A first palaeomagnetic study of Jurassic formations from the Qaidam basin, Northeastern Tibet, China—tectonic implications     

N. Halim  Y. Chen  J. P. Cogné 《Geophysical Journal International》2003,153(1):20-26

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Geomorphic Effects of Monsoon Floods on Indian Rivers   总被引：1，自引：0，他引：1  

Kale  Vishwas S. 《Natural Hazards》2003,28(1):65-84

The southwest summer monsoon contributesthe bulk of India's rainfall. Consequently,almost all the geomorphic work by the rivers is carried out during the monsoonseason in general and the monsoon floods in particular. Indian rivers arecharacterized by high average flood discharges and large temporal variability. Thereis also significant spatial variation in the magnitude, frequency and power of floods, on account of regional variations in monsoon rainfall, basin characteristics andchannel geometry. As a result, the channel responses and the geomorphic effects also varyspatially. This paper describes the hydrological and geomorphological aspects, as well asthe geomorphic effects of monsoon floods in the Indian rivers. The geomorphic effects of floods are most impressive only in certainareas – the Himalaya, the Thar Desert, and the Indus-Ganga-Brahmaputra Plains. There are numerous instances of flood-induced changes in the channel dimension,position and pattern in these areas. In the Ganga-Brahmaputra Plains, the annualfloods appear to be geomorphologically more effective than the occasional large floods.In comparison, the rivers of the Indian Peninsula are, by and large, stable and thegeomorphic effects of floods are modest. Only large-magnitude floods that occur at aninterval of several years to decades are competent to modify the channel morphology in asignificant way. A synthesis of the various case studies available from the Indianregion indicates that often the absolute magnitude of a flood is not as important withrespect to the geomorphic effects as the flow stress and competence.  相似文献   

Perspectives of Natural Disasters in East and South Asia,and the Pacific Island States: Socio-economic Correlates and Needs Assessment   总被引：2，自引：1，他引：2  

Haque  C. Emdad 《Natural Hazards》2003,29(3):465-483

The regions of East and South Asia, and the Pacific Islands are among the most-hazardprone areas in the world. Because of this, during the last century, most of the humancasualties of `natural-triggered' disasters have taken place in this region. This circumstance therefore has become a major global humanitarian concern. Another major concern, specifically for the donor agencies, is the damage sustained by infrastructure resulting from environmental disasters. These recurrent loses take away a significant proportion of the cumulative economic gains accrued from development investments over many years.Stepwise multiple regression results substantiated the fact that many of the socio-economic and demographic variables significantly influence disaster-related deaths and injuries in this part of the world. A comparative temporal analysis has shown that, over the past two decades, demographic variables have become prominent predictors of disaster-loss in South, Southeast and East Asian and the Pacific states.Many countries of the region are lagging behind in understanding and recognizing thebroader scope of disaster mitigation and management. Emerging needs and awarenessamong the decision-makers and the general public, however, have prompted institutionsin many countries to initiate a critical review of the prevailing approaches. Thecountry-specific disaster-management capacities and needs in the region vary widely.There are many differences in historical courses, institutional and administrative settings, sociocultural characteristics, as well as political and economic systems. Development of a common institutional framework for the region, therefore, seems unfeasible. Based upon a regional review, it has become clear that the research calls for improving the understanding of the significance of disaster mitigation and management in light of sustainable development and the emerging global issues. In addition, aspects of human resource development to enhance institutional mitigation and response capacities are emphasized.  相似文献   

Microbial dolomite crusts from the carbonate platform off western India   总被引：1，自引：1，他引：1  

V. Purnachandra Rao  Pratima M. Kessarkar  W. E. Krumbein†  K. P. Krajewski‡  Robert J. Schneider§ 《Sedimentology》2003,50(5):819-830

Abstract The occurrence of Late Pleistocene dolomite crusts that occur at 64 m depth on the carbonate platform off western India is documented. Dolomite is the most predominant mineral in the crusts. In thin section, the crust consists of dolomitized microlaminae interspersed with detrital particles. Under scanning electron microscopy, these laminae are made up of tubular filaments or cellular structures of probable cyanobacterial origin. Dolomite crystals encrust or overgrow the surfaces of the microbial filaments and/or cells; progressive mineralization obliterates their morphology. Well-preserved microbial mats, sulphide minerals (pyrrhotite and marcasite) and the stable isotope composition of dolomite in the crusts indicate hypersaline and anoxic conditions during dolomite formation. The crusts are similar to dolomite stromatolites, and biogeochemical processes related to decaying microbial mats under anoxic conditions probably played an important role in dolomite precipitation. The dolomite is therefore primary and/or very early diagenetic in origin. The dolomite crusts are interpreted to be a composite of microbial dolomite overprinted by early burial organic dolomite. The results of this study suggest that a microbial model for dolomite formation may be relevant for the origin of ancient massive dolomites in marine successions characterized by cryptalgal laminites. The age of the crusts further suggests that the platform was situated at shallow subtidal depths during the Last Glacial Maximum.  相似文献   

