中国大陆地壳上地幔电性特征   总被引：9，自引：6，他引：9       下载免费PDF全文

李立 《地球物理学报》1996,39(Z1):130-140

根据大地电磁测深调查结果，编制了中国大陆３０，９０，１５０ｋｍ三个深度的电阻率图以及壳内低阻层和上地幔低阻层的顶面深度图。在９０ｋｍ深度的电阻率图上发现了一个自松辽盆地直到扬子地台西南缘的北东－南西向巨大低阻异常带．１５０ｋｍ深度的电阻率图上显示出在低阻的背景上镶嵌着一些高阻块体．中国大陆的壳内低阻层深度国基本上反映了地温场的特征，壳内低阻层上隆区基本上对应于高地温区．中国大陆的上地幔低阻层深度变化大．最浅处仅５０－６０ｋｍ，大多位于构造活动地区；最深处达２００ｋｍ以上，大部分对应于稳定地区．中国大陆的上地幔低阻层平均深度为１００－１２０ｋｍ，东部浅，西部深。  相似文献   

论中国大陆基底构造   总被引：31，自引：2，他引：31       下载免费PDF全文

袁学诚 《地球物理学报》1995,38(4):448-459

中国大陆基底可以分为４个区域：西域克拉通和东亚克拉通具太古代－早元古代变质基底，蒙藏增生陆块与华南增生陆块为元古代基底．在西域克拉通内可以识别出南塔里木陆核分布区、准噶尔陆核分布区与伊犁陆核；在东亚克拉通内可以勾画出燕吕陆核分布区、黄淮陆核分布区、秦岭陆核分布区和扬子陆核分布区．在克拉通形成的同时或稍后，在克拉通的两侧发育巨型基底断裂．在东亚克拉通的东西侧为郯庐断裂带与东亚克拉通西缘断裂带；在西域克拉通的南北为阿尔金断裂带与阿尔曼大断裂带．在蒙藏地块中部通过的日喀则－狼山断裂带是位于两克拉通之间的重要断裂带．  相似文献   

扬子大陆的陆内俯冲与大陆的缩小──由白云母（二云母）花岗岩推导     

邓晋福  赵海玲  莫宣学  刘厚祥  罗照华 《高校地质学报》1995,(1)

本文简要地论述了白云母（二云母）花岗岩是陆内俯冲的岩石学记录。华南大陆与天水－雅江地带广泛分布中生代王云母花岗岩侵作入体，已有的同位素年龄范围为２４５～１２２Ｍａ，与此同时，它们两侧的扬子大陆发育前陆褶皱带，这种成对性及其构造配置表明，扬子大陆曾向华南大陆与松潘—甘孜褶皱带发生过陆内俯冲作用。基于二云母花岗岩带的总宽度（约６５０ｋｍ），估算扬子大陆最小的总俯冲量（指宽度）至少为６５０ｋｍ，扬子大陆现今宽度约６８０ｋｍ，这样，扬子大陆在中生代时期由于陆内俯冲作用缩小了约一半；另一可能方案是，消失的主要是夹持于扬子大陆与江南造山带之间的一个微大陆。扬子大陆与秦岭造山带之间的壳内构造边界不具陆内俯冲性质，而是逆冲推复一走滑构造带。在大地构造单元上，江南造山带与龙门山造山带应分属两个上叠大陆，而不属于扬子大陆。华南中生代造山带内侧属于高喜马拉雅型陆内俯冲带（缝合带内无蛇绿岩伴生），而不是雅鲁藏布江型（缝合带内有蛇绿岩伴生）。陆内俯冲作用是华南大陆形成的主要机制之一。此外，它经受过晚元古代和加里东期陆内俯冲事件，可能属于雅鲁藏布江型。多次陆内俯冲和白云母（二云母）花岗岩的形成导致亲陆壳金属元素富集，可能是华南Ｗ－Ｓｎ  相似文献   

Land use and land rights in the West African savannah: The Senufo in northern C^ote d'Ivoire     

