PRELIMINARY GEOLOGIC MAPPING AND QUATERNARY FAULTING OF THE WESTERN MARGIN OF THE GRANT RANGE, EAST-CENTRAL NEVADA

The Grant Range in Nevada is bounded on the west by a west-dipping normal fault system. The footwall of this fault system contains an erosionally-embayed mountain front that is bordered by a pediment developed on Paleozoic bedrock. The fault system is manifest by discontinuous fault scarps that cut pediment alluvium. The fault scarps reflect renewed fault slip after a period of tectonic quiescence that produced the embayed mountain front. The hanging wall of the fault system contains Quaternary alluvial fans and pluvial lake deposits that delimit the margin of Railroad Valley. The alluvial fan deposits are cut by antithetic faults that produced graben in proximity of the range-bounding faults. Preliminary mapping of the alluvial fans, pluvial deposits, and fault scarps reveal four general fault segments in the central and northern part of the Grant Range. From north to south these are the Box Canyon, Heath Canyon, Grant Canyon, and Irwin Canyon segments. We recognize four general fan surfaces, Qf1 (oldest) to Qf4 (youngest) that record a basinward stepping of depo-centers with concomitant erosion of older surfaces. All fault segments cut and therefore postdate Qf1, and the faults are overlapped by, and hence predate, Qf4. Furthermore, in all areas Qf4 overlaps the oldest Late Pleistocene pluvial deposits that mark the lake highstand indicating that faulting predates the pluvial deposits. In the southern part of the range, the relative ages of the Grant, Heath, and Irwin Canyon segments can be further constrained because of cross cutting relationships with the development of multiple fan surfaces (Qf1, 2, 3 and 4). The Irwin Canyon segment appears to be the oldest because it doesn’t cut surfaces younger that Qf1. The Heath and Grant Canyon segments cut Qf2 and therefore are younger than the Irwin Canyon segment. The age of the Box Canyon segment relative to the other fault segments is unconstrained because of the lack Qf3 and Qf2 in proximity of this fault segment. In summary, the recognition that at least two fault segments have different ages indicates that fault scarps in the Grant Range were produced by at least two separate slip events.