The hypothesized presence of a detachment underlying the Lake Mead region has created a dichotomy in the interpretations of the roles of strike-slip faults of the Lake Mead fault system in accommodating regional deformation. Our detailed field mapping reveals a previously unnamed left-lateral strike-slip segment of the Lake Mead fault system and a dense cluster of dominantly west-dipping and related normal faults located near Pinto Ridge. We suggest that the strike-slip fault that we refer to as the Pinto Ridge fault: (1) was kinematically related to the Bitter Spring Valley fault; (2) was responsible for the creation of the normal fault cluster at Pinto Ridge; and (3) utilized these normal faults as linking structures between separate strike-slip fault segments to create a longer, through-going fault. Results from numerical models demonstrate that the observed location and curving strike patterns of the normal fault cluster are consistent with the faults having formed as secondary structures as the result of the perturbed stress field around the slipping Pinto Ridge fault, regardless of whether or not the Pinto Ridge fault merges into a regional detachment at depth. Calculations of mechanical efficiency of various normal fault geometries within extending terranes suggest that a preferred west dip of normal faults likely reflects a west-dipping anisotropy at depth, such as a detachment. The apparent terminations of numerous strike-slip faults of the Lake Mead fault system into west-dipping normal faults suggest that a west-dipping detachment may be regionally coherent.

ABSTRACT The Miocene Columbia River Basalt Group (CRBG) covers a large part of Oregon, Washington, and Idaho and is one of the youngest and perhaps the best studied flood-basalt province on Earth. Decades of study have established a regional strati-graphic framework for the CRBG, have demonstrated the CBRG flows can be correlated with dikes and vents, have documented a wide variety of physical features within the CRBG flows, and have demonstrated that many characteristics of the CRBG are recognizable throughout its extent. Detailed studies of individual flows and their feeder dikes have allowed the development of models for the emplacement of voluminous basaltic lava flows. The interplay between the regional structure, contemporaneous deformation, preexisting topography, and paleodrainage systems helped to control the emplacement of individual CRBG flows. These features have also affected the nature of late Neogene sedimentation in the region covered by basalt flows. Finally, the distribution of sediments within the CRBG and the character of the intraflow and interflow structures have played a significant role in the development of aquifers within the CRBG. In this paper we present an overview of the regional aspects of the stratigraphy, structural geology, tectonics, and hydrogeology of the CRBG.