Three-dimensional structure of inclined sheet swarms: Implications for crustal thickening and subsidence in the volcanic rift zones of Iceland     

Drew L. Siler  Jeffrey A. Karson 《Journal of Volcanology and Geothermal Research》2009

Inward-dipping (cone) sheet swarms and an associated central volcano are well-exposed in the deeply-eroded Tertiary crust of Vatnsdalur, Skagi Peninsula region, northern Iceland. Spatially registered orientations of 389 mafic sheets, mapped in three distinct sheet swarms define both the overall shape and magmatic source of each swarm. The Vatnsdalur sheet swarms consist of planar inward-dipping sheets that collectively define a conical shape rather than a bowl- or trumpet-shape as have been found in swarms in other locations. In the best exposed swarm, three-dimensional projection of mafic sheets into the subsurface defines two distinct foci, which are interpreted as the magmatic sources of two temporally distinct sub-swarms. These results help to establish the influence of inclined sheet intrusion on crustal accretion at central volcanoes. The geometry of the swarm constrains the thickness of material that was added to the crust during sheet intrusion. When combined with estimates of surface relief, we calculate that 2.2 to 4.1 km of subsidence were required beneath the central volcano in order to accommodate the intrusion of the sheet swarm. Similar processes of crustal thickening and subsidence likely occur in a wide variety of both continental rift and mid-ocean ridge systems where magmatic activity is focused at central volcanoes.  相似文献   

极地冰盖冰雷达探测技术的发展综述          下载免费PDF全文

崔祥斌  孙波  张向培  张栋  李鑫  唐学远  田钢 《极地研究》2009,21(4):322-335

分三个阶段,即早期冰雷达系统(20世纪60至80年代)、第二代冰雷达系统(20世纪8O年代至2000年)和新冰雷达系统(2ooo年之后),综述了极地冰盖冰雷达探测技术的发展。最后,展望了冰雷达探测技术未来的发展趋势。  相似文献   

《新北京新奥运地图集》的内容与特点     

张保钢  白云芳 《北京测绘》2009,(4):66-67

介绍了《新北京新奥运地图集》的内容。全图集由奥运之城、北京概况、北京中心城、北京新城、北京郊区县5个图组组成。图集的特点是主题突出，设色和图面配置新颖、大方，符号设计艺术性强，文字说明颇具特色，时空设计充满动感。  相似文献   

数字化技术在城镇地籍调查中的应用研究     

胡俊  王梁  文敬  施凤翔 《现代测绘》2012,35(1):45-47

传统的地籍调查方式已经很难适应现代化地籍调查的要求,本文提出采用数字化技术进行城镇地籍调查的方案,并对该方案进行了详细论述。阐述了建立数字化地籍调查表和实现宗地草图数字化的方法。  相似文献   

Origin and evolution of playas and blowouts in the semiarid zone of Tierra de Pinares (Duero Basin, Spain)   总被引：1，自引：0，他引：1  

Mateo Gutirrez-Elorza  Gloria Desir  Francisco Gutirrez-Santolalla  Cinta Marín 《Geomorphology》2005,72(1-4):177-192

A large number of blowouts and playas occur in the marginal sectors of the aeolian deposits located in the southern sector of the Duero Depression (Tierra de Pinares) in Spain. The blowouts are relict landforms that were developed on sand sheets by deflation during dry periods with lower vegetation cover and a deeper water table. The studied blowouts form complexes of NW–SE and NNW–SSE elongated hollows with accompanying dunes up to 4 km long in the leeward margins. Some hollows host lakes or swampy areas related to a shallow water table. The dunes formed by NE–ENE winds show steep windward slopes and gentle leeward slopes. The studied playas, with prevalent NNW–SSE orientations, result from the aeolian excavation of terrace deposits and the underlying marly bedrock. It is probable that the formation of these depressions in an initial stage was related to deflation processes affected preferentially NNW–SSE sandy channels perpendicular to the dominant wind direction. The precipitation of salts in the playas generates aggregates of clay particles (peloids) that are easily removed by the wind. Once the bottom reached the substratum, the deepening of the depressions progressed by the deflation of particles produced by weathering of the argillaceous bedrock.  相似文献   

Linking variable explosion style and magma textures during 2002 at Stromboli volcano,Italy     

