Low frequency observations of the scintillation of weak radio sources     

Vadim S. Artyukh  Barry Speed 《Astrophysics and Space Science》1979,61(1):229-241

Interplanetary scintillation observations of weak radio sources have been made at a frequency of 102.5 MHz. Sources have been chosen from the Bologna catalogue, with flux densities 0.5 Jy–4.0 Jy at 408 MHz.It has been found that the average scintillation visibility is in agreement with the results of the interferometer observations of Speed and Warwick (1978).A very wide spectral index distribution has been found for weak radio sources at low frequencies, perhaps indicating the presence of a new population of low luminosity, flat spectrum radio sources.  相似文献   

Stochastic dynamics of damage evolution in brittle rocks     

Vadim V. Silberschmidt  Vladimir G. Silberschmidt 《International Journal of Earth Sciences》1996,85(1):103-109

Development of defects in brittle rocks is analysed in terms of continuous damage mechanics with an emphasis on a shear damage evolution. Necessity to account for an effect of stochastic action leads to an introduction of kinetic relations for damage accumlation in a form of stochastic differential equations. Numerical algorithms for their solution for three types of noise are proposed. Results of simulation are compared with analytical solutions for a determiistic case (absence of stochastic action). Effect of various types of noise on damage accumulation and on time-to-fracture distributions is investigated. The study is limited to the case of temporal stochasticity; ways for unification of approach and schemes with spatial randomness are discussed.  相似文献   

近百年里海海平面的升降与伏尔加三角洲的发育     

李从先  V·依万诺夫  范代读  V·科洛达也夫  刘曙光  R·恰洛夫  杨守业 《海洋地质与第四纪地质》2002,22(3):1-9

位于里海北部的伏尔加三角洲属河控型三角洲，其水下坡度极度平缓。近百年来里海海平面升降幅度达3．46m，其变化可分为3个阶段：1900－1929年缓慢下降期，平均下降速率为1.1cm/a；1930－1977年快速下降期，平均速率为6.3cm/a；1978－1999年为快速上升期，平均上升速率达到8.8cm/a。在海平面下降阶段，伏尔加三角洲增长幅度达到每年2－180km^2，这是由于河流泥沙的沉积和浅水区出露水面而造成。海平面上升之时，较浅水下三角洲部分成为“缓冲带”，降低了海平面上升对三角洲的影响。尽管近20多年来海平面迅速上升，伏尔加三角洲并未发现海岸侵蚀、湿地损失、盐水入侵、海岸洪涝灾害等一般大河三角洲常见的现象。对比其它三角洲可以发现，各个大河三角洲各以不同的方式响应海平面的上升。  相似文献   

Geophysical models for the tectonic framework of the Lake Vostok region, East Antarctica   总被引：2，自引：0，他引：2  

Michael Studinger  Garry D Karner  Vadim Levin  Anahita A. Tikku 《Earth and Planetary Science Letters》2003,216(4):663-677

Aerogeophysical and seismological data from a geophysical survey in the interior of East Antarctica were used to develop a conceptual tectonic model for the Lake Vostok region. The model is constrained using three independent data sets: magnetic, seismic, and gravimetric. A distinct change in the aeromagnetic anomaly character across Lake Vostok defines a crustal boundary. Depth to magnetic basement estimates image a 400-km-wide and more than 10-km-deep sedimentary basin west of the lake. Analysis of teleseismic earthquakes suggests a relatively thin crust beneath Lake Vostok consistent with predictions from kinematic and flexural gravity modelling. Magnetic, gravity, and subglacial topography data reveal a tectonic boundary within East Antarctica. Based on our kinematic and flexural gravity modelling, this tectonic boundary appears to be the result of thrust sheet emplacement onto an earlier passive continental margin. No data presently exist to date directly either the timing of passive margin formation or the subsequent shortening phase. The preserved thrust sheet thickness is related to the thickness of the passive margin crust. Because a significant amount of time is required to erode the thrust sheet topography, we suggest that these tectonic events are Proterozoic in age. Minor normal reactivation of the thrust sheet offers a simple mechanism to explain the formation of the Lake Vostok Basin. A low level of seismicity exists in the vicinity of this tectonic boundary. The existence of a crustal boundary in the Antarctic interior provides new constraints on the Proterozoic architecture of the East Antarctic craton.  相似文献   

