On the rifting dynamics of plate divergence and magma accumulation at oceanic ridge axes

Rifting dynamics at spreading axes is governed by two processes: the large-scale plate divergence and the local magma accumulation in the crust-mantle transition layer. Both evolve simultaneously. A model is developed particularly for the situation in Iceland where a well studied rifting episode occurred in the Krafla volcanic system 1975–1984. Both the divergence and the buoyant rise of magma create tensile deviatoric stress in the axial region, but while divergence generates an altogether extensional stress field, uprising of buoyant melt produces tension only near the axis but compression of the sides. The buoyant rise is driven by the differential pressure gradient in rock and melt. The processes are studied with a two-dimensional finite-element routine. Presently thermal effects are neglected. The model parameters are density difference, size of the buoyant body, externally applied stress field, mechanical properties of rock and melt. Relatively small amounts of divergence and small increases of buoyancy are shown to generate axial tension which can overcome the tensile strength. Axial tension produced by buoyant bodies can even overcome lateral compression. Observed long intervals of quiescence require either large rock strength, quasi-continuous stress relaxation, small buoyant bodies, and/or a compressive deviatoric stress normal to the axis during much of the time between rifting episodes. Buoyant rise and injection of melt must be important in generating compression.     

Impact assessments and policy responses to sea-level rise in three US states: An exploration of human-dimension uncertainties

Uncertainties in the human dimensions of global change deeply affect the assessment and responses to climate change impacts such as sea-level rise (SLR). This paper explores the uncertainties in the assessment process and in state-level policy and management responses of three US states to SLR. The findings reveal important political, economic, managerial, and social factors that enable or constrain SLR responses; question disasters as policy windows; and uncover new policy opportunities in the history of state coastal policies. Results suggest that a more realistic, and maybe more useful picture of climate change impacts will emerge if assessments take more seriously the locally embedded realities and constraints that affect individual decision-makers’ and communal responses to climate change.     

Coastal processes and environmental hazards: the Buenos Aires (Argentina) and Venetian (Italy) littorals

The Buenos Aires (Argentina) and Venice (Italy) coastlands have experienced significant saltwater contamination of the phreatic aquifer, coastal erosion, hydrodynamic changes and relative sea level rise processes due to natural and man-induced factors. These factors expose coastal areas to morpho-hydro-geological hazards, such as soil desertification, frequency and degree of flooding, littoral erosion, and the silting of river mouths and channels. Man-made interventions and actions, such as beach mining, construction of coastal structures and exploitation of aquifers without an adequate knowledge of the hydrology setting and an adequate management program, worsen these natural hazards. Uncontrolled human activity induces environmental damage to the overall coastal plains. The coastal plains play an important role in the social/economic development of the two regions based on land use, such as agriculture, horticulture, breeding, and tourism, as well as industry. Results of investigations on saltwater contamination, sea level rise and morphological changes recently performed in these two coastal areas are presented here.     

An Estimate of Global Mean Sea-level Rise Inferred from Tide-gauge Measurements Using Glacial-isostatic Models Consistent with the Relative Sea-level Record

This study is concerned with the influence of the glacial-isostatic adjustment caused by the last Pleistocene deglaciation on the present-day sea level. The viscoelastic deformation caused by the time-variable ice and ocean loads is simulated by computing the resulting perturbations for a spherical, self-gravitating, incompressible, Maxwell-viscoelastic earth model. The associated variation of the earth rotation is described in terms of the Liouville equation, which is solved by means of the MacCullagh formulae. This allows the determination of the vertical displacement and geoid height and, thus, the solution of the sea-level equation. We test several viscosity and ice models and evaluate them by comparison of the computed response with the Holocene relative sea-level record. Using the optimum combination of viscosity and ice models, we then estimate the influence of the last Pleistocene deglaciation on the tide-gauge measurements. A comparison between the observational and residual linear trends for the tide-gauge measurements shows a significant reduction of the variance and geographical variability for the latter, in particular for the formerly ice-covered regions of North America and Scandinavia. The favoured value determined for the global mean sea-level rise is (1.46±0.2) mm a⁻¹.     

河北沿海海侵灾害初探

文中以包含海进、海岸侵蚀和海水入侵的广义“海侵”概念,论述河北沿海地区海侵现状及其危害,探讨海侵产生的自然原因和人为活动的影响,在此基础上,评估海侵发展趋势。     

渤海湾盆地早第三纪水平面升降和全球海平面升降的不一致性

本文通过对渤海湾盆地下第三系中半咸水化石纵向分布特点研究,并根据新的地层划分方案,利用古生物－岩相综合分析方法绘制了该地区早第三纪水平面升降曲线,发现半咸水生物出现在水进初期,在最大水进期消失,水平面升降曲线一全球海平面升降曲线无直接关系,即水平面的上升示受海平面上升的影响,从而证明此阶段无海侵发生,非海侵观点对于总结我国陆相地层的油气勘探理论及实践具有重要意义。     

四川甘孜生康剪切混杂岩带物质组成、变形特征及演化历史

甘孜生康剪切混杂岩带,它初始发育在陆棚与大陆基之间,是“再生”小洋盆关闭、俯冲－碰撞乃至斜冲剪切、左行走滑综合作用的产物。带内由不同岩性,不同层位,不同时代的构造岩块与遭受强烈剪切变形的粉砂岩、板岩“基质”组合而成。宏观上,无层无序,块体内有序,是造山带特有的非史密斯地层体。     

1992—2007全球海平面变化

引起全球海平面变化的因素是复杂多样的，大气压、风、大洋环流以及海水密度的变化，都会引起海平面在时间、空间上的变化，而海水温度的变化是海平面变化的主要原因。该文利用法国Archiving, Validation and Interpretation of Satellite Oceanographic data（AVISO）的海表面高度异常数据，计算了1992年10月至2007年1月间，全球海平面的平均上升速度，同时详细解算海平面上升速度的全球空间分布，分析全球海平面的变化趋势并将海平面变化同美国国家海洋大气署（NOAA）的Optimum Interpolation Sea Surface Temperature （OISST）海表面温度数据进行了比对和相关分析。