A Preliminary Review and Summary of the Potential for Tectonic, Seismic, and Volcanic Activity at the Nevada Test Site Defense Waste Disposal Site

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ft DESERT RESEARCH INSTITUTE (I I UNIVERSITY OF NEVADA SYSTEM A PRELIMINARY REVIEW AND SUMMARY OF THE POTENTIAL FOR TECTONIC, SEISMIC, AND VOLCANIC ACTIVITY AT THE NEVADA TEST SITE DEFENSE WASTE DISPOSAL SITE March 1983 Publication No. 45029 WATER RESOURCES CENTER

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This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com pleteness or usefulness of any information, apparatus, product or process disclosed, or represents tnat its use would not infringe privately-owned rights. Reference herein to any specific commercial product, process, or service by trade name, mark, manufacturer, or otherwise, does not neces sarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of autnors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Printed in the United States of America National Tecnnical Information Service U.S. Department of Commerce 5285 Port Royal Road

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A PRELIMINARY REVIEW AND SUMMARY OF THE POTENTIAL FOR TECTONIC, SEISMIC, AND VOLCANIC ACTIVITY AT THE NEVADA TEST SITE DEFENSE WASTE DISPOSAL SITE Water Resources Center Desert Research Institute University of Nevada System 1Research Assistant, Water Resources Center The work upon which this report is based was supported in part by the U.S. Department of Energy under Contract DOE-

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The potential for tectonic, seismic, and volcanic activ ity, as it relates to a radioactive waste repository in Frenchman Flat on the Nevada Test Site (NTS), is reviewed and A change from compressional to extensional tectonics, which occurred approximately 17 m.y. ago, marks the emergence of the present tectonic regime in the southern Great Basin. Crustal extension is continuing at the present time, oriented in a NW-SE direction in the NTS region. Concurrently with the onset of crustal extension a system of northwest- and northeast-trending shear zones developed, along which mutual offset has occurred. Present seismic and tectonic activity in the NTS region is concentrated along the intersections of the shear zones and in areas of deep basin formation, such as Natural historic seismicity of the NTS region has been low to moderate. Seismic hazard assessments suggest a maxi mum magnitude 6-7 earthquake, associated with a maximum peaK acceleration of 0.7-0.9 g, is probable for the NTS. A return period of 12,700-15,000 years for the maximum peak accelera tion indicates a relatively low seismic hazard. It is recom mended that a site specific seismic hazard analysis be con ducted for Area 5 which considers the Cane Spring, Rock Val ley, and Yucca-Frenchman fault zones potentially active. Silicic volcanism in the NTS region was active from 16 to 6 m.y. ago, followed by a transition to basaltic volca nism. The tectonic settings most favorable for Quaternary basaltic activity are areas of young basin-range extension, caldera ring fracture zones, and intersections of conjugate shear zones. Probability calculations for the Yucca Mountain waste repository result in a volcanic disruption hazard of 10“8-10“9/year. This value is extremely low and is probably representative of the hazard at Frenchman Flat. However, due to its tectonic setting, Frenchman Flat may be an area con ducive to future basaltic volcanism; further investigation is needed to properly assess volcanic hazard.

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Present Tectonic Situation 7 Seismic Activity from Nuclear Tests 13 Seismic Hazard Assessments • 15 Summary and Conclusions 25 VOLCANIC HAZARD EVALUATION 28 Cenozoic Volcanism in the NTS Region 28 Pattern and Tectonic Setting of Quaternary Volcanic Risk Assessments 32 Summary and Conclusions 34

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1. Active faults and fault zones in the vicinity of 2. Major faults within 320 km of Area 410 at NTS. 22 3. Stratigraphy of silicic volcanism in the NTS 1. Physiographic map of the Nevada Test Site 2. Strike-slip fault systems in the NTS area. 5 3. Faults and earthquakes in Frenchman Flat. 9 4. Seismic risk map. 11 5. U.S.G.S. seismic stations in the southern Great 6. Graph of acceleration versus return period for 7. Graph of acceleration versus hazard in 30-year exposure time for the RSSF. 16 8. Maximum expected peak acceleration in the NTS 9. Return period of peak horizontal acceleration for the Spent Fuel Test Site. 24 10. Location of eruptive centers in the NTS

