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the Mid-Atlantic Ridge (Original)

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Author Topic: the Mid-Atlantic Ridge (Original)  (Read 9645 times)
Carolyn Silver
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« Reply #60 on: July 28, 2008, 11:40:53 pm »


Figure 9. Sea-level changes for six continents. For each time interval, the sea-level elevations for the various continents differ widely, highlighting the importance of vertical tectonic movements on a regional and continental scale. (Reprinted with permission from Harrison et al. [9]. Copyright by the American Geophysical Union.)

    Van Andel asserts that 'plates' rise or fall by no more than a few hundred meters -- this being the maximum depth of most 'epicontinental' seas. However, this overlooks an elementary fact: huge thicknesses of sediments were often deposited during marine incursions, often requiring vertical crustal movements of many kilometers. Sediments accumulate in regions of subsidence, and their thickness is usually close to the degree of downwarping. In the unstable, mobile belts bordering stable continental platforms, many geosynclinal troughs and circular depressions accumulated sedimentary thicknesses of 10 to 14 km, and in some cases of 20 km. Although the sediments deposited on the platforms themselves are mostly less than 1.5 km thick, here too sedimentary basins with deposits 10 km or even 20 km thick are not unknown.
    Subsidence cannot be attributed solely to the weight of the accumulating sediments because the density of sedimentary rocks is much lower than that of the subcrustal material; for instance, the deposition of 1 km of marine sediment will cause only half a kilometer or so of subsidence. Moreover, sedimentary basins require not only continual depression of the base of the basin to accommodate more sediments, but also continuous uplift of adjacent land to provide a source for the sediments. In geosynclines, subsidence has commonly been followed by uplift and folding to produce mountain ranges, and this can obviously not be accounted for by changes in surface loading. The complex history of the oscillating uplift and subsidence of the crust appears to require deep-seated changes in lithospheric composition and density, and vertical and horizontal movements of mantle material.
    In regions where all the sediments were laid down in shallow water, subsidence must somehow have kept pace with sedimentation. In eugeosynclines, on the other hand, subsidence proceeded faster than sedimentation, resulting in a deep marine basin several kilometers deep. Examples of eugeosynclines prior to the uplift stage are the Sayans in the Early Paleozoic, the eastern slope of the Urals in the Early and Middle Paleozoic, the Alps in the Jurassic and Early Cretaceous, and the Sierra Nevada in the Triassic. Although plate tectonicists often claim that geosynclines are formed solely at plate margins at the boundaries between continents and oceans, there are many examples of geosynclines having formed in intracontinental settings.

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