MNP

MNP

MNP

EXPLANATION

Isostatic gravity, in milligals (mGal)

Gravity station (from Denton and 

Ponce, 2016)

Other gravity station

Boundary of Mojave National 

Preserve (MNP)

Gravity contours—Hachures indicate 

gravity low

Index (5 mGal intervals)

Intermediate (1 mGal intervals) −42 −40 −38 −36 −34 −32 −30 −28 −26 −24 −22

−20

<−42

−18 −16 −14 −12 −10−8−6−4−202

>2

−20

Mountain

Pass

I-15

Nipton

Ivanpah

Cima

Jean

McCullough Range

Clark Mountain Range

Shadow

Ivanpah

Mid Hills

New York Mountains

Mesquite Valley

Spring Mountains

Lucy Gray Mountains

Mescal

Range

Cima

Dome

PrimmValley

Mountains

Valley

MNP

MNP

MNP

I-15

NEVADA

CALIFORNIA

0 5 0 5 MILES10 KILOMETERS 

35°10’

35°15’

35°30’

35°45’

35°50’

115°50’ 115°45’ 115°30’ 115°15’ 115°05’

EXPLANATION

Quaternary alluvium

Cenozoic sedimentary rocks

Cenozoic volcanic rocks

Tertiary basalt

Granitic rocks, undivided

Cretaceous rocks

Paleozoic rocks

Proterozoic rocks

Figure 1. Index map showing simplified geology of eastern Mojave Desert (modified from 

Jennings and others, 1977; Stewart and Carlson, 1978). Red outline, study area; green line, 

boundary of Mojave National Preserve (MNP); gray lines, roads.

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

This map or plate is offered as an online-only, digital publication. Users should be 

aware that, because of differences in rendering processes and pixel resolution, 

some slight distortion of scale may occur when viewing it on a computer screen or 

when printing it on an electronic plotter, even when it is viewed or printed at its 

intended publication scale

Digital files available at https://doi.org/10.3133/sim3412A

Suggested citation: Ponce, D.A., and Denton, K.M., 2018, Isostatic gravity map of 

Mountain Pass and vicinity, California and Nevada: U.S. Geological Survey Scientific 

Investigations Map 3412–A, scale 1:62,500, https://doi.org/10.3133/sim3412A.

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3412A

Isostatic Gravity Map of Mountain Pass and Vicinity, California and Nevada

By

D.A. Ponce and K.M. Denton

2018

INTRODUCTION 

Gravity investigations of Mountain Pass and vicinity were begun as part of an effort to study 

regional crustal structures as an aid to understanding the geologic framework and mineral resources of 

the eastern Mojave Desert. The study area, which straddles the state boundary between southeastern 

California and southern Nevada (fig. 1), encompasses Mountain Pass, which is host to one of the 

world’s largest rare earth element carbonatite deposits. The deposit is found along a north-northwesttrending, fault-bounded block that extends along the eastern parts of the Clark Mountain Range, 

Mescal Range, and Ivanpah Mountains (fig. 1). This Paleoproterozoic block is composed of a 1.7-Ga 

metamorphic complex of gneiss and schist that underwent widespread metamorphism and associated 

plutonism during the Ivanpah orogeny (Wooden and Miller, 1990). The Paleoproterozoic rocks were 

intruded by a Mesoproterozoic (1.4 Ga) ultrapotassic alkaline intrusive suite and carbonatite body 

(Olson and others, 1954; DeWitt and others, 1987; Premo and others, 2016). The intrusive rocks 

include, from oldest to youngest, shonkinite, mesosyenite, syenite, quartz syenite, potassic granite, 

carbonatite, carbonatite dikes, and late shonkinite dikes (Olson and others, 1954).

GRAVITY METHODS

Gravity data were collected and processed to identify lateral changes in subsurface density. 

Generally speaking, gravity anomalies can be used to infer subsurface geologic structure, provided that 

a physical-property contrast is present across the geologic boundaries. Gravity anomalies can, for 

example, reveal variations in lithology and delineate geologic features such as faults, plutons, volcanic 

centers, calderas, and deep sedimentary basins, all of which may play an important role in defining the 

geologic framework of a region. 