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The Desert Research Institute (DRI) is currently evalu ating the suitability of Area 5 at the Nevada Test Site (NTS) for the disposal of low-level radioactive waste. The DRI study will document site characteristics and describe their bearing on site objectives. The Department of Energy, Nevada Operations Office, has specified that the characterization should consider, among other factors, the potential for tec tonic, seismic, or volcanic activity and the extent and fre quency with which these processes might affect the disposal operation. This report addresses these considerations. A number of organizations are presently participating in research projects which are investigating the seismic and volcanic hazards associated with radioactive waste disposal at the NTS. Among the principal participants are Los Alamos Scientific Laboratory, Lawrence Livermore National Labora tory, Sandia National Laboratories, the U.S. Geological Sur vey, and various other smaller organizations. As a result of their research, abundant information already exists pertain ing to Quaternary tectonic conditions (i.e. seismicity, faulting, volcanism, etc.) in the NTS region. Some of this information pertains directly to the Area 5 characterization, while some of it is not applicable. The primary purpose of this report is to review and summarize the existing data so that we can identify what research has already been done, what research is now in progress, and what additional research needs to be conducted for an assessment of the potential seismic and volcanic hazards of Area 5. The Nevada Test Site is located in the south central part of the Great Basin section of the Basin and Range physi ographic province (Figure 1). The topography of this

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province is characterized by north- to northeast-trending mountain ranges, separated by broad, linear valleys. Eleva tions range from 2000-3000 m in the mountain ranges and 9001500 m on the valley floors. In the vicinity of tne NTS, this series of ridges and valleys is locally disrupted t»y a large volcanic plateau and an associated complex of overlap ping collapse calderas. During the Paleozoic (600-225 million years before present (m.y.b.p.)), the NTS region was part of the Cordilleran miogeosyncline, a subsiding trough on the submerged western edge of the North American continent. This miogeo syncline, extending from Mexico to Alaska, received thousands of feet of shallow water deposition, derived from erosion of the nearby continental landmass. As a result, in excess of 30,000 feet of Paleozoic clastic and carbonate rocks were deposited in the NTS region. During the Mesozoic (225-65 m.y.b.p.), these rocks were complexly folded and thrust faulted in several periods of compressional deformation. Tne CP Thrust and the Mine Mountain Thrust are the major thrust faults formed during this time in tne NTS region. These episodes of mountain building were accompanied by intrusions of granitic plutons, which are represented by the Climax, Twin Ridge, and Gold Meadows stocks on the NTS. A major period of silicic volcanism began in the central portion of the Great Basin approximately 40 million years (m.y.) ago (McKee, 1971) and spread outward through time. The dominant volcanic activity in the NTS region began about 16 m.y. ago and continued at least until 0.25 m.y. ago. A complex of six collapse calderas, five of which overlap, was active along the western portion of the NTS between 16 and 6 m.y. ago. Ash-flow tuffs that erupted from these centers exceed 15,000 feet thickness under Pahute Mesa, a volcanic plateau in the northwestern portion of the NTS. A transition to basalt eruptions occurred approximately 6 m.y. ago.

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The crustal extension whicn produced nortn- to nortneast-trending normal faults began between 17 and 14 m.y. ago In southern Nevada (Stewart, 1980; Noble, 1972). Oplirt and subsidence along these faults resulted in the present-day system of mountain ranges and topographically closed basins. Alluvium and colluvium from the mountain ranges has filled the basins to depths of several hundred meters or more. The Area 5 Radioactive Waste Management Site is located in the southeastern portion of the NTS, on Frenchman Flat. This northeast-trending basin is underlain by approximately 1300 ft. of alluvium and colluvium, intercalated with basalt lava flows (Carr et al., 1975). The alluvial fill rests on faulted ash-flow tuffs, which are in turn underlain by the Paleozoic basement rocks. Frenchman Flat is surrounded by fault-block mountain ranges consisting of Cenozoic volcanic rocks to the north and west, and Paleozoic carbonate and clastic rocks to the south and east. The structural situ ation is discussed in more detail in tne tectonic section of The change from compressional to extensional tectonics which began approximately 17 m.y. ago marks the emergence of the present tectonic regime in the southern Great Basin. Concurrent with the onset of crustal extension and block faulting, a series of northwest-striking lineaments developed along the west-southwest margin of the Great Basin. These lineaments form the Walker Lane-Las Vegas Valley snear zone (Figure 2), a strike-slip fault system along which at least 40 km of right lateral offset has occurred. Although vol canic deposits obscure the trace of the zone in the vicinity of the NTS, the Walker Lane and Las Vegas Valley snear zones

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Figure 2 - Major strike-slip fault systems on the NTS and surrounding areas (modified from Greensfelder

