全文获取类型
收费全文 | 1099篇 |
免费 | 142篇 |
国内免费 | 347篇 |
专业分类
测绘学 | 179篇 |
大气科学 | 148篇 |
地球物理 | 124篇 |
地质学 | 649篇 |
海洋学 | 154篇 |
天文学 | 13篇 |
综合类 | 67篇 |
自然地理 | 254篇 |
出版年
2024年 | 4篇 |
2023年 | 27篇 |
2022年 | 60篇 |
2021年 | 54篇 |
2020年 | 55篇 |
2019年 | 81篇 |
2018年 | 61篇 |
2017年 | 52篇 |
2016年 | 72篇 |
2015年 | 43篇 |
2014年 | 76篇 |
2013年 | 80篇 |
2012年 | 55篇 |
2011年 | 58篇 |
2010年 | 67篇 |
2009年 | 65篇 |
2008年 | 61篇 |
2007年 | 81篇 |
2006年 | 70篇 |
2005年 | 47篇 |
2004年 | 52篇 |
2003年 | 25篇 |
2002年 | 22篇 |
2001年 | 36篇 |
2000年 | 31篇 |
1999年 | 29篇 |
1998年 | 29篇 |
1997年 | 18篇 |
1996年 | 25篇 |
1995年 | 20篇 |
1994年 | 28篇 |
1993年 | 25篇 |
1992年 | 17篇 |
1991年 | 23篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 4篇 |
1987年 | 9篇 |
1985年 | 4篇 |
1984年 | 1篇 |
排序方式: 共有1588条查询结果,搜索用时 15 毫秒
971.
T. OKUDAIRA 《Journal of Metamorphic Geology》1996,14(4):427-440
Garnet crystals from low-pressure/high-temperature (LPHT) Ryoke metamorphic rocks in the Yanai district, south-western Japan, show several kinds of chemical zoning patterns that systematically vary with grain radius between c . 0.1 and 0.5 mm. Large grains (> c . 0.4 mm) show normal zoning and small grains (< c . 0.4 mm) show unzoned or reversely zoned cores. Observations of the chemical zoning and of the spatial and size distributions of the garnet grains between c . 0.1 and 0.5 mm in radius suggest that they were formed by continuous nucleation and diffusion-controlled growth.
A previously estimated temperature–time path ( T – t path) for the Ryoke metamorphism, using 1-D numerical simulation, is characterized by a rapid increase in temperature, 0.0017 °C yr−1 on average, and a period of high temperature (>600 °C) shorter than 0.5 Myr, which was presumably caused by the intrusion of a granodiorite sheet. Chemical zoning of garnet grains with different radii simulated for the T – t path using a numerical model of continuous nucleation and diffusion-controlled growth, in combination with intracrystalline diffusion, compares well with the observed zoning patterns in garnet grains with different radii. This is in spite of the fact that the simulated zoning patterns vary greatly, depending on subtle differences in the T–t history. Therefore, they suggest that the T–t path gives a good explanation for the LPHT Ryoke metamorphism. Although this study only refers to the Ryoke metamorphism, the technique may be applicable to thermal modelling of other metamorphic terranes. 相似文献
A previously estimated temperature–time path ( T – t path) for the Ryoke metamorphism, using 1-D numerical simulation, is characterized by a rapid increase in temperature, 0.0017 °C yr
972.
Abstract Retrograde and prograde mineral assemblages from metapelitic and metabasic rocks of the Iforas Granulitic Unit (Mali) were generated by the superimposition of two granulite facies metamorphic events. They clearly result from a polycyclic evolution and can be related to a late Eburnean unroofing followed by a Pan-African burial.
