The Fragmented Death of the Farallon Plate

Abstract

The processes that accompany the death of an oceanic plate, as a ridge nears a trench, remain enigmatic. How the plate might reorganize, fragment, and eventually be captured by one of the bounding plates are among the unresolved details. We present a tomographic model of the Pacific Northwest from onshore and offshore seismic data that reveals a hole in the subducted Juan de Fuca plate. We suggest that this hole is the result of a tear along a preexisting zone of weakness, is causing volcanism on the North American plate, and is causing deformation in the Juan de Fuca plate offshore. We propose that in the final stages of an oceanic plate's life, deformation on the surface can be driven by deeper dynamics and that the fragmentation and the eventual capture of oceanic plate fragments may be governed by a process that operates from the bottom up.

Detailed Structure of the Subducted Nazca Slab into the Lower Mantle Derived From Continent‐Scale Teleseismic P Wave Tomography

“…For simplicity, we infer in our slab model that the slab is continuous and coherent in the lower mantle, but we are unable to rule out the possibility that the relative incoherency of the slab anomaly is not representative of true slab structure. A fragmented slab at these depths is feasible given the variety of recent studies identifying fragmented slabs in both the upper and lower mantle in other subduction zones (Hawley & Allen, ; Obrebski et al, ; Peng et al, ; Portner et al, ).…”

The Case for a Long-Lived and Robust Yellowstone Hotspot

“…Several workers attribute the High Lava Plains trend to mantle upwelling associated with slab rollback (e.g., Long et al, 2009;Ford et al, 2013), but this may be difficult to reconcile with evolving seismic data that reveal a shortened and highly fragmented slab in this region (Long, 2016). Slab fragmentation is described by Hawley and Allen (2019) as a propagating tear responsible for westward mantle flow beneath the High Lava Plains trend. Following Jordan et al Colgan and Henry (2009).…”

Body Wave Tomography of the Cascadia Subduction Zone and Juan de Fuca Plate System: Identifying Challenges and Solutions for Shore‐Crossing Data

“…Segmentation of slab high-velocity anomalies at depth appears to be a robust feature but it is unclear what the exact spatial extent is, an important distinction to understanding if this is a tear, a large gap, or an imaging artifact. The most robust reduction in both models at 45°N places a segment boundary further to the north than suggested by recent S-wave imaging (Hawley & Allen, 2019).…”

“…High‐velocity anomalies tend to decrease in amplitude beneath Oregon. Some have attributed this to a slab‐plume interaction (Geist & Richards, 1993; Obrebski et al., 2010) and/or a tear in the slab (Hawley & Allen, 2019), while others have suggested that it is an imaging artifact (Roth et al., 2008). If the slab is nonexistent in this region, that has implications for dynamic models relying on slab rollback to drive mantle circulation (Long, 2016).…”

“…Many studies focus on properties of the overriding crust, incoming oceanic crust, or properties along the plate interface to explain along-strike segmentation in megathrust processes (Audet et al, 2009;Brudzinski & Allen, 2007;Cloos, 1992;Delph et al, 2018;Littel et al, 2018;Ruff, 1989). However, there is evidence that heterogeneity within the oceanic asthenosphere plays an important role in subduction phenomena (Bodmer et al, 2018(Bodmer et al, , 2020Hawley & Allen, 2019). Nearby hotspots and/or local flow dynamics may emplace buoyant mantle beneath the subducting slab (Bodmer et al, 2018;Gao, 2018;Portner et al, 2017), creating localized anomalies in the subslab.…”