flightline artifact

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116 ̊22'30" 116 ̊15' 116 ̊07'30" 116 ̊ 115 ̊52'30" 115 ̊45' 115 ̊37'30" 115 ̊30' 115 ̊15'

116 ̊22'30" 116 ̊15' 116 ̊07'30" 116 ̊ 115 ̊52'30" 115 ̊45' 115 ̊37'30" 115 ̊30' 115 ̊22'30" 115 ̊15'

Preliminary Aeromagnetic Map of Joshua Tree National Park and Vicinity, Southern California

By

V.E. Langenheim and P.L. Hill

2010

SCALE 1:100,000 

2 0 2 4 6 Miles 

2 0 2 4 6 Kilometers 

Topographic base from U.S. Geological Survey Palm Springs, Big Bear Lake, Sheephole Mountains,

and Eagle Mountains 1:100,000-scale quadrangles.

U.S. Department of the Interior

U.S. Geological Survey Open-File Report 2010-1070

DISCUSSION

 This aeromagnetic map of Joshua Tree National Park and vicinity is intended to promote further understanding of the geology and structure in the 

region by serving as a basis for geophysical interpretations and by supporting geological mapping, water-resource investigations, and various topical studies. 

Local spatial variations in the Earth's magnetic field (evident as anomalies on aeromagnetic maps) reflect the distribution of magnetic minerals, primarily magnetite, in the underlying rocks. In many cases the volume content of magnetic minerals can be related to rock type, and abrupt spatial changes in the amount of 

magnetic minerals commonly mark lithologic or structural boundaries. Bodies of mafic and ultramafic rocks tend to produce the most intense magnetic anomalies, but such generalizations must be applied with caution because rocks with more felsic compositions, or even some sedimentary units, also can cause measurable magnetic anomalies.

 Total-field aeromagnetic data from multiple surveys (table 1, index map) were used to construct the aeromagnetic map of Joshua Tree National Park and 

vicinity. All surveys, except for one, have been published previously. To eliminate poor coverage of the western Joshua Tree National Park, total-field aeromagnetic data were collected in September 2008 along north-south flight lines spaced 800 m apart and at a nominal terrain clearance of 305 m (table 1, Fig. 1, Joshua 

Tree survey). East-west tie lines were flown 8,000 m apart. Data were adjusted for tail-sensor lag and diurnal field variations. Further processing included microleveling using the tielines and subtraction of the reference field defined by IGRF2005 extrapolated to August 1, 2008.

 Data from all the surveys were transformed to a Universal Transverse Mercator Projection (Base Latitude 0°, Central Meridian -117° W.) and interpolated to 

a square grid with a grid interval of 250 m using the principle of minimum curvature (Briggs, 1974). The magnetic base levels of the surveys were then adjusted 

to bring them onto a common magnetic datum; a 1-km wide gap separates the various surveys.

 The red circles on the map (brown dots on fig. 2) indicate possible locations of abrupt lateral changes in magnetization and may represent lithologic or 

structural boundaries, although note that the northeast-trending alignments of circles in the southwest corner of the map are flightline artifacts. Their locations 

were determined as follows: 

 (1) The total-field magnetic anomaly data were mathematically transformed into pseudogravity anomalies (Baranov, 1957); this proce

 dure effectively converts the magnetic field to the "gravity" field that would be produced if all the magnetic material were replaced 

 by proportionately dense material.

 (2) The pseudogravity field was continued upward a distance of 100 m and subtracted from the original pseudogravity field. This proce

 dure emphasizes those components of the pseudogravity field that are caused by the shallow parts of the magnetic bodies, which 

 are most closely related to the mapped geology.

 (3) The horizontal gradient of the pseudogravity field difference was calculated everywhere by numerical differentiation.

 (4) Locations of locally steepest horizontal gradient (circles) were determined by numerically searching for maxima in the 

 horizontal gradient grid (Blakely and Simpson, 1986).

 Boundaries between bodies having different densities are characterized by steep gradients in the gravity field they produce and, if the boundaries have 

moderate-to-steep dips (greater than 45°), locally the maximum horizontal gradients will be located over the surface traces of the boundaries (Blakely and 

Simpson, 1986). Similarly, boundaries between bodies having different magnetizations are characterized by steep gradients in the pseudogravity field and, 

therefore, the procedure described above can be used to locate these boundaries. For example, the circles coincide locally with strands of the Pinto Mountain, 

Blue Cut and Chiriaco Faults and can be used to map bedrock fault strands concealed by young deposits, such as shown by Langenheim and Powell (2009).

REFERENCES CITED

Baranov, V.I., 1957, A new method for interpretation of aeromagnetic maps—Pseudo-gravimetric anomalies: Geophysics, v. 22, p. 359-383.

Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or gravity anomalies: Geophysics, v. 51, p. 1494-1498.

