Along‐Trench Structural Variations of the Subducting Juan de Fuca Plate From Multichannel Seismic Reflection Imaging

Abstract

To characterize the along‐strike structural variations of the Juan de Fuca (JdF) Plate as it enters the Cascadia subduction zone, we present prestack time migrated multichannel seismic reflection images of the JdF Plate along a 400‐km‐long trench‐parallel transect extending from 44.3°N to 47.8°N. Beneath the 1.8–3.0‐km‐thick sediment cover, our data reveal basement topographic anomalies associated with a 1.2‐km‐high seamount and in the vicinity of propagator wakes (390–540‐m relief). Weak Moho reflections are imaged beneath the propagator wakes and coincide with reduced Vp in the lower crust and/or uppermost mantle. The inferred locations of propagator wakes in the downgoing plate collocate with some of the boundaries of episodic tremor and slip events. We propose that the structural and hydration heterogeneities associated with these features could lead to anomalous plate interface properties and contribute to episodic tremor and slip segmentation. Intracrustal reflections with apparent dips (20°–30°) consistent with subduction bending normal faults change near 45.8°N, from northward dipping reflections confined to the middle crust in the north to antithetic reflections through the crust in the south, coinciding with a Vp reduction in the lower crust. These observations indicate more extensive faulting deformation and associated hydration of the JdF Plate south of 45.8°N, which likely results from variations of slab dip and resistance to subduction across 46°N. Basement offsets and abrupt depth/amplitude changes in Moho reflections are imaged beneath the four major WNW trending strike‐slip faults that cross the Cascadia deformation front, providing strong evidence of a lower plate origin for these faults.

Pn Tomography of the Juan de Fuca and Gorda Plates: Implications for Mantle Deformation and Hydration in the Oceanic Lithosphere

“…Such velocity reductions are commonly observed in the outerrise region of subduction zones where bend‐related normal faults penetrate many kilometers beneath the Moho (Cai et al, ; Faccenda et al, ; Fujie et al, ; Ranero et al, ; Shillington et al, ; Van Avendonk et al, ). In Cascadia, bend‐related faulting is observed to extend 6–7 km beneath the Moho in ~8.5‐Myr lithosphere (Han et al, , ). If these depths coincide with the brittle‐ductile transition, they imply a lower thermal limit to faulting of ~500 °C (estimated from a HSCM) or ~700 °C if the insulating effects of a sediment blanket are considered (Horning et al, ).…”

“…These anomalies occur at the northern extent of significant outerrise seismicity (Figure b) where paleospreading fabrics more closely parallel the deformation front. Here, Han et al () image outerrise faults that extend through the crust and into the upper 6–7 km of the mantle. Therefore, the 0.2–0.4 km/s reduction in Vp with respect to our HSCM likely results from faulting and the incorporation of H 2 O. Canales et al () recover nearly identical mantle velocities in the same region (Figure S5a) and show that they may be explained by 0.1–0.2 wt.% H 2 O stored primarily as pore water.…”

“…Active source studies have identified anomalously slow crustal and uppermost mantle velocities beneath seafloor created at migrating ridge offsets interpreted as regions of increased plate hydration (Han et al, ; Horning et al, ; McClymont & Clowes, ; Soule et al, ; Weekly et al, ). In reflection studies, propagator wakes often appear as regions of anomalous crustal and mantle reflectivity (Han et al, , ; Nedimović et al, ). Crust formed within and near tectonic offsets may also be compositionally distinct as suggested by generally more evolved basalt compositions (Sinton et al, ; Wanless et al, ) and denser crust (Marjanović et al, ; Toomey & Hooft, ).…”

“…The propagator wakes are more heavily fractured regions of seafloor formed between nontransform tectonic offsets along the mid‐ocean ridge (Wilson et al, ). As heavily tectonized features, they may provide a greater density of pathways for fluid circulation resulting in local hydration anomalies that may influence subduction zone processes (Han et al, , ; Horning et al, ). Abundant seismicity and deformed seafloor magnetic and basement lineations in southern Cascadia (Figure ) has led to the speculation that the Gorda may also be a lithospheric hydration anomaly (Canales et al, ; Nedimović et al, ; Walowski et al, ).…”

“…While a number of active‐source experiments in Cascadia have constrained the detailed structure of the crust and shallowest mantle (Canales et al, ; Han et al, , ; Horning et al, ; McClymont & Clowes, ; Nedimović et al, ), these observations are spatially limited and only exist for the JdF plate. Recent studies using CI data have primarily targeted upper mantle structure.…”