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Summary. The existence of an anomalously low-velocity, low-density zone within the upper mantle beneath the Kenya Dome has been deduced on the basis of previous gravity and seismic studies. This paper describes an experiment to measure teleseismic delay times across the Gregory Rift near the equator and along a SE radius of the Kenya Dome. The delay times have been determined with good relative accuracy and provide further independent evidence for the existence of the anomalous zone. The pattern of delay times along the two profiles and at other stations indicates that the zone thins rapidly to the SE away from the rift axis, mirroring the attenuation observed, from Kaptagat, for the same zone to the NW. The trend is for the thinning to become very much less rapid with distance, but there is also clear evidence for localized thickening of the zone under the Kilimanjaro–Chyulu volcanic area.
Significantly smaller delay times are measured at the centre of the rift than at the edges. This is shown to indicate that the anomalous zone penetrates the crust to form an intrusion of relatively high-velocity material along the rift axis. The clear correlation of the delay time low with the axial Bouguer high indicates that they are both manifestations of the same underlying structure. Thus the delay time results provide independent confirmation of the existence of the axial intrusion previously inferred from gravity data. The width of this intrusion at the normal base of the crust is well defined by the data as 30 km. 相似文献
Significantly smaller delay times are measured at the centre of the rift than at the edges. This is shown to indicate that the anomalous zone penetrates the crust to form an intrusion of relatively high-velocity material along the rift axis. The clear correlation of the delay time low with the axial Bouguer high indicates that they are both manifestations of the same underlying structure. Thus the delay time results provide independent confirmation of the existence of the axial intrusion previously inferred from gravity data. The width of this intrusion at the normal base of the crust is well defined by the data as 30 km. 相似文献
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Ultramafic and mafic granulites from Archaean gneisses in N.W. Scotland (the Scourian) show evidence of two periods of granulite facies mineral growth. The first produced a high pressure clinopyroxene +garnet±plagioclase assemblage at an estimatedP-T of 12–15 kb and 1,000° C. Uplift of the complex caused partial breakdown of the garnet by reaction with clinopyroxene to produce orthopyroxene +plagioclase ±spinel±amphibole symplectites, at an estimatedP-T of 10–14 kb and 800°–900° C. Garnet stability is shown to depend on both whole-rock Fe/Mg ratios and onP-T conditions. The pressures imply crustal thicknesses in the Archaean of least 35–45 km. 相似文献
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Artificial tarballs were made in the laboratory from four crude petroleums. Relative to the parent petroleum, a decrease in aromatic hydrocarbon content for all samples was observed, as was a decrease in saturate content for three of four samples and an increase in compounds containing nitrogen, sulphur and oxygen for three samples. The asphaltene content also increased for three samples. The saturate, aromatic and asphaltic content of the artificial tarballs was compared to values for natural tarballs reported in the literature. 相似文献
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Modeling regional initiation of rainfall-induced shallow landslides in the eastern Umbria Region of central Italy 总被引：3，自引：6，他引：3
Diana Salciarini Jonathan W. Godt William Z. Savage Pietro Conversini Rex L. Baum John A. Michael 《Landslides》2006,3(3):181-194
We model the rainfall-induced initiation of shallow landslides over a broad region using a deterministic approach, the Transient Rainfall Infiltration and Grid-based Slope-stability (TRIGRS) model that couples an infinite-slope stability analysis with a one-dimensional analytical solution for transient pore pressure response to rainfall infiltration. This model permits the evaluation of regional shallow landslide susceptibility in a Geographic Information System framework, and we use it to analyze susceptibility to shallow landslides in an area in the eastern Umbria Region of central Italy. As shown on a landslide inventory map produced by the Italian National Research Council, the area has been affected in the past by shallow landslides, many of which have transformed into debris flows. Input data for the TRIGRS model include time-varying rainfall, topographic slope, colluvial thickness, initial water table depth, and material strength and hydraulic properties. Because of a paucity of input data, we focus on parametric analyses to calibrate and test the model and show the effect of variation in material properties and initial water table conditions on the distribution of simulated instability in the study area in response to realistic rainfall. Comparing the results with the shallow landslide inventory map, we find more than 80% agreement between predicted shallow landslide susceptibility and the inventory, despite the paucity of input data. 相似文献
30.
Far-UV echelle spectroscopy of the radio-quiet QSO H1821+643 (zem=0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) at approximately 7 km s-1 resolution, reveals four definite O vi absorption-line systems and one probable O vi absorber at 0.15相似文献