共查询到20条相似文献，搜索用时 15 毫秒
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Bright Spots, Structure, and Magmatism in Southern Tibet from INDEPTH Seismic Reflection Profiling 总被引：8，自引：0，他引：8
LD Brown W Zhao KD Nelson M Hauck D Alsdorf A Ross M Cogan M Clark X Liu J Che 《Science (New York, N.Y.)》1996,274(5293):1688-1690
INDEPTH seismic reflection profiling shows that the decollement beneath which Indian lithosphere underthrusts the Himalaya extends at least 225 kilometers north of the Himalayan deformation front to a depth of approximately 50 kilometers. Prominent reflections appear at depths of 15 to 18 kilometers near where the decollement reflector apparently terminates. These reflections extend north of the Zangbo suture to the Damxung graben of the Tibet Plateau. Some of these reflections have locally anomalous amplitudes (bright spots) and coincident negative polarities implying that they are produced by fluids in the crust. The presence of geothermal activity and high heat flow in the regions of these reflections and the tectonic setting suggest that the bright spots mark granitic magmas derived by partial melting of the tectonically thickened crust. 相似文献
3.
Lorenz RD Wall S Radebaugh J Boubin G Reffet E Janssen M Stofan E Lopes R Kirk R Elachi C Lunine J Mitchell K Paganelli F Soderblom L Wood C Wye L Zebker H Anderson Y Ostro S Allison M Boehmer R Callahan P Encrenaz P Ori GG Francescetti G Gim Y Hamilton G Hensley S Johnson W Kelleher K Muhleman D Picardi G Posa F Roth L Seu R Shaffer S Stiles B Vetrella S Flamini E West R 《Science (New York, N.Y.)》2006,312(5774):724-727
The most recent Cassini RADAR images of Titan show widespread regions (up to 1500 kilometers by 200 kilometers) of near-parallel radar-dark linear features that appear to be seas of longitudinal dunes similar to those seen in the Namib desert on Earth. The Ku-band (2.17-centimeter wavelength) images show approximately 100-meter ridges consistent with duneforms and reveal flow interactions with underlying hills. The distribution and orientation of the dunes support a model of fluctuating surface winds of approximately 0.5 meter per second resulting from the combination of an eastward flow with a variable tidal wind. The existence of dunes also requires geological processes that create sand-sized (100- to 300-micrometer) particulates and a lack of persistent equatorial surface liquids to act as sand traps. 相似文献
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Cratering flow calculations for a series of oblique to normal (10 degrees to 90 degrees ) impacts of silicate projectiles onto a silicate halfspace were carried out to determine whether or not the gas produced upon shock-vaporizing both projectile and target material would form a downstream jet that could entrain and propel SNC meteorites from the Martian surface. The difficult constraints that the impact origin hypothesis for SNC meteorites has to satisfy are that these meteorites are lightly to moderately shocked and yet have been accelerated to speeds in excess of the Martian escape velocity (more than 5 kilometers per second). Two-dimensional finite difference calculations were performed that show that at highly probable impact velocities (7.5 kilometers per second), vapor plume jets are produced at oblique impact angles of 25 degrees to 60 degrees and have speeds as great as 20 kilometers per second. These plumes flow nearly parallel to the planetary surface. It is shown that upon impact of projectiles having radii of 0.1 to 1 kilometer, the resulting vapor jets have densities of 0.1 to 1 gram per cubic centimeter. These jets can entrain Martian surface rocks and accelerate them to velocities greater than 5 kilometers per second. This mechanism may launch SNC meteorites to earth. 相似文献
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Showalter MR 《Science (New York, N.Y.)》1995,267(5197):490-493
Careful reprocessing of the Voyager images reveals that the Uranìan lambda ring has marked longitudinal variations in brightness comparable in magnitude to those in Saturn's F ring and Neptune's Adams ring. The ring's variations show a dominant five-cycle (72-degree) periodicity, although additional structure down to scales of about 0.5 degree is also present. The ring's shape is defined by a small overall eccentricity plus a six-cycle (60-degree) sinusoidal variation of radial amplitude around 4 kilometers. Both of these properties can be explained by the resonant perturbations of a moon at a semimajor axis of 56,479 kilometers, but no known moon orbits at this location. Unfortunately, the mass required suggests that such a body should have been imaged by Voyager. 相似文献
