Geochemistry, Geophysics, Geosystems

RESEARCH ARTICLE Initiation of Strike-Slip Faults, Serpentinization, 10.1029/2018GC007851 and Methane: The Nootka Fault Zone,

Key Points: the Juan de Fuca-Explorer • Strike-slip faulting across the Juan de Fuca plate began in a zone at least Plate Boundary 80 km wide; after 4 Ma it is only 8 to 18 km wide Kristin M. M. Rohr1 , Kevin P. Furlong2 , and Michael Riedel3 • Uplift of oceanic crust after 2 Ma is likely an isostatic adjustment from 1Geological Survey of Canada, Sidney, British Columbia, Canada, 2Department of Geosciences, Pennsylvania State serpentinization as the volume of 3 upper mantle increased University, University Park, PA, USA, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany • Methane in sediments may have biogenic and abiogenic sources; the latter may be associated with Abstract The Nootka fault zone is a ridge-trench-trench transform fault that was initiated ~4 Ma when the serpentinization Explorer ridge became independent of the Juan de Fuca ridge. Multibeam data around the fault zone and a compilation of several seismic reflection surveys provide insight into initiation of strike-slip faults. Previous interpretations assumed that the two faults seen cutting the seafloor are subparallel to shear between the Correspondence to: Explorer and Juan de Fuca plates and formed instantaneously at 4 Ma. Increased data density shows that K. M. M. Rohr, these faults are subparallel to seafloor magnetic anomalies and appear to have utilized extensional faults [email protected] formed at the ridge. They are surrounded by numerous buried steeply dipping, small-offset growth faults; at least some of which are likely still active. Our observations corroborate analogue models of strike-slip fault Citation: initiation that predict formation of Riedel-like shears within a zone of faulting and that displacement localizes Rohr, K. M. M., Furlong, K. P., & Riedel, M. (2018). Initiation of strike-slip faults, over time. The existence of several long subparallel faults and a very wide zone of faulting has been predicted serpentinization, and methane: The by models of distributed shear at depth. Along the Nootka fault zone basement has risen by several Nootka fault zone, the Juan de hundred meters and bright reversed-polarity reflectors some of which are interpreted to be methane hydrate Fuca-Explorer plate boundary. fl Geochemistry, Geophysics, Geosystems, re ectors are common. Hydration, likely as serpentinization, of the upper mantle could explain both sets of 19, 4290–4312. https://doi.org/10.1029/ observations: Serpentinization can result in a 30–50% volume expansion and methane is observed in 2018GC007851 vents driven by this process. Biogenic sources of methane are likely to be present and concentrated by currently active fluid flow in the faulted sediments. Received 24 JUL 2018 Accepted 6 OCT 2018 Plain Language Summary Four million years ago the Juan de Fuca oceanic plate offshore British Accepted article online 13 OCT 2018 Published online 8 NOV 2018 Columbia was sheared and began to split off its northern section. This occurred along a shear zone we now call the Nootka fault zone. It generates many small earthquakes in the oceanic rocks, and its extension under Vancouver Island has generated sizable earthquakes felt by residents of the island. Understanding the composition and form of the fault zone allows a greater understanding of the seismic hazards associated with this fault. By studying detailed acoustic profiles of the seafloor morphology, sediments’ structure, and the underlying hard rocks, we learned that the split of the Juan de Fuca plate was not a clean break but began in a zone at least 80 km wide and is now just 8 to 18 km wide. The faults have likely changed the density and composition of rocks at depth by allowing water to percolate through the fractures. We also observe high-amplitude reflections from gas trapped in shallow sediments. Methane can be generated by bacteria in the sediments and is also often observed when deep rocks are being hydrated.

1. Introduction The Nootka fault zone (NFZ) is a transform plate boundary linking the northern end of the Juan de Fuca (JdF) ridge to the Cascadia subduction zone and separating the JdF plate and Explorer plate (EXP). The NFZ devel- oped in the last 4 Ma within the existing JdF plate and thus has had an evolutionary history significantly dif- ferent from that of mid-ocean ridge (MOR) transform faults. The latter that are continuously produced as plates diverge in a steady state process have characteristic structures and evolution (e.g., Behn et al., 2002; Cann et al., 1997; Furlong et al., 2001; Parson & Searle, 1986; Searle, 1986); their histories reflect the links and feedbacks between thermal and rheological processes along the plate boundary. In contrast, transform faults such as the NFZ, which have developed within an existing plate by strain localization and fault forma-

©2018. American Geophysical Union. tion in response to external forces, have evolutions that are less well understood but may be more analogous All Rights Reserved. to strike-slip fault systems in continental crust.

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Figure 1. Magnetic anomalies draped on hill-shaded bathymetry. Red denotes positive magnetic anomalies; ages are based on identifications by Wilson (1993, 2002). Gray outline is the extent of the Nootka fault zone (NFZ) as defined by Hyndman et al. (1979); white box outlines map areas shown in Figures 2 and 11. Cyan line is rift propagator 7 (Wilson, 1993); dashed cyan line is a proposed propagator (Wilson, 1993). EXP is the Explorer plate, JdF is the Juan de Fuca plate, and NAM is the North American plate. Arrow shows direction of relative motion between the JdF and NAM plates computed using the MORVEL model (DeMets et al., 2010).

Forming in a more homogeneous oceanic plate the NFZ provides a natural laboratory to investigate initiation of strike-slip faults. Whereas in continental plates, it can be difficult to separate the influences of previous tec- tonic events and heterogeneous lithologies from the process of initiation (Norris & Toy, 2014). Physical and numerical models have been used successfully to help unravel complex histories of evolving strike-slip fault systems, yet these models have some limits because they ignore pore fluid pressure, thermal, and isostatic effects (Dooley & Schreurs, 2012). In this paper we interpret numerous, existing single-channel seismic data sets, in which the deposition of thick distal Pleistocene turbidites allows us to track deformation in the last 2.6 Ma providing a detailed image of the present-day structure of the NFZ. This allows us to place important constraints on its present-day kine- matics and the tectonic pathway from initiation as a broad shear zone to its current ~20-km-wide structure. Angular unconformities within the sediments as well as significant bright reversed-polarity reflectors provide further information about hydration and fluid flow within the fault zone. When interpreted in concert with an understanding of the implications of the plate tectonic history, these detailed seismic interpretations provide an improved understanding of how a transform plate boundary can form and evolve within the lithosphere.

2. Regional Geologic and Tectonic Setting The NFZ (Figure 1) links the northern end of the JdF spreading center and the Sovanco Fracture Zone to the deformation front (DF) of the Cascadia subduction zone and separates oceanic lithosphere of the JdF plate

