Arc Explorer
Themes visible in the above map: STRUCTURE,
LATITUDE/LONGITUDE and COASTLINE. Descriptions of the units are
available in the TYPE field of the STRUCTURE theme.
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Tertiary and
Quaternary structures of the eastern Juan de Fuca Strait:
interpreted map
Samuel Y. Johnson, David C. Mosher, Shawn V. Dadisman, Jon R.
Childs and Susan B. Rhea, 2000.
This map shows the locations of faults
and folds in the eastern Juan de Fuca Strait region. For onshore
areas except Whidbey Island, the structures are based on geologic
mapping of Tabor and Cady (1978), Muller
(1983), and Whetten and others (1988).
Structures in offshore areas are based mainly on interpretation
of industry and high-resolution seismic-reflection data (Figure
2). Structures on Whidbey Island are based on integrated investigations
of onshore seismic-reflection data, onshore outcrops, borehole
analysis, geophysical anomalies, and seismic-reflection data (e.g.,
Johnson and others, 2000). The map is considered
"preliminary" because we have not conducted detailed, systematic
investigations in all parts of the map area.
On seismic-reflection data, faults
are recognized on the basis of truncated reflections and (or)
abrupt changes in reflection dip or seismic facies, such as amplitude,
frequency, continuity, and geometry. Fault identification was
greatly facilitated by the availability of diverse and complementary
seismic-reflection data sets with variable depth imaging and resolution.
There are a significant number of relatively high-angle contacts
bounding reflections or sets of reflections within the Quaternary
section (see depth to base of Quaternary Map)
in the upper ~ 1 s TWT that could be interpreted either as faults
or as steep depositional contacts associated with glaciofluvial
erosion and deposition (e.g., Figures
3, 6, 9,
and 13). Differentiating between
the tectonic and nontectonic origins is possible with deeper seismic
data that reveal whether potential structures are "rooted" within
the basement and are faults or, in contrast, are confined to the
Quaternary section and are sedimentary features.
Faults and folds are color-coded on
the Preliminary Neotectonics Map based on evidence for Quaternary
activity. Structures or segments of structures shown in shades
of blue appear to deform (fault or warp) reflections in the inferred
Quaternary section, whereas there is no obvious evidence for Quaternary
deformation associated with the structures or structure segments
shown in shades of pink. Information on amounts and rates of Quaternary
deformation are available for only a few structures (Johnson
and others, 1996, 2000), thus the significance of structures
in the eastern Juan de Fuca Strait region for earthquake hazard
assessments is largely unknown at this time.
At present, the southern Whidbey Island
fault (SWIF; Johnson and others, 1996) and
the Devils Mountain fault and related structures, the Strawberry
Point fault (SPF) and the Utsalady Point fault (UPF) appear to
be the most active and significant structures in the eastern Juan
de Fuca Strait region (Johnson and others, 2000).
The southern Whidbey Island fault, a zone of contractional and
(or) transpressional deformation, extends northwest from southern
Whidbey Island through Admiralty Inlet into the eastern Juan de
Fuca Strait, and appears to die out west of 123°. Most likely,
offset and deformation associated with this zone is being transferred
onto a set of northwest-trending structures that lie a few kilometers
to the southwest.
The Devils Mountain fault (DMF) is
a continuous structure that extends westward from the western
foothills of Washington's Cascade Range across the eastern Juan
de Fuca Strait, possibly merging with the Leech River fault on
southern Vancouver Island (Whetten and others,
1988; Johnson and others, 1996, 2000).
The Devils Mountain fault is bounded by northwest-trending en-echelon
folds and faults, a map pattern that strongly suggests it is a
left-lateral, oblique-slip, transpressional "master fault." En-echelon
faults that cut northern Whidbey Island include the Strawberry
Point and Utsalady Point faults. These high-angle faults form
the northern and southern boundaries of an uplift of pre-Tertiary
basement rock on western Whidbey Island. To the east, these faults
break into several splays that have the opposite sense of vertical
displacement. Based on their steep dips, reversals of offset,
and associated contractional deformation, Johnson
and others (2000) suggested that these faults are also oblique-slip
transpressional faults.
Faults and folds in the map region
are illustrated on interpreted seismic-reflection profiles and
photographs of coastal bluffs (Figures
3-16). For all seismic-reflection profiles, the tops of dots
indicate the inferred base of Quaternary deposits; the tops of
triangles indicate the inferred base of latest Pleistocene to
Holocene (postglacial) deposits; and prominent water-bottom multiples
are shown by "m." Solid and long dashed bold red lines show faults
and inferred faults, respectively. Locations of anticline and
syncline axes are shown above the profiles. We include examples
of both high-resolution shallow data and lower-resolution industry
data to demonstrate how the different types of seismic-reflection
data complement each other and contribute to characterization
of regional structure and tectonics.
Figure
3 shows the southern Whidbey Island fault zone on U.S.
Geological Survey Line P183 (Figure
2), a high-resolution, multichannel line in Holmes Harbor
in the southwestern part of the map area. Quaternary strata within
this broad (~7 km) zone are both folded and faulted. Figure
4 shows faulted and fractured Quaternary glacial outwash sand
within the fault zone from exposures in coastal bluffs on the
east side of Holmes Harbor. Folded Quaternary glaciomarine strata
within the southern Whidbey Island fault zone, exposed in coastal
bluffs on the west coast of Whidbey Island, are show in Figure
5.
