Emplacement and arrest of sheets and dykes in central volcanoes   总被引：1，自引：0，他引：1  

Agust Gudmundsson   《Journal of Volcanology and Geothermal Research》2002,116(3-4)

Sheet intrusions are of two main types: local inclined (cone) sheets and regional dykes. In Iceland, the inclined sheets form dense swarms of (mostly) basaltic, 0.5–1 m thick sheets, dipping either at 20–50° or at 75–90° towards the central volcano to which they belong. The regional dykes are (mostly) basaltic, 4–6 m thick, subvertical, subparallel and form swarms, less dense than those of the sheets but tens of kilometres long, in the parts of the volcanic systems that are outside the central volcanoes. In both types of swarms, the intrusion intensity decreases with altitude in the lava pile. Theoretical models generally indicate very high crack-tip stresses for propagating dykes and sheets. Nevertheless, most of these intrusions become arrested at various crustal depths and never reach the surface to supply magma to volcanic eruptions. Two principal mechanisms are proposed to explain arrest of dykes and sheets. One is the generation of stress barriers, that is, layers with local stresses unfavourable for the intrusion propagation. The other is mechanical anisotropy whereby sheet intrusions become arrested at discontinuities. Stress barriers may develop in several ways. First, analytical solutions for a homogeneous and isotropic crust show that the intensity of the tensile stress associated with a pressured magma chamber falls off rapidly with distance from the chamber. Thus, while dyke and sheet injection in the vicinity of a chamber may be favoured, dyke and sheet arrest is encouraged in layers (stress barriers) at a certain distance from the chamber. Second, boundary-element models for magma chambers in a mechanically layered crust indicate abrupt changes in tensile stresses between layers of contrasting Young’s moduli (stiffnesses). Thus, where soft pyroclastic layers alternate with stiff lava flows, as in many volcanoes, sheet and dyke arrest is encouraged. Abrupt changes in stiffness between layers are commonly associated with weak and partly open contacts and other discontinuities. It follows that stress barriers and discontinuities commonly operate together as mechanisms of dyke and sheet arrest in central volcanoes.  相似文献   

Statistical characteristics of chlorophylla concentration in Hong Kong’s coastal waters     

Chen Xiaoling  Yok Sheung Li  Li Zhilin 《地理学报(英文版)》2002,12(3):331-342

In this paper seven of the ten Water Control Zones (WCZs) in Hong Kong’s coastal waters with monthly or bi-weekly sampling data of 17 parameters collected at 37 monitoring stations from 1988 to 1999 were selected to analyze the spatial and temporal variations of chlorophyll-a and its influencing factors. Cluster analysis was employed to group the monitoring stations based on the structure of the data set. Multiple step regression was employed to determine the significant influencing factors of chlorophyll-a level. The results suggest that all the monitoring stations could be grouped into two clusters. Cluster I with frequent red tide incidents comprises two WCZs which are semi-enclosed bays. Cluster II with less red tide occurrence comprises the other five WCZs in an estuarine environment in the west. For both clusters, the organic contents indicator, BOD5, was a common significant influencing factor of the chlorophyll-a level. Nitrogen and light penetration condition related to turbidity, total volatile solids and suspended solids had more influence on the cholophyll-a level in Cluster I than in Cluster II, while phosphorus and oceanographic conditions associated with salinity, temperature, dissolved oxygen and pH were more important in Cluster II than in Cluster I. Generally, there was a higher average chlorophyll-a level in winter and autumn in a year. The chlorophyll-a level was much higher in Cluster I than in Cluster II among all seasons. Although the chlorophyll-a concentration had great variations from place to place in Hong Kong’s coastal waters, it seemed to have a common long term fluctuation period of 8–10 years with a high-low-high variation in the period in the whole region, which might be influenced by other factors of global scale.  相似文献   

非规则网格下二维浅水流动的数值模拟          下载免费PDF全文

陈祖华  赖冠文  王光谦  王志石 《水科学进展》2002,13(6):657-664

建立了一种在非规则结构化网格上求解平面二维浅水流动的有限体积方法。通过采用地形在离散网格内双线性变化及离散网格界面间地形连续的地形逼近方法和应用可以有效处理间断问题的Roe格式来离散浅水方程中的对流项,并通过VanLeer提出的状态插值法提高格式精度。在计算原始变量在网格内的插值梯度时,采用最小二乘方法求变量的最优梯度代替差分计算梯度,从而可采用任意形状的不规则四边形网格离散计算域。计算实例表明,该方法能够计算间断问题并能够处理各种复杂流态的过渡,具有较好适应性和计算精度,能够满足不同实际问题的计算要求。  相似文献   

Investigation of earthquake mechanisms and their impact on certain basic concepts in earthquake engineering and seismology     

Fulu Men 《地震工程与工程振动(英文版)》2002,1(2):281-291

In this paper, mantle circulation flow, continental drift, earthquake origin and other mechanical principles are examined as they apply to earthquake engineering, seismology and dynamics of fluid saturated porous medium. The relationship of mantle flow to earthquakes is examined and clarified, and a new model, different from Haskell’s, is proposed for the earthquake mechanism. The proposed new model is based on the discovery that two pairs of jump stress and jump velocity will start to act from the fault plane. Records obtained directly from recent earthquakes nearby and right on the fault break show a very large velocity impulse, which verify, indirectly, the new mechanism proposed by the author. Further, at least two physical parameters that characterize the seismic intensity must be specified, because according to the discontinuous (jump) wave theory, at the earthquake source, the stress jump and the velocity jump of particle motion should act simultaneously when a sudden break occurs. The third key parameter is shown to be the break (fracture) propagation speed together with the break plane area. This parameter influences the form of the unloading time function at the source. The maximum seismic stress in and displacement of a building are estimated for two unfavorable combinations of the building and its base ground in terms of their relative rigidity. Finally, it is shown that Biot’s theory of wave propagation in fluid saturated porous media is valid only when fluid flow cannot occur.  相似文献   