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and EXP (Davis & Currie, 1993). It was first recognized by its seismicity (Barr & Chase, 1974) and later inter- preted from magnetic anomalies and seismic reflection data to be an ~20-km-wide zone of active faulting extending more than 100 km to the Cascadia subduction zone (Hyndman et al., 1979) (Figure 1). Sediments in the region consist of Pliocene hemipelagic deposits overlain by Pleistocene turbidite deposits interlayered with hemipelagic sediments (Davis & Hyndman, 1989; McManus et al., 1972; Underwood et al., 2005). The sedimentation rate from glacial melting can be up to 5 times the rate of hemipelagic sedimenta- tion off Vancouver Island (Su et al., 2000). Vancouver Gap funneled outflow from a number of canyons off Queen Charlotte Sound and northern Vancouver Island onto the EXP and the NFZ. East of the NFZ individual canyons have fed sediments to the abyssal plain, but no channels are evident in the bathymetry. Distal flow from the Nitinat fan (Andrews et al., 2012; Underwood et al., 2005) may have reached this far west after its initiation at 0.76 Ma. Analysis of a reversed refraction line within the NFZ failed to observe a Moho refractor (~8 km/s) although the line should have been sufficiently long enough to do so for typical oceanic crust; instead, velocities of 7.5 km/ s were detected from 5.5 to 10 km below the sediment-basement interface (Au & Clowes, 1982). Similar velo- city structures nearby were interpreted by McClymont and Clowes (2005) to be the result of serpentinization associated with rift propagator 7 (Rp7; Wilson, 1993). A broad swath of earthquakes from Middle Valley (MV), Middle Ridge (MR), and below the continental shelf has also been attributed to the NFZ (Barr & Chase, 1974; Hyndman et al., 1979). Routine locations of earthquakes in the Geological Survey of Canada catalogue that occurred after 1992 (Figure 2d) are con- sidered to have more accurate locations (errors of 5–10 km, J. Cassidy, personal communication, June 2009) than events before 1992 because a denser network of permanent seismometers is in place and a velocity function that compensated for paths through oceanic lithosphere is applied. Focal mechanism solutions in the region are strike slip and interpreted to be left lateral (Braunmiller & Nábělek, 2002; Ristau, 2004; Rogers, 1975). More recently, the NFZ has been the subject of a 3-month long ocean bottom seismometer experiment that showed a large amount of microseismicity (Obana et al., 2015) up to 15 km below the top of basement. Various kinematic models were proposed for the initiation and evolution of the NFZ based on the com- plex pattern of magnetic anomalies (Figure 1). Barr and Chase (1974) proposed that a regional unconfor- mity at ~ 0.7 Ma corresponds with initiation of the Nootka fault (although it was not named as such until 1979) as well as initiation of West Valley spreading center and abandonment of MV as a spreading center (Davis & Villinger, 1992). Hyndman et al. (1979) dated initiation of the NFZ around 7 Ma and placed its eastern terminus off the coast of Washington state south of its current location. More recent interpreta- tions agree that shear initiated between the JdF plate and EXP around 4 Ma (Botros & Johnson, 1988; Riddihough, 1984; Wilson, 1993). Adjacent to the NFZ magnetic anomalies on the JdF plate have been identified, but anomalies on the EXP within 55 km of the NFZ have proven difficult to identify (Botros & Johnson, 1988; Riddihough, 1984; Wilson, 1993). As a result there is no direct estimate of the amount of total displacement between the EXP and JdF plate accommodated by the NFZ. Wilson (1993) further noted a 2- to 4-km discrepancy in the distance between anomalies 3 and 3A (4–6.5 Ma) on the JdF plate and EXP and interpreted it to reflect internal deformation or changes in behavior along the northern JdF ridge prior to the formation of the NFZ. The evolution of the NFZ may be further complicated by the northward propagation of the MV segment of the JdF ridge between 2.5 and 1 Ma (Botros & Johnson, 1988). RP7 in the magnetic anomalies is evidence of this migration; it intersects the NFZ at 48° 500 (Figure 1). Various lines of evidence point to the idea that since its formation the EXP has been gradually behaving kinematically more similar to North America with respect to the JdF plate. Riddihough (1984) used mag- netic anomalies to interpret that subduction of the EXP had ceased by 2.5 Ma. Because the general strike of the NFZ is similar to the direction of JdF-North American relative plate motion (Figure 1), an underlying factor in NFZ formation was likely the incorporation or partial incorporation of the northernmost section of the JdF plate (the Explorer) by North America as interpreted by Rohr and Furlong (1995). The NFZ does not appear to have exploited existing fracture zones associated with transforms on the JdF Ridge but rather runs at a high angle to such structures. As described by Botros and Johnson (1988), prior to the formation of the NFZ the northern JdF Ridge hosted numerous ridge propagators resulting in a

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Figure 2. Multibeam data (Dziak, 2006; GeoMapApp, 2018) over the NFZ is shown at different scales in (a), (b) and in a regional context in (c), (a) hill-shaded relief of multibeam data shows locations of fault segments well; arrows point to many of them. The gray box indicates the extent shown in (b) in which bathymetry has been draped over hill-shaded relief, the bathymetric range is only 50 m (2,490 over the fault zone and 2,540 on the JdF plate), the scale is shown in (c), a line interpretation of the principal faults in regional context; the deformation front of Cascadia subduction zone is shown by the thrust fault line. The locations of seismic reflection lines in Figures 4–10 and 13 are shown by blue boxes; figure numbers are annotated beside them, and the gray box shows the extent of (a), (d) earthquakes from 1992 to 2018 (Earthquakes Canada, 2018). Interpretation of NFZ has been modified from Dziak (2006). Two main fault traces are seen: northwestern Nootka fault and a southeastern Nootka fault. Red arrows are average direction of left-lateral slip from fault plane solutions in the NFZ (Braunmiller & Nábělek, 2002); cyan line is rift propagator 7. MV is Middle Valley segment of the JdF spreading center; MR is Middle Ridge. NFZ = Nootka fault zone; JdF = Juan de Fuca.

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complex pattern of MOR transform faults and fracture zone formation, precluding the development of lengthy long-lived transform faults. In contrast, in the southern JdF plate where the adjacent Gorda plate is similarly becoming increasingly independent of the JdF plate, the plate separation appears to be aligned with the eastward extension of the Blanco Fracture Zone into the JdF plate, which separates oceanic litho- sphere of differing ages, formed at different spreading centers (Chaytor et al., 2004). This extension also separates segments with dis- tinctly different slab behavior at subduction (Obrebski et al., 2010). This implies that at this southern plate breakup, the existing Blanco Fracture Zone, east of the Gorda Ridge, is acting as a developing plate boundary. The development of a crosscutting structure that is generally aligned with JdF-North America motions supports the assumption that the underlying driver for EXP formation was the increasing capture of the northernmost JdF by North America. The kinematic evolution of such a scenario is shown in a plate motion velocity triangle (Figure 3). Pacific-North America and Pacific-JdF plate Figure 3. Kinematic evolution of the Juan de Fuca (JdF), Pacific (PAC), and motions are well constrained (DeMets et al., 2010). Completing the velo- North American (NAM) plates. As the Explorer plate (EXP) becomes city triangle produces the North America-JdF motions shown; these attached to North America, the EXP to JdF motion approaches that of the JdF motions are compatible with the Euler poles for Explorer motion deter- to NAM motion in direction and magnitude. The top inset depicts ridge mined by Braunmiller and Nábělek (2002). Since 4 Ma, the Explorer configuration at ~4 Ma when the Nootka fault zone was initiated; the bottom ridge has rotated clockwise relative to the JdF plate (Botros & inset is the present configuration (both insets slightly modified from Botros and Johnson (1988). Johnson, 1988) implying a clockwise rotation of the motion of the EXP with respect to the Pacific plate. At formation of the NFZ, the relative motion between the EXP and JdF plate was likely very small. Hence, the position of the EXP in the velocity triangle of Figure 3 initially would be close to the JdF plate (i.e.,

EXPold). As the Explorer ridge rotates clockwise, the associated rotation of the motion of the EXP with respect to the Pacific would lead to increasing plate motion rates on the NFZ. The rotation of the Explorer ridge is preserved in the magnetic anomalies of the Pacific plate and EXP, but that signal likely represents the effect, not the cause. Rather, the increasing capability of the NFZ to accommodate relative plate motion and/or the increasing capture of the EXP by North America would lead to the migration of the location of the EXP on the velocity triangle (along the NAM/JdF leg) toward the North American posi- tion, driving the rotation of the Explorer ridge (Figure 3). This plate motion analysis implies that at present, the NFZ is accommodating approximately 80–90% of the motion between JdF and North America, or approximately 35–40 mm/year of the ~43 mm/year derived from plate motion models (Figure 3). If this relative motion rate has been linearly increasing since formation at 4 Ma (i.e., the increase in EXP-JdF motion shown in Figure 3 occurred at a constant rate which is a simplistic assumption), then the average rate of motion over the past 4 Ma would be ~20 mm/year, implying a maxi- mum of 80 km of relative displacement on the NFZ. This is most likely an overestimate, as the feedbacks between NFZ development and relative motions on the NFZ would likely lead to an increase in the rate of change of the plate motions over time. Additionally, as the position of the EXP on the velocity triangle in Figure 3 approaches the NAM vertex, this implies that the subduction rate between the Explorer and North America has been decreasing.

3. Multibeam and Seismic Reflection Data 3.1. Multibeam Data Multibeam data collected in systematic surveys as well as transit lines were compiled by National Ocean and Atmosphere Agency (Dziak, 2006) and more recently by GeoMapApp (2018; Figures 2a–2c). Transit lines can be noisy, and in some the base level does not agree with the regional base level; nevertheless, they provide information in areas that otherwise would have none. In the southeastern corner of the map single transit lines were cropped out.

