleo-lacustrine deposit could have recorded important geological and geoenvironmental events (earthquakes, paleo-climate changes, etc.) during the period from the late Pleistocene to Holocene. The formation and evolution of the Diexi paleo-landslide and the landslide-dammed lake are poorly understood. Through detailed field investigation and LiDAR measurements, we constructed a 3D geological model and found the geomorphological and geological evidence to determine the boundary and volume of the paleo-landslide that formed the Diexi paleo-lake deposit. Using geophysical measurements (ERT) and laser grain size analysis, we determined the internal structure of the landslide deposit. We then reconstructed and analyzed the extent, volume, and sedimentological features of the paleo-lake based on a survey of outcrops and boreholes, and discussed the possible failure processes of the paleo-dam and its implications for the migration of prehistoric settlements. The results indicate that the Diexi paleo-landslide not only completely blocked the Minjiang River but also blocked its upstream branches, with the deposit volume reaching 1400×106-2000×106 m3. The paleo-lake extended upstream to about 26 km behind the dam; the maximum surface area and volume of the lake was approximately 21.4 km2 and 1670 × 106 m3, respectively. The paleo-landslide caused a knickpoint that may have a long-term impact on the mountain landscape evolution in the region.
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    The origin and evolution of the Temi paleolandslide-dammed lake in the upper Jinsha River
    CHEN Jian, CUI Zhijiu, CHEN Ruichen, ZHENG Xinxin
    2021, 28(2): 85-93. 
    DOI: 10.13745/j.esf.sf.2020.9.9

    Abstract ( 23 )   PDF (8065KB) ( 57 )  
    In the Batang-Zhongza segment of the upper Jinsha River on the southeastern margin of the Tibetan Plateau, major plaeolandslides were widely developed along either bank of the river, some had blocked the river flow to form dammed lakes. The large scale Temi paleolandslide-dammed lake is one of them, however, its origin and evolution have not yet been reported. In this study, we investigated the geomorphology and sedimentology of the Temi paleolandslide-dammed lake through field investigation, remote sensing interpretation and optical stimulated luminescence dating, and analyzed its origin and evolutionary processes. The study results indicated that the Teimi paleolandslide-dammed lake was caused by a major landslide blocking the Jinsha River, triggered by the intensive paleoearthquakes in this area.  The maximum lake area and volume were about 1.42×107 m2 and 1.46×109 m3, respectively. The formation time of this paleolake was ~1.8 ka BP, and the time of its breaching was ~1.4 ka BP, with a peak discharging of 55858 m3/s, implying that the Temi paleolandslide-dammed lake were stable for about 400 years.
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    Cognization, characteristics, age and evolution of the ancient landslides along the deep-cut valleys on the eastern Tibetan Plateau, China
    ZHANG Yongshuang, LIU Xiaoyi, WU Rui’an, GUO Changbao, REN Sanshao
    2021, 28(2): 94-105. 
    DOI: 10.13745/j.esf.sf.2020.9.10

    Abstract ( 23 )   PDF (7087KB) ( 90 )  
    The eastern Tibetan Plateau is a region with the most developed ancient landslides on earth. Based on ground surveys, remote sensing interpretation, and age dating, we summarize here the identification methods and the main developmental  characteristics, formation age, and distribution pattern of the ancient landslides on the eastern Tibetan Plateau. The ancient landslides have the characteristics of large scale, high starting point, and complex material composition and structure, and their spatial distribution is closely related to factors such as topography, lithology, and active structure. The ancient landslides are obviously affected by climate change in the region, and are generally formed at the stage of strong river undercuts. They have a good corresponding relationship with river terraces, with the formation ages mostly equivalent to the age of the T2 terrace. The age span is 40-10 ka, concentrated in 30-20 ka. The distribution patterns of the ancient landslides differ in different areas  because of tectonic activities and earthquakes as landslides generally developed intensely near active fault zones. This study will be helpful for understanding the evolution of ancient landslides and for predicting future catastrophe risk.
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    Observations on the sedimentary structure of prehistoric rock avalanches on the Tibetan Plateau, China
    WANG Yufeng, CHENG Qiangong, LIN Qiwen, LI Kun, SHI Anwen
    2021, 28(2): 106-124. 
    DOI: 10.13745/j.esf.sf.2020.9.12

    Abstract ( 9 )   PDF (20777KB) ( 42 )  
    Rock avalanche kinematics is a forefront major challenge in the field of engineering geology globally. To explore the kinematics of rock avalanches, three typical prehistoric cases occurring on the Tibetan Plateau were investigated in this study. Detailed field surveys and analyses of their surface and internal sedimentary structures revealed the following: (1) Obvious sequential distributions of toreva blocks, longitudinal/transverse ridges, and hummocks were observed, and the rock avalanches could be divided into three zones from rear to front: the source area, transition zone, and accumulation zone, according to the spatial distribution of these landforms. (2) Inverse grading was observed in the deposits, with tree facies, carapace facies, body facies, and basal facies, divided vertically from top to bottom. In the carapace and body facies distribute retained stratigraphic sequences, jigsaw structures, etc.; in the basal facies, diapiric structures, convoluted laminations, etc. can be observed, indicating an intensive interaction between the basal facies and substrate. Based on these observations, we propose a plausible mechanism for the emplancement and accumulation of rock avalanches as follows: As avalanche mass became detached from the source area, it underwent first a rapid laminar flow-like motion, then an extensional sliding motion in the transition zone, followed by an extensional sliding motion as it arrived at the accumulation zone; and if  the water content of the substrate were high, a rapid radial spreading process could occur at its frontal edge due to the rapid decrease in basal friction.
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    Tectonic interpretation of the main paleoseismic landslides and their distribution characteristics in the Chenghai fault zone, Northwest Yunnan
    HUANG Xiaolong, WU Zhonghai, LIU Feng, TIAN Tingting, HUANG Xiaojin, ZHANG Duo
    2021, 28(2): 125-139. 
    DOI: 10.13745/j.esf.sf.2020.9.11