Magnetic properties of sediments in the Bay of Bengal and the Andaman Sea: impact of rapid North Atlantic Ocean climatic events on the strength of the Indian monsoon   总被引：3，自引：0，他引：3  

C Colin  C Kissel  D Blamart  L Turpin 《Earth and Planetary Science Letters》1998,160(3-4):623-635

The results of a high-resolution mineral magnetic study combined with major element geochemistry analysis, oxygen isotopes and ¹⁴C AMS stratigraphy are reported for deep-sea gravity cores MD77-169 and MD77-180 located in the Andaman Sea and the Bay of Bengal, respectively. Core MD77-169 covers the last 280 kyr and core MD77-180 covers the last 160 kyr. In both cores, rock magnetic parameters indicate that the magnetic assemblage is dominated by pseudo-single domain titanomagnetite grains, with grain-size variations following a strong 23 kyr periodicity. Smaller magnetic grain sizes are observed during periods characterized by a strong summer monsoon. In addition, in core MD77-180, we observe a correlation between magnetic grain size and a chemical index of alteration. This suggests that these magnetic grain-size changes are related to chemical weathering driven by summer monsoon rainfall. A comparison of the GISP2 ice core isotopic record and the magnetic grain-size record of the Bay of Bengal shows that rapid temperature variations documented in the ice core (Dansgaard–Oeschger cycles and Heinrich events), during the last glacial period are also present in the magnetic grain-size record. Heinrich events and cold stadial events are characterized by relatively large magnetic grain sizes. Furthermore, Heinrich events are characterized by lower values of the chemical index of alteration implying a lower degree of chemical weathering related to significantly drier conditions on the continent. We suggest that rapid cold events of the North Atlantic (Heinrich events) during the last glacial stages are characterized by a weaker summer monsoon rainfall over the Himalaya via an atmospheric teleconnection.  相似文献   

印度人口地理特征及其对经济的影响     

张庆辉 《世界地理研究》2004,13(3):103-107,65

印度是世界上仅次于中国的第二人口大国，同时也是个发展中大国，经济并不富裕。近几十年来印度人口的过度增长，对其国家现今的政治稳定和经济发展带来了巨大沉重的压力，并影响着印度综合国力和人民生活水平的提高。本文在阐述印度人口结构、区域分布等地理特征的基础上，分析了印度人口特征对经济发展的影响，指出印度解决人口问题的当务之急是控制人口和大力发展经济。  相似文献   

Early Proterozoic 87Rb-86Sr Model Ages of Pegmatitic Muscovite from Rare Metal-bearing Granite-Pegamtite System of Kawadgaon, Bastar Craton, Central India     

Yamuna Singh  T. Chabria 《Gondwana Research》2002,5(4):889-893

Seven muscovite mica separates from the rare metal pegmatites of Kawadgaon, Bastar Craton, Central India, give model 87Rb-86Sr ages ranging from 2330 to 1850 Ma. The oldest age of the muscovite almost overlaps within 20 error with the age (2497k152 Ma) of the parent fertile granites. The data suggest possible derivation of pegmatites shortly after the emplacement of Kawadgaon granites at ca. 2500 Ma. Most of the muscovite ages (n = 6) indicate tectonomagmatic ages after pegmatite injections. The 87Sr/86Sr ratio (0.7142) of granites suggests their derivation from crustal material.  相似文献   