The gravity stations, over 2,400 of which were collected as part of this study, were concentrated 

in areas of poor control and along traverses of interest. Gravity stations were distributed across parts of 

Shadow Valley, Clark Mountain Range, Mescal Range, Ivanpah Mountains, and Ivanpah Valley. All 

gravity data were tied to primary base stations in Primm, Nevada, as well as a secondary base station 

established near Mountain Pass, California. These stations ultimately were tied to high-precision 

gravity base station PB1021 near Baker, California, as part of a southern California high-precision 

gravity base-station network (Roberts and Jachens, 1986), as well as to a World Relative Gravity 

Reference Network of North America gravity base station at Nipton, California (Jablonski, 1974; see 

also, Denton and Ponce, 2016).

Gravity data, which were processed using standard geophysical methods (see, for example, 

Blakely, 1995), include the following corrections: (1) earth-tide correction, which accounts for tidal 

effects of the Moon and Sun; (2) instrument-drift correction, which compensates for an assumed linear 

drift in the gravity meter’s spring each day; (3) latitude correction, which accounts for variation in the 

Earth’s gravity with latitude; (4) free-air correction, which accounts for the variation in gravity owing 

to elevation relative to sea level; (5) Bouguer correction, which corrects for the attraction of material 

between the station and sea level; (6) curvature correction, which adjusts the Bouguer correction for 

the effect of the Earth’s curvature; (7) terrain correction, which removes the effect of topography to a 

radial distance of 167 km from the station; and (8) isostatic correction, which removes longwavelength variations in the gravity field related to the compensation of topographic loads.

Observed gravity values were referenced to the International Gravity Standardization Net 1971 

(IGSN 71) gravity datum (Morelli, 1974, p. 18). Free-air gravity anomalies were calculated using the 

Geodetic Reference System 1967 formula for theoretical gravity on the ellipsoid (International Union 

of Geodesy and Geophysics, 1971) and Swick’s (1942) formula for the free-air correction. Bouguer, 

curvature, and terrain corrections (Godson and Plouff, 1988) were added to the free-air correction to 

determine the complete Bouguer anomaly at a standard reduction density of 2,670 kilograms per cubic 

meter (kg/m

3

). Finally, a correction from the Airy-Heiskanen model for isostatic compensation of 

topographic loads (Jachens and Roberts, 1981), using an assumed crustal thickness of 25 km, a crustal 

density of 2,670 kg/m

3

, and a density contrast across the base of the model of 400 kg/m

3

, was made to 

remove a regional isostatic gravity field from the Bouguer gravity field.

New gravity data (Denton and Ponce, 2016) were combined with preexisting gravity data (Ponce, 

1997; Langenheim and others, 2009) from the surrounding areas in California and Nevada. All gravity 

data, which were gridded using a minimum curvature algorithm at an interval of 200 m, are displayed 

as a color-contour isostatic gravity map. Observed gravity values are accurate to about 0.05 milligals 

(mGal), and gravity anomalies are accurate to about 0.1 mGal, where 1 mGal is equal to 10-5

centimeters per second squared (cm/s

2

). 

DISCUSSION

Generally speaking, carbonatites have distinctive gravity signatures because these deposits are 

relatively denser than the surrounding host rocks. Previous gravity studies in the eastern Mojave Desert 

carbonatite terrane are limited in areal extent (Carlisle and others, 1980; Swanson and others, 1980; 

Hendricks, 2007; Langenheim and others, 2009). From west to east across the study area, Shadow 

Valley is characterized by a 20-mGal gravity low that is associated with relatively low density basinfill material and also, in part, a buried pluton that is relatively lower in density as compared to carbonate and dolomitic rocks. Gravity data indicate that the maximum depth to basement is probably about 

1.5 km, on the basis of a semi-infinite sheet (Nettleton, 1976, p. 193) and assuming a 20-mGal gravity 

anomaly and an average density contrast between basin fill and basement rocks of 400 kg/m

3

. This 

density contrast is based on rock-sample measurements made throughout the study area (Denton and 

Ponce, 2016). An iterative depth-to-basement study that incorporated geology, gravity, and drill-hole 

data applied to the Mojave National Preserve (green outline on map and on fig. 1) indicated that 

Shadow Valley reaches a maximum depth of about 1.5 km (Langenheim and others, 2009). 

