A computer-assisted thin-section study of Lake Baikal sediments: a tool for understanding sedimentary processes and deciphering their climatic signal

A freeze-drying technique for cutting thin-sections of soft sediments without disturbance is used to study several Lake Baikal sedimentary microstructures. Image analysis methodology is applied to selected thin-sections. This new technique provides quantification of the size, shape, orientation and packing of the objects forming the sedimentary structures. Sedimentary processes, which were previously poorly documented, have been identified, and others are better understood. Spheroidal lens-like pure aggregates of the diatom genus Synedra are found in hemipelagic sediments, providing a new insight into their traditional paleoecological interpretation. They are possibly related to a transportation mechanism from the littoral zone or to lacustrine snow. Laminae of Aulacoseira have also been recorded. Evidence of rapid sedimentation suggests they are due to massive algal blooms. The depositional mechanism that was suggested by other studies for explaining the laminations at the Buguldeika uplift is confirmed: the hemipelagic sedimentation is interrupted by terrigenous pulses due to discharge events. The sedimentation rate appears to be increasing during these pulses. Preliminary results from the Academician Ridge show stronger microbioturbation during cold periods. This observation strengthens the hypothesis of intense water circulation during colder times. Thin-section image analysis provides crucial information for deciphering lacustrine records and their regional and palaeoclimatic significance. Received: 27 November 1998 / Accepted: 13 September 1999     

The Anomaly Field of Subsurface Fluids and Its Formation and Evolution Mechanism Before Three Strong Earthquakes in the Northern North China Area

The data of pre-seismic subsurface fluid anomalies of such earthquakes as Datong-YanggaoM_s6.1 event on Oct.19,1989,western Baotou M_s6.4 event on May 3,1996 and Zhangbei-Shangyi M_s6.2 event on Jan.10,1998 are systematically collected and arranged.Then thefeatures of patterns,spatial distribution,time variation and time-spatial evolution of theseanomalies are compared and comprehensively analyzed.Then the formation and evolutionmechanism of medium-and short-term anomaly field of subsurface fluids in the northernNorth China area is proposed.The results show that the medium-term anomaly field is causedby regional tectonic activities,which further strengthen the local tectonic activities andpromote the formation and evolution of the seismic source body.The enhancement of loealtectonic activities causes the formation of anomaly field of short-term subsurface fluids,andthe evolution of source body engenders the source-precursor anomalies of subsurface fluids inthe epicenters at imminent stage.     

利用TLD减小高柔结构多振型地震反应的研究

本文提出了利多多个调谐液体阻尼器（简称TLDS）减小高柔结构多振型地震反应的方法,给出了TLD的布置方案和计算公式,通过钢框架模拟地震振动台试验和理论分析,验证了本文提出的方法的正确性．     

The airflow field and dynamic processes of pyramid dunes

Dune sedimentary structure and wind tunnel experimental results show that the pyramid dunes on the cliff top surface of Dunhuang Mogao Grottoes are formed under three wind directions (NE, SW and NW); the deformation and separation variety of chain dune. The local air circulation (SW) is key to the development of the pyramid dunes. Analysing the morphological dynamic processes indicates that inherent relations exist between trihedral pyramid dunes and other complex sand dune types (linear, reticulate and polyhedral pyramid dunes). The wind regime of multidirection, in conjunction with the connected main and subsidiary sand ridges, resulted in the geomorphological forms adapted to the near-surface air streams. It is concluded that integrated studies, including sedimentary structures and wind tunnel experiments, can make more efficient contributions to an overall, systematic and thorough understanding of the developmental processes of the pyramid dunes than previously used single-study methods.     

泉州市中心城区区域地壳稳定性分析

在前人工作成果的基础上,对泉州市中心城区的区域地壳结构、区域地质构造、新构造运动、区域构造应力场和区域地震活动性进行综合分析.分析结果表明,泉州市中心城区在第四纪中更新世之前位于环太平洋中、新生代构造一岩浆活动带中,地壳运动频繁而又强烈,区域地质构造复杂;现代处于菲律宾海板块对欧亚板块挤压形成的"台湾动力触角"影响区,但区内的断裂不具全新世活动性,新构造运动不强烈,有史以来未见≥5.0级地震记载,属于地壳相对稳定区,未来遭遇6.0级地震的危险性不大,其地震破坏性影响主要来自周边孕震区,特别是东部海域,与"台湾动力触角"的作用和滨海断裂带的活动有关.     

