The Water Cycle in Subduction Zones as Constrained by Seismology
Abstract
The water cycle in subduction zones is poorly constrained because of large uncertainties in the hydration of the subducting mantle. A study of the Mariana trench using ambient noise and surface wave analysis of ocean bottom seismograph (OBS) data shows that the seismic velocity of the incoming plate is reduced to depths of 30-40 km below seafloor at the trench axis (Cai et al., 2018). Strong anisotropy is observed, with fast axes parallel to the trench, consistent with shape preferred orientation caused by slow velocities along faults. Active-source data reveal a P velocity structure of the crust and uppermost mantle that is consistent with the passive S-wave structure (Eimer et al., in prep). Earthquake studies using the same OBS dataset show that seismic activity in the incoming plate increases approaching the trench axes, consistent with increased seafloor fault offsets, and extensional earthquakes occur to depths of ~ 35 km below seafloor (Eimer et al., in prep). These results indicate that the incoming plate is progressively hydrated by water transported along faults, with a portion of the velocity reduction due to free water. The slow velocities in the incoming plate mantle increase somewhat beneath the forearc as water is forced out of pore spaces and moves upward, serpentinizing the outer forearc mantle. The remaining velocity anomaly suggests an incoming plate serpentinization of about 20%. Other old, cold subducting slabs likely contain correspondingly thick layers of hydrous mantle, suggesting the global water flux into the sub-arc mantle should be increased by about a factor of three over previous estimates. Since a long-term net influx of water to the Earth's interior is inconsistent with the geological record, estimates of water expelled at volcanic arcs and back arc basins also need to be revised upward. In particular, some hydrous primitive arc magmas likely contain more water than previously estimated, increasing the water output of arcs.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T32C..07W
- Keywords:
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- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8158 Plate motions: present and recent;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY