A model for the active origin and development of source-bordering dunefields on a semiarid fluvial plain: A case study from the Xiliaohe Plain, Northeast China

Source-bordering dunefields have been reported in some drylands of the planet, but scarcely in China where there are extensive drylands. This article reports them in China for the first time, and presents a model for their active origin and development on a semiarid fluvial plain by means of satellite image analyses and field investigations. Local- and regional-scale examples are chosen to analyze the spatial patterns of dunefields, as well as the relationships with the fluvial systems in the central part of Naiman Banner where the Jiaolai River runs, and the lower Laoha River, and the middle and lower Ulijimulun River (principal tributaries of the Xiliaohe River). The active origin and development of source-bordering dunefields can be divided into four stages in terms of the spatial patterns of dunefields and channel dynamics: Stage I — individual dunes on the downwind margins of river valleys where running water constantly erodes the steep slopes of valley and where the downwind slopes orient to local dominant winds; Stage II — individual local-scale dunefields formed by deflation of the steep valley slopes and extending antecedent dunes downwind, together with the downstream displacement of meanders; Stage III — individual large-scale dunefield belts along the downwind margins of river valleys formed through frequent lateral migrations of channel; Stage IV — regional-scale dunefields formed mainly by river diversions due to climatic changes or tectonic movements. On the one hand, it is the running water's lateral migration, especially meandering, that prepares suitable places for aeolian systems in terms of both wind flow fields and sand sources, and subsequently it can further cause separate local-scale source-bordering dunefields to link together as a regional-scale dunefield belt given sufficient time. On the other hand, diversions of the river are bound to occur following changing hydrologic regimes resulting from tectonic movements or significant climate change (at regional and millennium scales). As a result, when some dunefield belts as well as the adjacent channels are abandoned, new channels work elsewhere in the same way to actively form new source-bordering dunefields and even dunefield belts at a regional scale.     

海温异常对东亚夏季风及长江流域降水影响的分析及数值试验

对夏季的向外长波辐射(OLR)场及前期热带海温之间的奇异值分解(SVD)分析表明,当自前冬开始的热带海温呈La Nia 型异常分布,相应的对流的最佳耦合模态将是:从印度半岛经孟加拉湾直至南海以及西太平洋暖池都为强对流活动区,而在中国江淮流域地区则为弱区.当海温呈El Nio分布时,则相反.数值试验的结果是:在1998年实际海温强迫下,模式十分成功地模拟了该年夏季流场的基本特征,南海季风爆发的日期及强度皆与实际十分符合,印度季风也偏弱.特别是模式成功地模拟出了1998年长江流域6～7月的强降水,距平百分率达到100%以上.改变热带海温,分别用气候海温代替赤道东太平洋和西印度洋实际海温.结果发现,在用赤道东太平洋气候海温取代实际海温时,与1998年实况相比,印度季风与南海季风明显加强,长江流域降水趋于正常值.即如果1998年不出现El Nio现象,或者海温异常的强度减弱,则长江流域降水就会大大减弱.1998年热带西印度洋海温异常对东亚季风的影响与热带东太平洋十分相像.暖洋面将有利于长江流域的强降水,而不利于南海季风的加强.关键词:海面温度;长江流域;洪涝;南海季风;数值试验     

冰雪融水沟谷纵剖面的形态与演化模式

冰雪融水是冰川泥石流的一种主要激发动力。冰雪融水沿程(s)流量Qs=q0(S1/2-s^2/2S)。其沟谷纵剖面形态模式近似为抛物线型：h/H=(s/S)^N。冰雪融水泥石流流域水文地貌系统的熵可用沟谷纵剖面形态指数N来表征：信息熵P=In[(N 2)/2]-N/(N 2)，超熵δxP=N^3(N^2-4)(N 2)，[32(6-N)]。冰雪融水沟谷纵剖面演化的最小能耗模式表现为沿程流速平均值u↑-与N值成正比：u↑-∝(N)。f(N)为流速函数，f(N)={2/3—2/[(N 1)(N 3)]}^1/2；冰雪融水泥石流沟的各地貌发育阶段开始时的u↑-值之比为：泥石流发展阶段(N=0．62)1．0；泥石流旺盛阶段(N=1．23)1．182；泥石流衰减阶段(N=2．0)1．280；流域稳定阶段(N=3．71)1．361。以藏东南典型冰雪融水泥石流沟谷为例，检验了上述沟谷纵剖面之形态与演化模式。     

速流结构防治泥石流的理论及应用

作为一种公路沿线发育的地质灾害，泥石流是路基、路面及相关建、构筑物最强烈的毁损动力。为了防治泥石流公路灾害，作者开发了速流结构。该结构由汇流槽和速流槽组成。其中汇流槽的侧墙长度、高度’、厚度等参数通过泥石流沟形态及其冲击力综合确定；而速流槽的宽度、侧墙及纵横断面则由泥石流排泄量、流速、锁固桩及流体性质综合确定。根据公路穿越速流结构的方式，将速流结构分为底越式和顶越式2种。速流结构对于解决具有大冲大淤特性的冲淤变动型沟谷泥石流灾害具有重要应用价值。据此可以确保穿越泥石流沟的公路构、建筑物的安全与稳定．确保公路交通有序进行。此外，文章对速流结构的一些子结构，如速流槽侧墙及锁固桩进行了力学分析。位于川西南西昌-木里干线公路的平川泥石流属于典型的冲淤变动型沟谷泥石流。该沟长度8．2km，沟床平均比降0．181，平均流速9．0m／s，沉积区宽度500m左右。为了确保该段公路交通的有序进行，作者干1999年实施了平川泥石流速流结构。通过2000年以来的研究及现场测试，显示了该防治结构的优越性。但是，作为一种特殊的水工结构，泥石流对速流结构的磨蚀作用是比较强烈的。因此，开展速流结构的抗冲、抗撞研究是进一步研究具有重要价值。     

