全文获取类型
收费全文 | 38598篇 |
免费 | 1131篇 |
国内免费 | 1061篇 |
专业分类
测绘学 | 969篇 |
大气科学 | 2948篇 |
地球物理 | 7949篇 |
地质学 | 14245篇 |
海洋学 | 3440篇 |
天文学 | 8463篇 |
综合类 | 245篇 |
自然地理 | 2531篇 |
出版年
2022年 | 293篇 |
2021年 | 494篇 |
2020年 | 466篇 |
2019年 | 511篇 |
2018年 | 941篇 |
2017年 | 900篇 |
2016年 | 1085篇 |
2015年 | 748篇 |
2014年 | 1089篇 |
2013年 | 1927篇 |
2012年 | 1382篇 |
2011年 | 1850篇 |
2010年 | 1614篇 |
2009年 | 2063篇 |
2008年 | 1757篇 |
2007年 | 1813篇 |
2006年 | 1757篇 |
2005年 | 1270篇 |
2004年 | 1182篇 |
2003年 | 1078篇 |
2002年 | 1040篇 |
2001年 | 882篇 |
2000年 | 855篇 |
1999年 | 697篇 |
1998年 | 759篇 |
1997年 | 703篇 |
1996年 | 597篇 |
1995年 | 575篇 |
1994年 | 497篇 |
1993年 | 435篇 |
1992年 | 438篇 |
1991年 | 397篇 |
1990年 | 471篇 |
1989年 | 383篇 |
1988年 | 365篇 |
1987年 | 453篇 |
1986年 | 356篇 |
1985年 | 447篇 |
1984年 | 563篇 |
1983年 | 468篇 |
1982年 | 473篇 |
1981年 | 423篇 |
1980年 | 439篇 |
1979年 | 381篇 |
1978年 | 370篇 |
1977年 | 361篇 |
1976年 | 325篇 |
1975年 | 311篇 |
1974年 | 323篇 |
1973年 | 356篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
851.
Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating 总被引:1,自引:0,他引:1
Metapelitic rock samples from the NE Shackleton Range, Antarctica,include garnet with contrasting zonation patterns and two agespectra. Garnet porphyroblasts in K-rich kyanitesillimanitestaurolitegarnetmuscovitebiotite schistsfrom Lord Nunatak show prograde growth zonation, and give SmNdgarnet, UPb monazite and RbSr muscovite ages of518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively.Geothermobarometry and PT pseudo-section calculationsin the model system CaONa2OK2O TiO2MnOFeOMgOAl2O3SiO2H2Oare consistent with garnet growth during prograde heating from540°C/7 kbar to 650°C/7·5 kbar, and partial resorptionduring a subsequent PT decrease to <650°C at <6kbar. All data indicate that rocks from Lord Nunatak were affectedby a single orogenic cycle. In contrast, garnet porphyroblastsin K-poor kyanitesillimanite staurolitegarnetcordieritebiotite-schistsfrom Meade Nunatak show two growth stages and diffusion-controlledzonation. Two distinct age groups were obtained. Laser ablationplasma ionization multicollector mass spectrometry in situ analysesof monazite, completely enclosed by a first garnet generation,yield ages of c. 1700 Ma, whereas monazite grains in open garnetfractures and in most matrix domains give c. 500 Ma. Both agegroups are also obtained by UPb thermal ionization massspectrometry analyses of matrix monazite and zircon, which fallon a discordia with lower and upper intercepts at 502 ±1 and 1686 ± 2 Ma, respectively. SmNd garnet datingyields an age of 1571 ± 40 Ma and RbSr biotiteanalyses give an age of 504 ± 1 Ma. Integrated geochronologicaland petrological data provide evidence that rocks from MeadeNunatak underwent a polymetamorphic Barrovian-type metamorphism:(1) garnet 1 growth and subsequent diffusive garnet annealingbetween 1700 and 1570 Ma; (2) garnet 2 growth during the RossOrogeny at c. 500 Ma. During the final orogenic event the rocksexperienced peak PT conditions of about 650°C/7·0kbar and a retrograde stage at c. 575°C/4·0 kbar. KEY WORDS: garnet microtexture; PT pseudosection; geochronology; polymetamorphism; Shackleton Range; Antarctica 相似文献
852.
