A 2‐D tomographic model of the Juan de Fuca plate from accretion at axial seamount to subduction at the Cascadia margin from an active source ocean bottom seismometer survey

Abstract

We report results from a wide‐angle controlled source seismic experiment across the Juan de Fuca plate designed to investigate the evolution of the plate from accretion at the Juan de Fuca ridge to subduction at the Cascadia margin. A two‐dimensional velocity model of the crust and upper mantle is derived from a joint reflection‐refraction traveltime inversion. To interpret our tomography results, we first generate a plausible baseline velocity model, assuming a plate cooling model and realistic oceanic lithologies. We then use an effective medium theory to infer from our tomography results the extent of porosity, alteration, and water content that would be required to explain the departure from the baseline model. In crust of ages >1 Ma and away from propagator wakes and regions of faulting due to plate bending, we obtain estimates of upper crustal hydration of 0.5–2.1 wt % and find mostly dry lower crust and upper mantle. In sections of the crust affected by propagator wakes we find upper estimates of upper crustal, lower crustal, and upper mantle hydration of 3.1, 0.8, and 1.8 wt %, respectively. At the Cascadia deformation front, we find that the amount of water stored at uppermost mantle levels in the downgoing JdF plate is very limited (<0.3 wt %), with most of the water carried into the subduction zone being stored in the oceanic crust.

Catalog of Offshore Seismicity in Cascadia: Insights Into the Regional Distribution of Microseismicity and its Relation to Subduction Processes

“…In the outer rise region of the incoming plates, our data reveal an abrupt transition across 46°N from extremely low seismicity in the north to high seismicity in the south. In contrast, offshore Oregon, large offset normal faults that transect the oceanic crust and extend~6-7 km into the uppermost mantle were imaged, accompanied by a reduction in V p in the lower crust Horning et al, 2016). This inference is consistent with structural observations from active source seismic studies (Canales et al, 2017;Gulick et al, 2001;Han et al, 2016Han et al, , 2017Horning et al, 2016).…”

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

“…Several large V‐shaped propagator wakes disrupt the seafloor age progression (Figure b) as well as the structure of the lithosphere. 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, ).…”

“…Prior to subduction, the plate is <10 Myr old, which has led to the inference of a largely dry lithosphere particularly at sub‐Moho depths where elevated temperatures limit the depth extent of brittle deformation. Indeed, recent estimates of Juan de Fuca (JdF) mantle hydration from 2‐D active‐source imaging indicate less than 0.25 wt% H 2 O within the uppermost 1 km of mantle (Canales et al, ; Horning et al, ). A relatively dry subducting slab is also consistent with a general lack of deep intraslab seismicity (McCrory et al, ) and eruption of dry arc magmas (Ruscitto et al, ; Walowski et al, ).…”

“…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.…”