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3.2. Seismic Reflection Data We focus on data within and 15 km either side of the NFZ and north of MV to limit the effects of subsidence and shear within the EXP as well as structural complications of ridge propagation. When describing the reflec- tion profiles we will reference the horizontal axis in kilometers along the line. Single channel seismic reflection profiles dating from 1971 to 2003 have been compiled and interpreted with modern digital software. Data collected between 1971 and 1979 used either a 300-in.3 air gun or an electro- magnetic source (Davis & Lister, 1977; Hyndman et al., 1979); paper records were scanned and converted into SEGY format. Records from 1994 to 1996 were collected using either a 40- or a 120-in.3 air gun and were digi- tized at sea with a 2-ms sampling rate; a few lines used a lower rate of 4 ms. Data collected in 2001 and 2003 used a 40-in.3 air gun and were recorded with 0.1-ms sampling rate. Acoustic penetration through the section is variable depending on the seismic source, and ambient noise, but signals up to 500–700 ms below the sea- floor are typically observed. Below that time, the signal-to-noise ratio tends to be 1:1, but reflectors can be detected if they are laterally coherent. Navigation progressed from Loran with occasional satellite fixes between 1971 and 1979 to GPS in 1994–1996 to real-time GPS in 2001 and 2003. One multichannel profile, 89-09 (Hyndman et al., 1994), is on the eastern edge of our study area. One-way travel time was converted into depth by using a velocity gradient of 0.6 km/s/km, the value obtained by a refraction study of the JdF plate (Horning et al., 2016). Computing travel time to a given depth using this gradient allowed us to construct the inverse equation to compute depth (z) from one-way travel time (t): z = 642 t2 + 1,660 t.

4. Interpretation 4.1. Multibeam Data Interpretation Multibeam data show subtle bathymetric changes in and around the fault zone (Figure 2; Dziak, 2006); a map of hill-shaded relief delineates the fault segments (Figure 2a). Topography across individual segments is rarely more than 10 m and can be as low as 1–4 m, the level of noise within the multibeam data sets. Coherence and a different strike from noise allow more subtle offsets to be identified (Figure 2b). Two main traces of rupture of the seafloor were first described by Hyndman et al. (1979) from 3.5-kHz data; Dziak (2006) interpreted a number of short en echelon fault segments to comprise the NFZ in his interpretation of regional shear. We interpret that the faults consist of longer en echelon segments, but detailed structure between the tips of faults is often obscured by noise. The northwestern Nootka fault (NNF) lies on the eastern side of MR and a subparallel fault trace, the southeastern Nootka fault (SNF), lies ~13 km to the east. The NNF had pre- viously been interpreted to wrap around MR and into the northern end of MV; instead, the NNF cuts MR at 48.6°N into two distinct sections. The northern MR is higher (1,860–2,300 m below sea level [mbsl]) and cut by northeasterly trending faults, whereas the southern MR (2,200–2,400 m bsl) consists of ridge-parallel abys- sal hills. In addition, a parallel segment of the NNF steps to the left and further north merges gradually with the dominant trend of the NNF. The SNF is observed over 40 km; irregular data coverage and processing do not allow its southern terminus to be well imaged (Figure 2b). Both faults terminate to the north in northwest curving tips. Broad-scale bathymetry (Figure 2c) shows a swell 25–50 m high over the southwestern NFZ, whereas in the northeast bathymetry slopes down from west to east. A step down in bathymetry trends north in the NFZ and connects right steps in both the NNF and the SNF; it appears to be a cross fault between the NNF and the SNF. During interpretation of the reflection data the multibeam data were visible simultaneously allowing for a joint interpretation. 4.2. Stratigraphy Seafloor, four sedimentary horizons and the top of oceanic crust (basement) horizon were picked wherever data quality permitted; a fifth horizon was visible on just a few profiles but proved useful for interpreting ages of horizons. Several tie lines exist west of the NFZ, but on the JdF plate there was only one short tie line. Regional occurrence of reflectors could be recognized but sedimentary sources from different locations and plate bending change the patterns somewhat; an interval of nondeposition over the southern NFZ also created challenges in horizon interpretation. An approximate age of sedimentary layers around the NFZ can be inferred from a tie of line 95-01 to line 89- 09 (Figure 4) and a reflection line that crossed a 2.6-Ma isochron. Acoustically the Pliocene (2.6–5.3 Ma) and

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Figure 4. Interpretation of single-channel line 95-01 and multichannel line 89-09: white, orange, brown, green, and blue are sedimentary horizons and red is basement. Faults are shown in dark blue. Horizontal axis is in kilometers; vertical axis is two-way travel time in seconds. Axis on far right is approximate depth in meters below seafloor (mbsf). Note 90° difference in orientation between the two lines. Red arrows indicate high-amplitude reflectors. Multichannel line 89-09 was published uninterpreted by Hyndman et al. (1994); stratigraphic interpretations were done for this paper. DF = deformation front.

older hemipelagic sediments are transparent to weakly reflective, whereas Pleistocene turbidite deposits are highly layered and generate numerous reflectors (Davis & Hyndman, 1989; McManus et al., 1972; Underwood et al., 2005). Multichannel line 89-09 imaged both sedimentary units well; crust is ~5.5 Ma here. The blue horizon separates relatively transparent from more reflective sections, and we interpret it to be the Pliocene-Pleistocene contact (~2.6 Ma). Without dates to constrain any of the other sedimentary horizons, we assume that sedimentation rates were constant in the Pleistocene in order to estimate ages of the horizons. We converted picked times to depth using the formula described above and obtained ages of 0.4, 0.8, 1.2, and 1.9 for the white, orange, brown, and green horizons, respectively. We also examined line 94-03 that crossed the young side of the anomaly 2A isochron (2.6 Ma) 14 km southwest of RP7. This yields ages of 0.3, 0.6, 1.3, and 2.0 Ma for the white, orange, brown, and green horizons; these ages are within 0.1–0.2 Ma of values on 89-09, a reasonable agreement considering the simplistic assumptions. The orange reflector is just after or at the transition from Early to Late Pleistocene time (0.8 Ma). These dates on the horizons should be considered only as rough guides to age; Su et al. (2000) have shown that deposition rates are highly variable on the JdF plate depending on location relative to the shelf break, local basement topography, and geometry of sediment delivery systems.

4.3. Imaging of Faults Faults were readily recognized by vertical offsets of horizons on adjacent traces (Figure 5). The ability to detect fault offsets depends on dominant frequency, bandwidth, seismic velocity, and signal-to-noise ratio. Theoretically, horizontal resolution is constrained by the size of the Fresnel zones (Sheriff, 1977) for the effec- tive frequencies; the Fresnel radius of a 70- to 200-Hz source ranges from 108 to 182 m at 50 m below seafloor (mbsf) and 128–206 m at 400 mbsf. In practice, one can often do better than the predicted Fresnel limit in

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Figure 5. Detail of line 94-02 over NNF; location shown in Figures 2 and 6. Steeply dipping faults offset the seafloor and deeper horizons. Annotation as in Figure 4. NNF = northwestern Nootka fault; TWTT = two-way travel time. mbsf = meters below seafloor.

shallow sediments where the high-frequency content was strong and noise levels were low (Yilmaz, 1987). The sections shown here were not migrated so diffractions often indicate the presence of a fault but can obscure its exact location. This effect increases as the acoustic velocity increases with depth and diffractions broaden. Vertical resolution also varies with frequency, bandwidth, signal-to-noise ratio, and sampling interval of the data and is often taken as one eighth of the dominant wavelength. With dominant wavelengths of 70 to 200 Hz the wavelength/8 values can be from 1 to 3 m for velocities in the top 1,000 ms (~1,600 m/s to 2,100 m/s). (Sheriff, 1977; Yilmaz, 1987). In practice, the bandwidth is highly significant because it determines whether the source signal is impulsive or broad. Differences of 2-ms two-way time (1.6–2.2 m) on a horizon between traces could usually be detected in the data. To summarize, it is often obvious in the data that a fault exists, but horizontal resolution of its location may be on the order of ±100 m and vertical resolution of offset can be 2 ± 1 m. Apparent fault dips were obtained by using the vertical and horizontal separation between two points along a given fault in an inverse tangent calculation. Two-way travel times were converted to depth using the for- mula described above. Measurements were made in the upper 700 m of sediment where offsets are better defined. Two thirds of the faults measured have dips over 60°; dip of the NNF is over 75°. The surface strike of the NNF and SNF is well determined by multibeam data, but the available reflection lines are too widely spaced to determine the strike of subsurface faults that are typically closely spaced (~1 km apart).

