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  • 1
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    Non-Volcanic Rifting of Continental Margins : A Comparison of Evidence from Land and Sea (2001)
    London : The Geological Society
    Details
    Description / Table of Contents: Non-volcanic continental margins may form up to 300f all present-day passive margins, and remnants of them are preserved in mountain belts. The papers in this volume demonstrate the benefits of integrating offshore and onshore studies, and illustrate the range of information obtained at different scales when comparing evidence from land and sea. Data sets collected across a range of spatial scales are evaluated: thin sections, cores, outcrops, seismic reflection profiles, and other geophysical data. The outcrop scale is crucial because it enables the spatial gulf to be bridged between DSDP and ODP cores and marine seismic data. There is also the problem that basins on land and beneath the sea inevitably have had different post-rift histories resulting in their contrasting present-day elevation. In mountain belts, portions of continental margins and oceanic crust are superbly exposed, but dismembered by subsequent compressional tectonics. Off present-day passive margins, extensional features have only been slightly deformed, if at all, by compressional movements, but are buried beneath significant thicknesses of post-rift sediments and so can only be sampled by ocean drilling at a small number of points. The first paper reviews the synergies that have occurred between investigations of the eastern North Atlantic non-volcanic margins and remnants of similar Mesozoic margins preserved in the Alps, and some later papers return to this theme. However, papers describing margins from other parts of the world show that it may be premature to use models based on the Atlantic and the Alps as the paradigm for all non-volcanic margins. The following 25 papers in the book are grouped under the following headings: (1) Margin overviews; (2) Exhumed crust and mantle; (3) Tectonics and stratigraphy; (4)Numerical models of extension and magmatism.
    Pages: Online-Ressource (585 Seiten)
    ISBN: 1862390916
    URL: http://sp.lyellcollection.org/content/187/1.toc
    Language: English
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    E-BOOK
  • 2
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    Unknown
    A lithospheric seismic refraction profile in the western North Atlantic Ocean (1983)
    Oxford University Press
    Details
    Publication Date: 1983-10-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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    PAPER CURRENT
  • 3
    Electronic Resource
    Electronic Resource
    Evolution of magma-poor continental margins from rifting to seafloor spreading (2001)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 413 (2001), S. 150-154 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The rifting of continents involves faulting (tectonism) and magmatism, which reflect the strain-rate and temperature dependent processes of solid–state deformation and decompression melting within the Earth. Most models of this rifting have treated tectonism and magmatism separately, and ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/35093085
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  • 4
    Electronic Resource
    Electronic Resource
    Cǒcte d'Ivoire-Ghana margin: seismic imaging of passive rifted crust adjacent to a transform continental margin (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: During May 1990 and January-February 1991, an extensive geophysical data set was collected over the Côte d'Ivoire-Ghana continental margin, located along the equatorial coast of West Africa. The Ghana margin is a transform continental margin running subparallel to the Romanche Fracture Zone and its associated marginal ridge—the Côte d'Ivoire-Ghana Ridge. From this data set, an explosive refraction line running ∼ 150 km, ENE-WSW between 3°55′N, 3°21′W and 4°23′N, 2°4′W, has been modelled together with wide-angle airgun profiles, and seismic reflection and gravity data. This study is centred on the Côte d'Ivoire Basin located just to the north of the Côte d'Ivoire-Ghana Ridge, where bathymetric data suggest that a component of normal rifting occurred, rather than the transform motion observed along the majority of the equatorial West African margin.Traveltime and amplitude modelling of the ocean-bottom seismometer data shows that the continental Moho beneath the margin rises in an oceanward direction, from ∼ 24 km below sea level to ∼ 17 km. In the centre of the line where the crust thins most rapidly, there exists a region of anomalously high velocity at the base of the crust, reaching some 8 km in thickness. This higher-velocity region is thought to represent an area of localized underplating related to rifting. Modelling of marine gravity data, collected coincident with the seismic line, has been used to test the best-fitting seismic model. This modelling has shown that the observed free-air anomaly is dominated by the effects of crustal thickness, and that a region of higher density is required at the base of the crust to fit the observed data. This higher-density region is consistent in size and location with the high velocities required to fit the seismic data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb06023.x
