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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 119 (1994), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We present results that constrain the crustal structure across a typical ‘non-volcanic’ rifted continental margin, the Goban Spur in the SE Atlantic. Traveltime and amplitude modelling of wide-angle ocean-bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop. the crustal-velocity structure, the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin.The seismic-velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust. the adjacent oceanic crust is abnormally thin (5.4 km). This is explained most readily by accretion at a very slow spreading centre (approx 11 mm yr−1 full rate), although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere, together with minor igneous intrusion into, and surface flows onto, the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent-ocean transition.
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  • 2
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 252 (1974), S. 676-679 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Unfortunately, analysis of variation of the gravity field gives little information about structures in isostatic equilibrium with small lateral variations. Beyond the fact that the underlying volumes are of equal weight, the only information it gives comes from the influence of the lateral ...
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  • 3
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 252 (1974), S. 464-465 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Fig. 1 Seismic profile across the Chain Fracture Zone south of Togo (location on Fig. 2). Note the difference in level of the acoustical basement under a thick sedimentary cover. The free air gravity anomaly is of the order of 40 mGal. The arrow indicates the theoretical location used for the ...
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 233 (1971), S. 257-258 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] We wish to show how these two hypothetical rotations agree with the magnetic data of Williams and McKenzie6. Fig. 1 shows a rotation of the Iberian peninsula (contour at the 1,000 fathom isobath) by 23 about the first pole. Intermediary rotated positions every 5 are also shown. Similarly, Fig. 2 ...
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  • 5
    ISSN: 1573-0581
    Keywords: Pasisar ; deep-towed seismic device ; high resolution seismic
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The Pasisar seismic acquisition system combines a source at the sea surface and a deep-towed single channel streamer. This unconventional device geometry reduces the width of the first Fresnel zone which increases the lateral resolution. However, the device acquisition geometry generates artifacts on seismic profiles and induces large incidence angles of the seismic signal. A specific processing sequence must be applied to the data to obtain a readable seismic section. Penetration of the seismic signal depends on the energy of the signal reaching the seafloor and on its incidence angle. Because of smaller source energy, 800 Joules Sparker data cannot be acquired in water depth larger than 1500 m for example, whereas there is no depth limit for the use of this system with air gun sources. Differential acoustic absorption of seismic frequencies (below 1000 Hz) in the water column is negligible when compared with wave fronts expansion. Thus, the horizontal resolution of any seismic system strongly depends on the frequency spectrum of the seismic source and on the travel distances. Pasisar and conventional seismic profiles being usually simultaneously recorded, we illustrate the interest of using a hybrid seismic device by comparing horizontal resolutions as well as signal-to-noise ratio obtained with both the Pasisar and conventional systems. In addition, by carefully picking time arrivals of a reflection on simultaneously recorded surface and deep-towed seismic records, it is possible to estimate the average interval seismic velocity. We present the simplified example of a horizontal reflector.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Marine geophysical researches 20 (1998), S. 251-252 
    ISSN: 1573-0581
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Marine geophysical researches 8 (1986), S. 243-264 
    ISSN: 1573-0581
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Spectral analysis techniques have been applied to data sets of gravity and topography selected across fracture zones with large offset in the Equatorial and Central Atlantic Ocean and processed independently for each fracture zone. Three simple compensation models, two in local isostatic equilibrium (Airy I and II) and one in regional equilibrium (Plate model) have been tested. It is found that the free-air anomalies are primarily controlled by the topography and its isostatic response. For short wavelengths, admittance can be explained either by the effect of uncompensated sea floor topography with high density basement or by the effect of uncompensated sea floor topography with normal oceanic basement density but accompanied by a crust of constant thickness. For intermediate wavelengths, admittance for the Romanche fracture zone agrees best with a local isostatic model in which compensation is achieved by a less dense material in the upper mantle. No such evidence exists for the Vema and Chain fracture zones for which topography could just as well be regionally supported by an elastic plate 3 to 10 km thick. For longer wavelengths, the admittances computed for the three fracture zones are compatible with a thermal compensation of the topography.