    Abstract ( 14 )   PDF (19274KB) ( 289 )  
    The Chenghai fault zone, located at the northwest end of the Red River fault zone and the east side of the Northwest Yunnan fault depression zone, has significant Quaternary activity. Geological disasters occur frequently along the fault where basin range and high valley landforms are apparant. Based on visual interpretation and field investigations, a total of 940 landslides were found along the Chenghai fault zone, which included 61 giant landslides, 125 large landslides, 316 medium-sized landslides, and 438 small-sized landslides. Among them, 32 giant and 61 large landslides are located on the fault scarps and fault triangles within about 5 km from the footwall of the Chenghai fault zone. These large scale landslides often have long sliding distance, high speed and long-distance debris flow as the main characteristics, and belong to the paleoseismic landslides triggered by earthquakes in the Chenghai fault zone. According to the investigation of the main paleoseismic landslides in this area, these landslides are mainly distributed in the areas east of the Jinguan and Chenghai Basins, south of the Qina Basin at the intersection of the Jinsha River and the Chenghai fault zone, and across the western Midu Basin. They are obviously controlled by the activities of different sections of the Chenghai fault zone. The giant and large paleoseismic landslides are mostly developed at the north end of the Chenghai fault zone where it has the highest vertical activity.  According to the analyses of the late Cenozoic activity of the Chenghai fault zone and crustal deformation characteristics of the southeastern margin of the Tibetan Plateau, it is the clockwise rotation of the Northwest Yunnan fault depression zone that led to more intense fault activity and more frequent earthquakes. As a result, more paleoseismic landslides occurred at the end of the Chenghai fault zone. The power of the clockwise rotation of the Northwest Yunnan fault depression zone comes from the clockwise rotation of the Sichuan Yunnan inner arc zone and the strike-slip pull-apart of the Nantinghe, Wanding, and Litang faults.
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    The sedimentology of high-energy outburst flood deposits: an overview
    WANG Hao, CUI Peng, Paul A.CARLING
    2021, 28(2): 140-167. 
    DOI: 10.13745/j.esf.sf.2020.9.17

    Abstract ( 35 )   PDF (15095KB) ( 51 )  
    Discoveries of cataclysmic glacial lake outburst floods have expanded the scale and frequency of known mass transport events on Earth and other planets. It has been realized that sedimentary deposits related to high-energy freshwater floods are widespread across continents and these deposits share some commonalities when compared carefully. However, the identification and sophisticated reconstruction of these high-energy flood events require well-conditioned stratigraphic models of depositional successions, yet such models are still lacking. Thus, the primary purpose of this paper is to examine and condense the published stratigraphic and sedimentological evidence to identify the key signatures of megaflood successions. The deposits are often the only record of a flood; therefore, the secondary purpose is to interpret the sedimentary sequences in order to reconstruct the behavior and dynamics of a single and multiple flood events. The deposits of high-energy floods are usually massive and extensive, exhibiting sedimentary structures such as amalgamation, rhythmicity, outsized clasts, intraclasts, and obstacle marks. Typical sedimentary successions of high-energy floods include basal coarse parallel-bedded units, large-scale clinoforms, horizontally bedded thin laminated units, ripple and dune cross-beds, silt beds, and debris flow deposits. Such a succession is usually dominated by high concentration suspension rain-outs, representing a single cycle of waxing and waning flood flow. Clay and silt are generally sparse or absent and are transported further down the system. At the scale of the hydrograph, the successions indicate that flows initially accelerate and then decelerate, with significantly shorter periods of flow pulses also evident in the sedimentary signatures.
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    Progress in the reconstruction of palaeoflood events in the mountain canyon valleys around the Tibetan Plateau
    GUO Yongqiang, GE Yonggang, CHEN Xiaoqing, LIU Weiming, MAO Peini, LIU Tao
    2021, 28(2): 168-180. 
    DOI: 10.13745/j.esf.sf.2020.9.13

    Abstract ( 20 )   PDF (9009KB) ( 33 )  
    Palaeoflood reconstruction is a frontier research topic in the field of geomorphology and environmental change. This paper reviews paleohydrologic techniques and approaches and discusses the hydrological reconstruction and geomorphological effects of palaeoflood events. Paleostage indicators (PSIs) and geomorphologic evidences (e.g., slackwater deposit (SWD)) were identified along the mountain canyon valleys of southwestern China using paleohydrological criteria. Geomorphologic evidences of the palaeoflood events should be systematically investigated at the watershed scale. Palaeoflood SWD, suspended during the high water stage in the slackwater/backwater area, provides a complete sedimentary sequence of PSI; while sand/gravel bars, the bed-load deposits during the flood’s low water stage, and run up deposits above the bar top provide a potential PSI. The magnitude of paleoflood was estimated using multiple one- and two-dimensional hydraulic models in the bedrock reach. Different flood geomorphologic evidences were dated by multiple geochronological methods (e.g., 14C dating, OSL dating, cosmogenic 10Be and 26Al exposure age dating). Floods of different origins were checked on a watershed scale. The magnitude-frequency relationship and spatiotemporal pattern of the palaeoflood events were established. The formation age, geomorphic evidence, and hydrological model of the catastrophic paleofloods around the Tibetan Plateau show obvious spatiotemporal characteristics and geomorphologic effects, which has significance for understanding global megaflood geomorphology during the last glaciation. The hydrodynamic conditions and geomorphologic effects of the palaeoflood were assessed on a special scale, and flood geomorphology has been extended from a continental to a planetary scale. In addition, modern large flood investigations can serve as a key reference for understanding the palaeoflood sedimentary processes and hydrological parameters.
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    Archaeological discoveries and research on the remains of an ancient flood event at the Majie Site in the Chengdu Plain
    ZHU Cheng, XU Jiajia, HUANG Ming, YANG Zhanfeng, ZHANG Na, JIANG Zhanghua, BAI Tieyong, LU Fuzhi
    2021, 28(2): 181-201. 
    DOI: 10.13745/j.esf.sf.2020.9.5

