HR: 1330h
AN: T22A-0501    [PDF]
TI: Secondary Normal Faulting Near the Terminus of a Strike-Slip Fault Segment in the Lake Mead Fault System, SE Nevada
AU: * Marshall, S T
EM: mars0776@uidaho.edu
AF: Dept. of Geological Sciences, University of Idaho, PO Box 443022, Moscow, ID 83844-3022 United States
AU: Kattenhorn, S A
EM: simkat@uidaho.edu
AF: Dept. of Geological Sciences, University of Idaho, PO Box 443022, Moscow, ID 83844-3022 United States
AB: The 95 km long Lake Mead Fault System (LMFS), located about 50 km east of Las Vegas and about 100 km west of the relatively undeformed Colorado Plateau, consists of a group of NE/SW-trending Miocene left-lateral strike-slip faults with a total offset of 65-110 km. Previous work suggests that the LMFS acted as a transform zone to accommodate differential extension between the southern Basin and Range to the north and the metamorphic core complexes of the Colorado River extensional corridor to the south. Studies of individual faults of the LMFS have shown that strike-slip faulting was the dominant mode of deformation while normal faulting, pull-apart basins, and push up structures formed as localized secondary structures related to strike-slip faults. This study focuses on the portion of the LMFS west of the Overton Arm of Lake Mead, which consists of the Bitter Spring Valley Fault (BSVF) and the Hamblin Bay Fault (HBF). Both faults have estimated offsets of 20-60 km, but past mapping efforts have been inconsistent with respect to the BSVF trace locations and degree of fault complexity. In order to demonstrate that the apparent complexity of the BSVF is the result of segmentation and secondary normal faults associated with individual segments, we focused field mapping efforts on an apparent segment of the BSVF near Pinto Ridge, located southwest of the Echo Hills and about 5 km NW of the more prominent HBF. We have identified nine normal faults that initiate near the SW tip of a segment of the BSVF and die out to the south before reaching the HBF. The offset on all these faults is a maximum at their northern intersection with the BSVF, then steadily decreases to zero away from the BSVF. These normal faults range from 0.6 km-2.25 km in length and have variable fault trace patterns. The normal fault originating closest to the SW tip of the BSVF segment curves with increasing distance away towards parallelism with the BSVF. The eight other normal faults are all oriented approximately N/S. These secondary faults all intersect the BSVF at angles of 40-60 degrees and the intersection angles typically decrease away from the BSVF segment tip. Linear elastic fracture mechanics predicts that when a mode II fault slips, it causes stress concentrations near the tips of the sliding fault. Furthermore, resultant secondary tensional fracturing is predicted to be oriented at 70.5 degrees to the main fault in the extensional quadrant. This angle can be larger or smaller for mixed mode cases and is also affected by the frictional properties of the fault. Typically, these features are referred to as tailcracks, and have mostly been documented at the cm to m scale as mode I joints. Given strike-slip faults like the BSVF where cumulative slip is on the order of tens of km, large-scale tailcracks can be manifested as normal faults. We thus interpret these normal faults to be tailcracks that formed in a locally perturbed stress field near the SW tip of a segment of the BSVF. We conclude that in the vicinity of Pinto Ridge, the left-lateral BSVF is less complex than has been previously mapped, occurring as a strike-slip fault segment flanked by secondary normal faults near the SW tip.
DE: 8010 Fractures and faults
DE: 8020 Mechanics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting