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A NTS -99 USGS-4 74-136 1971

UNIT1CD STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY

Federal Center, Denver, Colorado 80225

GRAVITY SURVEY OF THE AMARGOSA DESERT AREA OF NEVADA AND CALIFORNIA

D. L. Healey and C. H. Miller

D,,Sn"U, - w1r

.. .{ . ._ .. . . CONTEh"S

Page

Abstract ------_-______-__- 1

Introduction ------a------I

Acknowledgments------2

Geologic setting------3

Density data------5

Gravity survey------8

Field work------8

Reduction of data------8

Interpretation------9------9

Depch control------9------9

Discussion of gravity anomalies------9

Gravity highs------11

Funeral Mountains-Bare Mountain-Bullfrog Hills------11

Spring Mountains-Lathrop Wells area------12

Mount Shador area------12

Resting Spring and Nopah Ranges------12

Highs in alluviated areas------13

Gravity lows------_ 14

Northern Amargosa Desert------14

Southern Amargosa Desert------15

Crater Flat-Yucca Mountain area------19

Mount Shador brasin------20

Pabrump Valley-Stewart Valley------20 CONTENTS--Continued

Page

Interpretation--Continued

Gravity lows--Continued

Greenwater Range------22

Indian Springs Valley------22

Gi:avity trends------23

References------27

-- - - - a- .-. ------I

ILLUTSTRArIONS

Page

Figure l.--Location map showing area of this report (shaded)

and regional distribution of high-angle faults in

southern Nevada and southeastern California------In pocket

2.--Generalized geologic and complete Bouguer gravity

anomaly map of the Amargosa Desert area, Nevada

and California------In pocket

3.--Generalized geologic cross sections determined by

two-dimensional analysis of gravity data------In pocket

4.--Trend map of major gravity anomalies, faults, and

inferred faults in the Amargosa Desert area of

Nevada and California------In pocket

TABLES

Table l.--Range and average dersity of rocks in the vicinity

of the Amargosa Desert------6

2.--Principal facts for gravity base stations in the

Amargosa Desert area------10

iii UNITED STATES NTS-99 DEPARTMENT OF THE INTERIOR USGS-474-136 1971 GEOLOGICAL SURVEY

Federal Center, Denver, Colorado 80225

GRAVITY SURVEY OF THE AMARGOSA DESERT AREA OF NEVADA AND CALIFORNIA

D. L. Healev and C. H. Miller

ABSTRACT

A gravity survey consisting of about 1,800 observations within the 2,400-square-mile area of the Amar-03a Desert has provided information regarding the basin-fill geology and buried normal fdults. Analysis of five two-dimensional geologic cross sections indicates that the valley fill ranges in thickness from seversa hundred to tlore than 7,OOC feet. The thickest fill apparently occurs between Lathrop Wells and 'Death Valley Junction and also in Mount Shador basin. At two locations steep gravity gradients are interpreted to represent high-angle normal faults hidden beneath the alluvium. Near Lathrop Wells one of these inferred faults may have been tha cause for the abrupt western terminus of outcropping pre-Cenozoic rocks in both the Striped and Skeleton Hills. The second inferred fault, near Death Valley Junction, may have been the cause of the southeast termination of the Funeral Mountains. The agreement between outcrops of pre-Cenozoic roc-a a-d the gravity anomalies is very good. Exceptions do occur, F3waver, and in these places the gravity anomalies are useful in delinea.ing the buried geologic structure.

INTRODUCTION

The southern and southeastern Amargosa Desert is an important discharge area of ground water coming from the Nevada Test Site. To

provide the basic knowledge for determining semiquantitatively the

relative proportions of ground water being contributed from the Nevada

Test Site and from other areas, it has been necessary to compile the

available geologic, geophysical, and hydrologic data and to supplement

these data by limited field investigations where necessary. This

report describes the results of the gravity survey. - - *--.--.----- * -t - _-- _'~..- I -

The gravity survey of the Nevada Test Site was expaL.lec into the

Amargosa Derert area to provide data regarding the subsuiface structure

and thicknesses oL Tertiary and Quaternary fill in the various basinc.

These data also add to our knowledge of the regional gravity gradient,

which aids in interpreting gravity data at the Nevada Test Site aud

northward. The purpose of this report is to make available the

additional knowledge of the buried structure gained through analysii

if these gravity data. The known and Infrred structural features are

compared with known structural features in adjacent areas. These data

will aid in synthesizing the structure of both the Amargosa Desert and

the Nevada Test Site.

ACKNOWLEDGMENTS

The work was done on behalf of the Nevada Operations Office of the

U.S. Atomic Energy Commission. We are indebted to the many people who

helped to make this report possible. Maut of the gravity observatiou:3

were made by F. E. Currey, who aio assisted with the .eduction of

these data. D. G. Murray, E. D. Seals, G. S. Erickson, R. C. Smith,

Ppter Dobrovolny, and D. C. Jackson also aided in the acquisition and

reduction of the gravity duta. R. M. Haziewood assisted with the

gravity survey of the Mount Shade., quadrangle and adjacent areas.

Particular thanks go to I. J. Winograd for his assistance in obtaining

the drill-hole data for the area.

We thank D. R. Mabey, U.S. Geological Survey, Denver, CAlo., or

permission to use some of his reconnaissance gravity Zata in this

. . . e~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ report and M. F. Kane, U.S. Geological Survey, Woods Hole, Mass., for

permission to use some of his gravity data in the western part of

Clark County.

GEOLOGIC SETTING

The Amargosa Desert area is in southern Nevada and eastern

California and roughly parallels the State line (fig. 1). The area

of this report, about 2,400 square miles, is approximately 90 miles

long (in a northwest direction) by a maximum of 35 miles wide and

covers all or parts of fifteen 15-minute quadrangles. It lies within

the Basin and Range physiographic province and is characterized by

north- and northwest-trending mountain ranges and intermontane valleys.

The topography differs from the typical Basin and Range topography in

that the valleys are not closed. The Amargosa River drains all but

the southeast corner of the area C(alker and Eakin, 1963, p. 5 and

pl. 1).

The northwest trend of the mountains and valleys has apparently

resulted from tectonic activity. A right-lateral shear zone, the

Walker Lane (fig. 1), which is inferred to trend southeastward across

the Amargosa tesert, was postulated by 'ianella and Callaghan (1934,

p. 3) and by Locke, Billingsley, and Mayo (1940). They project this

shear zone more than 300 miles from north of Reno into Las Vegas

Valley along a topographic trough. This topographic trough also

marks the area where the Basin and Range structural alinement changes

from north (on the north) to northwest (on the south). Longwell

Ii (1960, p. 202) traces the Las Vegas shear zone from a point east of

Las Vegas northwesterly into a topographic depression west of Mercurv

(fig. 1), but does not recognize the shear zone beyond this area.