Figure
6 shows faulted and folded Quaternary strata within the southern
Whidbey Island fault zone on US Geological
Survey Line P127, a high-resolution, multichannel seismic-reflection
line in the southeastern Juan de Fuca Strait. Figures
7 and 8 (from Johnson
and others, 1996) show the southern Whidbey Island fault on
two industry seismic-reflection profiles from the eastern Juan
de Fuca Strait, and illustrate the variable along-strike geometry
in the fault zone. High-angle northwest-trending faults that cut
the Quimper Peninsula (Figure 2)
are also shown in Figures 6 and
7.
Figure
9 shows the Devils Mountain and Strawberry Point fault zones
in Skagit Bay east of Whidbey Island, on US
Geological Survey Lines P176. Each zone is represented by
several high-angle faults. The Strawberry Point fault zone cuts
across Strawberry Point on northeastern Whidbey Island, where
late Quaternary strata are folded and fractured (Figure
10). Figure 11 shows folded
and fractured late Quaternary strata at Utsalady Point within
the Utsalady Point fault zone, on northwestern Camano Island.
Figure
12 shows a short segment of a high-resolution Geopulse profile
(US Geological Survey Line P168) from
the eastern Juan de Fuca Strait (Figure
2), which shows a break in the slope of the seafloor above
the Devils Mountain fault, corresponding with a contrast in the
reflectivity of postglacial (latest Pleistocene to Holocene) sediments.
This contrast indicates juxtaposition of sediments with different
physical properties, such as grain size, bedding, and(or) gas-filled
porosity. The high-resolution multichannel seismic-reflection
profile shown in Figure 13 (US
Geological Survey Line P166; Figure
2) shows the Devils Mountain fault truncating reflections
within the Tertiary section on the south-dipping limb of a hanging-wall
anticline, but not obviously breaking the Quaternary section.
Dips in Quaternary beds on the flanks of this fold are as steep
as 13°. Farther south, the Strawberry Point and Utsalady
Point faults are imaged as subvertical structures that bound a
non-reflective horst block and truncate moderate to high amplitude,
subparallel, continuous to discontinuous reflections in the Everett
basin. This horst block is inferred to consist of pre-Tertiary
basement rock overlain by a thin postglacial section (latest Pleistocene
to Holocene).
Figures
14 and 15 provide deeper
images of structures in the northeastern Juan de Fuca Strait.
Figure 14 is a segment of an
industry line that lies nearly parallel to US
Geological Survey Line P166 (Figure
2) and provides a slightly deeper view of structure in this
area. The Devils Mountain fault is not well imaged, but appears
to juxtapose relatively non-reflective rock north of the fault
and a section characterized by warped discontinuous reflections
to the south. As on Figure 13,
the Strawberry Point and Utsalady Point faults appear as vertical
faults bounding a generally non-reflective uplift. The uplift
is onlapped by warped Quaternary strata. The Strawberry Point
and Utsalady Point faults appear to die out a few km west of this
profile (Figure 2). Figure
15 shows a segment of SHIPS line JDF4
(Figure 2) that images the Devils
Mountain fault as a north-dipping (~ 50°) blind structure
that truncates and juxtaposes different seismic facies in inferred
pre-Quaternary strata. This fault forms the core of an asymmetric
anticline and folds (but does not rupture) the prominent reflection
at the base of the Quaternary section. The dip on the base of
the Quaternary horizon on the south limb of this fold is ~15°.
Quaternary beds on the south-dipping limb of the fold dip south
and thin toward the anticline axis, indicating they were deposited
during fold growth.
A segment of Geological
Survey of Canada (1996) Line 35 (Figure
2), a high-resolution, multichannel profile, is shown on Figure
16. On this line, we interpret the Devils Mountain fault as
a north-dipping (~50°) structure that offsets the inferred
contact between pre-Tertiary rock and Quaternary deposits (marked
by a set of two high-amplitude reflections) about 330 m. South
of the fault, Pleistocene sediment is relatively flat and fills
in an irregular faulted and folded surface at the top of inferred
pre-Tertiary basement. Immediately north of the fault in the hanging
wall, the pre-Tertiary basement surface dips gently south for
about 1200 m and is draped by latest Pleistocene to Holocene (postglacial)
beds.
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References
Johnson, S.Y., Potter, C.J., Armentrout,
J.M., Miller, J.J., Finn, C., and Weaver, C.S., 1996, The southern
Whidbey Island fault, an active structure in the Puget Lowland,
Washington: Geological
Society of America Bulletin, v. 108, p. 334-354 and oversize
insert.
Johnson,
S.Y., Dadisman, S.V., Mosher, D.C., Blakely, R.J., and Childs,
J.R., 2000, Late Quaternary tectonics of the Devils Mountain fault
and related structures, northern Puget Lowland: Geological
Society of America Abstracts with Programs, v. 32, p. XX.
Muller, J.E., 1983, Geology, Victoria:
Geological Survey of Canada Map 1553A, scale 1:100,000.
Tabor, R.W., And Cady, W.M., 1978,
Geologic Map of the Olympic Peninsula, Washington: US Geological
Survey Map I-994, scale 1:125,000.
Whetten, J.T., Carroll, P.I., Gower,
H.D., Brown, E.H., And Pessl, F., Jr., 1988, Bedrock geologic
map of the Port Townsend 30- by 60-minute quadrangle, Puget Sound
Region, Washington: US Geological Survey Map I-1198-G, scale 1:100,000.
Reference citation:
Johnson, S.Y., Mosher, D.C., Dadisman, S.V., Childs, J.R., and
Rhea, S.B., 2000. Tertiary and Quaternary structures of the eastern
Juan de Fuca Strait: interpreted map, 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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