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We addressed the question of bottom-up versus top-down control of marine ecosystem trophic interactions by using annual fish catch data and satellite-derived (SeaWiFS) chlorophyll a measurements for the continental margin of western North America. Findings reveal a marked alongshore variation in retained primary production that is highly correlated with the alongshore variation in resident fish yield. The highest productivity occurs off the coasts of Washington and southern British Columbia. Zooplankton data for coastal British Columbia confirm strong bottom-up trophic linkages between phytoplankton, zooplankton, and resident fish, extending to regional areas as small as 10,000 square kilometers. 相似文献
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Gene flow and the geographic structure of natural populations 总被引：76，自引：0，他引：76
M Slatkin 《Science (New York, N.Y.)》1987,236(4803):787-792
There is abundant geographic variation in both morphology and gene frequency in most species. The extent of geographic variation results from a balance of forces tending to produce local genetic differentiation and forces tending to produce genetic homogeneity. Mutation, genetic drift due to finite population size, and natural selection favoring adaptations to local environmental conditions will all lead to the genetic differentiation of local populations, and the movement of gametes, individuals, and even entire populations--collectively called gene flow--will oppose that differentiation. Gene flow may either constrain evolution by preventing adaptation to local conditions or promote evolution by spreading new genes and combinations of genes throughout a species' range. Several methods are available for estimating the amount of gene flow. Direct methods monitor ongoing gene flow, and indirect methods use spatial distributions of gene frequencies to infer past gene flow. Applications of these methods show that species differ widely in the gene flow that they experience. Of particular interest are those species for which direct methods indicate little current gene flow but indirect methods indicate much higher levels of gene flow in the recent past. Such species probably have undergone large-scale demographic changes relatively frequently. 相似文献
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Expansion and contraction of the sahara desert from 1980 to 1990 总被引：1，自引：0，他引：1
Data from polar-orbiting meteorological satellites have been used to determine the extent of the Sahara Desert and to document its interannual variation from 1980 to 1990. The Sahara Desert ranged from 8,633,000 square kilometers in 1980 to 9,982,000 square kilometers in 1984. The greatest annual north-south latitudinal movement of the southern Saharan boundary was 110 kilometers from 1984 to 1985 and resulted in a decrease in desert area of 724,000 square kilometers. 相似文献
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Reuter DC Simon-Miller AA Lunsford A Baines KH Cheng AF Jennings DE Olkin CB Spencer JR Stern SA Weaver HA Young LA 《Science (New York, N.Y.)》2007,318(5848):223-225
Several observations of Jupiter's atmosphere made by instruments on the New Horizons spacecraft have implications for the stability and dynamics of Jupiter's weather layer. Mesoscale waves, first seen by Voyager, have been observed at a spatial resolution of 11 to 45 kilometers. These waves have a 300-kilometer wavelength and phase velocities greater than the local zonal flow by 100 meters per second, much higher than predicted by models. Additionally, infrared spectral measurements over five successive Jupiter rotations at spatial resolutions of 200 to 140 kilometers have shown the development of transient ammonia ice clouds (lifetimes of 40 hours or less) in regions of strong atmospheric upwelling. Both of these phenomena serve as probes of atmospheric dynamics below the visible cloud tops. 相似文献
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土默特川旱涝基本规律的研究（摘要） 总被引：1，自引：0，他引：1
本文利用前人成果资料，结合地区旱涝历史资料，采用持续和间隔年法、百分数法及太阳黑子低值周期，得出土默特川旱涝基本规律，并用岱海涨落验证所得规律的正确性。 相似文献
11.