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are considered continuous (Carr, 1974). Movement along the Las Vegas Valley shear zone is believed to have taken place between 17 and 11 m.y. ago (Carr, 1974; Longwell, 1974; Ekren, 1968). Structural evidence suggests the Walker Lane may have been active prior to basin and range faulting and may be currently active (Carr, 1974). The Furnace CreekDeath Valley fault zone (Figure 2) , parallel to and southwest of the Walker Lane, is a related right-lateral shear zone along which recent movement has occurred. A second system of strike-slip faults in the southern Great Basin strike to the northeast. These faults are lertlateral and are complementary to the right-lateral shear zones. The most prominent zone, the Garlock-Caliente linea ment, extends from the end of the left-lateral GarlocK Fault in California, northeastward to Caliente, Nevada. Among the faults related to this shear zone are the Pahranagat shear system to the east of the NTS, and the Mine Mountain (not to be confused with the Mine Mountain Thrust), Cane Spring, and Rock valley fault zones in the southern portion of the NTS The mutual offsets of the right-lateral and left-lateral fault zones indicate the strike-slip systems developed simul taneously. It appears that movement on the three left-later al faults on the NTS may be responsible for the offset in the The high-angle normal faults (basin and range faults) in the vicinity of the NTS consist of two systems. The older system strikes northeast and northwest and was active prior to 17 m.y. ago. The younger system strikes north-south and is responsible for the present configuration of basins and ranges on the NTS. This system developed between 17 and 14 m.y. ago and is considered to be still active (ZobacK and Zoback, 1980). Most of these normal faults occur along the range fronts, but some also occur beneath valley alluvium. The Yucca Fault, in the central portion of Yucca Flat on the

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NTS, displaces alluvium at the surface, indicating movement on this fault occurred within the last few thousand years. Volcanism in the southern Great Basin is related both spatially and temporally to extension and normal faulting. The silicic volcanism in the NTS region, wnich resulted in a complex of six collapse calderas, was coincident with the movement along the previously described shear zones and nor mal faults. Carr (1974) suggests that the volcanism tends to be concentrated in areas where the right-lateral faults die out, particularly where the right-lateral zones step to the right several miles and their ends are connected by north east-trending left-lateral faults. Carr indicates that the northeast-trending faults act as rifts or spreading centers and he believes eruptive centers and calderas on the NTS occur along these "rifts". The Wanmonie-Salyer volcanic center occurs to the west of Frenchman Flat, where the north east-trending Cane Spring Fault intersects the southeast projection of the vtfalker Lane. Present Tectonic Situation on the NTS Alluvial Basins Much of the information presented here is summarized from Carr's (1974) investigation of stress orientation at the NTS. The present structural configuration of the NTS prob ably began to develop within the last few million years. Although this structural style was largely controlled by tne older established system of basin and range faults, the deformation involved a new phase of normal faulting oriented in a north-south direction. In the alluvial basins of the NTS, the new faulting resulted in the formation of deep detritus^-filled troughs, superimposed on the earlier north east-striking basins. Yucca and Frenchman fiats are two sucn areas of deep-basin formation. Tectonic evidence indicates that the structural style responsible for the deep-basin formation is continuing at the

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present time. This style is controlled by the general stress orientation in which the NTS region is being extended along an axis trending about N 50°W. The deep basins of the NTS region are, as a result, being extended in a northwest-south east direction. Furthermore, the current seismic and struc tural activity in the southwestern Great Basin appears to be concentrated in the subsiding alluvial basins, particularly along the conjugate right- and left-lateral shear zones dis On the NTS the left-lateral shear zones of particular interest are the northeast-trending Mine Mountain, Cane Spring, and Rock Valley fault zones, all of which are locally active (Rogers et al., 1981). These fault zones and the mutually offset one another. Carr indicates, however, that the majority of stress relief is presently occurring on the northeast-trending faults, particularly in the areas where they approach intersections with the northwest-trending faults. All faults on the NTS which show Quaternary (<2m.y.) movement strike either north or northeast. The fault zones which occur in the vicinity of the Area 5 site are shown in Figure 3. The Yucca - Frenchman Flexure is a right-lateral zone which has in the past been active concurrently with the Cane Spring fault zone. The seismicity associated with these zones is discussed in detail below. The NTS region is influenced by two zones of high seis micity: the San Andreas system along the California coast line and the Nevada Seismic Zone which extends from Owens Valley, California, northward past Dixie Valley in central Nevada. Although the NTS lies east of the Nevada Seismic Zone, the tectonic settings of the two areas are very simi lar; the Nevada Seismic Zone is currently experiencing crust al extension in a NW-SE direction and the majority of seismic

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Contact batwoon bedrock and alluvium «•«*Fault or tracturo ot Quaternary age Figure 3 - Southern Yucca Flat and Frenchman Flat area, showing structural pattern. Quaternary …