Initiation of Strike‐Slip Faults, Serpentinization, and Methane: The Nootka Fault Zone, the Juan de Fuca‐Explorer Plate Boundary

Abstract

The Nootka fault zone is a ridge‐trench‐trench transform fault that was initiated ~4 Ma when the 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 Explorer and Juan de Fuca plates and formed instantaneously at 4 Ma. Increased data density shows that these faults are subparallel to seafloor magnetic anomalies and appear to have utilized extensional faults 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 initiation that predict formation of Riedel‐like shears within a zone of faulting and that displacement localizes over time. The existence of several long subparallel faults and a very wide zone of faulting has been predicted by models of distributed shear at depth. Along the Nootka fault zone basement has risen by several hundred meters and bright reversed‐polarity reflectors some of which are interpreted to be methane hydrate reflectors are common. Hydration, likely as serpentinization, of the upper mantle could explain both sets of observations: Serpentinization can result in a 30–50% volume expansion and methane is observed in 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.

Focused Fluid Flow Along the Nootka Fault Zone and Continental Slope, Explorer‐Juan de Fuca Plate Boundary

“…The northern arm of the NFZ shows earthquake depths of around 16-23 km, deeper than those along the southern arm (~12-16 km; Figure 2b). The conjugate faults (azimuth~165°) were defined by Hutchinson et al (2019) and Rohr et al (2018), and a prominent cluster of earthquakes delineating this trend shows shallower depths of~8-12 km. Tracing individual faults from line to line, we were able to define a possible second set of conjugate faults (labeled "Conjugate B" in Figures 2a and 2b) trending with an azimuth of~10°(dashed purple lines in Figure 2).…”

“…Two distinct zones of faulting along the NFZ were previously interpreted in the study by Hyndman et al (1979) from single-channel and 3.5 kHz subbottom profiler data. The fault traces were subsequently remapped in more detail using multibeam data from 2001 and 2002 (Rohr et al, 2018). In this study we additionally interpret detailed seismic surveys around both fault traces and the seafloor mounds.…”

“…As noted by Hyndman et al (1979) and Rohr et al (2018), around and between the two Nootka fault traces, there is a basement high of up to 400 m. We added basement depths from the tightly spaced seismic sections around the seafloor mounds ( Figure 2d). The basement depth below seafloor gradually increases landward as expected from a higher sediment input due to the proximity to the slope and onset of subduction.…”

“…Vertical exaggeration is 12. Two horizons with approximate ages (from Rohr et al, 2018) are superimposed together with fault interpretation. The high-amplitude feature underneath the mound is irregularly shaped and asymmetric to the donut-shape seafloor expression (see Figure 9b).…”

“…Active faulting can reach into the upper mantle as shown from the seismicity study by Hutchinson et al (2019) and is likely to partake in, if not, facilitate hydrothermal circulation. Serpentinization of the top 0.3-2 km of upper mantle was proposed to be possible by Au and Clowes (1982), Hutchinson et al (2019), and Rohr et al (2018). If these BSR-like reflections are indeed gas hydrate related and if the source of methane and excess salt is produced at depths from 10.1029/2020GC009095 serpentinization reactions (as also postulated at other gas hydrate provinces, such as off Svalbard, Johnson et al, 2015), this could be tested by coring (or deep drilling) and linked geo-chemical analyses.…”