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| Professional Paper 1643: SEISMIC-REFLECTION PROFILES |
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A series of seismic-reflection profiles extending from east to west, across the eastern Strait of Juan de Fuca and through Skagit Bay and Saratoga Passage (figs. 2, 5) document the structural style of the Devils Mountain fault and related structures. Selected lines from this data base are shown in figures 8-26. Velocities used to estimate Quaternary unit thickness and dip, and vertical exaggeration, are based on analysis of well logs and seismic-reflection data from the region (Johnson and others, 1994, 1996; Pratt and others, 1997; Brocher and Ruebel, 1998): uppermost Pleistocene and Holocene strata – 1,600 m/s; uppermost Pliocene(?) to Pleistocene strata – 1,800 m/s. All high-resolution airgun and industry profiles are plotted at about the same horizontal distance scale and vertical time scale in order to facilitate comparison, and both interpreted and uninterpreted sections are included. To simplify the discussion, the uppermost Pliocene(?) to Pleistocene section and the overlying postglacial uppermost Pleistocene and Holocene section are in some places collectively referred to as “Quaternary.”
Faults on seismic-reflection profiles (figs. 8-26) are recognized on the basis of truncated reflections and abrupt changes in reflection dip or seismic facies, characterized by reflection amplitude, frequency, continuity, and geometry (Sangree and Widmier, 1977; Stoker and others, 1997). Fault dip is generally defined in the upper part of a seismic-reflection profile where the record is best, then projected downdip to the base of the profile. Undeformed beds are characterized by flat reflections and no abrupt changes in dip or seismic facies. Faults are also numbered on the profiles so that text and figures can be easily compared. This numbering scheme is specific to each profile; faults with the same number on different profiles are generally not the same structure.
Aeromagnetic profiles parallel to seismic-reflection tracklines are also shown in figures 8-26. For practical purposes, these profiles have a relatively compressed vertical scale. Thus, they highlight only the largest anomalies; lower amplitude anomalies are more clearly displayed in the figure 3 map.
Seismic-reflection and aeromagnetic data (figs. 3, 5, 8-26) were used together to map the faults and folds across the eastern Strait of Juan de Fuca region shown in figure 2. However, not every structure in figure 2 is shown on every crossing seismic-reflection profile displayed in figures 8-26. All of the structures shown in figure 2 are inferred to cut or warp pre-Tertiary or Tertiary “basement,” but many do not obviously deform Quaternary deposits. In some cases, these older structures are deeper than the level imaged by shallower high-resolution seismic-reflection profiles (figure 5A, C) and mapping their trends is based entirely on deeper industry data (figure 5B). Conversely, in places the deeper industry data lack the resolution to image structures in the upper 1 km, and mapping of faults and folds is based mostly on the shallower high-resolution profiles.
Line 176 (figure 8) trends northwest and extends through Skagit Bay on the east side of northern Whidbey Island (figure 5A). On this profile, the combination of shallow water (< 50 m), shallow pre-Tertiary basement (inferred from nearby outcrops), and proximity to land has resulted in a seismic record with numerous water-bottom multiples, reverberations, and diffractions. Despite this “noise,” numerous faults can be identified on the profile on the basis of truncated and warped reflections, and juxtaposition of panels with disparate seismic facies. Among these faults, the Devils Mountain fault zone is identified based on extension of bedrock faults mapped onland by Whetten and others (1988) 1-2 km east of the profile. The Devils Mountain fault is imaged as a 2-km-wide zone of four subvertical structures. The juxtaposition of disparate seismic facies is largest along the southern fault (6) in the Devils Mountain zone, which correlates with an aeromagnetic low (figs. 3, 8A) and is inferred to be the main splay. Both this structure and the fault immediately to the north (7) truncate reflections in the Quaternary section within about 20-30 m (20-30 ms) of the seafloor. At least three other faults are imaged north of the Devils Mountain zone (10, 11, 12), cutting an aeromagnetic high formed by rocks of the Fidalgo ophiolite (Whetten and others, 1988). Of these, only the northernmost fault (12) appears to offset the Quaternary section.