Simanovich I. M. Golovin D. I. Bujakaite M. I. Vinogradov V. I. Sakharov B. A. Matsapulin V. U. Sokolova A. L. Pokrovskaya E. V. 《Lithology and Mineral Resources》2004,39(6):555-565
The influence of geodynamic factors on the postsedimentary lithogenesis of terrigenous sequences during the replacement of the passive continental-margin regime to the active regime is considered with the Toarcian–Aalenian complexes of southern Dagestan as an example. It has been established that clayey rocks were lithified under conditions of subsidence lithogenesis, whereas the orogenic catagenesis of sandy rocks was related to stress initiated by the initial (amagmatic) subduction phase. The Kübler index has been used to reveal the postsedimentary zonality that is consistent with the intensity of folding and cleavage. The metagenesis zone fits the cleavage zone. The Rb–Sr and K–Ar datings of clayey rocks from the metagenesis zone yielded 180–190 Ma that can be interpreted as the timing of the maximal manifestation of postsedimentary lithogenesis. 相似文献
853.
Beds comprising debrite sandwiched within co-genetic turbidite: origin and widespread occurrence in distal depositional environments 总被引:11,自引:0,他引:11
Co‐genetic debrite–turbidite beds occur in a variety of modern and ancient turbidite systems. Their basic character is distinctive. An ungraded muddy sandstone interval is encased within mud‐poor graded sandstone, siltstone and mudstone. The muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite. The mud‐poor sandstone, siltstone and mudstone show features indicating progressive layer‐by‐layer deposition and are thus called a turbidite. Palaeocurrent indicators, ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event. Detailed field observations are presented for co‐genetic debrite–turbidite beds in three widespread sequences of variable age: the Miocene Marnoso Arenacea Formation in the Italian Apennines; the Silurian Aberystwyth Grits in Wales; and Quaternary deposits of the Agadir Basin, offshore Morocco. Deposition of these sequences occurred in similar unchannellized basin‐plain settings. Co‐genetic debrite–turbidite beds were deposited from longitudinally segregated flow events, comprising both debris flow and forerunning turbidity current. It is most likely that the debris flow was generated by relatively shallow (few tens of centimetres) erosion of mud‐rich sea‐floor sediment. Changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition. The association with distal settings results from the ubiquitous presence of muddy deposits in such locations, which may be eroded and disaggregated to form a cohesive debris flow. Debrite intervals may be extensive (> 26 × 10 km in the Marnoso Arenacea Formation) and are not restricted to basin margins. Such long debris flow run‐out on low‐gradient sea floor (< 0·1°) may simply be due to low yield strength (? 50 Pa) of the debris–water mixture. This study emphasizes that multiple flow types, and transformations between flow types, can occur within the distal parts of submarine flow events. 相似文献
854.
Sedimentology and kinematics of a large, retrogressive growth-fault system in Upper Carboniferous deltaic sediments, western Ireland 总被引:2,自引:0,他引:2
Growth faulting is a common feature of many deltaic environments and is vital in determining local sediment dispersal and accumulation, and hence in controlling the resultant sedimentary facies distribution and architecture. Growth faults occur on a range of scales, from a few centimetres to hundreds of metres, with the largest growth faults frequently being under‐represented in outcrops that are often smaller than the scale of feature under investigation. This paper presents data from the exceptionally large outcrops of the Cliffs of Moher, western Ireland, where a growth‐fault complex affects strata up to 60 m in thickness and extends laterally for ≈ 3 km. Study of this Namurian (Upper Carboniferous) growth‐fault system enables the relationship between growth faulting and sedimentation to be detailed and permits reconstruction of the kinematic history of faulting. Growth faulting was initiated with the onset of sandstone deposition on a succession of silty mudstones that overlie a thin, marine shale. The decollement horizon developed at the top of the marine shale contact for the first nine faults, by which time aggradation in the hangingwall exceeded 60 m in thickness. After this time, failure planes developed at higher stratigraphic levels and were associated with smaller scale faults. The fault complex shows a dominantly landward retrogressive movement, in which only one fault was largely active at any one time. There is no evidence of compressional features at the base of the growth faults, thus suggesting open‐ended slides, and the faults display both disintegrative and non‐disintegrative structure. Thin‐bedded, distal mouth bar facies dominate the hangingwall stratigraphy and, in the final stages of growth‐fault movement, erosion of the crests of rollover structures resulted in the highest strata being restricted to the proximity of the fault. These upper erosion surfaces on the fault scarp developed erosive chutes that were cut parallel to flow and are downlapped by the distal hangingwall strata of younger growth faults. 相似文献
855.
