共查询到20条相似文献，搜索用时 31 毫秒
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D.E. Fresnillo Fedorenko O.A. Fernández C.A. Busso O.E. Elia 《Journal of Arid Environments》1996,33(4):409-416
The phenology ofMedicago minimavar.minimaandErodium cicutariumwas studied at two different field sites in the Calden District (approx. 10 million ha), a temperate semi-arid phytogeographical region in central Argentina. One site had been protected from wild and domestic herbivores for the 6 previous years. The other adjacent site had been exposed to continuous grazing by cattle for several decades. A phenological key was made for each species in 1988, and this key was improved during the 1989 growing season when phenological observations were made every 1–3 weeks.The growth cycle ofM. minimaandE. cicutariumwas similar at the two studied sites. Initiation of the cycle occurred in autumn and appeared to be associated with soil water availability. Phenological patterns were very dynamic at the reproductive stage in both species. This stage started earlier and was faster inE. cicutariumthan inM. minima. The end of the growing season occurred during late spring, concomitant with high maximum air temperatures. Our results suggest that these species hasten their development when air temperature increases and soil water availability decreases. This could be an important strategy in these species which allows them to persist as seeds, and produce a new generation under favourable environmental conditions. 相似文献
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Summary. Twenty-two plateau ages obtained by the 40 Ar/39 Ar step-heating method span most of the time of formation of eastern Iceland. With one exception the dates are consistent with the stratigraphy, the exception having a very unusual argon release pattern. It is concluded that the 40 Ar/39 Ar step-heating method can give correct dates on hydrothermally altered samples the total fusion ages of which are not correct; however, it is not possible to conclude from the results obtained that the method is inherently more accurate than the conventional method using carefully selected samples.
Though it is known from geological evidence that minor irregularities of spreading have occurred the results show that, within the resolution of the dates, spreading has been steady and symmetrical; in particular, neither a large eastward jump nor repetitive westward jumps can have occurred. The average half spreading rate for the period 2Myr to 12 Myr ago was 0.78 ± 0.16 cm yr-1 . 相似文献
Though it is known from geological evidence that minor irregularities of spreading have occurred the results show that, within the resolution of the dates, spreading has been steady and symmetrical; in particular, neither a large eastward jump nor repetitive westward jumps can have occurred. The average half spreading rate for the period 2Myr to 12 Myr ago was 0.78 ± 0.16 cm yr-
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Mark A. Riedesel Duncan Agnew Jon Berger Freeman Gilbert 《Geophysical Journal International》1980,62(2):457-471
Summary. Using nine IDA records for the Indonesian earthquake of 1977 August 19, we have formed an optimal linear combination of the records and have measured the frequency and Q of 0 S 0 and 1 S 0 . The frequency was measured using the moment ratio method. The attenuation was measured by the minimum width method and by the time-lapse method. The frequency and attenuation were measured simultaneously by varying them to obtain a best fit to the data. A 2000-hr stack, the sum of nine individual records, for 0 S 0 gave a frequency of 0.814664 mHz±4 ppm. The values for the Q of 0 S 0 for the three different methods of measurement were 5600,5833 and 5700, respectively. The error in the estimates of Q -1 is about 5 per cent for the minimum power method. For 1 S 0 a 300-hr stack yielded a frequency of 1.63151 mHz±30 ppm. The values of Q for this mode were 1960, 1800 and 1850, respectively, with an error in Q -1 of about 12 per cent for the minimum power method. 相似文献
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G. F. Gentile G. Bressan L. Burlini & R. De Franco 《Geophysical Journal International》2000,141(2):457-478
3-D images of P velocity and P - to S -velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V P and V P / V S 1-D models were computed before the 3-D inversion. V P was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic V P model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep V P and V P / V S anomalies that are associated with the complex geological structure. High V P / V S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the V P / V S anomalies. The V P images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong V P / V S heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area. 相似文献
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Jean Goslin Jean-Louis Thirot Olivier Noël & Jean Francheteau 《Geophysical Journal International》1998,135(2):700-710
Among the mantle hotspots present under oceanic areas, a large number are located on—or close to—active oceanic ridges. This is especially true in the slow-spreading Atlantic and Indian oceans. The recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models, and allow one to study interactions between mantle plumes and active slow-spreading ridges. The observed correlations allow us to draw preliminary conclusions on the general links between surficial processes, which shape the detailed morphology of the ridge axes, and deeper processes, active in the upper mantle below the ridge axial domains as a whole. The interactions are first studied at the scale of the Atlantic (the Mid-Atlantic Ridge from Iceland to Bouvet Island) from the correlation between the zero-age free-air gravity anomaly, which reflects the zero-age depth of the ridge axis, and Sr isotopic ratios of ridge axis basalts. The study is then extended to a more global scale (the slow ridges from Iceland to the Gulf of Aden) by including geoid and upper-mantle tomography models. The interactions appear complex, ranging from the effect of large and very productive plumes, almost totally overprinting the long-wavelength segmentation pattern of the ridge, to that of weaker hotspots, barely marking some of the observables in the ridge axial domain. Intermediate cases are observed, in which hotspots of medium activity (or whose activity has gradually decreased) located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle. Such observations tend to preclude the use of a single hotspot/ridge interaction model and stress the need for additional observations in various plume/ridge configurations. 相似文献
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Neobuxbaumia tetetzo 《Journal of Arid Environments》1996,32(4):395-405
A supposed hybrid population betweenNeobuxbaumia tetetzoandCephalocereus columna-trajani(Cactaceae) in a semi-arid ecosystem at Zapotitlán de La Salinas, in the Mexican State of Puebla, was investigated by analysing number of ribs, number of areoles per cm and number of branches. Tests for equality of means using the Games and Howell method and Stepwise Discriminant Analysis, indicated that the number of ribs and number of areoles per cm supported our hypothesis for the existence of these naturally occurring hybrids, which are intermediate and distinct from the parental species. Other qualitative characteristics which distinguished hybrids and parental species are highlighted, and the possible meaning of the phenomenon is discussed. 相似文献
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Trond H. Torsvik Elizabeth A. Eide Joe G. Meert Mark A. Smethurst & Harald J. Walderhaug 《Geophysical Journal International》1998,135(3):1045-1059
The Bøverbru and Lunner dyke ages are statistically concordant. However, the palaeomagnetic signature of the Bøverbru dyke is complex, and directions from the margins and the interior of the dyke differ in polarity. Therefore, the new Early Triassic palaeomagnetic pole for Baltica (Eurasia) is exclusively based on the less complex Lunner dykes and contacts (palaeomagnetic pole: latitude=52.9°N, longitude=164.4°E, dp / dm =4.5 ° /7.3°). The early Triassic palaeomagnetic pole [mean age: 243±5 Ma (2 σ )] is slightly different from the Upper Carboniferous–Permian (294–274 Ma) and Kiaman-aged poles from the Oslo Rift. 相似文献
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Ants of the speciesMessor arenariusandM. ebeninusare known as common granivores in the deserts of the Middle East and North Africa. The relationship between the spatial distribution of nests of these ant species and the distribution of vegetation was examined in the Negev highlands. Using the method of nearest neighbour analysis, it was found that nests ofM. arenariusare evenly dispersed while the dispersion of nests ofM. ebeninusdoes not differ from a random distribution. It is suggested that in desert surroundings there is a connection between the distribution pattern of nests of harvester ants and their foraging type. It seems that nests of individually foraging ants are usually evenly dispersed while nests of trunk-trail foraging ants are usually distributed in a random or aggregated dispersion. 相似文献