WRENCH-FAULT TECTONICS

Extending the work of E. M. Anderson, M. K. Hubbert, and W. Hafner on faulting, the authors develop the hypothesis that anticlinal folds, thrust faults, and wrench faults can be generated as a result of movement on a large wrench fault such as the San Andreas of California. Extension of this concept leads to the conclusion that for any given tectonic area, at least eight directions of wrench faulting and four directions of anticlinal folding and/or thrusting should accommodate the structural elements of that region; these directions should have a more or less symmetrical disposition relative to the direction of the primary compressive stress. The angles α, β, and γ are defined to describe the geometry of such a wrench-fault tectonic system relatively completely. The authors' interpretations of tectonics in various areas indicate that wrench-fault tectonic systems do exist and are aligned systematically over large portions of the earth's crust as indicated by Hobbs, Vening Meinesz, Sonder, and others. Eight principal wrench directions are defined in terms of major elements of the earth's crust such as the Alpine fault of New Zealand. Structural elements aligned in these eight directions constitute major features of the regmatic shear pattern of Sender. The authors conclude that the shear pattern may have resulted from stresses which are oriented essentially meridionally and have been acting in nearly the same direction throughout much of crustal history. It is concluded that major wrench faults, which penetrate the entire outer crust of the earth and result in wholesale segmentation of the outer crust into polygonal blocks, constitute a fundamental type of yielding in the crust. Possible origins of the stresses involved, formation of geosynclines, island arcs, volcanism, and crustal evolution are discussed in terms of these ideas. Some possible objections and weak points in the argument are pointed out, and suggestions for further study are included.