Journal Article

Constraints on Bend‐Faulting and Mantle Hydration at the Marianas Trench From Seismic Anisotropy

Lizarralde, Daniel, Mark, Hannah F., Wiens, Douglas A.

2023

Overview

Subduction zones are a key link between the surface water cycle and the solid Earth, as the incoming plate carries pore water and hydrous minerals into the subsurface. However, water fluxes from surface to subsurface reservoirs over geologic time are highly uncertain because the volume of water carried in hydrous minerals in the slab mantle is poorly constrained. Estimates of slab mantle hydration based on seismic tomography assume bulk serpentinization, representing an upper bound on water volume. We measure azimuthal seismic anisotropy near the Marianas Trench, use spatial variations in anisotropy to constrain the extent and geometry of bend‐related faulting, and place a lower bound on slab mantle water content for the case where serpentinization is confined within fault zones. The seismic observations can be explained by a minimum of ∼0.85 wt% water in the slab mantle, compared to the upper bound of ∼2 wt% obtained from tomography. Plain Language Summary The global water cycle extends into Earth's interior at subduction zones, where tectonic plates carrying water chemically bound in rocks and minerals descend into the mantle. The amount of water cycled into the mantle by subduction is not well known. Part of the water flux can be estimated by measuring seismic velocities in the subducting plate, since the water‐bearing minerals tend to have slower seismic velocities, but this is an upper bound because it assumes that the water‐bearing minerals are evenly distributed when in reality they are more likely to be localized within fault zones. We use seismic anisotropy, variations in wavespeed with propagation direction, to study the degree of faulting near the Marianas Trench and estimate a lower bound on the water flux from surface to subsurface assuming that water‐bearing minerals are only within faults. Key Points We measure spatial variations in upper mantle anisotropy that indicate bend‐faulting near the Marianas Trench Hydration localized to bend‐faults places a lower bound on the amount of water carried in the subducting slab mantle Synthetic seismograms compared to the observed anisotropy indicate a minimum of 0.85 wt% water in the slab mantle

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Anisotropy / Constraints / Earth / Earth mantle / Fault zones / Geological faults / Geological time / Hydration / Hydrologic cycle / Hydrological cycle / Lower bounds / Marianas Trench / Minerals / Moisture content / Plate tectonics / Plates (tectonics) / Pore water / Seismic activity / Seismic tomography / Seismic velocities / Serpentinization / solid‐Earth water cycle / Spatial variations / Subduction / Subduction (geology) / Subduction zones / Surface water / Tomography / Upper bounds / Volume transport / Water content / Wave propagation