Briggs, I.C., 1974, Machine contouring using minimum curvature: Geophysics, v. 39, p. 39-48.

Grauch, V.J.S., 1984, Aeromagnetic map of the Eagle Mountains area, Riverside County, California: U.S. Geological Survey Open-File Report 84-502, scale

 1: 62,500.

Langenheim, V.E., and Powell, R.E., 2009, Basin geometry and cumulative offsets in the Eastern Transverse Ranges, southern California: Implications for 

 transrotational deformation along the San Andreas fault system: Geosphere, v. 5, no. 1, p. 1-22.

LK&B Resources, Inc., 1980, NURE aerial gamma-ray and magnetic reconnaissance survey, Colorado-Arizona area, Salton Sea NI 11-9, El Centro NI11-

 12, Phoenix NI 12-7, Ajo NI12-10, and Lukeville NH 12-1, quadrangles: U.S. Department of Energy, Grand Junction Office Report GJBX-012(80), 5 vols.

Sweeney, R.E., 2002, Six aeromagnetic surveys in California, Nevada, and Arizona: A web site for distribution of data: U.S. Geological Survey Open-

 File Report 02-486 (http:// pubs.usgs.gov/of/2002/ofr-02-486).

U.S. Geological Survey, 1981, Aeromagnetic map of the Needles 1o by 2o quadrangle, California and Arizona: U.S. Geological Survey Open-File 

 Report 81-085, scale 1:250,000.

U.S. Geological Survey, 1983, Aeromagnetic map of the Salton Sea area, California: U.S. Geological Survey Open-File Report 83-664, scale 1:250,000.

U.S. Geological Survey, 1985, Aeromagnetic map of the Palen area, Riverside County, California: U.S. Geological Survey Open-File Report 85-281, scale 1:62,500.

U.S. Geological Survey, 1990, Aeromagnetic map of parts of the San Diego, Santa Ana, and adjacent 1o by 2o quadrangles, California: U.S. Geological 

Table 1. Surveys used to create aeromagnetic map

 Year Flight elevation Flightline Flightline 

Survey Flown above ground spacing direction Reference 

Eagle Mountains 1954 Variable Variable Variable Grauch (1984)

NURE 1978 122 m 4800 m N-S LK&B Resources, Inc. (1980)

Needles 1980 305 m 800 m E-W U.S. Geological Survey (1981)

Salton Sea 1981 305 m 800 m E-W U.S. Geological Survey (1983)

Palen 1955 152 m 400 m Variable U.S. Geological Survey (1985)

San Diego 1989 305 m 800 m N55°E U.S. Geological Survey (1990)

Twentynine Palms 2002 245 m 530 m E-W Sweeney (2002)

San Bernardino Mts 2002 245 m 530 m N-S Sweeney (2002)

Joshua Tree 2008 305 m 800 m N-S This study

Manuscript approved for publication March 31, 2010

116 ̊15' 116 ̊  115 ̊45' 115 ̊30' 115 ̊15'

33 ̊45'

34 ̊ 

Needles

Survey

Joshua Tree

Survey

Joshua Tree

Survey

San Diego

Survey

Eagle

Mountains

Survey

Palen

Survey

Salton Sea Survey

Twentynine

Palms Survey

Salton Sea Survey

San Bernardino

Mts Survey

NURE

SAN ANDREAS FAULT

 Pinto Mountain Fault Zone

Blue Cut Fault Zone

Chiriaco Fault Zone

0 5 10 KM

Figure 1. Shaded-relief topographic map showing major faults (magenta lines; modified from Langenheim and Powell, 2009) and extent of aeromagnetic 

surveys listed in table 1.

Little San Bernardino 

Mountains

Coachella

Valley

Pinto

Mountains Coxcomb

Mountains

-300

-250

-200

-150

-100

 -50

 0

 50

 100

 150

 200

 250

 300

 350

 400

 450

 500

 550

 600

 650

116 ̊15' 116 ̊  115 ̊45' 115 ̊30' 115 ̊15'

33 ̊45'

34 ̊ 

0 10 km

park boundary

Figure 2. Color-contour aeromagnetic map with magnetization boundaries shown in brown, survey boundaries shown in white.

nT

Joshua Tree National Park boundary, 

locally generalized for cartographic display

Contours of total magnetic field intensity relative to the International

Geomagnetic Reference Field. Contour interval is 20 nanoteslas. Hachures 

indicate closed magnetic lows. 

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.

Available at http://pubs.usgs.gov/of/2010/1070

Aeromagnetic survey boundary

UTM grid convergence (GN)

and 1986 magnetic declination

at center of Eagle Mountains 

1:100,000-scale map. Diagram 

is approximate

MAP EXPLANATION

Location of maximum horizontal gradient, with amplitudes greater than the mean for the study 

area

Location of maximum horizontal gradient, with amplitudes less than the mean for the study area

Index Map