Regional Variation in Mobility Levels and Timing in the United States     

《The Professional geographer》2013,65(3):483-493

The age patterns of U.S. internal migration, while examined extensively at the national level, have not been fully explored at the regional scale. This study examines, using 1985 - 1990 census data, the state-level variations in two aspects of lifecycle mobility: the mobility level, or the average number of moves made over the lifecycle, and the mobility timing, or the age at which half the lifetime moves are completed. It further delineates typologies of states based upon their age structure of mobility. The study found that regional patterns in the mobility level showed some evidence of the Snowbelt-Sunbelt patterns that characterized economic restructuring in the 1980s. Geographic patterns of mobility timing were less clear; however states in the West and the South showed somewhat more young distributions than the other regions. Further, there was a statistically significant relationship between mobility levels and timing: states with higher mobility levels also exhibited older mobility profiles, as a consequence of disproportionately high elderly mobility rates. The study highlights the regional differences in mobility behavior, and the interplay between the “how much” and the “when” of mobility.  相似文献   

Age models and tephrostratigraphy from two lakes on Adak Island,Alaska     

《Quaternary Geochronology》2013

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Effects of Disturbance Frequency on Canopy Age Structure and Species Composition in a Managed Midwestern Oak Savanna     

《自然地理学》2013,34(6):519-533

In the Midwest, the oak savanna is a unique plant community located between the western prairies and northern forests, but, once the most common plant community in Wisconsin, it is now one of the region's rarest plant communities. To assess the success of varying intensities of management practices, standard dendrochronology and vegetation surveying methods were used to determine the age structure and species composition in oak savanna sites that varied from intensely managed to unmanaged. Size and age distributions revealed that the number of distinct cohorts of oaks increased in less managed sites. Seedlings and saplings were present in high frequencies in all sites; however, management practices such as fire applications that were too frequent apparently decreased oak survivorship, as did the competition in the closed canopies of the unmanaged sites. A fire frequency of 2-3 years eliminates competition, but a frequency of 4-6 years, typical of fire-adapted forest communities, allows for slower-growing species and younger oaks to survive, while still maintaining a low density. In these conditions, where growth rates vary in time, the use of dendrochronology is necessary to obtain an accurate canopy structure for fire-adapted forest communities. Quercus macrocarpa was dominant in the more managed sites, but shade-tolerant species were dominant in the unmanaged site. At present, the canopy composition in the managed sites represented an oak savanna assemblage; however, the unmanaged composition was shifting to a woodland assemblage. Thus, remnant oak savanna can be over- or under-managed, and unmanaged oak savanna can be overlooked as woodlands.  相似文献   

Surface description for urban climate studies: A gis based methodology     

C.S.B. Grimmond  C. Souch 《国际地球制图》2013,28(1):47-59

In order to understand the nature of the urban climate, predict the effects of urbanization, or attempt to ameliorate some of the negative hydroclimatic effects of urbanization, it is necessary to have a good understanding of the role and significance of the urban surface. This paper presents a methodology which uses GIS to represent the characteristics and morphology of the urban surface, which can be used to describe a site objectively, model fluxes, or ensure spatial consistency between measured and modelled data, all of which can vary through time. The methodology is illustrated with respect to Chicago, Illinois. Surface data collected at three spatial scales were used to construct a georeferenced database which was linked to an objective, dynamic accessing system. Spatial variability of surface cover, derived hydroclimatic attributes, and modelled fluxes associated with changes in the urban environment are used to illustrate potential applications of the approach.  相似文献   

新疆西准噶尔地区花岗岩类年代学及其构造意义     

尚兆聪  王怀涛  张伟  刘昆鑫  何彦彬  魏东伟 《甘肃地质》2012,(1):1-5

本文在野外地质调查基础上,通过对西准噶尔花岗岩类年代学的研究,厘定各期构造岩浆事件的时限,反演西准噶尔造山带构造演化过程。研究结果表明,西准噶尔岩浆活动划分为3个时期。晚志留世—早泥盆世花岗岩类主要为谢米斯台花岗岩类、赛尔花岗岩类及阿克乔克花岗岩类,结合区域地质背景及阿克乔克花岗闪长岩具有典型的埃达克岩地球化学特征,推测阿克乔克埃达克岩可能是大洋板片俯冲过程中经过脱水发生部分熔融形成的;早石炭世花岗岩类主要分布在扎尔马—萨吾尔岩浆弧地区,这一时期的花岗岩类可能是额尔齐斯蛇绿岩所代表的古大洋向南俯冲脱水引发上覆地幔楔部分熔融或者部分熔融形成的玄武岩浆底侵作用引起中、下地壳物质部分熔融的结果,指示俯冲的古大洋在早石炭世期间未闭合碰撞;晚石炭世—早二叠世的花岗岩类在整个西准噶尔地区都有分布,形成于后碰撞构造环境,表明西准地区进入了陆内构造演化阶段。  相似文献   

黑龙江虎林岩体的锆石U——Pb 年龄及地质意义   总被引：1，自引：0，他引：1  

胥嘉  邢德和  杨浩  张彦龙 《世界地质》2012,31(4):631-637

通过虎林石青山采石场角闪花岗闪长岩的锆石U-Pb LA-ICPMS 年龄测定,确定了虎林岩体形成于118 Ma 的早白垩世晚期。根据该岩体的岩石学和地球化学特征及区域构造演化研究资料,认为虎林岩体的形成与太平洋板块的俯冲作用有关。  相似文献   

