Geological and geophysical evidence of structures in northwest-trending washes, Yucca Mountain, southern Nevada, and their possible significance to a nuclear waste repository in the unsaturated zone

Abstract

Geological and geophysical evidence suggests that five prominent linear northwest-trending washes in the northeastern part of Yucca Mountain, Nevada, are underlain by zones of right-lateral strike-slip faults. Northwest-striking faults exposed along the washes are nearly vertical, have essentially horizontal striations on slickensides, and have small vertical offsets. Cores from drill holes within Drill Hole Wash contain northwest-striking steeply dipping fault and fracture planes. Higher conductances and lower resistivities in zones within Drill Hole Wash are interpreted as zones of fractured rocks that are more highly altered or contain more water than adjacent, less fractured rocks. Little measurable horizontal offset of geomorphic features has occurred along these strike-slip faults, and strike-slip motion was probably small, even along the longest of these faults. The strikes, sense of motion, geographic position, and age of these Yucca Mountain strike-slip faults are similar to those of the regional Walker Lane-Las Vegas Valley shear zones. Strike-slip faults in the northeastern part of Yucca Mountain will affect the stability of mined openings of the potential high-level nuclear waste repository at Yucca Mountain where brecciated or highly fractured zones are encountered. Because the repository is planned above the water table in unsaturated rocks, such faults may be favorablemore » features where they allow recharge to drain rapidly from the repository. However, at greater depths these faults may be adverse features where they provide potential hydrologic conduits through unsaturated sorptive zeolitized nonwelded tuffs below the repository and within rocks below the water table in the saturated zone. Although these potentially favorable and adverse factors must be investigated and evaluated, present information does not rule out extension of the repository into the area northeast of Drill Hole Wash.« less