青藏高原古高速远程滑坡沉积学特征研究

高速远程滑坡运动学机理是国际工程地质领域亟待解决的重大前沿性关键科学问题.为探索高速远程滑坡的运动学机理,以青藏高原不同构造背景下的三大高速远程滑坡为研究对象,通过现场工程地质调查与分析,详细探讨了滑坡运动路径上所揭露出的各种表面与剖面沉积学特征,结果显示:(1)沿滑坡运动路径上依次可见大型堆积平台、纵向脊、横向脊、堆...     

从席底构造到第五类原生沉积构造:沉积学中具有重要意义的概念

发育微生物席或微生物膜的沉积底被形象地称为席底,在席底上由于微生物席的发育使沉积物富黏性而抗水流改造,并在沉积面和沉积层内形成一系列奇形怪状的原生沉积构造,如变余波痕、皱饰构造、微生物席碎片、纺锤状或次圆状收缩裂缝等等,这些构造都归为席底构造;与席底相对应,那些不发育微生物席或微生物膜的沉积面则称为混合底,在混合底上单个沉积颗粒受到水流剪切与变形作用的改造而形成诸如正常波痕等原生沉积构造,这些沉积构造则归为混合底构造。席底和席底构造的概念很重要,用来专指微生物沉积作用的产物而区别于混合底和混合底构造。从沉积学概念的演变方面分析,混合底构造就是1964年Pettijohn和Potter所定义的原生沉积构造,他们曾经将其归为4大类并且沿用至今;席底构造也就是微生物形成的(或相关的)原生沉积构造,由于其在形成机理和形态上与Pettijohn和Potter所定义的原生沉积构造存在较大的区别,Noffke 2001年认为其应该具有合适的分类位置,所以就将席底构造归为“第五类原生沉积构造”并纳入Pettijohn和Potter 1964年的原生沉积构造的分类体系之中。从席底构造到第五类原生沉积构造,概念的演变表明了万能的微生物在沉积作用过程中的重要性以及沉积学家对微生物沉积作用认识的深入。这些相关的沉积构造及其所形成的席底,就像叠层石(被当作正生长原生沉积构造)一样在前寒武纪地层之中较为普遍。随着后生动物的大规模出现而造成的寒武纪“农艺革命”(Agronomic revolution)的开始,显生宙的席底及其相关的席底构造多出现在那些不适宜后生动物居住的地方并大多被混合底及其相关的混合底构造所替代;因此,与微生物相关的原生沉积构造,除熟知的叠层石以外,在席底上发育的一系列原生沉积构造,由于其具有特别的指相意义并有助于古环境重塑而被很自然地归到“第五类原生沉积构造”。该类构造由于其在前寒武纪的普遍性和显生宙的局限分布性,特别是在生物大灭绝事件之后的灾难性泛滥,对于研究地球表层环境的演变具有重要意义。所以说,从席底构造到第五类原生沉积构造,是近年来沉积学发展过程中所出现的较为重要的基本概念。根据这一重要概念,对前寒武纪地层中一些常常被认为是遗迹化石、长期以来存在的“臼齿状构造谜”(Molar-tooth structure enigma)等奇形怪状的沉积构造的成因以及从前寒武纪到显生宙地层中均有分布的特殊的纹理化构造等,将会得到一些更加合理的认识;另一方面,关于微生物形成的原生沉积构造的研究,对于了解地球表层生物变化与沉积变化之间的复杂关系将会发现更多的新问题而得到进一步的思考与研究。     

