共查询到20条相似文献，搜索用时 31 毫秒
1.
Gurnis M 《Science (New York, N.Y.)》1990,250(4983):970-972
A numerical model of mantle convection shows that sea level fluctuations are not simply associated with temporal changes in ocean c plate spreading. In the dynamic model, sea level rises rapidly and then falls toward a steady value (but one still higher than the initial) following increased ridge spreading; this time dependence results from profound changes in the deep thermal structure under ocean and continent. The use of past variations in oceanic spreading to infer sea level fluctuations is called into question. With more realistic models and better continental stratigraphy, constraints may be placed on the viscosity structure of the mantle. 相似文献
2.
Transform faults appear in the process of stretching during freezing of the surface films on liquid wax. These films are composed of a warp yarn of wax fibers with optical anisotropy. This fabric is absent in materials that fail to produce transform faults. The mechanical anisotropy of these wax films (with high tensile strength and low shear strength in the direction of spreading) is responsible for the initiation of the transform faults. It is suggested that the anisotropy of the ocean upper mantle recorded seismically may likewise be responsible for the creation of the ridge-ridge transform faults in the oceans. 相似文献
3.
Spreading of the ocean floor: new evidence 总被引：3，自引：0，他引：3
Vine FJ 《Science (New York, N.Y.)》1966,154(3755):1405-1415
It is suggested that the entire history of the ocean basins, in terms of oceanfloor spreading,is contained frozen in the oceanic crust. Variations in the intensity and polarity of Earth's magnetic field are considered to be recorded in the remanent magnetism of the igneous rocks as they solidified and cooled through the Curie temperature at the crest of an oceanic ridge, and subsequently spread away from it at a steady rate. The hypothesis is supported by the extreme linearity and continuity of oceanic magnetic anomalies and their symmetry about the axes of ridges. If the proposed reversal time scale for the last 4 million years is combined with the model, computed anomaly profiles show remarkably good agreement with those observed, and one can deduce rates of spreading for all active parts of the midoceanic ridge system for which magnetic profilesor surveys are available. The rates obtained are in exact agreement with those needed to account for continental drift. An exceptionally high rate of spreading (approximately 4.5 cm/year) in the South Pacific enables one to deduce by extrapolation considerable details of the reversal time scale back to 11.5 million years ago. Again, this scale can be applied to other parts of the ridge system. Thus one isled to the suggestion that the crest of the East Pacific Rise in the northeast Pacific has been overridden and modified by the westward drift of North America, with the production of the anomalous width and unique features of the American cordillera in the western United States. The oceanicmagnetic anomalies also indicate that there was a change in derection of crustal spreading in this region during Pliocene time from eastwest to southeast-northwest. A profile from the crest to the boundary of the East Pacific Rise, and the difference between axial-zone and flank anomalies over ridges, suggest increase in the frequency of reversal of Earth's magnetic field, together, possibly, with decrease in its intensity, approximately 25 million years ago. Within the framework of ocean-floor spreading, it is suggested that magnetic anomaliesmay indicate the nature of oceanic fracture zones and distinguish the parts of the ridge system that are actively spreading. Thus data derived during the past year lend remarkable support to thehypothesis that magnetic anomalies may reveal the history of the ocean basins. 相似文献
4.
Wilson DS Teagle DA Alt JC Banerjee NR Umino S Miyashita S Acton GD Anma R Barr SR Belghoul A Carlut J Christie DM Coggon RM Cooper KM Cordier C Crispini L Durand SR Einaudi F Galli L Gao Y Geldmacher J Gilbert LA Hayman NW Herrero-Bervera E Hirano N Holter S Ingle S Jiang S Kalberkamp U Kerneklian M Koepke J Laverne C Vasquez HL Maclennan J Morgan S Neo N Nichols HJ Park SH Reichow MK Sakuyama T Sano T Sandwell R Scheibner B Smith-Duque CE Swift SA Tartarotti P Tikku AA Tominaga M Veloso EA 《Science (New York, N.Y.)》2006,312(5776):1016-1020
Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D. 相似文献
5.
Phillips JD 《Science (New York, N.Y.)》1967,157(3791):920-923
Ten magnetic profiles across the mid-Atlantic ridge near 27 degrees N show trends that are parallel to the ridge axis and symmetrical about the ridge axis. The configuration of magnetic bodies that could account for the pattern supports the Vine and Matthews hypothesis for the origin of magnetic anomalies over oceanic ridges. A polarity-reversal time scale inferred from models for sea-floor spreading in the Pacific-Antarctic ridge and radiometrically dated reversals of the geomagnetic field indicates a spreading rate of 1.25 centimeters per year during the last 6 million years and a rate of 1.65 centimeters per year between 6 and 10 million years ago. A similar analysis of more limited data over the mid-Atlantic ridge near 22 degrees N also indicates a change in the spreading rate. Here a rate of 1.4 centimeters per year appears to have been in effect during the last 5 million years; between 5 and 9 million years ago, an increased rate of 1.7 centimeters per year is indicated. The time of occurrence and relative magnitude of these changes in the spreading rate, about 5 to 6 million years ago and 18 to 27 percent, respectively, accords with the spreading rate change implied for the Juan de Fuca ridge in the northeast Pacific. 相似文献
6.
