西昆仑依格孜也尔河阶地沉积特征及其对河流演化的指示

对西昆仑山前依格孜也尔河阶地进行野外地质调查,获得阶地河拔、分布、发育及沉积特征等资料.在此基础上,分析阶地沉积相及沉积环境,进一步研究西昆仑山前河流阶地发育成因及河流演化过程.依格孜也尔河发育基座阶地T4和T3,堆积阶地T2和T1,阶地T1、T2、T3和T4时代依次为5~8 ka B.P.、18~30 ka B.P.、40~50 ka B.P.和80~100 ka B.P..阶地形成主要受控于间歇性构造抬升,受气候变化影响较小,阶地类型差异主要受河流演化影响.阶地沉积相分析表明,T4和T3阶地分别为泥石流相和冲洪积扇相,河流侵蚀作用强,间歇性堆积;T2阶地沉积期河流系统出现重大转变,呈多旋回辫状河沉积,堆积作用增强;T1阶地为主河道-漫滩相沉积,侵蚀-堆积作用趋于平衡.     

川西硕曲河流阶地及其对山地抬升和气候变化的响应

山地抬升和气候变化是影响内陆地区河流地貌发育的两个关键因素,不同地区的河流对它们的响应方式多种多样.位于川西高原西部活动构造区的硕曲河谷下游亚金段保留有6级河流阶地,运用ESR和TL技术对其中T₂到T₆级阶地的砾石层及邻近的冰碛物进行了测年,并结合阶地沉积物特征分析了阶地对山地抬升和气候变化的响应.结果表明:亚金T₂～T₆级阶地砾石层均堆积于冰期;阶地基座均形成于间冰期-冰期过渡时段;形成这些阶地的下切过程均开始于冰期晚期,可分别与深海同位素阶段的MIS 2、6、8、14和16阶段晚期相对应.自MIS16阶段晚期以来,硕曲河谷亚金段平均下切速率约为1.22 mm·a^-1,小于该地区山地同期平均抬升速率2 mm·a^-1,这与"河谷下切速率不大于山地抬升速率"一致.     

活动造山带地区河流阶地与下切速率及其时空分布样式

河流下切带动区域地貌的演化,因此河流下切速率及其时空分布样式是解析构造、地表过程和气候变化之间广泛相互作用的关键切入点之一。根据阶地的保存状况,利用河流阶地的累积下切量和下切时间可以直接测得千年到百万年时间尺度的下切速率。鉴于此河流下切速率是链接不同时间和空间尺度上地貌侵蚀速率的关键所在,并具有较精细的时空分辨率。计算河流下切速率时,累积下切量上限是阶地基座的拔河高度,下界最优选择是低级阶地基座,而累积下切时间的最佳约束是尽量靠近阶地基座的冲洪积物的沉积时间;运用河流下切速率获得岩石抬升速率和区域构造变形量时,通常假定区域地貌处于均衡状态,河流下切速率代表着岩石抬升速率。但是由于气候变化,河流下切过程具有较强的时空不稳定性,导致这些假设经常失效。此外,河流加积和下切的转换与第四纪冰期-间冰期的变化并不一定存在时间上的一一对应关系,这可能是由于河流裂点的溯源迁移以及河流对气候变化的响应方式与强度存在差异。因此,实际工作中需要多方面获得不同时空尺度和分辨率的构造变形、气候变化以及地表过程的基本信息,借助地貌数值模拟程序,定量解析构造、气候和地表过程之间的广泛而复杂的相互作用。

     

