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

Langenheim, V. E.; Biehler, Shawn; McPhee, D. K.; McCabe, C. A.; Watt, J. T.; Anderson, M. L.; Chuchel, B. A.; Stoffer, P.

Abstract:

This isostatic residual gravity map is part of an effort to map the three-dimensional distribution of rocks in Joshua Tree National Park, southern California. This map will serve as a basis for modeling the shape of basins beneath the Park and in adjacent valleys and also for determining the location and geometry of faults within the area. Local spatial variations in the Earth‘s gravity field, after accounting for variations caused by elevation, terrain, and deep crustal structure, reflect the distribution of densities in the mid- to upper crust. Densities often can be related to rock type, and abrupt spatial changes in density commonly mark lithologic or structural boundaries. High-density basement rocks exposed within the Eastern Transverse Ranges include crystalline rocks that range in age from Proterozoic to Mesozoic and these rocks are generally present in the mountainous areas of the quadrangle. Alluvial sediments, usually located in the valleys, and Tertiary sedimentary rocks are characterized by low densities. However, with increasing depth of burial and age, the densities of these rocks may become indistinguishable from those of basement rocks. Tertiary volcanic rocks are characterized by a wide range of densities, but, on average, are less dense than the pre-Cenozoic basement rocks. Basalt within the Park is as dense as crystalline basement, but is generally thin (less than 100 m thick; e.g., Powell, 2003). Isostatic residual gravity values within the map area range from about 44 mGal over Coachella Valley to about 8 mGal between the Mecca Hills and the Orocopia Mountains. Steep linear gravity gradients are coincident with the traces of several Quaternary strike-slip faults, most notably along the San Andreas Fault bounding the east side of Coachella Valley and east-west-striking, left-lateral faults, such as the Pinto Mountain, Blue Cut, and Chiriaco Faults (Fig. 1). Gravity gradients also define concealed basin-bounding faults, such as those beneath the Chuckwalla Valley (e.g. Rotstein and others, 1976). These gradients result from juxtaposing dense basement rocks against thick Cenozoic sedimentary rocks.

Genre:

USGS Numbered Series

ProdID:

80396

Citation Author:

Langenheim, V. E.; Biehler, Shawn; McPhee, D. K.; McCabe, C. A.; Watt, J. T.; Anderson, M. L.; Chuchel, B. A.; Stoffer, P.

Citation Contributing Office:

USGS Geophysics Unit of Menlo Park, California

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Citation Edition:

Version 1.0

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Citation Language:

ENGLISH

Citation Larger Work Title:

Citation LatN:

0341500

Citation LatS:

0333000

Citation LonE:

-1150000

Citation LonW:

-1163000

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Citation Phsyical Description:

Map: 65 x 35 inches; Data Files

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Citation Publisher:

Geological Survey (U.S.)

Citation Series:

Open-File Report

Citation Series Code:

OFR

Citation Series Number:

2007-1218

Citation Search Results Text:

Preliminary Isostatic Gravity Map of Joshua Tree National Park and Vicinity, Southern California; 2007; OFR; 2007-1218; Langenheim, V. E.; Biehler, Shawn; McPhee, D. K.; McCabe, C. A.; Watt, J. T.; Anderson, M. L.; Chuchel, B. A.; Stoffer, P.