Deccan traps mantle degassing in the terminal Cretaceous marine extinctions     

Dewey M. McLean 《Cretaceous Research》1985,6(3):235-259

Mantle degassing continually releases gases onto the earth's surface. Over geologically long time intervals, a general equilibrium probably exists between mantle CO₂ release and uptake by surficial sinks. However, during periods of rapid plate movement, or continental flood basalt volcanism, the increased rate of mantle CO₂ release may exceed that of uptake, leading to CO₂ accumulation in the atmosphere and the marine mixed layer (top 50–100 m). This in turn triggers chemical changes in the mixed layer, climatic warming, and bioevolutionary turnover. The Cretaceous/Tertiary ($\text{K}\text{T}$) transition at 65 Ma seems to have been a time of major mantle degassing which induced a perturbation of the carbon cycle. During the $\text{K}\text{T}$ transition, Deccan Traps volcanism, perhaps the greatest episode of continental flood basalt volcanism in the Phanerozoic, flooded an estimated 2.6 × 10⁶ km² of India with basaltic lavas, releasing 5 × 10¹⁷ moles of CO₂ into the earth's atmosphere over a duration 0.53–1.36 Ma at the rate of 3.9 × 10¹¹ to 9.6 × 10¹¹ moles CO₂ per year. The modern mean annual rate of mantle CO₂ release from all sources is 4.1 × 10¹² moles CO₂ per year; assuming a comparable rate of release prior to the Deccan Traps volcanism, the Deccan Traps addition would have elevated the rate of mantle CO₂ release by 10–25%. Sluggish marine circulation and warm, deep, oceans (14–15°C) would have exacerbated CO₂ buildup in the atmosphere, accounting for the Cretaceous to Tertiary drop in oxygen-18 via climatic warming, and, in the marine mixed layer (top 50–100 m), explaining the selective nature of the terminal Cretaceous marine extinctions via a pH change. The extinctions were most severe amongst the calcareous microplankton of the mixed layer; calcareous microplankton (planktonic foraminifera and coccolithophorids) begin to have pH problems at 7.8 and 7.5, respectively. Failure of the coccolithophorids would have disrupted the Williams-Riley pump (algal productivity-gravity pump of CO₂ from the atmosphere and mixed layer into the deep oceans) producing dead ocean conditions (severely reduced photosynthesis and CaCO₃ production). Failure of the Williams-Riley pump is reflected in the extinctions themselves, and in the loss of biogenic CaCO₃ to the sea floor, causing the $\text{K}\text{T}$ boundary hiatus and (or) the $\text{K}\text{T}$ boundary clay. Failure of the pump today would elevate atmospheric pCO₂ severalfold; the $\text{K}\text{T}$ failure would have responded comparably. Dead ocean conditions would, in themselves, have produced a major CO₂ buildup. Early Tertiary “Strangelove” conditions in the mixed layer, characterized by a dominance of the thoracosphaerids, braarudosphaerids and small planktonic foraminifera, were coeval with the main pulse of Deccan Traps volcanism. Overall, the record is one of gradual $\text{K}\text{T}$ bioevolutionary turnover during a period of disequilibrium between the rate of mantle CO₂ degassing and uptake by sinks. Mantle degassing during the Deccan Traps volcanism unifies the $\text{K}\text{T}$ biological and physicochemical records.  相似文献   

Late Quaternary geomorphic evolution of the lower Narmada valley, Western India: implications for neotectonic activity along the Narmada–Son Fault     

L. S. Chamyal  D. M. Maurya  S. Bhandari  Rachna Raj 《Geomorphology》2002,46(3-4)

Geomorphic data combined with stratigraphic studies provide significant information to constrain timing and amount of fault movement. The lower Narmada valley lies astride the Narmada–Son Fault (NSF), an important ENE–WSW-trending tectonic element responsible for the current intraplate seismicity being experienced in the central part of the Indian plate. Varying nature and degree of tectonic movements along the NSF during Late Pleistocene and Holocene have produced four geomorphic surfaces in the lower Narmada valley: the alluvial plain (S₁), ravine surface (S₂), a gravelly fan surface (S₃) and the valley fill terrace surface (S₄). Two major phases of tectonic movements in a compressive stress regime are recorded along the NSF: slow synsedimentary subsidence of the basin during Late Pleistocene due to differential movement, followed by inversion of the basin during the Holocene marked by differential uplift along the NSF. The study suggests that the inversion of the basin is in response to the significant increase in the intensity of compressive stresses in the Indian plate mainly during the Early Holocene. The present incisive drainage and recent seismic activity indicate that the compressive stresses continue to accumulate along the NSF due to continued northward movement of the Indian plate.  相似文献   

Melt-producing and melt-consuming reactions in the Achankovil cordierite gneisses, South India   总被引：7，自引：0，他引：7  

B. Cenki  L. M. Kriegsman  I. Braun 《Journal of Metamorphic Geology》2002,20(6):543-561

Migmatitic cordierite gneisses within the Achankovil Zone (AZ) of southern Pan‐African India record melt‐producing and subsequent melt‐consuming mineral reactions. Early mineral assemblages Bt‐Sil‐Qtz and Bt‐Sil‐Spl, deduced from inclusion textures in garnet prophyroblasts, break down via successive dehydration melting reactions to high‐T phase assemblages (e.g. Grt‐Crd‐Liq, Opx‐Liq, Spl‐Crd‐Liq). Later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome, and partial resorption of garnet to Opx‐Crd or Crd‐Bt‐Qtz symplectites. Leucosomes generally display a moderate (low‐strain gneisses) to strong (high‐strain gneisses) depletion of alkali feldspar attributed to mineral‐melt back reactions partly controlled by the degree of melt segregation. Using a KFMASH partial petrogenetic grid that includes a melt phase, and qualitative pseudosections for microdomains of high and low Al/Si ratios, the successive phase assemblages and reaction textures are interpreted in terms of a clockwise P–T path culminating at about 6–7 kbar and 900–950 °C. This P–T path is consistent with, but more detailed than published results, which suggests that taking a melt phase into account is not only a valid, but also a useful approach. Comparing P–T data and lithological and isotopic data for the AZ with adjacent East Gondwana fragments, suggests the presence of a coherent metasedimentary unit exposed from southern Madagascar via South India (AZ) and Sri Lanka (Wanni Complex) to the Lützow–Holm Bay in Eastern Antarctica.  相似文献