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481.
The Quaternary glacial sequence proposed for the Ecuadorian Andes by Walter Sauer is critically reviewed. Examination of his field evidence at sections exposing Quaternary sediments east of Quito has led to a fundamental reinterpretation. Deposits which Sauer considered as glacial, glacio-fluvial, glacio-lacustrine and pluvio-glacial in origin appear to have been formed mainly by volcanic, volcano-loessic, laharic, fluvial, colluvial and pedogenic processes.  相似文献   
482.
Summary. Linear-programming methods are powerful and efficient tools for objectively analysing seismic focal mechanisms and are applicable to a wide range of problems, including tsunami warning and nuclear explosion identification. The source mechanism is represented as a point in the six-dimensional space of moment-tensor components. Each observed polarity provides an inequality constraint, linear with respect to the moment tensor components, that restricts the solution to a half-space bounded by a hyperplane passing through the origin. The intersection of these half-spaces is the convex set of all acceptable solutions. Using linear programming, a solution consistent with the polarity constraints can be obtained that maximizes or minimizes any desired linear function of the moment tensor components; the dilatation, the thrust-like nature, and the strike-slip-like nature of an event are examples of such functions. The present method can easily be extended to fit observed seismic-wave amplitudes (either signed or absolute) subject to polarity constraints, and to assess the range of mechanisms consistent with a set of measured amplitudes.  相似文献   
483.
The depositional processes associated with late Devensian ice in areas bordering the Irish Sea basin have been the subject of considerable debate. Among the key areas around the Irish Sea, southwest Wales occupies a particularly crucial position because it is here that ice flowing from the north impinged upon the coast orthogonally and encroached inland. Two main hypotheses have emerged concerning deglaciation of the Irish Sea basin. The traditional hypothesis holds that sedimentation was ice‐marginal or subglacial, whereas an alternative hypothesis that emerged in the 1980s argued that sedimentation was glaciomarine. Southwest Wales is well‐placed to contribute to this debate. However, few detailed sedimentological studies, linked to topography, have been made previously in order to reconstruct glacial environments in this area. In this paper, evidence is presented from four boreholes drilled recently in the Cardigan area, combined with data from coastal and inland exposures in the lower Teifi valley and adjacent areas. A complex history of glaciation has emerged: (i) subglacial drainage channel formation in pre‐Devensian time, (ii) deposition of iron‐cemented breccias and conglomerates possibly during the last interglacial (or in the early/mid‐Devensian interstadial), (iii) late Devensian ice advance across the region, during which a glaciolacustrine sequence over 75 m thick accumulated, within a glacial lake known as Llyn Teifi, (iv) a second high‐level glaciolacustrine succession formed near Llandudoch, (v) outside the Teifi valley, ice‐marginal, subglacial and glaciofluvial sediments were also laid down, providing a near‐continuous cover of drift throughout the area. Glacial advance was characterized by reworking, deformation and sometimes erosion of the underlying sediments. The glaciomarine hypothesis is thus rejected for southwest Wales. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
484.
The expert system approach enables geochemical evidence to be integrated with geological, petrological and mineralogical evidence in identifying the eruptive setting of ancient volcanic rocks. This paper explains the development of ESCORT, an Expert System for Characterization of Rock Types. ESCORT uses as its knowledge base a set of dispersion matrices derived from a geochemical data bank of some 8000 immobile element analyses, together with tables of magma-type membership probabilities based on a-priori knowledge, the dominant flow type, the facies of intercalated sediment, the basement to the volcanic sequence, the presence or absence of an iron-enrichment trend and the phenocryst phase assemblage. Inferencing is based on the Bayes Decision Rule, adapted to take into account possible uncertainties in the geological evidence. Upper and lower probability thresholds are used to decide whether an interpretation is likely or unlikely. Evaluation of the system shows that it is able to overcome many of the well-publicized ambiquities attached to geochemical discrimination diagrams, as well as providing a unified method of decision making and a measure of the correctness of the interpretation.  相似文献   
485.