Bernstein RL 《Science (New York, N.Y.)》1974,183(4120):71-72
A set of subsurface temperature measurements in the trade wind region northeast of Hawaii reveals large perturbations about the mean state, with zonal wavelengths of 480 kilometers. The perturbations are identified as mesoscale baroclinic eddies, and they appear to drift westward at a rate of 4.7 +/- 2.0 centimeters per second. The large-scale ( 1000 kilometers) baroclinic flow at a depth of 200 meters is 1.5 +/- 0.7 centimeters per second, also toward the west, and comparable in magnitude with the eddy drift velocity; this finding suggests that the eddy drift is strongly influenced by the large-scale flow. Mesoscale eddies have been discovered in the tropical and subtropical Atlantic Ocean. Their existence in the Pacific Ocean is now confirmed. 相似文献
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Silliman BR van de Koppel J Bertness MD Stanton LE Mendelssohn IA 《Science (New York, N.Y.)》2005,310(5755):1803-1806
Salt marshes in the southeastern United States have recently experienced massive die-off, one of many examples of widespread degradation in marine and coastal ecosystems. Although intense drought is thought to be the primary cause of this die-off, we found snail grazing to be a major contributing factor. Survey of marsh die-off areas in three states revealed high-density fronts of snails on die-off edges at 11 of 12 sites. Exclusion experiments demonstrated that snails actively converted marshes to exposed mudflats. Salt addition and comparative field studies suggest that drought-induced stress and grazers acted synergistically and to varying degrees to cause initial plant death. After these disturbances, snail fronts formed on die-off edges and subsequently propagated through healthy marsh, leading to cascading vegetation loss. These results, combined with model analyses, reveal strong interactions between increasing climatic stress and grazer pressure, both potentially related to human environmental impacts, which amplify the likelihood and intensity of runaway collapse in these coastal systems. 相似文献
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Streamflow and water well responses to earthquakes 总被引：9，自引：0，他引：9
Earthquake-induced crustal deformation and ground shaking can alter stream flow and water levels in wells through consolidation of surficial deposits, fracturing of solid rocks, aquifer deformation, and the clearing of fracture-filling material. Although local conditions affect the type and amplitude of response, a compilation of reported observations of hydrological response to earthquakes indicates that the maximum distance to which changes in stream flow and water levels in wells have been reported is related to earthquake magnitude. Detectable streamflow changes occur in areas within tens to hundreds of kilometers of the epicenter, whereas changes in groundwater levels in wells can occur hundreds to thousands of kilometers from earthquake epicenters. 相似文献
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The time history of energy release during earthquakes illuminates the process of failure, which remains enigmatic for events deeper than about 100 kilometers. Stacks of teleseismic records from regional arrays for 122 intermediate (depths of 100 to 350 kilometers) and deep (depths of 350 to 700 kilometers) earthquakes show that the temporal pattern of short-period seismic radiation has a systematic variation with depth. On average, for intermediate depth events more radiation is released toward the beginning of the rupture than near the end, whereas for deep events radiation is released symmetrically over the duration of the event, with an abrupt beginning and end of rupture. These findings suggest a variation in the style of rupture related to decreasing fault heterogeneity with depth. 相似文献
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For more than 50 years, observations of earthquakes to depths of 100 to 650 kilometers inside Earth have been enigmatic: at these depths, rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces. Laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep-focus seismicity could originate from dehydration and high-pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle. Thus, seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet. 相似文献
16.