长江中下游成矿带及邻区地壳结构——MASH成矿过程的P波接收函数成像证据？

利用长江中下游成矿带多学科深部探测剖面于2009年11月至2011年3月间采集的天然地震数据,通过天然地震接收函数成像等分析研究,得到了研究区地壳和上地幔结构的清晰图像。接收函数成像结果显示研究区内Moho面深度存在着明显的起伏变化,在长江中下游成矿带(指剖面穿过的长江中下游成矿带宁芜矿集区,下同)下方存在着"幔隆构造"。在剖面东南端(即扬子克拉通北缘),Moho面相对稳定,深度约为30km;在茅山和江南断裂附近,Moho面存在上下起伏现象;在剖面中部或宁芜矿集区下方,Moho面存在明显隆起,深度只有28km;在郯庐断裂带下方,Moho面明显加深,深度达到36km;进一步向北到华北地台南缘,Moho面深度逐渐恢复到了32km左右的平均深度水平。其次,我们在接收函数成像结果中发现,长江中下游成矿带与其周边下地壳结构存在着明显的差异,成矿带的下地壳具有显著的地震波方位各向异性。扬子克拉通北缘的下地壳呈高速的近水平状结构,地震波各向异性特征不明显;与此相比,长江中下游成矿带的下地壳虽然也呈近水平状结构特征,但是,对于沿成矿带走向方向传播的地震波,其下地壳具有高速特征,而对于垂直于成矿带走向方向上传播的地震波,其下地壳却又表现为低速特征,这意味着成矿带的下地壳存在着平行于成矿带走向(即近北东—南西)方向的地震波各向异性,我们解释其是下地壳熔融并沿成矿带走向水平流动导致矿物晶体定向排列的结果。最后,在郯庐断裂以西的华北地台南缘观测到一条从上地壳延伸到中下地壳的南南东向倾斜的转换震相,我们推测它可能是合肥盆地内地壳伸展构造的反映。此外,我们发现接收函数成像结果中观测到的"幔隆构造"与远震P波层析成像结果在成矿带下方150km深度上显示的上地幔低速异常(江国明等,另文发表)存在着良好的对应关系,我们解释它们是软流圈物质上涌的遗迹。综合天然地震接收函数成像、远震P波层析成像和前人关于岩浆岩等方面的研究成果,我们认为长江中下游成矿带现今的下地壳可能是中生代发生成矿作用的多级岩浆房系统的一部分,成矿带的形成可能是类似MASH过程的产物。首先,软流圈物质上涌导致了长江中下游成矿带及其周边拉张环境的形成,在其上部地壳中形成了一系列伸展构造;然后,软流圈物质通过底侵进入长江中下游成矿带的原下地壳并与原下地壳物质发生同化作用,形成类埃达克质岩浆;接着,类埃达克质岩浆沿着伸展、拆离构造上升到地壳浅部形成不同层次的岩浆房和侵入岩体,并与围岩作用形成矿床。     

东海陆架两期沙脊的时空对比

基于高分辨率的单道地震和多波束测深数据,识别并对比了东海陆架中部同一海区相距20余万年的层U₁₄和层U₂两期沙脊群,其中层U₁₄期沙脊属于埋藏沙脊,位于东海海底以下90m深处,推测属于距今320~200ka的海侵体系域(TST),沙脊顶界面是该期海侵的最大洪泛面(MFS);层U₂期沙脊位于东海陆架,属于衰退沙脊,系末次盛冰期(LGM)以来的TST,顶界面是LGM以来的MFS。尽管两期沙脊形成年代相距20余万年,地层层位相距近90m,但是沙脊群总体走向一致,表明距今2×105/a以来东海陆架潮波基本格局稳定。从层U₂期可识别出4个亚期沙脊,通过多波束海底地形图可识别出4组走向的沙脊,多亚期、多走向沙脊是LGM以来海平面阶梯状波动在海底地形演变过程中的响应证据。     

A virtual laboratory for stability tests of rubble-mound breakwaters

The prediction of rubble-mound breakwater damage under wave action has usually relied on costly and time-consuming physical model tests. In this work, artificial neural networks (ANNs) are applied to estimate the outcome of a physical model throughout an experimental campaign comprising of 127 stability tests. In order to choose the network best suited to the problem data, five different activation function options and 38 network architectures are compared. The good agreement found between the physical model and the neural network shows that an ANN may well serve as a virtual laboratory, reducing the number of physical model tests necessary for a project.