GEOLOGIC SETTING OF THE PROPOSED HIGH-LEVEL RADIOACTIVE WASTE REPOSITORY, YUCCA MOUNTAIN, NEVADA

Yucca Mountain (YM), in southern Nevada, is the proposed site of the nation’s sole high-level radioactive waste repository and lies within the middle to upper Miocene southwestern Nevada volcanic field (SWNVF) in the Walker Lane tectonic zone near the SW margin of the Basin and Range province. Rocks of the SWNVF were erupted from the Timber Mountain caldera complex; they unconformably overlie an older Tertiary nonmarine section, which in turn overlies a highly deformed Paleozoic and Proterozoic section along a profound angular unconformity. The Paintbrush Group dominates the near-surface stratigraphy at YM, comprising two thick, mainly densely welded rhyolite tuffs – the 12.8-Ma Topopah Spring Tuff and the 12.7-Ma Tiva Canyon Tuff – and a thinner intervening interval of nonwelded and bedded tuffs. The Tiva Canyon is the most extensively exposed bedrock unit at YM, whereas the underlying Topopah Spring includes the "repository host horizon," which would be excavated for waste storage. The proposed repository volume (in north-central YM) is in the structurally simplest part of YM: a 4-km-wide fault block of gently E-dipping strata. This "central block" is one of several 1- to 4-km-wide blocks that are delineated by moderately to steeply W-dipping, block-bounding normal faults. Within the central block, intrablock faults are commonly short and discontinuous, except for one prominent 7-km-long fault. Block margins are more intensely deformed, particularly in the hanging-walls of block-bounding faults. The mesoscale fracture network in the tuffs is strongly controlled by lithologic properties of the volcanic stratigraphy (degree of welding, lithophysae development, etc.), and locally the fracture network has a strong influence on the nature of intrablock faulting. In the southern part of YM, the intensity of deformation increases markedly, and block-bounding faults commonly are linked by northwest-striking relay faults. Block-bounding faults were active at Yucca Mountain during and after eruption of the Paintbrush Group, and significant motion on those faults postdated the 11.6-Ma Rainier Mesa Tuff. Roughly half of the stratal tilting in the repository-site area occurred after 11.6 Ma. In addition, 0 to 4 m of Quaternary displacement are associated with each block-bounding fault.