Thermal stability of a layer with permeable boundaries and variable gravitational field     

A. K. Singh  Mohammad Yusuf 《Astrophysics and Space Science》1990,167(1):69-80

The aim of this paper is to investigate the thermal stability of a fluid layer with permeable boundaries and a variable gravitational field. It is observed that the principle of exchange of stabilities is valid when the layer is heated form below and the complex growth rate of an arbitrary oscillatory mode exists outside of a circle whose radius depends upon the permeability parameter, Prandtl number and wavelength of the mode. In the case of a layer heated from below, gravity increasing upward has a destabilizing effect whereas the permeability parameter has a stabilizing effect.  相似文献   

Stability of the periodic solutions of the restricted three-body problem representing analytic continuations of Keplerian rectilinear periodic motions     

Abdul Ahmad 《Celestial Mechanics and Dynamical Astronomy》1995,61(2):181-196

The periodic solutions of the restricted three-body problem representing analytic continuations of Keplerian rectilinear periodic motions are well known (Kurcheeva, 1973). Here the stability of these solutions are examined by applying Poncaré's characteristic equation for periodic solutions. It is found that the isoperiodic solutions are stable and all other solutions are unstable.  相似文献   

Cretaceous Environments of Afghanistan:A Synthesis Based on Selected Sections     

Abdul Rahman Ashraf  Ashok Sahni 《东北亚地学研究》2003,(1)

1IntroductionThe Late Mesozoic sequences of Afghanistan are important in understanding the geodynamic evolu-tion of the northwestern part of South Asia, in par-ticular the transgressive and regressive history of the northwestern part of the Tethys connecting Iran to the west and the northern Indian Subcontinent to the east. Jurassic to Upper Cretaceous sedimentary seque-nces were initially studied by Griesbach (1885 ~1887) and Hayden (1880~1901) through several traverses in northern an…  相似文献   

Revised age of proximal deposits in the Zagros foreland basin and implications for Cenozoic evolution of the High Zagros     

Mohammad D. Fakhari  Gary J. Axen  Brian K. Horton  Jamshid Hassanzadeh  Abdolhossein Amini 《Tectonophysics》2008,451(1-4):170-185

The regionally extensive, coarse-grained Bakhtiyari Formation represents the youngest synorogenic fill in the Zagros foreland basin of Iran. The Bakhtiyari is present throughout the Zagros fold-thrust belt and consists of conglomerate with subordinate sandstone and marl. The formation is up to 3000 m thick and was deposited in foredeep and wedge-top depocenters flanked by fold-thrust structures. Although the Bakhtiyari concordantly overlies Miocene deposits in foreland regions, an angular unconformity above tilted Paleozoic to Miocene rocks is expressed in the hinterland (High Zagros).

The Bakhtiyari Formation has been widely considered to be a regional sheet of Pliocene–Pleistocene conglomerate deposited during and after major late Miocene–Pliocene shortening. It is further believed that rapid fold growth and Bakhtiyari deposition commenced simultaneously across the fold-thrust belt, with limited migration from hinterland (NE) to foreland (SW). Thus, the Bakhtiyari is generally interpreted as an unmistakable time indicator for shortening and surface uplift across the Zagros. However, new structural and stratigraphic data show that the most-proximal Bakhtiyari exposures, in the High Zagros south of Shahr-kord, were deposited during the early Miocene and probably Oligocene. In this locality, a coarse-grained Bakhtiyari succession several hundred meters thick contains gray marl, limestone, and sandstone with diagnostic marine pelecypod, gastropod, coral, and coralline algae fossils. Foraminiferal and palynological species indicate deposition during early Miocene time. However, the lower Miocene marine interval lies in angular unconformity above ~ 150 m of Bakhtiyari conglomerate that, in turn, unconformably caps an Oligocene marine sequence. These relationships attest to syndepositional deformation and suggest that the oldest Bakhtiyari conglomerate could be Oligocene in age.

