The Northern Terminus of Cascadia Subduction

Abstract

The Juan de Fuca (JDF) plate system is pivoting and breaking apart as a result of resistance to subduction. At the northern end, the Explorer (EXP) plate moves independently of the JDF plate along the Nootka Fault Zone (NFZ), which forms an unstable triple junction with the JDF ridge and the Sovanco Fracture Zone. We trace the subsurface extension of the NFZ using tomography by combining results from a new nearshore study with a previous ocean-bottom study and other geophysical constraints. Cross sections from the offshore NFZ through central Vancouver Island suggest a strong fold in the subducting lithosphere (∼40 •). Near the deformation front, the NFZ is defined by microseismicity that extends into oceanic mantle with low V P /V S (<1.7) values. This anomaly extends to the NE below the continental shelf with a more northerly trajectory than common NFZ depictions through the seismicity concentration off Nootka Island. Tremor locations and receiver functions indicate that this more northerly trajectory persists across Vancouver Island. We propose a revised tectonic evolution for the NFZ that leads to (i) an EXP microplate confined to shallow depths below the convergent margin and which underthrusts North America no farther landward than Brooks Peninsula and (ii) a northern edge of Cascadia subduction that cuts between Brooks Peninsula and Nootka Island. The plate fold offshore Nootka Island and abundant seismicity there and at Brooks Peninsula are ascribed to stress concentrations on either side of the NFZ associated with deformation at the edges of more competent JDF, EXP, and North American plates.

Complex Structure in the Nootka Fault Zone Revealed by Double‐Difference Tomography and a New Earthquake Catalog

“…The centroids of the 2004 and 2011 clusters locate at approximately 25 and 35 km depth, respectively, despite appearing to belong to a single strike-parallel structure in map section. The 2004 cluster appears near 90 km along profile CC' immediately beneath the McCrory slab interface model, whereas the 2011 cluster, near 80 km on profile DD', appears more tightly constrained and lies >10 km below the slab interface model, consistent with the results of Savard et al (2020). Both clusters are also evident on section GG' where seismic velocities in the source region of the two clusters differ.…”

“…Following Savard et al (2020), we advocate a revised interpretation for the location of the Nootka Fault zone, defined as the zone of maximum strain accumulation between the Explorer and Juan de Fuca plates. This definition adopts a more northerly trajectory than traditional descriptions (e.g., Barr & Chase, 1974;Hyndman et al, 1979) and widens landward as depicted in Figure 3.…”

“…The NFZ separates the Juan de Fuca and Explorer plates at a region of complex left-lateral strike-slip faulting where fluids are interpreted to have penetrated deep into the oceanic plate (McClymont & Clowes, 2005;Riedel et al, 2020;Rohr et al, 2018). The NFZ is believed to have developed within the Juan de Fuca plate since 4 Ma as the present-day Explorer plate is captured by the North American plate (Hyndman et al, 1979;Rohr et al, 2018;Savard et al, 2020). Several large (M > 6) right-lateral strike-slip events have occurred in or near the NFZ in 2004NFZ in , 2011NFZ in , and 2014 with aftershocks that reveal structures sub-parallel to the continental margin.…”

“…The Savard et al (2020) model further implies that seismicity concentrations originally used to define the NFZ trajectory below and seaward of Nootka Island instead reside within the Juan de Fuca plate. For example, aftershocks from the 2004 Mw 6.3 event locate at depths as shallow as 20 km and were interpreted to lie beneath the top of a bending JdF plate (Savard et al, 2020). Less than 50 km to the northwest, the deepest aftershocks of the 2014 Mw 6.6 event locate at depths >35 km and suggest that locally the oceanic plate also dips to the northwest (Hutchinson et al, 2020).…”

Deep Coseismic Slip in the Cascadia Megathrust can be Consistent with Coastal Subsidence

“…The Cascadia Subduction Zone (Figure 1, CSZ) is a 1200 km plate boundary that extends from the Mendocino Triple Junction in Northern California to the tectonically complex region surrounding the Explorer Plate offshore of Northern Vancouver Island (Braunmiller and Nabelek, 2002;Savard et al 2020). It accommodates roughly 4 cm of annual convergence between the Juan de Fuca and North American plates (DeMets et al, 2010).…”