    Abstract ( 11 )   PDF (12410KB) ( 215 )  
    We investigated an ancient flood event at the Majie site in Pixian County, Chengdu, Sichuan Province. The Majie site, first discovered in May, 2014, is located at 30°55′37.1″ N, 103°55′39″ E and has an elevation of 499.758 m. In the 2014CPMT1-T2 excavation section, we found  ancient relics of an underground tree root system, a canoe brought by scouring, and nine tree trunks. The ancient tree relics were mainly ancient Chinese autumn maple (Bischafia javanica). Most of the woody sediments were deposited along the NW-NE-SW plane, indicating they were washed mainly from the direction of the Dujiangyan area of northwestern Pixian County. We determined the AMS 14C ages of three tree trunks and a wood rafter to be (2420±70)-(2566±78) a BP. Since the Dujiangyan irrigation system was built by the Warring States period official Li Bing in 256 BC, the woody sediments at the Majie site should be the result of a southward ancient flooding of the Minjiang River in the Zhou Dynasty (1122-256 BC), before the construction of Dujiangyan. The results of zircon morphological identification and whole sectional analysis show that the morphological composition of zircon varied with burial depth, especially in the 2.6-2.4 ka BP detrital zircon populations, with larger proportion of round columnar zircon—evidence of long distance transport—and slightly smaller proportion of square biconical zircon, suggesting flooding events likely occurred during this period, and the detrital zircon had been transported to the Majie site area from afar.
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    The geomorphic process and sedimentary characteristics of the “11·3” Baige Dammed Lake outburst flood event in the upper reaches of the Jinsha River from Benzilan to Shihku
    SU Huai, SHI Zhengtao, DONG Ming, YE Liaoyuan, YE Lei
    2021, 28(2): 202-210. 
    DOI: 10.13745/j.esf.sf.2020.9.19

    Abstract ( 8 )   PDF (14407KB) ( 28 )  
    Reconstruction of paleo-dammed lake outburst floods is one of the hotspots in current Earth science research. Collecting enough reference cases of modern outburst floods is the basis for this research. The outburst flood on November 13, 2008, referred to as the “11·3” Baige Dammed Lake outburst flood, is a once-in-1000-year event when the outburst flood discharged from the Baige Dammed Lake, blocking the mainstream of the Jinsha River. As the flood event occurred during the dry season, the flood peak was solely from the outburst flood; therefore, this rare case provides a great reference for evaluating the impact of dammed lake outburst flood on the basin landform and sedimentary system. In this flood event, the upper reaches of the Jinsha River from Benzilan to Shihku, the focus area of this study, was most devastated. Based on detailed field investigation and preliminary hydrodynamic estimation, we found that the geomorphic process of the outburst flood was mainly manifested in bank slope collapse and sediment accumulation, with no significant bedrock erosion observed in the flooded area. The outburst flood sediments are mainly composed of well sorted, horizontally laminated sand.  Due to relatively low slope gradient of the Jinsha River, the bed shear stresses exerted by the outburst flood, ranging from 27 to 142 N/m2 , were relatively weak and could not suspend and transport gravels larger than 5 cm, nor could it produce obvious abrasion and impact marks. Some collapsed banks show gravels falling from the bank slope into the outburst flood sediments; even turbidity sequences appear. These phenomena provide new geomorphologic evidence for further understanding the complex geomorphological process and sedimentary characteristics of dammed lake outburst floods.
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    Paleoseismic investigation of the recurrence behavior of large earthquakes on active faults
    LIU Jing, YUAN Zhaode, XU Yueren, SHAO Yanxiu, WANG Peng, XU Jing, LIN Zhou, HAN Longfei
    2021, 28(2): 211-231. 
    DOI: 10.13745/j.esf.sf.2020.9.8

    Abstract ( 33 )   PDF (5122KB) ( 62 )  
    Paleoseismology is a subdiciplinary study closely linked to active tectonics, earthquake geology, and tectonic geomorphology. It is field-based, aiming to reconstruct the location, time, and magnitude of paleoseismic events through the detailed documentation of the deformations caused by the events. Paleoseismic studies utilize the methods and means commonly used in the study of sedimentary stratigraphy, geomorphology, and structural geology to identify the evidence of prehistoric strong earthquakes preserved in well-dated Quaternary deposits. The instrumental record of earthquakes is much shorter than their return time on faults. Paleoseismology thus extends the record, providing fundamental data constraints on the spatio-temporal repetition of earthquake ruptures on faults, and facilitates the evaluation of the probability of earthquakes and their hazards. In this paper, we review the history, frontier research results, and future directions of the field. We first introduce the principle of paleoseismology, the history of its developments in the world and in China, and the basic methods used in paleoseismic research. We then summarize recent progress and frontiers in paleoseismic research, including developing new trenching techniques, quantifying the robustness of evidence in event identification, revealing the pattern of paleo-earthquake recurrence and coseismic displacement, and exploring shaking-related effects such as paleo-liquefaction and paleo-landslides. Finally, we briefly outline some future trends in paleoseismology. In future, we should further strengthen the application of quantitative evaluation of the uncertainty in event identification, continuously explore new dating techniques, and experiment with paleoseismology in virtual reality settings. In China, especially in North China where there is a long and precious historical record of earthquakes, innovative approaches combining historical accounts of shaking-related damages and field trenching will be fruitful.
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    Characteristics of lake sediment response to earthquakes and the reconstruction of paleoseismic sequences
    LI Dewen, LI Linlin, MA Baoqi, ZHANG Jian
    2021, 28(2): 232-245. 
    DOI: 10.13745/j.esf.sf.2020.9.15

