Gravity Data

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Raw Gravity Data Map

Arc Explorer
Themes visible in the above map: GRAVRAW (Image), LATITUDE/LONGITUDE and COASTLINE. When the gravity data is viewed in Arc Explorer, please note that the GRAVRAW(Image) theme is an image and therefore not able to be queried. The GRAVRBAR (Image) provides a scale for GRAVRAW (Image).

Processed Gravity Data Map

Arc Explorer
Themes visible in the above map: GRAVPROC (Image), LATITUDE/LONGITUDE and COASTLINE. When the gravity data is viewed in Arc Explorer, please note that the GRAVPROC (Image) theme is an image and therefore not able to be queried. The GRAVPBAR (Image) provides a scale for GRAVPROC (Image). However, the GRAVITY CONTOURS theme may be queried. The contour values are contained in the CONTOUR field.

Free-air and Bouguer gravity anomalies of the eastern Juan de Fuca Strait region
Carmel Lowe and Richard J. Blakely

Gravity maps provide information on density variations within the Earth. Rock densities are dependent on bulk composition and, in general, increase with decreasing quartz content. Felsic igneous rocks and many sedimentary rocks have low densities, whereas mafic igneous and volcanic rocks tend to have high densities. The density of common rock types rarely varies by a factor of more than two, in contrast to their magnetic susceptibilities (the parameter to which magnetic data are sensitive) which may vary by several orders of magnitude (Carmichael, 1982). In addition, gravity fields are monopolar whereas magnetic fields are dipolar. For both of these reasons the gravity map for a given area is usually dominated by long-wavelength features related to regional anomalies, whereas the corresponding magnetic map is typically dominated by short-wavelength, high-amplitude features related to a multitude of local anomalies.

The 4200 gravity measurements in this compilation were acquired for the most part in the 1960's and 1970's (Stuart, 1965; Walcott, 1967; Stacey and Steele, 1970; MacLeod et al., 1977) and are available from the Canadian National Geophysical Data Centre (Geological Survey of Canada) and/or the United States National Geophysical Data Center (NOAA). The distribution of these measurements is highly variable: onshore, data are randomly distributed with an average station interval of 5 km, approximately; offshore, data were acquired along ship tracks which are predominantly north-trending and spaced 4 - 7 km apart. Measurement intervals along ships tracks range from <200 m to ~1300 m. No data are available for Saratoga Passage or Holmes Harbour, and very few measurements are available for the shallow-water coastal areas immediately north of the Olympic Peninsula. All gravity measurements have been adjusted to the International Gravity Standardization Net 1971 and gravity anomalies computed using the Geodetic Reference System 1967 theoretical gravity formula. For onshore measurements, Bouguer anomalies were computed using a standard density of 2670 kg/m3, and in areas of rugged topography, terrain corrections were applied. Although the terrain correction procedures applied to onshore Canadian data differed slightly from those applied to onshore United States data, resulting differences are generally less than 1 mGal and do not noticeably influence anomalies and trends observed in the gravity maps included on this CD.

Gravity data are commonly displayed as free-air anomalies in offshore areas and as Bouguer anomalies in onshore areas, with high gravity values shown in hot colours and low gravity values in cool colours. See Blakely (1996) for a description of these anomalies. In general, northwestern and western portions of this map are dominated by high gravity values whereas eastern and southwestern areas are dominated by low gravity values. The highest amplitude anomalies (up to 66 mGal) are observed in areas underlain by high-density (MacLeod et al., 1977) basalts of the Metchosin Formation (Crescent Terrane), the Sooke gabbro, and older mafic volcanic rocks in Wrangellia exposed on southern Vancouver Island and the adjacent offshore. Smaller isolated regions of elevated gravity values occur on the northwestern Olympic Peninsula, where pillow basalts of the Crescent Formation (correlative with the Metchosin Formation) are exposed, as well as on Fidalgo Island where ultramafic rocks are mapped.

