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Tectonic Framework of Southwestern United States and Possible Continental Rifting1

By
Chas. B. Hunt
Chas. B. Hunt
Baltimore, Maryland
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Published:
January 01, 1963

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, the San Andreas rift, is conspicuous enough. Displacement on this fault system is right lateral and has been estimated as great as 350 miles.

Another structure parallel to the San Andreas rift is 150 miles to the northeast. In part, it coin-cides 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 to the 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 south-eastward across the Panhandle of Texas to the Wichita Mountains in southwestern Oklahoma. If these structures have right-lateral displacement comparable to that along the San Andreas rift, an aggregate displacement of 1,200 to 1,500 miles is indicated.

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 ones at the surface. This perhaps could be tested by determining whether the foci of deep-seated earthquakes (± 65 kms) under the Rocky Mountains have a different distribution pattern from the shallow ones (± 45 kms).

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 Mohorovičić 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 ex-plaining 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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Contents

AAPG Memoir

Backbone of the Americas: Tectonic History from Pole to Pole

Orlo E. Childs
Orlo E. Childs
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B. Warren Beebe
B. Warren Beebe
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American Association of Petroleum Geologists
Volume
2
ISBN electronic:
9781629812359
Publication date:
January 01, 1963

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