大凉山区比尔河晚更新世以来阶地级序及其演化

河流阶地详细记录了区域构造活动和地貌演化信息,可为第四纪地貌阶段性隆升提供证据。位于四川大凉山区的昭觉河流域及其支流地处青藏高原东南部,横断山脉与四川盆地西南缘的过渡带,发育了河流阶地等层状地貌。以昭觉河的支流比尔河为研究对象,通过野外河流阶地级序及阶地沉积物调查,结合14C测年结果,厘定了比尔河的3级河流阶地,其中T2和T3为基座阶地,T1为堆积阶地,形成时间为27.75 ka B.P.、12.20 ka B.P.以及6.65 ka B.P.,其中T3阶地与T2阶地的河流下切速率分别为0.59~1.29 mm/a和1.26~4.50 mm/a。认为川西大凉山地区30 000 a以来地貌至少经历了2次间歇性抬升,在整体隆升过程中,河流上游较中下游的地壳抬升速度更快,具有掀斜式抬升特征。T3、T2、T1阶地的形成与研究区气候变化有一定相关性,但新构造运动是比尔河河流阶地形成的主导因素,这对了解川西地区晚更新世以来河流阶地的级序展布及演化具有重要意义。     

河流阶地演化与构造抬升速率——以天山北麓晚第四纪河流作用为例

构造地貌学重点关注构造和地表过程对于地形地貌演化的差异化作用,构造活动速率则是评估这种影响的一个重要指标。利用河流阶地数据计算河流下切速率从而约束构造抬升速率是常用的方法,但由于阶地成因复杂,这一方法具有不确定性。对于山前河流地貌序列,基于背斜段与未变形段的阶地拔河高度差以及阶地面形成年龄,计算得到的河流下切速率可在一定程度上消除气候等因素的影响,因此可用于估算背斜自阶地形成以来的平均抬升速率。基于该方法,本文通过研究天山北麓乌鲁木齐河、塔西河、玛纳斯河、金钩河、安集海河及奎屯河等河流在背斜段发育的主要阶地,分析了背斜抬升速率及其时空特征。天山北麓发育3排逆断裂-背斜带,结果表明位于第Ⅱ排逆断裂-背斜带的吐谷鲁背斜自约13ka以来的抬升速率为3.52mm/a,同时期霍尔果斯背斜构造抬升速率为4.8mm/a,玛纳斯背斜东端的抬升速率相对较小,为<2mm/a; 第Ⅲ排构造带中的独山子背斜全新世抬升速率仅为1.2~1.9mm/a。这可能表明,自山前向盆地方向晚第四纪背斜抬升速率大致呈减小趋势,与背斜地壳缩短量的空间分布规律基本一致。更多的阶地年龄数据有助于更好地揭示天山北麓晚第四纪背斜构造活动特征。     

黄河兰州段河谷演化研究与认识

兰州盆地作为黄河自青藏高原流入黄土高原的转折点,其河谷演化对黄河水系在不同地貌单元的演化研究有承上启下的意义。兰州黄河河谷演化研究至今已有近100年的历史,在研究过程中取得了重要的研究成果和认识:①兰州盆地黄河河谷之上发育1级剥蚀面,河谷中发育了宽广连续的9级阶地;利用多种测年技术较系统地测定了1级剥蚀面和9级黄河阶地的年代。②提出兰州黄河基座阶地的形成模式——在长期地面抬升的背景下,气候变化控制阶地形成的年代,抬升速率影响阶地的级数。③兰州至其上游各盆地内黄河最高级阶地年代逐渐年轻化,揭示现代黄河上游水系格局的形成是黄河自兰州盆地不断溯源侵蚀,逐渐贯通上游内流盆地的结果。同时,阶地序列年代学仍需继续深入,区域构造抬升变形特征缺乏直接证据,阶地发育模式需要模型模拟进行验证,水系变化的地貌响应不清楚,以及阶地河漫滩沉积记录的河流过程缺乏关注,这些问题将是今后研究的重点。     

