The pathways of fluids and mantle-originated carbon dioxide in the seismically active Ohře (Eger) Rift system appearing as mofettes at the surface are currently subject to investigation, especially by the International Continental Scientific Drilling Program “Drilling the Eger Rift”. If the aquifers show significant contrast in electrical resistivity to the host rocks, they can be investigated with geo-electromagnetic methods. However, imaging complex fluid and CO2 pathways in detail in near-surface structures is challenging, because, in contrast to the background stratigraphy, they are often oriented in near-vertical directions. Therefore, we aim to investigate how the shallow aquifer structures can be examined best with an inductive electromagnetic method. For this purpose, we collected radio-magnetotelluric data in the Hartoušov mofette field and evaluated them by two- and three-dimensional inversions. Data from a nearby magnetotelluric station, drill hole data, gas flux measurements and electrical resistivity tomography models were used to assess the reliability and robustness of our inversion results. We concluded that the near-surface fluid reservoirs are adequately depictable, while the migration paths of gaseous CO2 cannot be traced properly due to a lack of resistivity contrast. Our model analyses suggest that imaging the given geological setting with fluids and gases ascending in anastomosing pathways benefits from a fine-scale three-dimensional inversion approach because the fluids mostly appear as local conductive reservoir-like anomalies, which can be falsely projected onto the profiles during inversion in two dimensions. The resistivity models contribute with detailed images of the near-surface aquifers to the geodynamic model of the Ohře Rift.

The Bakreswar geothermal province represents a medium enthalpy geothermal system with its Bakreswar and Tantloie hot springs. It lies within the Chotanagpur Granite Gneissic Complex in the eastern part of the Indian Peninsula. The province has a high heat flow and a high geothermal gradient of 90 degrees C/km. Magnetotelluric data from 95 sites in a frequency range of 10 kHz-10 Hz were acquired over the Bakreswar geothermal province to obtain an electrical conductivity model and map the geothermal reservoir with its fluid pathways and related geological structures. Subsurface conductivity models obtained from three-dimensional inversions of the Magnetotelluric data exhibit several prominent anomalies, which are supplemented by gravity results. The conductivity model maps three features which act as a conduit (a) a northwest-southeast trending feature, (b) an east-west trending feature to the south of the northwest-southeast trending feature (which lies 1 km north of the Oil and Natural Gas Corporation fault marked by previous studies) and (c) shallow conducting features close to Bakreswar hot spring. The northwest-southeast trending feature coincides with the boundary of the high-density intrusive block. This northwest-southeast trending feature provides the pathway for the meteoric water to reach a maximum depth of 2.7 km, where it gets heated by interacting with deep-seated structures and then it rises towards the surface. The radiogenic process occurring within the granites of Chotanagpur Granite Gneissic Complex provides the heat responsible for heating the meteoric water. The northwest-southeast and east-west trending features are responsible for the transport of meteoric water to deeper depths and then towards the shallow regions of the Earth. The near surface features close to the Bakreswar hot spring are responsible for carrying the water further towards the hot spring. The resistivity of these structures plotted as a function of salinity and temperatures for saline crustal fluids suggests the involvement of meteoric water. Further, applying Archie's law to this resistivity suggests that the conduit path has a porosity greater than 10%. This study successfully maps the anomalous structures which might foster the migration of geothermal fluid in Bakreswar geothermal province.

