Abstract

At Hoover Dam, 40 km southeast of Las Vegas, Nevada, well-exposed, highly faulted Miocene rocks provide an excellent opportunity to study the paleostress history of a very small area within a region where previous geologic studies indicate clockwise rotation of paleostress and a nearness to a major strike-slip fault-zone boundary. Within <0.5 km3 of rock, the sense of slip was determined on almost 500 separate faults. The fault-slip data show internal consistency with respect to lithology, size of faults, and location within the small area. With respect to fault slip, however, the data represent an in-homogeneous mixture of primarily strike-slip and dip-slip motions. From this mixture, it is possible to resolve two distinct stress fields with directions of extension that differ by ∼60°. Each stress field corresponds to a mixture of strike-slip and dip-slip faults, and thus the orientations of σ1 and σ2 are not tightly constrained. If, however, the data are first separated into strike-slip and dip-slip faulting modes and subsequently searched for sub-populations that correspond to contrasting paleostress orientations, the computations yield tightly constrained tensors that illustrate two distinct stress fields with subhorizontal σ3 axes that trend N50°E and N75°W. The σ1 and σ2 axes permutate in two vertical planes that strike N40°W and N15°E, because σ1 and σ2 are close in value, relative to σ3. These relationships suggest that strike-slip and dip-slip faulting belong to the same tectonic regime.

Qualitative observations of polyphase slip, fault-fault offsets, and fault-bedding geometric relationships, when evaluated in the context of changes in σ3 orientation and permutations of σ1 and σ2, provide a basis for a two-stage, late Cenozoic structural evolution at Hoover Dam. These stages are (1) strike-slip faulting (partly pre-tilt) and dip-slip faulting and associated stratal tilting and (2) mostly post-tilt, complexly interrelated strike-slip, oblique-slip, and dip-slip faulting. The qualitative evaluations indicate that strike-slip and dip-slip faulting alternated in time during the second stage of deformation and may have done so during the first. These mixed-mode movements probably represent stress oscillations in time and space rather than discrete stress reorganizations. In contrast, the two different orientations of σ3 either represent a major clockwise rotation of the stress field or a major counterclockwise rotation of the rocks during the faulting history. Each alternative is consistent with regional geologic relationships, and the choice of which is correct cannot be made within the small area that was studied.

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