兰州东盆地最近1.2Ma的黄河阶地序列与形成原因

毗邻青藏高原的兰州地区的黄河阶地系列是地面抬升和气候变化的信息载体.通过古地磁、光释光测年及黄土-古土壤地层序列对比,初步确定了兰州东盆地1.2Ma以来主要有两个阶地发育时期.第1个时期是1.24～0.86Ma, 黄河至少发育了4级阶地,其形成时代分别为:1.24Ma, 1.05Ma, 0.6Ma和0.86Ma; 第2时期为最近0.13Ma,黄河发育3级阶地,时代分别为0.13Ma, 0.05Ma和0.01Ma.每级阶地河漫滩顶部都有一层古土壤发育,表明黄河下切形成阶地发生在古土壤开始发育的冰期向间冰期的过渡阶段,但是并非1.2Ma以来的每次冰期-间冰期的气候交替都能引起黄河下切形成阶地.气候变化只是阶地形成的必要条件之一,不是充分条件.对比黄河下切速率和阶地年代序列发现,地面上升仍然是影响黄河下切的重要因素.只有在地面上升速率达到一定程度的时候 (例如:1.2～0.8Ma和最近0.13Ma),气候变化才能导致黄河堆积与下切交替形成阶地;而在地面上升缓慢时期(例如:0.80～0.13Ma),即使发生了大幅度的气候变化,黄河也没有阶地记录.     

北天山山前安集海河阶地形成的时代及意义

北天山山前几条主要河流普遍发育河谷阶地。安集海河发育６～８级阶地,通过年代测定及区域对比,可得出安集海河阶地形成于中更新世晚期一晚更新世早期（约１２～１４万年左右）。第四纪以来构造活动及气候变化控制着河流下切和侧蚀作用的进行,安集海河阶地的形成和发育明显受第四纪晚期构造活动和气候变化等因素的影响。     

湟水河流域条带状水成黄土层倾斜率与青藏高原--祁连山隆升

在湟水河流域发育夷平面、阶地不对称的山脉、沟谷等地貌,对阶地、夷平面上的条带状水成黄土层的倾向和倾斜率的系统测量,反映出黄地表的抬升。根据对称性可分为４个阶段,隆升在大的时间尺度上是均匀的,在小尺度上是不均匀的,当高原隆升到一定高度后,受印度板块侧向挤压,高原向四周的应力的共同作用使祁隆升,并伴随着气候恶化。     

西昆仑山前河流阶地的形成及其构造意义

西昆仑山前河流普遍发育6级阶地,利用光释光（OSL）与热释光（TL）方法对采自西昆仑山前几条主要河流的低阶地堆积物样品进行年代测定。研究结果显示,主要河流低阶地的形成具有同时性,构造活动是河流阶地形成的主要控制因素。河流阶地的年龄测定结果表明,西昆仑山前河流阶地最早形成于约1.2Ma,T4、T3、T2阶地分别形成于约39ka、18ka和5ka。多级阶地的形成反映了河流自早更新世中期开始下切于活动抬升的西昆仑山。河流阶地的发育及区域对比揭示了西昆仑第四纪晚期以来的隆升过程,区域构造活动明显地影响河流的形态与行为。河流阶地的分布、地貌特征及区域对比表明,河流阶地的形成与演化受新构造活动、山脉隆升、气候变化等多种因素的影响。     

Evaluating the role of climate and tectonics during non-steady incision of the Yellow River: evidence from a 1.24 Ma terrace record near Lanzhou,China

The competing roles of bedrock uplift and climatic change in the formation of fluvial terraces remain uncertain. Most of recent studies have attributed terrace formation to climatic changes and held that, even in tectonically active settings, climate variations control cycles of terrace planation and abandonment. Based on field investigations of loess-paleosol sequences, magnetostratigraphy and optically stimulated luminescence (OSL) dating, we develop a new chronology for a spectacular flight of terraces along the Yellow River near Lanzhou, China over past 1.24 Ma. All the terraces are strikingly similar in that they have several meters of paleosol developed directly above fluvial deposits on the terrace treads, suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates. However, the ages of terraces cluster in two relatively short time periods (1.24–0.86 Ma and 0.13 Ma – present). During the intervening time between 0.86 Ma and 0.13 Ma, terraces either did not form or were not preserved. We suggest that this record indicates that rock uplift rates varied through time and influenced terrace formation/preservation. Thus, our results demonstrate the utility of deep chronologic records from fluvial terraces for deconvolving the effects of tectonics and climate on fluvial incision.     