The new age information constrains the timing of initial foreland-basin development and proximal Bakhtiyari deposition in the Zagros hinterland. These findings reveal that structural evolution of the High Zagros was underway by early Miocene and probably Oligocene time, earlier than commonly envisioned. The age of the Bakhtiyari Formation in the High Zagros contrasts significantly with the Pliocene–Quaternary Bakhtiyari deposits near the modern deformation front, suggesting a long-term (> 20 Myr) advance of deformation toward the foreland.  相似文献   

Structural elements of the Makran region, Oman sea and their potential relevance to tsunamigenisis     

Mohammad Mokhtari  Iraj Abdollahie Fard  Khaled Hessami 《Natural Hazards》2008,47(2):185-199

The character of convergence along the Arabian–Iranian plate boundary changes radically eastward from the Zagros ranges to the Makran region. This appears to be due to collision of continental crust in the west, in contrast to subduction of oceanic crust in the east. The Makran subduction zone with a length of about 900 km display progressively older and highly deformed sedimentary units northward from the coast, together with an increase in elevation of the ranges. North of the Makran ranges are large subsiding basins, flanked to the north by active volcanoes. Based on 2D seismic reflection data obtained in this study, the main structural provinces and elements in the Gulf of Oman include: (i) the structural elements on the northeastern part of the Arabian Plate and, (ii) the Offshore Makran Accretionary Complex. Based on detailed analysis of these data on the northeastern part of the Arabian Plate five structural provinces and elements—the Musendam High, the Musendam Peneplain, the Musendam Slope, the Dibba Zone, and the Abyssal Plain have been identified. Further, the Offshore Makran Accretionary Complex shown is to consist Accretionary Prism and the For-Arc Basin, while the Accretionary Prism has been subdivided into the Accretionary Wedge and the Accreted/Colored Mélange. Lastly, it is important to note that the Makran subduction zone lacks the trench. The identification of these structural elements should help in better understanding the seismicity of the Makran region in general and the subduction zone in particular. The 1945 magnitude 8.1 tsunamigenic earthquake of the Makran and some other historical events are illustrative of the coastal region’s vulnerability to future tsunami in the area, and such data should be of value to the developing Indian Ocean Tsunami Warning System.  相似文献   

National ocean policy—new opportunities for Malaysian ocean development     

Abdul Hamid Saharuddin   《Marine Policy》2001,25(6)

Being a littoral state, Malaysian maritime sectors provide the basis for the growth and development of marine-related activities. As the country enters the new millennium along with the ‘2020 Vision’ which serves as the basic template against which the economic performance of these sectors should be judged, it poses a new challenge for Malaysia in developing these sectors. This paper attempts to address three basic issues: Firstly, is there any national ocean policy in place? Secondly, are the maritime sectors adequately organized to play an important role in achieving the objectives? Thirdly, what are the opportunities available in developing the ocean?It can be seen that existing policies to develop comprehensive ocean governance have not received the full attention they deserve. Organizational structures governing the ocean for implementing national policies are well in place but in a fragmented and uncoordinated fashion. As a result, sectoral and intersectoral management problems were created such as multiple-use conflicts, overlapping of jurisdiction and duplication of efforts. Environmental problems have also not been properly addressed. A few sectors have been identified as offering opportunities to further develop the Malaysian maritime areas. Among these are marine education and human resources development, marine tourism and the seafood industry.  相似文献   

Crustal xenoliths from Calbuco Volcano,Andean Southern Volcanic Zone: implications for crustal composition and magma-crust interaction   总被引：1，自引：1，他引：0  

Rosemary Hickey-Vargas  Mohammad J. Abdollahi  Miguel A. Parada  Leopoldo López-Escobar  Fred A. Frey 《Contributions to Mineralogy and Petrology》1995,119(4):331-344

Crustal xenoliths in the 1961 andesite flow of Calbuco Volcano, in the southern Southern Volcanic Zone (SSVZ) of the Andes, consist predominantly of pyroxene granulites and hornblende gabbronorites. The granulites contain plagioclase+pyroxene+magnetite±amphibole, and have pronounced granoblastic textures. Small amounts of relict amphibole surrounded by pyroxene-plagioclase-magnetite-glass symplectites are found in some specimens. These and similar textures in the gabbronorites are interpreted as evidence of dehydration melting. Mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism: a moderate pressure, high temperature stage accompanied by melting and melt extraction from some samples, followed by thermal metamorphism after entrainment in the Calbuco andesite lavas. High _Nd ^T values (+4.0 to +8.6), Nd-isotope model ages of 1.7–2.0 Ga, and trace element characteristics like chondrite normalized La/Yb< and La/Nb1 indicate that the protoliths were oceanic basalts. Similar oceanic metabasalts of greenschist to amphibolite facies are found in the Paleozoic metamorphic belt that underlies the Chilean coastal ranges. Mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate, crystallizing from the basaltic andesite magma at Calbuco. Crystallization pressures for the gabbros based on total Al contents in amphibole are 6–8 kbar. These pressures point to middle to lower crustal storage of the Calbuco magma. Neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of Calbuco andesites, although an ancient sedimentary contaminant is indicated by the lava compositions. The presence of oceanic metabasaltic xenoliths, together with the sedimentary isotopic imprint, suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt, which is an accretionary complex of intercalated basalts and sediments that formed along the Paleozoic Gondwanan margin. If this is the case, the geochemical composition of the lower and middle crust beneath the SSVZ is significantly different from that of most recent SSVZ volcanic rocks.  相似文献   

Composition-related microstructures in zinc-bearing carbonate assemblages from Broken Hill,New South Wales     

Dr. Mohammad Riaz Khan  Dr. D. J. Barber 《Mineralogy and Petrology》1990,41(2-4):229-245

Summary Analytical transmission electron microscopy has been used to investigate two zincian dolomites occurring as overgrowths on calcite from the oxidized zone of the ore bodies at Broken Hill, New South Wales. Particular attention has been focussed on the nature of the microstructures, and any correlations between microstructure and local composition, especially zinc content. The possible occurrence of tiny amounts of the zinc dolomite, minrecordite, has also been explored.The most notable features in the zincian dolomite are fine-scale distributions of ribbonlike calcitic second phase, growth bands, and modulated microstructures (strong modulations in diffraction contrast). Their occurrence is correlated with calcium in excess of stoichiometric requirements, but is not directly influenced by zinc content. The associated calcites are low in zinc, but they also exhibit carbonate second phase, which is thought to be dolomitic although giving no detectable order (b-type) reflections. Long-exposure electron diffraction patterns, however, revealed the presence of diffuse c-type superstructure reflections, which are thought to arise from smaller volumes in which Ca and Mg cations are ordered, for which a model is proposed.

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Zusammensetzungsabhängige Mikrogefüge in Zn-haltigen Karbonat-Vergesellschaftungen von Broken Hill, New South Wales

Zusammenfassung Analytische Transmissionselektronenmikroskopie wurde benutzt um zwei Zn-haltige Dolomite zu untersuchen, die als Überzüge von Calcit aus der Oxidationszone der Erzkörper von Broken Hill, New South Wales, auftraten. Besondere Aufmerksamkeit wurde der Art der Mikrogefüge und möglichen Korrelationen zwischen Mikrogefüge und der lokalen Zusammensetzung, vor allem dem Zn-Gehalt, gewidmet. Dem möglichen Auftreten von geringen Mengen des Zn-Dolomit, Minrecordit, wurde ebenfalls nachgegangen.Die bemerkenswertesten Erscheinungen im Zn-haltigen Dolomit sind feine bandartige Verteilung einer calcitischen Phase, Wachstumsbänder und modulierte Strukturen (starke Modulationen im Beugungskontrast sichtbar). Ihr Auftreten ist mit über das stöchiometrische Verhältnis hinaus vorhandenem Ca korreliert; es ist aber vom Zn-Gehalt nicht direkt beeinflußt. Die mitvergesellschafteten Calcite haben einen niedrigen Zn-Gehalt, zeigen aber eine weitere Karbonatphase. Diese wird für dolomitsch gehalten, obwohl keine entsprechenden Ordnungsreflexe (b-Reflexè) nachweisbar waren. Sehr lange belichtete Elektronenbeugungs-Aufnahmen zeigen jedoch diffuse c-Typ-Über-struktur-Reflexe. Diese sind wahrscheinlich auf kleine Bereiche beschränkt, in dener die Ca und Mg Kationen geordnet sind, wofür ein Modell vorgeschlagen wird.