    Abstract ( 19 )   PDF (2345KB) ( 56 )  
    Lake sediment is considered a “natural seismograph” because of its sensitivity to ground motion. Paleoseismic study of lake sediments, an important direction of paleoseismic study, makes it feasibleBecause of its sensitivity to ground motion, lake sediments are considered a “natural seismograph”. The paleoseismic study of lake sediments, which gives us the opportunity to reconstruct long-term seismic sequences. The purpose of this paper is to summarize the main progress, existing problems, and future prospects of this important research field. The paper first briefly introduces the relative advantages of lacustrine paleoseismic records in terms of formation and preservation potentials, spatial distribution, and sensing ability over the  traditional paleoseismic records. It then summarizes the main response mechanism of lake sediments to ground motion from the process viewpoint, with emphasis on the similarities and differences of different mechanisms, such as liquefaction, fluidization, and sediment resuspension, in control factors, process features, and response threshold. Next, combined with process characteristics and research status, the paper summarizes the seismic records of different types of lake sediments, compares and analyzes the sedimentary and dynamic characteristics from four types of records, including deformation structure, mass-transported deposits, turbidity deposits, and resuspension deposits, and discusses the paleoseismic meaning and research methods of  different types of records. Lastly, the paper summarizes the applicability of popular research methods, such as geophysical exploration, sedimentary analysis, and physical and chemical proxies, in the identification of paleoearthquakes and reconstruction of paleoseismic sequences of lake sediments on different spatial scales. However, each method has its limitations due to the complexity of the lacustrine system itself and diversity of external disturbances. In the final analysis, it is clear that the main problems in paleoseismic investigation of lake sediments are the lacks of a universal response model, identification bases, and discrimination criteria. In future, we should concentrate our effort toward an in-depth understanding of the response process of lake sediments to ground motion, actively introducing theoretical tools and methods in expanding mathematical or experimental simulation from 2D observation to 3D reconstruction; whilst data interpretation should attempt to combine macro and micro aspects, e.g., changing from single to comprehensive combined indexes, to facilitate the establishment of universal diagnostic indicators and criteria.
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    Research progress in the reconstruction of paleotsunami in the South China Sea and the tsunami deposit characteristics
    YANG Wenqing, XIE Zhouqing, SUN Liguang
    2021, 28(2): 246-257. 
    DOI: 10.13745/j.esf.sf.2020.9.6

    Abstract ( 19 )   PDF (4081KB) ( 38 )  
    The East and South China Seas are adjacent to the Ryukyu and Manila Trenches, where earthquakes occur frequently. Tsunami researches in the past were focused on analyzing historical documents and numerical simulation to assess the risk and history of tsunamis striking the coastline, but the knowledge of historical tsunami events was uncertain. In the past decade, by investigating coastal, marine sediments and their depositional processes, tsunami relics, and historical documents, we have developed research approaches to reconstruct paleotsunami. We first recognized a 1000-year-old tsunami based on sedimentary evidences from Dongdao and Xisha Islands and the South China Sea, and proposed using lake sediment characteristics as a criterion for identifying paleotsunami. Sampling and dating methods, including validation from both the age of the event layer and historical literature, were also proposed. We suggest that a tsunami occurred in the South China Sea in the past 1300 years, and the year of this event is AD 1076.To expand the age span of the reconstruction, a 2200-year-long sediment record from the East China Sea was studied. Disturbance caused by the South China Sea tsunami could be seen in the record, whereas the absence of a stronger abrupt change indicated the East China Sea has not been hit by catastrophic tsunamis in the past 2200 years. The 1076 tsunami hit the coastal regions of  South China. Tsunami deposits containing shards of Song ceramics were preserved on the coast of Nan’ao Island; the culture of Nan’ao experienced a 500-year decline after the South China Sea tsunami and did not recover until the late Ming Dynasty. We used radiocarbon ages of plant remains and shell carbonates, OSL analysis of sand, and ceramic dating to determine the age of the paleotsunami; based on the dating results, we put forth the methods for tsunami deposit dating. In addition to these dating methods, proxies of geomorphology, geochemistry, and sedimentary sources can also be used under different depositional environments. The east coast of Hainan Island is also vulnerable to tsunamis, and future studies are needed to reconstruct its paleotsunami records.
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    Submarine landslides in the northern South China Sea: characteristics, geohazard evaluation and perspectives
    SUN Qiliang, XIE Xinong, WU Shiguo
    2021, 28(2): 258-270. 
    DOI: 10.13745/j.esf.sf.2020.9.3

    Abstract ( 20 )   PDF (15060KB) ( 109 )  
    Submarine landslides and the tsunamis they trigger can cause catastrophic damage to the infrastructure, economy, and lives increasingly expanding on the sea and along the coastlines. Therefore, quick technological progress is needed to guard against geohazards triggered by submarine landslides. Submarine landslides are identified by a large amount of geophysical data (seismic and multibeam bathymetrical data) in the petroliferous basins (the Pearl River Mouth Basin and the Qiongdongnan Basin) along the margin of the northern South China Sea. These submarine landslides formed from the Pliocene to Quaternary periods, ranging in size from several to over ten thousand square kilometers. They are mostly characterized by chaotic and blank seismic reflections; some contain a large number of collapse blocks. The seismic characteristics, sizes, origins, and geohazard potentials of these submarine landslides vary greatly because of their different developing environments. In this study, the seismic expression of submarine landslides is characterized, and their direct and indirect geohazard potentials (including those in the Xisha Massif) are evaluated. Finally, the important scientific (formation mechanism of submarine landslide) and practical questions (possible submarine landslides and landslide-generated tsunami) arising from this research are discussed.
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    Discussions on the bathymetric segmentation and tectonogenesis of the oblique spreading Southwest Indian Ridge
    ZHANG Huatian, LI Jianghai, TAO Chunhui
    2021, 28(2): 271-283. 
    DOI: 10.13745/j.esf.sf.2020.12.7