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  • 5
    Electronic Resource
    Electronic Resource
    The ocean–continent boundary off the western continental margin of Iberia—II. Crustal structure in the Tagus Abyssal Plain (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 109 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An 80 km long reversed seismic refraction line (Line 5) was shot over the Tagus Abyssal Plain off Portugal. The main P-wave reflected and refracted phases were modelled both for traveltime and amplitude. The resulting P-wave velocity/depth model has the following features: (a) an extremely thin crust of about 2 km; (b) the absence of oceanic layer 3; and (c) very low upper mantle velocities between 7.6 and 7.9 km s-1. This very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at Atlantic fracture zones, and in particular to the structures found in profiles shot along the transform valley and near ridge-transform intersections. A magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of Line 5.As an alternative to the fracture zone hypothesis we show that if the ocean–continent transition in the Tagus Abyssal Plain is located at about 11°30'W, in a symmetric position with respect to the ocean–continent transition in the conjugate South Newfoundland Basin, then magnetic anomalies can be modelled simply by assuming sea-floor spreading west of 11°45'W at 10 mm yr-1 beginning at M11 time (133 Myr BP), and blocks of rifted continental crust to the east. The location of the proposed ocean–continent transition in the Tagus Abyssal Plain is marked by a well-defined N–S linear magnetic anomaly which is adjacent to the oldest sea-floor spreading block. East of the proposed ocean–continent transition there is an increase in the depth to basement similar to that found east of the ocean–continent transition in the Iberia Abyssal Plain and elsewhere. This model also allows us to explain why Purdy's (1975) seismic refraction line A–AR in the Tagus Abyssal Plain cannot be interpreted as a conventional reversed pair because most of Line A was shot over the ocean–continent transition zone and most of Line AR over thinned continental crust.Remarkably similar velocity/depth structures to that under Line 5 are found close to the ocean–continent transition zone off the whole of western Iberia, in areas which show no clear evidence of fracture zones. Therefore it appears more likely that the seismic structure of Line 5 is due to its proximity to the ocean–continent transition than to a local association with a fracture zone and further, that its structure is typical of this transition off the western margin of Iberia. We also suspect that the low upper mantle velocities associated with the ocean–continent transition indicate the widespread occurrence of serpentinized peridotite.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00082.x
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  • 6
    Electronic Resource
    Electronic Resource
    The seismic velocity structure of some NE Atlantic continental rise sediments; a lithification index? (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 101 (1990), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Two separate sets of experiments with digital ocean-bottom seismographs (DOBS) and airguns, on continental rise areas off Madeira and west of Portugal, produced en echelon second arrivals from the sediment layer on record sections. Traveltime and synthetic seismograrn modelling indicate that the arrivals represent multiply-reflected refracted phases which have undergone reflection within the sediment layer itself. Further, although the P-wave contrast at the intrasediment reflecting horizon is relatively small, the modelling indicates a large downward increase in S-wave velocity from 100–250 m s−1 (Poisson's ratio of at least 0.42) to about 1200 m s−1 (Poisson's ratio of about 0.25). A reflection event can usually be found on reflection profiles along the refraction lines at almost exactly the same ‘depth’ as the intrasediment reflector. In one case such an event can be traced to a nearby Deep Sea Drilling Project (DSDP) borehole where it is associated with the transition from ooze to chalk. This, and other circumstantial evidence, suggests that the intrasediment reflector marks an important increase in lithification within the sediment layer. If so it means that, in future, straightforward OBS experiments may be used to measure the depth of this increase without resorting to the drill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb06575.x
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  • 7
    Electronic Resource
    Electronic Resource