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  • 8
    ISSN: 1573-0581
    Keywords: Arc-arc collision ; backarc extension ; along-axis propagating trench ; trench segmentation ; Gaguaridge ; Luzon arc ; Taiwan ; Okinawa trough
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Located between the Okinawa trough (OT) backarc basin and the collisional zone in Taiwan, the southernmost Ryukyu subduction zone is investigated. This area, including the southwestern portions of the OT and Ryukyu island arc (RA) and located west of 123.5° E, is named the “Taiwan-Ryukyu fault zone” (TRFZ). West of 123.5° E, the OT displays NNW-SSE structural trends which are different in direction from the ENE-WSW trending pattern of the rest of the OT. Using joint analysis of bathymetric, magnetic, gravity and earthquake data, three major discontinuities, that we interpret as right-lateral strike-slip faults (Faults A, B and C), have been identified. These faults could represent major decouplings in the southern portion of the Ryukyu subduction zone: each decoupling results in a decrease of the horizontal stress on the portion of the RA located on the eastern side of the corresponding fault, which allows the extension of the eastern side of OT to proceed more freely. We demonstrate that the 30° clockwise bending of the southwestern RA and the consecutive faulting in the TRFZ are mainly due to the collision of the Luzon arc with the former RA. After the formation of Fault C, the counterclockwise rotated portion of the ancient RA located west of the Luzon arc was more parallel to the Luzon arc. This configuration should have increased the contact surface and friction between the Luzon arc and the ancient RA, which could have reduced the northward subduction of the Luzon are. Thus, the westward component of the compressive stress from the collision of the Luzon arc should become predominant in the collisional system resulting in the uplift of Taiwan. Presently, because the most active collision of the Luzon arc has migrated to the central Taiwan (at about 23° N; 121.2° E), the southwestern OT has resumed its extension. In addition, the later resistent subduction of the Gagua ridge could have reactivated the pre-existing faults A and B at 1 M.y. ago and present, respectively. From 9 to 4 M.y., a large portion of the Gagua ridge probably collided with the southwestern RA. Because of its large buoyancy, this portion of the ridge resisted to subduct beneath the Okinawa platelet. As a result, we suggest that a large exotic terrane, named the Gagua terrane, was emplaced on the inner side of the present Ryukyu trench. Since that period, the southwestern portion of the Ryukyu trench was segmented into two parallel branches separated by the Gagua ridge: the eastern segment propagated westward along the trench axis while the western segment of the trench retreated along the trench axis.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Marine geophysical researches 14 (1992), S. 88-92 
    ISSN: 1573-0581
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
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  • 10
    facet.materialart.
    Unknown
    In:  Geological Society Special Publication 369: 429-452.
    Publication Date: 2013-07-24
    Description: Two different types of ‘transitional lithosphere’ have been documented along magma-poor rifted margins. One consists of apparently subcontinental mantle that has been exhumed, brittlely deformed, and serpentinized during late stages of rifting. A second is thinned (〈10 km) continental crust, which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth-dependent extension. In both cases, it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter, and thus that transitional lithosphere has relatively limited width. Here three representative cases of transitional lithosphere are examined: one in the Newfoundland–Iberia rift and one at Goban Spur (both exhumed mantle), and one off the Angola–Congo margin (thin continental crust flanked seaward by apparently exhumed lower continental crust±exhumed mantle). Considering the geological and geophysical evidence, it appears that depth-dependent extension (riftward flow of weak lower continental crust and/or upper mantle) may be a common phenomenon on magma-poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed. Transitional lithosphere in this wide zone may consist of subcontinental mantle, lower continental crust or some combination thereof, depending on the strength profile of the pre-rift continental lithosphere. Transitional lithosphere ceases to be emplaced (i.e. ‘final break-up’ occurs) only when emplacement of heat and melt from the rising asthenosphere becomes dominant over lateral flow of the weak lower lithosphere. This model implies a two-stage break-up: first, the rupture of the brittle continental crust; and, second, the eventual separation of the ductile subcontinental lithosphere which is coincident with emplacement of normal oceanic crust. Well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpentinized, exhumed mantle, and such anomalies therefore are not diagnostic of oceanic crust. Where present, the anomalies can be helpful in interpreting and dating the rifting history.
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