新疆白干湖钨锡矿田东北部花岗岩锆石SIMS U-Pb 年龄、地球化学特征及构造意义   总被引：6，自引：0，他引：6  

李国臣  丰成友  王瑞江  马圣钞  李洪茂  周安顺 《地球学报》2012,33(2):216-226

白干湖矿田东北部出露钾长花岗岩和二长花岗岩,具有相近的成岩年龄和相似的地球化学特征,表明为同源演化的复式岩体。利用SIMS方法获得二者锆石U-Pb谐和年龄是422±3Ma和421±3.7Ma,为晚志留世。岩石主量元素地球化学特征二者表现为弱过铝质(A/CNK为0.99～1.02)或准铝质(A/CNK为0.92～0.94)、高钾(K2O/Na2O分别为1.08～1.31和1.03～1.22)、高碱((Na2O+K2O)分别为8.59%～9.38%和9.54%～9.69%)、钙碱性或弱碱性(δ分别为2.39～3.17和4.02～4.22)、Fe#(TFeO/MgO分别为9.58～12.26和8.94～9.96)高。稀土元素总体含量(分别为228×10-6～448×10-6和271×10-6～392×10-6)较高,微量元素亏损Ba、Nb、Sr、Ti、P和富集La、Ce、Zr、Sm,总体显示A型花岗岩的特征。R1-R2图解显示岩体构造背景落入晚造山区域。Sr-Yb图解显示岩体落入低Sr高Yb的区域,暗示拉伸的地壳减薄的环境。Pearce图解显示构造环境为板内为主。综合区域背景资料,认为该岩体形成于加里东造山旋回的后碰撞阶段。  相似文献   

西藏甲玛-卡军果推-滑覆构造系特征   总被引：5，自引：0，他引：5  

钟康惠  李磊  周慧文  白景国  李伟  钟婉婷  张勇强  蔺吉庆  郑凡石  黄小雨  陆彪  雷波 《地球学报》2012,33(4):411-423

通过矿区填图和路线调查,结合室内研究,指出:①甲玛-卡军果推-滑覆构造系由印度-欧亚板块碰撞引发,具有递进变形、自盆地中心向边部推覆、新盖老等特点,强变形于50Ma±。②推覆体下盘为叶巴组构造岩系;推覆体由上侏罗统和下白垩统碳酸盐-碎屑沉积岩系构成,主体为轴面北倾的复式褶皱,自南而北分为前部带、中部带和后部带。滑覆构造系推覆形成的复背斜轴部高位岩块失稳向北滑覆形成,自南而北分为后部带、中部带和前部带。③导致矿区蚀变的主要岩浆活动发生于推覆强变形后的松弛期,上侵受近南北向基底断裂和推覆断裂联合控制,就位受控于前部带推-滑覆褶断系,其后发生多次构造-岩浆-成矿液叠加,造就甲玛铜多金属矿床。  相似文献   

黄陵野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成   总被引：1，自引：0，他引：1  

赵敏  魏君奇  王建雄 《华南地质与矿产》2012,28(2):124-131

采用激光剥蚀-等离子质谱(LA-ICP-MS)分析技术测定野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成,以探讨黄陵地区TTG片麻岩原岩的形成及变质时间、是否存在比崆岭群更古老的地壳等问题。野马洞辉绿岩脉(1850 Ma)侵入TTG片麻岩,并从TTG片麻岩中捕获了大量捕掳晶锆石。捕掳晶锆石岩浆结晶核部的U-Pb年龄分别为2842 Ma、2900 Ma和2949 Ma,指示TTG花岗岩体为复式岩体,其至少经历了2949 Ma、2900 Ma和2842 Ma三期岩浆作用。捕掳晶锆石变质边部的U-Pb年龄为2557 Ma,指示TTG花岗岩体转变为TTG片麻岩,是"水月寺运动"及其构造热事件共同作用的结果,其变形变质的时间为2557～2511 Ma。捕掳晶锆石的εHf(t)为-9.85～0.89、平均值为-4.07,亏损地幔模式年龄TDM为3.6～3.2 Ga,指示黄陵地区存在比崆岭群(3.2 Ga)更古老的陆壳。  相似文献   

新疆北部大地构造演化阶段与斑岩－浅成低温热液矿床的构造环境类型   总被引：5，自引：0，他引：5       下载免费PDF全文

王玉往王京彬  龙灵利邹 滔 王莉娟 《中国地质》2012,39(3):695-716

本文系统总结了新疆北部斑岩-浅成低温热液矿床的成矿时代,按构造环境将该类矿床归为三大类型:洋-陆俯冲型、碰撞造山型、板内型,其中碰撞造山型又可分为碰撞型和后碰撞型。4类矿床的差别主要在于矿床金属元素组合,以及同期相伴出现的矿床类型不同:俯冲型斑岩矿床以斑岩Cu-Au矿-浅成低温热液Au矿组合为主,以伴有海相火山岩有关的VMS矿床和铁矿为特征;碰撞型和后碰撞型矿床以斑岩Cu-Mo-Au组合为主,伴有构造蚀变岩型复合/叠加的浅成低温热液型Au矿出现;板内型矿床以斑岩型单Mo(或Mo-Re)组合为主。斑岩矿床与浅成低温热液矿床虽为同一成矿系统,但二者基本不共生,且后者成矿时代一般晚于前者10～20 Ma。斑岩-浅成低温热液矿床的含矿岩石和成矿特征并不随构造环境类型不同而出现特征性差别。不同时期的斑岩矿床在分布上具有继承性和"同位成矿"特点,并表现出一定的分带性,从早到晚逐渐由靠近缝合带向外扩展、由线型分布逐渐趋于面型分布。  相似文献