Nicole C. Lautze  Bruce F. Houghton 《Bulletin of Volcanology》2007,69(4):445-460

Microtextural characteristics of fresh ejecta from Stromboli volcano were examined from three periods of differing eruption style and intensity in 2002. Activity shifted from relatively weak and infrequent ash-charged explosions during January through May into two broad cycles of waxing activity in June through late September, and late September through December, followed by the onset on 28 December of the 2002/2003 effusive eruption. Analyzed sets of lapilli from May, September/October, and 28 December show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity. Three distinct textures are observed among the analyzed pyroclasts: low density (LD) with an abundance of subspherical bubbles, the presence of large, irregularly shaped bubbles, and a light-to-transparent glass matrix; transitional texture (TT) with an intermediate number of subspherical bubbles, a high frequency of large, irregularly-shaped bubbles, and a honey colored glass matrix; and high density (HD) with sparse relatively small bubbles, conspicuous large irregular bubbles, and a dark glass matrix. Observational and quantitative data (density, vesicle size) indicate that these textures are linked through variable residence time in Stromboli’s shallow conduit, with an ongoing evolution from LD to HD magma. Calculations suggest that residual LD magma will evolve to HD texture in a period of hours to days. Contrasting amounts of the LD, TT, and HD magmas are present in each sample, with the most TT in May, the most LD in September/October, and the most HD in December. This implies that the shallow magma had a different rheology at each collection period. The viscosity of LD and HD magmas are calculated to be in the range of 2,000 to 2,600 and 3,000 to 5,000 Pa s, respectively, which, with their changing proportions, must have implications for rates of bubble slug ascent and processes of fragmentation. This study suggests that an increasing maturity of magma in Stromboli’s shallow conduit (with resultant increase in viscosity) feeds back to reduce the intensity of explosions, whereas a steady flux of LD magma favors more powerful explosions.  相似文献   

Inception of the Northern European ice sheet due to contrasting ocean and insolation forcing   总被引：1，自引：0，他引：1  

Bjørg Risebrobakken  Trond Dokken  Eystein Jansen  Yongqi Gao  Helge Drange 《Quaternary Research》2007,67(1):128-135

About 115,000 yr ago the last interglacial reached its terminus and nucleation of new ice-sheet growth was initiated. Evidence from the northernmost Nordic Seas indicate that the inception of the last glacial was related to an intensification of the Atlantic Meridional Overturning Circulation (AMOC) in its northern limb. The enhanced AMOC, combined with minimum Northern hemisphere insolation, introduced a strong sea-land thermal gradient that, together with a strong wintertime latitudinal insolation gradient, increased the storminess and moisture transport to the high Northern European latitudes at a time when the Northern hemisphere summer insolation approached its minimum.  相似文献   

基于线要素的图幅自动接边问题的探讨   总被引：1，自引：0，他引：1  

张昭云  夏启兵  许志海 《测绘学院学报》2001,(Z1)

文中介绍了基于线要素的图幅自动接边的基本算法 ,主要分 3步完成 :待拼接数据链的提取 ,属性匹配和图形拼接 ,给出了具体的实现过程 ,并应用某区的地貌数据进行了实际检验。同时还进一步探讨了交通信息管理中图幅之间道路要素的拓扑连接问题  相似文献   

Landscape preservation under Fennoscandian ice sheets determined from in situ produced Be and Al     

Derek Fabel  Arjen P. Stroeven 《Earth and Planetary Science Letters》2002,201(2):397-406

Some areas within ice sheet boundaries retain pre-existing landforms and thus either remained as ice free islands (nunataks) during glaciation, or were preserved under ice. Differentiating between these alternatives has significant implications for paleoenvironment, ice sheet surface elevation, and ice volume reconstructions. In the northern Swedish mountains, in situ cosmogenic ¹⁰Be and ²⁶Al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces, provide consistent last deglaciation exposure ages (∼8-13 kyr), confirming ice sheet overriding as opposed to ice free conditions. However, these ages contrast with exposure ages of 34-61 kyr on bedrock surfaces in these same relict areas, demonstrating that relict areas were preserved with little erosion through multiple glacial cycles. Based on the difference in radioactive decay between ²⁶Al and ¹⁰Be, the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845₋₄₁₈⁺⁴⁶¹ kyr. These results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models, and in landscape development models. Subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets. These relict areas also have significance as potential long-term subglacial biologic refugia.  相似文献   

用CSS样式动态整形地震目录查询结果     

熊志强  许云廷 《华南地震》2002,22(4):74-78

在开发地震目录查询系统的过程中，通过CSS样式和PHP程序的结合运用。可动态地控制地震目录查询系统的返回结果，使查询结果的显示可以随获取的地震目录的条数而改变。极大地方便查询者。  相似文献