Oxygen isotope fractionations between zoisite and water have been studied at 400–700°C, PH2O = 13.4 kbar, using the three-isotope method described by Matsuhisaet al. (1978) and Matthewset al. (1983a). The zoisite-waier exchange reaction takes place extremely slowly and consequently direct-exchange calibration of equilibrium 18O16O fractionation factors was possible only at 600 and 700°C. Fractionation factors at 400–600°C were determined from samples hydrothermally crystallized from a glass of the anhydrous zoisite composition. At 600°C, both exchange procedures gave identical fractionations within experimental error. Scanning electron microscope studies showed that the zoisite-water exchange reaction occurs largely by solution-precipitation mass-transfer mechanisms. The slow kinetics of zoisite-water exchange may be typical of hydrous silicates, since additional experiments on tremolite-water and chlorite-water exchange also showed very low rates. When the zoisite-water fractionation factors determined in this study are combined with the quartz and albite-water data of Matsuhisaet al. (1979) and the calcite-water data of O'Nellet al. (1969), mineral-pair fractionations are obtained for which the coefficients “A” in the equation 1000 In α = A × 106T?2 are:
  相似文献   
486.
Zusammenfassung Das Hesperische Massiv wurde von mehreren Bruchsystemen erfaßt, die durch ihre Häufigkeit und besonderen Charakter recht bedeutsam sind. Das wichtigste davon ist ein sich kreuzendes Bruchsystem mit NW-SE- und NE-SW-Streichen. Es handelt sich um spätvariszische Frakturen, die während des Mesozoikums und Tertiärs weiterhin aktiv gewesen sind. Über den ganzen variszischen Iberischen Block verbreitet findet man sehr häufig an diese Brüche gebundene Kluftsysteme.Die Störungen gaben Raum für Diabasgänge, sie haben die Intrusion von späten kalkalkalischen Graniten begünstigt, und sie haben Scherungen und Mylonitisierungen in den variszischen Gesteinen hervorgerufen.Setzt man die Lageveränderung des gesamten Hesperischen Massivs in Beziehung zum Kräftespiel an diesen Brüchen, so ist als verantwortlich für diese Deformation eine relative Drehbewegung des iberischen variszischen Sockels gegenüber dem französischen durchaus vorstellbar.
The Hesperian Massif is affected by several systems of faults which are relevant both by their frequency and by their characteristics. Among these the most important is the system of conjugated faults striking NW-SE and NE-SW from late hercynian age which kept working during the Mesozoic and Tertiary periods. Systems of joints linked to these faults may be found very often throughout all the Iberian hercynian block.These features have originated diabasic dikes, have helped the intrusion of late calcoalcaline granite and have produced shearings and mylonitizations in the hercynian rocks.The analyse of the deformation of the Hesperian Massif as a whole in accordance with the work of these faults reveals a relative rotation of the Iberian hercynian socle by respect to the French one as the reason for this deformation.
Resumen El Macizo hespérico se halla afectado por varios sistemas de fracturas que son importantes por su frequencia y caracteristicas. De estos el más importante es el sistema de fracturas conjugadas con direcciones NW-SE y NE-SW, de edad tardihercinica, que han seguido jugando durante el Mesozoico y el Terciario. Ligados a estas fracturas se encuentran con gran frecuencia sistemas de diaclasas distribuidos por todo el bloque hercmico ibérico. Estos accidentes han dado lugar a diques diabásicos, han favorecido la intrusión de granitos calcoalcalinos tardios y han producido cizallamientos y milonitizationes en las rocas hercínicas. Analizando la deformación del conjunto del Macizo Hespérico en función del juego de estas fracturas, puede suponerse como responsable de esta deformación una rotación relativa del zócalo hercínico ibérico respecto al francés.
Résumé Le Massif Hespérique se trouve affecté plusieurs systèmes fractures qui sont importants par leur fréquence et par leur caractère. Parmi eux le plus important est le système de fractures conjugées en direction NW-SE et NE-SW de l'époque tardihercynienne qui s'est prolongé pendant les époques mesozoïque et tertiaire. Très souvent ces fractures sont liées à des systèmes de joints sur toute l'étendue du bloc hercynien Ibérique. Ces accidents ont été la raison de la formation de diques diabasiques, ont favorisé l'intrusion de granites calcoalcalins tardifs et ont produit des cisaillements et des mylonitisations dans les roches hercyniennes.En analysant la déformation de l'ensemble du Massif Hespérique en fonction du jeu de ces fractures, ont peut supposer comme cause de cette déformation une rotation relative du socle hercynien Ibérique par rapport au Français.