Late Quaternary fluvial terraces of the Romagna and Marche Apennines,Italy: Climatic,lithologic, and tectonic controls on terrace genesis in an active orogen

We synthesize a new fluvial terrace chronostratigraphy of the Bidente and Musone Rivers cast within a broader European framework, which forms the basis of a terrace genesis and river incision model for the northern Apennines, Italy. Our model, supported by terrace long profiles, correlation to Po foreland sediments, 15 new radiocarbon dates, and published numeric and relative stratigraphic ages, highlights how drainage basin substrate drives concurrent formation of strath terraces in the Bidente basin and fill terraces in the Musone basin. Quaternary climate change paces the formative geomorphic processes through unsteady discharges of water and sediment. In the weathering-limited setting represented by the Bidente basin, siliciclastic detritus carves broad strath surfaces during glacial climates that are preserved as terraces as the river incises during the transition to an interglacial climate. In contrast, the transport-limited and carbonate detritus dominated Musone basin sees valleys deeply buried by aggradation during glacial climates followed by river incision during the transition to an interglacial climate. Incision of these rivers over the past ～1 million years has been both unsteady and non-uniform. These and all Po-Adriatic draining rivers are proximal to a base level defined by mean sea level and have little room for increasing their longitudinal profile concavities through incision, particularly in their lower reaches despite periodic glacio-eustatic drawdowns. As a result, the observed incision is best explained by rock uplift associated with active local fault or fold growth embedded in the actively thickening and uplifting Apennine foreland.     

OSL‐based chronostratigraphy of river terraces in mountainous areas,Dunajec basin,West Carpathians: a revision of the climatostratigraphical approach

The technique of optically stimulated luminescence (OSL) dating applied to fluvial sediments provided a geochronological framework of river terrace formation in the middle part of the Dunajec River basin – a reference area for studies of evolution of river valleys in the northern part of the Carpathians (West Carpathians). Fluvial sediments at 18–90 m above valley bottoms were dated in the valleys of the Dunajec River and one of its tributaries. The resulting ages range from 158.9±8.3 to 12.2±1.3 ka. This indicates that some of the terrace sediments were deposited much later than previously assumed on the grounds of a combined morphostratigraphical and climatostratigraphical approach. The OSL‐based chronostratigraphy of terrace formation consists of seven separate phases of fluvial aggradation, separated by periods of incision and lateral erosion. Some of the ages determined correspond to warm stages of the Pleistocene – Marine Isotope Stage 3 (MIS 3) and MIS 5 – demonstrating that some terraces were formed during interstadial or interglacial periods. The results provide a key for evaluating rates of neotectonic uplift, allowing us to decipher the response of a fluvial system to climate change within the context of the glacial–interglacial scheme.     

黄河源地区的河谷地貌特征及其对黄河上游形成和演化的启示

黄河的形成与演化对于认识我国宏观地貌格局的形成、青藏高原及黄土高原的区域构造活动历史、华北平原及黄渤海陆架的形成和演化等问题具有重要意义。目前对黄河演化历史的研究主要集中在龙羊峡以下的河段,对于黄河源段的关注较少。文章基于黄河源地区河谷地貌的实地考察,并利用SRTM1-DEM数据,分析了黄河源段干流及支流河谷橫剖面的地貌特征,并与该区典型的冰川谷和兰州附近黄河的河谷横剖面进行了对比。结果表明:黄河源地区的河谷规模巨大,并呈现出谷底开阔、河床窄小、阶地不明显、谷坡陡立、河谷横剖面左右对称的U型谷特征。这些特征与该区冰蚀谷的特征相似,但与兰州段黄河成型河谷的特征相差甚远,且其河谷规模更大。我们推断,黄河源地区的河谷可能主要为冰期时的冰蚀作用所塑造,而非单纯的流水侵蚀形成。由于冰蚀作用的存在,该区早期的河流阶地可能被随后冰期的冰蚀作用所破坏,当前基于黄河源地区现存河流阶地年代的研究很可能低估了该区水系的发育历史。此外,反复的冰川进退也可能导致黄河源水系自上而下贯通,而非溯源侵蚀形成。     

