Evidence of Strong Upper Oceanic Crustal Hydration Outboard the Alaskan and Sumatran Subduction Zones

Abstract

The hydration state of subducting oceanic crust has been proposed to influence subduction zone processes like seismic coupling at the megathrust interface and arc magmatism downdip. Plate bending in the outer rise region is thought to help rehydrate the incoming oceanic lithosphere before subduction. Although numerous seismic refraction studies provide constraints on the amount of water stored in the lower crust and uppermost mantle, little information exists about how much free water is present in the upper 1–2 km of oceanic crust. Here, we present results from the application of advanced techniques to long‐offset multi‐channel seismic data acquired outboard the Alaskan and South Sumatran subduction zones. Our results show that the incoming upper crustal seismic layer, layer 2A, is significantly hydrated in both areas. Favorable conditions offshore the Alaska Peninsula promote the creation of a dense system of bending‐related faults, facilitating the infiltration of fluids that increases average water estimates to ∼3.9 wt.% H2O in the outer rise. As the crust subducts and temperature increases, some free water may react with the host rocks to form mineral‐bound water that is carried to greater depths, in agreement with elevated water contents found in arc lavas of Shumagin Gap volcanoes. Offshore Sumatra, we propose that similar layer 2A water estimates (∼3.2 wt.% H2O average) and heterogeneous hydration within 2B are associated with the ongoing, slow, complex deformation occurring in the Wharton Basin and thus potentially contribute to the presence of a long‐lived slow slip event recently inferred there at seismogenic depths.

Structural Heterogeneity of the Alaska‐Aleutian Forearc: Implications for Interplate Coupling and Seismogenic Behaviors

“…However, lower pore pressure of the subducted sediments has been reported in the Shumagin Gap than that in the Semidi segment, indicating a much drier sedimentary layer atop the slab (J. Li et al., 2015), inconsistent with the highly serpentinized slab mantle. In contrast, strong hydration in a form of free water is suggested to be retained in the upper portion of the oceanic crust at depths <10 km beneath the Shumagin Gap (Acquisto et al., 2022). At greater depths, water in the crust can be dominantly bounded in hydrous minerals.…”

“…Observations from an exhumed, basalt‐hosted megathrust on the Kenai Peninsula in south‐central Alaska indicate megathrust weakening due to chlorite‐bearing altered basalts coming from the oceanic plate (Braden & Behr, 2021). Although it is unable to directly estimate the hydration state of the oceanic crust due to the limited resolution of our tomography, the oceanic crust is quite likely hydrated coherently with the underlying lithospheric mantle, with constraints from multi‐channel seismic data (Acquisto et al., 2022). The changes in hydration state of these different parts of the subducting slab may contribute to the variation of the megathrust slip behavior.…”

“…Li et al, 2015), inconsistent with the highly serpentinized slab mantle. In contrast, strong hydration in a form of free water is suggested to be retained in the upper portion of the oceanic crust at depths <10 km beneath the Shumagin Gap (Acquisto et al, 2022). At greater depths, water in the crust can be dominantly bounded in hydrous minerals.…”

Subducting volcaniclastic-rich upper crust supplies fluids for shallow megathrust and slow slip

“…Pwave velocities increase gradually with depth to ~3 to 4.5 km/s at ~1.5 to 2 km below the top of the Hikurangi Plateau (Fig. 5A), reaching values that are consistent with altered and fractured lavas at the top of basaltic oceanic crust (11,59,60). The low-velocity (<4 km/s) upper crust contains extensive subhorizontal intracrustal reflectors (ICRs) with lateral dimensions of ~1 to 5 km (Figs.…”

“…Where fluids are released has important consequences for crustal strength, deformation, and earthquake processes within the subduction system. Subducting lithosphere that is hydrated by outer rise bend-faulting and fracture zones (11)(12)(13) is suggested as a source of fluids that contribute to intermediate-depth intraplate earthquakes (14,15) and influence the chemical composition of arc magmas (16,17). Shallow slow earthquakes, including slow slip events (18,19), tectonic tremor (20), and low-frequency earthquakes (21), can occur when faults with slightly rate-weakening friction experience low-effective normal stresses induced by fluid overpressures (22)(23)(24); the major sources of these fluids are often assumed to be compacting and dehydrating marine sediments (1,25,26).…”