Thermobarometry on Pan-African garnet-bearing assemblages yields ( P, T ) estimates of 620±50°C and 5± Ikbar. The nearly anhydrous conditions produced in the Eburnean appear to be the direct cause of the unusually lowtemperature granulite-facies metamorphism in the Pan-African. These P, T estimates are compared with those obtained on the underlying unit (Kidal Assemblage) upon which the Iforas Granulitic Unit was thrust. A P-T-t path, during the Pan-African orogeny, is proposed and discussed for both the Iforas Granulites and Kidal Assemblage. 相似文献
Thermobarometry on Pan-African garnet-bearing assemblages yields ( P, T ) estimates of 620±50°C and 5± Ikbar. The nearly anhydrous conditions produced in the Eburnean appear to be the direct cause of the unusually lowtemperature granulite-facies metamorphism in the Pan-African. These P, T estimates are compared with those obtained on the underlying unit (Kidal Assemblage) upon which the Iforas Granulitic Unit was thrust. A P-T-t path, during the Pan-African orogeny, is proposed and discussed for both the Iforas Granulites and Kidal Assemblage. 相似文献
973.
Abstract Portions of three Proterozoic tectonostratigraphic sequences are exposed in the Cimarron Mountains of New Mexico. The Cimarron River tectonic unit has affinities to a convergent margin plutonic/volcanic complex. Igneous hornblende from a quartz diorite stock records an emplacement pressure of 2–2.6 kbar. Rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4–5 kbar and 330 ± 50° C. The Tolby Meadow tectonic unit consists of quartzite and schist. Mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 ± 20° C. A low-angle ductile shear zone separates this succession from gneisses of the structurally underlying Eagle Nest tectonic unit. Gneissic granite yields hornblende pressures of 6–8 kbar. Pelitic gneiss records regional metamorphic conditions of 6–7 kbar and 705 ± 15° C, overprinted by retrogression at 4 kbar and 530 ± 10° C. Comparison of metamorphic and retrograde conditions indicates a P–T path dominated by decompression and cooling. The low-angle ductile shear zone represents an extensional structure which was active during metamorphism. This extension juxtaposed the Tolby Meadow and Eagle Nest units at 4 kbar and 520° C. Both units were later overprinted by folding and low-grade metamorphism, and then were emplaced against the Cimarron River tectonic unit by right-slip movement along the steeply dipping Fowler Pass shear zone. An argon isotope-correlation age obtained from igneous hornblende dates plutonism in the Cimarron River unit at 1678 Ma. Muscovite associated with the greenschist facies metamorphic overprint yields a 40 Ar/39 Ar plateau age of 1350 Ma. By contrast, rocks within the Tolby Meadow and Eagle Nest units yield significantly younger argon cooling ages. Hornblende isotope-correlation ages of 1394–1398 Ma are interpreted to date cooling during middle Proterozoic extension. Muscovite plateau ages of 1267–1257 Ma appear to date cooling from the low-grade metamorphic overprint. The latest ductile movement along the Fowler Pass shear zone post-dated these cooling ages. Argon released from muscovites of the Eagle Nest/Tolby Meadow composite unit, at low experimental temperatures, yields apparent ages of c. 1100 Ma. Similar ages are not obtained north-east of the Fowler Pass shear zone, suggesting movement more recently than 1100 Ma. 相似文献
974.
975.