Till Förster 《GeoJournal》1998,46(2):101-111

The change in property rights in the West African savannah has been analysed in particular from the perspective of development policy, i.e. from the perspective of state regulation. This contribution, however, attempts to trace property rights from the perspective of the farming population in the savannah in the northern part of Côte d'Ivoire. For peasants, the change of law is part of a long-term process which had already started before the colonialisation at the end of the 19th century and is still continuing today. The changing law is not analysed per se but is rather analysed with reference to people's actions, out of which the sphere of life of the local population is constituted. The changing forms of farm work and of land usage play an important role in this respect. The change of property rights is inscribed in the conflicts with the state and state power on the one hand, and the changes of social forms of organisation on the other.  相似文献   

Re‐enlivening the Indian Ocean through contemporary trade: East African traders searching for new markets in Jakarta          下载免费PDF全文

Julia Verne 《Singapore journal of tropical geography》2017,38(1):123-138

While much has been written about the history of mobility and trade in the Indian Ocean, recent trading connections between Eastern Africa and Asia have so far only gained very little attention. However, in Zanzibar alone, hundreds of traders regularly embark on journeys reviving old routes and further develop them, in order to take advantage of the way the global economy works today. By providing an ethnographic account of such a journey undertaken by four Zanzibari traders to Jakarta, this article gives an insight into the organization, calculations and imaginations involved in contemporary Indian Ocean trading networks. Although this trade journey clearly marks a new form of translocal mobility among Zanzibari traders, this mobile ethnography highlights how the journey is at the same time closely linked to, evokes and enlivens very old processes of mobility across the Indian Ocean, thus highlighting the dialectics between old and new mobilities, and between familiarity and strangeness.  相似文献   

African Fire Cultures,Cattle Ranching,and Colonial Landscape Transformations in the Neotropics          下载免费PDF全文

Andrew Sluyter  Chris Duvall 《Geographical review》2016,106(2):294-311

Fire regimes emerge partly from human activities that reflect cultural‐ecological knowledge of the relationships among fire, vegetation, grazing, climate, and other variables, as well as social relations. More knowledge of such “fire cultures,” past and present, therefore remains necessary to better understand the causes and persistent consequences of landscape burning. In the neotropics, people have used fire for centuries to manage livestock pastures. Conventional wisdom has long posited that such practices derived solely from antecedent European and indigenous, Native American fire cultures. Analysis of accounts of rangeland burning from throughout the neotropics during colonial times, however, demonstrates that ranchers incorporated African fire cultures and that the timing of burning shifted from early during the dry season in the sixteenth century to late during the dry season by the nineteenth century.  相似文献   

Take the edge off: A hybrid geographic food access measure     

《Applied geography (Sevenoaks, England)》2017

Measures of geographic food access overlook an important source of statistical biases, termed the edge effect. The edge effect refers to the fallacy that events contributing to the spatial pattern of an analysis unit may be outside of that unit; thus merely summarizing events within the unit may lead to distortion of the estimation. Food procurement activities can happen beyond existing administrative boundaries. Delineating food access using unit-based metrics may misrepresent the true space within which food stores are accessible. To overcome this problem, this paper proposes a gravity-based accessibility measure to improve unit-based statistical approaches in food access research. In addition, this method accounts for the spatial interaction between food supply (e.g., food items in stock) and demand (e.g., population) as well as how this interaction is mediated by the spatiotemporal separation (e.g., travel time, modality). The method is applied to the case of Franklin County, OH and has revealed the food access inequity for African Americans by modes of transport, including walking, biking, and driving. The analysis of the correlation between mode-specific food access and socioeconomic status (SES) variables reveals that using a single modality in food access research may not fully capture the travel behavior and its relationship with local food environments. With modifications, the proposed method can help evaluate food access for a target population group, such as Supplemental Nutrition Assistance Program (SNAP) users or selected ethnic minorities who may face acute difficulties in procuring economically affordable and culturally appropriate foods.  相似文献   

