Structural analysis of the Basin Range province in terms of wrench faulting
AuthorSales, John Keith
Geological Sciences and Engineering
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A structurally, chronologically, and geographically integrated hypothesis for the origin of the Basin Range province and critical adjacent areas is attempted, based on the premise that horizontal fault couple is the prime structure-generating mechanism. Pre-Tertiary structure of the North American Cordillera was generated by relative overriding of North America over the Pacific block. Tertiary structural history is the result of a right-lateral couple between the same first-order blocks. This rotation of stresses (perhaps much less than 90°) is all that is necessary to integrate Paleozoic-Tertiary structural history. In terms of this stress history, specific detailed hypotheses are developed for most major structural features in the province. Emplacement of the huge (originally continuous and straight) Late Mesozoic batholiths occurred as the result of decreased pressure and increase in temperature as the center of the eugeosyncline began to shear due to the stress reorientation. Formation of Basin Range structure is the result of dominantly right-lateral horizontal couples in vertically uncoupled crustal layers, each of which has a different structural style and couple orientation. The style in the upper (I 30 km) crust in the Basin Range province is the tension wrench on which movement is both right lateral and apart, allowing gravity tectonics to operate. When unmodified, these tension wrenches have the Nevada (N 15° E) trend. They are generated from underneath by the distributive, northwest-oriented San Andreas regional couple. This layer is probably confined between the Moho and Gutenberg's low-velocity layer. The deformational style is that of both a distributive couple and compression wrenches, across which there is a considerable component of compression and therefore little gravity tectonics. This deformational layer intersects the surface along the continental margin, where it is represented by the San Andreas fault. Tectonics in this layer are in turn generated from underneath by the first-order couple between the ocean and continent. To the northwest of the Basin Range province the same San Andreas regional couple is responsible for formation of the Idaho-Mendocino oroclines, the volcanic provinces of the Pacific Northwest, and the subocean Ridge and Trough province. Besides the basic tension-wrench mechanism, three modifying Basin Range structure-generating mechanisms are isolated and described. These are related to configuration of the province and anisotropy (especially the distribution of batholithic segments) in the crust. Basin Range province seismic zones can be explained and structural subprovinces isolated on the basis of interplay of these basic mechanisms. The well-studied Dixie Valley-Fairview Peak earthquake and several other features of the Basin Range structure are analyzed in terms of this theory and found to fit amazingly well. Both table-top and natural (intermediate-scale) models are used to advantage in developing the hypotheses.
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