Abstract
The Lake Mead Region is recognized as unique within the Basin and Range due to both major normal and strike-slip faults that accommodate large-magnitude extension. Much debate has focused on the relative importance of each type of fault, the role of young contractional features, and the tectonic drivers for deformation in this region. We have begun to address these issues by mapping the Bitter Spring Quad just north of the left-lateral Lake Mead Fault System (LMFS) at 1:10,000 scale. Specifically, we have mapped and collected structural data in much of the Longwell Ridges Area (LRA) to 1) determine how the White Basin and Longwell Faults are related to major strands of the LMFS, 2) ascertain the southern extent of the Roger's Spring Fault, 3) examine the relation between varying sizes of faults and, 4) attempt to constrain the timing of various sets of faults. Faults are well-exposed in Paleozoic miogeoclinal carbonates and the Jurassic Aztec and Miocene Horse Spring Formations. To date, we have identified three sets of faults. Northeast-trending faults run throughout the area, and include the largest faults; these generally dip west and indicate left-lateral oblique offset. North-south trending faults typically dip to the west, but are usually smaller in length and magnitude of offset. Northwest-trending faults cluster in certain areas; in the southern LRA, these faults dip to the southwest and in the northern LRA, they dip to the northeast. Although mapping is not yet complete, the northeast-trending White Basin and Longwell faults appear to be en-echelon faults accommodating and transferring strain between two larger strands of the LMFS and thus, the presence of the Longwell Ridges may be explained by a strike-slip duplex mechanism. Somewhat smaller north-south trending faults found between the White Basin and Longwell faults may play a similar role but at a smaller scale. Clusters of northwest and north-south trending faults found in the LRA appear to be related to the termination of the White Basin and Longwell faults. Sets of faults do not consistently offset each other; instead, offsets are variable and complex. Recent work by Henza et al. (2007) suggests that such patterns are characteristic of areas that have undergone multiple phases of extension.