ALBERT

All Library Books, journals and Electronic Records Telegrafenberg

feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    ISSN: 1437-3262
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Description / Table of Contents: Abstract Seismic reflection data from three areas of the Middle America Trench provide insights into the behavior of the decollement that separates subducted and offscraped sediment. The range of responses observed along this single subduction system provides clues as to how the decollement forms and is influenced by local conditions. The location of the decollement and whether or not the subducting basement topography influences the decollement are apparently controlled by the type and thickness of sediment in the trench. Where subducting basement topography and oceanic plate hemipelagic sediments are buried by sandy terrigenous turbidites, such as in the trench axis off Mexico, the decollement is localized near the base of the turbidite section. Subducting normal fault blocks in the oceanic crust control thrust ramps, and hanging wall anticlines form above the ramps. In regions of the trench where coarse terrigenous sediment is thin or absent, the decollement is localized within the incoming sediment section. Where muddy trench turbidites bury subducting topography (e.g. off Guatemala), the decollement is approximately 100 m deep and is little affected by the underlying subducting topography. The lower 200–300 meters of trench sediment and all of the pelagic sediment are subducted. Where there is no trench sediment overlying carbonate-rich oceanic plate sediments (e.g. off Costa Rica), the decollement is located within the subducting plate sediment section. The decollement is localized at a single stratigraphic level and rides up and over subducting horst blocks.
    Abstract: Résumé Des prospections par sismique-réflexion, effectuées dans trois régions de la fosse d'Amérique Centrale, éclairent le comportement du décollement qui sépare les sédiments subductés des sédiments «raclés». Les donnés recueillies le long de ce système de subduction simple fournissent des indications sur la manière dont le décollement prend naissance et est influencé par les conditions locales. L'emplacement du décollement et le fait qu'il est, ou non, influencé par la topographie du substrat en subduction dépendent, semble-t-il, de la nature et de l'épaisseur des sédiments de la fosse. Lorsque, dans la plaque océanique en subduction, la topographie du socle et les sédiments hémipélagiques surincombants sont enfouis sous des turbidites terrigènes arénacées, comme c'est le cas au large du Mexique, le décollement se produit près de la base de la série turbiditique. La subduction de blocs découpés par des failles normales dans la croûte océanique détermine des rampes de charriage au-dessus desquelles prennent naissance des structures anticlinales. Dans les parties de la fosse qui sont pauvres en sédiments terrigènes grossiers ou qui en sont dépourvues, le décollement est situé à l'intérieur de la série sédimentaire. Lorsque le relief de la plaque en subduction est enfoui sous des turbidites boueuses (p. ex. au large du Guatemala), le décollement se situe à 100 m sous la surface supérieure des sédiments et est peu affecté par la topographie du substrat subducté. La subduction affecte alors la partie inférieure (200 à 300 m) des sédiments de la fosse, ainsi que l'entièreté des sédiments pélagiques. Lorsqu'il n'y a pas de sédiments terrigènes recouvrant les dépôts carbonates de la plaque océanique (p. ex. au large de Costa Rica), le décollement se situe au sein de ces derniers; il correspond à un niveau stratigraphique et contourne par en-haut les blocs en horst de la plaque descendante.
    Notes: Zusammenfassung Reflektionsseismische Daten aus drei Bereichen des Mittel-Amerika-Grabens geben Einblick in das Verhalten entlang einer Abscherung in der subduziertes und »abgeschabtes« Sediment voneinander getrennt werden. Die Daten, die entlang dieses Subduktionssystems erhalten wurden, liefern Anhaltspunkte darüber, wie sich die Abscherung bildet und wie sie von lokalen Bedingungen beeinflußt wird. Der Entstehungsort der Abscherung wird, unabhängig davon, ob die abtauchende Basementtopographie die Abscherung beeinflußt oder nicht, anscheinend von Typ und Dicke der Grabensedimente kontrolliert. Dort, wo das abtauchende Basement und die hemipelagischen Sedimente der ozeanischen Platte von sandigen, terrigenen Turbiditen überlagert werden, wie im Grabenabschnitt vor Mexico, liegt die Abscherung nahe der Basis des Turbiditprofiles. Abtauchende, normal gestörte Schollenblöcke der ozeanischen Kruste kontrollieren die Bildung von Überschiebungsrampen unter Bildung von Antiklinalen oberhalb dieser Rampen. Wo schlammige Grabenturbidite die subduzierende Topographie überdecken (z.B. vor Guatemala), liegt die Abscherung etwa 100 m unter der Sedimentoberfläche und wird kaum von der unterlagernden, abtauchenden Topographie beeinflußt. Die unteren 200–300 m der Grabensedimente sowie die gesamten pelagischen Ablagerungen werden subduziert. Wo karbonatreiche Sedimente der ozeanischen Platte nicht von Grabensedimenten überlagert werden (z.B. vor Costa Rica) liegt die Abscherung innerhalb der Sedimentsäule der abtauchenden Platte. Die Abscherung bewegt sich in einem einzigen stratigraphischen Niveau und gleitet dabei auf und über abtauchende Horstschollen.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 2
    ISSN: 1573-0581
    Keywords: Costa Rica ; Kirchhoff ; migration ; OBS ; seismic reflection ; seismic refraction ; seismogenic zone ; subduction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 3
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 211 (1966), S. 1315-1317 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Every supraliminal stimulus presented to the eye affects its adaptation level, as was particularly emphasized by DeLange1. While the stimulus level can be use 〈l to control the adaptation level1, so also can the level of the surrounding intensity2. Furthermore, it is possible to determine the ...