Evolution of the Mongol–Okhotsk Ocean as constrained by new palaeomagnetic data from the Mongol–Okhotsk suture zone, Siberia     

Vadim A. Kravchinsky  Jean-Pascal Cogné  William P. Harbert   Mikhail I. Kuzmin 《Geophysical Journal International》2002,148(1):34-57

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Crustal structure of the Middle Urals based on reprocessing of Russian seismic reflection data     

David N. Steer  James H. Knapp  Larry D. Brown  Alexander V. Rybalka  Vadim B. Sokolov 《Geophysical Journal International》1995,123(3):673-682

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Porphyry copper deposits of the CIS and the models of their formation     

Vadim S. Zvezdov  Igor F. Migachev  Mikhail M. Girfanov 《Ore Geology Reviews》1993,7(6)

Deposits of the “porphyry” family (essentially porphyry copper and gold-porphyry copper, gold-bearing porphyry molybdenum-copper, gold-containing porphyry copper-molybdenum and porphyry molybdenum deposits) are associated in time and space with granitoid magmatism mainly in Phaerozoic volcano-plutonic belts. Whatever their age, the deposits belong to two types of belts: basaltic belts, representing axial zones of island arcs, or andesitic belts formed within active continental (Andean-type) margins.The petrochemistry of ore-bearing magmatism related to the nature of the substratum of volcano-plutonic belts, reveals a number of essential characteristics, both in composition and zonation of wallrock alteration and ore mineralization. These characterisics enabled previous researchers to establish four models of porphyry copper deposits based on their lithologic associations, e.g., “diorite”, “granodiorite”, “monzonite” and “granite”.Pophyry copper deposits are thought to be the product of self-generating “two-fluid mixing” ore-magmatic systems. Porphyry intrusions are pathways for energy and metals from deep-seated magma chambers, of which the upper mineralized parts are accessible for observation. The relationship between magmatic fluids and meteoric water participating in the ore-forming processes (dependent on the structural-petrophysical conditions of formation), provide a subdivision for the porphyry copper ore-magmatic systems into three types: “open”, “closed” and “transitional”.Concurrently, a common trend in the evolution of the systems has been established, from a nearly autoclave regime of structural-and ore-forming processes to a gradual increase in the importance of hydrothermal recycling. The completeness of the OMS (ore-magmatic system) development according to this scheme, which determines the existence of various OMS types, depends on many factors, the most important being the depth of formation of porphyry intrusive bodies, the petrophysical peculiarities of the host rocks and the palaeohydrogeological conditions of ore deposition.Although rock fracturing (especially defluidization: second boiling) and contraction are caused by the same mechanisms, the stockwork growth in “open” and “closed” systems, relative to the wall rock, takes place in opposite directions, primarily due to different petrophysical parameters of the near-stock environment.In “open” systems structural and ore metasomatic processes are finalized. Fractures extend progressively from porhyry stocks into the marginal parts of the intrusive framework and extension of large-scale recycling of magmatic and activated meteoric water, in the same direction, result in the formation of ore-bearing stockworks. These are large in all dimensions, cover mainly hanging-wall zones and are characterized by clearly defined concentric mineral zoning and extensive geochemical haloes.In a “closed” OMS with centripetal growing fractures, hydrothermal convection is stunted. The vertical extension of recycling cells is restricted and the volume of meteoric water involved in circulation during the period of ore deposition is relatively small. As a result, relatively small intra-intrusive lenticular stockworks are developed which are characterized by close co-existence of several generations of mineralization with fragmentary preservation of the earliest ones. These are characterized by the elements of “reverse” zoning, increased density of the veinlets and metal content, as well as poorly developed hanging-wall dispersion haloes.  相似文献   