South of the Devils Mountain fault zone, a variably thick (100-250 m; 110-275 ms) uppermost Pliocene(?) to Pleistocene section overlies nonreflective pre-Quaternary strata and is overlain by a relatively thin (< 135 m; 150 ms) uppermost Pleistocene to Holocene (postglacial) section that is largely obscured by water-bottom multiples. At least one fault (5) that extends up into the lower part of the uppermost Pliocene(?) to Pleistocene section occurs in the Everett basin between the Devils Mountain zone and a zone of faults offshore of Strawberry Point.
The structural zone offshore from Strawberry Point consists of at least 4 subvertical faults (1, 2, 3, 4) that span a distance of 3 km along Line 176, about 2 km perpendicular to the inferred strike of the zone (figure 2). Although water-bottom multiples are prominent in the upper part of the record in this zone, all four structures appear to propagate upward into the Quaternary section and within about 45-90 m (50–100 ms) of the seafloor. Fault 1 juxtaposes discontinuous, moderate-amplitude, warped reflections (on the southeast) and a largely nonreflective domain, probably cored by uplifted bedrock, to the northeast. Faults 2 and 3 truncate low-amplitude reflections, most obvious between ≈300 and 400 ms. Fault 4 juxtaposes disparate domains of subhorizontal reflections. Overall, uplift of basement rock within and north of the fault zone is suggested by the juxtaposition across fault 1 of relatively nonreflective material to the north and more reflective material to the southeast. No obvious aeromagnetic anomaly is associated with this fault zone along the profile (figs. 3, 8A).
Line 177 (figure 9) trends northeast and extends through Saratoga Passage between eastern Whidbey Island and northern Camano Island (figure 5A). On this line, a 2.3 km-wide fault zone (2 km normal to the inferred fault trend; figure 2) lies offshore from Utsalady Point. This zone comprises five faults (1 to 5), all of which extend upward into at least the lower part of the Quaternary section. The faults warp, tilt, and truncate reflections, but vertical offset does not appear to be significant across individual faults within the zone or across the fault zone as a whole. Northeast and southwest of this zone, subhorizontal to locally hummocky reflections characterize the Quaternary strata of the Everett basin. Southwest of the zone, significant erosional relief occurs at the base of the inferred postglacial uppermost Pleistocene to Holocene section. The fault zone occurs along the southwest margin of a gentle aeromagnetic high (figs. 3, 9A).
Line 167-168 (figs. 10, 11, 12) trends north and extends from the eastern Strait of Juan de Fuca into southern Rosario Strait, from about 1 to 5 km west of Whidbey Island (figure 5A). On the airgun profile (figure 10), the Devils Mountain fault is imaged as a zone of three structures (faults 4, 5, and 6) about 1.5 km wide that correlate with a gentle aeromagnetic high (figs. 3, 10A). The northernmost structure (6) is subvertical and is inferred to offset the Tertiary-Quaternary contact about 100–120 m (110–130 ms). This fault juxtaposes a panel of gently south dipping, moderate-amplitude continuous reflections on the north and more discontinuous, variable-amplitude, higher frequency reflections on the south. The fault is overlain by a break in slope in the seafloor, and higher resolution geopulse data (figure 11) show a contrast in the reflectivity of postglacial (uppermost Pleistocene to Holocene) sediments coinciding with this break. This contrast indicates juxtaposition of sediments with different physical properties, such as grain size, bedding, and (or) gas-filled porosity (Hovland and Judd, 1988; Fader, 1997). Thus, the fault may project to the surface. The geometry of the break in slope (south side up) is opposite to the inferred sense of vertical slip on the fault at depth, suggesting a possible reversal of slip. Alternatively, uplift could have exposed more erodible sediments on the uplifted northern block, leading to topographic inversion.
The two southern faults in the Devils Mountain fault zone on this profile (4, 5) are subvertical, truncate subhorizontal Quaternary reflections, juxtapose domains of different reflection properties, and propagate upward into at least the lower part of the postglacial uppermost Pleistocene to Holocene section. The amount of vertical separation on the base of the Quaternary across these two structures appears minimal. The juxtaposition of different seismic facies with relatively small vertical offset on both these faults and on fault 4 to the north suggests out-of-plane (strike-slip) movement on these structures.
