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Gravity and magnetic maps compiled for southwest Nevada reveal important geologic features of an area that includes a potential nuclear waste disposal site at Yucca Mountain. High-amplitude gravity lows characterize major valleys and nonresurgent calderas in the study area, whereas gravity highs reflect Paleozoic rocks at or near the surface. Short-wavelength magnetic anomalies reflect volcanic rocks in the study area, and high-amplitude magnetic highs are associated with known or inferred plutonic rocks. Magnetic anomalies also correlate with cinder cones in Crater Flat and northwest of the town of Amargosa Valley. Isolated magnetic anomalies south of the town of Amargosa Valley may reveal the presence of buried volcanic centers. INTRODUCTION The study area, about 110 km northwest of Las Vegas, includes a potential nuclear waste disposal site at Yucca Mountain (fig. 1) . Surface outcrops consist predominantly of Tertiary silicic volcanic and intrusive rocks (fig. 2) . Precambrian rocks, consisting of the Johnnie Formation and Stirling Quartzite primarily crop out in the easternmost part of the study area in the Halfpint Range. Paleozoic rocks crop out near the central part of the study area in an arcuate band in the Eleana Range, at Syncline Ridge, and at Mine Mountain. Paleozoic rocks also occur in the Striped Hills, Specter Range, and northeast of Mercury. At the southwest edge of the map, Paleozoic rocks crop out at Bare Mountain. Rocks of Mesozoic age are represented by scattered granitic plutons that include quartz monzonite stocks in the northern end of Yucca Flat. Rocks of Tertiary age mostly consist of voluminous ash-flow tuffs that range in age from about 26 to 7 Ma (Ekren, 1968). Two small exposures of Tertiary granitic rocks occur northwest of Wahmonie. GRAVITY AND MAGNETIC MAPS Isostatic gravity (Ponce and others, 1988) and aeromagnetic maps (Kirchoff-Stein and others, 1989) of the study area (fig. 1) have been compiled at a scale of 1:100,000. The isostatic gravity map (fig 3) is controlled by about 16,000 gravity stations reduced using a Bouguer reduction density of 2.67 g/cm3 . A regional isostatic gravity field was removed from the gravity anomalies by assuming an Airy-Heiskanen model for isostatic compensation of topographic loads (Simpson and others, 1983). On the basis of seismic refraction data, this model assumed a crustal -density of 2.67 g/cm3 , a normal crustal thickness at sea-level of 25 km, and a density contrast across the base of the In addition, an isostatic gravity map model crust of 0.4 g/cm3 . reduced using a Bouguer reduction density of 2.20 g/cm3 (fig 4) was compiled to aid in the interpretation of areas covered by relatively lower-density volcanic rocks. An aeromagnetic map of the study area was compiled by merging eight separate aeromagnetic surveys (fig 5). Each survey was compiled and merged by Kucks and Hildenbrand (1987), as part of a statewide aeromagnetic data compilation of Nevada (Hildenbrand and Kucks, 1988). Each data set was gridded at a 1-km interval, and either upward or downward continued to 305 m above the ground. Because of the large grid interval and the merging and continuation processes, caution was exercised in interpreting local anomalies and anonialies that cross survey boundaries. GRAVITY ANOMALIES The isostatic gravity map (fig 3) shows ci range of values from -65 mGal over Silent Canyon caldera, the lowest value in Nevada, to + 14 mGal over Bare Mountain in the southwest corner of the study area. Major negative gravity valued occur over the Timber Mountain caldera moat (-42 mGal), Yucca Mountain (-38 mGal), and Emigrant Valley (-40 mGal). Significant gravity highs with amplitudes of +6 to +10 mGal are associated with Paleozoic argillites and carbonate rocks. Major valleys in the study area, which! include Emigrant Valley, Yucca Flat, and Frenchman Flat, are characterized by highamplitude gravity lows that reflect thick alluvial basins. These gravity lows are caused by large density contrasts between basement rocks and lower-density basin fill. In addition, calderas are characterized by moderateto lar^e-amplitude gravity lows that reflect lower-density volcanic rocks, or superimposed local gravity highs that reflect higher-density resurgent domes. t The gravity low associated with the Silent Canyon caldera is one of the most conspicuous gravity features in Nevada. A largeamplitude elliptical gravity low trending N. 35° E. is associated with the caldera. The gravity low ov$r Silent Canyon caldera extends southward into the northern part of the younger Timber Mountain caldera, suggesting that the Timber Mountain caldera overlies the southern part of the Silent Canyon caldera. In contrast to the major low over the $ilent Canyon caldera, a nonresurgent caldera, a local gravity high of about 8 mGal in amplitude occurs over a resurgent dome i[n the central part of the Timber Mountain caldera (fig. 4) (Kane and others, 1981). A gravity high dominates the Black Mountain caldera