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Ilya N. Bindeman Axel K. Schmitt John W. Valley 《Contributions to Mineralogy and Petrology》2006,152(6):649-665
Large volumes of silicic magma were produced on a very short timescale in the nested caldera complex of the SW Nevada volcanic field (SWNVF). Voluminous ash flows erupted in two paired events: Topopah Spring (TS, >1,200 km3, 12.8 Ma)–Tiva Canyon (TC, 1,000 km3, 12.7 Ma) and Rainier Mesa (RM, 1,200 km3, 11.6 Ma)–Ammonia Tanks (AT, 900 km3, 11.45 Ma; all cited ages are previously published 40Ar/39Ar sanidine ages). Within each pair, eruptions are separated by only 0.1–0.15 My and produced tuffs with contrasting isotopic values. These events represent nearly complete evacuation of sheet-like magma chambers formed in the extensional Basin and Range environment. We present ion microprobe ages from zircons in the zoned ash-flow sheets of TS, TC, RM, and AT in conjunction with δ18O values of zircons and other phenocrysts, which differ dramatically among subsequently erupted units. Bulk zircons in the low-δ18O AT cycle were earlier determined to exhibit ∼1.5‰ core-to-rim oxygen isotope zoning; and high-spatial resolution zircon analyses by ion microprobe reveal the presence of older grains that are zoned by 0.5–2.5‰. The following U–Pb isochron ages were calculated after correcting for the initial U–Pb disequilibria: AT (zircon rims: 11.7 ± 0.2 Ma; cores: 12.0 ± 0.1 Ma); pre-AT rhyolite lava: (12.0 ± 0.3 Ma); RM: 12.4 ± 0.3); TC: (13.2 ± 0.15 Ma); TS: (13.5 ± 0.2). Average zircon crystallization ages calculated from weighted regression or cumulative averaging are older than the Ar–Ar stratigraphy, but preserve the comparably short time gaps within each of two major eruption cycles (TS/TC, RM/AT). Notably, every sample yields average zircon ages that are 0.70–0.35 Ma older than the respective Ar–Ar eruption ages. The Th/U ratio of SWNVF zircons are 0.4–4.7, higher than typically found in igneous zircons, which correlates with elevated Th/U of the whole rocks (5–16). High Th/U could be explained if uranium was preferentially removed by hydrothermal solutions or is retained in the protolith during partial melting. For low-δ18O AT-cycle magmas, rim ages from unpolished zircons overlap within analytical uncertainties with the 40Ar/39Ar eruption age compared to core ages that are on average ∼0.2–0.3 My older than even the age of the preceding caldera forming eruption of RM tuff. This age difference, the core-to-rim oxygen isotope zoning in AT zircons, and disequilibrium quartz–zircon and melt-zircon isotopic fractionations suggest that AT magma recycled older zircons derived from the RM and older eruptive cycles. These results suggest that the low-δ18O AT magmas were generated by melting a hydrothermally-altered protolith from the same nested complex that erupted high-δ18O magmas of the RM cycle only 0.15 My prior to the eruption of the AT, the largest volume low-δ18O magma presently known.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. 相似文献
104.
Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2-2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1-100 m thick units intercalated with shallow-water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K2O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (LaN/YbN = 2.84-4.45) and no significant Eu anomaly (Eu/Eu* = 0.93-0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (LaN/YbN = 0.69-2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering. 相似文献
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106.
中国西南山区蒸散发能力的季节和空间变化 总被引:3,自引:0,他引:3
AxelThomas 《山地学报》2002,20(4):385-393
从全球范围来看,中国西南山区位地热带与亚热带,印度洋季风与中国季风的过渡带,属于季风气候。5-9月的降水占全年平均降水量的70%,整个山区随海拔高度的变化形成了很多气候带,与中国东部相比,中国西南山区旱季(冬季)温暖,光照充足,天空晴郎。根据中国西南山区36个(主要是云南的)气象站观测资料,利用彭曼修正式估算了中国西南山蒸散发能力(PET)。观测资料统计显示这些气象站位于同一气候区的不同类型区域,本文中估算了不同类型区域的月平均PET的递减率。各种气候类型区域的季节变化反映了季风影响的程度,PET的年内变化可划分为季风前期,季风期和季风后期。PET的区域月递减率在1mm/10mm0--5mm/100mm间变动,较低的云雾或云层等局部气候将太阳辐射控制在低水平,影响PET的递减率的正,负变化,由太阳辐射和风速主导大部分区域PET的最大值出现在季风来临前的2-5月份。在6-9月的季风期,中国西南山区受西南季风控制,整个区域气候变化比较一致,PET的值较小,其空间变化也较小。 相似文献
107.