<p>The Radio-Magnetotelluric (RMT) method is a geophysical near-surface imaging technique with a broad range of possible applications. In 2020, the GFZ Potsdam has acquired a newly developed horizontal magnetic dipole transmitter that allows the application of the RMT method even in regions with an insufficient coverage of radio transmitters which normally serve as source signal. First controlled-source RMT measurements were conducted at three different locations in Chile in 2020. Further measurements were recently conducted in Ireland.  As we are able to store the raw time series, we have full control over the subsequent data processing. The processing tools at GFZ include the modular processing suite EMERALD, which was originally designed for MT processing, but has recently been adapted to process RMT data. One main difference is that in RMT the transmitter data is considered as signal, while in natural source MT this would be regarded as electromagnetic noise that needs to be removed using automated robust statistical approaches. However, processing the entire time series in an automated manner has a large drawback: The different emitted frequencies are transmitted in a sweep implying that only a smaller fraction of the time series contains the required signal for a particular target frequency and leading to an unfavourable signal-to-noise ratio. Since it is technically impossible to have the same time base for the data logger and the transmitter with an accuracy of a few nanoseconds, an automated detection scheme is required to find time segments that contain the transmitter signal. Usually, several Gigabytes of raw time series are collected during field measurements, making manual editing and supervision of the time series virtually impossible. However, a careful selection of appropriate time segments is essential for the success of the data processing. To address the challenge, machine learning algorithms have a high potential to solve both problems. Initial experience was gained with a recurrent neural network approach in order to identify suitable time segments (Patzer & Weckmann, EMTF 2021 – conference contribution and personal communication). However, many questions remained open, e.g. if other machine learning algorithms can result in better performances, which machine learning algorithms are in principle suitable for the characteristics and properties of RMT time series and which parameters should be used as input variables (features) for the algorithms. A large number of machine learning algorithms exist, which can be divided into different groups according to their operating principle and their activity fields. We will test unsupervised methods, especially for clustering the data, to identify a set of suitable input variables. Subsequently, we will use these features to train supervised algorithms as logistic regression, support vector machine and different kinds of neural networks to find the best performing algorithm. We will mainly use the RMT data from Chile within the training process. Furthermore, we will test if the trained algorithm is applicable to other new data sets measured at different locations (e.g. Ireland) and/or with different equipment.</p>

The 100km wide Mérida Andes extend from the Colombian/Venezuelan border to the Coastal Cordillera. The mountain chain and its associated major strike-slip fault systems in western Venezuela formed due to oblique convergence of the Caribbean with the South American Plates and the north-eastwards expulsion of the North Andean Block. Due to the limited knowledge of lithospheric structures related to the formation of the Mérida Andes research projects have been developed to illuminate this zone with deep geophysical data. In this study, we present three-dimensional inversion of broadband magnetotelluric data, collected along a 240km long profile crossing the Mérida Andes and the Maracaibo and Barinas–Apure foreland basins. The distribution of the stations limits resolution of the model to off-profile features. Combining 3D inversion of synthetic data sets derived from 3D modelling with 3D inversion of measured data, we could derive a 10 to 15km wide corridor with good lateral resolution to develop hypotheses about the origin of deep-reaching anomalies of high electrical conductivity. The Mérida Andes appear generally as electrically resistive structures, separated by anomalies associated with the most important fault systems of the region, the Boconó and Valera faults. Sensitivity tests suggest that the Valera Fault reaches to depths of up to 12km and the Boconó Fault to more than 35km depth. Both structures are connected to a sizeable conductor located east of the profile at 12–15km depth. We propose that the high conductivity associated with this off-profile conductor may be related to the detachment of the Trujillo Block. We also identified a conductive zone that correlates spatially with the location of a gravity low, possibly representing a SE tilt of the Maracaibo Triangular Block under the mountain chain to great depths (>30km). The relevance of these tectonic blocks in our models at crustal depths seems to be consistent with proposed theories that describe the geodynamics of western Venezuela as dominated by floating blocks or orogens. Our results stress the importance of the Trujillo Block for the current tectonic evolution of western Venezuela and confirm the relevance of the Boconó Fault carrying deformation to the lower crust and upper mantle. The Barinas–Apure and the Maracaibo sedimentary basins are imaged as electrically conductive with depths of 4 to 5km and 5 to 10km, respectively. The Barinas–Apure basin is imaged as a simple 1D structure, in contrast to the Maracaibo Basin, where a series of conductive and resistive bodies could be related to active deformation causing the juxtaposition of older geological formations and younger basin sediments.

The Bohemian Massif is part of the geodynamically active European Cenozoic Rift System and represents its easternmost termination. The study area is situated at the junction of three different Variscan tectonic units and hosts beside the Eger Rift a series of different fault systems. The entire region is characterized by ongoing magmatic processes in the intra-continental lithospheric mantle expressed by, e.g., the occurrence of repeated earthquake swarms, the presence of Quaternary volcanoes, and massive degassing of mantle-derived CO2 in mineral springs, mofettes as well as. Several geoscientific studies suggest that fluid circulation along the deep-reaching faults seems to play an important role in explaining the underlying geodynamic processes. As part of an ICDP drilling programme, we applied the Magnetotelluric (MT) method with the goal to contribute to the understanding of the physical and chemical processes and interaction that led to the magma and fluid transport by mapping potential fluid pathways from the crust-mantle boundary up to the surface. Here, we present 3D inversion models of two different overlapping regional and one local MT experiments located in the Cheb basin close to the Hartoušov mofette field. The most prominent large-scale conductivity features of the regional models are several channels from the lower crust to the surface, possibly representing pathways for fluids into the earthquake swarm region, mofette fields, and known spas. However, such a conductive channel is absent in the local model beneath the surface expression of the mofettes. We will test two different hypotheses, namely a vertical ascending channel versus lateral fluid migration. Results from synthetic modelling studies and available geoscientific constraints hint that such a channel might exist directly beneath the mofette field, but due to the given data quality, station distribution, and the subsurface conductivity structure within a conductive sediment basin, it might be challenging to resolve.