4.4. Profile Interpretations Line 94-02 (Figure 6) imaged a broad swell (~350 m of relief) of basement that is cut either side of its apex by the NNF and the SNF; both faults can be traced to offsets of several hundred meters on base- ment. The NNF and SNF are the only faults to cut the seafloor within the fault zone, but several faults come within 100 m of the seafloor. Within and either side of the NFZ faults are spaced ~0.5 to 1 km part and most cut up to the orange horizon (~0.8 Ma). On many faults offset increases with depth (e.g., at 130 km) indicating that faulting was active during deposition. At 102 km, the sense of offset changes from down to the southeast on the orange horizon to up to the southeast on the green horizon which is not unusual for strike-slip faults. Sedimentary layers onlap the basement high with most of the thinning occurring between the green and orange horizons, in the early Pleistocene. Seafloor rises ~20 m in a gra- dual slope across the NFZ and steps down ~30 m to the JdF plate over the SNF indicating that uplift is an ongoing process. Folds with heights of tens of meters occur between faults in the NFZ and adjacent EXP; they indicate that a small amount of compression affected the Pleistocene section. The youngest sedi- ments are flat lying implying that the compression is small or no longer present. The basement reflector has smaller amplitudes on the EXP and into the middle of the NFZ possibly from greater roughness on this surface.

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Figure 6. Interpretation of line 94-02; location shown in Figure 2. Two main faults intersect the seafloor; basement and green horizon (~2 Ma) have been uplifted across the fault zone. Yellow box shows data in Figure 5. Annotation as in Figure 4. NNF = northwestern Nootka fault; SNF = southeastern Nootka fault; TWTT = two-way travel time; mbsf = meters below seafloor.

Line 96-01 (Figure 7) crosses the intersection of the NFZ with RP7 and imaged the greatest topographic relief in the NFZ: a basement high that rises 550 ms (~500 m) above basement of the JdF plate. In this crossing the NNF is a small extensional zone where a subparallel fault has stepped to the left (see multibeam maps in Figure 2). Extension in a left-stepping strike-slip fault indicates that the sense of motion is left lateral (Dooley & Schreurs, 2012). The green horizon could not be confidently tracked all the way across the NFZ. The orange to basement interval is thinner over the basement high and the brown horizon onlaps the high. These observations indicate that uplift has been active since at least deposition of the green horizon; slight uplift of the seafloor indicates that uplift continues into present times. Fault offsets increase with depth in the EXP (17.5–32 km) and JdF plate (53–60 km). A series of bright spots lie in the top 150 m of sediment in the NFZ (42–46 km) and above faults in the JdF plate (55–58 km); a slight depression of reflectors including the sea- floor between the two bright spots on the JdF plate (56 km) indicates that their formation is a recent phe- nomenon. The basement reflector has smaller amplitudes on the EXP and through the NFZ than on the JdF plate. Twenty kilometers to the north Line 95-02 (Figure 8) imaged a broad swell in basement around the NFZ, but basement is rough on a 10-km horizontal scale. Sedimentary units below the white horizon thin toward the swell but could be traced through the fault zone. The seafloor is slightly uplifted in the NFZ and slopes down

Figure 7. Interpretation of line 96-01; location shown in Figure 2. Faults intersect the seafloor; the NNF consists of two strands. Basement is high in the fault zone; uplift is again evident from angular unconformities in the sediments. Bright reflectors are indicated by red arrows. Yellow box shows data in Figure 13. Annotation as in Figure 4. NNF = northwestern Nootka fault; SNF = southeastern Nootka fault; TWTT = two-way travel time; mbsf = meters below seafloor.

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Figure 8. Interpretation of line 95-01; location shown in Figure 2. Faults intersect the seafloor; basement topography is a bit rougher than seen on 94-02 (Figure 6). Bright reflectors are indicated by red arrows. Annotation as in Figure 4. NNF = northwestern Nootka fault; SNF = southeastern Nootka fault; TWTT = two-way travel time; mbsf = meters below seafloor.

to the JdF plate over a distance of ~15 km. A series of shallow bright spots are adjacent to and within 10 km of the SNF (28–38.5 km). Faulting is dense in the EXP and the NFZ but is obscured by attenuation under the bright spots on the JdF plate. Sediments in the southeast end of the line dip more steeply to the east than on lines to the south because this line is closer to the DF. Growth faults are visible at 5 and 12 km in the EXP and between 23 and 28 km in the NFZ. Between faults, the green and blue horizons have been gently folded. In reflection profile 95-03 (Figure 9) all sedimentary horizons can be interpreted across the NFZ; sedimentary units thicken to the southeast toward the DF, which is just 2 km from the eastern end of the line. Outside the NFZ only the brown and the green horizons have been cut by faults. Attenuation under shallow bright spots around the NNF obscures deeper reflections. Folding of horizons is evident between the NNF and the SNF; amplitudes of the folds decrease upsection. Line 95-07 (Figure 10) was collected along the axis of the NFZ: basement subsides approximately 1,300 ms (~1,500 m) and sedimentary units thicken as the plate moves closer to the sediments’ source and into the subduction zone. Amplitudes of the basement reflector are weak below 5 s but have some coherence that allows picking. A ~300 ms (360 m) step down in the basement reflector occurs at 12.5 km; the green reflector is offset by only about 60 ms (40 m) there so the fault must predate deposition of this horizon. Offset continues to decrease upsection so that the white reflector is offset by only 12 ms (10 m).

Figure 9. Interpretation of line 95-03; location shown in Figure 2. Closer to the deformation front the sedimentation rate is higher and basement is too deep to be observed. Bright reflectors are indicated by red arrows. Annotation as in Figure 4. TWTT = two-way travel time; mbsf = meters below seafloor.

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Figure 10. Interpretation of line 95-07; location shown in Figure 2. The deformation front is at the northeastern end of the line. Increased sedimentation thicknesses are evident as is deflection of oceanic crust into the subduction zone. Annotation as in Figure 4. TWTT = two-way travel time; mbsf = meters below seafloor.

The seafloor ramps down over the fault by 38 ms (~28 m) indicating that this fault is still active, if at a low rate. In the multibeam data (Figure 2c) this ramp is 25–30 m high and runs between segments of the NNF and the SNF. The fault appears to be a conjugate fault within the NFZ. Landward of this fault the density of faulting and amplitude of folds decrease. The SNF occurs in the subsurface at 32–33 km but does not intersect the seafloor.

5. Initiation of Strike-Slip Faulting and Serpentinization 5.1. Fault Occurrences Faults in and around the NFZ are typically steeply dipping (>60°) and have small apparent vertical offsets (<50 ms); these observations are consistent with characteristics of strike-slip faults imaged elsewhere and with strike-slip focal mechanisms computed for earthquakes for the NFZ (Braunmiller & Nábělek, 2002; Ristau, 2004). Observations on reflection lines allow us to extend the southern terminus of the SNF 15 km to the south. Microearthquakes were recorded during a 3-month deployment of ocean bottom seismometers (Hutchinson & Kao, 2015; J. Hutchinson, personal communication, June 8, 2018); even at depths of 10–15 km below sea level the earthquakes follow the surface traces of faults in the NFZ quite closely. These observa- tions imply that the faults are steep within the crust and upper mantle. Previous interpretations of left-lateral motion (Hyndman et al., 1979) are corroborated by extensional deformation between the NNF and a left- stepping fault (Figures 2 and 6, 32–36 km). Around the NFZ, we interpret that most faults are strike-slip, but some normal or reverse faults are likely present as well. In seismic reflection sections, which record events primarily in the vertical plane, lateral offset of sediments is impossible to detect directly; depending on the geometry of the sedimentary layers, lateral movement may juxtapose different thicknesses of similar layers. Other characteristics such as steep dip, small offsets, apparent reversal of offset with depth, and flower struc- tures are often seen in strike-slip settings. In addition, the stress conditions between overlapping strike-slip faults can be complex and vary locally between extension and compression over relatively short distances. Bending stresses created by sediment loading and subduction are also in play here. Bending stresses in the JdF plate from both the sedimentary load and subduction were found to occur approximately 120 and 75 km from the DF, respectively, on a reflection profile off Washington State (Han et al., 2016). The NFZ is dominated by two main faults (Figure 11a) that cut from the basement to seafloor and is sur- rounded by numerous faults that offset the green horizon (~2 Ma) at a density of about one fault per kilo- meter. Faults occur at a similar density further west in the EXP, but in the JdF plate fault density decreases to every 4–10 km at distances of 5–10 km away from the NFZ. Fault density decreases upsection such that faults that offset late Pleistocene sediments (above the orange horizon) are spaced 2–5 km apart

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Figure 11. Maps of time to (a) seafloor and (b) orange horizon, the approximate location of the early-late Pleistocene con- tact (0.8 Ma). Each red cross represents an intersection of a fault with the horizon. Note that the orange horizon could not always be identified in the fault zone. Two-way time to the horizons is shown by the color bar in (a).