北半球夏季海洋性大陆区域气候与EP型ENSO:直接与间接联系          下载免费PDF全文

方陆俊  管兆勇  王美  夏阳 《大气科学学报》2016,39(3):289-299

利用1979—2009年的NECP资料、Hadley海温月平均资料和CMAP降水资料,采用Kao and Yu(2009)的方法定义了夏季EP型ENSO指数EPI,用合成分析的方法分析了东部型ENSO与海洋性大陆降水的关系。结果表明:EPI与MC(Maritime Continent,海洋性大陆)区域降水变化间存在非常弱的负相关。造成这一弱相关的原因是EPI与MC区域降水在某些年份存在同号变化。在剔除Nio4信号后,海洋性大陆区域降水序列与EPI与存在着同号和反号两种关系。反号关系是通常所认为的,当经典的El Nio(La Nia)发生时MC区域降水出现显著地减少(增多)。此时,沿赤道的异常Walker环流建立了EP型ENSO与MC区域气候间的直接联系。而在同号关系时,菲律宾以东异常加热和SPCZ区域异常冷却引起的西北—东南走向的垂直环流圈削弱了MC区域与赤道东太平洋之间的异常Walker环流所建立的直接联系,或者说,赤道东太平洋区域SSTA与MC区域降水异常的形成是通过SPCZ区域SST的反号异常而产生间接联系的。这种机制的揭示为深刻认识ENSO影响海洋性大陆区域甚至东亚地区气候变动的联系提供了新的线索。  相似文献   

2015/2016年超强El Nio在成熟/衰减阶段对澳洲夏季风环流与降水异常的影响          下载免费PDF全文

陈蔚  管兆勇 《大气科学学报》2016,39(6):801-812

2015/2016年发生了超强El Nio事件,Nio3区海温异常在2015/2016年冬季超过了2.5℃,其对全球气候异常的产生具有重大影响。此次El Nio事件可归类为东部型。本文利用NCEP/NCAR再分析资料、Hadley中心海温资料及CMAP降水资料等,通过诊断分析,揭示了2015/2016年强El Nio事件盛期和衰减期海温异常对澳洲夏季风环流异常和降水变化的影响及其途径。澳洲地区受此次El Nio事件影响,大部分地区降水显著减少,某些地区降水减少达60%~80%。影响途径主要有:1)海温异常通过Gill型响应造成水平环流异常并进而影响澳洲夏季风减弱。2015/2016年ENSO事件成熟期及衰减期,在西太平洋赤道地区出现海温负异常,导致此处热带地区产生了负异常的热源分布,并由此激发产生了位于赤道北侧和南侧的异常反气旋式环流。位于海洋性大陆南侧的异常反气旋性环流引起澳洲夏季风减弱,从而利于抑制上升运动,造成了澳大利亚降水量显著减少;2)通过赤道中东太平洋地区上升和澳洲地区下沉的垂直环流异常导致澳洲降水显著减少。在El Nio事件盛期,因海温异常偏暖导致赤道中东太平洋地区产生显著的异常上升运动,其补偿性的下沉运动因Walker环流而出现在海洋性大陆地区,以及因澳洲大陆夏季风减弱而出现在澳大利亚中东部地区,由此构成了中东太平洋—澳洲副热带的垂直环流圈,此东北—西南向的垂直环流圈对澳洲降水减少的维持起到了重要作用。另外,中国南方的上升运动与海洋性大陆及澳洲地区的下沉运动之间通过局地经圈环流产生了联系,表明东亚冬季风异常减弱对澳洲夏季风减弱可能存在间接影响。这些结果对深刻认识超强ENSO事件对亚澳季风的影响机理以及寻找澳洲降水预测线索具有重要意义。  相似文献   

Linked and Independent African American Migration from Los Angeles     

Curtis C. Roseman  Seong Woo Lee 《The Professional geographer》1998,50(2):204-214

This paper examines African American migration from Los Angeles County, 1985–1990, utilizing Census Public Use Microdata Samples (PUMS) data to identify linkages between Los Angeles migrants and others in destination households. Increased migration to suburban counties and to regions outside of the South, since 1975–1980, suggests an overall diversification of African American migration. The majority of outmigrants were “independent” because they moved into 1990 destination households that contained only migrants from the same origin, Los Angeles County. Others were linked either to nonmovers or to migrants from other places in destination households. A discriminant analysis suggests that many migrants linked to nonmigrants were moving for assistance, depending on others at the destination for housing and financial resources. In contrast, independent migrants have the personal resources to set up their own destination households.  相似文献