(). , .: NW-SE NE-SW. , . . - , . , , , .

Für die Herausgabe dieser Arbeit in deutscher Sprache ist der Autor Herrn Dr. G.Nollau in aufrichtigem Dank verbunden.  相似文献   
487.
Many geophysical problems involve solving for the time derivative of the Laplacian of a streamfunction, particularly three-dimensional ocean circulation studies. This paper demonstrates the existence of two instabilities when relaxation methods are used to solve the resulting Poisson equation. One occurs as a result of over-relaxation. The other occurs even if under-relaxation is used, provided that there are variations in Coriolis parameter and that extrapolation provides an initial guess for the relaxation process. Both instabilities equilibrate with (erroneous) amplitudes determined by the degree of dissipation present. In the case of the latter instability, the amplitude can be six orders of magnitude larger than the error criterion for the relaxation; its structure resembles basic modes, which persist indefinitely. Some ways of circumventing the problem are discussed.  相似文献   
488.
Within a period of a few weeks toward the end of the Allerød Interstadial, the major Plinian eruption of the Laacher See volcano produced some 20 km3 of eruptiva, covering and preserving the late-glacial landscape in the German Central Rhineland over an area of more than 1000 km2. Correlation of terrestrial archives with the Greenland ice-core records and improved calibration of the radiocarbon timescale permit a precise, accurate age determination of the Laacher See event some 200 yr before the onset of the Younger Dryas cold episode. Carbonized trees and botanical macrofossils preserved by Laacher See Tephra permit detailed regional paleoenvironmental reconstruction and show that open woodland were typical for the cool and humid hemiboreal climatic conditions during the late Allerød. This woodland provided the habitat for a large variety of animal species, documented at both paleontological and Final Paleolithic archeological sites preserved below Laacher See deposits. Of special interest are numerous animal tracks intercalated in Middle Laacher See deposits at the south of the Neuwied Basin. This knowledge may help to evaluate possible supraregional impacts of this volcanic event on northern hemispheric environment and climate during the late Allerød.  相似文献   
489.
The terranes which outcrop to the north and south of the westward extension of the Jebha Fault show considerable differences in their nature and structural organisation. The pre-Oligocene basement is composed of synorogenic synclines filled with detrital deposits. In the North, the synclinal troughs are oriented North-South and filled with siliciclastic material. The synclinal structures on top of the western extension of the Jebha Fault are orientated N 140° and are also filled with the same material. South of this westward extension of the Jebha Fault, the orientation of the synclinal structures is north-south; they are filled with bioclastic calcareous material.The synorogenic synclines, situated on top of the Jebha-Arbaoua limit show characteristics of the detached sedimentary cover by their position and orientation. This cover formed the Arbaoua-Jebha lateral ramp during its movement towards the west. Developement of this ramp is explained by the crustal discontinuity inherited from the period of rifting.The resulting palaeomorphological relief allows the difference between the zones situated at each part of the limit to be understood, in particular, it permits the understanding of the differences in the detrital material, in its role as a barrier opposing the arrival of bioclastic calcareous material from the African margin (situated southwards of the limit).The Arbaoua-Jebha lateral ramp appears as a major and original feature of the Rift chain.  相似文献   
490.
The most widely adopted approach for estimating the wave-induced fatigue damage of the tubular joints of offshore structures relies on a wave scatter diagram to characterise the wave environment. This paper shows that the commonly adopted approach to compiling the wave scatter diagram and the subsequent selection of characteristic individual wave height and period pairs for analysis purposes will almost invariably lead to bias in the estimation of fatigue lives. A statistical argument is developed to show the sources and nature of this bias. This is supported by fatigue calculations based on individual wave data collected in the northern North Sea. It is found that the current practice in most cases introduces a substantial conservative bias into estimated fatigue lives. An alternative procedure for producing wave scatter diagrams and selecting wave heights and periods for subsequent finite-element analysis is presented which should lead to unbiased (or at least much less biased) estimates of fatigue lives.  相似文献   
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