S. ELVEVOLD H. REGINIUSSEN E. J. KROGH F. BJØRKLUND 《Journal of Metamorphic Geology》1994,12(4):539-556
The Seiland Igneous Province of the North Norwegian Caledonides consists of a suite of deep-seated rift-related magmatic rocks emplaced into paragneisses during late Precambrian to Ordovician time. In the south-eastern part of the province, contact metamorphism of the paragneisses and later reworking of intrusives and associated contact aureoles have resulted in the development of three successive metamorphic stages. The contact metamorphic assemblage (M1) Opx + Grt + Qtz + Pl + Kfs + Hc + Ilm ± Crd is preserved in xenolithic rafts of paragneiss within metagabbro. Geothermobarometric calculations yield 930-960d? C and 5-6.5 kbar for the contact metamorphism. M1 was followed by cooling, accompanied by strong shearing, formation of the gneiss foliation and recrystallization at intermediate-P granulite facies conditions (M2). Stable M2 phases are Cpx + Opx + Pl +Ilm ± Hbl in metagabbro and Grt ± Sil ± Opx + Kfs + Qtz + Pl ± Bt + Ilm in host paragneiss. The M2 conditions are estimated to 700-750d? C and 5-7 kbar. A subsequent pressure increase is recorded in the M3 episode, which is associated with recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. M3 is defined by the assemblages Grt + Cpx ± Opx + Pl + Ru + Qtz in metagabbro, and Grt ± Ky + Qtz + Pl ± Kfs + Bt + Ru in host paragneiss. M3 conditions are estimated to 650-700d? C and 8-10 kbar. The substantial pressure increase related to the M2 → M3 transition is interpreted to be a result of (early?) Caledonian overthrusting. Chemical zoning in cordierite and biotite suggest rapid cooling following the M3 event. The proposed P-T-t evolution implies that the tectonic evolution of the Seiland Igneous Province was long (at least 330 Ma) and complex and involved initial rifting and extension followed by crustal thickening and compression. 相似文献
976.
石羊河流域水资源利用与经济发展系统分析 总被引:12,自引:7,他引:5
对石羊河流域1995—2005年水资源利用与经济发展指标进行相关性分析及通径分析,结果表明,农业生产对水资源的压力巨大,必须建立科学合理的水资源利用机制和上中下游协调发展模式,通过生活、工业、农业和生态环境等各用水部门之间协调,使整个区域的水资源得到合理充分的利用,从而达到最大供水效益。研究表明,在经济可持续发展的情况下,人口的增长使得发展林业对水资源的压力巨大,为显著减轻经济和社会发展对水资源的压力,应优先大力发展第三产业,其次发展第二产业、畜牧业和林业。科学分配各产业、各部门的供需水量,实现流域或区域的社会、经济和生态环境的协调发展。 相似文献
977.
旅行社网络化路径研究 总被引:1,自引:0,他引:1
随着中国旅行社数量的增多和市场竞争日趋激烈,网络化战略已成为数量众多中小型旅行社持续发展和提升自身竞争力的趋势性选择。在对国内旅行社的经营现状和网络化发展态势分析的基础上,提出旅行社进行网络化的5种路径,即旅行社战略联盟、旅行社虚拟企业、旅行社联合体、旅行社企业集团、旅行社间“准一体化”,及其3种构建方式,即横向一体化、纵向一体化、横纵向混合一体化,以期实现中国旅行社业健康持续发展。 相似文献
978.
An improved recursive decomposition algorithm for reliability evaluation of lifeline networks 总被引:1,自引:1,他引:0
The seismic reliability evaluation of lifeline networks has received considerable attention and been widely studied.In this paper,on the basis of an original recursive decomposition algorithm,an improved analytical approach to evaluate the seismic reliability of large lifeline systems is presented.The proposed algorithm takes the shortest path from the source to the sink of a network as decomposition policy.Using the Boolean laws of set operation and the probabilistic operation principal,a recursive deco... 相似文献
979.
980.
软土在不同应力路径下的力学特性分析 总被引:7,自引:0,他引:7
通过固结不排水剪应力路径试验对广州南沙典型软土在不同固结条件的力学特性进行了系统研究。分析了初始固结状态对软黏土应力路径依赖性的影响,比较了不同应力路径下土的应力-应变关系特征和孔隙水压力变化规律,探讨了孔压分布与土体变形特征的关系,认为侧向卸荷会造成剪应力增加、体应力减小,从而使土体产生剪胀趋势。研究结果还表明,不排水条件下的有效应力路径主要与土样初始固结状态有关,因而同一固结状态下的有效应力路径具有唯一性。另外,试验中的剪切控制方式对有效应力路径有明显影响,但对土体抗剪强度的影响可以忽略。 相似文献