单发性脑转移瘤的CT诊断     

孙立新  袁令锦 《CT理论与应用研究》1996,5(2):21-22

笔者收集２９例经ＣＴ诊断单发性脑内转移瘤，其中经手术病理证实的２３例，通过对２９个单发病灶的ＣＴ表现进行分析，着重讨论该病灶的出现位置，并对其原因进行了探讨性分析，旨在进一步提高对颅内单发病灶的诊断水平。  相似文献   

锥束重建的Smith—Grangeat方法的分析     

尤江生  包尚联 《CT理论与应用研究》1996,5(2):46-51

ＣＴ机、ＳＰＥＣＴ及ＰＥＴ等医疗设备是当前重要的医疗诊断工具，但这些设备都跟某些射互有关，当放射性元素衰变而发射某种射线时，如果它们在４立体角范围内是等概率分布的，则扇形采集比在上方向上有集的效率提高了许多。  相似文献   

Probability　forecast　of　earthquake　magnitude　in　Chinese　mainland　before　A．D．　2005     

王晓青  傅征祥  蒋铭 《地震学报(英文版)》1996,9(4):545-552

ＰｒｏｂａｂｉｌｉｔｙｆｏｒｅｃａｓｔｏｆｅａｒｔｈｑｕａｋｅｍａｇｎｉｔｕｄｅｉｎＣｈｉｎｅｓｅｍａｉｎｌａｎｄｂｅｆｏｒｅＡ．Ｄ．２００５ＸＩＡＯ－ＱＩＮＧＷＡＮＧ（王晓青），ＺＨＥＮＧ－ＸＩＡＮＧＦＵ（傅征祥）ａｎｄＭＩＮＧＪＩＡＮＧ（蒋铭）...  相似文献   

西南极乔治王岛北海岸第三纪火山岩的岩石化学和地球化学研究          下载免费PDF全文

郑祥身 《极地研究》1996,8(2):4-18

乔治王岛北海岸的第三纪火山岩包括菲尔德斯半岛组（ＦＰＧ）熔岩、Ａｄｍｉｒａｌｔｙ湾组（ＡＢＧ）岩颈和Ｗｅｇｇｅｒ峰组（ＷＰＧ）侵入岩。大多数中基性熔岩为斑状结构，斜长石和普通辉石是岩石中最主要的斑晶相。安山岩中尚含有少量斜方辉石，英安岩质熔岩则为细晶质结构。岩颈相岩石的物质组成与熔岩相似，但斑晶含量往往较少。岩石化学成分表明，所有岩石均富铝，为亚碱性火山岩，属于钙碱性岩石系列。岩石富集大离子亲石元素，轻稀土元素略为富集，稀土元素总含量落在钙碱性岩石的含量范围之内并基本随硅含量的增加而增加。岩石具有很低的８７Ｓｒ／８６Ｓｒ比值（０．７０３４左右）和较高的１４３Ｎｄ／１４４Ｎｄ比值（０．５１２７８－０．５１２８８），εＮｄ平均值在＋６左右。８７Ｓｒ／８６Ｓｒ比值与Ｓｉ、Ｋ、Ｒｂ、１／Ｓｒ等元素呈水平线性关系。在Ｃｅ／Ｙｂ－Ｃｅ和Ｔｈ／Ｔａ－Ｔｈ图解上所有岩石的投影点分布趋势相同，趋势线的斜率＞０。上述特征表明，乔治王岛北海岸第三纪火山岩均由上地幔物质直接生成而未受壳源物质的混染。推测在古太平洋板块向南极板块下俯冲的过程中，上地幔发生的不同程度部分熔融所生成的玄武岩岩浆从深部岩浆囊沿构造薄弱带穿过地壳直接喷出形成  相似文献   

东南极普里兹湾海冰面地磁考察          下载免费PDF全文

焦丞民  孔祥儒  刘成恕 《极地研究》1996,8(2):55-61

中国第九次南极考察期间在中山站北部普里兹湾中面积约１３２ｋｍ２的测区内进行了地磁测量，为日后的南极内陆考察作前期准备。磁测资料表明，这里有一个宽约５－８ｋｍ、走向ＮＷ的磁异常，其幅度达５００ｎＴ。磁异常可能是由深成岩浆的侵入体或富含磁铁矿的地质体引起的，磁源物体的埋深在海冰面以下０．３－２．０ｋｍ。考察实践证明：只要经过充分准备，利用普里兹湾冬季冰封的有利条件，开展大范围的海冰面地球物理考察，在技术上是可行的。  相似文献   

南极拉斯曼丘陵周围地区卫星影象的冰貌信息提取   总被引：1，自引：0，他引：1       下载免费PDF全文

刘继琳  国晓港 《极地研究》1996,8(4):23-34

本文依据南极地区Ｌａｎｄｓａｔ－ＴＭ影象各波段不同的光谱和空间特征，在ＡＲＩＥＳ数字图象处理系统上，提取高程信息和冰貌信息，并将栅格数据转换成矢量数据，建立图形间的拓扑关系。经ＲＡＭＳ图形信息处理系统编辑和修饰，由计算机输出线划冰貌图。最后对该地区冰貌的形成、分布特点及其变化和对环境的影响等作了初步的分析和探索  相似文献