Western Palaearctic palaeoenvironmental conditions during the Early and early Middle Pleistocene inferred from large mammal communities,and implications for hominin dispersal in Europe     

Ralf-Dietrich Kahlke  Nuria García  Dimitris S. Kostopoulos  Frédéric Lacombat  Adrian M. Lister  Paul P.A. Mazza  Nikolai Spassov  Vadim V. Titov 《Quaternary Science Reviews》2011,30(11-12):1368-1395

Large-scale fluctuations in global climate and resulting changes in ecology had a profound effect on human evolution and dispersal. Though hominin remains are scarce, studies focussing on the more abundant records of fossil land mammal communities can contribute greatly to our knowledge of the palaeoenvironmental circumstances that influenced and directed the global spread of hominins. To produce a comprehensive and accurate account of the evolution of western Palaearctic habitat diversity between 2.6 and 0.4 Ma BP, information generated from large mammal communities from 221 key sites has been included in this study.The palaeoecological conditions of the western Palaearctic during the Early and early Middle Pleistocene were principally controlled by the following key factors: (1) a widespread trend of temperature decrease, (2) the periodicity of the global temperature record, (3) the intensity of single climatic stages, (4) the temporal pattern of climatic variation, (5) geographical position, and (6) the distribution of continental water resources. A general picture of the evolution of western Palaearctic habitat diversity saw the replacement of extensive forested terrain by an alternating sequence of varied savannah-like and forested habitats during the 2.6–1.8 Ma span, as well as an alternation between different types of predominantly open habitats between 1.8 and 1.2 Ma. Both of these processes were governed by 41 ka temperature periodicity. During the 1.2–0.9 Ma time span, irregular climatic fluctuations were more common and habitat variability increased. The subsequent 0.9–0.4 Ma interval, a period controlled by 100 ka periodicity, was by comparison more stable, with longer climatic cycles alternating between open and forested landscapes. During the entire Early and early Middle Pleistocene, assemblages of large mammal communities reveal a distinct trend of decreasing continentality between Eastern and South-Eastern Europe on the one hand, and South-Western and North-Western Europe on the other. This trend was due to the effect of the Atlantic Ocean, while in Southern Europe the relatively low continentality was balanced by influences from the Mediterranean Sea.When plotted against evidence of hominin occurrence, the data on western Palaearctic habitat diversity inferred from large mammal communities indicate clear environmental stimuli for the earliest human dispersal in Europe. These are: (1) a wide range of habitats, implying a high diversity of resources; (2) mild climates with low seasonality, implying a lack of strong environmental fluctuations. Around 1.8 Ma at the latest, hominins of African origin entered the western Palaearctic for the first time, taking advantage of the diversity of habitats and resources, particularly along large river systems. Their subsequent westward spread between 1.7 and 1.3 Ma was restricted to Mediterranean-influenced areas, which offered a high variability of habitats and relatively low seasonality. The increase in environmental diversity, which occurred from 1.2 Ma onwards, opened up South-Eastern and Eastern Europe for hominin occupation. According to the available records, North-Western and Central Europe were initially colonized during late Early to early Middle Pleistocene interglacials, when these regions experienced periods of low seasonality and considerable habitat diversity.  相似文献   

Palaeomagnetic results from Palaeocene basalts from Mongolia reveal no inclination shallowing at 60 Ma in Central Asia     

Fatim Hankard  Jean-Pascal Cogné  France Lagroix  Xavier Quidelleur  Vadim A. Kravchinsky  Amgalan Bayasgalan  Purevdorj Lkhagvadorj 《Geophysical Journal International》2008,172(1):87-102

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Cyclonic circulation and upwelling in Lake Baikal   总被引：1，自引：0，他引：1  

Elena Troitskaya  Vadim Blinov  Vyacheslav Ivanov  Andrey Zhdanov  Ruslan Gnatovsky  Ekaterina Sutyrina  Mikhail Shimaraev 《Aquatic Sciences - Research Across Boundaries》2015,77(2):171-182

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