(Figure 11b). As the subduction zone is approached and sedimentation rates increase fewer faults offset shallower layers until even the two main strands do not intersect the seafloor. Within 5 km of RP7 more faults intersect the shallower sediments perhaps indicating that this crustal heterogeneity has been reactivated (McClymont & Clowes, 2005; Nedimovic et al., 2009). Apparent offsets here often increase with depth showing that these are growth faults, that is, active during deposition of the sediments. Faults observed within and 10–20 km either side of the NFZ may have ceased to be active during the late Pleistocene or remained active but at such a low rate that offsets cannot be resolved by these data. In most geologic interpretations a fault that did not penetrate into late Pleistocene strata would be considered inactive during that time interval. However, Nedimovic et al. (2009) have made a case that high sedimentation rates can overwhelm faults with small rates of movement such that no offset can be distinguished in seismic reflection records—depending on vertical resolution of the data. This process is likely in effect within 10–15 km of the subduction zone where even the NNF and the SNF do not offset the top 100 m of sediments. Within 25 km of the DF (Figure 10) the SNF only offsets horizons deeper than 250 m. Ocean bottom seismometers deployed around the NFZ (Obana et al., 2015) detected several hundred earthquakes of M < 4 on the NFZ up to the DF and under the continental rise as well as on either side of the NFZ indicating that at least some of the faults seen outside of the NFZ are active, if at small rates.

5.2. Initiation of a Strike-Slip Fault System Structural features of the recently formed NFZ are similar to those seen in analogue models of initiation of strike-slip faulting: Several active fault strands overlap each other and strike at a small angle to the orientation of the shear driving the deformation (Dooley & Schreurs, 2012). In addition, they are surrounded by faults that are either defunct or active at a much slower rate. Numerical models have also shown that displacement can be spread over an array of faults that forms initially and later concentrates into a single principal deformation zone (e.g., Cowie et al., 2007); numerous geologic observations in continents provide evidence for this kind of evolution (e.g., Norris & Toy, 2014). The NNF and SNF strike at small angles (7–12° and 14–21°, respectively) to the JdF-NA relative plate motion (Figure 12) indicating that they are behaving like Riedel shears. As discussed above the JdF-NA motion is essentially equivalent to the JdF-EXP relative motion (Figure 3). In analogue experiments with soft materials layered over a rigid basal layer Riedel shears can form at angles of 8–23° relative to the slip direction (Dooley & Schreurs, 2012, and references therein), depending on mechanical properties of materials used. Braunmiller

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Figure 12. Interpretations of Nootka fault zone (NFZ) from multibeam and seismic reflection data shown on magnetic anomaly map from Figure 1. The NFZ is behaving like Riedel shears within a broad zone of seismicity. Red arrows show average direction of left-lateral slip from fault plane solutions in the NFZ (Braunmiller & Nábělek, 2002); black arrow is Juan de Fuca-North American relative plate motion. Dotted line is shelf break. Large historic earthquakes are shown in gray with year of occurrence beside them (Cassidy et al., 1988). Earthquakes of M4 are shown as green circles, M5 as yellow stars, and M6 as red stars.

and Nábělek (2002) computed an 18° difference between slip vectors from earthquakes in the NNF and the regional trend of earthquake locations but could not explain the difference. The average of slip vectors from 13 events on the southwestern NNF was 37° and is very similar to the average strike of the NNF, 34°. Ristau (2004) also found that slip vectors were at an acute angle to the presumed direction of relative plate motion. Earthquakes from 1957 to 1986 landward of the DF (Figure 12) have been interpreted to occur in the sub- ducted NFZ at depths of 15–30 km (Cassidy et al., 1988); more recent M > 6 events occurred in a similar depth range (Earthquakes Canada, 2018). The initiation of strike-slip faulting has been shown by analogue modeling to evolve differently depending on the mechanics of the underlying shear. When particulate matter (sediments) overlies a single moving fault in the baseplate (basement) short initial faults (Riedel shears) form at a small angle to the actual shear direc- tion. They typically overlap with each other in short zones and occur in a zone in map view; with increased displacement some faults lengthen and link up to become the main displacement zone (e. g. Dooley & Schreurs, 2012 and references therein; Tchalenko, 1970). Occurrence of strike-slip faults over more than 80 km across the NFZ that currently carry little displacement is similar to the results of these experiments. Analogue experiments modeling deformation above a broad basal ductile zone (Schreurs & Colletta, 2002) showed that a few initial Riedel shears lengthened and were joined by more faults until the entire volume above the basal shear was faulted. The long subparallel Riedel-like shears coexisted over long zones and were connected by faults at a high angle to the shear. In the NFZ, two long subparallel faults coexist over a distance of 55 km and are connected by at least one high angle shear suggesting that at depth a broad ductile region exists, as might be expected in young oceanic lithosphere. The traces of the NNF and the SNF appear to be utilizing extensional faults formed in the oceanic crust; they are subparallel to the strike of magnetic anomalies which to have formed between 4 and 5 Ma

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(Wilson, 1993; Figures 1 and 12). Extensional faults in the JdF plate imaged by multichannel reflection pro- files (Han et al., 2016) dip 50–60° were spaced mostly 4 km apart (with occasional smaller spacing) and penetrated 1,100–1,700 m into upper oceanic crust. Such faults likely existed in the crust when the NFZ was initiated; they could easily be reactivated to nucleate crustal strike-slip faults. Faults observed around the NFZ are typically spaced more densely, about 0.5 to 1 km apart showing that rupture was more widespread. Sediments have been arched into low-amplitude folds between faults in the NFZ and to a lesser degree on either side. They are seen over a broad enough area to indicate that they are not just local fault interactions but must be from a regional, if small, component of compression. Their expression dies out upsection implying a lesser degree of compression over time. The small folds between faults are similar to deforma- tion observed in transpressional models of Schreurs and Colletta (2002) where the underlying shear was distributed across the whole model and had 7° of compression. In cross-section layers appear to form sim- ple arcs between faults in contrast to sinusoidal folding in pure compressional models with unfaulted layers. Models with 15° of transpression developed thrust faults as well as strike-slip faults. Transpression above a single basement fault is more likely to produce flower structures where the faults root into the basement fault but spread out at the surface. Some small degree of compression must be active in the NFZ; thrust faults are not observed so the degree of compression is likely less than 15°. The clockwise rota- tion of the Explorer with respect to the JdF ~4 Ma implies a small degree of compression between the plates if the pivot point was south of the plate. As the EXP became attached to the North American plate (NAM) and faults in the NFZ were affected by the JdF-NA direction, the amount of compression would decrease.

