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

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

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

Fluids entering the subduction zone are a key factor in the subduction process. They determine the onset of melting, weakening and changes in the dynamics and thermal structure of subduction zones and trigger earthquakes when being released from the subducting plate in a series of metamorphic processes. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data and are subject of vigorous current research in SFB574 (Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters). Electromagnetic methods like magnetotellurics have been used widely to recognize fluid release and melt production through enhanced electrical conductivities. Here we present an image of the hydration and dehydration cycle down to 120 km depth in one setting derived by an onshore-offshore transect of magnetotelluric soundings in Costa Rica. 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 possibly causing serpentinization. Along the downward subducting plate distinct conductive anomalies identify fluids from dehydration of sediments, crust and mantle. A conductivity anomaly at a depth of approx. 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. The position of the conductivity anomaly correlates with geochemical and seismic evidence stating that mid-slope fluids are originated at >=12 km depth before rising up through deep faults to the seeps. The conductivity anomaly is therefore associated with a fluid accumulation feeding the mid-slope seeps. 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. While we are at the moment not able to attribute one specific process causing the anomaly based on electromagnetic data alone, this feature is however of fundamental importance. 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. Based on very simplified assumptions we are able derive rough estimates for the amount of water being stored in the overriding plate. Relating seismic evidence as well as petrological results collected in the multi-disciplinary study on the Costa Rican subduction zone we introduce budget estimations for the water cycle in the subduction zone

Approximations for the 2-D coast effect on marine magnetotelluric data

Marine natural source electromagnetic data acquired on continental margins are often of considerable scientific and commercial interest. However, the large conductivity contrast between the ocean and coast causes this type of data to be severely distorted. For a 2-D coastal model, this distortion is most pronounced for the marine magnetotelluric and geomagnetic response function derived from induced currents flowing parallel to the coast. A maximal distortion occurs for a given period at a specific distance from the coast and causes severe anomalies in the magnitude and phase of the response functions. Based on a modelling study, we empirically relate the characteristic period and characteristic distance to physical parameters such as the ocean depth and the host resistivity. Based on a simple analytical approach, we test these approximations and show that maximum distortion occurs when destructive interference between the ocean and host response is at its highest. While the coast effect causes a large distortion in the marine responses we show through a resolution analysis that it does not mask subsurface conductivity anomalies but in fact increases the sensitivity to the seafloor

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 metamorphism. Fluids are delivered to the subduction zone by the oceanic crust and also enter as the oceanic plate 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 mantle, 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 zones5–14 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