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Origin of Mobile Belts—Ouichitas Emphasized: Abstract
No one, to the present writer's knowledge, has satisfactorily explained the underlying reason for the continent-bordering mobile belts, and the following hypothesis may fall in the same category. It is thought reasonable, however, to assume that the thermal, and therefore density, differential between the sub-continental and sub-oceanic basaltic layer, is responsible for the turbulent zones referred to as the mobile belts.
Conflicting evidence points to a difference of temperatures between the basaltic layer beneath the ocean floor and the subsurface of the continent. It is argued, convincingly, that sub-continent basalts are cooler than those beneath the oceans. It is just as convincingly argued that sub-continent basalts are warmer than those beneath the oceans.
Inadequacy of measuring devices and difficulty in obtaining sufficient coverage of data is the present barrier to proof in either direction. It appears, to the writer, more logical to assume warmer sub-continent temperatures for several reasons. The material of the continents is less dense and less apt to lose its heat. Also, the oceanic cover of water would appear to act as a reflecting shield to prevent the degree of heating of the submarine surface by solar radiation as that being absorbed by the continents. In any case conclusive evidence is still lacking, and it fits the general hypothesis to follow the latter view.
Attention is called to the fact that oceanic waters, subjected to flow by various factors, the principal of which is the earth's rotation, seek levels based upon their temperature and density differentials. Although there is some mixing, the major feature observable is the fact that bodies of water of even slightly different temperatures and densities collide with the result that the warmer or lighter current overrides the cooler or denser. At their zone of collision a surface of discontinuity is set up much like that of an overthrust fault zone, and the ensuing up-welling creates a pronounced zone of turbulence.
A generalized diagram of a cold front in the atmosphere is illustrative of the same principle. As in the case of ocean currents, the triggering mechanism is, in the main, a great and extensive force—the earth's rotation. The warmer and lighter air overrides the cooler and denser along a surface of discontinuity between two air masses of only slightly different temperature and composition. It is along, and in proximity to, that surface of collision that a zone of turbulence with its ensuing thunderstorms is created.
It is therefore postulated that an analogous situation occurs between the warmer, lighter continental mass and that of the cooler, denser sub-oceanic mantle, thus creating a zone of turbulence in the form of volcanoes and mobile belts. Although the specific differences between air, earth, and water are great, there exists a much lesser difference in the principles involving two masses of the same media. It is considered less than reasonable to deny the existence of the subjacent and superjacent relationships between contrasting masses in all three media. In each case there is a major cause which is not immediately apparent upon specific or particular examination. In all cases temperatures and densities are involved, and the major intermediate cause of overriding in air and water is the earth's rotation. A major, intermediate cause for overriding in the earth's crust is held to be the crustal shrinkage as diagrammed. The only apparent, great difference between the three cases of turbulence is one of time and the time required is apparently proportional to the density and friction of the medium involved.
That an overthrusting relationship exists between continent and ocean, is indicated by the depth of earthquake foci which are most shallow at the island arcs and progressively deeper toward, and beneath the continents. Diagrams are constructed to illustrate this concept of how the continents have grown by accretion due to encroachment of auxiliary welts and subsequent marginal mountain building. The bordering geosynclines are thus formed, primarily by compressional forces oriented horizontally, and are filled with sediments the character of which mainly depends upon source—cratonal or extra-cratonal.
Acknowledgments and Associated Footnotes
1 Continental, Oklahoma City
Copyright © 2006 by the Tulsa Geological Society