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-121°0'0"

-122°0'0"

-121°7'30"

-121°7'30"

-121°15'0"

-121°15'0"

-121°22'30"

-121°22'30"

-121°30'0"

-121°30'0"

-121°37'30"

-121°37'30"

-121°45'0"

-121°45'0"

-121°52'30"

-121°52'30"

-121°0'0"

36°22'30"

36°22'30"

36°15'0"

36°15'0"

36°7'30"

36°7'30"

36 ̊0’0”

36 ̊30’0”

36 ̊30’0”

-122 ̊0’0”

36 ̊0’0”

CALIF.

MAP LOCATION

14°

APPROXIMATE MEAN

DECLINATION, 2011

MAGNETIC NORTH

TRUE NORTH

Shaded-relief map of the study area showing isostatic gravity and Quaternary faults (in red, U.S. Geological Survey and California Geological Survey, 2009). SAF, San 

Andreas Fault. This map represents the same data as shown in the contour map.

Isostatic Gravity

Anomaly

(mGal)

-121°0'0"

-121°7'30"

-121°7'30"

-121°15'0"

-121°15'0"

-121°22'30"

-121°22'30"

-121°30'0"

-121°30'0"

-121°37'30"

-121°37'30"

-121°45'0"

-121°45'0"

-121°52'30"

-121°52'30"

36°30'0" 36°22'30"

36°22'30"

36°15'0"

36°15'0"

36°7'30"

36°7'30"

36°0'0"

36°0'0"

2 0 2 4 6 8 10 1

Kilometers

Tularcitos Fault

San Gregorio-Hosgri Fault

Reliz-Rinconada Fault

Palo Colorado Fault

SAF

Sierra de Salinas

Santa Lucia Range

Gabilan Range

Salinas Valley

Arroyo Seco

Pacific Ocean

Sur Basin

-121 ̊52’30” -121 ̊45’0” -121 ̊37’30” -121 ̊30’0” -121 ̊22’30” -121 ̊15’0” -121 ̊7’30” -121 ̊0’0”

-36 ̊30’0” -36 ̊22’30” -36 ̊15’0” -36 ̊7’30” -36 ̊0’0”

Soledad

Greeneld

King City

Big Sur

Carmel Valley

San Lucas

Open-File Report 2011–1036

INTRODUCTION

This isostatic residual gravity map is part of a regional effort to investigate the tectonics and water 

resources of the central Coast Range. This map serves as a basis for modeling the shape of basins and 

for determining the location and geometry of faults in the area. Local spatial variations in the Earth’s 

gravity field (after removing variations caused by instrument drift, earth-tides, latitude, elevation, 

terrain, and deep crustal structure), as expressed by the isostatic anomaly, reflect the distribution of 

densities in the mid- to upper crust, which in turn can be related to rock type. Steep gradients in the 

isostatic gravity field often indicate lithologic or structural boundaries. 

Gravity highs reflect the Mesozoic granitic and Franciscan Complex basement rocks that comprise 

both the northwest-trending Santa Lucia and Gabilan Ranges, whereas gravity lows in Salinas Valley 

and the offshore basins reflect the thick accumulations of low-density alluvial and marine sediment. 

Gravity lows also occur where there are thick deposits of low-density Monterey Formation in the hills 

southeast of Arroyo Seco (>2 km, Marion, 1986). Within the map area, isostatic residual gravity values 

range from approximately -60 mGal offshore in the northern part of the Sur basin to approximately 22 

mGal in the Santa Lucia Range.

DATA SOURCES, REDUCTIONS, AND ACCURACIES

The isostatic gravity map was compiled from 880 gravity stations that have been gridded at 400 m. 

The distribution of stations throughout the map is not consistent, however. North of lat 36.25° N, station 

spacing in the valleys is approximately 1 station per 2 km2, whereas station spacing in the ranges and 

in the offshore areas is approximately only 1 station per 8 km2. South of lat 36.25° N, station spacing is 

consistent at 1 station per 1.6 km2 and represents data collected by the Defense Mapping Agency. Previously published data are from a regional compilation (Chapman and others, 1990). During 2009, the 

U.S. Geological Survey (USGS) collected data at 210 gravity stations throughout Salinas Valley and 

surrounding ranges, as well as along the Monterey Peninsula.