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Biological cybernetics 10 (1972), S. 181-189 
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract The Mach bands are directly related to the size and the shape of on-center off-surround neural units in human vision. The effects of various stimulus parameters were studied on both bright and dark bands of equal plateau intensities. At low overall intensities, the dark band increases markedly in width, while the bright band does not. However, the bandwidth is more affected by the brightness slope, than by the plateau intensity per se. In this case, both bands vary approximately linearly and inversely with the log of the slope. The bright bands are slightly wider (4′) than the dark bands, for matched intensities. Both bands almost double in width with only a ±30′ para-foveal fixation. Optical blur enlarges the bands as predicted from the spread function. A comparable enlarging effect found with pupil size increase is not so readily understood. The apparent centers of the bright bands are positioned significantly more asymmetrically between the two edges than are the dark band centers. Eccentric neural units are considered as possible explanations for some of these non-linearities.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 5
    Publication Date: 2017-05-17
    Description: Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to 〉90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.
    Type: Article , PeerReviewed
    Format: text
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 6
    Publication Date: 2017-04-10
    Description: The interrelation between deformation styles and behavior of fluids in accretionary prisms is under debate, particularly the possibility that overpressuring within the basal decollement may enable mechanical decoupling of the prism from the subducting material. Anisotropy of magnetic susceptibility (AMS) data from sediments spanning the basal decollement of the Barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is markedly overpressured. In the accretionary prism, above the decollement, the minimum AMS axes are subhorizontal and oriented nearly east-west, whereas the maximum AMS axes are oriented nearly north-south and shallowly inclined. At the top of the decollement, the minimum AMS axes orientations abruptly change to nearly vertical; this orientation is maintained throughout the decollement and in the underthrust sediments below. The AMS orientations in the prism sediments above the decollement are consistent with lateral shortening due to regional tectonic stress, as the minimum axes generally parallel the convergence vector of the subducting South American plate and the maximum axes are trench-parallel. Because the orientations of the AMS axes in deformed sediments usually parallel the orientations of the principal strains, the AMS results indicate that the incremental strain state in the Barbados prism is one dominated by subhorizontal shortening. In contrast, the AMS axes within and below the decollement are consistent with a strain state dominated by vertical shortening (compaction). This abrupt change in AMS orientations at the top of the decollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sediments from the underthrust sediments. The decoupling horizon occurs at the top of the decollement zone, coinciding with the location of flowing, high-pressure fluids.
    Type: Article , PeerReviewed
    Format: text
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 7
    Publication Date: 2017-06-15
    Description: Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to 〉90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.
    Type: Article , PeerReviewed
    Format: text
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 8
    Publication Date: 2016-04-26
    Description: Anomalous reflections in marine seismic reflection data from continental slopes are often correlated with the base of gas hydrated sedimentary rocks. Examination of University of Texas Marine Science Institute reflection data reveals the possible presence of such gas hydrates along the east coast of the United States, the western Gulf of Mexico, the coasts of northern Colombia and northern Panama, and along the Pacific side of Central America in areas extending from Panama to near Acapulco, Mexico. Suspected hydrates are present in water depths of 700 to 4,400 m and extend from 100 to 1,100 m subbottom. Geometric relations, reflection coefficients, reflection polarity, and pressure-temperature relations all support the identification of the anomalous reflections as the base of gas hydrated sediments. In most places, gas hydrate association is related to structural anomalies (anticlines, dipping strata), which may allow gas to concentrate and migrate updip into pressure and temperature conditions suitable for hydrate formation. The gas hydrate boundary can be used to estimate thermal gradients. In general, thermal gradients estimated from the gas hydrate phase boundary are higher than reported thermal gradients measured by conventional means.
    Type: Article , PeerReviewed
    Format: text
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 9
    Publication Date: 1982-01-01
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 10
    Publication Date: 2000-01-01
    Print ISSN: 0096-3941
    Electronic ISSN: 2324-9250
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...