Mid-ocean ridges: discontinuities, segments and giant cracks 总被引：1，自引：0，他引：1
Geological observations reveal that mid-ocean ridges are segmented by numerous rigid and nonrigid discontinuities. A hierarchy of segmentation, ranging from large, long-lived segments to others that are small, migratory, and transient, determines the pattern and timing of creation of new ocean floor. To the extent that spreading segments behave like giant cracks in a plate, the crack propagation force at segment tips increases with segment length, which may explain why long segments tend to lengthen and prevail over shorter neighboring segments. Partial melting caused by decompression of the upper mantle due to plate separation and changes in the direction of spreading result in the spawning of new short segments so that a balance of long and short segments is maintained. 相似文献
7.
低洼重度盐碱地滴灌垄作模式土壤水盐运移规律研究 总被引：1，自引：0，他引：1
8.
选择钙质紫色土和中性紫色土为工程试样,通过室内多级三轴剪切试验测定了含水率和干密度交互作用对土坎稳定性重要指标——土壤抗剪强度指标的影响,确定了紫色土土坎坎高及外边坡稳定角的取值,以优化土坎设计.结果表明:干密度和含水率交互作用对土壤粘聚力的影响下,钙质紫色土和中性紫色土土坎土质的最优含水率分别为11%,8%,最优干密度分别为1.7,1.68g/cm3;在紫色土最优干密度和含水率下,考虑占地和稳定性因素,钙质紫色土土坎高度不宜超过2.0m,其外边坡坡度角可设计为80°;中性紫色土土坎高度不宜超过1.5m,其外边坡坡度角可设计为60°.本文旨在为紫色土区国土整治土坎修筑工程提供理论指导. 相似文献
9.
New potassium-argon age data from the Columbia Plateau suggest a basalt production rate of 108 cubic meters per year during a middle Miocene volcanic episode. This is two to three times the production rate in some oceanic islands, and about four to six times the production rate in spreading mid-oceanic ridge systems. 相似文献
10.
Urabe T Baker ET Ishibashi J Feely RA Marumo K Massoth GJ Maruyama A Shitashima K Okamura K Lupton JE Sonoda A Yamazaki T Aoki M Gendron J Greene R Kaiho Y Kisimoto K Lebon G Matsumoto T Nakamura K Nishizawa A Okano O Paradis G Roe K Shibata T Tennant D Vance T Walker SL Yabuki T Ytow N 《Science (New York, N.Y.)》1995,269(5227):1092-1095
A survey of hydrothermal activity along the superfast-spreading (approximately 150 millimeters per year) East Pacific Rise shows that hydrothermal plumes overlay approximately 60 percent of the ridge crest between 13 degrees 50' and 18 degrees 40'S, a plume abundance nearly twice that known from any other rige portion of comparable length. Plumes were most abundant where the axial cross section is inflated and an axial magma chamber is present. Plumes with high ratios of volatile ((3)He, CH(4), and H(2)S) to nonvolatile (Mn and Fe) species marked where hydrothermal circulation has been perturbed by recent magmatic activity. The high proportion of volatile-rich plumes observed implies that such episodes are more frequent here than on slower spreading ridges. 相似文献
11.
Dott RH 《Science (New York, N.Y.)》1969,166(3907):874-876
The hypothesis of sea floor spreading and lithosphere plates seems to unify the origins of both oceanic ridges and volcanic arc-trench systems; therefore knowledge of well-known land areas should shed light upon sea floor tectonics. Impressive evidence of a major mid-Cenozoic discontinuity in the tectonic history of circum-Pacific land areas suggests a roughly synchronous change in sea floor development, more evidence for which may be anticipated in the future. 相似文献
12.
Menard HW 《Science (New York, N.Y.)》1967,157(3791):923-924
Local sea floor topography and also the thickness of the second layer of the oceanic rise-ridge system appear related to the spreading rate in the region. Slow spreading, away from the ridge center at 1 to 2 centimeters per year, is associated with a thick second layer, a central rift, and adjacent rift mountains. Fast spreading, 3 to 4.5 centimeters per year, is associated with a thin second layer and subdued topography lacking any central rift. The volume of lava discharged in this layer per unit time and per unit length along the crest of the whole active system is relatively constant regardless of the spreading rate. Total second layer discharge of the system has been about 5 to 6 cubic kilometers per year during the last several million years. 相似文献
13.