The datum of observed gravity for this map is the International Gravity Standardization Net of 1971 

(IGSN 71) as described by Morelli (1974), and the reference ellipsoid is the Geodetic Reference System 

of 1967 (GRS67; International Union of Geodesy and Geophysics, 1971). Observed gravity data were 

reduced using standard.

gravity methods (Blakely, 1995), and were reduced to isostatic anomalies (Jachens and Roberts, 

1981) that emphasize features in the mid- to upper crust, having removed long-wavelength variations 

due to compensated topography (Simpson and others, 1986). Bouguer, curvature, and terrain corrections were applied to the free-air anomaly at each station to determine the complete Bouguer anomalies 

at a standard reduction density of 2.67 g/cm3. Finally, a regional isostatic gravity field was removed 

from the Bouguer field assuming an Airy–Heiskanen model for isostatic compensation of topographic 

loads (Jachens and Roberts, 1981) with an assumed crustal thickness of 25 km (16 mi), a crustal density 

of 2.67 g/ cm3, and a density contrast across the base of the model of 0.4 g/ cm3. 

Offshore gravity data were reduced to isostatic anomalies in two different ways. Ocean-bottom measurements reduced to complete Bouguer gravity values (Woodson, 1973) were corrected to isostatic 

anomalies. The ship-track gravity data were reduced by digitizing trackline points intersecting simple 

Bouguer gravity anomaly contours (Chapman and others, 1990), and then applying the complete Bouguer and isostatic corrections.

The main sources of error in these gravity data result from imprecise elevations and inaccurate terrain 

corrections. Total terrain corrections for the stations represented in this map average 5.89 mGal, which 

translates to an estimated uncertainty of 0.3 mGal (or 5% of average total terrain correction). Terrain 

correction errors for stations in the rugged Santa Lucia Range may be as high as 2.5 mGal. However, 

the likely uncertainty due to the terrain correction is small (less than 0.2 mGal) for most of the stations. 

Errors resulting from elevation uncertainty are likely less than 0.5 mGal for most of the previously 

collected onshore and ocean-bottom stations and less than 0.2 mGal for stations collected by the USGS 

in 2009. In general, 

the uncertainty due to both terrain and elevation errors is less than 1 mGal, although in many areas, 

particularly the valleys, the data are considerably more accurate. Shiptrack data are notably less accurate, with errors in the gravity measurement alone in excess of 3 mGal (Oliver, 1980, p. 4).

*The gravity data in this map are provided in comma delimited ascii format. Please see accompanying readme file for detailed database information.

ACKNOWLEDGMENTS

We thank David Ponce and Daniel Scheirer for their helpful reviews of this map. We are also grateful 

to Edie Morin for her help in the field.

REFERENCES

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

University Press, 441 p.

Chapman, R.H., Chase, G.W., and Youngs, L.G., 1990, Bouguer gravity and magnetic anomaly map of 

the central California continental margin: California Continental Margin Geologic Map Series Central California Continental Margin Map no. 5C, scale 1:250,000.

International Union of Geodesy and Geophysics, 1971, Geodetic Reference System 1967: International 

Association of Geodesy Special Publication no. 3, 116 p.

Jachens, R.C., and Roberts, C.R., 1981, Documentation of a FORTRAN program, ‘isocomp’, for computing isostatic residual gravity: U.S. Geological Survey Open-File Report 81-574, 26 p.

Ludington, S., Moring, B.C., Miller, R.J., Flynn, K.S., Stone, P.A., and Bedford, D.R., 2005, Preliminary integrated databases for the United States-Western States: California, Nevada, Arizona, and 

Washington:

 U.S. Geological Survey Open-File Report 2005-1305. http://pubs.usgs.gov/of/2005/1305.

Marion, R.C., 1986, Neogene stratigraphy and hydrocarbon generation in the Salinas Basin, California: 

Stanford University Masters Thesis, 104 p. 

