Arc Explorer
Themes visible in the above map:
MAGPROC (Image), LATITUDE/LONGITUDE and COASTLINE. The theme which
presents the legend for MAGPROC (Image) is MAGPRBAR (Image).When
the magnetic data is viewed in Arc Explorer, please note that this
theme is an image and therefore not able to be queried.
However, the MAGNETICS CONTOURS theme
may be queried. The contour values are contained in the CONTOUR
field. |
Aeromagnetic anomalies
of the eastern Juan de Fuca Strait
Richard Blakely and Carmel Lowe
The Raw Magnetic data map (top) shows
the relative intensity of the magnetic field as measured near
the earth's surface. The colors on the maps reflect the variable
magnetic properties of rocks in the upper crust, which in turn
reflect subsurface lithology. Volcanic and ultramafic rocks, for
example, are often strongly magnetic and produce high-amplitude
magnetic fields when measured near the earth's surface. Glacial
and fluvial deposits, on the other hand, are only weakly magnetic
and produce subdued patterns of magnetic anomalies.
These maps are based on five airborne
magnetic surveys, flown at different times and with different
specifications. Most of the region (the area east of longitude
123°W) is represented by a high-resolution aeromagnetic survey
flown in 1997 by the U.S. Geological Survey over the U.S. part
of the Puget Lowland (Blakely and others, 1999).
These data were collected at an altitude of 245 m (800 ft) above
terrain along north-south flight-lines spaced 400 m (0.25 mi)
apart. The region west of 123°W longitude is covered by four
separate surveys, three flown in 1976 by the Canadian Geological
Survey and one flown in 1974 by the U.S. Geological Survey (1974).
All of the western aeromagnetic surveys were flown along north-south
lines but at different constant altitudes ranging from 305 m (1000
ft) to 1370 m (4500 ft), and at different line spacings, ranging
from 1200 m (0.75 mi) to 1600 m (1 mi).
To produce the Raw Magnetic Data map,
the original digital data from each survey were projected to a
common rectangular grid. The five gridded surveys were merged
digitally, allowing no changes to the 1997 Puget Lowland survey
and allowing only first-order baseline changes to the other four
surveys. No attempt was made to continue the surveys to a uniform
altitude. Due to the disparate flight specifications, certain
artifacts are apparent along survey sutures, notably along meridian
123°15'W.
The most magnetic rocks in this part
of the Puget Lowland are Eocene basalts of the Crescent Formation
and other mafic rocks of similar age, which crop out in the western
and southwestern parts of the map, and pre-Tertiary ultramafic
rocks located in the northeastern part of the map. Linear alignments
of anomalies and anomaly gradients sometimes indicate important
crustal structures. The linear east-west gradient extending from
the eastern edge of the map to about longitude 123°15'W,
for example, is associated with the western part of the Devils
Mountain fault, a regionally important fault that juxtaposes highly
magnetic ultramafic rocks north of the fault against less magnetic
Tertiary rocks to the south (e.g., Whetten and
others, 1980; Johnson and others, 2000).
West of longitude 123°15'W, the Devils Mountain fault merges with
the Leech River fault, which also is apparent in the magnetic
data.
The Processed Magnetic Data map shows
the aeromagnetic data digitally processed in order to emphasize
shallow magnetic sources. The original merged data (the Raw Magnetic
Data map) were continued to a higher flight surface 100 m above
the original flight surface. Subtracting this "regional field"
from the original data tends to suppress anomalies originating
at deep levels relative to those generated by shallower sources.
Thus, the high-amplitude anomalies at the western edge of the
Raw Magnetic Data map are suppressed in the Processed Magnetic
Data map, while anomalies caused by shallow sources, like the
anomalies associated with the Devils Mountain fault, are emphasized.
This presentation is especially useful in identifying various
linear magnetic contacts, some of which reflect faults in the
shallow subsurface. Note that this technique also emphasizes "noise"
in the original data. Flight-line artifacts in the original data,
for example, cause the north-south striation in the western part
of the map.
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References
Blakely, R.J., Wells, R.E., and Weaver,
C.S., 1999, Puget Sound aeromagnetic maps and data: U.S.
Geological Survey Open-File Report 99-514.
Johnson, S.Y., Dadisman, S.V., Mosher,
D.C., Blakely, R.J., and Childs, J.R., 2000, Active tectonics
of the Devils Mountain fault and related structures, northern
Puget Lowland and eastern Juan de Fuca Strait region, Pacific
Northwest, in review.
Whetten, J.T., Zartman, R.E., Blakely,
R.J., and Jones, D.L., 1980, Allochthonous Jurassic ophiolite
in northwest Washington: Geological
Society of America Bulletin, v. 91, p. 359-368.
U.S. Geological Survey, 1974, Aeromagnetic
map of part of the Puget Sound area, Washington: U.S. Geological
Survey Open-File Report 74-1106, scale 1:125,000.
Reference
citation:
Blakely, R., and Lowe, C., 2000. Aeromagnetic 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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