    Abstract ( 14 )   PDF (4484KB) ( 24 )  
    Oblique spreading is a characteristic feature of ultra-slow spreading ridges. As its bathymetric segmentation pattern is distinct from classical fast-slow spreading ridge models, it can provide insight into the geological process of ultra-slow spreading ridges. Based on multibeam bathymetric data, the segmentation patterns (with variable spreading obliquity (α)) of the Southwest Indian Ridge (SWIR)  Indomed-Gallieni and Shaka-DuToit zones are analyzed. Results show that the SWIR segments, between 46.5°E and 47.5°E (α=5°), 16°E and 25°E (α=10°), and 48.5°E and 52°E (α=15°), respectively, are quasi-orthogonal spreading segments, which have developed en échelon axial volcanic ridges. Along the axis of the oblique spreading SWIR, between 47.5°E and 48.5°E (α=50°) and between 16°E and 25°E (α=60°), only one axial volcanic ridge has formed at the center of the axis. The distribution of the strain rates along the mid-ocean ridges with variable obliquity (α) is calculated by a ‘finite difference+markers in cells’ (FD+MIC) technique. Combined with bathymetry analysis, two factors influencing the segmentation pattern of oblique spreading results, namely, the spreading obliquity and distribution of temperature disturbance, are proposed. Along the quasi-orthogonal spreading segments (α<20°), temperature disturbance led to localization of extensional strain and development of  en echelon axial volcanic ridges. In the oblique spreading segments (α>20°), strain localization was controlled by oblique geometry so the axial volcanic ridges only developed in the middle of the segment. These volcanic ridges were insensitive to temperature disturbance, therefore, they could form lasting fixed local magmatic centers.
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    Re-estimating the depth of shear wave splitting anisotropy in the Yunnan region by using a mantle convection model based on lithospheric thickness and lateral mantle viscosity variations
    ZHU Tao, MA Xiaoxi
    2021, 28(2): 284-295. 
    DOI: 10.13745/j.esf.sf.2020.6.28

    Abstract ( 5 )   PDF (5658KB) ( 6 )  
    In the present paper, we propose a more realistic mantle convection model by introducing the lithospheric thickness and lateral asthenospheric viscosity variations into a previous model and re-estimate the depth of shear wave splitting (SWS) anisotropy. Our results indicate that the variations greatly affected the depth of asthenospheric source responsible for SWS anisotropy and the intensity and mechanism of asthenospheric deformation in the Yunnan region. Asthenospheric anisotropy obviously contributed to a SWS residing at 90-180, 170-330, and 200-320 km depths in southwestern Yunnan, eastern Yunnan south of 26°N, and the Sichuan Basin and its western margin, respectively. Asthenosphere responsible for a SWS in southwestern Yunnan and eastern Yunnan south of 26°N likely experienced large shear deformation and was primarily controlled by mantle flow-direction/flow-plane mode; whereas asthenosphere resposible for a SWS in the Sichuan Basin and its western margin likely experienced small shear deformation and was mainly controlled by strain mode.
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    A rock thin section analysis system based on continuous extinction characteristics
    HU Xiaoqing, GAO Shichen, ZHANG Xin, WANG Wei, ZHONG Hongru, ZHOU Heng
    2021, 28(2): 296-304. 
    DOI: 10.13745/j.esf.sf.2020.6.36

    Abstract ( 13 )   PDF (5452KB) ( 19 )  
    Continuous extinction refers to a phenomenon whereby the observed extinction intensity of a rotating rock thin section changes continuously under polarizing microscope observation. As an important reference for thin section identification, the extracted characteristics of continuous extinction can be used in the automatic thin section analysis. Combining digital image processing technology and clustering algorithm, we developed a thin section analysis system for grain segmentation, grain identification and quantitative data analysis. How to precisely extract the characteristics of continuous extinction—based on the extinction characteristics of minerals—became the vital part of grain identification. Therefore, we set up the control groups for extinction position alignment and baseline correction during the extraction operation. The resulting angular domain sequence images of sedimentary rock thin section under crossed polarizer show that the extinction position alignment and baseline correction contributed to more accurate continuous extinction characteristics, and the results of thin section analysis by our method matched those by expert evaluation.
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    Characteristics and prospecting significance of thermoluminescence patterns and cell parameters of quartz from the undersea gold deposit  off northern Sanshandao, Jiaodong Peninsula
    SONG Yingxin, LI Shengrong, SHEN Junfeng, ZHANG Long, LI Wentao, ZENG Yongjie
    2021, 28(2): 305-319. 
    DOI: 10.13745/j.esf.sf.2020.6.27

