The tectonic-dynamic mechanisms that formed N-S-directed rifts in Southern Tibet remain unknown. Their formation is believed to be related to the subduction pattern of the Indian Plate; however, this pattern remains highly debated. For example, different studies have proposed various ideas about where a slab tear might be located. We created lithospheric-scale 3-D electrical resistivity model along a line that crosses the Xainza-Dinggye rift and Yadong-Gulu rift. The results reveal crustal low-resistivity zones beneath the rifts and high-resistivity zones adjacent to them. The low-resistivity zones are interpreted to indicate a weak middle-lower crust caused by the upwelling of mantle-derived materials, and leading to deformation of the crust, thereby contributing to the development of the rifts. High-resistivity zones between the rifts may be ancient crust in the central Lhasa terrane. In the southern region of the high-resistivity zones it is speculated that, due to break-off of the frontier of the eclogitized Indian lower crust, the Indian lithospheric mantle delaminated from the Indian crust and rolled back, causing steep-angled subduction. The results are interpreted to indicate that, because of the roll-back of the Indian Plate at depth in the regions between the two adjacent rifts, the subduction angle in these regions is steeper than that on both sides. The findings of our study offer an alternative to the traditional view of the subduction pattern of the Indian Plate in southern Tibet, and are in contrast to previous models that suggested smooth and monotonic variations from west to east. ... Read more

Reservoirs of interest for resource exploration, including geothermal and hydrocarbon reservoirs, commonly have an impermeable cap, which traps fluids below. Identifying this boundary is important for resource development. The cap rock for hydrocarbon reservoirs in southwest Iran contains evaporites and thus some geophysical exploration methods, specifically seismic reflection, have faced problems recovering subsurface information in this environment. As an alternative, we generate an electrical resistivity model from magnetotelluric (MT) data. Furthermore, we consider three-dimensional triaxial electrical anisotropy, which is rarely done. The study objectives are to a) define and map the boundary between the cap rock and the principal reservoir, b) characterize geological and tectonic formations in the area, and c) analyze the tectonic factors influencing the evolution of the region. A total of 359 MT measurements were acquired across the Sarab field in an array consisting of five profiles separated by >2000 m with a measurement spacing of >200 m. Transient electromagnetic (TEM) measurements were co-located with the MT measurements at 181 locations and used to correct for static shifts. Isotropic and anisotropic inversions of the MT data were performed, using all impedance tensor elements. The anisotropic electrical resistivity model exhibits both a significantly better alignment with the depths of geological formations known from drilling data and a better fit to the data. Therefore, the boundary between the primary cap rock and principal reservoir, the Gachsaran and Asmari formations, is defined and mapped across the survey area. In addition, major tectonic and fault-related features in the region are identified. ... Read more

The formation of large and superlarge mineral deposits results from the accumulation of significant quantities of mineralized materials, driven by multistage geological processes. However, the complete migration pathway of orogenic gold systems—ranging from deep magmatic sources to crustal mineralization—remains elusive. Here, we integrate multiscale electrical resistivity models with lamprophyre isotopic analyses to establish a coherent metallogenic framework for the Daping gold deposit in the southeastern margin of the Tibetan Plateau. High-resolution magnetotelluric data reveal interconnected low-resistivity anomalies, delineating a continuous, translithospheric pathway. These vertically aligned anomalies represent (1) metasomatized mantle domains enriched in volatiles and Au-bearing melts; (2) transcrustal magmatic-hydrothermal channels facilitating melt ascent; and (3) shallow fluid exsolution loci controlling ore precipitation. Geodynamic simulations further suggest that migration efficiency is influenced by the overlying lithospheric structure—specifically, crustal-scale fault networks channel the metal-rich fluids, while rheological contrasts around the lithosphere-asthenosphere boundary regulate the migration pathway of melt. This study, by unveiling the multilevel structural framework for mineralizing fluid migration for the first time, establishes a unified model that structurally connects mantle reservoirs with near-surficial gold deposits and systematically integrates isotopic signature, geophysical evidence, and translithospheric fluid pathways. Recognizing such a multilevel lithospheric control system not only bridges the critical observational gap between mantle sources and surface mineralization but also offers a predictive framework for gold exploration. ... Read more

