Seismic evidence of tectonic control on the depth of water influx into incoming oceanic plates at subduction trenches.

Lefeldt, Marten, Ranero, César R. and Grevemeyer, Ingo (2012) Seismic evidence of tectonic control on the depth of water influx into incoming oceanic plates at subduction trenches. Open Access Geochemistry Geophysics Geosystems, 13 . Q05013. DOI 10.1029/2012GC004043.

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Abstract

Water transported by slabs into the mantle at subduction zones plays key roles in tectonics, magmatism, fluid and volatiles fluxes, and most likely in the chemical evolution of the Earth's oceans and mantle. Yet, incorporation of water into oceanic plates before subduction is a poorly understood process. Several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and/or slab pull, and display anomalously low seismic velocities. The low velocities are interpreted to arise from a combination of fluid-filled fractures associated to normal faulting and mineral transformation by hydration. Mantle hydration by transformation of nominally dry peridotite to water-rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle. The depth of fracturing by normal-fault earthquakes is usually not well constrained, but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle, restrained only by serpentine stability. Yet, interpretation of deep intraplate mineral alteration remains speculative because active-source seismic experiments have sampled only the uppermost few kilometers of mantle, leaving the depth-extent of anomalous velocities and their relation to faulting unconstrained. Here we use a joint inversion of active-source seismic data, and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor. We found that anomalous velocities are restrained to the depth of normal-fault micro-earthquake activity recorded in the network, and are considerably shallower than either the rupture depth of teleseismic, normal-fault earthquakes, or the limit of serpentine stability. Extensional micro-earthquakes indicate that each fault in the region slips every 2–3 months which may facilitate regular water percolation. Deeper, teleseismic earthquakes are comparatively infrequent, and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study. Our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration.

Document Type: Article
Keywords: Meeresgeologie; Geodynamics; Geophysics; bending ; microseismicity ; oceanic plates ; serpentinization ; subduction zone ; water influx
Research affiliation: OceanRep > SFB 574 > A5
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1029/2012GC004043
ISSN: 1525-2027
Projects: Future Ocean, SFB574
Contribution Number:
ProjectNumber
SFB 574242
Expeditions/Models/Experiments:
Date Deposited: 05 Jun 2012 08:21
Last Modified: 28 Feb 2018 11:48
URI: http://oceanrep.geomar.de/id/eprint/14394

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