Burchfiel (1960, p. 138) suggests that the Las Vegas shear zone may trend west through the central portion of the Specter Range (west of Mercury) and then swing northwest at a point southwest of Lathrop

Wells. F. G. Poole (oral commun., 1964) believes that the right-lateral displacement on the shear zone is taken up by a series of en echelon high-angle faults in the Specter Range. R. L. Christiansen (oral commun., 1964) thinks the right-lateral displacement is taken up by flexure.

Other major high-angle fault zones in adjacent areas are also shown on figure 1. The structural complexity of the surrounding region is well illustrated by the number and extent of these faults.

The apparent lack of faults in the area of this report is due mainly to a lack of geologic information. The fault pattern in figure 1 was taken from the Tectonic Map of the United States (Cohee, 1962), and most of the area of this report was unmapped, except in reconnaissance fashion, when the tectonic map was published.

The rocks that crop out range in age from Precambrian through

Cenozoic. A generalized summary of the geology of the region has been given previously by Healey and Miller (1962, p. 8). The mountains are composed almos; entirely of Precambrian and Paleozoic carbonate and clastic rocks, whereas the basins are filled with

Cenozoic volcanic rocks and alluvium (fig. 2). The pre-Cenozoic

k~~~~~~~~~~ 'Al

carbonate and clastic rocks form a nearly continuous ring around the

Amargosa Desert basin. Exceptions occur on the northwest, west of

Beatty; at Crater Flat, east of Bare Mountain; and in the Greenwater

Range, to the south. At these locations igneous intrusive and

pyroclastic rocks of Tertiary age crop out.

DENSITY DATA

Bulk-density determinations of the outcropping rocks and valley

fill of the Amargosa Desert area are lacking. Certain assumptions of

the rock densities, therefore, are made. Bulk densities determined

for similar rocks in adjacent areas and values taken from the literature

are applied. The average values determined from laboratory analyses

of samples (largely from the Nevada Test Site) and those taken from

the literature are listed in table 1. On the basis of these data the

valley fill is assigned a bulk density of 2.20 g/cc. This density

assumes that approximately 40 percent of the valley fill is alluvium

(1.90-2.00 g/cc) atnd the remainder is Cenozoic volcanic rocks (2.36 g/cc).

This assigned density (2.20 g/cc) is in the range used by other

writers in adjacent or similar areas. In the Death Valley region,

Habey (1963) assigned values of 2.4 g/cc and 2.3-2.4 g!cc to the

Cenozoic volcanic and sedimentary rocks, respectively. In southern

Owens Valley, California, Kane and Pakiser (1961, p. 13) assigned a

density of 2.0-2.4 g/cc to the poorly consolidated valley fill. In

northwestern Nevada, Thompson and Sandberg (1958, p. 1271) used

^-r 2.2 g/cc for the intermontane basin depofits in the Virginia City-Mount

.. i n~~~~~~~~~J *le 5

| %t _ . . _. . . z_.'.. .. .'''_! . _.- I

Table l.-m-Range and aver'setensity of rocks in the vicinity of the Amargosa Desert

Age and Range in Average Area where Source of rock type density density measured information (g/cc) (g/cc)

Quaternary: Alltivium 1.55-2.39 L.92 Depths of 540 D. D. Dickey to 1,500 ft (written co un., in Yucca Flat 1961-1963). Sand 1.4 -1.8 1.6 ------Jakosky (1950). Gravel ------1.89 ------Berman, Daly, and ______Spicer______(1942). Tertiary: Tuff 1.49-2.34 2.0±0.1 Nevada Test D. D. Dickey Site (written commun., 1961-1963). Altered tuffs 2.18-2.52 2.37 North Jackass Do. Flats Rhyolite 2.0 -2.70 2.36 Nevada Test Do. Site Basalt 2.50-2.87 2.68 Nevada Test F. M. Byers Site area (written coummtn., ______1961.2. Mesozoic(?): Granodiorite 2.66-2.68 2.67 Climax stock Izett (1960). Quartz mon- 2.60-2.63 2.61 Gold Meadows D. D. Dickey zonite stock (written camun., 1961 and 1962). Granite 2.52-2.81 2.67 ------Berman, Daly, and Spicer (1942). Paleozoic: DolVmite 2.83-2.85 2.84 N.W. Yucca C. H. Roach (oral Flat commun., 1959). Limestone 2.62-2.70 2.67 Nevada Test D. D. Dickey Site (written commun., 1961 and 1962). ; Quartzite 2.52-2.71 2.62 do. Do. Argillite ------2.65 do. Do. Precambrian: Schist ------2.68 ------Berman, Daly, and Spicer (1942). Slate 2.59-2.83 ------Do. Gneiss 2.6 -3.3 2.80 ------Jakosky (1950).

6 Rose area. These data suggest that 2.2 g/cc is probably a good

average value for the intermontane basin deposits.

The pre-Cenozoic rocks have a wide range in bulk density

(2.52-3.3 g/cc) and are assumed to average about 2.70 ghc. At the

Nevada Test Site these rocks are found to :iverage 2.67 g/cc. However, because of the Precambrian metamorphic rocks present locally the

2.70-g/cc value may be low in some areas.

The average density contrart between the pre-Cenozoic rocks and

the valley fill is estimated tt be 0.5 g/cc. This value and an alternate, which assumes a density contrast of 0.7 g/cc (2.70-2.00 g/cc), are applied in the interpretation of the gravity data. These two contrasts are thought to bracket the actual density contrast, as the

0.7-g/cc value is applicable for valley-fill density as low as

2.0 g/cc (R. M. Hazlewood, D. L. Healey, and C. H. Miller, written commun,, 1963). However, owing to the wide density range possible within the Cenozoic volcanic rocks, the possibility exists that some may have a density that approaches that of the pre-Cenozoic rocks

(as a- Pahute Mesa, north of the Nevada Test Site). if these dense

Cenozoic rocks should be appreciably thick the density contrast might be several tenths of a gram less than 0.5 g/cc and, therefore, the pre-Cenozoic surface might be much deeper than shown in the following interpretations.

7 GRAVITY SURVEY

Field work

Within the approximately 2,400-square-mile area of this report

about 1,800 gravity observations were mwde for an average station

density of 0.8 station per square mile. Most of these gravity

observations were made on benchmarks, section corners, or on photo-

grammetric spot elevations listed on the available topographic maps.

However, some horizontal and vertical position control was provided

by the Nevada State Highway Department. Approximately 75 miles of

planetable surveying was done by members of the gravity crew to fill

in areas not covered by adequate topographic maps or where insufficient

elevations were listed.