The Tibetan Plateau is undergoing east-west extension manifested by north-trending rifts. Rift dynamics have been attributed to both mantle convection, which induces vertical motion causing general extension, and plate convergence, with northward motion causing along-strike extension, driven by the subducted Indian slab. However, the cause of lithospheric extension remains debated. We carried out electrical resistivity modeling of the entire Tibetan Plateau and present a quantitative interpretation of low-resistivity structures in terms of high fluid fraction and low viscosity. The model reveals low-resistivity features intruding and overlying the resistive lithosphere of Lhasa and Qiangtang. The low-resistivity features show a transition from vertically oriented to horizontally oriented positions at ∼50−70 km depth and appear to be oriented north-south below the Himalaya and Lhasa and east-west below Qiangtang. The anomalies can be explained by partial melts and fluids and may represent the signatures of material migration and locally weakened lithosphere. This material migration must have been significant enough to sustain rifting and drive the rift tips northward, despite the complex tectonic setting of the Tibetan Plateau, which is composed of a number of independent blocks. The results suggest that north-trending rifts were formed in response to fluid flow, after or during lithospheric foundering below Lhasa. Furthermore, fluid flow can explain the surface distribution of rifts in bands and the variations in rift formation and development between Qiangtang and Lhasa, which are attributed to the local rheological differences and specific regimes of vertical and/or horizontal stresses that are induced by fluid migration. ... Read more

The capital of Mongolia is Ulaanbaatar (UB), which is situated in the central region of the country. Over the past few decades, the city has expanded and developed, establishing itself as the most extensively developed metropolis in Mongolia regarding infrastructure and commerce. While this impact has resulted in development for the nation, it has also led to environmental and social concerns, including traffic congestion and air pollution. The resolution of these issues necessitates a more comprehensive understanding of the geological formation of the region, which can be achieved through the sustainable development of renewable energy and road construction. UB is situated at the confluence of the Tuul River and is enveloped by mountains significantly higher than the surrounding terrain. One is the Bogd Uul intrusive/plutonic granite, situated south of Ulaanbaatar. It spans a 200 km2 area and is believed to have been formed during the Late Triassic (Khishigsuren et al., 2006, 2009) to the early Jurassic period, with an age of 208 Ma. One of the strategies to mitigate the challenges previously identified is to drill Bogd Uul for the purpose of constructing the road. ... Read more

Since the Cenozoic, a series of extensional south-north normal faults and gneiss-granite domes evolved in the southern Tibet Plateau, the formation mechanism of which is of scientific interest and which has implications for the tectonic dynamics of the plateau. Typical of such features are the Xainza-Dinggye rift and the Mabja gneiss dome, which are located in the Xainza-Xietongmen-Dinggye region in central Tibet. In this study, Magnetotelluric measurements across this region are used to generate a high-resolution 3-D electrical resistivity model of the subsurface and to analyze the cause of the conductive zones. The large-scale conductive zones identified in the middle-lower crust may result from aqueous melt partial melting, whereas the smaller-scale conductive zones in the upper-middle crust may result from saline fluids, possibly with varying minor volumes of melts. Subsequently, based on the electrical resistivity model, and combined with the spatiotemporal coupling of the geological, geochemical and geophysical data, the state and migration features of crustal materials are discussed. The results show that the upwelling of mantle materials along subduction channels and slab-windows related to the tearing of the Indian lithospheric plate contributed to the partial melting of the middle-lower crust in the Lhasa terrane. Furthermore, partial melting of the upper-middle crust in the Tethys-Himalaya terrane resulted from southern extrusion of crustal materials in the Lhasa terrane. These two mechanisms can significantly reduce the effective viscosity. We speculate that the deformation of the brittle upper crust that is controlled by large-scale ductile layers characterized by weak rheology is the main dynamic mechanism of rift evolution. Meanwhile, the metamorphism and anatexis in the upper-middle crust of the Tethys-Himalaya terrane related to the southern extrusion of materials contributed to the evolution of the Mabja gneiss dome. During the middle Miocene, the southern extrusion of crustal materials may have been influenced by the cooling events beneath the Mabja gneiss dome, which can explain why the deep areas beneath the Mabja gneiss dome have middle-high resistivity. In addition, our study region is located in the Mediterranean-Himalayan seismic belt, and mainly includes shallow-focus earthquakes and intermediate-depth earthquakes. In the north, shallow-focus earthquakes are mainly controlled by the accumulation of stress in the brittle layer of the overlying crust related to the ductile layer of the middle and lower crust. In the south, shallow-focus earthquakes (e.g., Dingri Ms 6.8 earthquake) mainly occur in the rigid, resistive block, which is surrounded by conductive zones, possibly because fluid migration may be hindered by these resistive blocks. The intermediate-depth earthquakes are mainly related to the detachment of the Indian lithospheric mantle from the Indian crust. ... Read more