    Abstract ( 10 )   PDF (4844KB) ( 18 )  
    The Jiaodong Peninsula is one of the most important bases of gold resources in China. In recent years, great breakthroughs have been made in deep prospecting of gold deposits, especially a super large undersea gold deposit off northern Sanshandao has been discovered. Presently only limited research has been done on this deposit. Based on the genetic and mineral prospecting theory, we systematically studied the temporospatial distribution patterns of thermoluminescence and cell parameters of connective mineral quartz from the deposit. We found the following typomorphic characteristics of quartz very important for gold prospecting: (1) The quartz thermoluminescence curves showed characteristic singlet and shoulder-like peaks at the mid range temperatures in the early and late mineralization stages, and an asymmetric doublet peak with the strongest emission intensity over the mid-to-high temperature range in the main mineralization stage. (2) The peak temperatures and emission intensities on the thermoluminescence curves showed vertical fluctuations in volatility correlating with gold mineralization. (3) Quartz (Δa0/a0)/(Δc0/c0) values ranged between 1.92 and 3.30, indicating gap filling was the major way of incorporating trace elements into quartz. (4) Spatiotemporal variation of quartz cell parameters followed a clear pattern of increasing a0 and V0,  decreasing c0/a0 from the early to late mineralization stages, indicating temperature and  elemental impurity had an influence on quartz cell parameters. (5) In the depth of the ore body, the quartz thermoluminescence curves showed higher peak temperature, larger emission intensity fluctuation, singlet and shoulder-like peaks,  and large a0 and V0 values, suggesting a good downward mineralization prospect.
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    Characteristics of fluid inclusions in and metallogensis of the Zhuanshanzi gold deposit in Inner Mongolia
    OUYANG Xin, ZHANG Yongmei, GU Xuexiang, LIU Li, WANG Luzhi, GAO Liye
    2021, 28(2): 320-332. 
    DOI: 10.13745/j.esf.sf.2020.6.30

    Abstract ( 18 )   PDF (9802KB) ( 18 )  
    The Zhuanshanzi gold deposit is located on the northern margin of the eastern section of the Inner Mongolia Plateau, in the boundary area between the North China Craton and the Xingmeng orogenic belt. Its ore-bodies are hosted in the Lower Permian limestone (Yujiabeigou Formation), Middle Permian rhyolite and Middle Triassic granite-porphyry, intercalated by Late Triassic diorite. Based on the studies of fluid inclusions in hydrothermal minerals in various mineralization stages of the Zhuanshanzi gold deposit, we discuss in this paper the property and evolution of ore-forming fluids in the deposit. The ore-forming process can be divided into three metallogenic stages: quartz-pyrite, quartz-sulfide, and quartz-calcite-fluorite mineralization stages. Petrographic observation indicates fluid inclusions within the deposit can be divided into six types: biphasic vapor-aqueous, multiphasic CO2-H2O, daughter mineral-bearing multiphasic, monophasic CO2, monophasic vapor, and monophasic liquid inclusions. The temperature and salinity of the ore-forming fluids showed a decreasing trend from the first to the last metallogenic stage. The ore-forming fluid system is a CO2-NaCl-H2O system characterized by medium high temperature, medium low to high salinity and medium low density. In the main ore-forming stage, the ranges of hydrostatic pressures within the biphasic vapor-aqueous salt water system and triphasic CO2-H2O inclusions were 15.7-28.6 MPa and 59.0-88.7 MPa, respectively, and the ore-forming depth ranged between 1.6 and 3.3 km. It is considered that the immiscible (boiling) effect of the ore-forming fluid is the main mechanism of mineral precipitation, and the deposit is closely related to the magmatic activity caused by the collision between the North China and Siberian plates from the Middle to Late Triassic. The Zhuanshanzi gold deposit is a typical medium high temperature magmatic hydrothermal deposit in Inner Mongolia.
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    Geochemical characteristics of tourmalines from the Dayakou emerald deposit in Yunnan Province: implications for emerald mineralization
    LONG Zhengyu, YU Xiaoyan, ZHENG Yuyu, GUO Bijun
    2021, 28(2): 333-347. 
    DOI: 10.13745/j.esf.sf.2020.10.30

    Abstract ( 12 )   PDF (6319KB) ( 25 )  
    Emerald is the green variety of beryl and its color is predominantly associated with trace chromium and/or vanadium. The Dayakou emerald deposit in Malipo County, Yunnan Province is a significant emerald deposit in China. A series of researches have been done in recent years on a variety of minerals in the deposit, but the emerald-associated tourmaline has not been investigated. Here, we studied the tourmalines from emerald-bearing and barren veins in the Dayakou deposit. Based on detailed field survey, petrographic investigation and component analysis, we aimed to investigate the origin of tourmaline to ascertain its fluid source and evolution in order to further understand the metallogenic mechanism of the Dayakou emerald deposit. The results show that, in tourmalines from emerald-bearing and barren veins, the measured Na contents were 0.62-0.79 and 0.64-0.76 apfu, respectively; the Al contents were 5.36-6.17 and 5.66-6.38 apfu, respectively; and the Fe/(Fe+Mg) ratios were 0.31-0.41 and 0.14-0.34, respectively. The Dayakou tourmaline belongs to alkline dravite, rich in Mg, with low or zero Al content in the Y site, whereas tourmaline from emerald-bearing veins has a comparably high Fe/(Fe+Mg) ratio. These chemical compositional differences in tourmalines may be related mainly  to the ore-forming condition, which makes tourmaline a potential indicator of emerald mineralization. The Dayakou tourmaline contains 0.65%-4.76% V2O3 in component zonation, formed by metasomatism during sustained fluid-rock interactions. The early fluids of the Dayakou deposit were granite-derived fluids consisting of Si, Al, Be, F, and P. As hydrothermal alkali-metasomatism progressed, the Ca and V (extracted from wall rock) contents gradually increased. We believe beryllium fluoride (BeF) or Be-F complex is a significant Be migration mode in the Dayakou ore-forming fluids. The crystallization of F-containing minerals such as fluorite and fluorapatite resulted in the decomposition of BeF or Be-F complex, and the reduction of fluorine in the ore-forming fluid may be one of the important emerald mineralization mechanism of the Dayakou emerald deposit.
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    Redefining the Heishantou Formation and defining the Aketamu Formation in West Junggar, Xinjiang
    LI Yongjun, LI Hai, NING Wentao, XU Qian, REN Pengfei, TAO Xiaoyang
    2021, 28(2): 348-361. 
    DOI: 10.13745/j.esf.sf.2020.12.10

