包头地区二次大雪过程的成因分析

文章应用常规资料,从天气环流形势演变、物理量等方面,对2000年1月10—11日包头地区普降大雪过程作了客观的分析,揭示了大雪过程的发生、发展机制,对今后分析大雪过程具有一定指导意义。     

2008年前汛期广东长连续暴雨过程的500 hPa环流场特征

2008年5月下旬末～6月18日,广东遭受1951年以来最严重的“龙舟水”袭击。通过分析这次长连续暴雨过程开始、持续和结束期的500hPa侯平均环流特征和稳定维持的大尺度环流场特征,发现欧亚中高纬维持两槽两脊或两槽一脊稳定的大环流形势,为暴雨的持续提供了弱冷空气;孟湾低槽为华南地区提供了充足的水汽条件;西太平洋副热带高压的加强西伸,使华南沿海地区暖湿气流加强而产生强降水。     

冲绳海槽岩浆物理化学性质及其地质意义

借助冲绳海槽火山岩地球化学分析结果,讨论了海槽不同区段岩浆的温度、压力、密度和黏度等物理化学参数,以确定海槽岩浆的物理性质及其对海槽不同区段岩浆演化过程的影响.海槽北段的岩浆相对于中段表现出高压力、低温度的特征.由于海槽岩浆挥发分含量较高,北段岩浆的密度和黏度要高于中段的.但总的来说,海槽岩浆具有低密度、低黏度的特点,因此岩浆更容易穿透岩石圈,而且有利于岩浆结晶分异作用的进行.对海槽岩浆相对运移速度的研究显示,在外部条件及含水量相同的前提下,在海槽中段岩浆上升速度要大于北段的,这从岩浆物理化学性质的角度解释了为何海槽中段火山活动要强于北段的.     

Historical tsunamis and storms recorded in a coastal lowland,Shizuoka Prefecture,along the Pacific Coast of Japan

Four sand units deposited by tsunamis and one sand unit deposited by storm surge(s) were identified in a muddy marsh succession in a narrow coastal lowland along the Pacific coast of central Japan. Tsunamis in ad 1498, 1605, 1707 and 1854 that were related to large subduction‐zone earthquakes along the Nankai Trough, and storm surges in 1680 and/or 1699 were responsible for the deposition of these sand units. These sand units are distinguished by lithofacies, sedimentary structures, grain‐size and mineral composition, and radiocarbon ages; their ages are supported by events in local historical records. The tsunami deposits in the study area are massive or parallel‐laminated sands, with associated intraclasts, gravels, draping mud layers and, rarely, a return‐flow subunit. The storm surge deposits are devoid of these characteristics, and are composed of groups of thin, current ripple‐laminated sand layers. The differences in sedimentary structures between the tsunami and storm surge deposits are attributed to the different characteristics of tsunami and storm waves.     

The resistivity structure of the Penola Trough,Otway Basin from magnetotelluric data

We present inversion results for a 100 site, broadband magnetotelluric (MT) survey in the Penola Trough, Otway Basin, South Australia. The Penola Trough is host to several petroleum reservoirs and has more recently been a target for unconventional geothermal exploration. We present two interpretations of the MT data. A 1D anisotropic interpretation, where anisotropy is determined within the Otway Basin sequence and basement in the northeastern Penola Trough, fits the impedance tensor well. However, the anisotropy strike is inconsistent with the known orientation of electrically conductive fractures in the Penola Trough. On the other hand, a 3D interpretation, which incorporates lateral variations in resistivity, requires no anisotropy yet it matches the data equally well. Both the 1D and 3D inversions resolve several layers within the Otway Basin sequence, which correspond to stratigraphic units defined in wells and in the coincident Haselgrove–Balnaves 3D seismic survey. These include the Eumeralla and Dilwyn formations, which are poorly resolved in the seismic data. The basin architecture, defined in the 3D inversion, in particular the depth to basement, is consistent with previous interpretations based on seismic reflection data that show that the Otway Basin thins in the northeastern Penola Trough. This does not occur in the anisotropic model. We therefore conclude that the subsurface resistivity appears to be isotropic in the Penola Trough. This contrasts with the anisotropic resistivity structure determined in a previous study in the Koroit region, eastern Otway Basin. The difference in the MT responses between the two regions is supported by resistivity and permeability information from well logs and may reflect differences in the orientation of subsurface fractures, or differences in the present-day stress field, between the two regions.     

冲绳海槽黑色与白色浮岩特征及其对岩浆演化的指示

为了认识黑色与白色浮岩的岩石特征及成因,使用人工重砂、元素地球化学等分析技术,研究冲绳海槽两类浮岩中元素及矿物组成特征。研究显示,冲绳海槽黑色和白色浮岩具有相似全岩化学组成,都落入流纹岩区,稀土配分曲线明显呈右倾,具负Eu异常,富集不相容元素,岩石物理性质差别是导致其颜色差异的主要因素。两类浮岩岩石学特征的不同主要体现在:(1)白色浮岩锆石和辉石都具有两种类型,黑色浮岩仅具有其中一种;(2)黑色浮岩中发育钛铁矿、钛磁铁矿与基质构成的珠状构造,且基质玻璃中密集分布磁铁矿雏晶,而白色浮岩不具备上述岩石学特征。综合分析海槽两类浮岩岩石学特征所蕴含的岩浆演化信息,推测两类浮岩具有相同的玄武质岩浆源区,富挥发组分的玄武质母岩浆上升进入地壳,形成初级岩浆房,房内结晶分异后残留的酸性岩浆,沿海槽构造薄弱带向上运移,岩浆运移过程中有少部分进入分支断裂系统形成次级岩浆房,期间初级岩浆房接受多次基性岩浆注入;初级和次级岩浆房中的酸性岩浆分别独立喷出海底,前者形成白色浮岩,后者形成黑色浮岩。此外,演化过程中岩浆性质逐渐由氧化性向还原性转变。     