Since the Cenozoic, a series of extensional south-north normal faults and gneiss-granite domes evolved in the southern Tibetan Plateau, the formation mechanism of which is of scientific interest and which has implications for the tectonic dynamics of the plateau. Typical of such features are the Xainza-Dinggye rift and the Mabja gneiss dome, which are located in the Xainza-Xietongmen-Dinggye region in the central Tibetan Plateau. In this study, Magnetotelluric measurements across this region are used to generate a high-resolution 3-D electrical resistivity model of the subsurface and to analyze the cause of the conductive zones. The large-scale conductive zones identified in the middle-lower crust may result from aqueous melt partial melting, whereas the smaller-scale conductive zones in the upper-middle crust may result from saline fluids, possibly with varying minor volumes of melts. Subsequently, based on the electrical resistivity model and combined with the spatiotemporal coupling of the geological, geochemical, and geophysical data, the state and migration features of crustal materials are discussed. The results show that the upwelling of mantle materials along subduction channels and slab-windows related to the tearing of the Indian lithospheric plate contributed to the partial melting of the middle-lower crust in the Lhasa terrane. Furthermore, partial melting of the upper-middle crust in the Tethys-Himalaya terrane resulted from southern extrusion of crustal materials in the Lhasa terrane. These two mechanisms can significantly reduce the effective viscosity. We speculate that the deformation of the brittle upper crust that is controlled by large-scale ductile layers characterized by weak rheology is the main dynamic mechanism of rift evolution. Meanwhile, the metamorphism and anatexis in the upper-middle crust of the Tethys-Himalaya terrane related to the southern extrusion of materials contributed to the evolution of the Mabja gneiss dome. During the middle Miocene, the southern extrusion of crustal materials may have been influenced by the cooling events beneath the Mabja gneiss dome, which can explain why the deep areas beneath the Mabja gneiss dome have middle-high resistivity. In addition, our study region is located in the Mediterranean-Himalayan seismic belt, and mainly includes shallow-focus earthquakes and intermediate-depth earthquakes. In the north, shallow-focus earthquakes are mainly controlled by the accumulation of stress in the brittle layer of the overlying crust related to the ductile layer of the middle and lower crust. In the south, shallow-focus earthquakes (e.g., Dingri MS6.8 earthquake) mainly occur in the rigid, resistive block, which is surrounded by conductive zones, possibly because fluid migration may be hindered by these resistive blocks. The intermediate-depth earthquakes are mainly controlled by the response in the subsurface area, which is related to the detachment of the Indian lithospheric mantle from the Indian crust. ... Read more

