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.

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.

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.

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.

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.

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.

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.

Mongolia is a region of major scientific relevancy because it is a prime example of continental intraplate surface deformation, which is poorly studied and not well understood. There are open questions regarding the tectonic evolution of the region, including the closure of the Mongol-Okhotsk Ocean and subsequent re-arrangement, the development of the Central Asian Orogenic Belt, and the genesis of the Khangai Dome and Mongolian Plateau.

Previous magnetotelluric (MT) field campaigns (2016-2018: 328 MT sites) across the Khangai Dome (Central Mongolia) imaged a localized asthenospheric upwelling with a corresponding thin lithosphere and fluid-rich domains within the lower crust. In this study, we report on new MT data consisting of 378 MT sites installed across Mongolia, west and east of Central Mongolia, from 2020 to 2023. This extended survey area now includes approximately 700 magnetotelluric measurements collected over a total area of approximately 1000 km by more than 1150 km, similar in scope to other national survey programs.

We use MT responses (impedances) estimated from both the previous and new measurements to generate a new, regional-scale, 3-D electrical conductivity model of more than half of Mongolia, using an open-access forward and inverse solver (GEMMIE), based on an integral equation approach. The new data were processed by employing, in particular, a multi-taper approach to improve the estimated MT responses at long periods.

The new 3-D model reveals lithospheric high-conductivity anomalies consistent with the main geological and tectonic features of Mongolia and indicates that the lithospheric anomalies previously imaged below Central Mongolia extend further westward but are bounded to the east by the Mogod fault system. It also reveals significant lithospheric-scale boundaries separating the northern and southern regions within Eastern and Western Mongolia. Furthermore, it establishes links between high-conductivity anomalies in the lower lithosphere with features of scientific and economic interest, such as fault or suture systems, important mineral zones, and intraplate volcanism.

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.

Continental intraplate volcanic systems, with their locations far from plate tectonic boundaries, are not well understood: the crustal and lithospheric mantle structure of these systems remain enigmatic and there is no consensus on the mechanisms that cause melt generation and ascent. The Cenozoic saw the development of numerous volcanic provinces on the African plate, including within the Central Hoggar, located in Northwest Africa, part of the Tuareg shield. The magmatic activity began at approximately 34 Ma and continued throughout the Quaternary. In order to understand the origins and potential mechanisms that generated the intraplate volcanic activity in the Central Hoggar we aim to image the subsurface architecture, in terms of electrical resistivity, from the surface to the lithospheric mantle. To do so we use magnetotelluric measurements from 40 locations to generate a 3-D electrical resistivity model, over an area of about 100 km by 160 km. Low-resistivity features (i.e., conductors) are observed in the crust that are narrow, linear structures congruent with the boundaries of terranes and prominent fault zones (e.g., Azrou N’Fad). They likely reflect the Pan-African mega-shear zones, which were reactivated throughout the tectonic evolution of the region. The model reveals that these faults are lithospheric-scale. The low-resistivity features likely represent the signatures of past fluid pathways and mineralization. A deeper low-resistivity feature is observed in the upper lithospheric mantle directly beneath the Manzaz and Atakor volcanic districts. It may represent local, small-scale metasomatism of the sub-continental lithospheric mantle, and low-percent partial melting, that sits above a regional, large-scale asthenospheric upwelling associated with the Hoggar swell. It is likely the origin point of the fluids responsible for the overlying anomalies. The results highlight the control of the lithospheric-scale, mega-shear zones on the spatial distribution of the recent Cenozoic volcanic activity, which was influenced by the location of pre-existing structural weaknesses.

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.

In the framework of a mineral system approach, a combination of components is required to develop a mineral system. This includes the whole-lithosphere architecture, which controls the transport of ore-forming fluids, and favorable tectonic and geodynamic processes, occurring at various spatial and temporal scales, that influence the genesis and evolution of ore-forming fluids (Huston et al., 2016; Groves et al., 2018; Davies et al., 2020). Knowledge of the deep structural framework can advance the understanding of the development of a mineral system and the emplacement of mineral deposits. Deep geophysical exploration carried out with this aim is increasingly important for targeting new ore deposits in unexplored and underexplored regions (Dentith et al., 2018; Dentith, 2019). We analyze data and electrical resistivity models generated from magnetotelluric measurements acquired across Mongolia, part of the Central Asian Orogenic Belt, as part of a regional array (Käufl et al., 2020; Rigaud et al., 2023a, b; Comeau et al., 2024; see Fig. 1) and focus on several metallogenic zones. These zones contain significant resources of copper and gold, as well as rare earth elements. We interpret the results, with the help of geological and geochemical data, in addition to seismic velocity data, and discuss fluid transport pathways and links to the surface expressions of mineral deposits.

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.

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.

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.

We are investigating the lithospheric properties and lithospheric architecture beneath Mongolia with three-dimensional models of the electrical resistivity generated from magnetotelluric measurements. In addition, thermo-mechanical numerical modelling, with geophysically-guided constraints, is being used to provide valuable insights by testing the mechanical viability of different hypotheses for the temporal evolution and dynamic processes within this region.