The photogrammetric spot elevations are considered accurate to

within about one-fourth the contour interval of the map by the

Topographic Division of the U.S. Geological Survey. Most of the maps

used had 40-foot contours, so that the accuracy is t10 feet for most

of these spot elevations. An elevation error of 10 feet results in

a Bouguer anomaly error of t0.6 milligal.

Reduction of data

Standard geophysical methods were used throughout this survey

to reduce all gravity observations. Details of reduction procedures may be found in geophysics textbooks, such as Nettleton (1940),

Heiland (1946), Jakcsky (1950), and Dobrin (1960).

I8 All gravity observations were reduced to a sea-level datum through ai network of base stations which are referenced to a national network base station (Woollard, 1958) at McCarran Field in Los Vegas, Nevada.

Principal facts for these bases are listed in table 2. A combined free air-Bouguer (elevation) factor of 0.06 milli-al per foot, which corresponds te a density of 2.67 g/cc (Nettleton, 1940, p. 55), was used to reduce the data. The standard correction for latitude

(Nettleton, 1940) was made. Terrain corrections through zone L

(Hamwer, 1939), a radial distance of 9 miles, were applied to most stations. The terrain effect for some stations was interpolated from the computed values of adjacent stations. The terrain effect is, in most instances, small except for those stations located in the mountains, and the resulting values for most stations are thought to be accurate to within 1 milligal.

INTEPRFPTATION

Depth control

Several drill holes penetrate the pre-Cenozoic rocks below the valley fill in the area southwest of Lathrop Wells (Walker and Eakin,

1963, table 4). However, these holes are concentrated in a small area and provide depth control for only a small portion of the basin.

Discussion of gravity anomalies

The complete Bouguer gravity anomaly map obtained from the

Amargosa survey is presented on figure 2. These gravity data ore corrected for instrument drift, latitude, elevation, and terrain,

9 Table 2.--Principal facts for gravity base stations in the AmarRo3a Desert area

Base Quadrangle Geographic Observed Description Elevation station coordinates gravity (feet) (lat and long) (mgals)

1-0 Specter Range 36°38'26" 979,537.9 BM Y-327. 1.3 miles S. of 3,544 116°00'14"1 main gate at Mercury on Mercury highway.

13-0* Lathrop Wells 36038'40" 979,595.0 BM B-333 (1952), N. side 2,660 116°24'00h of Y Junction of U.S. 95 and Nevada 29 at Lathrop Wells.

23-0* Ash Meadows 36018 15" 979)617.9 B14 R-672, 0.5 mile W. of 2,060 116°25128" Death Valley Junction. I-$ 0 Beatty* Bullfrog 36°54'25" 979,582.4 BM M-16, 0.2 mile E. of 3,284 116045'22" Junction U.S. 95 and Death Valley Road in Beatty.

* Established by D. R. Mabey, U.S. Geological Survey.

I.. V .. l

but the regional gradient has not been removed. The contour interval

is 5 mgals and all values are negative. Station locations are shown but not the Bouguer values. The map scale is 1:125,000.

The areas where pre-Cenozoic rocks crop out are delineated in gross form on figure 2. Inspection of the gravity map shows that

these outcrop areas are associated with Gravity highs. The valleys or basin areas generally are associated with gravity lows, owing to the

low deasity of the valley fill. However, exceptions do occur and several gravity highs occur in alluviated areas, as mentioned below.

Cravity highs

Funeral Mountains Bare Mountain-Bullfrog Hills

The highest gravity anomaly (-80 ngals) in the area of this report

is associated with the northwest-trending Funeral Mountains along the

northwest edge of the map area (fig. 2). This very prominent high

extends the full length of the mountains. A prominent gravity nose

trends north from the Funeral Mountains across the Amargosa Desert to

the south end of Bere Mountain. This nose coincides with a nortL-

plunging anticline, the axis of which is seen in a series of small

outcrops of pre-Cenozoic rocks in the valley (fig. 2). Beyond a

saddle located near U.S. Highway 95, the gravity high of Bare Mountain

is arcuate to the northwest as far as the area of outcropping

pre-Cenozoic rocks in the Bullfrog Hills, west of Beatty.

11 Spring Mountains-Lathrop Wells area

The Spring Mountains lie along the east edge of the map area.

The mountains trend northwest, and the outcropping pre-Cenozoic rocks of the range are continuous westward through the Specter Range, the

Striped Hills, and the Skeleton Hills almost to Lathro2 Wells. The gravity high associated with this block extends almost continuously

from a point s chin Clark County, southeast of the map area

(M. F. Kane, written commun., 15..), to the west .nd of the block.

South and southeast of the Skeleton Hills the gravity high is connected by a gravity ridge broken by two saddles to a high associated with the pre-Cenozoic rocks in the Mount Shador area.

Mount Shador area

The Mount Shador area, at the northwest end of the Spring Mountains, includes the hills of outcropping pre-Cenozoic rocks that lie south of the Specter Range, west of the Spring Mountains, and north of the Resting

Spring and Nopah Ranges. The gravity anomaly consists of a prominent

Y-shaped gravity high that trends north-northeast into the northern

Spring Mountains and northwest toward the Skeleton Hills. This high is marked by two closed -105-mgal contours and a closed -110-mgal contour. The two closed -105-mgal contours are partially separated by a low (-110 mgals) that trends southwest from near Johnnie.

Resting Spring and Nopah Ranges

The Resting Spring and Nopah Ranges are largely in California; their locations are shown in the south-central part of figure 2. These

12 i...... ,,,,, ~~~~~~~~~~~~~~~~~~~1~ two ranges are separated from the Mount Shador area by Stewart Valley.

A topographic divide separates the two ranges at the north end of Chicago

Valley. Gravity data along the tops of both ranges are lacking, but available observations indicate a prominent high of at least -85 mgals along the Resting Spring Range and probably -95 mgals on the Ncpah

Range.

Highs in alluviated areas

Prominent gravity highs occur in two alluviated areas. The first and larger of these highs is defined by the -105-mgal contour, mentioned above, iv the northern part of the Mount Shador area, west of Johnnie.

This high may be accounted for by either one of two hypotheses. It may reflect buried Precambrian metamorphic rocks with a higher bulk density beneath the Johnnie Formation, which crops out at two places crossed by the -105-mgal contour. If this hypothesis is correct the Johnnie

Formation is not underlain by additional clastic strata of the Patrump

Group as suggested by R. H. Moench (written commun., 1965). On the other hand, the Precambrian Johnnie Formation could be the sole of a thrust block that has overriddcz the small outcrop of Cambrian Bonanza

King and Nopah Formations 1 mile north of this -105-mgal contour. If this second hypothesis is correct the high may be caused by a thick section of these carbonate rocks, which also have a higher bulk density than the Johnnie Formation. In either case, the rocks cauaing the anomaly are buried by only a thin veneer of alluvium.

