华北地区热均衡、重力均衡与深部构造

本文基于最新的全球重力场模型TopexV18.1数据,在区域背景尺度下,计算了华北地区的区域布格重力异常,并反演了Moho面分布特征;在此基础上结合区域地热流测量数据,讨论重力均衡和热均衡调整过程对现今地形的影响;最后,结合已有的速度结构模型和岩石圈热参数,对10个次级块体单元的热均衡特点开展了综合对比研究.研究结果表明:华北地区构造块体单元之间由于克拉通底部受到不同程度的破坏,不仅深部构造特征差异性明显,而且其构造热均衡特点也不尽相同.根据平均地震波速去掉重力均衡作用后恢复的高程-热流关系表明,均衡作用对于现今地形的改造作用十分显著,但根据该方法对燕山隆起和冀中坳陷两个块体的恢复效果不明显.     

海山的倾斜俯冲对上覆板块变形的影响

伴随洋壳的俯冲,驼伏其上的海山会导致上覆板块的强烈变形.为解释该构造变形特征,本文运用物理模拟实验的方法,着重分析海山的斜向俯冲对上覆板块变形的影响,并将模拟结果与正向俯冲过程进行对比.实验结果显示:海山开始进入俯冲,前缘楔体的增生会被阻止,同时楔体被抬升并出现脱顶构造,未被海山破坏的楔体会出现后冲断层的激活,后冲断层轴平行于海山的俯冲方向.海山进一步俯冲,突起项部发育一系列张扭性质的微断裂和走滑性质的共轭断裂,尾随突起之后的楔体由于重力会产生正断层系统.比起正向俯冲,斜向俯冲过程中所产生的后逆冲体、海山两侧的叠瓦状逆冲推覆构造都出现不对称分布,断裂和微断裂束的走向不规则散开,后冲断层的轴向及海山俯冲过后在楔体上产生的凹槽的轨迹都不断斜向迁移,且凹槽两侧的地势不一致等.最后利用文中的物理模拟结果,很好的解释了马尼拉海沟中段俯冲构造的构造特征,同时对其他俯冲大陆边缘的构造解释具有指导意义.     

Paleoenvironmental implications of high-density records in Co-rich seamount crusts from the Pacific Ocean

Co-rich seamount crusts have been shown to possess great potential for providing information on paleoceanographic and paleoclimatic changes. High resolution data are essential to decipher and correctly understand such high-density records. With the development of modern microprobe techniques, detailed sampling of crusts can be performed and it is possible to retrieve detailed information about envi- ronmental changes recorded in the seamount crusts. We report here geochemical results of more than 40 elements (including all rare earth elements) of four Co-rich seamount crust samples, which were collected from seamounts in the central and western Pacific Ocean. These data were obtained with two micro-probe techniques: Electron Probe Micro Analyzer and Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The chronological framework of the seamount crust samples was determined using the cos- mogenic 10Be and the Co-chronometer. Records of elemental composition, P, and Al/(Fe Mn) and Y/Ho ratios across the sections of the four samples are used to identify paleoceanographic and paleoclimatic events over the past ~30 Ma. These data show that: (1) Al/(Fe Mn) in the western Pacific seamount crust is a useful proxy for the assessment of changes of source materials related to the variability of the Asian monsoon; (2) P and Y/Ho can be used as proxies to infer biogenic episodes. Finally we discuss the methodology related to dating and micro-probe analysis used in crust study.     

利用大地测量和地热资料联合确定黑水—泉州地学断面东段岩石圈热及强度结构

黑水泉州地学断面因其所处的特殊地理位置和所受复杂地球动力学机制的影响,而备受地学界的关注.但要弄清控制岩石圈动力机制的重要影响因素之一——热动力,还需对岩石圈的热结构有更细致的研究.地热学研究作为一种独立的地球物理学方法,在地球内部结构及地球动力学研究中发挥着越来越重要的作用.根据地热资料来研究地球深部热结构是一种最为直接有效的方法,但是,我国地热资料的相对匮乏严重阻碍了地热研究的进展.本文将引入地形、重力等大地测量观测资料作为约束条件,基于岩石圈均衡原理,综合地热和重力学方法联合确定黑水泉州地学断面东段岩石圈热结构,弥补了传统地热研究方法的不足.基于黑水泉州地学断面岩石圈二维温度分布结果,本文计算了该岩石圈的强度分布,并对其岩石圈的热结构和流变结构特征展开了分析和讨论.     

Osmium isotope of the Co-rich crust from seamount Allison,central Pacific and its use for determination of growth hiatus and growth age

Based on its microstructure, Co-rich crust A1-1 from seamount Allison, central Pacific, was scraped at averaged interval of 1.3 mm to measure osmium isotopic composition, and subsequently to establish the 187Os/188Os profile of scraping section of the crust. By observing the variation of 187Os/188Os under 10Be chronology and matching it to the well-known seawater Os isotope evolution of the past 40 Ma, two growth hiatuses (H1 and H2) occurring in the periods respectively between 13.6 and 29.6 Ma and between 8 and 9.8 Ma in the crust were recognized. According to the two hiatuses, the dating scheme for each scraped layer of the crust was suggested. For the upper layers younger than 6.8Ma, their growth ages were calibrated under 10Be chronology; for the lower layers older than 6.8Ma, their growth ages were obtained from 187Os/188Os evaluation curve by linear interpolation. Hereby, the age for the most inner layer of the crust was determined to be 39.5 Ma. H1 and H2 exactly correspond to the boundary between phosphatization and non-phosphatization and volcanic ash layer in the crust, respectively.     