in the northwest corner of the study area (fig. 3) and hcts an amplitude of about 6 mGal. The Oasis Valley and Sleeping Butte caldera segments are characterized by gravity gradients with lows to the east, and the Claim Canyon caldron segment (fig. 1) is overlain by a local gravity high. South of Timber Mountain, a local gravity low with an amplitude of about 30 mGal occurs over Crater Flat. Gravity data indicate that Tertiary volcanic rocks thicken toward Crater Flat, to a depth of about 3,000 to 4,000 m. A part of the gravity low extends east to Yucca Mountain in a narrow east-west zone near Drill Hole Wash indicating the presence of a structural basement low. This structural low may influence ground-water movement within the Yucca Mountain area. Along the east edge of Yucca Mountain, a gravity high reflects pre-Cenozoic rocks near the surface at a depth of about 1,100 to 1,400 m. Drilling confirmed the presence of Paleozoic rocks at a depth of 1,244 m (Carr and others, 1986). MAGNETIC ANOMALIES Aeromagnetic data are useful for estimating depth to magnetic basement and locating and delimiting plutons, calderas, and faults. One of the most conspicuous aeromagnetic anomalies is a large amplitude magnetic high over Climax stock in the northeastern part of the area (fig. 5) . This magnetic high is part of a regional magnetic trend in the northern part of the study area that includes magnetic highs over intrusive rocks at Gold Meadows and Twinridge stocks, Pahute Mesa, and Black Mountain (Bath and others, 1983). In the northern part of Timber Mountain, aeromagnetic highs are associated with upper parts of the Ammonia Tanks Member of the Timber Mountain Tuff. In the southern part of Timber Mountain, magnetic anomalies are of lower amplitude, and negative or no anomalies occur over the lower parts of the Ammonia Tanks Member. In the central part of the Timber Mountain caldera and along the southeastern part of Timber Mountain, the magnetic relationship of the upper and lower parts of the Ammonia Tanks Member appears to be reversed, indicating that the magnetic properties of these rocks have been altered by heating (see Kane and others, 1981). Another conspicuous feature of the aeromagnetic map is a large amplitude high at Calico Hills, in the south-central part of the study area (fig. 5) . Although a part of the anomaly probably reflects a pluton at depth, strongly magnetized argillite of the Eleana Formation is the principal cause of the anomaly (Snyder and Oliver, 1981). Farther east, at Wahmonie, a relatively large amplitude magnetic high occurs south of two exposures of granitic rocks, each of which are locally associated with magnetic anomalies. This indicates that the larger anomaly is also related to a granitic intrusion (Ponce, 1984) . An aeromagnetic survey flown at a barometric elevation of 2,450 m (Boynton and Vargo, 1963a,b) shows that magnetic anomalies at Calico Hills and Wahmonie occur along the distal eastern parts of a southeasttrending magnetic high across the study area. Circular magnetic anomalies in Crater Flat and northwest of the town of Amargosa Valley (formerly Lathrop Wells) correlate to known cinder cones. In addition, similarly shaped anomalies occur over alluvium in the Amargosa Valley, including an anomaly just south of the town of Amargosa Valley \ These anomalies may be related to buried basaltic volcanic centers. In summary, gravity and magnetic studies in southwest Nevada reveal important geologic features of |an area that includes a potential nuclear waste disposal site at Yucca Mountian. These studies are particularly useful for delineating calderas, volcanic centers, granitic intrusions, and basement rocks. In addition, these studies show that the eastern part of Yucca Mountain is located at the east edge of a large depression in the pre-Tertiary basement rocks. REFERENCES CITED Bath, G.D., Jahren, C.E., Rosenbaum, J.G., and Baldwin, M.J., 1983, Magnetic investigations, jja Geologic and geophysical investigations of the Climax stock intrusive, Nevada: U.S. Geological Survey Open-File Report 83-377, p. 40-77. Boynton, G. R. , and Vargo, J. L. , 1963a, Aeromagnetic map of the Cane Spring quadrangle and parts of the Frenchman Lake, Specter Range, and Mercury Quandrangles, Nye County, Nevada: U.S. Geological Survey Geophysical Investigations Map GP-442, scale 1:62,500. ____1963b, Aeromagnetic map of the Topopah Spring quadrangle and part of the Bare Mountain Quadrangle, Nye County, Nevada: U.S. Geological Survey Geophysical Investigations Map GP-440, scale 1:62,500. Byers, F.M., Jr., Carr, W.J., Orkild, P.P., Quinlivan, W.D., and Sargent, K.A., 1976, Volcanic suites and related caldrons of Timber Mountain-Oasis Valley caldera complex, southern Nevada: U.S. Geological Survey Professional Paper 919, 70 p. Carr, M.D., Waddell, S.J., Vick, G.S., Stock, J.M., Monsen, S.A., Harris, A.G., Cork, B.W., and Byers, F.M., 1986, Geology of drill hole UE25p#l: A test ho

Gravity and magnetic anomalies in the vicinity of Yucca Mountain and their geologic implications