Two kilometers north of the Devils Mountain fault zone, another steep north-dipping fault (7) truncates reflections on the south-dipping limb of a gentle anticline. The fault appears to project into the lowest part of the uppermost Pliocene(?) to Pleistocene section, and Quaternary strata on the south-dipping limb of the anticline dip south and thin toward the fold axis, indicating deposition during fold growth. The north limb of the fold is also cut by a steep fault (8) that does not appear to extend into the inferred Quaternary section. Northwest-trending structures that occur north of fault 8 in Rosario Strait (figure 3) mainly affect basement rocks and are not clearly imaged on line 167-168 (figure 10). The locations of these structures are based on industry data, including profiles collected by Puget Power (1979).
Three faults (1, 2, and 3) occur south of the Devils Mountain fault zone. Based on their westerly traces determined from seismic-reflection data farther west (figs. 10, 12, 13, 14, 15) and on the pattern of aeromagnetic anomalies (figure 3), these structures are correlated and inferred to be continuous with the faults on the east side of Whidbey Island offshore of Strawberry Point and Utsalady Point (figs. 8, 9), locations for which these faults are here named. The subvertical Strawberry Point fault (3) juxtaposes north-dipping to subhorizontal reflections representing Tertiary and Quaternary strata on the north (largely obscured by multiples) and seismically opaque, uplifted basement rocks and a thin overlying sedimentary section including a small (≈850 m wide) postglacial basin on the south. The uppermost Pliocene(?) to Pleistocene section north of and adjacent to the fault dips north about 10°, and vertical separation on the base of this unit across the fault is inferred to be about 80 m (≈90 ms). The companion geopulse profile (figure 12) shows disrupted reflections in the upper ≈10 ms along the fault, but the underlying record is nonreflective.
The subvertical Utsalady Point fault forms the southern margin of the uplift bounded on the north by the Strawberry Point fault and is imaged by at least two splays (1 and 2; figure 10). The northern splay (within the uplift, fault 2) warps and truncates reflections, juxtaposing reflective and nonreflective zones. The southern splay juxtaposes the uplifted block and subhorizontal reflections in the Everett basin to the south. The faults project upward into at least the lower part of the uppermost Pliocene(?) to Pleistocene section, and vertical offset on the base of this unit across these two splays is about 200–225 m (220–250 ms). The companion geopulse profile (figure 12) shows these two faults truncating reflections in the inferred uppermost Pleistocene to Holocene section to within 5-10 m of the seafloor. Between the Strawberry Point and Utsalady Point faults, the geopulse profile (figure 12) also displays another subvertical fault that cuts and warps reflections within both the uppermost Pliocene(?) to Pleistocene and the postglacial uppermost Pleistocene to Holocene sections, to within about 15 m of the seafloor.
The shallow basement between the Strawberry Point and Utsalady Point faults is inferred to consist of pre-Tertiary sedimentary basement rock because of its nonreflective character and because pre-Tertiary sedimentary rock is exposed 1 km to the east at Rocky Point on the west coast of Whidbey Island (figure 2). The Rocky Point exposures of these basement rocks have surprisingly low bulk magnetic susceptibility (see “Evidence for Onshore Faulting, Whidbey and Camano Islands”) and form a prominent aeromagnetic anomaly that extends eastward and provides an important constraint for projection of the Strawberry Point and Utsalady Point faults across Whidbey Island.