The West Bohemian Massif represents the easternmost part of the geo-dynamically active European Cenozoic Rift System. This region hosts different tectonic units, the NE-SW trending Eger Rift, the Cheb Basin and a multitude of different faults systems. Furthermore, the entire region is characterised by ongoing magmatic processes in the intra-continental lithospheric mantle. These processes take place in absence of active volcanism at surface, but are expressed by a series of phenomena, including e.g. the occurrence of repeated earthquake swarms and massive degassing of CO2 in the form of mineral springs and mofettes. Active tectonics is mainly manifested by Cenozoic volcanism represented by different Quaternary volcanic structures e.g. the Eisenbühl, the Kammerbühl and different maars. All these phenomena make the Eger Rift a unique target area for European intra-continental geo-scientific research. Therefore, an interdisciplinary drilling programme advancing the field of earthquake-fluid-rock-biosphere interaction was funded within the scope of the ICDP. Magnetotelluric (MT) measurements are applied to image the subsurface distribution of the electrical conductivity from shallow surface down to depths of several tens of kilometres. The electrical conductivity is a physical parameter that is particularly sensitive to the presence of high-conductive phases such as aqueous fluids, partial melts or metallic compounds. First MT measurements within this ICDP project were carried out in winter 2015/2016 along two 50 km long perpendicular profiles with 30 stations each and a denser grid of 97 stations close to the mofettes with an extension of 10 x 5 km2. Muñoz et al. (2018) presented 2D images along the NS profile of one regional profile. They reveal a conductive channel at the earthquake swarm region that extends from the lower crust to the surface forming a pathway for fluids into the region of the mofettes. A second conductive channel is present in the south of the model. Due to the given station setup, the resulting 2D inversion allows ambiguous interpretations of this feature. 3D MT data and inversions are required to distinguish between different scenarios and to fully describe the 3D structure of the subsurface. Therefore, we conducted a large MT field experiment in autumn 2018 by extending the study area towards the south. Broad-band MT data were measured at 83 stations along three 50-75 km long profiles and some additional stations across the region of the maars, the Tachov fault and the suture zone allowing for 2D as well as 3D inversion on a crustal scale. To improve the data quality, advanced data processing techniques were applied leading to good quality transfer functions. Furthermore, the previously collected MT data were reprocessed using the new approaches. This entire MT data set across the Eger Rift environment together with old MT data collected within the framework of the site characterisation in the surrounding of the KTB drilling are used to compute 3D resistivity models of the subsurface, with combining different transfer functions. These 3D inversion results will be introduced and discussed with regard to existing geological hypotheses.

The weathering front is the boundary beneath Earth’s surface where pristine rock is converted into weathered rock. It is the base of the “critical zone”, in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow.

The Mérida Andes are a 100 km wide mountain chain that extends from the Colombian/Venezuelan border to the Caribbean coast. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Uplift of the mountains started in the late Miocene due to oblique convergence of the Caribbean and South American tectonic plates and the north-eastwards expulsion of the North Andean Block (NAB). This tectonic interaction fostered major strike-slip fault systems, with associated high seismicity, and the partitioning of the North Andean Block into smaller tectonic units, whose interaction accelerated the uplift of the Mérida Andes since the Plio-Pleistocene.We present the three-dimensional inversion results of broadband magnetotelluric (MT) data from 72 sites gathered along a 240 km long profile across the central part of the MA, the Maracaibo (MB), and Barinas-Apure (BAB) foreland basins. Directionality and dimensionality analyses suggested 3D structures for the MA section, with the induction vectors indicating off-profile structures, particularly at long periods. Since the distribution of sites predominantly along a single profile can have adverse effects on the outcome of the 3D inversion, we rigorously tested all model features for robustness and excluded artefacts.One of the main findings is a deep connection (> 10km) between the most prominent faults of the MA, the Valera and Boconó fault systems, with a deep off-profile conductor to the east of our profile. We interpret this conductive structure as a detachment surface of the Trujillo Block, which is part of the NAB and whose expulsion to the NE significantly influences the present-day geodynamic evolution of western Venezuela. A conductive zone under the Maracaibo Basin correlates spatially with the location of a Bouguer low. Both geophysical anomalies may be caused by a SE tilt of the Maracaibo Triangular Block under the Mérida Andes, bound by the north-western thrust system which could reach depths of 30 km.

