IMPLICATIONS OF SEISMIC-REFLECTION AND POTENTIAL-FIELD GEOPHYSICAL-DATA ON THE STRUCTURAL FRAMEWORK OF THE YUCCA MOUNTAIN CRATER FLAT REGION, NEVADA

Seismic reflection and gravity profiles collected across Yucca Mountai n, Nevada, together with geologic data, provide evidence against propo sed active detachment faults at shallow depth along the pre-Tertiary-T ertiary contact beneath this potential repository for high-level nucle ar waste, The new geophysical data show that the inferred pre-Tertiary -Tertiary contact is offset by moderate to high-angle faults beneath C rater Flat and Yucca Mountain, and thus this shallow surface cannot re present an active detachment surface. Deeper, low-angle detachment sur face(s) within Proterozoic-Paleozoic bedrock cannot be ruled out by ou r geophysical data, but are inconsistent with other geologic and geoph ysical observations in this vicinity, Beneath Crater Flat, the base of the seismogenic crust at 12 km depth is close to the top of the refle ctive (ductile) lower trust at 14 to 15 km depth, where brittle fault motions in the upper crust may be converted to pure shear in the ducti le lower crust, Thus, our preferred interpretation of these geophysica l data is that moderate- to high-angle faults extend to 12-15-km depth beneath Yucca Mountain and Crater Flat, with only modest changes in d ip. The reflection lines reveal that the Amargosa Desert rift zone is an asymmetric half-graben having a maximum depth of about 4 km and a w idth of about 25 km, The east-dipping Bare Mountain fault that bounds this graben to the west can be traced by seismic reflection data to a depth of at least 3.5 km and possibly as deep as 6 km, with a constant dip of 64 degrees +/- 5 degrees, Within Crater Flat, east-dipping hig h-angle normal faults offset the pre-Tertiary-Tertiary contact as well as a reflector within the Miocene tuff sequence, tilting both to the west, The diffuse eastern boundary of the Amargosa Desert rift zone is formed by a broad series of high-angle down-to-the-west normal faults extending eastward across Yucca Mountain. Along our profile the trans ition from east- to west-dipping faults occurs at or just west of the Solitario Canyon fault, which bounds the western side of Yucca Mountai n. The interaction at depth of these east- and west-dipping faults, ha ving up to hundreds of meters offset, is not imaged by the seismic ref lection profile. Understanding potential seismic hazards at Yucca Moun tain requires knowledge of the subsurface geometry of the faults near Yucca Mountain, since earthquakes generally nucleate and release the g reatest amount of their seismic energy at depth. The geophysical data indicate that many fault planes near the potential nuclear waste facil ity dip toward Yucca Mountain, including the Bare Mountain range-front fault and several west-dipping faults east of Yucca Mountain, Thus, e arthquake ruptures along these faults would lie closer to Yucca Mounta in than is often estimated from their surface locations and could ther efore be more damaging.