5.3. Basement Uplift, Bright Spots, and Serpentinization Basement uplift and methane hydrate reflectors are common in the densely faulted NFZ. One process that can explain both observations is serpentinization: deep faults could allow water to penetrate into the lower crust and upper mantle, resulting in hydration reactions, especially serpentinization. Significant volume increases could account for basement uplift. Methane is commonly detected in areas where serpentinization is active, which indicates that this process could at least partially account for presence of methane hydrate reflectors. The depth and intensity of hydrothermal activity are unknown in the NFZ; consequently, the ther- mal state of upper mantle is also unknown. 5.3.1. Basement Uplift and Serpentinization A refraction study over the NFZ (Au & Clowes, 1982) found velocities of 7.5 km/s 6–10 km below the base- ment surface where one would expect to find velocities of 8.0–8.1 km/s. Adjacent to the SNF a similar velo- city structure and supporting gravity model (McClymont & Clowes, 2005) were interpreted to indicate that 25–30% serpentinization had occurred at depths from 5 to at least 10 km below the basement surface around RP7. Dipping discontinuous crustal and subcrustal reflectors around this propagator imaged by a multichannel reflection line (Hasselgren & Clowes, 1995) were taken as further evidence of serpentinization. More recently, a tomographic study across the JdF plate (Horning et al., 2016) transected two rift propaga- tors and also found low-velocity anomalies in the upper mantle below them. The anomalies were attributed to presence of serpentinite or talc depending on whether the plate had been cooled by hydrothermal circulation or is simply a half-space cooling by conduction alone. Across the NFZ, basement can be characterized as a broad swell with local fault relief (e.g., Figure 6); the mag- nitude of relief increases slightly toward the DF. In line 94-02 relief on the basement surface between the NNF and SNF is 100 ms, whereas in line 95-02 it is up to 300 ms. The swell is highest at the intersection of the NFZ and RP7 rising 525 ms above the JdF plate. In 94-02 sediments below the green horizon are similar in thick- ness across the NFZ, but sedimentary units above the green horizon (younger than 2 Ma) thin toward the NFZ indicating that uplift was underway by 2 Ma (Figures 6–9). Onlap of late Pleistocene horizons (Figures 6–8) and topography on the seafloor (Figures 2 and 6–9) in all profiles indicate that uplift has continued into recent time. The older sedimentary section is quite thin where the NFZ and the rift propagator intersect (line 96-01) indicating that uplift was underway locally before 2 Ma. A 3-month deployment of ocean bottom seismometers detected microseismicity up to 15 km below the top of basement in the NFZ (Obana et al., 2015; J. Hutchinson, personal communication, June 8, 2018), well into

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upper mantle. These observations show that the fault zone is being fractured and therefore open to water flow, hydration, and more specifically, serpentinization. Serpentinization is accompanied by a 29–50% increase in volume of the altered peridotite depending on the original rock composition (Coleman, 1971; Coleman & Keith, 1971; O’Hanley, 1992; Plumper et al., 2012). Assuming isostatic equilibrium and a 40% volume increase, a 300-m basement swell (Figure 5) implies that a 333-m-thick layer of peridotite had been 100% altered into 467 m of serpentinite. Velocities of 7.5 km/s in the top several kilometers of upper mantle in the NFZ (Au & Clowes, 1982) imply serpentinization of ~20% (Carlson & Miller, 2004) implying that 1730 m of mantle would have been altered. Larger uplift was observed at the intersection of the NFZ and RP7 (Figure 6) implying that either a greater thickness of rocks could have been altered there or more intense alteration had occurred. Fault intersections are known to allow large volumes of fluids to circulate and are often the site of seafloor seeps or ore deposits (Hovland & Judd, 1988). Water flowing through oceanic crust and upper mantle will undoubtedly cause hydration of rocks along the entire flow path; an examination of data from Deep Sea Drilling Project and International Ocean Discovery Program drill sites concluded that faults, breccia, and fractured rocks are the primary fluid conduits on the scale to tens to hundreds of meters (Bach et al., 2004). Complex reflectivity seen in the multichannel profile across the altered rift propagator (Hasselgren & Clowes, 1995) has been interpreted as evidence that water flow and alteration have occurred. The simplifying assumption of McClymont and Clowes (2005) that altera- tion occurred primarily in the upper mantle below thin crust of the rift propagator allowed a gravity model to be computed and correlated to the refraction data and similarly allows us to compute a simple isostatic model (above). The question remains whether the thermal state of the NFZ would allow serpentinization to occur. The occur- rence of earthquakes to depths up to 15 km below the top of basement shows that the top ~9 km of upper mantle is brittle, that is, with temperatures <~800 °C, the transition temperature from brittle to plastic defor- mation in dunite. This transition may occur as low as 700 °C (Wiens & Stein, 1983) or 600 °C (McKenzie et al., 2005) in oceanic lithosphere. A model of a 5-Ma half-space cooling conductively predicts that temperatures 9 km into upper mantle are ~950 °C (e.g., Horning et al., 2016) so conductive cooling cannot be the only pro- cess affecting the thermal state of the NFZ. A thermal model of 5-Ma oceanic plate that includes 600 m of hydrothermally cooled basalt under 1.5 km of sediment (Rotman & Spinelli, 2013) predicts brittle behavior down to ~15 km below the top of basement and that temperatures at Moho would be ~400 °C, the upper limit for serpentinization. Given that in the NFZ faulting has been occurring over the last 4 Ma and has pene- trated up to 9 km below the regional Moho depth (Obana et al., 2015), it is likely that hydrothermal circulation has cooled the top few kilometers of upper mantle in the fault zone below these temperatures. We would, therefore, expect serpentinite to be present. Hyndman et al. (1979) interpreted uplift across the NFZ to be thermal in origin because of proximity to the ridge crest, but the better-determined geography of the region argues against this as the only or primary cause of uplift. Crust adjacent to the northern end of MV is a valley, and the uplift of the NFZ is offset to the east from MR, the faulted block that bounds the east side of MV (Figure 3). A thermal origin of uplift related to the rift propagator would result in a pattern of uplift parallel to the propagator, whereas the observed uplift is along the axis of the NFZ. 5.3.2. Free Gas and Potential Gas Hydrates Bright reflectors are observed in the shallow sediments of the NFZ: between 42 and 46 km of line 96-01 (Figures 7 and 13), adjacent to the SNF: 28–39 km of line 95-01 (Figure 8), 9–12 km of line 95-03 (Figure 9), and in the JdF plate: 54–58 km of line 96-01 (Figure 7), 8–9 km line 95-01 (Figure 4). They typically lie between the white and orange horizons; their shallowest levels are between 60 and 130 ms bsf, (52 to 112 mbsf). The phase of the reflectors is reversed (Figure 13), and arrival times of deeper reflectors are delayed (sags) below them. These observations indicate lower velocities and the presence of free gas below the bright-amplitude reflections. Many of these reflections are parallel to stratigraphy suggesting free gas lies in a more permeable layer; others crosscut stratigraphy (45–46 km, Figure 13) suggesting that they are bottom-simulating reflec- tors (BSRs) from methane hydrate. Attenuation of reflectors’ amplitudes underneath the bright spots is observed in all but one of the lines shown. Attenuation can be created by a variety of factors not the least of which is the bright reflector itself

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Figure 13. Detail of line 96-01 (Figure 7). These high-amplitude reflections (red arrows) are phase reversed compared to the seafloor and equal to or larger in amplitude than the seafloor reflector. Reflectors underneath them appear to sag i.e. have increased travel times because of low-velocity material below the bright reflectors. Annotation as in Figure 4. TWTT = two-way travel time; mbsf = meters below seafloor.

(because it prevents energy from penetrating into the section) but also the presence of free gas in sediments (Riedel et al., 2002). Depending on the morphology of the attenuated zone such blanking is often taken as evidence of migrating gas (e.g., Petersen et al., 2010). For example, in line 95-01 (Figures 8, 28–38 km) reflectors can be observed immediately beneath the bright spots so the underlying attenuation is not entirely due to the high amplitude of the bright spots. Deeper in the section the degree of attenuation here varies laterally: A curtain of blanking between 32 and 38 km may well be from migrating gas. 5.3.3. Faults and Fluid Circulation The link between bright spots and fault systems (Figure 14) leads us to infer that methane has been advected toward the surface through the fault systems. Widespread biogenic processes utilizing organic carbon in the

Figure 14. Bright spot occurrence in and around the Nootka fault zone is concentrated within 40 km of the deformation front and on rift propagator 7 (cyan line).