The Bohemian Massif represents the easternmost part of the geodynamically active European Cenozoic Rift System. This region hosts the contact between three tectonic units of the Variscan Belt, the NE-SW trending Eger Rift and the NNW-SSE striking Marianské Lázne fault. It is characterised by ongoing magmatic processes in the intra-continental lithospheric mantle, repeated earthquake swarms, extensive CO2 degassing in mineral springs and mofettes and the presence of Quaternary volcanoes. While the ICDP drilling programme utilizes information gathered within shallow boreholes in the region, we applied the Magnetotelluric (MT) method to obtain site characterizations in the vicinity of the proposed drill sites. The electrical conductivity has proven to be an important parameter to image the above-mentioned tectonic from the lower crust to the shallow subsurface as well as on a regional and a local scale. Here, we present 2D and 3D inversion models of different MT and Radio-MT (RMT) experiments to study e.g. the Hartouŝov mofette fields, the Quaternary scoria cones, the regional faults and their interplay. Thereby the experiments were designed that we can use lower frequency data from MT to support shallow 3D inversions of e.g. the scoria cones in the regions. The most prominent large-scale conductivity features map channels from the lower crust to the surface possibly forming pathways for fluids into the region of earthquake swarms, mofette fields and know spas. However, the locations of the scoria cones seem to be bound to regional fault zones.

Abstract The geodynamic settings of north-western Venezuela are conditioned by the expulsion of the North Andean Block (NAB), and the deformational structures related to the Caribbean – South America plate interactions. The most prominent feature is the 100 km wide Mérida Andes (MA) that reach from the Colombian border to the Caribbean coast over more than 500 km. We present the analysis of a magnetotelluric (MT) profile acquired at 72 broadband sites along a 240 km long profile with station spacing between 3 and 5 km across the central part of the MA, including the sedimentary Maracaibo (MB) and Barinas-Apure (BAB) basins. Phase Tensors (PT) are consistent with 1D/2D dimensionality above the MB and the BAB, however, stations above the MA show a clear 3D dimensionality, with the induction vectors indicating the presence of off-profile structures. Following the dimensionality analysis, 3D inversions of the entire dataset were performed, employing the finite differences code ModEM. The results are in agreement with prominent geological structures, and are particularly effective in modelling the depth extensions of major fault systems. A conductive structure east from the profile at mid crustal levels seems to correlate with the tectonic escape of the Trujillo block, which is part of the NAB.

Abstract The geodynamic settings of north-western Venezuela are conditioned by the expulsion of the North Andean Block (NAB), and the deformational structures related to the Caribbean – South America plate interactions. The most prominent feature is the 100 km wide Mérida Andes (MA) that reach from the Colombian border to the Caribbean coast over more than 500 km. We present the analysis of a magnetotelluric (MT) profile acquired at 72 broadband sites along a 240 km long profile with station spacing between 3 and 5 km across the central part of the MA, including the sedimentary Maracaibo (MB) and Barinas-Apure (BAB) basins. Phase Tensors (PT) are consistent with 1D/2D dimensionality above the MB and the BAB, however, stations above the MA show a clear 3D dimensionality, with the induction vectors indicating the presence of off-profile structures. Following the dimensionality analysis, 3D inversions of the entire dataset were performed, employing the finite differences code ModEM. The results are in agreement with prominent geological structures, and are particularly effective in modelling the depth extensions of major fault systems. A conductive structure east from the profile at mid crustal levels seems to correlate with the tectonic escape of the Trujillo block, which is part of the NAB.