13 rhe second high over an alluviaced basin vccurs about 6 miles southwest of Lathrop Wells, almost midway between the Funeral Mountains

and the Skeleton Hills. This high (-115 mgals) is apparently caused by pre-Cenozoic rocks at shallow depth. Thi. high will be discussed

further in conncction with the gravity lows.

Two small gravity-high noses occur near this closed -115-mgal high; both are associated with small eatcrops of pre-Cenozoic rocks that protrude through the alluvium. One is located at Leeland (abandoned) and the second is about 7 mileE south and extends across the California

State line into Nevada.

Gravity lows

Northern Amargosa Desert

The northern Amargosa Desert, as herein defined, includes the area enclosed by the Funeral Mountains, Bare Mountainr, and the Bullfrog Hills.

The gravity anomaly within this bs in is a low that trends southeastward from southwest of the Bullfrog Hills to a closed -120-mgal low south of

Bare Mountain (fig. 2). In an attempt to define the approximate thickness of valley fill, a two-dimensional cross section (after 1ibrin,

1960) was constructed along iine A-A' (figs. 2 and 3). In this interpretation, and those that follow, density contrasts of 0.5

(2.70-2.20) and 0.7 (2.70-2.00) g/cc were applied. As mentioned earlier, the 0.5-g/cc contrast is believed to be a good average figure and the 0.7-g/cc contrast should provide the minimum thicknesses of valley fill t6 be expected.

~~~~~~~~. - .* - - - Cross section A-A' extends from the Funeral Mountains to Bare

Mountain. The residual anomaly (Bouguer anomaly minus the regional

gradient) attains a maximum value of 19 mgals, and the indicated

thicknesses of valley fill are 2,300 (0.7 g/cc) and 3,500 (0.5 g/cc)

feet, respectively. The subtraction of the regional gradient removes

the gravitational effects caused by deep-seated masses. The removal

of these effects leaves only those caused by near-surface density

contrasts. By assuming that the low-density valley-fill material causes

all the low anomalies, a straight-line regional gradient connecting the

gravity highs can be drawn. This assumption of the regional gradient

was made on four of the five geologic cross sections included. On

cross section C-C' it was necessary to curve the regional gradient

near the west end to show the small pre-Cenozoic outcrop at the

California-Nevada State line in a geologically reasonable configuration.

A cross section through the closed -120-mgal low, south of Bare

Mountainhas been reported previously (Healey and Miller, 1962). This

earlier interpretation was made with an assumed density contrast of

0.7 gicc and indicated a thickness of-2,400 feet for the fill. If a

contrast of 0.5 g/cc were applied to these data the indicated depth

of fill would be about 3,500 feet, providing a range similar to that along A-A'.

Southern Amargosa Desert

The southern Amargosa basin includes the area bounded by the

Funeral Mountains, Bare Mountain, U.S. Highway 95 east to the Skeleton I 15 }tills, and a line south along the line of pre-Cenozoic outcrops to

the Resting Spring Range, west to Eagle Mountain, and then northwest

to the Funeral Mountains.

The low-gravity anomalies found in this and the adjacent basins

are generally alined northwest or north. A major exception occurs

about 6 milas south-southwest cf Lathrop Wells. Here the gravity-low

anomaly has a northeast lineation that is at right angles to adjacent

lowb.

To interpret the southern Amargosa area, three two-dimensional

cross sections were constructed and analyzed (fig. 3). Cross section

B-B' extends from the Funeral Mountains northeastward to the Striped

Hills. A maximum residual anomaly of 15 mgals occurs in the low

saddle south of Lathrop Wells. In this low area the valley fill is

interpreted to be 2,000 and 2,800 feet thick, respectively, assuming

density contrasts of 0.7 and 0.5 g/cc. The analysis implies the

possibility of a high-angle fault, with from 100 to 1,500 feet of

vertical displacement about 2 miles east-southeast of Lathrop Wells,

where the profile crosses U.S. Highway 95.

At the closed -115-mgal high (near center of cross section B-B')

the pre-Cenozoic rocks may be buried by 600-1,200 feet of valley fill.

East of this high the buried surface of the pre-Cenozoic rocks appears

to be nearly level throughout a horizontal distance of about 5 miles.

Depth of burial for this segment of the cross section is interpreted

to be 1,400 and 1,800 feet (for the two contrasts). About 4 miles

I~~_ . ~ ~. ~ ~. ~.. ~~ ~. _ ...... ,_,_...... _,...... east of the west end of the section a fault with vertical displacements

of 600 and 1,000 feet is inferred from the interpretation.

Cross section C-C' extends from the Funeral Mountains rortheastwagd

to the outcropping pre-Cenozoic rocks located south of the Skeleton

Hills. This cross section intersects an area of outcropping pre-Cenozoic

rocks at the California-Nevada State line. It is assumed that the

gravity anomaly associated with this outcrop is on, or nearly on, the

regional gradient. Therefore, from this point eastward the regional

gradient is taken to be level. To the west the gradient is curved

upward until it merges with the Funeral Mountains anomaly. The

resulting residual anomaly of C-C' attains a maximum of 14 mgals

inside the closed -125-mgal contour. This interpretation indicates

the valley fill to be 1,500 and 2,300 feet thick, respectively, for the

two density contrasts (0.7 and 0.5 g/cc). The steep slope on the

bedrock surface about 5,000 feet from the east end of the cross section

implies a high-angle fault with about 1,200-2,200 feet of displacement.

At the California-Nevada State line a high-angle fault with a displacement of 1,009-1,500 feet (down to the west) is inferred from the

interpretation.

Cross section D-D' extends northeastward from a point near the

southern end of the Ftneral Mountains to a series-of northwest-trending

hills of pre-Cenozoic rocks. The edges of two gravity lows are

intersected by this line and the residual anomaly is about 24 mgals

at each low. The computed depths of the low near the California-Nevada

State line along D-D' are 3,000 and 4,000 feet for the two density