Salt diapirs are of interest due to their unique properties that make them ideal for secure, long-term subsurface storage, including for CO ₂, natural gas, and radioactive waste. However, their utilization requires an understanding of their structure, which can be achieved with geophysical imaging. It is often a challenge to delineate salt diapirs with seismic reflection methods; therefore, we employ electromagnetic methods. We aim to a) highlight how magnetotellurics can identify the subsurface structure of salt diapirs, b) characterize the key tectonic structures and stratigraphic layers in the area, and c) investigate the role of faults on the distribution of diapirs. To do this we analyze an array of 253 magnetotelluric measurements and generate electrical resistivity models. The study area lies in the Shurab region, Central Iran, where numerous salt diapirs are observed near the surface. Overall, the models show a deformed southwestern zone and an undisturbed northeastern zone. Throughout the area, a thin (∼100 m) surface layer (1–100 Ωm) is underlain by a thick (up to 1000 m) low resistivity (<1 Ωm) layer, interpreted to be sediments of the Upper Red Formation. Below this is a higher resistivity (3–30 Ωm) layer that is complex and variable in depth and thickness, particularly in the southwest, where it shallows. This corresponds to the Lower Red Formation, which is the main salt layer and encompasses the diapirs. The electrical resistivity models successfully determine the locations, boundaries, and depths of salt diapirs within the area. Furthermore, they reveal that the salt diapirs are laterally extended along fault zones. This result provides valuable insights into the area's tectonic evolution and structural framework. Based on these subsurface images and geological information, we conclude that the tectonic activity along the Sen-Sen, Ab-Shirin, and Dehnar faults had a primary role in the formation of the salt diapirs. ... Read more

Mogod Soum, Bulgan aimag, is located in the eastern part of Khangai Dome. During the winter, the soum is heavily affected by air pollution due to coal burning. Using geothermal resources in the region, manifested by hot springs, could dramatically reduce air pollution. To understand the nature of the geothermal reservoir feeding the hot springs, we conducted Magnetotelluric surveys in the Mogod hot spring region during the fieldwork in 2020, 2021, 2022 and 2024. To obtain a subsurface electrical conductivity model of the hot spring area with magnetotellurics, we inverted data from 60 unique sites. As a tool for inversion, we used a high-order finite element code available to locally refined unstructured meshes to ensure numerical accuracy with a sufficiently fine discretization of the inversion domain while keeping the computational cost feasible. We inverted the full impedance tensor to recover a 3-D electrical conductivity model. The best-fitting model provides important new insights into the subsurface structure of the Mogod region. ... Read more

The Qitianling pluton in southern Hunan, China, has spatially and genetically influenced the formation and distribution of a series of polymetallic deposits, including Xintianling, Baoshan, Huangshaping, and Furong. These deposits host a variety of tungsten- and tin-related deposits, often regarded as strategic and critical rare metals, and comprise one of the most prominent reserves globally. A thorough understanding of the structure of the Qitianling pluton is essential for insights into the development and evolution of the metallogenic system in southern Hunan. Working towards the goal of investigating regional structural features and magma emplacements model, we have generated three-dimensional (3-D) electrical resistivity models of the Qitianling pluton and its surrounding areas to upper-crustal depth using magnetotelluric (MT) data that range from 1000 Hz to 0.001 Hz. The results reveal that the upper-crust of southern Hunan is mainly characterized by high resistivity with multiple unique conductive zones. The high-resistivity anomalies (>1000 Ω·m) are interpreted to represent the Qitianling pluton. In addition, they correspond very well to a negative residual Bouguer gravity anomaly. Moreover, the morphology of the feature aligns with low-velocity obtained by modelling reflected seismic waves. Conductive anomalies (<30 Ω·m) near the sides of the pluton that extend through the upper crust likely indicate the presence of the Chenzhou-Linwu deep-seated fault system, which may have served as a pathway for the upward migration and emplacement of magma/hydrothermal fluids. Conductive features (<30 Ω·m) beneath the Qitianling pluton are inferred to represent ancient magma reservoirs where assimilation and mixing processes occurred before magma emplacement. Based on the geophysical models and the available geological data, a multi-stage magma emplacement model of the Qitianling pluton is proposed, which provides new insights into the W-Sn polymetallic mineralization system and the regional magmatic evolution within southern Hunan. ... Read more

Volcán Uturuncu is a volcano located in the southwestern corner of Bolivia, near the borders with Chile and Argentina. It sits above the Andean subduction zone and is part of the Altiplano-Puna Volcanic Complex (APVC). Volcán Uturuncu is situated on top of the Altiplano-Puna Magma Body (APMB), which is currently the world's largest continental silicic partial melt reservoir. This reservoir is estimated to hold a total volume of 500,000 km3 of 20-30% partial melt and is located about 15 to 20 kilometers below sea level.