Hynds RJ Cowley SW Sanderson TR Wenzel KP VAN Rooijen JJ 《Science (New York, N.Y.)》1986,232(4748):361-365
During the encounter with comet Giacobini-Zinner, the energetic particle anisotropy spectrometer on the International Cometary Explorer spacecraft observed large fluxes of energetic ions, believed to result principally from ionization of the cometary atmosphere followed by pickup and acceleration by the ambient flow of the solar wind. These heavy cometary ions were observed from approximately 1 day before closest approach to about 2(1/2) days afterward. Three regimes of differing ion characteristics have been identified. An outer region with a scale of approximately 10(6) kilometers contains variable fluxes of antisolar-streaming pick-up ions in the undisturbed solar wind. In the middle region, of approximately 10(5) kilometers, fluxes have less large-scale variability and broader angular and energy distributions. This region is separated from the outer zone by a sharp transition. The inner region has a scale of approximately 10(4) kilometers and is characterized by reduced fluxes and complex angular distributions. 相似文献
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Cirtain JW Golub L Lundquist L van Ballegooijen A Savcheva A Shimojo M Deluca E Tsuneta S Sakao T Reeves K Weber M Kano R Narukage N Shibasaki K 《Science (New York, N.Y.)》2007,318(5856):1580-1582
Coronal magnetic fields are dynamic, and field lines may misalign, reassemble, and release energy by means of magnetic reconnection. Giant releases may generate solar flares and coronal mass ejections and, on a smaller scale, produce x-ray jets. Hinode observations of polar coronal holes reveal that x-ray jets have two distinct velocities: one near the Alfvén speed ( approximately 800 kilometers per second) and another near the sound speed (200 kilometers per second). Many more jets were seen than have been reported previously; we detected an average of 10 events per hour up to these speeds, whereas previous observations documented only a handful per day with lower average speeds of 200 kilometers per second. The x-ray jets are about 2 x 10(3) to 2 x 10(4) kilometers wide and 1 x 10(5) kilometers long and last from 100 to 2500 seconds. The large number of events, coupled with the high velocities of the apparent outflows, indicates that the jets may contribute to the high-speed solar wind. 相似文献
18.
应用6个微卫星分子标记位点评价了扎龙自然保护区包括野生、散养和笼养丹顶鹤3个小群体的遗传变异水平,以及野生与散养小群体间的基因流水平。结果表明,3个丹顶鹤小群体均表现出较高且相近的遗传变异水平,杂合度期望值(HE)为野生0.83300,散养0.80000,圈养0.81000,F-Statistics结果有97.5%以上的遗传多样性存在于群体内。同时还应用两种计算方法计算了野生与散养丹顶鹤小群体间基因流水平:应用稀有等位基因法计算两个小群体间的基因流值为每代1.9个个体;应用FST指数计算方法,并根据基因流值计算公式得出基因流值Nm为1个个体。这个基因流水平和机制能够维持扎龙自然保护区野生与散养丹顶鹤小群体间相近且较高的遗传变异水平。 相似文献
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Tomographic imaging indicates that slabs of subducted lithosphere can sink deep into Earth's lower mantle. The view that convective flow is stratified at 660-kilometer depth and preserves a relatively pristine lower mantle is therefore not tenable. However, a range of geophysical evidence indicates that compositionally distinct, hence convectively isolated, mantle domains may exist in the bottom 1000 kilometers of the mantle. Survival of these domains, which are perhaps related to local iron enrichment and silicate-to-oxide transformations, implies that mantle convection is more complex than envisaged by conventional end-member flow models. 相似文献
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Large-scale, electric currents flowing along magnetic field lines into the polar regions of Earth are thought to be the main contributors of the energy that powers the ionospheric aurora. However, we have found evidence for global contributions from electromagnetic waves (Alfvén waves). Data that were collected from the Polar spacecraft over the course of 1 year show that the flow of wave electromagnetic energy at altitudes of 25,000 to 38,000 kilometers delineates the statistical auroral oval. The Poynting flux of individual events distributed along the auroral oval was larger than 5 ergs per square centimeter per second, which is sufficient to power auroral acceleration processes. This evidence suggests that in addition to magnetic field-aligned currents, the dayside and nightside aurora is globally powered by the energy flow of these high-altitude Alfvén waves. 相似文献