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

of Geodesy Special Publication no. 4, 194 p.

Oliver, H.W., 1980, Interpretation of the gravity map of California and its continental margin: California 

Division of Mines and Geology Bulletin 205, 52 p.

Simpson, R.W., Jachens, R.C., Blakely, R.J., and Saltus, R.W., 1986, A new isostatic residual gravity 

map of the conterminous United States with a discussion on the significance of isostatic residual 

gravity anomalies: Journal of Geophysical Research, v. 91, no. B8, p. 8,348-8,372.

U.S. Geological Survey and California Geological Survey, 2009, Quaternary fault and fold database for 

the United States, accessed June 10, 2009, from USGS web site:

 http://earthquake.usgs.gov/regional/qfaults/.

Woodson, W.B., 1973, A bottom gravity survey of the continental shelf between Point Lobos and Point 

Sur, California: Naval Postgraduate School Master’s thesis, 110 p.

-121°0'0"

-121°7'30"

-121°7'30"

-121°15'0"

-121°15'0"

-121°22'30"

-121°22'30"

-121°30'0"

-121°30'0"

-121°37'30"

-121°37'30"

-121°45'0"

-121°45'0"

-121°52'30"

-121°52'30"

36°30'0" 36°22'30"

36°22'30"

36°15'0"

36°15'0"

36°7'30"

36°7'30"

36°0'0"

36°0'0"

2 0 2 4 6 8 10 1

Kilometers

Palo Colorado Fault

Reliz-Rinconada Fault

Sierra de Salinas

Santa Lucia Range

Gabilan Range

Salinas Valley

Arroyo Seco

Pacific Ocean

Tularcitos Fault

San Gregorio-Hosgri Fault

SAF

-121 ̊52’30” -121 ̊45’0” -121 ̊37’30” -121 ̊30’0” -121 ̊22’30” -121 ̊15’0” -121 ̊7’30” -121 ̊0’0”

-36 ̊30’0” -36 ̊22’30” -36 ̊15’0” -36 ̊7’30” -36 ̊0’0”

Soledad

Greeneld

King City

Big Sur

Carmel Valley

Shaded relief map of the study area showing simplified geology (modified after Ludington and others, 2005) and Quaternary faults (in red, U.S. Geological Survey and 

California Geological Survey, 2009). 

Tertiary sedimentary rocks

Cretaceous granitic rocks

Jurassic/Cretaceous basement rocks

Quaternary alluvium

Tertiary volcanic rocks

LEGEND

San Lucas

Topographic base from U.S. Geological Survey 1:100,000 Point Sur quadrangle, 1982.

Isostatic Gravity Map of the Point Sur 30’ x 60’ Quadrangle and Adjacent Areas, California

By

J.T. Watt, R.L. Morin, and V.E. Langenheim

2011

SCALE 1:100 000

4 2 0 4 8

4 2 0 4 8 12 KILOMETER

Mapping compiled in 2009–2010

Manuscript approved for publication February 3, 2011

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 was printed on an electronic plotter directly from digital files. Dimensional 

calibration may vary between electronic plotters and between X and Y directions on the 

same plotter, and paper may change size due to atmospheric conditions; therefore, scale 

and proportions may not be true on plots of this map.

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, 

Denver, CO 80225, 1–888–ASK–USGS

Digital files available at http://pubs.usgs.gov/of/2011/1036/

12 MILE

CONTOUR INTERVAL 40 FEET

SUPPLEMENTARY CONTOUR INTERVAL 10 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

U.S. Department of the Interior

U.S. Geological Survey

GRAVITY EXPLANATION

STATION LOCATIONS

#

"

!

USGS (collected in 2009)

Previously published (Chapman and others, 1990)

Defense Mapping Agency

Gravity anomaly contours. Contour interval, 2 mGal. Hatchered lines indicate 

gravity low. Contours were computergenerated based on a 400-m grid. 

Although the data have been edited, caution should be exercised when interpreting anomalies controlled by only a single 

gravity station.