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the Mid-Atlantic Ridge (Original)

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Author Topic: the Mid-Atlantic Ridge (Original)  (Read 9780 times)
Carolyn Silver
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« Reply #60 on: July 28, 2008, 11:38:15 pm »


Figure 6. Two views of marine magnetic anomalies. Top: a textbook cartoon. (Reprinted with permission from McGeary & Plummer [2]. Copyright by The McGraw-Hill Companies.). Bottom: magnetic anomaly patterns of the North Atlantic (Reprinted with permission from Meyerhoff & Meyerhoff, 1972. Copyright by the American Geophysical Union.)

    A remarkable fact concerning oceanic magnetic anomalies is that they are approximately concentric with respect to Archean continental shields (i.e. continental nuclei more than 2.5 billion years old). This implies that instead of being a 'taped record' of seafloor spreading and geomagnetic field reversals during the past 200 million years, most oceanic magnetic anomalies are the sites of ancient fractures, which partly formed during the Proterozoic and have been rejuvenated since. The evidence also suggests that Archean continental nuclei have held approximately the same positions with respect to one another since their formation -- which is utterly at variance with continental drift.
    Benioff zones are distinct earthquake zones that begin at an ocean trench and slope landward and downward into the earth. In plate tectonics, these deep-rooted fault zones are interpreted as 'subduction zones' where plates descend into the mantle. They are generally depicted as 100-km-thick slabs descending into the earth either at a constant angle, or at a shallow angle near the earth's surface and gradually curving round to an angle of between 60° and 75°. Neither representation is correct. Benioff zones often consist of two separate sections: an upper zone with an average dip of 33° extending to a depth of 70-400 km, and a lower zone with an average dip of 60° extending to a depth of up to 700 km. The upper and lower segments are sometimes offset by 100-200 km, and in one case by 350 km. Furthermore, deep earthquakes are disconnected from shallow ones; very few intermediate earthquakes exist. Many studies have found transverse as well as vertical discontinuities and segmentation in Benioff zones. The evidence therefore does not favor the notion of a continuous, downgoing slab.




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