856.
Metasomatism accompanying kimberlite emplacement is a worldwide phenomenon, although infrequently described or recognised. At the Cambrian-aged Murowa and Sese kimberlite clusters located within the Archean Zimbabwe Craton just north of the boundary with the Limpopo Mobile Zone in southern central Zimbabwe, the metasomatism is intense and well exposed and the processes can be readily studied. Dykes, sills and the root zones of pipes are exposed at the current erosion level. Kimberlite lithologies present are hypabyssal macrocrystic kimberlite (“HMK”), HMK breccia, and tuffisitic kimberlite breccia (“TKB”) including minor lithic tuffisitic kimberlite breccia (“LTKB”). Country rocks are 2.6 Ga Chibi and Zimbabwe granite batholiths emplaced into 2.6–2.9 Ga or earlier Archean tonalitic gneiss and greenstones. During initial metasomatism, the granites become spotted with green chlorite, needles of alkaline amphiboles (winchite, riebeckite, arfvedsonite) and pyroxenes (aegirine–augite) with minor carbonate and felts of talc. Oligoclase feldspar becomes converted to albite, extensively altered, dusted and reddened with hematite, whereas K-feldspar remains unaffected. The granites become converted to syenite through removal of quartz. More intense metasomatism at Murowa and Sese results in veins of green metasomatite which cut and disrupt the granite. Progressive disruption entrains granite blocks, breaking down the granite still further, spalling off needle-like granite slivers, and so giving rise to LTKB. This process of disruption and entrainment appears to be the manner of initial development of the pipe structure. The chemistry of the metasomatite is intermediate between granite and kimberlite. Compared to granite country rock it has markedly higher Mg, Cr, Ni, CO2 and H2O+, higher Ca, Mn, Nb, Sr, P, Fe3+/Fe2+ ratio, U, Co, and Cu, approximately equal TiO2, K2O, Na2O, La, Ta, Rb, Zr, Zn and resultant lower SiO2, Al2O3, Ga and Y. The metasomatite Na2O/K2O ratio is slightly higher than that of the granite. The metasomatic process is broadly analogous to fenitisation of granitic wall rock accompanying carbonatite complex emplacement. The metasomatism at Murowa and Sese was caused by fluids from the rising but confined proto-kimberlite melt penetrating into cracks and matrix of granite country rock and reacting with it. These fluids were CO2-rich, hydrous, oxidising, enhanced in ultramafic elements and carried low levels of Na. 相似文献
857.
We perform a global statistical investigation into the problem of Coulomb stress triggering of earthquakes by using the Harvard CMT catalogue of shallow earthquakes from 1976 to 2001. We consider ‘earthquake pairs’, i.e., pairs of successive earthquakes occurring near each other with similar focal mechanisms, and address the problem of whether or not the change of the Coulomb failure stress (CFS) produced by the first earthquake of the pair ‘encourages’ the occurrence of the second one. An interesting feature is that such a Coulomb stress-triggering phenomenon has an apparent focal mechanism dependence: for thrust earthquakes, a more evident triggering effect can be observed. 相似文献
858.
