Abstract
The distribution and geologic environment of magnetic smooth zones in the world's oceans are examined in order to determine their possible origins. The smooth zones are divided into two categories: (1) marginal smooth zones associated with the sedimentary deposits of continental rises, and (2) abyssal smooth zones located far from the continental margins in the deep sea. Abyssal smooth zones appear to result from two uniform polarity intervals, one in the Cretaceous and the second in the Jurassic, combined with some process, such as a fast spreading rate or crustal subsidence, that eliminates topographic anomalies. Many marginal smooth zones having ages coincident with the polarity intervals might have the same origins as abyssal smooth zones. However, marginal smooth zones are found almost solely on rifted continental margins, thus suggesting that they may owe their origin in part to the breakup of Pangea. As an alternate origin for marginal zones not having ages coincident with polarity intervals or as an additional process for those that do, we suggest that they are formed during the initial rifting of continents. Spreading rates are at first slow, producing narrow magnetic bodies. When these narrower bodies subside to continental margin depth, their anomalous magnetic field is attenuated at the sea surface. Marginal smooth zones might result from either or both processes, but all are accompanied by basement subsidence and deep sedimentary burial. Until the actual time spans (maximum and minimum ages) for certain marginal zones are known, sea floor spreading magnetic models cannot meaningfully be applied to test the various proposed origins.
Published Version
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