The Clark Mountain Range, the Mescal Range, and the northeastern part of the Ivanpah 

Mountains are characterized by gravity highs that are associated with relatively dense bodies of 

Proterozoic gneiss and Cambrian dolomite. The southwestern part of the Ivanpah Mountains is 

characterized by a gravity low that reflects relatively lower density granitoid rocks. The central and 

western parts of Ivanpah Valley are characterized by a prominent gravity high that decreases from west 

to east, which suggests that dense rocks are present at relatively shallow depths. In the northwestern 

part of Ivanpah Valley, bodies of Proterozoic gneiss and the Cambrian to Devonian Goodsprings 

Dolomite are exposed in small outcrops (see, for example, Hewett, 1956). On the basis of geophysical 

data, Carlisle and others (1980) suggested that Ivanpah Valley is an asymmetric graben, deeper along 

its southeastern margin, and that the depth to basement or thickness of sediment is as much as about 

2.4 km. This compares well to drill-hole data that show depths to basement of 1.9 km in the 

southeastern part of the valley, as well as depths of 0.7 and 1.1 km in the central part of the valley 

(Hodgson, 1980; Carlisle and others, 1980); basin depths inferred from the inversion of gravity data 

(Langenheim and others, 2009) agree with these values and show that most of Ivanpah Valley is quite 

shallow, less than about 500 m near the Clark Mountain Range, but it may reach depths of about 3 km 

south of Nipton.

In the vicinity of Mountain Pass, a local gravity high of several milligals reflects relatively more 

dense carbonatite and shonkinite rocks; the high is within a broader terrace (that is, an area of 

flattening) in the regional gravity field. The terrace in the gravity field probably reflects a decrease in 

subsurface rock densities owing to the termination of thrusted sheets of relatively dense Paleozoic 

carbonate rocks to the west along the Clark Mountain Range and also to relatively more dense 

Proterozoic rocks to the east. In addition, this area likely is underlain by relatively less dense granitoid 

rocks of unknown age.

The diverse physical properties (Denton and Ponce, 2016) of rocks that underlie the study area are 

well suited to geophysical investigations. The contrasts in density between Proterozoic crystalline 

basement, Mesozoic granitoids, Cenozoic volcanic rocks, and Cenozoic unconsolidated alluvium, for 

example, produce a distinctive pattern of gravity anomalies that can be used to infer subsurface 

geologic structure, which in turn aids in the understanding of the geologic framework and mineral 

resource potential of the eastern Mojave Desert.

ACKNOWLEDGMENTS

We thank the following individuals and agencies for facilitating our efforts or granting us access: 

John Landreth and Dan Cordier of Molycorp Inc., Debra Hughson and David Nichols of the Mojave 

National Preserve, and Doug Davis and Amanda Sieb of the Ivanpah Solar Power Facility. We also 

thank Dan Scheirer and Geoff Phelps of the U.S. Geological Survey (USGS) for their reviews, and 

map editor Taryn Lindquist (USGS) for comments and suggestions.

REFERENCES CITED

Blakely, R.J., 1995, Potential theory in gravity and applications: New York, Cambridge University 

Press, 441 p.

Carlisle, C.L., Luyendyk, B.P., and McPherron, R.L., 1980, Geophysical survey in the Ivanpah Valley 

and vicinity, eastern Mojave Desert, California, in Fife, D.L., and Brown, A.R., eds., Geology and 

mineral wealth in the California Desert: South Coast Geological Society, Dibblee Volume, no. 8, 

p. 485–494.

Denton, K.M., and Ponce, D.A., 2016, Gravity and magnetic studies of the eastern Mojave Desert, 

California and Nevada: U.S. Geological Survey Open-File Report 2016–1070, 20 p., 

https://doi.org/10.3133/ofr20161070. 

DeWitt, E., Kwak, L.M., and Zartman, R.E., 1987, U-Th-Pb and 40Ar/39Ar dating of the Mountain Pass 

carbonatite and alkalic igneous rocks, southeastern California: Geological Society of America, 

Abstracts with Programs, v. 19, no. 7, p. 642.

Godson, R.H., and Plouff, D., 1988, BOUGUER version 1.0, a microcomputer 

gravity-terrain-correction program: U.S. Geological Survey Open-File Report 88–644–A, 22 p.

Hendricks, J.D., Geophysics, 2007, in Theodore, T.G., ed., Geology and mineral resources of the East 

Mojave National Scenic Area, San Bernardino County, California: U.S. Geological Survey 

Bulletin 2160, p. 81–87, https://pubs.usgs.gov/mf/2007/2414/.

Hewett, D.F., 1956, Geology and mineral resources of the Ivanpah quadrangle, California and Nevada: 

U.S. Geological Survey Professional Paper 275, 172 p.

Hodgson, S.F., ed., 1980, Oil and gas prospect wells drilled in California through 1980: California 

Division of Oil and Gas Publication TRO 1, 258 p.