末次冰期河流下切行为对气候变化的响应模式

末次冰期是距离人类最近的一次冰期,气候异常寒冷且存在高频高幅波动,河流系统如何响应冰期气候的变化值得关注与研究。基于河流系统对气候变化的敏感响应,传统的经典地貌理论认为河流下切在河流阶地形成过程中起着至关重要的作用,河流的下切行为发生在间冰期或者冰期向间冰期的过渡阶段,堆积行为发生于冰期,然而近年来最新的河流地貌研究成果表明,末次冰期河流下切较为普遍。首先对河流阶地形成的传统模式进行总结分析,认为单纯的构造驱动模式存在不合理性,气候也发挥着重要的调节作用;单一的气候变化无法驱动多级且高差较大阶地的形成,地壳抬升往往是必要因素;气候变化是引发河流堆积-下切行为转换形成河流阶地的关键因素。其次通过前人的研究案例总结出末次冰期河流下切行为响应气候变化的三种模式：(1)气候的高度不稳定性引发大规模的洪水事件驱动河流快速下切;(2)快速隆升区气候高频波动叠加构造抬升驱动河流下切;(3)沿海平原地区海平面大幅度下降驱动河流下切。这三种模式对于理解末次冰期河流系统对短尺度高频气候变化的响应以及对河流阶地成因的判断具有十分重要的指导意义。     

Late Quaternary river terrace sequences in the eastern Kunlun Range,northern Tibet: A combined record of climatic change and surface uplift

The Kunlun Range, a reactivated orogenic belt, constitutes the northern margin of the Tibetan Plateau. The extreme relief and major landforms of the Kunlun Range are a product of late Cenozoic tectonics and erosion. However, well-developed late Quaternary terraces that occur along the northern slope of the Kunlun Range probably resulted from climatic change rather than surface uplift. The terrace sequences formed in thick Quaternary valley fills and have total incision depths of 50–60 m. Optically stimulated luminescence dating was employed to place time controls on the valley fills and associated terraces. Dating results suggest that periods of significant aggradation were synchronous between different rivers and correspond to the last glacial stage. The abrupt change from aggradation to incision occurred between 21.9 ± 2.7 and 16 ± 2.2 ka, coincident with the last glacial–interglacial transition. Additional terraces developed during the late glacial period and early to middle Holocene. Based on a broader set of chronological data in northern Tibet, at least four regional incision periods can be recognized. Chronological data, terrace elevation profiles, and climate proxy records suggest that these terracing periods were triggered by cool and/or wet climatic conditions. A geometric survey of the riverbed longitudinal profile suggests that surface uplift serves as a potential dynamic forcing for long-term incision. A model is proposed for terrace formation as a response to climatic perturbation in an uplifted mountain range.     

Late Quaternary fluvial terraces near the Daocheng Ice Cap,eastern Tibetan Plateau

The timing of terrace formation relative to the glacial–interglacial cycle and what factors control that timing, such as changes in climate and/or uplift, are controversial. Here we present a study of the terraces along the Yazheku River using electron spin resonance (ESR) dating and analysis of the sedimentary characteristics in order to establish the timing of terrace formation and to assess the forcing mechanisms that have been proposed. The Yazheku River flows in glacial trough leading from the Haizi Shan, on the eastern Tibetan Plateau. The range was uplifted during the Quaternary and repeatedly glaciated by ice caps. The four highest major terraces (T5, T4, T3, and T2) are the result of both climatic and tectonic influences. Strath terraces T5–T2 were created during Haizi Shan glacial expansions during MIS 16, 12, 6 and 3–4, respectively. The major aggradation phases of the four terraces occurred during the deglaciations at the ends of MIS 16, 12, 6, and 2. Down-cutting, which led to the generation of the four terraces, immediately followed the deposition of the T5–T2 gravel units. These incisions occurred during the transitions between MIS 16–15, MIS 12–11, MIS 6–5, and MIS 2–1.     