The Caribbean and South American tectonic plates bound the north-eastwards expulsion of the North Andean Block in western Venezuela. This complex geodynamic setting resulted in the formation of major strike-slip fault systems and sizeable mountain chains. The 100-km-wide Mérida Andes extend from the Colombian/Venezuelan border to the Caribbean coast. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Knowledge of lithospheric structures, related to the formation of the Mérida Andes, is limited though, due to a lack of deep geophysical data. In this study, we present results of the first broad-band magnetotelluric profile crossing the Mérida Andes and the Maracaibo and Barinas–Apure foreland basins on a length of 240 km. Geoelectrical strike and dimensionality analysis are consistent with 1-D or 2-D subsurface structures for the sedimentary basins but also indicate a strong 3-D setting for the Mérida Andes. Using a combination of 2-D and 3-D modelling we systematically examined the influence of 3-D structures on 2-D inversions. Synthetic data sets derived from 3-D modelling allow identification and quantification of spurious off-profile features as well as smoothing artefact due to limited areal station coverage of data collected along a profile. The 2-D inversion models show electrically conductive basins with depths of 2–5 km for the Barinas-Apure and 2–7 km for the Maracaibo basins. A number of resistive bodies within the Maracaibo basin could be related to active deformation causing juxtaposition of older geological formations and younger basin sediments. The most important fault systems of the area, the Boconó and Valera Faults, cross-cut the Mérida Andes in NE–SW direction along its strike on a length 400 km and N–S direction at its centre on a length 60 km, respectively. Both faults are associated with subvertical zones of high electrical conductivity and sensitivity tests suggest that they reach depths of up to 12 km. A sizeable conductor at 50 km depth, which appears consistently in the 2-D sections, could be identified as an inversion artefact caused by a conductor east of the profile. We speculate the high conductivity associated with the off-profile conductor may be related to the detachment of the Trujillo Block. Our results partially support the ‘floating orogen hypothesis’ developed to explain the geodynamic evolution of western Venezuela and they highlight the relevance of the Trujillo Block in this process.

As part of the Bohemian Massif, the Cheb Basin is one of the most active areas of the European Cenozoic Rift System. Separated from the ENE-WSW striking Eger Rift to the west by the morphological prominent Mariánské Lázne Fault Zone (MLF), the basin shows presently no active volcanism at the surface. Nonetheless it is characterized by degassing of mantle derived CO2 in mofettes and mineral springs and by repeated occurrences of swarm earthquakes along the Pocátky-Plesná Zone (PPZ) and MLF near Nový Kostel. All these activities are vivid signs of ongoing magmatic processes in the lithospheric mantle. Over the last 15 years four potential maar diatreme structures were discovered and join the two known scoria cones Komorní hurka and Zelezná hurka in the western part of the Cheb Basin. Unlike scoria cones there are no prominent morphological indications for maar diatreme structures, why only modern approaches in remote sensing and systematic gravimetrical surveys led to the discovery of the Mýtina Maar in 2007 (Mrlina et. al., 2007), the Neualbenreuth Maar in 2017 (Rohrmüller et. al., 2017) and recently the two potentials Ztracený rybník maars close to Libá (Hosek et. al., 2019; Mrlina et. al. 2019). All these quaternary volcanic structures are located very close along the Tachov Fault Zone (TFZ), one of the major NNW-SSE striking fault zones of the Bohemian Massif. Maar volcanoes were formed when rising magma interacts explosively with groundwater. Advancing explosions left a cone-shaped diatreme that has been filled with post-eruptive sediments which could conduce as a climate archive for the last 300.000 years in central Europe. An interdisciplinary Project "Drilling the Eger Rift" within the International Continental Scientific Drilling Program (ICDP) targets the interactions between fluids, deep biosphere, CO2 degassing and earthquake activity to shed light on the tectonic structure and related geodynamic processes. As a part of this project, Radio-Magnetotelluric (RMT) measurements were applied to image the near-surface electrical conductivity structure of these maar volcanoes. From May 2018 on, we conducted field experiments encompassing six 500 m RMT profiles across the Neualbenreuth maar, three 700 m profiles across Mýtina Maar and finally eight 400 - 1200 m long profiles over both Ztracený rybník maars. Compared with geo-electric resistivity tomography (ERT), our RMT measurements are more sensitive to conductors such as fluids or metallic compounds and were done with an areal coverage for 3D inversion and interpretation. With advanced and statistically robust data processing techniques typically applied to MT data resulted in impedance tensors in a period range of 10 kHz to 250 kHz. This RMT data sets are then modelled using inversion. The resulting 3D electrical conductivity models across the maar diatreme structures show distinct contrasts between the resistive rocks of the diatreme and the rather conductive post-eruptive sediments. The inversion results will be compared and discussed, in particular regarding a position for a potential core drilling in one of the maar structures.

