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Cordilleran detachment faults, as defined here, are extensional faults of low initial dip, probably less than 30°, and subregional to regional scale. Some detachment faults have large translational displacements, i.e., in excess of several tens of kilometers. First interpreted as Tertiary extensional structures in the eastern Great Basin by Armstrong (1972), they are now known to be widespread throughout those Cordilleran regions that have undergone greatest Cenozoic extension. Detachment faults are commonly, but not necessarily, associated with lower-plate mylonitic gneisses that compose the so-called “metamorphic core complexes.” Probably nowhere in the U.S. Cordillera are detachment faults more widely and spectacularly developed than in the region that borders the lower Colorado River in southernmost Nevada, southeastern California, and southwestern Arizona. We believe that our studies and those of numerous other workers in this region, the Colorado River extensional corridor of Howard and John (1987), provide a number of new perspectives on the origin, geometry, and evolution of Cordilleran detachment faults.

Detachment faults are best explained as evolving shallow-dipping shear zones that have accommodated Tertiary crustal extension (Wernicke, 1981). The fault zones are believed to root at midcrustal or lower upper crustal depths into broad zones of intra-crustal flow, the tectonic regime in which mylonitic gneisses form. At their upper ends, major detachment faults either reach the surface directly or terminate at shallow depth into pull-apart complexes of closely spaced normal faults. Along these evolving shear zones, lower-plate mylonitic gneisses are drawn upward and out from beneath upper-plate rocks. As footwall gneisses rise structurally upward, they are retrograded, sheared, and shattered at progressively colder and shallower crustal levels to form the chloritic breccias and microbreccias characteristic of many major detachment faults. At advanced stages of detachment fault evolution, lower-plate mylonitic gneisses formed at depths >12 km are tectonically juxtaposed beneath unmetamorphosed supracrustal rocks and exposed at the surface through combinations of crustal upwarping, tectonic denudation, and erosion.

Contrary to popular belief, the master detachment faults exposed today are probably not in their entirety those faults that formed at the start of extensional deformation, but rather are only the youngest in a succession of major detachment faults. Detachment faults undergo warping at high angles to the direction of crustal extension, probably in large part related to isostatically induced distortions of originally more planar faults. Such warping leads to the development of younger, more planar fault splays that either cut upward into former upper-plate rocks (excisement) or downward into former lower-plate rocks (incisement).

Recognition of such geometric complexities offers fresh insights into deciphering the evolving strain patterns within major detachment terranes. Studies in the Whipple Mountains region of southeastern California indicate that: (1) detachment faults have formed by both excisement and incisement tectonics; (2) northeast-southwest-trending “folds” of major detachment faults, oriented parallel to the direction of extension, are in reality primary corrugations or flutes in the fault surface (a conclusion previously reached by other workers in nearby areas); (3) most normal faults in the upper plates of major detachments originally had listric geometries before losing their flattened lower segments as the consequence of excisement tectonics; and (4) detachment faults can transect upper crustal rocks as primary, low-dipping shear zones without pre-existing, shallow-dipping structural controls (e.g., thrust faults) on their localization; the northeast-southwest-trending curviplanar geometry of the Whipple fault does, however, seem to mimic preexisting fold structure in lower-plate mylonitic gneisses crossed by the fault. Finally, the rate of translation along master faults of some evolving detachment systems apparently can be very rapid (>1 cm/yr), much faster than rates that we (and perhaps other workers) once deemed reasonable. A very good case can be made on the basis of geochronologic and field studies that footwall mylonitic gneisses were transported upward along the Whipple detachment system from lower upper crustal depths to near surface levels in less than 2 m.y. (between 18 and 20 Ma).

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