The eastern and southwestern map areas dominated by low gravity values are predominantly underlain by Cretaceous to Recent clastic sedimentary rocks. Gravity values are a minimum (<-122 mGal) south of the Devils Mountain Fault where thick accumulations of sedimentary rocks and overlying Quaternary deposits infill the Everett and Port Townsend basins (Johnson, 1985). North of the fault, higher relative gravity values reflect thinner accumulations of sedimentary rocks, as well as outcrops of volcaniclastic rocks in the Chilliwack Group. The low gravity values in the southwestern map area correlate with clastic sedimentary rocks in the Twin River and older formations exposed on the Olympic Peninsula.

In Map B a first-order trend has been removed in order to enhance more subtle anomalies. In this map, zones of relatively steep gravity gradients clearly demarcate the east-trending Devils Mountain fault and northwest- and west-trending segments of the Leech River fault. The northwest-trending belt of high gravity values that extends from the western Olympic Peninsula across the Juan de Fuca Strait to southern Vancouver Island supports continuity of the Crescent terrane beneath these areas (Shouldice, 1971; MacLeod et al., 1977; Tiffin and Riddihough, 1977), and is consistent with surface geologic maps (Muller, 1977) and interpretations of seismic reflection data (Clowes et al., 1987). Accumulations of similar rocks in the northwestern Olympic Peninsula must be less voluminous, as gravity values are significantly lower here.

References

Blakely, R.J., 1996, Potential Theory in Gravity and Magnetic Applications: Cambridge University Press, 441 p.

Carmichael, R.S. (Ed.), 1982. Handbook of physical properties of rocks, CRC Press, Boca Raton, Florida.

Clowes, R.M., Brandon, M.T., Green, A.G., Yorath, C.J., Sutherland Brown, A., Kanasewich, E.R., and Spencer, C., 1987. LITHOPROBE - southern Vancouver Island; Cenozoic subduction complex imaged by deep seismic reflections.

Green, A.G., Yorath, C.J., Sutherland Brown, A., Kanasewich, E.R., and Spencer, C., 1987. LITHOPROBE - southern Vancouver Island: Cenozoic subduction complex imaged by deep seismic reflections.

Johnson, S.Y., 1985. Eocene strike-slip faulting and non-marine basin formation in Washington, in: Strike-slip deformation, basin formation, and sedimentation. K.T. Biddle and N. Christie-Blick (Eds.), Society of Economic Paleontologist and Mineralogists Special Publication 37, 283-302.

MacLeod, N.S., Tiffin, D.L., Snavely, Jr., P.D., and Currie, R.G., 1977. Geological interpretation of magnetic and gravity anomalies in the Juan de Fuca Strait, U.S.-Canada, Canadian Journal of Earth Sciences, 14, 223-238.

Muller, J.E., 1977. Geology of Vancouver Island. Geological Survey of Canada, Open File 463, scale 1:250 000.

Shouldice, D.H., 1971. Geology of the western Canadian continental shelf. Canadian Society of Petroleum Geologists Bulletin, 19, 405-424.

Stacey, R.A., and Steele, J.P., 1970. Geophysical measurements in British Columbia with Maps: No. 120 Strait of Georgia, No. 121 Juan de Fuca Strait, Gravity Map Series, Earth Physics Branch, Department of Energy and Mines and Resources, Ottawa, Canada, 17p.

Stuart, D.J., 1965. Gravity data and Bouguer gravity map for western Washington. United States Geological Survey Open File Report.

Tiffin, D.L., and Riddihough, R.P., 1977. Gravity and magnetic survey off Vancouver Island, 1975; in Report of Activities, Part A, Geological Survey of Canada, Paper77-1A, 311-314.

Walcott, R.I., 1967. The Bouguer anomaly map of southwestern British Columbia. University of British Columbia, Institute of Earth Sciences, Scientific Report 15, 74p.

 

Reference citation:
Lowe, C. and Blakely, R., 2000. Free-air and Bouguer gravity anomalies of the eastern Juan de Fuca Strait region, in: Mosher, D.C. and Johnson, S.Y. (Eds.), Rathwell, G.J., Kung, R.B., and Rhea, S.B. (Compilers), Neotectonics of the eastern Juan de Fuca Strait; a digital geological and geophysical atlas. Geological Survey of Canada Open File Report 3931

48°30'
25'
20'
15'
10'
05'
48°00'
123°45' 40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 20' 15' 122°10'
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