Line 166 (figure 13) trends north and extends from the eastern Strait of Juan de Fuca into southern Rosario Strait, about 6 km west of Whidbey Island (figure 5A). On this profile, the Devils Mountain fault is imaged as two splays (4 and 5) that dip steeply (≈60° - 80°) to the north. The southern splay (4) truncates south-dipping reflections within the uppermost Pliocene(?) to Pleistocene section on the south limb of a hanging-wall anticline, as well as nearly horizontal reflections in the Everett basin to the south. The northern splay (5) truncates reflections within the Tertiary section on the south-dipping limb of this fold, but does not obviously break into the Quaternary section. Dips in uppermost Pliocene(?) to Pleistocene beds on the south and north flanks of this fold are about 11° and 7°, respectively. There appears to be about 200-225 m (220–250 ms) of structural relief on the base of the uppermost Pliocene(?) to Pleistocene from the crest of the hanging-wall anticline across the fault zone to the floor of the Everett basin. This inference and estimates of offset on the Devils Mountain fault on this profile are speculative because reflections in the Everett basin are partly obscured by water-bottom multiples. These multiples also partly mask hummocky reflections in the uppermost Pliocene(?) to Pleistocene section south of the Devils Mountain fault, which either could be depositional in origin (such as moraines) or could represent disruption by faulting or folding. A small aeromagnetic anomaly is associated with the Devils Mountain fault on this profile (figs. 3, 13A).
Farther south, the Strawberry Point (3) and Utsalady Point (2) faults are imaged as subvertical structures that bound a nonreflective horst block and truncate moderate- to high-amplitude, subparallel, continuous to discontinuous reflections in the Everett basin. This horst block coincides with an aeromagnetic low (figs. 3, 13A) and is inferred to consist of pre-Tertiary sedimentary rock overlain by a thin uppermost Pleistocene to Holocene (postglacial) section. Juxtaposition of this horst block and Everett basin Tertiary and Quaternary strata requires significant uplift. This seismic profile suggests about 150-200 m (165–220 ms) of vertical separation on the base of the uppermost Pliocene(?) to Pleistocene section across the Strawberry Point fault, and about 250-300 m (275–330 ms) across the Utsalady Point fault.
Industry Line 1 (figure 14) is subparallel to U.S. Geological Survey Line P166 (figure 5A, B) and provides a slightly deeper view of structure in this area. The Devils Mountain fault is not well imaged, but it appears to juxtapose relatively nonreflective material north of the fault and a section characterized by warped discontinuous reflections to the south (best seen at ≈0.7 s). As on Line 166 (figure 13), the Strawberry Point and Utsalady Point faults appear as subvertical faults bounding a generally nonreflective uplift that correlates with an aeromagnetic low (Fig. 14A). The uplift has relief above the seafloor and is onlapped or draped by Quaternary strata, which are warped across the axis of the uplift.
Line 165 (figure 15) trends west, extending partly across the opening to Rosario Strait, between Lopez and Fidalgo Islands (figure 5A). This line images a gentle asymmetric anticline with a subvertical fault or faults along its axis, coinciding with a bathymetric rise and east-facing scarp on the seafloor. The fault propagates upward into the lower part of the uppermost Pliocene(?) to Pleistocene section, and these strata are involved in the folding. Because water-bottom multiples obscure much of the uppermost part of the record, the amount of deformation in the upper part of the Quaternary section is unclear.
Line 164 (figs. 16, 17) trends north and extends from the eastern Strait of Juan de Fuca into southern Rosario Strait (figure 5A). On the airgun profile (figure 16), the contact between the inferred Tertiary and uppermost Pliocene(?) to Pleistocene sections is characterized by an upward decrease in frequency, amplitude, and continuity of reflections. This contact is offset about 45-60 m (50-65 ms) across the steeply (≈70°-75°) north-dipping Devils Mountain fault (3), which correlates with a gentle aeromagnetic high (figs. 3, 16A). The offset contact is most clear where disparate seismic facies are juxtaposed between about 100 and 300 ms. The companion higher frequency seismic-reflection profile (figure 17) similarly shows an abrupt vertical termination of reflections at this location, extending up to within 10 m of the surface. This abrupt termination likely indicates a significant contrast in the physical properties of the sediment, such as grain size, stratification, or gas-filled porosity (Hovland and Judd, 1988; Fader, 1997). The steep fault dips and the contrasts in reflection properties across the fault on both images is consistent with out-of-plane (strike-slip) displacements.
