Abstract

Low-angle thrust faults and décollement folds are characteristic features of the supracrustal rocks along the foreland margin of the North American Cordillera from northern Alaska to southeastern California. They form a distinctive structural zone more than 3000 mi long that is an integral component of the Cordilleran orogenic belt.

There is an intuitive appeal to the notion of a simple basic relationship between gravity and the large horizontal translations which produced these structures, but the popular-concept of gravitational sliding is not an acceptable model for such a relationship. The critical data of the regional geology are not compatible with the two basic phenomena implicit in this concept—a simple, deeply rooted uplift which involved vertical displacement without any large horizontal components and produced the slope for the sliding; and a tectonic gap or horizontal stretching in the supracrustal rocks behind the allochthonous mass, which is of equivalent magnitude and compensates for the tectonic overlap or horizontal shortening that is involved in the thrusting and folding along the front of the mass. The concept of lateral gravitational spreading from the area of up-welling of the hot, mobile, metamorphic infrastructure in the core of the Cordillera does lead to an acceptable model. The large horizontal translations that carried the allochthonous supracrustal rocks out of the Cordillera onto the flank of the craton also involved the metamorphic and plutonic rocks that had welled up within the Cordillera. The thrusting and folding are a result of a lateral gravitational spreading up the flank of the craton which affected the entire mass. The gravitational potential for the large horizontal translations existed because of the sloping surface of the whole mass, and not because of the slope of an autochthonous basement beneath it.

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