Distribution and Forming Model of Fluvial Terrace in the Huangshui Catchment and its Tectonic Indication

<正>The Huang Shui River,a main tributary of the Yellow River,crosses a series of tectonically subsided and uplifted areas that show different patterns of terrace formation.The distribution of fluvial terrace of the Huang Shui River is studied through topographic and sedimentologic terrace mapping.Three terraces in the Haiyan Basin,four terraces in the Huangyuan Basin,19 terraces in the Xi'ning Basin(the four high terraces may belong to another river),nine terraces in the Ping'an Basin, five terraces in the Ledu Basin and 12 terraces in the Minhe Basin are recognized.Sedimentology research shows that the geomorphologic and sedimentological pattern of the Huang Shui River,which is located at the margin of Tibet,are different from that of the rivers at other regions.The formation process of the terrace is more complicated at the Huang Shui catchment:both accumulation terrace and erosion terrace were formed in each basin and accumulation terraces were developed in some basins when erosion terraces were formed in other basins,indicating fluvial aggradation may occur in some basins simultaneously with river incision in other basins.A conceptual model of the formation process of these two kinds of fluvial terraces at Huang Shui catchment is brought forward in this paper.First,the equilibrium state of the river is broken because of climatic change and/or tectonic movement,and the river incises in all basins in the whole catchment until reaching a new equilibrium state.Then,the downstream basin subsides quickly and the equilibrium state is broken again,and the river incises at upstream basins while the river accumulates at the subsidence basin quickly until approaching a new equilibrium state again.Finally,the river incises in the whole catchment because of climatic change and/or tectonic movement and the accumulation terrace is formed at the subsidence basin while the erosion terrace is formed at other basins.The existence of the accumulation terrace implied the tectonic subsidence in the sub-basins in Huang Shui catchment.These tectonic subsidence movements gradually developed from the downstream Minhe Basin to the upstream Huangyuan Basin.Dating the terrace sequence has potential to uncover the relationship between the subsidence in the catchment and the regional tectonic at the northeastern Tibetan Plateau.     

青藏高原东部牙着库河流阶地研究

构造运动和气候变化是制约内陆地区河流阶地发育的两个关键因素,而不同地区的河流对它们的响应方式多种多样.研究区海子山位于青藏高原东部的沙鲁里山中段,在第四纪期间经历了大幅度构造抬升及第四纪冰川作用.海子山北缘牙着库河谷保留着6级河流阶地,南缘稻城河谷完好地保留着第四纪冰川作用遗迹.本研究运用电子自旋共振技术对牙着库4级高阶地(第3～第6级)的砾石层及稻城河谷的第四纪冰川沉积物进行了测年,并对这4级阶地的形成过程进行了分析.结果表明,牙着库3～6级阶地基座及相应的砾石层均形成于冰消期,分别与深海氧同位素2、6、12、16阶段晚期相对应.待气候进一步变暖而逐渐进入间冰期,海子山冰川消融殆尽,下伏地壳负荷锐减,构造抬升效应的释放结合冰川均衡抬升使得牙着库河谷梯度增大,而同期的河流沉积物通量较小,结果导致流水切割前期加积的沉积物及其下伏基座形成一级新的河流阶地.牙着库河谷自深海氧同位素16阶段后期以来的平均下切速率为0.43 mm/a左右,小于海子山的平均抬升速率2 mm/a,与"河谷下切速率不大于山地抬升速率"一致.