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

Abstract

Subduction zones are a locus of material transfer between the surface and the solid Earth and play a key role in global cycling of water and other volatiles over geologic time. Balancing the global water budget requires knowing the amount of water recycled to the mantle by subduction, but current estimates span an order of magnitude (Hacker, 2008;Rupke, 2004;Schmidt & Poli, 2003;van Keken et al., 2011). The largest source of uncertainty for subduction zone water fluxes is the amount of water carried in the subducting slab mantle. Hydrous minerals in the slab mantle are the reservoir most likely to carry water past the volcanic arc and into the deep mantle (Hacker, 2008), so this level of uncertainty represents roughly an order of magnitude difference in the rate at which water moves from surface to subsurface reservoirs, which has implications for eustatic sea level change (Parai & Mukhopadhyay, 2012).Oceanic lithosphere approaching deep ocean trenches tends to form faults due to plate bending, and these bend-related faults promote water circulation and hydration of the incoming plate crust and mantle (Faccenda, 2014;Faccenda et al., 2009;Hatakeyama et al., 2017;Korenaga, 2007). Plate bending can reactivate normal faults inherited from mid-ocean ridge tectonism or, if the incoming plate is subducting oblique to existing weak zones, new faults are broken parallel to the trench (Fujie et al., 2018;Zhou et al., 2015). Serpentinized fault zones may be up to several hundred meters wide, with the extent of serpentinization strongly controlled by permeability, which varies with depth (Hatakeyama et al., 2017). Water carried in the subducting slab can take different paths through the subduction zone: hydrous minerals in the crust tend to dehydrate into the mantle wedge (Jarrard, 2003), generating melts that feed arc volcanoes and potentially contributing to intermediate depth earthquakes (Boneh et al., 2019;Faccenda et al., 2012;Jung et al., 2004), while hydrous phases in deeper and colder parts of the slab can remain stable past the volcanic arc and subduct into the deeper mantle.Observational constraints on the amount of water carried by subducting oceanic lithosphere come primarily from geophysical data and rely on relationships between serpentinization, porosity, and geophysical observables

Along-Strike Variations of Alaska Subduction Zone Structure and Hydration Determined From Amphibious Seismic Data

“…Cai et al (2018) attributed part of the large VS reduction in the Mariana outer-rise mantle to water in cracks and joints. Mark et al (2023) found evidence from seismic anisotropy for water in crack-like pores in the upper 1 km of the Mariana outer rise mantle using active-source data. If the water in crack-like porosity exists in the mantle, the percent serpentinization determined by VS values and the VP/VS ratios will be overestimated.…”

“…The actual situation may be more complex, as the serpentinization may be localized in narrow regions surrounding discrete faults (Hatakeyama et al, 2017). In this case, there will be frequency-dependent wave propagation through the mantle at the frequencies used in active source studies (Miller & Lizarralde, 2016;Miller et al, 2021;Mark et al, 2023). Estimates of serpentinization taking this effect into account generally result in smaller percentages of serpentinization, but require analysis of azimuthal anisotropy, which is not available in this case.…”

“…These estimates necessarily involve a number of assumptions and are thus only very approximate, but are improvements on previous estimates that made ad-hoc assumptions about the hydration of the subducting mantle (e.g., van Keken et al, 2011), which had no constraint on the depth extent of the serpentinized layer. The largest uncertainty in these estimates is associated with the volume percent of serpentinization, due to the uncertainty of interpreting the discrepant estimates from VP, VS, and VP/VS, as well as the possible effects of liquid water in crack-like porosity (Korenaga et al, 2017) and anisotropy (Miller & Lizarralde, 2016;Mark et al, 2023). All of the assumptions made in our estimations are conservative and thus result in a minimum estimate of subducting water in each segment.…”

“…The velocities decrease towards the trench and show significant along-strike variations. Low-velocity zones at the top of the incoming plate mantle have been observed at many other subduction zones and are generally attributed to the serpentinization of mantle peridotite (Ivandic et al, 2008;Van Avendonk et al, 2011;Shillington et al, 2015;Cai et al, 2018;Mark et al, 2023) and/or to the effects of water in platebending faults (Miller & Lizarralde, 2016;Korenaga, 2017). The Shumagin segment shows a distinct low-velocity zone (~3.65 km s -1 ) at the top of the incoming plate mantle, suggesting strong hydration if the velocity reduction is due to serpentinization.…”

Along‐Strike Variations of Alaska Subduction Zone Structure and Hydration Determined From Amphibious Seismic Data

“…The velocities decrease toward the trench and show significant along‐strike variations. Low‐velocity zones at the top of the incoming plate mantle have been observed at many other subduction zones and are generally attributed to the serpentinization of mantle peridotite (Cai et al., 2018; Ivandic et al., 2008; Mark et al., 2023; Shillington et al., 2015; Van Avendonk et al., 2011) and/or to the effects of water in plate‐bending faults (Korenaga, 2017; Miller & Lizarralde, 2016). The Shumagin segment shows a distinct low‐velocity zone (∼3.65 km s −1 ) at the top of the incoming plate mantle, suggesting strong hydration if the velocity reduction is due to serpentinization.…”