Farther south on Line 164 (figure 16), two faults are mapped as the Strawberry Point (2) and Utsalady Point (1) faults. Both structures are imaged as high-angle faults that truncate subhorizontal reflections, but only fault 1 appears to continue upward into the lower part of the uppermost Pliocene(?) to Pleistocene section. These faults are not recognized on seismic-reflection profiles to the west (for example, figs. 18, 19, pl. 2), and their more subtle seismic and aeromagnetic character (figs. 3, 16A) provides an indication that they are dying out (figure 2). South of these faults, the inferred Tertiary section is warped into a gentle anticline and syncline.
Line 162 (figs. 18, 19) is a north-trending line south of Lopez Island (figure 5A). The airgun line (figure 18) shows three faults (1, 2, 3) in a 1,200-m-wide zone representing the Devils Mountain fault. The faults dip 60°-80° to the north in the upper ≈300-500 m. On reflections within the Quaternary section across 1, structural relief amounts to about 150–200 m (160–220 ms), and we attribute the bathymetric relief (50-60 m) within the fault zone to Quaternary uplift. All three faults extend upward into the lower part of the inferred uppermost Pliocene(?) to Pleistocene section. The southern strand (1) juxtaposes relatively flatlying beds in the Everett basin and a panel of south-dipping beds in the fault zone. The southerly dips extend upward into the uppermost Pleistocene to Holocene section, indicating that some structural and topographic growth has occurred in the last ≈13,000 years. The middle fault (2) juxtaposes the domain of south-dipping beds to the south with a zone of less reflective north-dipping beds and represents a faulted anticline axis. The northern fault (3) occurs within the north limb of this fold, truncates and warps reflections on its trace, and forms a boundary between the less reflective horizon to the south and a panel of more continuous higher amplitude reflections to the north. Dips north of this fault are about 10°-15° in the uppermost Tertiary section and about 5°-8° in the middle part of the uppermost Pliocene(?) to Pleistocene section. A small positive aeromagnetic anomaly is associated with the fault zone on this profile (figs. 3, 18A).
The companion higher frequency profile (figure 19) provides additional details from the uppermost part of the section. On this profile, the inferred postglacial uppermost Pleistocene to Holocene section is represented by relatively continuous, high-amplitude, high-frequency, parallel reflections; the underlying Pleistocene section is characterized by nonreflective or low-amplitude, discontinuous reflections. The southern strand of the Devils Mountain fault (1 in figure 18) appears as two splays (1A, 1B). These faults truncate and warp reflections in the upper part of the uppermost Pliocene(?) to Pleistocene section (at ≈170 ms) and warp but do not obviously cut the uppermost Pleistocene to Holocene section. Fault 1B forms the boundary between parallel reflections on the south and a region of warped reflections (dips as much as 8°) and diffractions on the north. The middle fault (2) is not obvious - its location in Figure 19 is based on the companion airgun profile (figure 18). The inferred postglacial uppermost Pleistocene to Holocene section between faults 2 and 3 to the north is very gently warped. Fault 3 truncates and warps the upper part of the uppermost Pliocene(?) to Pleistocene section and cuts the lower and middle parts of the inferred postglacial section to within about 6-7 m of the seafloor (above the stratigraphic level shown in figure 18).
North-trending Line 158 (figure 20) is located south of southwestern Lopez Island (figure 5A). The line shows three faults (1, 2, 3) dipping about 45° to 60° to the north (above ≈450 ms). This 1,200-m-wide zone of faults is located about 1,000 m north of the projected westerly trace of the Devils Mountain fault, where no obvious fault is imaged in the upper ≈400 m (≈450 ms). Industry Line 2 (figure 21A, see next section), located 1.5 km to the west of line 158 (figure 5), reveals “blind” faulting below about 1 km along the projected fault trace, the basis for mapping the fault in this area in Figure 2. Faults 1, 2, and 3 all extend upward into the uppermost Pliocene(?) to Pleistocene section, and we infer about 50-100 m (55-110 ms) of structural relief on the base of this unit within the fault zone. Faults 1 and 2 bound a panel of steep north-dipping reflections in basement (inferred Tertiary sedimentary rocks). Fault 3 juxtaposes north- and south-dipping panels and represents a faulted anticline axis. Farther north, the basement and at least the lower part of the uppermost Pliocene(?) to Pleistocene section are warped into a series of gentle anticlines and synclines that have wavelengths of 1-2 km. South of the projection of the Devils Mountain fault, the contact at the base of the uppermost Pliocene(?) to Pleistocene section in the Everett basin is picked mainly on the basis of nearby industry data and is nearly flat. Figure 3 shows that the Devils Mountain fault zone and the fold belt on its north flank correlate with low-amplitude aeromagnetic anomalies.