Volcán Uturuncu has not produced any eruption during the last 250,000 years, effectively making it an "extinct" volcano. However, the presence of active fumarole fields and the discovery of a consistent uplift pattern suggest that this volcano remains, up until this day, a dynamic system. Hence, numerous geophysical and geochemical surveys have been conducted during the past decades to understand the physical processes behind the recent unrest of this "zombie" volcano. Thay also aimed to shed light on the dynamics between the APMB and the near-surface volcanic-hydrothermal activity. Recent seismological studies worked on constraining the crustal stress distribution, by mapping the faults below Volcán Uturuncu and studying the seismic anisotropy distribution in the surrounding area. Findings from these studies reveal a complex network of fractures with a strong NW-SE-directed seismic attenuation and anisotropy, seeming to indicate the preferential pathway of fluids (Hudson et al. [2022, 2023]).

With this new information in mind, we aim to re-assess the previous electrical resistivity model of Volcán Uturuncu, which was obtained from isotropic inversion of magnetotellurics (MT) data by Comeau et al. [2016]. This model shows a pattern of low resistivity and high resistivity structures, which was interpreted as a series of magmatic dykes. However, this interpretation may overlook the inherent anisotropy of the system. Thus, we aim to generate electrical resistivity models allowing for isotropic and anisotropic zones and assess the results in the context of the newly available scientific data. We will also present preliminary results from the joint inversion of MT and gravity data. Such joint modeling allows us to delineate the density signature of the resistivity anomalies in the subsurface. This can help us in determining whether low resistivity structures represent either saline brines, partial melt or dense sulfide mineralization. ... Read more

Strong earthquakes have been mapped within the Ulaanbaatar region, Mongolia, near the capital city of Ulaanbaatar. From 1994 to 2016, 120 earthquake events were recorded between 3.4 and 5.6, and 978 earthquake events were recorded with a magnitude between 2.5 and 3.4 (Adiya, 2016; Al-Ashkar et al., 2022). Residents of Ulaanbaatar have felt several of these earthquakes.

Historical records dating back to 1905 show that Mongolia as a whole has experienced four major earthquakes with magnitudes larger than 8 and many moderate earthquakes with magnitudes larger than 5.5 (e.g., Adiya, 2016). However, the seismicity in Mongolia is mostly concentrated along the Mongolian-Altai and Gobi-Altai (south and west of the Khangai mountains), the Bulnay fault (north of the Khangai mountains), and around the Mogod area (east of the Khangai mountains) (e.g., Adiya, 2016), which are remote and sparsely populated areas. In contrast, the region around Ulaanbaatar is home to a large population; today, about 1.7 million inhabitants, or half of the country’s total population.

In the west of the Ulaanbaatar region, there are several prominent fault zones, some of which have only recently been identified. The majority of the seismic events in this region are related to three of these: the Khustai, Sharkhai, and Avdar fault zones (Adiya, 2016; Al-Ashkar et al., 2022). Seismicity is typically detected in the upper crust (above ~16 km depth; Ferry et al., 2010; Adiya, 2016). These fault zones are quasi-parallel and are ~100 km long (Figure 1). Historical seismic events are predicted to have produced vertical offsets of up to 10 m, with some sections showing cumulative horizontal offsets of up to 100 m (Al-Ashkar et al., 2022). Based on paleo-seismic surveys, it is estimated that these fault zones could produce earthquakes of magnitude 7+ (Ferry et al., 2010, 2012; Schlupp et al., 2013; Al-Ashkar et al., 2022).

