Marine magnetotellurics on a continental margin: imaging the hydration and dehydration cycle of the Costa Rican subduction zone.

Worzewski, Tamara W. D. (2011) Marine magnetotellurics on a continental margin: imaging the hydration and dehydration cycle of the Costa Rican subduction zone. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, IX, 162 S. pp.

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Abstract

At continental margins, the water content and its distribution play an important role in the subduction process. Water is released from the subducting slab in a series of metamorphic reactions and may trigger the onset of melting, cause crustal weakening and changes in the dynamics and thermal structure of subduction zones. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data. They are subject of vigorous current research in the special research initiative (SFB 574) at University of Kiel and IFM-GEOMAR “Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms
for Natural Disasters”.
Electromagnetic methods like magnetotellurics have been widely used to recognize fluid release and melt production through enhanced electrical conductivities. In the framework of SFB 574, an offshore magnetotelluric experiment was performed in 2007-2008 along a profile crossing the trench, where the Cocos plate is thrust beneath the Caribbean plate. The marine profile was extended onshore by the Free University of Berlin, yielding a large-scale amphibious data set across the subduction zone with a profile length of 370 km. The main goal of the experiment is to image the fluid content and its distribution along the subducting plate and deeper Earth structure.
The recorded electromagnetic time series have been processed to electromagnetic sounding curves (apparent resistivity & phase, and Tipper) at each station. As most of the stations lay on a cliffy continental shelf, they were highly susceptible to water enforced movement (tidal currents hitting the shelf). The data quality of the recorded electromagnetic time series therefore ranges from very good to noisy, depending on the instrument’s position and stability. Only quiet sections are used for the processing. In the subsequently derived marine transfer functions a distortion due to the so-called “coast effect” is visible at specific period and distance to the coast (apexes in apparent resistivity curves occur in the transvers-electric (TE) mode, accompanied by phases wandering through all four quadrants and abnormally high Tipper values).
A detailed modeling study is performed in order to explain and quantify the coastal distortion. The modeling study reveals that the presence of a coast affects the marine transfer functions with a specific signature, which depends on several physical parameters, such as distance from the coast, period, ocean depth and bulk resistivity.
Approximations are derived that define a “characteristic period” and “characteristic distance” from the coast at which the distortion is expected to be most pronounced in the transfer functions. The distortion due to the coast is shown to be helpful as it allows the estimation of the bulk resistivity of the subsurface and furthermore increases the sensitivity of the electromagnetic response to conductivity anomalies at depth.
The recorded marine transfer functions were inverted together with the land transfer functions to an electrical resistivity model of the subduction zone down to a depth of approximately 120 km. Based on the model the hydration and dehydration cycle of a subduction zone may be derived. An electrically conductive zone in the incoming plate outer rise is associated with sea water penetrating down extensional faults and cracks into the upper mantle. Along the downward subducting plate, distinct conductive anomalies identify fluids from dehydration processes in the sediments, crust and mantle.
A conductivity anomaly at a depth of approximately 12 km and at a distance of 65 km from the trench is associated with a first major dehydration reaction of minerally-bound water. This is of importance in the context of mid-slope fluid seeps which are thought to significantly contribute to the recycling of minerally-bound water. Another fluid accumulation is revealed by a conductivity anomaly at 20-30 km depth and a distance of approximately 30 km seaward from the volcanic arc. This lower crustal fluid accumulation could likely be caused by trapping of fluids released due to de-serpentinization processes or due to other mineral dehydration processes. A comparison with other electromagnetic studies from subduction zones around the world reveal that such a conductivity anomaly is a global feature suggesting the presence of a global fluid sink. This sink may help to explain the general observed deficit between water input and output in a subduction cycle. By relating seismic evidence as well as petrological results collected in the multi-disciplinary study of Costa Rica, budget estimations for the water cycle in the subduction zone are introduced.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: UNSPECIFIED
Additional Information: Referentin/Referent: Dr. Marion Jegen-Kulcsar Koreferentin(-nen) / Koreferent(-en): Prof. Dr. Heidrun Kopp / Print-Ausg. vorhanden in der Bibliothek Ostufer
Keywords: Geodynamics; Geochemistry; Magnetotellurics, Subduction Zone, Coast Effect, Costa Rica, Fluids
Research affiliation: OceanRep > SFB 574
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
OceanRep > SFB 574 > N01
Expeditions/Models/Experiments:
Date Deposited: 01 Jun 2011 10:17
Last Modified: 06 Jul 2012 14:50
URI: http://oceanrep.geomar.de/id/eprint/11887

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