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With 10 Figures  相似文献   

Tectono-sedimentary processes along an active marine/lacustrine half-graben margin: Alkyonides Gulf, E. Gulf of Corinth, Greece     

M. R. Leeder  R. E. Ll. Collier  †  L. H. Abdul Aziz  M. Trout  †  G. Ferentinos  ‡  G. Papatheodorou‡  E. Lyberis‡ 《Basin Research》2002,14(1):25-41

ABSTRACT The Alkyonides half‐graben is separated from the Gerania Range to the south by active faults whose offshore traces are mapped in detail. The East Alkyonides and Psatha Faults have well‐defined, Holocene‐active tip zones and cannot be extrapolated from the onshore Skinos Fault into a single continuous surface trace. During the late Quaternary, catchments draining the step‐faulted range front have supplied sediment to alluvial fans along a subsiding marine ramp margin in the hangingwall of the Skinos Fault, to shelf ledge fans on the uplifting footwall to the East Alkyonides Fault and to the Alepochori submarine fan in the hangingwall of the latter. During late Pleistocene lowstand times (c. 70–12 ka), sediment was deposited in Lake Corinth as fan deltas on the subsiding Skinos shelf ramp which acted as a sediment trap for the adjacent 360 m deep submarine basin plain. At the same time, the uplifting eastern shelf ledge was exposed, eroded and bypassed in favour of deposition on the Alepochori submarine fan. During Holocene times, the Skinos bajada was first the site of stability and soil formation, and then of substantial deposition before modern marine erosion cut a prominent cliffline. The uplifting eastern shelf ledge has developed substantial Holocene fan lobe depositional sequences as sediment‐laden underflows have traversed it via outlet channels. We estimate mean Holocene displacement rates towards the tip of the Psatha Fault in the range 0.7–0.8 mm year^?1. Raised Holocene coastal notches indicate that this may be further partitioned into about 0.2 mm year^?1 of footwall uplift and hence 0.5–0.6 mm year^?1 of hangingwall subsidence. Holocene displacement rates towards the tip of the active East Alkyonides Fault are in the range 0.2–0.3 mm year^?1. Any uplift of the West Alkyonides Fault footwall is not keeping pace with subsidence of the Skinos Fault hangingwall, as revealed by lowstand shelf fan deltas which show internal clinoforms indicative of aggradational deposition in response to relative base‐level rise due to active hangingwall subsidence along the Skinos Fault. Total subsidence here during the last 58 kyr lowstand interval of Lake Corinth was some 20 m, indicating a reduced net displacement rate compared to estimates of late Holocene (< 2000 bp ) activity from onshore palaeoseismology. This discrepancy may be due to the competition between uplift on the West Alkyonides Fault and subsidence on the onshore Skinos Fault, or may reflect unsteady rates of Skinos Fault displacement over tens of thousands of years.  相似文献   

VORTEX FLOW FIELD IN A SCOUR HOLE AROUND ABUTMENTS     

Abdul Karim BARBHUIYA  Subhasish DEY 《国际泥沙研究》2003,18(4)

The three-dimensional flow field in a scour hole around different abutments under a clear water regime was experimentally measured in a laboratory flume, using the Acoustic Doppler Velocimeter (ADV). Three types of abutments used in the experiments were vertical-wall (rectangular section), 45° wing-wall (45° polygonal section) and semicircular. The three-dimensional time-averaged velocity components were detected at different vertical planes for vertical-wall abutment and azimuthal planes for wing-wall and semicircular abutments. The velocity components were also measured at different horizontal planes. In the upstream, presentations of flow field through vector plots at vertical / azimuthal and horizontal planes show the existence of a primary vortex associated with the downflow inside the scour hole. On the other hand, in the downstream, the flow field shows a reversed flow near the abutments having a subsequent recovery with a passage of flow as a part of the main flow. The data presented in this p  相似文献