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

The Nootka fault zone is a ridge-trench-trench transform fault that was initiated 4Ma 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 4Ma. 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. Plain Language Summary Four million years ago the Juan de Fuca oceanic plate offshore British 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 80km wide and is now just 8 to 18km 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.