These fault zones pose a serious threat and risk of damage to Ulaanbaatar. Because of this, we aim to characterize the subsurface structure of the active fault zones near Ulaanbaatar in order to better understand them. To do this, we measure magnetotelluric data and generate electrical resistivity models. We aim to give an integrated interpretation of the electrical conductivity structure of the subsurface with geomorphological and geological knowledge, in addition to geodetic measurements, paleo-seismic trenching, fault mechanical models, and near-surface ground-penetrating radar surveys. Understanding the subsurface structure of the region and characterizing the active faults is an important step for assessing seismic hazards.
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Salt diapirs are prominent geological features, formed by the piercing of buoyant salt within overlying strata, with implications for basin evolution, tectonic deformation, and resource accumulation. In this study, we investigate the Shurab salt diapirs in northwestern Central Iran—an area with five known near-surface diapirs—whose subsurface geometries and interconnections at depth remain unclear due to the complex structural settings. To address these challenges, we generated a 3D electrical resistivity model from an array of 183 magnetotelluric (MT) measurements. Phase tensor and resistivity phase tensor analyses confirmed the presence of multidimensional conductivity structures. A range of modeling tests were performed to ensure a robust result, and final models were validated against seismic data and borehole logs, as well as previous 2D electric modeling. The resulting 3D resistivity model provides new insight into the geometry, depth, and interconnectedness of the salt diapirs and superior resolution of diapir flanks compared to seismic data. High resistivity zones at shallow depths correspond to dry salt, while lower resistivity at greater depths indicates brine-saturated regions. Notably, Diapirs No. 4 and 5 were found to be interconnected at depth, sharing a root zone and likely originating from a common evaporite layer. Tectonic analysis suggests that active fault systems—including the Sen-Sen, Ab-Shirin, and Dehnar faults—have played key roles in guiding salt migration and shaping diapir structures. This study highlights the effectiveness of using MT data to image complex salt structures and underscores the importance of integrated geophysical approaches in tectonically active regions. ... Read more

The tectonic evolution of Northeast Africa, particularly the interaction between the Saharan Metacraton and the Arabian-Nubian Shield in Egypt, remains poorly understood due to the lack of deeply-penetrating geophysical data that can shed light on lithospheric structures. We present magnetotelluric data along a 700 km profile that was acquired to reveal the lithosphere's composition, thickness, and thermal state, thereby contributing to a better understanding of the tectonic evolution in the region.

The generated 3-D electrical resistivity model illustrates the resistivity distribution along the tectonic boundaries. The Nile Valley region, characterized by significant sedimentary deposits, shows a complex resistivity pattern. The upper crust is highly conductive, consistent with thick sedimentary layers, while deeper sections reveal heterogeneous resistivity indicative of tectonic reactivation and sedimentary basin evolution. The Saharan Metacraton shows as a massive resistive feature interlocated with a more conductive feature revealing the location of the cratonic remnants that still hold some of the cratonic signature. The electrical signature of the Arabian Nubian Shield shows a resistive upper crust corresponding to the predominantly crystalline and igneous rocks, such as granitoids and gneisses, which form the bulk of the shield.

Magnetic and gravity data were combined with the electrical resistivity model in a joint inversion approach to enhance the accuracy and confidence in the interpretations by cross-verifying the findings from multiple sources. The magnetotelluric survey across Northeast Africa, integrating joint inversion with magnetic and gravity data, has provided detailed insights into the lithospheric structures, revealing complex resistivity patterns indicative of tectonic reactivation and sedimentary basin evolution. ... Read more

There are indications that some long-dormant or seemingly inactive volcanoes may have potentially active magma storage systems. One such system is Ciomadul volcano, which is located at the south-eastern terminus of the Carpathian volcanic chain (Romania). With the last eruption occurring at ~30 ka, this is the youngest volcano in eastern-central Europe. Understanding the nature and structure of the magma plumbing system is crucial to elucidating the evolution of the volcano and to assessing its hazard potential. This includes the depth, size, and geometry of the magma storage region, the amount and composition of the melt present, and the link between mantle and crustal processes.

Ciomadul is situated in a geodynamically active region about 50 km from the Vrancea zone, where deep earthquakes are frequent. These earthquakes may represent the descent of a dense lithospheric slab beneath a continental collision zone and this may imply an asthenospheric upwelling due to return flow of mantle material. To the north-west of Ciomadul lies a chain of older volcanic complexes, the Călimani–Gurghiu-Harghita volcanic complex; about 40 km west of Ciomadul towards the Transylvanian Basin, a monogenetic basaltic volcanic region was developed at 1.2–0.5 Ma (Perşani volcanic field). Seismic tomography has revealed low-velocity columns through the lithosphere beneath both Ciomadul and Perşani. However, high-resolution images of the complex geometry of the system are lacking.

