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.
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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
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