North-trending industry Line 2 (figure 21) lies about 1 km west of U.S. Geological Survey line 158 (figure 5A, B) and provides a deeper image of structure south of Lopez Island. In contrast to the shallower data of figure 20, this profile shows two north-dipping faults (1, 2) along the projected westerly trace of the Devils Mountain fault. Each occurs below about 0.5 s, deeper than the level imaged in figure 20, and neither of these faults obviously disrupts the Quaternary section. Faults 1 and 2 form the structural boundary between subhorizontal beds in the Everett basin to the south and gently folded Tertiary and Quaternary strata in an uplifted block to the north. This broad boundary is overlain by a gentle aeromagnetic high (figs. 3, 21A). The folds on the uplifted northern block are cut by at least one north-dipping thrust fault (3) overlain by a hanging-wall anticline. This folded terrane coincides with the southeast portion of a band of aeromagnetic anomalies that extend northwest along the west coast of San Juan Island (figure 3).
Geological Survey of Canada Line 15 (figure 22) extends southwest of southern Lopez Island into the eastern Strait of Juan de Fuca (figure 5C). The southernmost fault identified on this profile (1) lies on strike with the westerly projection of the Devils Mountain fault and is inferred to be its main strand. This structure dips about 50°-55° to the north and juxtaposes subhorizontal reflections in Tertiary and uppermost Pliocene(?) to Pleistocene strata (to the south) and a panel of north-dipping reflections in correlative beds to the north. The north-dipping reflections are in turn truncated by another fault (2) about 700 m north of the main strand. Both faults extend upward into the lower part of the uppermost Pliocene(?) to Pleistocene section, and fault 1 appears to offset the base of the postglacial uppermost Pleistocene to Holocene section. South of fault 1, the contact at the base of the uppermost Pliocene(?) to Pleistocene section is a locally angular unconformity.
About 3,600 m north of the inferred main strand of the Devils Mountain fault, another north-dipping fault (3) truncates the south limb of an asymmetric anticline. The fault extends upward into the Quaternary section and about 100 m (110 ms) of structural relief is present on the inferred base of the uppermost Pliocene(?) to Pleistocene section across the structure. North of this fault, inferred Tertiary and lower Quaternary strata are gently folded and Tertiary strata are cut by at least one fault (4).
SHIPS Line JDF4 (figure 23) extends south from offshore southern San Juan Island (figure 5B). This profile images the Devils Mountain fault as a north-dipping (≈45°-55°?) blind structure that truncates and juxtaposes different seismic facies in inferred pre-Quaternary strata. This fault forms the core of an asymmetric anticline that coincides with an aeromagnetic high (figs. 3, 23A). The prominent reflection at the base of the uppermost Pliocene(?) to Pleistocene section is folded above the fault, but not ruptured. The dip on the base of this unit on the south limb of the fold is ≈11°, and about 400-450 m (450-500 ms) of inferred structural relief is present on this reflection from the crest of the fold north of the blind fault to the floor of the basin south of the blind fault. Reflections in inferred uppermost Pliocene(?) to Pleistocene strata on the south-dipping limb of the fold dip south and converge toward the anticline axis, suggesting that they were deposited during fold growth. The aeromagnetic high at the south end of the profile probably reflects a change in the lithology of basement rocks, possibly associated with the northwest extent of the southern Whidbey Island fault (Johnson and others, 1996).
Industry Line 3 (figure 24) extends south from Rosario Strait into the eastern Strait of Juan de Fuca (figure 5C). This profile images the Devils Mountain fault as a north-dipping structure (~60°-70°) that truncates the south limb of an asymmetric anticline. Quaternary beds are gently folded (dips as much as 10°) on the flanks of the anticline but do not appear to be truncated by the fault. The anticline is overlain by an aeromagnetic high (figs. 3, 24A) bounded on the south by a steep, west-trending gradient that correlates with the Devils Mountain fault and on the north by a steep, northwest-trending gradient that correlates with a belt of northwest-trending folds.