青藏高原的地幔结构:地幔羽、地幔剪切带及岩石圈俯冲板片的拆沉

正断层位移长度关系是近年来研究正断层的一种常用方法 ,它将断层的研究从二维扩展到三维。研究正断层位移长度关系对于研究正断层演化、盆地演化以及油气勘探等方面具有重要意义。讨论了影响正断层位移距离剖面形态的主要因素 :断层端的脆韧性变形、围岩强度、远程应力以及断层之间的相互作用和连接等。根据正断层位移距离剖面的几何形态和断层发育阶段 ,可将断层的位移模式分为 3个类型 :(1)对称的椭圆状或钟状代表简单的单条断层 ;(2 )不对称的似椭圆状或钟状代表断层间的相互作用 ;(3)不规则的锯齿状代表由多条断层连接而成的断层。介绍了伴随正断层发育的传递斜坡、盆内高地等构造单元以及断层位移距离剖面的测量制作方法。测量时应注意恢复其剥蚀部分。     

Differentiating the effect of episodic tectonism and eustatic sea-level fluctuations in foreland basins filled by alluvial fans and axial deltaic systems: insights from a three-dimensional stratigraphic forward model

Many studies of foreland basins have recognized a hierarchical organization in the stacking of sequences deposited by axial‐deltaic and alluvial fan systems. The hierarchy is often explained in terms of the competing control of eustasy and pulsed tectonic subsidence and the different frequencies at which these processes operate. Unravelling the relative contributions of tectonic and eustatic controls on the sequence stacking pattern is a fundamental question in foreland basin analysis, yet this is difficult because of the lack of independent stratigraphic evidence. In this study, a three‐dimensional numerical model is presented, which aids in the interpretation of alluvial successions in foreland basins filled by transverse and axial depositional systems, under conditions of variable tectonism and eustatic sea‐level change. The tectono‐sedimentary model is capable of simulating the hierarchical stratigraphic response to both eustatic and tectonic forcing, and is of higher resolution than previous models of foreland basin filling. Numerical results indicate that the onset of tectonic activity is reflected by rapid retrogradation of both depositional systems and by widespread flooding and onlap of carbonate sediments. Syntectonic fluvial patterns on the axial‐deltaic plain are dominated by bifurcating channels, swiftly relocating in response to the general rise in relative sea level induced by flexural subsidence. The resulting surface morphology of the axial delta is convex upwards. Syntectonic eustatic sea‐level fluctuations result in parasequence‐scale packages of retrograding and prograding fan and delta sediments bounded by minor flooding surfaces and type 2 sequence boundaries. Incised channels are rare within the syntectonic parasequences and are formed only during phases of tectonic quiescence when eustatic falls are no longer compensated by the subsidence component in the rise in relative sea level. Suites of amalgamating, axial channels corresponding to multiple eustatic falls delineate the resulting type 1 unconformities. Coarse‐grained, incised‐channel fills are found in the zone between the alluvial fan fringes and the convex‐upward body of the axial delta, as the axial streams tend to migrate towards this zone of maximum accommodation.     

Asymmetrical valleys created by the geomorphic response of rivers to strike-slip fault

Offset fluvial valleys, including rivers beheaded and deflected by strike-slip faults, have long been used to estimate horizontal displacements on the faults. Larger rivers crossing such faults, however, sometimes show either no offset or only a small amount of offset compared to smaller rivers crossing the same faults. The larger rivers with higher erosional rates may widen their valleys asymmetrically downstream of strike-slip faults, rather than being beheaded or deflected. Examples are described from the Yellow River near the NE margin of the Tibetan Plateau. River beheading and asymmetrical widening are two end-members of a fluvial valley's response to strike-slip faulting, whereas deflection is a combination of both. Recognition of the formation of such asymmetrical valleys related to strike-slip faulting will help to understand fault activity better over longer time spans and enable a re-evaluation of many fault histories worldwide.     

Geomorphologic characteristics and origin of the valley bottom troughs at the site of Three Gorges Dam

This paper describes valley bottom troughs of the Changjiang River and infers the geomorphologic development of troughs. Based on the morphology of the troughs, the following conclusions are drawn. (1) The deep troughs on the Three Gorges valley bottom are formed by river downcutting along the structural zones on the background of regional tectonic uplift at about 40-30 ka BP. (2) When river downcutting occurred in the river bed of Changjiang, the jets current (particularly eddy current) with a large number of pebbles ground and eroded the valley bottom, resulting in trough formation and deepening. Meanwhile, water currents with gravels and pebbles eroded the bank and the left wall of No.76 trough as well as the right wall of No.77 trough by striking, scouring, horizontal and vertical grinding. (3) The depth of the trough is mainly determined by the intensity of the water current and the consistency of bedrock against erosion, and is not controlled by the altitude of the sea level as the base level of erosion.     

沟谷系统中流域面积与周长关系及其地貌学意义

通过对《中国泥石流数据库》中３５３２个流域的形态数据的处理,得出了一个流域面积与周长的关系,并对其指数地貌学意义进行了初步的探讨．