    The ocean–continent boundary off the western continental margin of Iberia—I. Crustal structure at 40°30'N (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 103 (1990), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The western continental margin of the Iberian peninsula has the characteristics of a rifted non-volcanic margin with half-graben and tilted fault blocks seen in several places. The ocean–continent boundary (OCB) is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed, as at many similar margins worldwide. It is particularly useful to locate the OCB off western Iberia in order to constrain the pre-rift fit of North America to Iberia and, by implication, the shape of the proto-Bay of Biscay. The fit is only marginally constrained by sea-floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the OCB and it is even possible that all older oceanic crust was created during the Cretaceous constant polarity interval. The best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure. Therefore in 1986 a series of seismic refraction lines was shot parallel to, and normal to, the continental margin. These lines enabled us to bracket the location of the OCB. A further constraint on the location was obtained by modelling an east–west magnetic profile which included the enigmatic J-anomaly. This anomaly can be explained as either just pre-anomaly MO or as part of the Cretaceous constant polarity interval, depending on whether spreading began about 127 or after 118 Myr ago, respectively. The evidence favours the former explanation. Lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles. This indicated a new fracture zone at 41°15'N which offsets the OCB. A few key reflection profiles also suggest that the OCB can be identified by an abrupt landward step-down in acoustic basement. We conclude that the OCB in the eastern Iberia Abyssal Plain lies between 12°10’ and 12°30'W and has a trend just east of north. This westerly location is consistent with recent estimates of the location of the OCB off the Grand Banks but brings into question the proposed location at about 11°W of the OCB in the Tagus Abyssal Plain.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb01788.x
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  • 8
    Electronic Resource
    Electronic Resource
    Marine Magnetic Data Processing In Equatorial Regions Off Ghana (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Total field magnetic values recorded during a survey by RRS Charles Darwin off Ghana yielded large track-crossover errors of up to 120 nT (RMS value of 58.7 nT), which masked the weak magnetic anomalies in this equatorial region. the heading effect of the ship's magnetic field and strong diurnal variation in the Earth's field are likely causes of the errors. A heading effect experiment shows differences of up to 30 nT for Charles Darwin on different headings, which have been corrected for. the diurnal variation has been calculated by using the magnetic field observations themselves, because observatories are either too distant or were inoperative at the time of the survey. A method that uses the anomalies corrected for heading effect and differences at track crossovers was found to produce an acceptable curve, with an amplitude of 120 nT and a shape similar to that of equatorial observatories. Fully corrected anomalies have crossover errors of up to only 40 nT with an RMS value of 17.5 nT. These anomalies reveal a linear magnetic anomaly low along the continental slope off Ghana.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb06539.x
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  • 9
    Electronic Resource
    Electronic Resource
    Synthesis of the crustal structure of the transform continental margin off Ghana, northern Gulf of Guinea (1997)
    Springer
    Geo-marine letters 17 (1997), S. 12-20 
    Details
    ISSN: 1432-1157
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  Results of a detailed geophysical transect across the transform continental margin off Ghana, at the eastern end of the Romanche Fracture Zone in the Equatorial Atlantic, are presented. Seismic refraction, single-channel seismic reflection, gravity, and magnetic data were collected, and seismic, gravity, and magnetic models along the transect are shown. The 6- to 11-km-wide ocean–continent transition (OCT) is characterized by a high-velocity, high-density, high-magnetization crustal zone. The models show no evidence for any underplating of the continental crust adjacent to the margin but minor melting and intrusion of the continental crust may have occurred in the vicinity of the OCT.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00007202
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  • 10
    Electronic Resource
    Electronic Resource
    Median Valley Refraction Line, Mid-Atlantic Ridge at 37° N (1973)
    [s.l.] : Nature Publishing Group
    Nature 246 (1973), S. 297-299 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Fig. 1 Bathymetric chart of the median valley in part of the FAMOUS area showing the seismic refraction lines reported here; contours in uncorrected fathoms (from an unpublished chart produced by the Woods Hole Oceanographic Institution, courtesy of J. D. Phillips). The receiver position for each ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/246297a0
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