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sediments would create hydrate reflectors throughout the region but this is not observed (Ferguson et al., 2018; Han et al., 2016; Hyndman et al., 1994). Faults from bending stresses alone tend to deform the lower sediment section and are not associated with bright spots or hydrate reflectors. Thermal studies of the JdF plate have shown that these sediments have temperatures conducive to biogenic production of methane —less than 80 °C (Davis et al., 1992; Hutnak et al., 2006). Active faults in plate boundaries can be good path- ways for migration of fluids; repeated ruptures can create fracturing and permeability along and around the fault plane. Although attenuation and sagging of reflectors preclude good observations of faults immediately under the bright spots, the ubiquity of faults suggests that they have played a role in fluid migration. Under one bright spot without attenuation faults can be observed (Figures 7, 55–58 km). Fluid flow in faults of the NFZ activated by strike-slip motion as well as bending stresses are likely entraining whatever biogenic methane has been generated in the sediments. In addition, we note that methane plumes are associated with serpentinization in ophiolites (Vacquand et al., 2018), highly extended rift margins (Klein et al., 2013), and slow-spreading MORs (Cannat et al., 2010; Früh-Green et al., 2004; Proskurowski et al., 2008). The processes thought to generate methane and other simple hydrocarbons found in vents may be as simple

as reduction of dissolved CO2 by H2 liberated by the serpentinization reaction (e.g., Charlou et al., 2002), but more recent studies indicate that methane may be flushed out of fluid inclusions in intrusive rocks by the extensive fracturing that brings sea water into the oceanic plate (McDermott et al., 2015; Wang et al., 2018). Methane in the inclusions may have evolved from primary volatiles (Kelley et al., 2004), and fracturing would release it to the surface. Some of the high-amplitude reflections resemble BSRs in that they crosscut the layered stratigraphy (45– 46 km, Figure 13); however, they are not always parallel to the seafloor. Such reflectors are commonly observed on continental margins in gas-hydrate environments (e.g., MacKay et al., 1994; Shipley et al., 1979) but rare in the abyssal plain. The changes in subseafloor depth of the crosscutting reflections could be the results of changes in heat flow, which is strongly controlled by advecting fluids along faults as well as depth to basement. In Figure 13, the eastern portion of the BSR that crosscuts stratigraphy is within the NFZ and above a locally steep gradient in basement topography (Figure 7). Regionally lateral variation in heat flow across short distances (5–10 km) has been observed (Davis & Lister, 1977) and attributed to geothermal circulation within the oceanic crust. Bright spots above local basement highs are also observed 6 and 20 km (Figure 4, 6–14 km) either side of the NFZ. Similar to the NFZ but to a lesser degree angular unconformities attest to the timing of uplift—mostly between basement and brown horizons or approximately 4.5–1.2 Ma. The base of Pleistocene reflector, the blue horizon, has been uplifted ~100 m since deposition, and the white horizon has only been uplifted by ~4 m (50 ms). Differential compaction can explain a small percentage of the 100-m height differential on the blue horizon, but not all. For example, sediments above a basement high of similar magnitude have no more than 15-ms (15-m) difference at a similar height above basement (Figure 17 in Hutnak et al., 2006). A similar set of features is also seen about 6 km west of the NNF in lines EX-01 and 95-08, not displayed here; three reflection lines show that this basement high is ~7 km in diameter. The bright spot is deeper at 200 ms (~150 m bsf), and uplift is also most evident in the thinning of early Pleistocene sediments, specifically between basement and the brown horizon. These examples are spatially smaller than in the NFZ, but the association of uplift and bright spots is the same. Widespread deformation during initiation of shear faulting could have created enough displacement on regional faults that they cut into mantle allowing water to pene- trate and serpentinization to occur. Alternatively, the basement topography has focused flow in the sedi- ments upward concentrating methane above basement highs. This process, however, does not explain progressive thinning of sediments above the basement high.

5.4. Faulting and Serpentinization Because, in general, increased amounts of displacement would result in greater amounts of hydration, which in turn generates crustal uplift, uplift can be taken broadly as a qualitative proxy for displacement. With high rates of seismicity and predicted net displacements across the fault zone on the order of 80 km, repeated fracturing of individual faults is to be expected. In addition, the process of serpentinization (in part because of volume expansion) creates local fractures (Plumper et al., 2012; Tutolo et al., 2016). The approximately 50- km breadth of uplift as seen on 94-02 (Figure 6) implies that serpentinization could have occurred not just on the two main faults but around and between them. Faults at some time must have been active over this

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width with displacement focusing over time into the central, highest 15 km and eventually onto two faults: the NNF and the SNF. Closer to the DF three-dimensional aspects of plate interactions come into play. Flexure caused by the increasing sediment load as the plate approaches the shelf break and enters the subduction zone bend the subducting plate downward and put the shallower region (upper 10 km or so) of the plate into extension. Computation of isostatic stresses on the central and southern JdF plate showed that bending stresses from subduction are present within approximately 75 km of the DF (Han et al., 2016). Greater vertical relief on the basement surface observed closer to the DF implies that bending stresses may be allowing normal dis- placements on existing strike-slip faults of the NFZ. This extension could permit a greater volume of water to penetrate and hydrate the lower crust and upper mantle. Bright spots from free gas and methane hydrate reflectors kilometers long occur within 40 km of the DF (Figure 14), which indicates that this is a region of greater water circulation and possibly, serpentinization. We further note that recent uplift is greater around the SNF than the NNF resulting in asymmetry of the sea- floor across the NFZ. This implies that alteration is slightly greater on the SNF—the shorter fault—whereas one would predict a greater degree of serpentinization and uplift on the NNF, the longer fault. It is generally accepted that longer faults in similar settings carry greater displacement (Cowie & Scholz, 1992), implying that the NNF would have a greater degree of serpentinization and uplift. However, extension associated with plate bending close to the DF could augment serpentinization around the SNF, ultimately resulting in greater swelling and uplift. Over time, increasing serpentinization of the crust and uppermost mantle along the developing Nootka fault system would likely enhance processes of fault localization. The rheological differences between serpentinite minerals and the precursors should lead to a dramatic decrease in overall lithospheric strength (Escartin et al., 1997) and also favor localized fault slip and fault zone development (Hirth & Guillot, 2013). Further, the feed- back between fault displacements (aided by serpentinization) and the additional interactions with fluids along those faults will favor localization along the main fault segments. From moment tensor solutions Ristau (2004) estimated that the NFZ is slipping at only 1.5 ± 0.5 mm/year out of a possible 40 mm/year, while Obana et al. (2015) estimated that two thirds of the slip were taken up aseismically but did not specify an actual rate (13.2 mm/year if the rate is 40 mm/year). The proportion of aseismic slip is similar to oceanic ridge-ridge transforms that have been highly serpentinized (Boettcher & Jordan, 2004).

5.5. Evolution of the NFZ Over its short history the NFZ has evolved from a broadly distributed shear (Figure 15a) with a minimal rate of displacement to a two-fault system with a rate of displacement that may be as high as 40 mm/year. The NFZ (~4 Ma) initiated in the JdF plate that was 0–1 Ma. Sedimentation at the time consisted of hemipelagic clays; nearby on the JdF plate the hemipelagic deposition rate was determined to be ~100 m/Ma in this time (Su et al., 2000). Faulting would have been widespread and fairly shallow; with little in the way of overlying sedi- ments hydrothermal circulation would have been vigorous. As circulation cooled the plate, faults would have penetrated more deeply. By ~2 Ma (green horizon), about 500 m of sediment had accumulated (e.g., Figures 6 and 15b), which could have sealed off the circulation (Underwood et al., 2005); we expect that repeated rup- turing on this plate boundary kept the faults permeable. Using the simplistic assumption presented in section 2, the rate of movement across the fault zone could have been ~20 mm/year. After 2 Ma, angular unconformities around the NFZ indicate that the upper surface of basement had begun to rise (Figure 15c), an isostatic response to hydration of the crust and upper mantle. We expect that serpentiniza- tion in the upper 1–2 km of upper mantle would have the most effect volumetrically, because olivine occurs in far greater proportion than in lower oceanic crust. The shape of the swell implies greatest amounts in the central 15 km of the NFZ with lesser amounts either side. The swell diminished the amount of sediment deposited in the NFZ in comparison to adjacent crust. Displacements were becoming more localized such that fewer faults intersect the white horizon (~0.3 Ma) until the present day when only two fault sets, the NNF and the SNF, offset the seafloor (Figure 15d). Present-day bathymetry indicates that the uplift and pre- sumably the hydration continues. As discussed in section 5.4, motion across the plate could be as much as 40 mm/year but most of it is now accommodated aseismically. The rate of uplift appears to have decreased perhaps as a result of fewer active faults, but a gradual increase in sedimentation rate as the plate approaches the DF would also yield a similar looking section. Both processes are likely active. The plate close to the DF