We report here on a 3-D electrical resistivity model of the region that was generated from 41 magnetotelluric measurements acquired in 2022 that form a 75 km by 75 km array. The data typically had reliable periods from 128 Hz to 4,100+ s. Choosing appropriate locations for measurement was critical, away from sources of cultural electromagnetic noise that can contaminate the signals, as was careful data processing, including applying data pre-selection schemes and manual time windows in addition to standard approaches using robust statistics.

Phase tensor analysis suggests that the data are 3-D at all scales. The 3-D electrical resistivity model reveals conductive anomalies (<10 ohm-m) in the subvolcanic crust. These are interpreted as melt-bearing magma reservoirs distributed in the mid-lower crust (depths of ~10–25 km) and a quasi-vertical conduit extending to the near surface. The crustal reservoir is oriented north-south, has its western margin beneath the surface vent of Ciomadul, and extends ~20 km eastward. These results are consistent with the quantitative petrological models placing the upper melt-bearing silicic crystal mush reservoir at a depth of 5–20 km beneath Ciomadul, and a magma-generation area in the asthenosphere (85–105 km depth). In contrast, no strong conductive anomaly is observed in the crust below Perşani, which fits the magma evolution model, i.e. small batches of mantle-derived magmas ascend rapidly through the crustal column. Our results suggest that Ciomadul, a seemingly inactive volcano, is still underlain by a melt-bearing magma body and therefore can be regarded as having potential for reactivation and further volcanic eruptions. ... Read more

In order to attain good quality transfer function estimates from magnetotelluric field data (i.e., smooth behavior and small uncertainties across all frequencies), we compare time series data processing with and without a multitaper approach for spectral estimation. There are several common ways to increase the reliability of the Fourier spectral estimation from experimental (noisy) data; for example to subdivide the experimental time series into segments, taper these segments (using single taper), perform the Fourier transform of the individual segments, and average the resulting spectra. To further reduce the bias of spectral estimation, a multitaper approach can be adopted. In this approach, a number of orthogonal taper functions are used to generate independent estimates, from the same time segment, which are subsequently averaged. We apply multitaper spectral analysis to magnetotelluric time series data and we show examples of responses from field data. The results clearly show that this approach improves the transfer function responses, often significantly, particularly at the long periods. ... Read more

New evidence worldwide has linked the surface locations of mineral deposits and their crustal-scale electrical conductivity footprint. We examine the relationship between the Gangdese Miocene porphyry copper deposits, Tibetan Plateau, and the electrical conductivity signature from a three-dimensional model generated from 311 magnetotelluric measurements. The distribution of electrical resistivity throughout the crust and the conductance within the mid-lower crust (depth range of 25–70 km) is analyzed. The results clearly show that the large and ultra-large Miocene porphyry copper deposits coincide spatially with conductive zones and areas of very-high conductance (>10,000S) in the mid-lower crust. Computations are undertaken to determine the influence of water-bearing silicate melts and alkali-bearing (Na+ and K+) fluids on conductivity. Based on this, the bulk conductivity is interpretated to be caused by a system of alkali-rich volatile-rich partial melt. The alkali-rich volatile-rich magmatic-hydrothermal fluids facilitate the migration and concentration of metal ions originating in deep areas. The volumes necessary are much less than partial melt alone and can thus help to reconcile large conductivity variations with small seismic velocity variations. The electrical structure indicates the magma source area of anatexis in the lower crust, a multi-stage magmatic system with large mid-crustal and small upper-crustal magma reservoirs, and complex pathways related to rift zones. We determine that the conductive zones in the mid-lower crust have an influence on the development of the mineralization and the location of the mineral belt. ... Read more

The whole-lithosphere architecture controls the genesis, evolution, and transport of ore-forming fluids. Transient tectonic and geodynamic processes, occurring at various spatial and temporal scales, control the structure of the lithosphere. However, there remains questions about the source mechanism for ore-forming fluids and their depth of genesis. Thus knowledge of the deep structural framework can advance understanding of the development and emplacement locations of mineral systems. Deep geophysical exploration studies carried out with this in mind may be crucially important for targeting new ore deposits in unexplored and underexplored regions.

