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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 45 (1961)

Issue: 3. (March)

First Page: 412

Last Page: 413

Title: Tectonic Framework of Southwestern United States, and Possible Continental Rifting: ABSTRACT

Author(s): Chas. B. Hunt

Article Type: Meeting abstract

Abstract:

Major structural features in southwestern United States mostly trend northerly, but a study of the seismic, gravity, and geologic maps of the region suggests there may be four or more southeast-trending structures obliquely crossing and largely obscured by the northerly ones. The most southwesterly of these is conspicuous enough, the San Andreas rift. Displacement on this fault system is right-lateral and has been estimated as great as 350 miles.

Another structure parallel with the San Andreas rift is 150 miles northeast. In part, it coincides with the front of the Sierra Nevada, but the gravity and seismic maps suggest it may continue northwestward across the center of northern California and southeastward across the southwest corner of Arizona.

A third parallel structure is about 100 miles farther northeast and in part coincides with the southwest edge of the Colorado Plateau. The seismic, gravity, and geologic maps show it extending northwestward across Nevada. It is lost in southwestern New Mexico, but the structurally disturbed Trans-Pecos Texas area is aligned with it as are a few scattered epicenters.

The fourth southeast-trending structure is represented by the well known late Paleozoic troughs and highlands that extend diagonally across the Rocky Mountains. This alignment extends southeastward across the Panhandle of Texas to the Wichita Mountains in southwestern Oklahoma. If these structures have right-lateral displacement comparable with that along the San Andreas rift, an aggregate displacement of 1,200-1,500 miles is indicated.

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It is suggested that the southeast-trending structures may be at the base and in the lower part of the crust, and may have controlled the shallower and more obvious structures at the surface. This perhaps could be tested by determining whether the foci of deep-seated earthquakes (ca. 65 kms.) under the Rocky Mountains have a different distribution pattern from the shallow ones (<45 km).

If similar structures can be identified elsewhere in the northern hemisphere, and if a counter set of northeast-trending structures with left-lateral displacement could be found in the southern hemisphere, this would be compelling structural evidence that the crust is drifting eastward on top of the mantle and that the Mohorovicic discontinuity is a shear plane, or rather a float plane.

The merit in this hypothesis of continental rifting is chiefly in the possibility it provides of explaining the forces required to form geosynclines and to form folded mountains and overthrusts. Too, it offers a mechanism for generating magmas, whether basalt derived from the base of the crust or silicic magmas derived by palingenesis of higher parts of the crust.

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