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Continental Drift: Implications of Paleomagnetic Studies, Meteorology, Physical Oceanography, and Climatology

1970; University of Chicago Press; Volume: 78; Issue: 1 Linguagem: Inglês

10.1086/627487

1537-5269

A. A. Meyerhoff,

Marine and environmental studies

Paleomagnetic methods apparently have little or no resolving power for studies of global tectonics. As the number of ancient pole determinations increases, the scatter of ancient pole positions has become so great, even from single localities and geologic provinces, that the circles of error for paleopoles from rocks of each age are wider than the Atlantic Ocean. From this fact it is apparent that paleomagnetic pole determinations cannot be used to show relative movements between continents. The use of paleoclimatology to interpret earth history in general-and global tectonics in particular-has not been successful because its practitioners have not utilized the results of present-day meteorology, physical oceanography, and climatology. The writer used marine evaporite, coal, desert eolian sandstone, and tillite deposits as paleoclimate indicators. From a plot of marine evaporite, coal, and proved desert eolian deposits on world maps-one for each age from the Proterozoic through the Miocene-a consistent geometric pattern emerged. Addition of proved tillite deposits to the maps brought out a second, but time-related, pattern. The geometric pattern on the maps shows that, since Devonian time, the major coal deposits of each age consistently are in two belts which extend to high latitudes. Equatorward from the two coal belts are two-or at times only one- evaporite belts. The widths of the coal and evaporite belts fluctuate through time. When the evaporite belts have the greatest latitudinal spread (up to 125°), the earth must have been very warm; these are termed "evaporite maxima." The coal belts are very narrow during evaporite maxima. When the coal belts are broadest, the evaporite belts are restricted (about 60°) and glaciation is most common. These periods are called "glacial maxima." The great fluctuation of the widths of these belts illustrates the second, or time-related pattern: specifically, the average world temperature has changed considerably during earth history, and the changes are episodic; ultimately a periodicity may be established. The climate changes do not appear to be related to orogeny. In the Northern Hemisphere, the northern limit of evaporite deposition through time fluctuates from about lat 40° N. to lat 83° N. In the Southern Hemisphere, the fluctuation is slight, ranging from lat 20° S. to lat 40° S. This northward "offset" of evaporite zones is similar to that of the present northward offset of the meteorological equator. The largest areas of desert eolian deposits are in two circumglobal belts: one extends from Spanish Sahara to India, and from Southern California to the Rio Grande Valley; the other crosses South America from Peru and Chile to southern Brazil, extends across southern Africa, and continues across most of Australia. The two belts where the known desert eolian sandstones are concentrated coincide with today's horse-latitude, or desert, belts. It was found that 95 percent of all ancient evaporites, Proterozoic to the present, are in areas which today receive less than 100 cm of annual rainfall. Thus the evaporites are in areas which today have the driest air currents. The significance of this discovery is that the planetary wind- and ocean-current circulation patterns have undergone little or no important change since middle Proterozoic time. This is not possible unless the positions of the rotational pole, ocean basins, and continents also have been the same for the past 800-1,000 m.y. An independent test of these statements is provided by a study of the tectonic history of the Arctic and North Atlantic Oceans and the relation of this history to evaporite deposition in Eurasia and North America. This study demonstrated that the Gulf Stream and North Atlantic Drift have been in existence since middle Proterozoic time. Thus the Atlantic and Arctic basins appear to be very ancient features. In view of the results presented here, modern concepts of continental drift (the new global tectonics) and polar wandering require complete reevaluation unless some other explanationcan be found for (1) the evaporite- and coal-distribution patterns and (2) the causal relations of these patterns to the modern planetary ocean- and wind-circulation system, and to the present rotational pole.