    Abstract ( 21 )   PDF (9689KB) ( 33 )  
    The Heishantou Formation is widely distributed in the stratigraphic regions of East and West Junggar and can be used as a landmark unit for stratigraphic correlation. The Heishantou Formation was initially established as marine terrigenous clastic rock strata; however, the geological section that defined the Heishantou Formation was renamed the Nalinkala Formation with the addition of the volcanic strata in the alternate section, leading to serious disagreement between the classification and timing of the Heishantou Formation. As a result, the use of stratigraphic units overlying and underlying the Heishantou Formation created many confusions. Through a substantial field geological survey, we found the lithology of the initially defined marine terrigenous clastic rocks of the Heishantou Formation are significantly different from that of the volcanic strata added to the upper part of the section. Their lithologic interfaces are clear and comparable in the Junggar area; in addition, there is an evident eruptive unconformity between the two strata. Thus, we suggest to disintegrate and redefine the Heishantou Formation. The newly defined Heishantou Formation retains only the lower strata of the marine terrigenous clastic rocks; the volcanic strata, believed to be the Early Carboniferous (not the Middle Devonian as previously considered), is defined as the Aketamu Formation. This new scheme provides important evidence for the division of the Carboniferous strata in the Junggar stratigraphic region and for the correlation of regional strata, which in turn improves the comparative value of the Heishantou Formation in the two stratigraphic regions.
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    Evolutionary characteristics and controlling factors of deep-sea fans in Cretaceous passive continental margin basin, Northern Subbasin, Senegal Basin
    WANG Hongyu, ZHANG Feng, YANG Xiongbing
    2021, 28(2): 362-375. 
    DOI: 10.13745/j.esf.sf.2020.12.12

    Abstract ( 10 )   PDF (15214KB) ( 21 )  
    Deep-sea fan reservoirs in passive continental margin basins are among exploration hotspots in offshore petroleum exploration. Understanding  the spatiotemporal evolution of deep-sea fans can help refining geological models  by enhancing the exploration and development of related reservoirs. Based on seismic and drilling data, different types of deep-sea fans with characteristic lithologies, shapes, and sizes are identified along the passive continental margin of the Senegal Basin. Factors such as sea-level changes, sediment supply, and paleo-geomorphology were integrated into the interpretation. This study shows that the passive continental margin dates back to the Middle Jurassic. During the Cretaceous Age, the study area underwent three stages of tectonic-sedimentary evolution: (1) carbonate platform build-up stage during the Neocomian and Aptian Ages; (2) sedimentary stage of clastic deposition on a continental fault-slope from the Albian to Santonian Ages; and (3) sedimentary stage of clastic deposition on a gentle continental slope (slope-break) during the Campanian and Maastrichtian Ages. During these periods, five types of deep-sea fans   with varied lithologies, morphologies, and sizes developed in the study area. Several factors could have influenced the development and evolution of deep-sea fans. Among them, global sea-level changes and terrestrial clastic supply had the greatest influence on the texture and size of the fan, while continental slope geomorphology determined the shape and distribution of the fan reserviors.
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    Provenance characteristics and tectonic setting analysis of the 3rd Member of the Paleogene Funing Formation, Subei Basin
    WANG Xuying, JIANG Zaixing
    2021, 28(2): 376-390. 
    DOI: 10.13745/j.esf.sf.2020.6.31

    Abstract ( 11 )   PDF (6028KB) ( 19 )  
    Based on the analyses of debris component, gravel composition, rare earth element, sandstone content distribution and glutenite distribution characteristics, and drawing on the previous research results, it can be concluded that there are six main provenance areas in the 3rd Member of the Funing Formation, Subei Basin. The south and north sides of the Jianhu Uplift (areas Ⅰ, Ⅱ) provide sources, respectively, southward to the Eastern Jinhu, Northern Gaoyou and Hai’an Sags, with the parent rocks including mainly medium-acidic igneous rock, quartzite and granite, and northward to the Yancheng Sag, with the addition of basic igneous rock to the parent rocks from area Ⅰ. The Zhangbaling Uplift (area Ⅲ) provides source eastward to the Western Jinhu Sag, with the main parent rock types almost identical to that of area I but without granite.  The Sunan Uplift (area Ⅳ) provides source northward to the Southern Gaoyou Sag, supplying mainly carbonate and medium-acidic igneous rocks as the parent rocks that are very different from the parent rocks (supplied from area Ⅰ) in the Northern Garyou Sag by the presence of large amount of carbonate rocks. According to the literature reports, the Binhai Uplift (area Ⅴ) provides source southward to the Funing Sag, with medium-deep metamorphic rocks and Paleozoic sedimentary rocks as the parent rocks; while the eastern section of the Lusu Uplift (area Ⅵ) provides sources to the Lianbei Sag, with the parent rock mainly including metamorphic acidic igneous rock of the basement. Combined with the Bhatia REE characteristic parameters and Dicknson tectonic discrimination diagrams, it is considered that the tectonic settings of the source areas are mainly active continental margins associated with continental island arc and recycled orogenic belt.
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    Preliminary analysis on the paleogeography and paleoenvironment in the eastern Yanliao area during the Jurassic-Cretaceous tectonic transition
    LI Xiaobo, ZHANG Yan, TONG Yabo
    2021, 28(2): 391-411. 
    DOI: 10.13745/j.esf.sf.2020.6.29