17 contrasts. Because of the anomaly configuration, the low near the

east end of the cross section indicates a thicker section of Cenozoic

rocks. The computed thicknesses here are 3,400 and 5,200 feet,

respectively. The steep gravity gradient (about 10 mgals per mile)

at the east end of the line apparently represents major structure

and implies a high-angle fault of several thousand feet of vertical

displacement. Near the west end of this cross section a high-angle

fault with a-out 800 feet of vertical displacement may be inferred

from the interpretation.

The intermediate-high trend that separates these two lows occurs

in an area where undifferentiated Cenozoic igneous and sedimentery

rocks c'.i out. Part of this high might be associated with these

rocks if their bulk density is appreciably higher than the adjacent valley fill. A subsurface structural ridge of pre-Cenozoic rock could also cause this gravity high. The interpretatica that the buried ridge causes this anomaly is favored. Near Ash Meadows Lodge a gravity

low extends across an outcrop of these same Cenozoic igneous and sedimentary rocks and ter4d to discount their influence on the anomaly.

The cross sections described above do not show the thickest valley fill, as none of them intersects the lowest parts of the gravity anomalies. The valley fill in the lowest part of the three closed

low-gravity anomalies may be 500-800 feet thicker than shown on any of the cross sections.

18 Crater Flat-Yucca Mountain Area

The Crater Flat-Yucca Mountain area, at the north edge of the map, includes the terrain east of Bare Mountain, north of U.S. Highway 95, and west of the Striped Hills. The area is just south of the Timber

Mountain volcanic caldera, and a thick section of Cenozoic volcanic rocks was deposited here.

Cross section E-E' extends from Bare Mountain northeastward to near the Calico Hills, an area where pre-Cenozoic rocks crop out. The maximum residual anomaly is about 37 mgals and indicates thicknesses of about 5,600 and 7,900 feet based on the 0.7- and 0.5-g/cc density contrasts.

Cornwall and Kleinhampl (1961a) mapped a higt;-angle fault on the east side of Bare Mountain. The steep gravity gradient here also suggests a fault, and the interpretation implies about 1,200 feet of vertical displacement. A second interpretation (not shown) of this fault zone indicates that there is about 3,500 feet of displacement about 5 miles farther north. These data, although tenuous, may indicate increasingly greater vertical displacements to the north along this fault.

A band of altered volcanic rocks extends around the pre-Cenozoic rocks that crop out in the Calico Hills. This alteration, of unknown horizontal and vertical extent, complicates the interpretation of the gravity data in this area. Therefore, the configuration presented at the east end of the cross section may be somewhat in error.

19 Mount Shador basiu

The Mount Shador basin area, as herein defined, includes all the topographically low are& centered near lat 36°30' and long 11615'.

With the exception of two gaps on the west and northwest, the area is encircled by outcropping pre-Cenozoic rocks. Surface drainage otit of the basin is through the topographic low between Fairbanks and Longstreet

Springs on the southwest side.

The gravity data show a bifurcating low anomaly with the deepest low (-130mgals) centered near the northwest end of the playa. The main a=m of this low trends north and northwest to the vicinity of the

Skeleton Hills and is marked by a closed -125-mgal low. The second arm trends northeast towards the Specter Range. A north-trending gravity high occurs between the Mount Shador basin gravity low and a similar

-130-wyal closed low on the southwest that is centered near Jap Ranch.

No analysis of this area has been made to date. However, if the density contrast here is similar to that of the -130-mgal anomaly analyzed by cross sections C-C' and D-D' the depth to pre-Cenozoic rocks should be the same, or about 3,500-5,000 feet.

Pahrump Valley-Stewart Valley

Pahrump Valley, in the southeast part of map area, includes the topographic depression between Mount Sterling, the Mount Shador area, and the Nopah Range and extends southward off the map. Stewart Valley, along the California-Nevada State line, is also included in this area.

20 The gravity survey of Pahrump Valley is not complete. The limited number of observations, part of which were established by M. F. Kane as part of his Clark County survey (Kane and Carlson, 1964), suggests that a low may extend from near Johnnie southward the length of that part of the valley shown on figure 2.

The steep gravity gradient east of the northern end of the Nopah

Range implies steep relief on the buried pre-Cenozoic rocks or a high-angle fault. A high-angle fault extending the length of the range has been mapped (Wilhelms, 1962) and the gravity gradient may define this fault.

In Stewart Valley a closed -115-mgal gravity low extends across the State line. This low shows Stewart Valley to occupy a narrow structural trough. R. H. Moench (written commun., 1965) mapped the fault that trends northwestward along the east side of the valley.

This fault is down to the west, and Moench suggests that there is as much as 10,000 feet of vertical displacement.

The -120-mgal gravity low centered near Big Spring and Bole

Spring (northwest of Stewart Valley) appears to be a continuation of the Stewart Valley anomaly. This low may be caused by the continuation of the Stewart Valley fault, which projects into the area. Such a fault may continue northward and join the inferred fault shown near the east ends of cross sections B-B', C-C', and D-D'.

Five springs extend in a line southward across this anomaly.

Moench interpreted this alinement of springs to indicate near-surface carbonate rocks. However, as these springs occur within the gravity

21 low, near-surface carbonate rocks seem to be ruled out. The gravity

nose that trends northwest from the Resting Spring Range may represent

a buried ridge of pre-Cenozoic rocks that acts as a ground-water

barrier and forces the water from these springs to the surface. The

alinement of these springs suggests fault control.

Greenwater Range

The Greenwater Range area includes the area south and west of a

