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991.
992.
青藏高原及邻区大地构造单元初步划分 总被引:146,自引:59,他引:146
青藏高原及邻区大地构造单元的划分是当前该地区板块构造精细结构研究的关键,也是板内构造研究的基础理论问题。本文在前人工作的基础上,结合青藏高原及邻区新一轮1∶25万区域地质调查成果,基于多岛弧盆系的形成是大洋向大陆岩石圈构造体制演化转换的标志这一认识,将弧-弧、弧-陆及陆-陆碰撞结合带和夹持于其间的陆块、岩浆弧划分为一级单元,构成青藏高原及邻区构造单元的基本骨架;以南昆仑俯冲碰撞结合带和班公湖-丁青-碧土-昌宁-孟连结合带为界,划分出:泛华夏大陆早古生带秦祁昆构造区、泛华夏大陆晚古生代—三叠纪羌塘-三江构造区及冈瓦纳北缘晚古生代—中生代冈瓦纳-喜马拉雅构造区。这样的厘定不仅对青藏高原及邻区的地质科学研究具有重要的理论意义,而且对青藏高原空白区区域地质调查和成矿地质条件分析等也有重要的实际意义。 相似文献
993.
994.
To investigate the influence of microlites on lava flow rheology, the viscosity of natural microlite-bearing rhyolitic obsidians
of calc-alkaline and peralkaline compositions containing 0.1–0.4 wt.% water was measured at volcanologically relevant temperatures
(650–950 °C), stresses (103–105 Pa) and strain rates (10–5 to 10–7 s–1). The glass transition temperatures (T
g
) were determined from scanning calorimetric measurements on the melts for a range of cooling/heating rates. Based on the
equivalence of enthalpic (calorimetric) and shear (viscosity) relaxation, we calculated the viscosity of the melt in crystal-bearing
samples from the T
g
data. The difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal-bearing
samples is interpreted to be the physical effect of microlites on the measured viscosity. The effect of <5 vol.% rod-like
microlites on the melt rheology is negligible. Microlite-rich and microlite-poor samples from the same lava flow and with
identical bulk chemistry show a difference of 0.6 log10 units viscosity (Pa s), interpreted to be due to differences in melt chemistry caused by the presence of microlites. The
only major differences between measured and calculated viscosities were for two samples: a calc-alkaline rhyolite with 1 vol.%
branching crystals, and a peralkaline rhyolite containing crystal-rich bands with >45 vol.% crystals. For both of these samples
a connectivity factor is apparent, with, for the latter, a close packing framework of crystals which is interpreted to influence
the apparent viscosity.
Received: 14 March 1996 / Accepted: 30 May 1996 相似文献
995.
Flume experiments were conducted in order to monitor changes in flow turbulence intensity and suspended sediment concentration at seven stages across the ripple–dune transition and at three different positions above the bed surface. Three‐dimensional velocity measurements were obtained using an acoustic Doppler velocimeter (ADV). Suspended sediment concentration (SSC) was monitored indirectly using ADV signal amplitude. Although limited to time‐averaged parameters, the analysis reveals that SSC varies significantly with stage across the transition and with sampling height. The statistical analysis also reveals an apparent uniformity of suspended sediment concentration with height above the bed in the lower half of the flow depth at the critical stage in the transition from ripples to dunes. This is also the stage at which turbulence intensity is maximized. Statistically significant correlations were also observed between suspended sediment concentrations and root‐mean‐square values of vertical velocity fluctuations. These correlations reflect the various levels of shear‐layer activity and the distinct turbulent flow regions across the transition. Conversely, time‐averaged values of Reynolds shear stress exhibit a very weak relationship with suspended sediment concentrations. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
996.
997.
Mohammad M. Sohrabi Daniele Tonina Rohan Benjankar Mukesh Kumar Patrick Kormos Danny Marks 《水文研究》2018,32(19):2976-2989
Accurate snow accumulation and melt simulations are crucial for understanding and predicting hydrological dynamics in mountainous settings. As snow models require temporally varying meteorological inputs, time resolution of these inputs is likely to play an important role on the model accuracy. Because meteorological data at a fine temporal resolution (~1 hr) are generally not available in many snow‐dominated settings, it is important to evaluate the role of meteorological inputs temporal resolution on the performance of process‐based snow models. The objective of this work is to assess the loss in model accuracy with temporal resolution of meteorological inputs, for a range of climatic conditions and topographic elevations. To this end, a process‐based snow model was run using 1‐, 3‐, and 6‐hourly inputs for wet, average, and dry years over Boise River Basin (6,963 km2), which spans rain dominated (≤1,400 m), rain–snow transition (>1,400 and ≤1,900 m), snow dominated below tree line (>1,900 and ≤2,400 m), and above tree line (>2,400 m) elevations. The results show that sensitivity of the model accuracy to the inputs time step generally decreases with increasing elevation from rain dominated to snow dominated above tree line. Using longer than hourly inputs causes substantial underestimation of snow cover area (SCA) and snow water equivalent (SWE) in rain‐dominated and rain–snow transition elevations, due to the precipitation phase mischaracterization. In snow‐dominated elevations, the melt rate is underestimated due to errors in estimation of net snow cover energy input. In addition, the errors in SCA and SWE estimates generally decrease toward years with low snow mass, that is, dry years. The results indicate significant increases in errors in estimates of SCA and SWE as the temporal resolution of meteorological inputs becomes coarser than an hour. However, use of 3‐hourly inputs can provide accurate estimates at snow‐dominated elevations. The study underscores the need to record meteorological variables at an hourly time step for accurate process‐based snow modelling. 相似文献
998.
Li Feng Wei-Qun Gan Purple Mountain Observatory Chinese Academy of Sciences Nanjing Max Planck Institute for Solar System Research Max-Planck-Str. Katlenburg-Lindau Germany 《中国天文和天体物理学报》2006,6(5)
It was suggested by Parker that the solar corona is heated by many small energy release events generally called microflares or nanoflares. More and more observations showed flows and intensity variations in nonflaring loops. Both theories and observations have indicated that the heating of coronal loops should actually be unsteady. Using SOLFTM (Solar Flux Tube Model), we investigate the hydrodynamics of coronal loops undergoing different manners of impulsive heating with the same total energy deposition. The half length of the loops is 110 Mm, a typical length of active region loops. We divide the loops into two categories: loops that experience catastrophic cooling and loops that do not. It is found that when the nanoflare heating sources are in the coronal part, the loops are in non-catastrophic-cooling state and their evolutions are similar. When the heating is localized below the transition region, the loops evolve in quite different ways. It is shown that with increasing number of heating pulses and inter-pulse time, the catastrophic cooling is weakened, delayed, or even disappears altogether. 相似文献
999.
1000.
We analyze the spatial distribution of the intensity of radio emission from a cool filament in terms of the generalized Kippenhahn-Schluter model. Based on a numerical calculation of the centimeter radio emission and using the temperature transition layer model by Anzer and Heinzel (1999), we show that two symmetric brightening bands must be observed near the filament. The absence of any bands during observations with a sufficient angular resolution suggests that a different type of model is realized: a model with a narrow (unobservable) temperature transition layer across a magnetic field, in particular, a Kuperus-Raadu-type model. 相似文献