Alexandra Haase-Schramm Steven L. Goldstein Mordechai Stein 《Geochimica et cosmochimica acta》2004,68(5):985-1005
U-series dating can be an effective means to obtain accurate and precise ages on Quaternary carbonates. However, most samples require a correction for U and Th in admixed detritus. This complication is often addressed through generation of U-Th isochrons, requiring analyses of several coeval samples. In addition, presence of water-derived (hydrogenous) Th in the carbonate can cause inaccuracies in isochron ages.This study reports a high-resolution U-series chronology of sediments deposited by Lake Lisan, the last glacial precursor of the Dead Sea. The strategy employed combines multiple measurements from a few stratigraphic heights and fewer analyses from many heights in a single described and measured section. The resulting chronology is based on ages at 22 heights in a ∼40-m-thick section covering the interval of ∼70-14 calendar ka BP. The effects of admixed detritus are evaluated using trace elements. Nearly pure aragonite samples, indicated by very low abundances of insoluble elements such as Nb and Zr, were found to contain hydrogenous Th, which causes the uncorrected U-230Th age of a modern sample to be ∼2.5 ka. Nevertheless, accurate ages have been obtained by correcting for the detrital and aqueous interferences. The resulting ages are in stratigraphic order, and their accuracy is evidenced by consistency of Lisan Formation U-series and 14C ages with the coral-based calendar-radiocarbon age calibration.The U-Th ages provide a context to unravel the limnological history of Lake Lisan. Boundaries between the Lower, Middle, and Upper stratigraphic units correspond to the MIS 4/3 and 3/2 transitions, respectively. During MIS 2 and 4 the lake generally showed a stable two-layer configuration and a positive fresh-water balance, reflected by deposition of laminated aragonite-detritus. Dry intervals during MIS 2 and 4 are indicated by thick gypsum layers and an inferred depositional hiatus, which are temporally associated with Heinrich events H1 at ∼17 ka and H6 at ∼65 ka, respectively. During MIS 3 the lake level was unstable with intermittent dry periods indicated by abundant clastic layers and a significant hiatus between ∼43-49 ka. Clastic layers are associated with Dansgaard-Oeschger events during MIS 3, and indicate lake level declines during abrupt Northern Hemisphere warmings. Overall, the climate of the Eastern Mediterranean region shows a strong linkage to the Northern Hemisphere climate, with increasing lake size and stability during cold periods, and fluctuations and dessication during warmings and Heinrich events. 相似文献
859.
The exhumation of ultrahigh-pressure (UHP) metamorphic units from depths more than 100-120 km is one of the most intriguing questions in modern petrology and geodynamics. We use the diamondiferous Kumdy-Kol domain in the Kokchetav Massif to show that exhumation models should take into consideration initially high uplift velocities (from 20 down to 6 cm/year) and the absence of the deformation of UHP assemblages. The high rate of exhumation are indicated by ion microprobe (SHRIMP) dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds.Diamondiferous rocks in the Kumdy-Kol domain occur as steeply dipping (60°-80°) thin slices (few hundred metres) within granite-gneiss. Using geological, petrological and isotopic-geochemical data, we show that partial melting of diamondiferous metamorphic rocks occurred; a very important factor which has not been taken into account in previous models.Deformation of diamondiferous rocks at Kumdy-Kol is insignificant; diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation. All these facts could be explained by partial melting of metapelites and granitic rocks in the Kumdy-Kol domain. The presence of melt is responsible for an essential reduction of viscosity and a density difference (Δρ) between crustal rocks and mantle material and reduced friction between the upwelling crustal block, the subducting and overriding plates. Besides Δρ, the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle.We construct different models for the three stages of exhumation: a model similar to “corner flow” for the first superfast exhumation stage, an intermediate stage of extension (most important from structural point of view) and a very low rate of exhumation in final diapir+erosional uplift. 相似文献
860.
Recent investigations of a limestone solution cave on the Queen Charlotte Islands (Haida Gwaii) have yielded skeletal remains of fauna including late Pleistocene and early Holocene bears, one specimen of which dates to ca. 14,400 14C yr B.P. This new fossil evidence sheds light on early postglacial environmental conditions in this archipelago, with implications for the timing of early human migration into the Americas. 相似文献