line connecting the Funeral Mountains, Eagle Mountain, and the Resting

Spring Range.

The gravity anomaly is a large -115-mgal gravity low that is

centered near Deadman Pass. This low has been described briefly by

Mabey (1963), who feels the anomaly may reflect either relief on the

pre-Cenozoic bedrock surface or density variations in the overlying volcanic rocks.

These data are reproduced here to show the southward continuation of the gravity low into this area through a saddle west of Eagle

Mountain. The questionable Furnace Creek fault zone may extend across

the area just southwest of Eagle Mountain. Such a fault, if present, may exert some influence on the location and magnitude of this broad

gravity low.

Indian Springs Valley

Indian SprTngs Valley is the topographic depression that extends southeastward across the northeast corner of the map.

,~~~~~~~~~~~~~~~ 5 'I The gravity anomaly consists of a lineation of two closed

-130-mgal lows and one closed -135-mgal low. These lows apparently represent separate basins along the Las Vegas shear zone (C. H. Miller and D. L. Healey, written commun., 1963). Geologic interpretation of these gravity lows as reported by C. H. Miller and D. L. Healey shows the two -130-mgal lows to represent a structurally connected basin with the deeper area (about 1,900 feet) associated with the northwestern low.

The -135-mgal low was interpreted to define a bosIn containing about

1,000 feet of fill material. Separating these two basins is a buried ridge of pre-Cenozoic rock covered by about 650 feet of valley fill.

Gravity trends

The major gravity trends, both high and low, discernible in the gravity data, are plotted, along with some of the more prominent fault-s, on figure 4. The variable nature of these fault trends is well illustrated and a dominant trend is lacking. Generally speaking, in the western part of the Amargosa Desert area, the trend of the gravity anomalies is northwest; in the central portion it is north or northeast; and in the east and south the trend on the larger anomalies is north with components to the northwest and northeast. The shorter trends are usually northeast or northwest.

The highly variable pattern of trends points out the structural complexity of the region. The many thrust faults mapped in the area

(some of which are shown on figures 2 and 4) indicate that the area has been subjected to very strong compressive forces, probably from

23 the northwest. The lateral movement associated with this thrusting

and the subsequent high-angle faults have produced the complex geologic

structure we are attempting to define by analysis of the gravity data.

The general agreement between the gravity data and the known

geology is very good. as was pointed out in the discussion of the

highs and lows. The plot of the high and low trends (fig. 4) helps

to shoa the position ef the highs in relation to the outcropping

pre-Cenozoic rocks and the lows to the b-asin areas.

The fault implied on cross sections B-B', C-C', and D-D' is shown by the hollow dashed line. A fault in this area, down t, the west, is a possible explanation for the abrupt terminus of the outcropping pre-Cenozoic rocks west of the Skeleton Hills-Striped Hills area (see generalized geology of fig. 2). A second fault, inferred from the

interpretation, is plotted near the west end of cress section D-D' and

is shown as extending south of Death Valley Junction to the projected

Furnace Creek fault, which may or may not extend through œere. This

fault inferred from gravity data may account for the topographic low that occurs between the Funeral Mountains and Eagle Mountain.

In the vicinity of Skeleton Hills three gravity trends, two highis separated by a low, cut across the trace of a postulated thrust fault, named the Specter Range thrust by R. H. Moench (written cowmun., 1965).

These gravity anomalies seem little affected by this postulated thrust.

Little or no density contrast is indicated in the pre-Cenozoic rocks on either thrust plate. The low in the center is thought so define a small syncline mapped in this area.

24 Only a small percentage of the faults recognizable in the field are shown on figures 2 and 4. A comparison of this fault pattern to that of the surrounding region may be made by referring again to figure 1. The fault pattern of figure 1 shows only the high-angle faults and shear zones, as the addition of the many thrust faults would clutter and confuse the illustration.

It is evident from figure 1 that this entire region has been complexly faulted--many faults extend tens or even hundreds of miles.

The dominant trends in the surrounding region are northwest and north.

Trends in the Amargosa Desert area are less consistenr except fir the area in California, aad perhaps the Las Vegas shear zone.

The Walker Lane (fig. 1), first described by Gianella and

Callaghan (1934), was named by Locke, Billingsley, and Mayo (1940).

These writers believed that the structural significance of the Walker

Lane would become as great as that of the San Andreas fault zone, which it parallels. This prediction has not yet been fulfilled, and the Walker Lane is still a controversial geological subject. The

Walker Lane might best be defined as a wide, right-lateral shear zone across which topographic and tectonic elements change abruptly. On the northeast side of this zone structural (and topographic) elements are generally parallel and are aline' approximately north-south. On the southwest side of the zone, these elements lack this strong parallelism and have no preferred directional orientation. Much of the ar-a of the Walker ;Lane between Beatty and Goldfield (fig. 1) bas

25 not been geologically mapped in detail, and the important right-lateral structures of the zone may be largely buried by Cenozoic rocks.

The course of the Walker Lane in the area of this resort is nct defined by any gravity trends along it that are obviously associated with the actual zone of right-lateral movement. It may, however, be indirectly defined by the relative regularity and subparallelism of gravity anomalies between the Calico Hills and Mercury as contrasted to the more varied arrangement of the anomalies to the southwest.