半干旱区沟垄集雨种植对紫花苜蓿出苗及草产量的影响 总被引：5，自引：0，他引：5
本研究利用沟垄集雨模式种植紫花苜蓿,观测不同沟垄比和垄覆盖方式对紫花苜蓿出苗率、土壤贮水量及干草产量的影响.结果表明:覆膜垄处理具有较高的出苗率、土壤贮水量和干草产量,分别比平作提高6.1%、16.1%和58.1%;土垄处理效果不明显,其出苗率比平作降低4.7%,土壤贮水量和干草产量分别比平作提高3.1%和8.4%;沟垄集雨种植增加了土壤贮水量,从而提高了干草产量,其中覆膜垄处理的增产效果显著;膜垄处理中,宽垄处理的增产效果要好于窄垄处理. 相似文献
14.
在长武县刺槐林选取６１块标准地，对其立地、现实生长状况进行系统调查研究。结果表明：８年生树高能反映立地差异显著性；地貌部位是影响刺槐林生长发育的主导性因子。立地要素聚类分析结果表明，长武县刺槐林立地可归纳为４个立地类型：梁峁—塬坡；台地；沟坡；沟底。两个立地类型组：梁峁—塬坡—台地和沟底—沟坡。根据立地类型间和立地类型组间的林木生长状况差异显著性检测结果，同时考虑目前当地生产经营水平，将长武县刺槐林划分为两个经营类型：（１）梁峁—塬坡刺槐林；（２）沟底—沟坡刺槐林。后者的生产力高于前者 相似文献
15.
Mid-ocean ridge basalts (MORBs) and ocean island basalts (QIBs) are derived by partial melting of the upper mantle and are marked by systematic excesses of thorium-230 activity relative to the activity of its parent, uranium-238. Experimental measurements of the distribution of thorium and uranium between the melt and solid residue show that, of the major phases in the upper mantle, only garnet will retain uranium over thorium. This sense of fractionation, which is opposite to that caused by clinopyroxene-melt partitioning, is consistent with the thorium-230 excesses observed in young oceanic basalts. Thus, both MORBs and QIBs must begin partial melting in the garnet stability field or below about 70 kilometers. A calculation shows that the thorium-230-uranium-238 disequilibrium in MORBs can be attributed to dynamic partial melting beginning at 80 kilometers with a melt porosity of 0.2 percent or more. This result requires that melting beneath ridges occurs in a wide region and that the magma rises to the surface at a velocity of at least 0.9 meter per year. 相似文献
16.
脲醛树脂稻壳板平面抗拉强度的研究 总被引：1，自引：0，他引：1
李兰亭 《东北林业大学学报》1988,(6)
脲醛树脂压制的稻壳板主要问题是平面抗拉强度低。从组成脲醛树脂的成分、树脂特性、增强剂及固化剂等几方面研究了稻壳板的平面抗拉强度。研究结果表明,采用适合稻壳特点的脲醛树脂并在适当增强剂和固化剂的配合下,可大幅度提高稻壳板的平面抗拉强度,即使施加较抵的胶量,稻壳板的平面抗拉强度仍然达到标准。 相似文献
17.
18.
In the outer regions of Saturn's main rings, strong tidal forces balance gravitational accretion processes. Thus, unusual phenomena may be expected there. The Cassini spacecraft has recently revealed the strange "flying saucer" shape of two small satellites, Pan and Atlas, located in this region, showing prominent equatorial ridges. The accretion of ring particles onto the equatorial surfaces of already-formed bodies embedded in the rings may explain the formation of the ridges. This ridge formation process is in good agreement with detailed Cassini images showing differences between rough polar and smooth equatorial terrains. We propose that Pan and Atlas ridges are kilometers-thick "ring-particle piles" formed after the satellites themselves and after the flattening of the rings but before the complete depletion of ring material from their surroundings. 相似文献
19.
Detrick RS Harding AJ Kent GM Orcutt JA Mutter JC Buhl P 《Science (New York, N.Y.)》1993,259(5094):499-503
Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models. However, the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate. These observations indicate that the paradigm of ridge crest magma chambers as small, sill-like, midcrustal bodies is applicable to a wide range of intermediate- and fast-spreading ridges. 相似文献
20.
Tolstoy M Cowen JP Baker ET Fornari DJ Rubin KH Shank TM Waldhauser F Bohnenstiehl DR Forsyth DW Holmes RC Love B Perfit MR Weekly RT Soule SA Glazer B 《Science (New York, N.Y.)》2006,314(5807):1920-1922
Two-thirds of Earth's surface is formed at mid-ocean ridges, yet sea-floor spreading events are poorly understood because they occur far beneath the ocean surface. At 9 degrees 50'N on the East Pacific Rise, ocean-bottom seismometers recently recorded the microearthquake character of a mid-ocean ridge eruption, including precursory activity. A gradual ramp-up in activity rates since seismic monitoring began at this site in October 2003 suggests that eruptions may be forecast in the fast-spreading environment. The pattern culminates in an intense but brief (approximately 6-hour) inferred diking event on 22 January 2006, followed by rapid tapering to markedly decreased levels of seismicity. 相似文献