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1.
The conodont colour alteration index (CAI) is potentially valuable in thermal history investigations, but there are problems in its use. An approach is proposed herein to overcome the problem of establishing reliable geothermal palaeogradients in areas with burial metamorphism, based on knowledge of the stratigraphic succession thickness, the boundaries of chronostratigraphic units, and the age and duration of deformation episodes. Although also problematical that a correlation between CAI and Kübler index (KI) values does not exist, there are areas where CAI and KI values are notably consistent for the anchizone boundaries; however, anomalous correlations between CAI and KI values have also been found in some areas. A CAI anchizone (or ancaizone) is defined in order to enable CAI values to be used independently to establish a metamorphic zonation. 相似文献
2.
Richard J. Lisle Jesús Aller Fernando Bastida Nilo C. Bobillo-Ares Noel C. Toimil 《地学学报》2009,21(1):14-20
Neutral surface folding is a significant contributor to fold development. This mechanism produces contrasting strains in the inner and outer arcs of the folded layer that arise from principal stress orientations that are approximately parallel and perpendicular to the layer. We demonstrate that such stress patterns imply significant gradients of mean stress across the folding layer, being more tensional on the outer arc and more compressive in the inner arc. This could pump fluids towards the outer arc during folding and result in heterogeneous volume changes. We conclude that the neutral surface folding model should be adapted to accommodate volumetric strains, in order to explain dilatational structures (e.g. open fractures, veining) on the extrados and volume-loss structures (e.g. pressure solution seams, stylolitic cleavages) on the intrados. This dilatation has economic implications as it allows prediction of sites of mineralization and zones of secondary permeability in fold-related hydrocarbon traps. 相似文献
3.
Conodont colour alteration index (CAI) data in Upper Ordovician rocks from several areas of the Variscan domain in the Iberian Peninsula indicate conditions ranging from diagenesis to low-grade metamorphism. In most of the areas, where studies using other indicators, such as illite crystallinity (IC) or where vitrinite reflectance are lacking, the CAI method has permitted a preliminary estimation of the metamorphic grade. In the Almadén syncline (Central-Iberian Zone), where IC studies are available, the thermal conditions inferred from CAI data agree with those obtained by the IC method. In the Puertollano–Almuradiel syncline, the thermal interval obtained primarily from fluid inclusions (270–370°C) overlaps considerably with that obtained from CAI data (180–340°C). In general, cleavage in rocks is present in anchizonal or epizonal conditions, whereas in diagenetic conditions with CAI 2.5, cleavage is scarce. The conodont texture changes with increasing metamorphism, and apatite recrystallisation appears in general with CAI 5. Variation of CAI values within a single sample and/or within short stratigraphic distances observed at several localities is due to hydrothermal activity. 相似文献
4.
Denuncio P Bastida R Dassis M Giardino G Gerpe M Rodríguez D 《Marine pollution bulletin》2011,62(8):1836-1841
Plastic debris (PD) ingestion was examined in 106 Franciscana dolphins (Pontoporia blainvillei) incidentally captured in artisanal fisheries of the northern coast of Argentina. Twenty-eight percent of the dolphins presented PD in their stomach, but no ulcerations or obstructions were recorded in the digestive tracts. PD ingestion was more frequent in estuarine (34.6%) than in marine (19.2%) environments, but the type of debris was similar. Packaging debris (cellophane, bags, and bands) was found in 64.3% of the dolphins, with a lesser proportion (35.7%) ingesting fishery gear fragments (monofilament lines, ropes, and nets) or of unknown sources (25.0%). PD ingestion correlated with ontogenetic changes in feeding regimes, reaching maximum values in recently weaned dolphins. Because a simultaneous increase in gillnet entanglement and the bioaccumulation of heavy metals take place at this stage, the first months after trophic independence should be considered as a key phase for the conservation of Franciscana dolphin stocks in northern Argentina. 相似文献
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Two-dimensional analysis of folded surfaces oblique to the mechanical layering can shed light on the kinematic mechanisms that operated during the development of folds. A new version of the program ‘FoldModeler’, developed in the Mathematica™ environment, is used to obtain the deformed configuration of an initial pattern of oblique surfaces deformed by any combination of the most common kinematic folding mechanisms: flexural flow, tangential longitudinal strain, with or without area change and heterogeneous simple shear. The layer can also undergo any form of homogeneous strain at any moment of the folding process. The outputs of the program provide complete information about the strain distribution in the folded layer that includes graphs of the angle between the oblique surfaces as a function of the inclination of the layering through the fold. These graphs can be very useful to discriminate between the mechanisms that operate in the development of natural folds, and they have been obtained and discussed for the most common combinations of strain patterns. The program is applied to obtain theoretical folds that give a good fit of some natural examples of folded oblique surfaces. 相似文献
7.
