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

Three-dimensional inversion of magnetotelluric data from the Central Andean continental margin

Magnetotelluric data were collected in the late 1990s in the Central Andes of Chile and Bolivia, with the aim to delineate the electrical conductivity distribution in the subsurface and its relations to subduction processes. In previous studies, these data were interpreted based on 2-D models. The principal result was a vast conductivity zone beneath the Altiplano high plateau at mid and lower crustal depths and a much smaller, though significant conductor associated with the Precordillera Fault System. However, there are some significant 3-D effects in the investigation area, in particular near the coast and on the eastern Altiplano. The aim of this work is to give a reinterpretation based on new 3-D inversion of these data. The 3-D inversion not only provides a better fit to the data compared to 2-D results but furthermore allows to include sites with strong telluric distortion which were ignored in previous studies. We are now able to explain anomalous phases above 90° and induction arrows pointing subparallel to the coast as observed at several sites in the Coastal Cordillera. These strongly distorted data are caused by highly conductive near-surface structures that are partly connected to the Pacific Ocean, forcing currents to flow around the sites. The lower crust beneath the Coastal Cordillera resembles a poorly conductive, nearly homogeneous half-space and is electrically unremarkable. Besides, we can now image the Precordillera conductor as a continuous, elongated feature. The volcanic arc of the Western Cordillera is highly resistive with the exception of a few conductive spots which may be associated with certain individual volcanoes or geothermal resources, respectively. The Altiplano conductor is again the dominant electrical feature in the Central Andes, indicating widespread melting of the middle and lower back-arc crust

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

Shallow and Deep Electric Structures in the Tolhuaca Geothermal System (S. Chile) Investigated by Magnetotellurics

The geoelectric properties of the geothermal system associated with the Tolhuaca volcano were investigated by three-dimensional (3D) inversion of magnetotelluric (MT) data. This study presents the first resistivity model of the Tolhuaca volcano derived from 3D MT inversion to have a better understanding of its magmatic and hydrothermal system. We selected data from 54 MT stations for 3D inversion. We performed a series of 3D MT inversion tests by changing the type of data to be inverted, as well as the starting model to obtain a model in agreement with the geology. The final 3D MT model presents a conductive body (1800 m from the surface) that had so far not been identified by previous resistivity models. The result of this study provides new insights into the complexity of the Tolhuaca geothermal system

Shallow and Deep Electric Structures in the Tolhuaca Geothermal System (S. Chile) Investigated by Magnetotellurics

The geoelectric properties of the geothermal system associated with the Tolhuaca volcano were investigated by three-dimensional (3D) inversion of magnetotelluric (MT) data. This study presents the first resistivity model of the Tolhuaca volcano derived from 3D MT inversion to have a better understanding of its magmatic and hydrothermal system. We selected data from 54 MT stations for 3D inversion. We performed a series of 3D MT inversion tests by changing the type of data to be inverted, as well as the starting model to obtain a model in agreement with the geology. The final 3D MT model presents a conductive body (1800 m from the surface) that had so far not been identified by previous resistivity models. The result of this study provides new insights into the complexity of the Tolhuaca geothermal system

Crustal structure of the Lazufre volcanic complex and the Southern Puna from 3-D inversion of magnetotelluric data: Implications for surface uplift and evidence for melt storage and hydrothermal fluids

The Central Andes are unique in the global system of subduction zones in that a significant, high-altitude plateau has formed above a subduction zone. In this region, both subduction and the associated magmatism have been shown to vary in both space and time. Geophysical data have been invaluable in determining the subsurface structure of this region. Extensive seismic studies have determined the regional-scale distribution of partial melt in the crust and upper mantle. Magnetotelluric studies have been effective in providing independent constraints on the quantity and composition of partial melt in the crust and upper mantle. Geodetic studies have shown that a small number of volcanic centers exhibit persistent, long-term uplift that may indicate the formation of plutons or future eruptions. This paper describes a detailed study of the Southern Puna using magnetotelluric (MT) data. This region is located at the southern limit of the Central Andes in a region where a recent transition from flat-slab subduction to normal subduction has caused an increase in magmatism, in addition to hypothesized lithospheric delamination. It is also a region where an extensive zone adjacent to the volcanic arc is undergoing surface uplift, located near Volcán Lastarria and Cordon del Azufre (collectively called Lazufre). The main goals of the work are to define the crustal structure and to investigate processes that may cause surface uplift of relatively large regions not associated with active volcanism. As part of the PLUTONS project, MT data were collected on an east-west transect (approximately along 25°S) that extended across the Southern Puna, from Lazufre to north of Cerro Galan. The data were combined with previously collected MT data around Lazufre and inverted to give a 3-D resistivity model of the crust. The low resistivity of the crust resulted in limited sensitivity to mantle structure. A number of major crustal conductors were detected and included (1) a mid-crustal conductor extending eastward from the volcanic arc as far as the Salar de Antofalla; (2) an upper- to mid-crustal conductor located north of Cerro Galan; and (3) a conductor that rises westward from (1) and terminates directly beneath the region of surface uplift at Lazufre. These conductors are broadly coincident with the location of crustal lowshear-wave anomalies. The conductive features were interpreted to be due to zones of partial melt stored in the crust, and petrological data were used to estimate melt fractions. Below Lazufre, it is likely that aqueous fluids contribute to the high conductivity, which is observed within the depth range of the inflation source, giving evidence that the surface uplift may be associated with both magmatic and hydrothermal processes

Three-dimensional electrical resistivity image of the South-Central Chilean subduction zone

Highlights • 3-D magnetotelluric image of the South Chilean arc and forearc. • High conductivity zones beneath active volcanoes. • New model explains all EM transfer functions in the arc and backarc. • Induction vectors in the forearc hint at large-scale anisotropy in the crust. Abstract Based on isotropic 3-D inversion, we re-interpret long-period magnetotelluric data collected across the geotectonic structures of the South-Central Chilean continental margin at latitudes 38°–41°S and summarize results of long-period magnetotelluric (MT) investigations performed between 2000 and 2005. The new 3-D conductivity image of the South-Central Chilean subduction zone basically confirms former 2-D inversion models along three profiles and complete the previous results. The models show good electrical conductors in the tip of the continental crustal beneath the Pacific Ocean, the frequently observed forearc conductor at mid-crustal levels, a highly-conductive zone at similar levels slightly offset from the volcanic arc and a – not well-resolved – conductor in the Argentinian backarc. The subducted Nazca Plate generally appears as a resistive but discontinuous feature. Unlike before, we are now able to resolve upper crustal conductors (interpreted as magma reservoirs) beneath active Lonquimay, Villarrica, and Llaima volcanoes which were obscured in 2-D inversion. Data fit is rather satisfactory but not perfect; we attribute this to large-scale crustal anisotropy particularly beneath the Coastal Cordillera, which we cannot include into our solution for the time being