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Figure 15. Simplified evolution of the Nootka fault zone. The vertical to horizontal scales are 1:1. Faults (near-vertical blue lines) deepen with time as hydrothermal circulation cools the plate. After 2-Ma faults have penetrated to upper mantle and serpentinization can begin. Serpentinization is represented as the pale region under Moho; gradational transparency is an effort to represent what we expect is a gradational process with differing degrees of serpentinization both with depth and laterally. The 200–350 m of swell in basement drawn in (c) and (d) is not obvious at a 1:1 scale. Present-day seismogenic zone extends up to 20-km depth below sea level (Obana et al., 2015). Water is kept at a constant thickness of 2.5 km, and three-dimensional effects of bending stresses from subduction are not included. Faults were drawn at about half their actual spacing; otherwise, they would overwhelm the diagram.

would be subject to extensional bending stresses possibly increasing existing faults’ vertical offset. Asymmetric uplift and evidence of methane within 40 km of the DF imply that fluid flow in this zone is greater than to the southwest.

6. Tectonic Implications 6.1. Implications for Megathrust Earthquake Behavior The formation and evolution of the NFZ have led to the separation of the EXP and JdF plate. Our analyses indicate (see, e.g., Figure 3) that the convergence rate with respect to North America of the Explorer region is significantly lower than JdF-North American convergence. This is seen in the relative motion across the NFZ discussed here, but the consequences of this difference in plate motion have also been invoked to explain deformational behavior of the overriding NAM. Staisch et al. (2018) invoke the separa- tion of the EXP and JdF plate as a mechanism to change convergence rates and orientations (with respect to the overriding NAM), leading to an increase in deformation rates in the Yakima Fold province at approximately 4–2 Ma. Both Staisch et al. (2018) and Mullen et al. (2018) further imply that this separation between the EXP and JdF plate must be a throughgoing plate boundary structure, perhaps even leading to the development of a slab window beneath Vancouver Island south of the onshore extension of the NFZ. If the EXP and JdF plate are acting as independent plates, this raises questions about how megathrust ruptures might proceed in the vicinity of the triple junction between the EXP, JdF plate, and NAM. Generally, triple junctions along plate boundaries are thought to act as segment boundaries for

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earthquake ruptures. The change in plate kinematics across the triple junction and the associated differ- ences in strain accumulation can provide a location for rupture termination. Additionally, the physical properties of the subducting fault zone itself—greater hydration, decreased density—over a 20- to 50- km width may affect rupture propagation. However, trench-transform-trench triple junctions similar to that produced by the development of the NFZ appear to not necessarily serve such a role. A similar triple junction—the Simbo triple junction between the Pacific (upper plate) and the Solomon Sea and Australia plates (subducting) along the Solomon Islands—experienced a Mw 8.1 megathrust earthquake in 2007. Although the plate motions around the Simbo triple junction are significantly faster than along the northern Cascade margin, in both cases there is subduction of extremely young oceanic plates, which might also be thought to inhibit rupture. In spite of the differences in kinematics across the triple junction and the extre- mely young age of the subducting plates, the triple junction did not serve as a segment boundary during the 2007 Mw 8.1 megathrust earthquake (Furlong et al., 2009). Rather, the slip orientation of the fault rupture changed at the location of the triple junction. In the Solomon Islands example, the coseismic slip on the megathrust changed to be consistent with the orientation of relative motion between the two plates at that location (i.e., Solomon Sea plate-Pacific plate motion NW of the triple junction and Australia plate-Pacific plate motion SE of the triple junction). Assuming this to be an analog to the NFZ, it implies that although the sub- duction plate motions change substantially (primarily in plate velocity) north and south of the NFZ, Cascadia ruptures could continue across this plate boundary structure. We would expect coseismic slip north of the triple junction to reflect EXP-NAM motion, which based on Figure 3 is substantially less than the relative plate motions south of the triple junction, to be either substan- tially smaller and/or rupture on the megathrust would only occasionally continue north of the triple junction. The magnitude of slip may be different and/or the recurrence rate for such cross triple junction ruptures may be longer than the recurrence of events on the JdF-NAM boundary—that is perhaps only ~10% of the events along the northern Cascadia margin would rupture the entire extent of the northern subduction zone. Alternatively, if most ruptures did continue across the triple junction, we might expect significantly smaller coseismic slip in each event along the EXP margin. Submarine slides originating at the Explorer DF (Riedel & Conway, 2015) were significantly older than slides originating from the JdF DF; in other words the Explorer DF has not experienced the same degree of shaking during historic megathrust events on the JdF DF, potentially indicating smaller coseismic slip in that region.

6.2. Comparison to Oceanic Transform Faults The formation of the NFZ differs significantly from the formation and evolution of MOR transforms. The initia- tion of the NFZ within existing oceanic lithosphere means that there is no age or rheological contrast across the fault. Additionally, the orientation of inherited structures (formed at the MOR or associated with propa- gating ridges) that are utilized at first is not necessarily well aligned for the kinematics of the newly formed plate boundary structure. This leads to the observed patterns of an initial very broad deformation zone that with continued relative motion and crustal modifications (both mechanical through faulting and chemical through fluid interactions) is localized onto the main structures with the orientations subparallel to original extensional faults. As a result, the newly forming plate boundary structure may be slightly oblique to the direction of relative plate motions as it is responding to local stress conditions as much as to the overall kine- matics. In contrast, MOR transforms and their geometry are dominated by plate motions and are modified principally by the effects of heat transfer (and its rheological effects) across the transform (e.g., Furlong et al., 2001). The evolution of a plate boundary structure such as the NFZ has features in common with transforms associated with offsets in subduction zones such as the San Cristobal trough along the Solomon subduc- tion zone (Neely & Furlong, 2018). In this case the Australia plate is being torn to form the southern mar- gin of the transform and the maturity of the fault system increases from west to east. One sees similar patterns of a broad deformation zone in the less mature segments that become progressively more loca- lized to the east as displacements across the fault system increase to approximately 90–100 km. This spa- tial pattern serves as a reasonable analog for the temporal change in fault zone behavior we have documented along the NFZ although the NFZ has a lesser relative plate motion and we cannot precisely quantify the net displacement across the NFZ.

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7. Summary Interpretation of single-channel seismic reflection data integrated with multibeam data and seismicity ana- lyses shows that initiation of a strike-slip fault system in young oceanic crust has similarities to analogue mod- els of deformation. The 4-Ma ago shear deformation in the JdF plate began on extensional faults formed at the ridge over a zone up to 100 km wide. Shortly after 2 Ma, deformation had concentrated into a zone approximately 50 km wide and in the present day just two faults 8–18 km apart are the most active. Deformation over such a very broad zone and coexistence of two long faults over 50 km are features observed in models of broad shear underlying the deforming media. The two main bounding faults trend 17° to the presumed relative plate motion across the fault and are therefore Riedel shears. Development of Riedel shears as well as progressive focusing of deformation are features observed in models of shear defor- mation above a single fault in a rigid baseplate. In this young oceanic plate shear within the ductile portion has likely become localized over time. At least one major conjugate fault and step over (sidewall ripout) are other features commonly seen in continental strike-slip zones. Broad uplift of the sediment-basement interface centered along the fault zone is interpreted as evidence of isostatic adjustment following volume expansion during hydration of the crust and upper mantle. Continual rupture on faults that penetrate up to 9 km into upper mantle has likely brought seawater into the plate cool- ing it to below temperatures predicted by a simple conductive cooling half-space model. Angular unconfor- mities in the sediment indicate that uplift occurred in the last approximately 2 Ma, perhaps indicating when faults had reached into the upper mantle. Methane hydrate reflectors are commonly seen within 40 km of the DF; they may be sourced from bacteria in the sediments, but we also note that methane is commonly observed in vents above serpentinization in ultramafic rocks.

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