Influence of habitat and host morphology on macrofaunal assemblages associated with the sponge Halichondria melanadocia in an estuarine system of the southern Gulf of Mexico 
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下载免费PDF全文 Sponges are inhabited by a wide variety of organisms and have been regarded as one of the richest biotopes in tropical seas. The aim of this study was to assess the influence of the host morphology and selected environmental conditions on macrofaunal assemblages associated with the sponge Halichondria melanadocia in an estuarine system of the southern Gulf of Mexico. This sponge exhibits different growth forms when it inhabits mangrove prop roots of Rhizophora mangle (thickly encrusting form) and adjacent seagrass beds (massive, amorphous or ramose form). From a total of 50 sponge specimens collected in each habitat, a total of eight taxa (of epi‐ and endobionts) was found associated with this sponge, with polychaetes, echinoderms and crustaceans the most abundant groups. In both morphotypes (thickly‐encrusting and massive‐ramose forms), taxon richness was positively related to sponge volume and oscular diameter. The overall mean abundance of associated fauna was also positively related to sponge volume in both morphotypes and with the oscular diameter (in the seagrass morphotype only). These findings suggest that H. melanadocia constitutes an important microhabitat for a wide variety of fauna, independent of its morphology and habitat type. However, when comparing the two morphotypes, the mangrove individuals, despite having smaller sizes, smaller oscular diameter and less structural complexity, displayed an overall mean abundance of fauna 17 times higher [0.36 ± 0.18 individuals (ind.)·ml·sponge?1] than that recorded in the seagrass individuals (0.021 ± 0.01 ind.·ml·sponge?1). There were also marked differences in the proportions of the major taxonomic groups; in fact, some of them were found exclusively in one morphotype. The variability recorded in the composition and abundance of associated fauna between the morphotypes seems to be influenced by differences in sponge morphology, environmental conditions (e.g. sedimentation rate and light intensity), substrate orientation and the fauna inhabiting the surrounding area. 相似文献
8.
The benthic community structure of the southernmost part of Samborombon Bay was studied at 13 stations for 2 yr. A detailed examination of the zoobenthos, using similarity indices and cluster analysis, is presented. The area was characterized by low diversity. The Shannon-Weaver information diversity index H′ showed low values of 0.28–1.60. The brackish part of the area was dominated by two opportunistic polychaetes species,Heteromastus similis andLaeonereis acuta. The only species found in the transitional marine zone was the bivalveMesodesma mactroides. Variability in benthic community has been interpreted in terms of spatial and temporal changes. Depending on the area, abundance appeared to be controlled by pollution and intense predation during the warmer months. The observed gradient in species composition and dominance of these benthic communities are compared with some other North and South Atlantic estuaries. 相似文献
9.
This paper analyses a major shear zone from the Iberian Hercynian belt which forms the basal thrust of the Mondoñedo Nappe. The shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle-ductile deformation in greenschists facies. Folds in the shear zone are asymmetric, very tight, 1C or similar class and frequently developing sheath geometries. The sheath folds originated by non-coaxial flow superimposed on earlier irregularities. The fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non-mylonitic tectonites. c-axis fabrics vary across the shear zone, but show a dominant monoclinic symmetry. The blastomylonitic rocks include the fabrics representing the highest temperatures. The main foliation of the schists results from flattening of an earlier foliation, recording occasional microfolds. The use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen. 相似文献
10.
F. Bastida C. Brime S. García-López G. N. Sarmiento 《International Journal of Earth Sciences》1999,88(1):38-48
The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian
Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC).
Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions
show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal
thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous
rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values
are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI
indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably
developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed
zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of
tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than
in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role
of the burial on the metamorphism.
Received: 11 May 1998 / Accepted: 19 January 1999 相似文献