    Abstract ( 28 )   PDF (6921KB) ( 42 )  
    During the transit period from the Middle and Late Jurassic to early Early Cretaceous, the tectonic pattern and paleogeography overturned in eastern China. This paper discusses some issues on the paleogeography and paleoenvironment in the eastern Yanliao area through this period. The preliminary results show that the aggravate plate convergence at the continental margin of eastern Asian from the Middle Jurassic induced the crustal thickening and tectonic uplifting in the region. With the continuance of the subduction of the Paleo-Pacific (Isanagi) Plate, deep magma upwelled, crust thinned, and multiphasic rifts developed, while the East Asian ocean-continent connection zone underwent tectonic escape. These processes shaped the high mountain and other basin-wide  paleo-landforms in the Yanliao area. During the early Early Cretaceous(137-131 Ma), an altitude difference—low in the west and high in the east—established along the extension line through the Nenjiang-Balihan Fault and the Kharatsin Metamorphic Core Complex. In the comparatively descending western part, the strata of the Northern Hebei-Great Xingan Mountain volcano-graben belt yielded the fossils of the early assemblage of the Jehol Biota, while the Western Liaoning-Songliao area rose to tectonic upland. As the rifting area expanded during the 130-110 Ma, the middle and late-phase assemblages of the Jehol Biota extended to the surrounding areas. The paleoclimate of the Jehol Biota was temperate continental, mostly arid and cold, thus not inconsistent with the notion that the paleoclimate was affected potentially by the   environment of the Mesozoic East China Plateau. The global paleoclimatic evolution and regional paleogeographical condition jointly caused the subartic climate   trend in the Yanliao area from the Late Jurassic to Early Cretaceous, and further affected the evolution of the paleo-ecosystem from the Middle-Late Jurassic Yanliao Biota to the Early Cretaceous Jehol Biota.
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    Geochemical characteristics and genesis of the polymetallic nodules in the Pacific survey area
    REN Jiangbo, DENG Yinan, LAI Peixin, HE Gaowen, WANG Fenlian, YAO Huiqiang, DENG Xiguang, LIU Yonggang
    2021, 28(2): 412-425. 
    DOI: 10.13745/j.esf.sf.2020.6.37

    Abstract ( 23 )   PDF (5947KB) ( 39 )  
    The polymetallic nodules in China’s polymetallic nodules exploration contract area in the East Pacific Basin (EP nodules) are largely buried or semi-buried in a sedimentary environment dominated by clay and siliceous components. The manganese mineral phases of the EP nodules mainly include vernadite and todorokite, with high REY and high Cu and Ni contents and Mn/Fe ratio. These characteristics indicate the nodules are affected by interstitial water thus falling into the range of diagenesis and hydrogenesis. In comparison, the polymetallic nodules in the Northwest Pacific Basin (WP nodules) are mostly exposed to seawater, and the surrounding sediments are mainly deep-sea clay. The manganese mineral phases of the WP nodules are almost all vernadite, with high REY, high Co content and low Mn/Fe ratio. It shows that the WP nodules are of typical hydrogenic form. The NASC-normalized REY patterns of the polymetallic nodules in both regions show positive Ce and negative Y anomalies and no/weak Eu anomaly, while the REY patterns of ambient seawater show obvious negative Ce and positive Y anomalies and no obvious Eu anomaly. This inverse coupling relationship implies that the REY patterns of the polymetallic nodules are the result of selective enrichment from seawater. The WP nodules have higher Ce content and δCe than the EP nodules. Both Co and Ti correlate positively with Ce, which may collectively affect the selection of Ce and Y in seawater by polymetallic nodules. In this study, dissolved oxygen in seawater was not a factor controlling the level of positive Ce anomaly observed in the polymetallic nodules. Rather, elements such as Co and Ti and their related components can cause strong fractionation of Ce and other REY, which may be the controlling factor. The surface samples of polymetallic nodules and cobalt-rich crusts in the study area, with εNd values ranging from -6.6 to -2.5, represent the most radioactive ferromanganese samples of Nd in the global sea area. Combining the εNd result with REY patterns and Eu anomaly, we suggest that REY in polymetallic nodules mainly come from the surrounding continental crust with relatively high εNd and can be transported to the ocean via rivers or atmospheric deposition. However, the radioactive Nd isotopes released from basalt in the Northwest Pacific Basin have only weak effect on the seawater.
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    Distribution characteristics and enrichment mechanism of fluoride in the shallow aquifer of the Tacheng Basin
    LV Xiaoli, LIU Jingtao, ZHOU Bing, ZHU Liang, ZHANG Yuxi
    2021, 28(2): 426-436. 
    DOI: 10.13745/j.esf.sf.2020.10.29

    Abstract ( 10 )   PDF (3148KB) ( 26 )  
    The Tacheng Basin is located in the northwestern Xinjiang Hui Autonomous Region (XHAR), which is characterized by drought, less rain and strong evaporation. Compared with other basins in XAR, the concentrations of total dissolved solids (TDS) and fluoride (F- ) in the groundwater in the Tacheng Basin were relatively low. To explain this different distribution characteristics and enrichment mechanism of F-  in the groundwater, statistical analysis, including principal component analysis, was used based on the hydrochemical data of groundwater samples in this region. The results showed that the concentration of F in groundwater was generally low due to the climatic and geological factors in the region. The high-F-  groundwater was mainly distributed in the marsh lands of the front and central regions of the basin. The concentration of F-  was high in shallow and low in deep unconfined aquifers,  controlled by an upward recharge from deep confined aquifer. The groundwater runoff was relatively short and the groundwater circulation was fast in this region, causing a short interaction time between groundwater and rock. In addition, the deep confined aquifer of the plain was recharged from the groundwater in the mountainous area by an upward hydraulic gradient flow, which further replenished the upper shallow unconfined aquifer. Thus, the evaporation and concentration process was very weak in this recharge mode. The concentration of F-  in the groundwater was mainly controlled by the weathering and dissolution of fluoride-bearing minerals in the sedimentary strata in the piedmont alluvial-pluvial fan, with rock weathering, concentration by evaporation, cation exchange, and competitive adsorption as the main influencing factors.
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