26 REFERENCES

Berman. H., Daly, R. A., and Spicer, H. C., 1942, Density at room

temperature and 1 atmosphere, Sec. 2 of Birch, A. F., Shairer,

J. F., and Spicer, H. C., eds., Handbook of physical constants:

Geol. Soc. America Spec. Paper 36, p. 8-26.

Bowyer, Ben, Pampeyan, E. H., ond Longwell, C. R., 1958, Geologic map

of Clark County, Nevada: U.S. Coeol. Survey Mineral Investigation

Field Studies Map MF-138.

Burchfiel, B. C., 1960, Sttx ai..i stratigraphy of tthe Specter

Rangte quadrangle, Nye County, Nevada: Yale University, Unptub.

Ph.D. thesis.

Cohee, G. V., Chairman, 1962, Tectonic map of the United States:

*U.S. Gtol. Survey and Am. Assoc. Petroleum Geologists.

Cornwall, H. R., and Kleinhampl, F. J., 1961a, Geologic map of the

Bare Mountain quadrangle, Nevada: U.S. Geol. Survey Geol.

Quad. Map GQ-157,

1961b, Prelim'iary geologic map and sections of the Bullfrog

quadrangle, Nevada-California: J.S. Geol. Survey Mineral

Investigation Field Studies Map MF-177.

Dohrin, M. B., 1960, Introduction to geophysical prospecting, 2d. ed.:

; iNew York, McGraw-Hill Book Co., p. 435.

Gianella, V. P., and Callaghan, Eugene, 1934, The earthquake of

December 20, 1932, at Cedar Mountain, Nevada, and its bearing

on the genesis of Basin and Range structure: Jour. Geology,

| v. 42, no. 1, p. 1-22.

27 Hammer, Sigmund, 1939, Terrain corrections for gravimeter stations: Geophysics, v. 4, no. 3, p. 184-194.

Healey, D. L., and Miller, C. H., 1962, Gravity survey of the Nevada

Test Site and vicinity, Nye, Lincoln, and Clark Counties,

Nevada--Interim report: U.S. Geol. Survey TEI-827, open-file report, 36 p.

Heiland, C. A., 1964, Geophysical exploration: New York, Prentice- Hall, Inc., p. 1013.

Izett, G. A., 1960, "Granite" exploration hole, Area 15, Nevada Test

Site, Nye County, Nevada--Interim report, Part-C, Physical

properties: U.S. Geol. Survey TEM 836-C, open-file report, 37 p.

Jakosky, J. J., 1950, Exploration geophysics, 2d ed.: Los Angeles, Trija Publishing Co., p. 1195.

Jennings, C. W., 1958, Geologic map of California, Death Valley

Sheet: California Dept. Nat. Res., Div. Mines.

Kane, M. F., and Pakiser, L. C., 1961, Geophysical study of subsurface structure in southern Owens Valley, California: Geophysics, v. 26, no. 1, p. 12-26.

Kane, M. F., and Carlson, J. E., '964, Gravity observations and

N Bouguer anomaly values for Clark County, Nevada: U.S. Geol. Survey open-file report.

Locke, Augustus, Billingsley, Paul, and Mayo, E. B., 1940, Sierra

Nevada tectonic patterns: Geol. Soc. America Bull., v. 51, no. 4, p. 513-539.

A__Ub - - va - , Longwell, C. R., 1960, A possible explanation of diverse structural

patterns in southern Nevada, in Bradley Volume 258-A: Am.

Jour. Sci., p. 192-203.

Mabey, D. R., 1963, Complete Bouguer anomaly map of the Death Valley

region, California: U.S. Geol. Survey Geophys. Inv. Map GP-305.

Nettleton, L. L., 1940, Geophysical prospect-in Ah- -oil, 1st ed.:

New York, M~cGraw-Hill Book Cc.

Thompsou, C. A., and Sandberg, C. H., 1S58, Structural significance

of gravity surveys in the Virginia City-Mount Rose area, Nevada

and California: Geol. Soc. Amerlca Bull., v. 60, uc. 10,

p. 1269-1282.

Walker, G. E., and Eakin, T. E., 1963, Geology and ground water of

Amargosa Desert, fIevada-California: Nevada Div. Water Resources,

Water Resources-Reconn. Ser.e Rept. 14, _587 p.

Wilhelms, D. E., 1962, Geology of part of the Resting Spring and

Nopah Ranges, Inyo Cotmty, California: University of California

at Los Angeles, Ph.D. thesis.

Woollard, G. P., 1958, Results for a gravity control network at

airports in the United States: Geophysics, v. 23, no. 3,

p. 520-535.

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EXPLANATION

CENOZOIC ROCKS ,NCL.UDES QUATERNARY ALLUVIUM AND TERTIARY VOLCANIC

" i 4iL

.\ 14 ¼ PRE-CENOZOIC ROCKS :IPi .ixw INCLUDES MESOZOIC tND PALEOZOIC CARBONATE AND CLASTIC I. ROCKS AND PRECAMBRIAN METAMORPHIC ROCKS

(

NORMAL FAULT, BASED ON SURFACE GEOLOGY; DASHI 1) WHERE INDEFINITE; OOTTEO WHERE INFERRED. BAR AND BALL ON 'j. DOWNTHROWN SIDE 4 ~~ ~~AA & A . THRUST FAULT, BASEO ON SURFA^.E GEOLOGY; SAWTEETH 'v. h INDICATE UPPER PLATE. DOTTED WHERE INFERRED. .k ._I..F ...... E

N ,4 ANITICLINE, SHOWING DIRECTION OF PLUNGE; DOTTED WHERE j I INIFERRED.

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.0 f iI i I II I l EXPLANATION

CENOZOIC ROCKS INCLUDES QUATERNARY ALLUVIUM AND TERTIARY VOLCANIC ROCKS

PRE-CENOZOIC ROCKS INCLUDES MESOZOIC AND PALEOZOIC CARBONATE AND CLASTIC ROCKS AN4D PRECAMBRIAN METAMORPHIC ROCKS II

NORMAL FAULT, BASED ON SURFACE GEOLOGY; OASHETJ WHERE INDEFINITE; DOTTED WHERE INFERRED. BAR AND BALL ON DOWNTHROWN SIDE -A. A . . A . . 'I THRUST FAULT, BASEO ON SURFACE GEOLOGY; SAWTEETH INDICATE UPPER PLATE. DOTTED WHERE INFERRED. i

i m ...... >. -N i 3 I ANTICLINE, SHOWING DIRECTION OF PLUNGE; 001 WHERE 11,0 INFERRED.

0 LOCATION OF GRAVITY STATION.

125

GRAVITY CONTOUR CCNTOUR INTERVAL. 5 MILLIPALS; ALL VALUES NEGA~iVE, GP.AV'TY-LOW AREAS ARE HACHURED. 1 2 4 6 _ 1 : _ _i - __ ._. . - _ ..

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I'.. - I-~-: - C20z~ i ~ t Irita * * *x d DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY 1170 121- 120° I19° 1180

I I ) . i I II II

390

,/ qy 1 4 380

IFIELD

BEATTY 370 INDEPENDENCE

36e

35. a I ! .4I

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340 FAJL.' PATTERNS r RC% ' 25 50 75 10 MILES STATES WALKER LANE

-i h1

I Figure l.--Location map showing area of this report (shaded) and regional distribution of

t i t iI 4

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TfIk, p,~k ,~" r, V. C,4NOMN Q#;;) -FC-TONIC MAP OF THE UNITED WALKER _ANE GENERAL.IZED FROM i OCKE, BILLINGSLEY, AND MAYO (1940).

ribution of high-angle faults in southern Nevada and southeastern California I PI4 DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY

BOUGUER GRAVITY PROFILE

- 4, e~ 0 0 ILa- oi! w> o 0{,> w , z z iC,! I;!A. -J i i FUNERAL MTS. A bd A R C OS A

PRE- CECNOZOIC ROCKS I

SURFACE OF tat* CENOZOIC RtOCKSASSuM1kG A DEt4S1TY COOKYRST Of~ O.T(,/ CC} a

BOUGUER GRAVITY PROFILE (0 N% a>

V.~~~~~~~~~~~~~~~~~~~~~~~~~~~~ l i OS A D E S E R T

CENOZOIC VOLCANIC ROCKS AND ALLUVIUM

