The Hidden History of the South‐Central Cascadia Subduction Zone Recorded on the Juan de Fuca Plate Offshore Southwest Oregon

Abstract

New seismic reflection data collected and processed as part of early career scientist training at sea and in classroom projects fill gaps in seismic coverage of the Cascadia subduction zone and provide new insights into anomalous subduction behavior and mass wasting along the south‐central Cascadia Subduction Zone (CSZ) between 42°20’N and 44°15’N. The data reveal at least six distinct buried horizons of folded and faulted sediments similar to strata recently interpreted to result from in situ deformation induced by the load imposed by a large blocky mass transport deposit known as the 44°N slide. Although our results support prior studies indicating that the south‐central CSZ has experienced large slope instabilities, they indicate that the slides were more frequent but volumetrically smaller than previously thought. Similar strata have not been observed elsewhere beneath the abyssal plain adjacent to the Cascadia subduction zone. The structure of the deformation front along this segment is also indistinct, in contrast to the clear frontal faults outboard of folded trench strata observed immediately to the north and south (and generally throughout the rest of Cascadia). We attribute the anomalous nature of this segment of the margin to past subduction of shallow and rough seafloor, which resulted in greater uplift of the forearc than elsewhere along the margin. A consequence of this postulated history would be the shedding of older, more consolidated blocks onto the Juan de Fuca plate, resulting in the observed distinctive stratigraphy offshore southern Oregon.

Impact‐Induced Seafloor Deformation From Submarine Landslides: Diagnostic of Slide Velocity?

“…In fact, the 44‐N Slide is not unique to this margin. There have been at least five other similar events indicated along the south‐central Cascadia Subduction Zone (Tréhu et al., 2022). Additionally, we would expect to find similar in situ deformation where large blocky slides are common such as other subduction zones, volcanic islands, and margins affected by salt diapirism (Alves, 2015).…”

“…Cascadia is seismically active and estimated to produce megathrust earthquakes about once every 500 years (Goldfinger et al., 2017; Walton et al., 2021). The Oregon margin has a long history of slope failures ranging from small non‐cohesive slides to massive blocky slides (Goldfinger et al., 2000; Hill et al., 2022; Lenz & Sawyer, 2021; Lenz et al., 2019; McAdoo & Watts, 2004; McAdoo et al., 2000; Tréhu et al., 2022). Cascadia is therefore susceptible to many geohazards, including submarine landslides (Hill et al., 2022).…”