Geophysical ProspectingVolume 68, Issue 1 p. 3-6 Introduction Introduction to the special issue on "Cost-effective and innovative mineral exploration solutions" Alireza Malehmir, Corresponding Author Alireza Malehmir alireza.malehmir@geo.uu.se Department of Earth Sciences, Uppsala University, Uppsala, SwedenE-mail: alireza.malehmir@geo.uu.seSearch for more papers by this authorMusa Manzi, Musa Manzi University of the Witwatersrand, School of Geosciences, Johannesburg, South AfricaSearch for more papers by this authorDeyan Draganov, Deyan Draganov Department of Geoscience and Engineering, TU Delft, Delft, The NetherlandsSearch for more papers by this authorUte Weckmann, Ute Weckmann GFZ German Research Centre for Geosciences, Potsdam, GermanySearch for more papers by this authorEsben Auken, Esben Auken Department of Earth Sciences, Aarhus University, Aarhus, DenmarkSearch for more papers by this author Alireza Malehmir, Corresponding Author Alireza Malehmir alireza.malehmir@geo.uu.se Department of Earth Sciences, Uppsala University, Uppsala, SwedenE-mail: alireza.malehmir@geo.uu.seSearch for more papers by this authorMusa Manzi, Musa Manzi University of the Witwatersrand, School of Geosciences, Johannesburg, South AfricaSearch for more papers by this authorDeyan Draganov, Deyan Draganov Department of Geoscience and Engineering, TU Delft, Delft, The NetherlandsSearch for more papers by this authorUte Weckmann, Ute Weckmann GFZ German Research Centre for Geosciences, Potsdam, GermanySearch for more papers by this authorEsben Auken, Esben Auken Department of Earth Sciences, Aarhus University, Aarhus, DenmarkSearch for more papers by this author First published: 30 December 2019 https://doi.org/10.1111/1365-2478.12915Citations: 6Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume68, Issue1Special Issue: Cost-Effective and Innovative Mineral Exploration SolutionsJanuary 2020Pages 3-6 RelatedInformation

The Grunehogna Craton (GC) in the Atlantic sector of East Antarctica is situated in western Dronning Maud Land between approximately 2 and 15°W. The GC was part of or adjacent to the Archaean Kalahari Craton of southern Africa and became attached to Antarctica during the Jurassic breakup of Gondwana. We report on magnetotelluric (MT) measurements from 20 sites in the vicinity of Neumayer III research station, which is located on the Ekstrom Ice Shelf (EIS) at the eastern coast of the Weddell Sea. The MT data was collected using broad band instruments with an average site spacing of 5 km. Contrary to sea ice, ice shelves originate from a glacier or ice sheet that flows down to a coastline and onto the ocean surface. Therefore ice shelves are firmly attached to the land masses and the EIS overlays in parts the GC. The MT data, particularly the inductions vectors, clearly mark the transition from areas which are underlain by sea water and where the ice is directly grounded on the craton. The MT sites located on the GC show generally high resistivity, which can be expected for Archean rocks.

We present a model of the electrical resistivity structure of the lithosphere in the Central Andes between 20 degrees and 24 degrees S from 3-D inversion of 56 long-period magnetotelluric sites. Our model shows a complex resistivity structure with significant variability parallel and perpendicular to the trench direction. The continental forearc is characterized mainly by high electrical resistivity (>1,000m), suggesting overall low volumes of fluids. However, low resistivity zones (LRZs, <5m) were found in the continental forearc below areas where major trench-parallel faults systems intersect NW-SE transverse faults. Forearc LRZs indicate circulation and accumulation of fluids in highly permeable fault zones. The continental crust along the arc shows three distinctive resistivity domains, which coincide with segmentation in the distribution of volcanoes. The northern domain (20 degrees-20.5 degrees S) is characterized by resistivities >1,000m and the absence of active volcanism, suggesting the presence of a low-permeability block in the continental crust. The central domain (20.5 degrees-23 degrees S) exhibits a number of LRZs at varying depths, indicating different levels of a magmatic plumbing system. The southern domain (23 degrees-24 degrees S) is characterized by resistivities >1,000m, suggesting the absence of large magma reservoirs below the volcanic chain at crustal depths. Magma reservoirs located below the base of the crust or in the backarc may fed active volcanism in the southern domain. In the subcontinental mantle, the model exhibits LRZs in the forearc mantle wedge and above clusters of intermediate-depth seismicity, likely related to fluids produced by serpentinization of the mantle and eclogitization of the slab, respectively.