新疆精河6.6级地震周边地区密度构造、均衡异常以及岩石圈挠曲机理

依据EIGEN-6C4重力模型和ETOPO1高程模型数据,围绕新疆精河6.6级地震展开岩石圈均衡与挠曲机理研究,得到如下结论：（1）震中附近的布格与自由空气重力异常分别为-221和-92 mGal（10^-5m·s^-2）,震中位于重力异常高梯度带上;（2）震中周边地区地壳厚度约为50 km,密度结构总体变化平缓,东西方向地壳厚度变化较小,但自南向北地壳厚度逐渐变薄,精河6.6级地震初始破裂发生在上中地壳分界面附近;（3）震中附近岩石圈承载的垂向构造应力为20 MPa左右,震中位于岩石圈垂向构造应力极大值附近的高梯度带上;（4）地震周边地区岩石圈有效弹性厚度最优解为26 km,加载比最优解为F₁=1,F₂=F₃=0,表明该区域岩石圈相对坚硬,且导致岩石圈变形的初始加载全部来自地表.

     

Two-stage model of incorporation of seamount and oceanic blocks into sedimentary melange: Geochemical and biostratigraphic constraints in Jurassic Chichibu accretionary complex, Shikoku, Japan

Abstract The significance of timing and formation of mélange in accretionary prisms, particularly concerning basaltic and related rocks and pelagic sediments, is exemplified in the Sawadani area of the Jurassic Chichibu accretionary complex in Shikoku, southwest Japan. Major and trace element geochemistry of the basaltic and related rocks indicates that all are of a hot-spot origin which produced a seamount. Most of the rocks have a trend of differentiation from an alkalic parental magma. The time relationship between the blocks and matrices of the mélange deduced from radiolarian fossil evidence and macro- to microscopic characteristics of contacts between different lithologies indicates two stages of mixing of materials in the seafloor. The first mixing occurred on the flank of the seamount in the pelagic environments in the Late Permian, and the second occurred on the trench floor or in the accretionary prism after the Early Jurassic. These two stages show respectively the geological phenomena of a seamount within the Izanagi-Kula plate and its incorporation into the Asian continental margin.     

The relationship between the growth process of the ferromanganese crusts in the pacific seamount and Cenozoic ocean evolvement

Base on the Os isotope stratigraphy together with the empirical growth rate models using Co concentrations, the growth ages of the ferromanganese crusts MHD79 and MP3D10 distributed in the seamount of Pacific are confirmed. Through the contrast and research on the previous achievements including ODP Leg 144 and the crusts CD29-2, N5E-06 and N1–15 of the seamount of the Central Pacific, the uniform five growth and growth hiatus periods of them are found, and closely related to the Cenozoic ocean evolvement process. In the Paleocene Carbon Isotope Maximum (PCIM), the rise of the global ocean productivity promoted the growth of the seamount crust; the first growth hiatus (I) of the ferromanganese crust finished. In the Paleocene-Eocene Thermal Maximum (PETM), though the vertical exchange of seawater was weakened, the strong terrestrial chemical weathering led to the input of a great amount of the terrigenous nutrients, which made the bioproductivity rise, so there were no crust hiatuses. During 52–50 Ma, the Early Eocene Optimum Climate (EECO), the two poles were warm, the latitudinal temperature gradient was small, the wind-driven sea circulation and upwelling activity were weak, the terrestrial weathering was also weakened, the open ocean bioproductivity decreased, and the ferromanganese crust had growth hiatus again (II). From early Middle Eocene-Late Eocene, Oligocene, it was a long-term gradually cooling process, the strengthening of the sea circulation and upwelling led to a rise of bioproductivity, and increase of the content of the hydrogenous element Fe, Mn and Co and the biogenous element Cu, Zn, so that was the most favorable stage for the growth of ferromanganese crust (growth periods III and IV) in the studied area. The hiatus III corresponded with the Eocene- Oligocene boundary, is inferred to relate with the global climate transformation, celestial body impact event in the Eocene-Oligocene transition. From the early to the middle Miocene, a large-scale growth hiatus (hiatus period IV) of the ferromanganese crust in the studied area is inferred to relate with temporary warm up climate and ephemeral withdrawal of Antarctic bottom water in the early Miocene. After that, the Antarctic ice sheets extended, the bottom water circumfluence strengthened, the ocean fertility increased, and the once interrupted crust continued to grow in the late Miocene (growth period V). Supported by China Ocean Mineral Resources Research and Development Association “10th Five Year” Topic (Grant No. DY105-01-04-14)     