International Union of Geodesy and Geophysics, 1971, Geodetic reference system 1967: International 

Association of Geodesy Special Publication 3, 116 p.

Jablonski, H.M., 1974, World relative gravity reference network North America, Parts 1 and 2, with a 

supplementary section on IGSN 71 gravity datum values (rev. ed.): U.S. Defense Mapping 

Agency Aerospace Center Reference Publication 25, 1,261 p.

Jachens, R.C., and Roberts, C.W., 1981, Documentation of a FORTRAN program, ‘isocomp’, for 

computing isostatic residual gravity: U.S. Geological Open-File Report 81–574, 26 p.

Jennings, C.W., Strand, R.G., and Rogers, T.H., 1977, Geologic map of California: California Division 

of Mines and Geology, scale 1:750,000.

Langenheim, V.E., Biehler, S., Negrini, R., Mickus, K., Miller, D.M., and Miller, R.J., 2009, Gravity 

and magnetic investigations of the Mojave National Preserve and adjacent areas, California and 

Nevada: U.S. Geological Survey Open-File Report 2009–1117, 25 p., 

https://pubs.usgs.gov/of/2009/1117/.

Morelli, C., ed., 1974, The International Gravity Standardization Net 1971: International Association 

of Geodesy Special Publication 4, 194 p.

Nettleton, L.L., 1976, Gravity and magnetics in oil prospecting: New York, McGraw-Hill Company, 

464 p. 

Olson, J.C., Shawe, D.R., Pray, L.C., and Sharp, W.N., 1954, Rare-earth mineral deposits of the 

Mountain Pass District, San Bernardino County, California: U.S. Geological Survey Professional 

Paper 261, 75 p., 13 pls.

Ponce, D.A., 1997, Gravity data of Nevada: U.S. Geological Survey Digital Data Series DDS–42, 27 

p., CD-ROM, https://pubs.usgs.gov/dds/dds-42/. 

Premo, W.R., Miller, D.M., Moscati, R.J., Holm-Denoma, C., Neymark, L., and Ponce, D.A., 2016, 

Searching for the aerial extent of the Mountain Pass carbonatite event—Evidence from U-Pb 

zircon geochronology of Proterozoic rocks in southwestern United States [abs.]: Geological 

Society of America, Abstracts with Programs, v. 48, no. 7. 

Roberts, C.W., and Jachens, R.C., 1986, High-precision gravity stations for monitoring vertical crustal 

motion in southern California: U.S. Geological Survey Open-File Report 86–44, 76 p.

Stewart, J.H., and Carlson, J.E., 1978, Geologic map of Nevada: Nevada Bureau of Mines and 

Geology Map, scale 1:500,000.

Swanson, S.C., McPherron, R.L., Searls, C.A., and Luyendyk, B.P., 1980, A geological and 

geophysical investigation of the extension of the Clark Mountain fault into Ivanpah Valley, 

Ivanpah quadrangle, California, in Fife, D.L., and Brown, A.R., eds., Geology and mineral wealth 

in the California desert: South Coast Geological Society, Dibblee Volume, no. 8, p. 495–504.

Swick, C.A., 1942, Pendulum gravity measurements and isostatic reductions: U.S. Coast and Geodetic 

Survey Special Publication 232, 82 p.

Wooden, J.L., and Miller, D.M., 1990, Chronologic and isotopic framework for Early Proterozoic 

crustal evolution in the eastern Mojave Desert region, SE California: Journal of Geophysical 

Research, v. 95, p. 20,133–20,146.

APPROXIMATE MEAN CONTOUR INTERVAL 50 METERS DECLINATION, 2018

Base map from U.S. Geological Survey 

1:100,000-scale quadrangles: Ivanpah, 1985; 

Mesquite Lake, 1985

Universal Transverse Mercator projection, Zone 

11N, North American Datum of 1983 (NAD 83)

Data compiled in 2018

GIS database and digital cartography by D.A. 

Ponce and K.M. Denton

Edited by Taryn A. Lindquist; digital cartographic 

production by Katie Sullivan

Manuscript approved for publication July 20, 2018

115°45’ 115°15’

35°17’

35°37'

40’ 35’ 25’ 20’

20’

25’

30’

115°45’ 40’ 35’ 30’ 25’ 20’ 115°15’

30’

35’

35°17’

35°37’

20’

25’

30’

35’

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3412–A