- iRA-O ~ ENZI OK SJI- ~~~~~~~~~~~i-SURFACE Of PRE - CENOZOIC ROCKS ASSUIVIN 8

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100

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125 I- 012. 130 I ui4 40 BARE MTN. 6000

4000 w PRE-CENOZOIC ROCKS w 2000 , 3M- SYINCG A DENSITY ONTRAST OF 0a a it/cc SEA LEVEL

BI 90

100 REGIONAL GRADIENT / 105

46 110 40

113 :i

120

125

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-. -r - MAareACel.Ug..fA flei,TvnS -rstACT flh A T , *WuvnL yr - -…-- - - - ' '4 - I WI

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BOUGOJER GRAVITY PROFILE ? ¢i

* 0 AL 0 tro enI A i¢ l U.I 0 >. a w X I 'II _FUNERAL MITS. AMARGOSA DESE R 1

0 PRE-CENOZOIC ROCKS I I SURFACE Of PRE-CENOZOIC ROCKS ASSUMING A DENSITY CONTRAST OF i I

_%..S.68OUGUER GRAVITY PROFILE

,~~~~~~~~~~~~~~~~ Wo _ 3~ Yl I _qI@ Il * C

P~~~~AGEilI BLA3K ii' N:t .,'a*0>

110 46 Locs 0-4j 115 i1

. ~~~~~~~~~~~~~~~~~.-~- WI 3t

ZOr | A M A R G OSiIO ~~A OInSER I A MA RGO0S A 0 ES ER T CENOZOIC VOLCANIC ROCKS AND ALLUVIUM

SURFACE OF PRE-CENOZOIC ROCKS ASSUMING A DENSITY OF 0.5 t9/Cc)!

REGIONAL GRADIENT

/ ~~~~~~~~~~~~~~~~~

- '~~~s -a.

fA~~~~~~~~~~~~~~~~~~~~~~~~~

O', DESE RTr-J - - CEMOZ ~~~VOCANIC ROCKS AND ALLUVIUM

SURACEOF~u-CENOZOIC ROCKS ASSUIgNG A DENSITY CONTRAST OF O 5 (q/ eel

7 5 0~~15. CE~~40ZIC AN ALLVI IOCAIlOCS

oREGIONAL GRtADIENT10 = I~~~ 5~ t. <_ __, ~~~~~~~~~~~~~~~~~103 ~~~~~~~110 4-11- - -l.-,. - - ---. -$ A - i 4

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lolO4MA o o of a: I' 1/ Ca F9 V < 4 J'.t .2 .3 -j -j STR)PED HILLS 4000

I 2000 PRE-CENOZOIC ROCKS

SEA LLEVEL I

(-f 85

95 .

900

T 105 <

110 _ -J lb 195 :1 %0 120

125 'w1 0 § w~~~~~~~~~~~~~'

.loW IL ;1 -tI 4000 ,J J

-q 2000 w = N0Z C'NOZOIC w ROCKS SEA LEVE' 1. 8 cC)

EXPLANATION A t 100 4 Computed gravity value .1I

(0 I 05 i.

10 n I Location and number oa gravity station on or adjzcent to line 153 _j of section BULLFROG RAPP UiTN I

I I

I i

BLAN IIAPAGE l

I. I I

i. t-

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Parra - I I| I I ' ot I I STRIPED HILLS 4000 I

I 2000 PRE -CENOZOIC ROCKS w w i. SEA LEVEL UJ.

I Cj0

100

105 -j 4

I l IS

120

I 125 II t

I i I

I 4000 i II I 2000 IhJ SEA LEVEL I&a IC

E)'PLANATIOJN A Computed gravity value

Locatian I and number of gravity station on or adjacent to lIne of section BULLFROG BARE MTN Boundary of 7 112- or 15-m'inute quadrant ....A. _t~~~~~~~~~~~~~~~~s Age~~~~- BLANK PAGE

, .1

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.4 'I IIII

19M~po i _l~~E IV , - ' >* ' t , -K 4. I on bl)~~~~4 *t)II

FUNERAL. MTS. ______~~~~~AMARGOSA DESERT

CENOZOIC VOLCANIC RCCK(S PRE- CENOZOIC ROCKS

StAMIE Of PREt- Z:ENOZOIC ROCKS AISIJWIN4 A DENSITY CO$4T*AST 0OF .? (I/csI SuRFACE Of PIFt-CCENOZ0

fI I E 7

i I

SBOUGUER GRtAVITY PROFILE

N .r BARE MTN.

CRATER FLAT

I i I PRE-CENOZOIC ROCKS I I .I. I 0

Il t OF PM( -a"A0 ;$qu

to Su

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t4A

F. BLANK PAGE :I i hi I IS S!

;1~~~~~~ 170~~~~~~ x 7ct _-he I0 o I 130 N I t- nIn I - 4000

:RT 2000

NIC ROCKS AND ALLUVIUM PRE-CENOZOICROCKS SEA LEVEL

- 2000

PRE-CENOZOIC ROCKS ASSUMING A DfNSITY CONTRAST Of 0.5 (1/t.* - 4000

Adllmbh.

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YUCCA MTN.

--- 4- CEN020.(. 'OLCANIC ROCK AND ALLUVIUM

SUF C _rOP P*E-CE4PRE-CCE

lleN=;rZE - WIIn 1 Subsurface configurations of pre-Cenozoic rocks based on 1::

*1 - 1LA iNK G8E B!

\ COatLgULatlons of pre-Cenozoic rocks based on density contrasts it 0.7 g,'ct (upper)-and 0.5 g-vt (lower)

4000

2000

SEA LEVEL Ww LL 2000

I000

E 1(5

120

125

J GRADIENT (30

135 a -J 140 2

145 X A 150

155

160 I av

6000

4000

2000

SEA LEVEL w wt. PRE-CENOZOIC ROCKS - 2000 tE-CEV4OZOIC WXXCcS £SUKING A DEN4SITY Ce)"YR*S? OF OS04 t CC)

.m -i I I'm - I I --- .C2_11-:Wvlz---- --14*--!p m

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SOUGUER GRAVITY PROFILE

N I Cf BARE MTN.

SUMP"A" OF PRC-CIOZOIC ROCKS ASSUMING A DENSITY CONTRASTZ

Figure 3. ii

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CENOZOCC 'OLCANIC ROCK AND ALLUVIUM i

SLA/ZF A C E OF PR E - CE: O~C RO C M S~ URAE OF PRE -CENOZOiC ROCxS

l~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

;eneralized geologic cross sections dettrmrtied :i. two-dimenisional analysis of .raviti, datal

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I.~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~IL 560

6000

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2000

I.- SEA LEVEL w w I ~~~~~~~~~PRE-CENOZ~iC ROCXS 1A. Ic-o~t'xs £s~uV'S A ocosuTy coumlmAST of (%4I c - 2o00

. 4000

favttcs data

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p I i 0z NEVADA TEST SITE.,, N'. 0 ¾, II. C I z V t . -4 I, p -J I 0 . :r 0 '. E . ,. U. l'o"' ., I 1

1"/ I'' k s BLA'1K GE-

tiHe #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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K I -mR INV- K RTIV41LaS "U 11l6*15" 30oo0 -F --- I I

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.. i tI pNEVADA TEST Svl IE ,.. I i

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EXPLANATION \ -' '

OUTLINE OF HIGH TOPOGRAPHIC AREAS ,...

NOw)kMAL FAULT eA)ED UN SURFACE GEOLOGY; DASHED WHERE INDEFINITE; DOTTED WHERE INFERRED. eAR AND SALL ON DOWNTNROWN SDE.

THRUST FAULT BASED ON SURFACE GEOLOGY; S4WTEE rH INtJICATE UPPER PLATE. UOTTED WHERE INFEPRED

I a% .'- GRAVrrY TRENO: MAY INDICATE FAULT ZONE OR STEZP SLOPE Of p 1, ON I I PRE-CtNO701C ROCKS II- -" 4- V'.A _ ^*,/ 0A I 'II A~IS 0' GRAvJTY Htw, D4siEnl WHERE INFERREi.

I-

AXIS OF GRAViT' LOW-DASHED WHERE INFERRED.

L O C

LINE OF TW.)-PIMENSIONAL CROSS SEcriON.

I) 4 a 12 -e M.tES W IL~

FIGURE 4.--TREND MAP OF MAJOR GRAVITY ANOMALIES, FAULTS 1 CALlfORNiA

GRAITANM~IE, FU~ j 3.v i

-~~~~~~~~~a 7. "W1,95 - 1\1

4 3 LM3ft I *: I I ii

3- AND

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EAGLE - MTN.

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i. ,, N%

; 7. I I I i 0

rS, AND INFERRED FAULTS tN THE AMARGOSA DESERT AREA OF NE

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tANK~~~~P#.A%L

/ I

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360 15.

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WMARGOSA DESERT AREA OF NT-VADA AND