Least action principle,equilibrium states,iterative adjustment and the stability of alluvial channels

The energy based least action principle (LAP) has proven to be very successful for explaining natural phenomena in both classical and modern physics. This paper briefly reviews its historical development and details how, in three ways, it governs the behaviour and stability of alluvial rivers. First, the LAP embodies the special stationary equilibrium state of motion and so its incorporation with the principle of energy conservation explains why so many optimizing hypotheses have been proposed in fluvial geomorphology. Second, the variational approach underlying the LAP provides a more straightforward and simpler fuzzy‐object orientated method for solving river regime problems than do the various complex Newtonian formulations. Third, it is shown that in fluvial systems with surplus energy the surplus can be expended with slope and/or channel geometry adjustments, with the degree of channel geometry adjustment quantified by the dimensionless numbers F for depth dominated adjustment and H for width/depth dominated adjustment. Different planforms are preferred at different energy levels, with H providing a quantitative measure of the flow's efficiency for moving sediment. In rivers with insufficient energy, the interactions of endogenous and exogenous factors are shown to be capable, in certain circumstances, of achieving a stationary equilibrium condition which acts as the attractor state. Importantly, this study describes how iterative changes enable systems to achieve such a stable equilibrium. Copyright © 2007 John Wiley & Sons, Ltd.     

Using equilibrium temperature to assess thermal disturbances in rivers

Flow regulation is widely known to modify the thermal regime of rivers. Here, we examine the sensitivity of an empirical approach, the Equilibrium Temperature Concept (ETC), to detect both the effects of hydraulic infrastructures on the annual thermal cycle and the recovery of the thermal equilibrium with the atmosphere. Analysis was undertaken in a Pyrenean river (the Noguera Pallaresa, Ebro basin) affected by a series of reservoirs and hydropower plants. Equilibrium temperature (T_e) is defined as the water temperature (T_w) at which the sum of all heat fluxes is zero. Based on the assumption of a linear relationship between T_e and T_w, we identified changes in the T_e–T_w regression slope, used as an indicator of a thermal alteration in river flow. We also assessed the magnitude of the alteration by examining the regression slope and its statistical significance. Variations in the regression parameters were used as indicators of the influence of factors other than atmospheric conditions on water temperature. Observed T_w showed a linear relationship with T_e at all river stations. However, the slopes of the T_e–T_w relationship appeared to be lower in the reaches downstream from hydraulic infrastructures, particularly below large dams. A seasonal analysis indicated that T_e–T_w relationships had higher slopes and lower p‐values during autumn, while no significant differences were found at other seasons. Although thermal characteristics did not strongly depend on atmospheric conditions downstream of hydraulic infrastructures, the river recovered to pre‐alteration conditions with distance downstream, indicating the natural tendency of water to attain thermal equilibrium with the atmosphere. Accepting associated uncertainties, mostly because of the quality of the data and the lack of consideration of other factors influencing the thermal regime (e.g. discharge), ETC appears to be a simple and effective method to identify thermal alterations in regulated rivers. Copyright © 2015 John Wiley & Sons, Ltd.     

K–Ar ages from seamount chains in the back-arc region of the Izu–Ogasawara arc

The Izu–Ogasawara arc contains, from east to west, a volcanic front, a back-arc extensional zone (back-arc knolls zone), and a series of across-arc seamount chains that cross the extensional zone in an east-northeast and west-southwest direction and extend into the Shikoku Basin. K–Ar ages of dredged volcanic rocks from these across-arc seamount chains and extension-related edifices in the back-arc region of the Izu–Ogasawara arc were measured to constrain the volcanic and tectonic history of the arc since the termination of spreading in the Shikoku Basin. K–Ar ages range between 12.5 and 1 Ma. Andesitic to dacitic rocks of 12.5–2.9 Ma occur mainly on the western part of the chains. The western part of the chains are the locus of volcanism behind the front which erupted mainly calc-alkaline andesitic lavas. The youngest rocks (< 2.8 Ma), characterized by cpx-ol basalt, occur along the western margin of the back-arc knolls zone. Basaltic rocks of 12.5–2.9 Ma have relatively high concentrations of Na₂O (> 2.0 wt%), Zr (> 50 p.p.m.) and Y (> 20 p.p.m.) and low CaO (< 12 wt%). On the other hand, basalts of 2.8–1 Ma have lower Na₂O (< 1.8 wt%), Zr (< 50 p.p.m.) and Y (< 20 p.p.m.), but significantly higher CaO (> 12 wt%). The age inferred for the initiation of back-arc rifting (∼ 2.35–2.9 Ma: Taylor 1992 ) behind the current volcanic arc coincides with the time that basalt chemistry changed drastically (eruption of the low-Na₂O and high-CaO basalt). This implies that post-2.8 Ma volcanism in the back-arc knolls zone is associated with rifting. Similarly, the change in chemical composition might be explained by a different type of source mantle following rift initiation. Volcanism in the western seamounts ceased after the onset of rifting at ∼ 2.8 Ma.