冲绳海槽中部17 000 a以来沉积物中微量元素的组成特征及其对古环境的指示

基于加速器质谱仪AMS14C高精度定年,以及应用电感耦合等离子体质谱仪(ICP-MS)对S10孔岩芯沉积物进行微量元素测试,对冲绳海槽中部沉积物的微量元素组成及其记录的物源和古环境信息展开研究。研究结果显示,稀土元素、Th、Nb和Ta元素反映陆源碎屑混合矿物的化学组成特征,Zr和Hf元素指示锆石矿物的化学组成,锆石主要为火山来源,Sc元素可能与铁镁物质有关。稀土元素分析表明沉积物主要由陆源和火山源物质组成,陆源物质主要来自长江和黄河,不同时期,各源区物质贡献量不同:16 500~11 600 a,长江、黄河为沉积物的主要物质来源;11 600~7 750 a,长江物质贡献减少,黄河物质为主;7 750~6 450 a,K-Ah火山物质为主,长江、黄河物质供给骤减;6 450~3 900 a,长江、黄河陆源物质输入增加,4 000 a左右受火山作用影响;3 900~1 900 a,长江、黄河陆源物质输入持续增多;1 900 a以来长江、黄河物质仍有增加,且以黄河物质为主。此外,物源判别公式研究显示台湾物质输入量不能有效反映黑潮演变,而长江、黄河物质输入量对17 000 a以来东亚冬季风的强弱变化有很好的指示,可作为东亚冬季风演变的新证据。这些研究结果表明,冲绳海槽中部微量元素研究可得到海槽17 000 a以来物源及东亚冬季风的演化情况,有助于重建全新世以来沉积物物源及古环境的演变历史。     

西太平洋典型弧后盆地的地质构造、岩浆作用与热液活动

研究了西太平洋典型弧后盆地冲绳海槽和马努斯海盆的区域地质背景、岩浆岩、喷口流体、热液柱、沉积物以及多金属硫化物。结果表明,冲绳海槽在构造和地球物理特征上南北分异,从北向南地壳厚度减薄,南部重力异常变化大,发育条带状磁异常,中部则具有最高的热流值。马努斯海盆的重力异常变化较小,磁力异常呈东西向展布,海底扩张中心附近出现磁异常条带。马努斯海盆已出现洋壳,冲绳海槽的地壳属于过渡性地壳,在中南部的地堑中可能已出现洋壳。 冲绳海槽与马努斯海盆玄武岩的岩浆是由地幔源区部分熔融产生的原始岩浆与板块俯冲组分混合构成。中酸性岩与基性岩具有相同的岩浆物质来源,是玄武质岩浆结晶分异的产物。与马努斯海盆火山岩相比,冲绳海槽火山岩存在不同程度的地壳混染。 冲绳海槽的喷口流体为富含金属元素（Mn、Fe、Zn、Pb）的酸性高温（高达320 °C）流体,其热液柱中的Zn2 、Cd2 、Pb2 、Cu2 离子浓度明显高于正常海水。马努斯海盆的热液柱呈现出CH4、Mn、Al、δ3He正异常及pH负异常的特征。与马努斯海盆PACMANUS热液区的喷口流体（220–276 °C、pH = 2.5–3.5）相比,DESMOS热液区的喷口流体温度（≥88–120 °C）相对较低,酸性更强（pH ≤ 2.1）,二者较低的δ34SH2S和δD值表明岩浆演化过程中有酸性挥发分进入岩浆流体。冲绳海槽与马努斯海盆含金属沉积物的分布及其元素含量特征具有差异,相应地,两个弧后盆地含金属沉积物中的微生物群落也不同。冲绳海槽和马努斯海盆热液区均具富Zn型、Ba-Pb-Zn型、Si-硫化物型和Si-硫化物-硫酸盐型热液产物。与冲绳海槽的多金属硫化物相比,马努斯海盆中多金属硫化物的Pb和Au含量较低。     

冲绳海槽中段表层沉积物物质来源的定量分离:Sr-Nd同位素方法

利用1994年85－904航次获取的冲绳海槽中段表层沉积物样品，测定其硅酸盐相的^87Sr/^86Cr和^143Nd/^144Nd比值，以定量研究冲绳海槽中段表层沉积物硅酸盐物质的来源，结果表明，冲绳海槽中段陆坡区表层沉积物硅酸盐相的^87Sr/^86Sr/^143Nd/^144Nd与东海陆架沉物硅酸盐相的^87Sr/^86Sr和^143Nd/^143Nd相当，槽底和东坡沉积物硅酸盐相的^87Sr/^86Cr和^143Nd/^144Nd值介于中国大陆硅酸盐物质和海槽水山碎屑的^87Sr/^86Sr,^143Nd/^144Nd比值之间，并大致具有两端员混合特征，利用以Sr同位素为参数的二端员混合方程进行物源定量分析的结果表明，冲绳海槽中段的西坡和槽底表层沉积物硅酸盐相以陆源物质为主，最大值为91．07％，由西向东，陆源物逐渐减少，在海槽东坡含量最小，最小值为9．93％，火山源物质在东坡含量最高，量大值为64．92％，由东向西，由北向东，火山源物质逐渐减少为零。