As a case study, we investigate a gold-copper metal belt located at the margin of an Archean-Paleoproterozoic microcontinent in central Mongolia. We explore three-dimensional models of the electrical resistivity generated from a regional-scale array of magnetotelluric data. In addition, we examine models of shear-wave velocity throughout the lithosphere.

Directly beneath the metal belt, and the surface expressions of known mineral deposits and occurrences, the electrical resistivity model reveals narrow, vertical, finger-like low-resistivity features within the high-resistivity upper-middle crust, which are connected to a large low-resistivity zone in the lower crust. A broad low-resistivity zone is imaged in the lithospheric mantle. This is well aligned with a zone of low shear-wave velocity. We carry out a quantitative correlation analysis between electrical resistivity and shear-wave velocity and observe a close correlation within the zones of interest.

In the upper-middle crust, the low-resistivity signatures give evidence for ancient pathways of fluids below the metal belt constrained by structure along a tectonic boundary. In the lower lithosphere, the low-resistivity and low-velocity signatures are interpreted to represent a fossil fluid source region. We propose that these signatures are caused by a combination of factors. In particularly, factors related to refertilization and metasomatism of the lithospheric mantle by long-lived subduction at the craton margin, possibly including iron enrichment, F-rich phlogopite, and metallic sulfides, are analysed and discussed. ... Read more

We carried out long-term measurements of the electric potential in the laboratory and in the field and assessed the stability and temperature sensitivity of the recordings.

In the field, a robust design for long-term telluric recordings including a redundant parallel dipole so that consecutive stable time windows are more likely to be recorded was implemented in the Sauerland region of Germany (more than three months). Field testing is complicated by the fact that the system is no longer in isolation. However, the stability of the electric potential measured in the laboratory was a reasonable predictor of the stability of electric potential measured in the field. Nevertheless, instabilities in the form of spikes in the potential, steps, and spontaneous jumps (on the order of 1 mV) of unknown origin were observed.

The field measurements included a temperature-logging device. The temperature was monitored at two locations: a) the bottom-hole temperature at a depth of 80 cm below the surface, where the electrode was planted, and b) the top-hole temperature at a depth of 5 cm below the surface. The recorded temperatures in the electrode hole can be compared to the air temperature (as recorded in the nearby village). The results clearly show that planting the electrode deeper avoids the daily variations of temperature, which, in this case, were appreciable (up to 7°C), and which can affect the electric potential recordings. The bottom-hole temperature variation follows the long-term seasonal trend (e.g., 1–2°C/10 days), but is insensitive to short-term variations. Furthermore, installing electrodes at such depths can insulate them and avoid problems associated with the temperature going below the freezing point.
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In late 2022, 79 magnetotelluric (MT) measurements were acquired across the Hovsgol and Darhad region, in northern Mongolia, consisting of an array (200 km by 200 km) and several denser profiles (~10 km site spacing). Currently, little is known about the subsurface structure of the Hovsgol and Darhad region. However, it is an important region because it represents the transition from the thin lithosphere, thick crust, and high plateau of central Mongolia to the south to the thick lithosphere of the Siberian Craton to the north. The region contains three parallel, seismically-active, rift valleys, oriented approximately north-south only ~100 km south of the Siberian Craton and ~200 km west of the Baikal rift zone. This change in the style of crustal deformation from compression, as compared to central Mongolia, is very intriguing. Meanwhile, petrological studies indicate that Cenozoic magmatic activity in the region may possibly be related to that in the Hangai. Further MT measurements have been acquired across the Siberian craton west of Lake Baikal (i.e., to the north). The data will provide constraints for geodynamic modelling on the lithospheric architecture of the region, with respect to, for example, edge-driven convection in the mantle due to the edge of the Siberian Craton. In this presentation, we report on a new 3-D electrical resistivity model of the Hovsgol and Darhad region, northern Mongolia. ... Read more