Interpreted Structure Map (Quaternary and pre-Quaternary faults and folds) Back to Start >

123°45' 40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 20' 15' 122°10'
Structure Map
Faults and Fold Legend

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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.

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.

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

Sediment Deformation Features Map
Structure Point Map
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