Axel Bronstert 《水文研究》1999,13(1):21-48
Hillslope hydrological modelling is considered to be of great importance for the understanding and quantification of hydrological processes in hilly or mountainous landscapes. In recent years a few comprehensive hydrological models have been developed at the hillslope scale which have resulted in an advanced representation of hillslope hydrological processes (including their interactions), and in some operational applications, such as in runoff and erosion studies at the field scale or lateral flow simulation in environmental and geotechnical engineering. An overview of the objectives of hillslope hydrological modelling is given, followed by a brief introduction of an exemplary comprehensive hillslope model, which stimulates a series of hydrological processes such as interception, evapotranspiration, infiltration into the soil matrix and into macropores, lateral and vertical subsurface soil water flow both in the matrix and preferential flow paths, surface runoff and channel discharge. Several examples of this model are presented and discussed in order to determine the model's capabilities and limitations. Finally, conclusions about the limitations of detailed hillslope modelling are drawn and an outlook on the future prospects of hydrological models on the hillslope scale is given.The model presented performed reasonable calculations of Hortonian surface runoff and subsequent erosion processes, given detailed information of initial soil water content and soil hydraulic conditions. The vertical and lateral soil moisture dynamics were also represented quite well. However, the given examples of model applications show that quite detailed climatic and soil data are required to obtain satisfactory results. The limitations of detailed hillslope hydrological modelling arise from different points: difficulties in the representations of certain processes (e.g. surface crusting, unsaturated–saturated soil moisture flow, macropore flow), problems of small‐scale variability, a general scarcity of detailed soil data, incomplete process parametrization and problems with the interdependent linkage of several hillslopes and channel–hillslope interactions. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
108.
This paper investigates the transferability of calibrated HBV model parameters under stable and contrasting conditions in terms of flood seasonality and flood generating processes (FGP) in five Norwegian catchments with mixed snowmelt/rainfall regimes. We apply a series of generalized (differential) split-sample tests using a 6-year moving window over (i) the entire runoff observation periods, and (ii) two subsets of runoff observations distinguished by the seasonal occurrence of annual maximum floods during either spring or autumn. The results indicate a general model performance loss due to the transfer of calibrated parameters to independent validation periods of ?5 to ?17%, on average. However, there is no indication that contrasting flood seasonality exacerbates performance losses, which contradicts the assumption that optimized parameter sets for snowmelt-dominated floods (during spring) perform particularly poorly on validation periods with rainfall-dominated floods (during autumn) and vice versa. 相似文献
109.
Climate change effects on hydrological system conditions influencing generation of storm runoff in small Alpine catchments 总被引:1,自引:0,他引:1 下载免费PDF全文
Gertraud Meißl Herbert Formayer Klaus Klebinder Florian Kerl Friedrich Schöberl Clemens Geitner Gerhard Markart David Leidinger Axel Bronstert 《水文研究》2017,31(6):1314-1330
Floods and debris flows in small Alpine torrent catchments (<10 km2) arise from a combination of critical antecedent system state conditions and mostly convective precipitation events with high precipitation intensities. Thus, climate change may influence the magnitude–frequency relationship of extreme events twofold: by a modification of the occurrence probabilities of critical hydrological system conditions and by a change of event precipitation characteristics. Three small Alpine catchments in different altitudes in Western Austria (Ruggbach, Brixenbach and Längentalbach catchment) were investigated by both field experiments and process‐based simulation. Rainfall–runoff model (HQsim) runs driven by localized climate scenarios (CNRM‐RM4.5/ARPEGE, MPI‐REMO/ECHAM5 and ICTP‐RegCM3/ECHAM5) were used in order to estimate future frequencies of stormflow triggering system state conditions. According to the differing altitudes of the study catchments, two effects of climate change on the hydrological systems can be observed. On one hand, the seasonal system state conditions of medium altitude catchments are most strongly affected by air temperature‐controlled processes such as the development of the winter snow cover as well as evapotranspiration. On the other hand, the unglaciated high‐altitude catchment is less sensitive to climate change‐induced shifts regarding days with critical antecedent soil moisture and desiccated litter layer due to its elevation‐related small proportion of sensitive areas. For the period 2071–2100, the number of days with critical antecedent soil moisture content will be significantly reduced to about 60% or even less in summer in all catchments. In contrast, the number of days with dried‐out litter layers causing hydrophobic effects will increase by up to 8%–11% of the days in the two lower altitude catchments. The intensity analyses of heavy precipitation events indicate a clear increase in rain intensities of up to 10%. 相似文献
110.
Quality of reprocessed GPS satellite orbits 总被引:4,自引:2,他引:2
Peter Steigenberger Markus Rothacher Mathias Fritsche Axel Rülke Reinhard Dietrich 《Journal of Geodesy》2009,83(3-4):241-248
High-precision Global Positioning System (GPS) satellite orbits are one of the core products of the International GNSS Service (IGS). Since the establishment of the IGS in 1994, the quality and consistency of the IGS orbits has steadily been improved by advances in the modeling of GPS observations. However, due to these model improvements and reference frame changes, the time series of operational orbits are inhomogeneous and inconsistent. This problem can only be overcome by a complete reprocessing starting with the raw observation data. The quality of reprocessed GPS satellite orbits for the time period 1994–2005 will be assessed in this paper. Orbit fits show that the internal consistency of the orbits could be improved by a factor of about two in the early years. Comparisons with the operational IGS orbits show clear discontinuities whenever the reference frame was changed by the IGS. The independent validation with Satellite Laser Ranging (SLR) residuals shows an improvement of up to 30% whereas a systematic bias of 5 cm still persists. 相似文献