Mongolia is located between the relatively stable Siberian craton and the extensional regime near the Baikal rift zone to the north and to the south the North China and Tarim cratons that have a northward-directed compressional regime. Due to its location, it is an excellent region to study intracontinental deformation. Furthermore, enigmatic continental intraplate basaltic volcanism of the Cenozoic age exists across Mongolia. In addition, this region contains economically important mineral zones (copper and gold), with the origin and evolution of the mineral systems linked to the whole-lithosphere architecture, crust-mantle interactions, and mantle convection dynamics.

Magnetotelluric data has been collected across Western, Central, and Eastern Mongolia. Three field campaigns in 2016, 2017, and 2018 collected more than 328 sites on an array (50 km spacing) and along three dense profiles (3-15 km spacing) that focused on the Hangai Dome (plateau) and Gobi-Altai (Arkhangai, Bayankhongor) over an area of approximately 800 km (north-south) by 400 km (east-west). Between 2020 and 2022, the array was extended to the east with 77 sites collected across central-east Mongolia (Bulgan, Selenge, Tuv, Uvurkhangai, Dundgovi; 400 by 200 km), including 34 sites along an 810 km long north-south profile crossing the Mongol-Okhotsk suture zone. In late 2022, 79 measurements were acquired in northern Mongolia across the Hovsgol region and Darhad (200 by 200 km) with an array and several profiles, which connect to data west of Lake Baikal. In early 2023, 38 sites were collected in central-east Mongolia (Umnugovi; 200 by 200 km), completing the eastern array. Later in 2023, a major field campaign was launched that successfully collected 150 measurements in western Mongolia (Zavkhan, Uvs, Govi-Altai, Khovd) over an area of approximately 500 by 400 km. This included an array (50 km spacing) and three dense profiles (5-10 km spacing). This gives approximately 700 magnetotelluric measurements collected over a total area of approximately 1000 km (north-south) by more than 1150 km (east-west).

This is a large area that approaches the scope of several other regional and national magnetotelluric survey programs. What’s more, this dataset fills an important gap between the existing magnetotelluric data across China and the Tibetan Plateau with several profiles across the Siberian Craton, in principle completing a remarkable transect of 4000 km across a variety of tectonic domains.

In this presentation, we will report on the new measurements. They will be integrated into the previously collected dataset, and new models will be generated that incorporate all data. We will also present new models of western, central and eastern Mongolia that provide insights on the properties, structure, and evolution of the Hangai Dome, the Mongol-Okhotsk suture and the Central Asian Orogenic Belt.

The West African Craton constitutes a significant geological domain within the African Precambrian basement, characterized by two cratonic sedimentary basins delineating three distinct Archaean and Paleoproterozoic metamorphic and magmatic shields. Among these shields, the Reguibat Shield features a basement primarily composed of Palaeoproterozoic rocks dating between 2.21 and 2.07 billion years ago. The formation of the West African Craton followed from the amalgamation of Archean, Paleoproterozoic, and Neoproterozoic terranes during the Pan-African orogeny, an event which occured approximately 750 to 550 million years ago. Superimposed upon this basement are Neoproterozoic and Palaeozoic sediments, notably within the Taoudenni Basin (alternatively referred to as the Hank Basin) to the south, the Reggane Basin to the east, and the Tindouf Basin to the north. The Adrar region, southwest Algeria, falls within the Taoudeni Basin, specifically aligning with the Reggane Basin.

In this work we present, for the first time, magnetotelluric data from the Adrar region. This (deeply-penetrating) electromagnetic geophysical technique can provide multi-scale imaging: for example, both regional and local imaging. The study has two objectives; one is to image the architecture of part of the West African Craton, specifically to elucidate and characterize its eastern limit. Another objective is the exploration of the distribution of deep subterranean water reserves, known to exist across the region, with the aim to help mitigate potential water scarcities in the west Sahara Desert, one of the hottest and driest places on Earth.

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.

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.

Three-dimensional (3-D) modelling of magnetotelluric (MT) data is standard practice nowadays, with various 3D inverse solvers being available for commercial and scientific usage. Three approaches are commonly used to numerically solve Maxwell’s equations in practice: finite-differences, finite elements, and volume integral equations. Most standard forward and inverse MT solvers/approaches have been benchmarked against each other and tested on several synthetic data/models. However, there has been few comparisons of the electrical conductivity models recovered by different solvers from real datasets.

In this presentation, we tackle this issue by generating inverse models from MT impedances taken from a subset of a large regional array in Central Mongolia using different codes: MODEM, based on finite differences; GEMMIE, based on integral equations; and FEMALY, a solver based on finite elements. In addition, we compare the recovered models with a published model, which was obtained by the finite elements code GOFEM. We will discuss the obtained models considering the underlying fundamentals of each method, the different inversion strategies, and the corresponding inversion parameters used, such as mesh discretization and regularization.