Geological Survey of Canada Line 37 (figure 25) extends southwest from offshore of San Juan Island into the eastern Strait of Juan de Fuca (figure 5C). On this profile, the Devils Mountain fault is a north-dipping blind fault (1) overlain by an asymmetric hanging-wall anticline (fault-propagation fold of Suppe and Medwedeff, 1990). Reflections from uppermost Pliocene(?) to Pleistocene beds above the fault on the south-dipping limb of this anticline appear continuous, dip about 7°, and shallow upward. There is approximately 180 m of structural relief on the base of this unit from the crest of this anticline into the flanking footwall basin. Following the methods of Schneider and others (1996, their figure 10), the horizontal component of fold growth was determined to be about 135 m, and dip on the blind fault is inferred to be about 53°. Lowest uppermost Pliocene(?) to Pleistocene beds also appear to be very gently warped in the syncline and anticline that occur within 2 km north of the Devils Mountain fault, but they flatten upwards. For nearly 6 km north of the Devils Mountain fault, the Quaternary section has a relatively constant thickness of about 200 m (220 ms). Farther north, the basement appears irregular and faulted (faults 2, 3, and others?) and the thickness of the Quaternary section is highly variable.
The uplift on the north side of the Devils Mountain fault forms a prominent aeromagnetic high bounded by west- and northwest-trending gradients on its south and north sides, respectively (figs. 3, 25A). The steep west-trending gradient is associated with the Devils Mountain fault. The steep northwest-trending gradient occurs within folded terrane north of the Devils Mountain fault, does not correlate with any feature on the shallow seismic-reflection data, and therefore must have a deeper source. The aeromagnetic high at the south end of the profile similarly has a deeper source and correlates with a significant contrast in basement lithology (Johnson and others, 1996).
Geological Survey of Canada Line 35 (figure 26) extends southwest from about 4 km east of Vancouver Island into the eastern Strait of Juan de Fuca (figure 5C). On this line, our favored interpretation (Interpretation 1, figure 26A) is that the Devils Mountain fault (1) continues its west trend as a north-dipping (≈45°-55°) structure that offsets the contact between pre-Tertiary rock and uppermost Pliocene(?) to Pleistocene deposits (marked by a set of two high-amplitude reflections) about 250 m (280 ms). Alternatively (Interpretation 2; figure 26B) and less likely, the Devils Mountain fault is a blind structure on this profile (as in the nearest profiles to the east, figs. 23, 24, 25), and the north-dipping limb of the fold is poorly imaged on this profile. In this second scenario, Quaternary structural relief is about 500 m (550 ms), Quaternary shortening is about 450-500 m, and fault dip is 45°-50°.
Immediately north of the fault in the hanging wall (Interpretation 1), the pre-Tertiary basement surface dips gently south for about 1,200 m and is draped by uppermost Pleistocene to Holocene (postglacial) beds. Farther north, the pre-Tertiary basement surface is irregular, appears faulted (2 and 3), and is variably overlain by both uppermost Pliocene(?) to Pleistocene and uppermost Pleistocene to Holocene strata. Fault 3 coincides with a steep, continuous, northwest-trending aeromagnetic gradient (figs. 3, 26A ) and probably represents a significant splay of the Devils Mountain fault.
South of the Devils Mountain fault, uppermost Pliocene(?) to Pleistocene sediment is relatively flat and fills the irregular faulted and folded surface at the top of highly magnetic basement (figs. 3, 26A). The prominent anomaly formed by this magnetic basement extends southeast onto the northeastern Quimper Peninsula (Blakely and others, 1999b; Blakely and Lowe, 2001) where it can clearly be correlated with Eocene basaltic rocks of the Crescent Formation. Thus, the Devils Mountain fault in this location corresponds to a major crustal boundary between pre-Tertiary basement rocks and younger Tertiary basement rocks (Johnson, 1984b; Johnson and others, 1996).
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