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Magnetotelluric image of the fluid cycle in the Costa Rican subduction zone


Fluids entering the subduction zone play a key role in the subduction process. They cause changes in the dynamics and thermal structure of the subduction zone1, and trigger earthquakes when released from the subducting plate during metamorphism2,3. Fluids are delivered to the subduction zone by the oceanic crust and also enter the oceanic plate as it bends downwards at the plate boundary. However, the amount of fluids entering subduction zones is not matched by that leaving through volcanic emissions4 or transfer to the deep mantle2, implying possible storage of fluids in the crust. Here we use magnetotelluric data to map the entire hydration and dehydration cycle of the Costa Rican subduction zone to 120 km depth. Along the incoming plate bend, we detect a conductivity anomaly that we interpret as sea water penetrating down extensional faults and cracks into the upper mantle. Along the subducting plate interface we document the dehydration of sediments, the crust and mantle. We identify an accumulation of fluids at 20–30 km depth at a distance of 30 km seaward from the volcanic arc. Comparison with other subduction zones5,6,7,8,9,10,11,12,13,14 indicates that such fluid accumulation is a global phenomenon. Although we are unable to test whether these fluid reservoirs grow with time, we suggest that they can account for some of the missing outflow of fluid at subduction zones.

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Figure 1: Location of magnetotelluric (MT) profile on- and offshore Nicoya Peninsula in Costa Rica.
Figure 2: Two-dimensional inversion model of electrical resistivity below the Costa Rican subduction zone.
Figure 3: Schematic interpretation of inversion results.
Figure 4: Global overview of the forearc conductor (anomaly G) appearances in subduction zones.

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This publication is contribution No 187 of the Sonderforschungsbereich 574 ‘Volatiles and fluids in subduction zones’ at Kiel University. The logistical support of Guillermo Alvarado and the Instituto Costarricense de Electricidad is appreciated. We would further like to thank the Costa Rican Coast Guard for providing us with ship time and the German embassy in Costa Rica for intercession. We thank B. Lewitz, Y. Dzierma, R. Kroth, P. Schroeder, T. Brandt and C. Jung for their great help in the campaign. We also thank D. Scholl and two anonymous reviewers for comments that helped to substantially improve this paper. Thanks to A. Freundt and S. Kutterolf for discussions. The study was funded by the German Science Foundation.

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T.W. and M.J. designed this study, developed marine magnetotellurics instruments, carried out the marine experiment, analysed the data, interpreted the results and wrote the paper; H.K. interpreted the results and wrote the paper; H.B. designed this study, carried out the land experiment, analysed the data and interpreted the results; W.T. provided geologic background information and logistical support, which was vital for the whole experiment.

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Correspondence to Tamara Worzewski.

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The authors declare no competing financial interests.

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Worzewski, T., Jegen, M., Kopp, H. et al. Magnetotelluric image of the fluid cycle in the Costa Rican subduction zone. Nature Geosci 4, 108–111 (2011).

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