Volatiles and Fluids in Subduction Zones

Bending-related faulting and mantle serpentinization / hydration at deep sea trenches

Growing observational evidence suggests that the incoming oceanic lithosphere at subduction zones is modified in the so called outer rise - trench area. The outer rise is a consequence of the flexure of the subducting lithosphere, producing a prominent bathymetric bulge seaward of the trench axis. Seismic reflection imaging of the incoming oceanic lithosphere detected faults cutting through the entire crust into the upper most mantle off Middle America and Central Chile that may create pathways for seawater to penetrate to mantle depth. Seismic velocities are the best indicator for studying hydration of the lithosphere at mantle depth. With increasing degree of serpentinization seismic compressional wave velocity is reduced from ~8.0-8.2 km/s for pristine peridotite to ~4.5 km/s at 100% transformation of peridotite to serpentinite. Evidence for mantle rock alteration using seismic velocities derived rom seismic refraction and wide-angle data has previously been found at abandoned spreading ridges and interpreted in terms of hydration and hence serpentinization. Offshore Nicaragua we collected within SFB574 data from a seismic pilot experiment to study mantle velocity where multi-channel seismic data imaged faults that cut ~15 km into the mantle. Indeed, seismic velocities are with 7.6-7.8 km/s much lower than expected for "normal" oceanic crust, indicating plate hydration prior to subduction. This results have been publihed by Grevemeyer et al. (Passive and active seismological study of bending-related faulting and mantle serpentinization at the Middle America trench; Earth Planet. Sci. Lett, 258, 528-542, 2007).
To study if reduced mantle velocity are indeed caused by plate hydration in the trench-outer rise area and have not been caused by accretion at the spreading centre, a second line extending well into the outer-rise area was surveyed; yielding decreasing P-wave velocities in the crust and uppermost mantle as the plate approaches the trench. Seaward of the outer rise velocities are typical for ~24 Myr old oceanic lithosphere. In the near-trench region, however, crustal velocities are reduced by 0.2-0.5 km/s compared to normal mature oceanic crust. Seismic velocities of the uppermost mantle are 7.6-7.8 km/s and hence 5-7% lower than the typical velocity of mantle peridotite (Fig. A5-5a and b).

Fig. A5-4: Seismic velocity model from joint travel time inversion of seismic refraction and wide-angle data (a) shows and evolutionary process. Seismic velocities decrease systematically approaching the trench axis (a, b). Resolution tests indicates that observed features are real.

These systematic changes in P-wave velocity indicate an evolutionary process in the subducting plate consistent with percolation of seawater through the faulted and fractured lithosphere and serpentinization of mantle peridotites. Seismic resolution tests clearly suggest that reduced velocities in the crust and in the mantle could be resolved with the experimental setup (Fig. A5-4c). If hydration is indeed affecting seismic properties of the mantle, serpentinization reaches 12-17%. Based on the laboratory studies that relate the degree of serpentinization and water content of partially serpentinized peridotites to their seismic P-wave velocities, we estimate that the water stored in this region may range from ~1.5 to 2.5 wt% in the uppermost 3-4 km, where seismic data provide enough resolution. A revised paper has recently been submitted (Ivandic et al., Impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of Nicaragua, revised for J. Geophys. Res., December, 2007).