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.@en

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.@en

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.@en

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. @en

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.@en

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.@en

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.
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Within the Ulaanbaatar region, Mongolia, 120 earthquake events were recorded with a magnitude between 3.4 and 5.6 and 978 earthquake events had a magnitude between 2.5 and 3.4, for the period from 1994 to 2016. Several of these have been strongly felt by residents of Ulaanbaatar. Historical records, since 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. The seismicity in Mongolia is concentrated along the Mongolian-Altai and Gobi-Altai, the Bulnay (north of the Khangai mountains), and around Mogod (east of the Khangai mountains).

In the west of the Ulaanbaatar region there are several prominent fault zones, some only identified very recently. The majority of the seismic events in this region are related to the Khustai, Sharkhai, and Avdar fault zones. Seismicity is typically detected in the upper ~16 km of the crust. These fault zones are 100+ km long and historical events are predicted to have produced vertical offsets of up to 10 m; some sections show a cumulative horizontal offset of up to 100 m. Based on paleo-seismic surveys, it is estimated that these fault zones could produce earthquakes up to magnitude 7. Therefore, these faults pose a serious threat and risk of damage to Ulaanbaatar.

In this presentation we aim to characterize the active fault zones near Ulaanbaatar with electrical resistivity models generated from magnetotelluric data. In mid-2024 we carried out measurements across the Khustai, Sharkhai, and Avdar fault zones and modeled the local features near the fault traces and the regional crustal features of the region. Preliminary models show several low-resistivity features (approximately <100 Ωm) in the near-surface. The upper crust (0-25 km depth) appears to have a generally high-resistivity (~10,000 Ωm), whereas the lower crust (25–50 km depth) appears to have a lower resistivity (approximately <100 Ωm).

We aim to give an integrated interpretation of the electrical conductivity structure of the subsurface with geomorphological and geological knowledge, geodetic measurements, paleo-seismic trenching, and near-surface ground-penetrating radar surveys. We also aim to discuss the relation with fault mechanical models and local fault damage zones, and the relevance of the low slip rate. Understanding the subsurface structure of the region and characterizing the active faults is an important step for assessing the seismic hazard.
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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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The Mongol-Okhotsk suture and the Adaatsag ophiolite belt are associated with the closure of the Mongol-Okhotsk paleo-ocean and are located within the Central Asian Orogenic Belt (CAOB) and Mongolia. The suture zone is flanked by volcanic-plutonic belts that host significant metallogenic zones, containing deposits of copper and gold. The tectonic evolution of this region is not fully understood and the lithospheric structure has been poorly studied. We analyze magnetotelluric data and generate a model of the electrical resistivity distribution across this region. Whereas the northern segment has a sharp transition from a high-resistivity upper crust to a low-resistivity lower crust, as observed beneath the Hangai Dome, the southern segment does not show this transition. A wide, low-resistivity zone (1–100 Ωm) imaged in the crust and lithospheric mantle is coincident with the Mongol-Okhotsk suture and ophiolite, revealing a clear and significant lithospheric-scale feature. Across the profile, numerous narrow, vertically oriented, low-resistivity features (1–100 Ωm) are spatially associated remarkably well with the proposed boundaries of tectonic domains. These results confirm ideas about the development of the CAOB. Some of these low-resistivity features are beneath the surface locations of large mineral zones, and likely represent fossil fluid pathways. We show congruent seismic velocity models for comparison and the results show a large-scale low-velocity anomaly (decrease of 2%–3%) that correlates with the location of the low-resistivity anomaly below the Mongol-Okhotsk suture. The geophysical results, combined with geological and geochemical data, provide insights into the structure of this region and help shed light on unanswered questions.

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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.@en

Electrodes are used to measure a potential difference between two points. In geophysical and geotechnical applications they are often in the form of non-polarizable porous-pot electrodes. Here we describe the design, construction, and testing of modular and refillable electrodes, which facilitates repair as the electrodes degrade over time. We use a chemical composition based on a metal in contact with an over-saturated electrolyte that consists of a salt of that metal and an auxiliary salt. We compare characteristics when the electrolyte is stabilized in a clay or not, and with various states of ceramic porous plugs and two types of wood plugs. Next, we assess the long-term stability (more than 1 month), noise (periods of 1 s to 1 hr), and temperature sensitivity of different types of electrodes. Electrodes with an electrolyte and clay formula showed lower noise (0.2–0.4 μV at periods of 1–120 s), greater long-term stability (0.05–0.5 mV/month of smooth drift), and greater consistency between samples measured than those with no clay (noise and drift values up to four times larger). The effects from different porous plugs were negligible, with similar results for ceramic and wood types. The temperature sensitivity of the electric potential was assessed, from −3 to 35°C. All electrodes showed a temperature sensitivity of about −30 μV/°C. This is considered very low compared to some commercially available electrodes. Finally, continuous long-term laboratory and field measurements of the potential highlight the application of the new electrodes.@en

Ophiolite mélanges are commonly found in ancient accretionary orogenic belts. However, they may have diverse origins because they can form in various tectonic settings, including fore-arc, back-arc, active continental margins, mid-ocean ridges, and continental rifts. Identifying and characterizing suture zones in accretionary orogenic belts is crucial for understanding their tectonic evolution. The Beishan Orogenic Collage (BOC) is located in the southernmost part of the Central Asian Orogenic Belt (CAOB), one of the world's largest accretionary orogens, and formed through ongoing subduction and consumption of the Paleo-Asian Ocean and its branch ocean basins. It contains four east-west trending ophiolite mélange belts: (1) Hongshishan; (2) Shibanjing-Xiaohuangshan; (3) Hongliuhe-Niujuanzi-Xichangjing; and (4) Liuyuan, from north to south. Despite abundant geochemical, structural, and geochronological data, no consensus on the settings in which these ophiolite mélange belts formed and their subduction polarities remains elusive. This uncertainty has resulted in several contrasting models hypothesized for the tectonic evolution of the Beishan region.

In this study, we used 60 broadband magnetotelluric measurements and 16 long-period magnetotelluric measurements sites recently acquired across the Beishan region in northwest China to obtain a three-dimensional electrical resistivity model. The model reveals a generally high-resistivity upper crust with several low-resistivity features aligning with suture zones and tectonic boundaries. The high-resistivity lithosphere beneath Niujuanzi is compatible with northward and southward subduction of the Niujuanzi Ocean, potentially revealing remnants of a cold fossil oceanic lithosphere. In contrast, the deep lithosphere beneath the other three ophiolite belts is characterized by low-resistivity features. Since MORB-type rocks have lower iron, hydrogen, and carbon content, they tend to exhibit high resistivity characteristics, compared to a back-arc basin or rift. The model suggest that the Njiujuanzi Ocean was possibly the major ocean of the southern branch of the Palaeo-Asian Ocean, and that it had bi-directional subduction polarity.@en

Continental intraplate volcanic systems, with their location 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. This includes the Hoggar volcanic province, located in Northwest Africa, part of the Tuareg shield. It is composed of several massifs with contrasting ages and eruptive styles. The magmatic activity began at around 34 Ma and continued throughout the Neogene-Quaternary. Phonolite and trachyte domes as well as scoria cones and necks are found in the Manzaz and Atakor volcanic districts. In order to image the crustal and lithospheric mantle structure of this region, and to understand the origins and potential mechanisms of the continental intraplate volcanic activity in the Central Hoggar and specifically the Atakor/Manzaz area, we acquired magnetotelluric (MT) measurements from 40 locations and generated a 3-D electrical resistivity model. The model covers an area of about 100 km by 200 km. Images of the subsurface architecture, in terms of electrical resistivity, from the near-surface to the lithospheric mantle, allow us image the deep plumbing system of the volcanic system. Low resistivity features (i.e., conductors) in the crust that are narrow, linear structures trending approximately north-south, are revealed along the two boundaries of the Azrou N’Fad terrane, in the Manzaz area. 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. In addition, the low-resistivity features likely represent the signatures of past fluid flow. The location of the recent Cenozoic volcanic activity was likely influenced by the pre-existing structure. A deep feature of moderate conductivity is located in the upper lithospheric mantle directly beneath the Manzaz and Atakor Volcanic Districts. It may represent the origin of the overlying anomalies and may suggest metasomatism of the sub-continental lithospheric mantle.@en

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. @en

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. @en

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.@en

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.@en

Both low resistivity zones and low velocity zones are distributed in the middle-lower crust of the western Lhasa terrane, Tibetan Plateau, China. Some estimates from electrical resistivity data suggest large volume fractions of silicate melts that are difficult to reconcile with seismic velocity data that prefer lower volumes. A second conductive phase, such as saline fluids, that drastically reduces the conductivity but does not significantly affect the seismic velocity because of its low volume may be able to explain these differences. In this study, a 3-D model of the electrical resistivity structure is generated on a profile along longitude 85°E from a latitude of 29°N to 32.5°N. Based on experimental measurement of melts and alkali-rich fluids (e.g., H ₂O-NaCl), we estimate the volume fraction of each phase that is required to explain the conductive anomalies observed in the geophysical model. The model reveals that the maximum bulk conductivity of the mid-lower crust in the south (1.52 S/m) is much higher than the conductivity of the mid-lower crust in the north (0.18 S/m) when taking 31°N as a rough boundary, near Coqen region. We hypothesize that the conductive zones in the south of the Coqen region may result from a silicate melt and alkali-rich fluid (multicomponent) system. In contrast, partial melting alone can explain the conductive zones in the north. The hypothesis can reconcile the predictions from electrical resistivity data and seismic data, and it corresponds well with zircon Hf isotope data. For example, a combination such as the presence of <1% NaCl-bearing aqueous fluids in addition to 5-10% partial melt can reconcile electrical conductivity data and seismic data. We propose that the contributions from partial melt or saline fluids are controlled by the distinct tectonic dynamics in each region. Furthermore, the model compatible with the idea that the Indian lower crust subducted northwards beneath the Lhasa terrane and may not extend far beyond the Indus-Yarlung Zangbo suture (approximately 30-31°N). The widespread distribution and interconnection of crustal conductors at different depths is consistent with the lateral migration of materials. However, both geophysical data sets agree that some anomalies are discontinuous along the profile. Furthermore, the low-angle subducted Indian Plate with no obvious tearing feature and a low volume of melts may have contributed to the absence of long, continuous, N-S-trending normal faults in this region.

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Northeast Africa, which today includes the Arabian-Nubian Shield and the Saharan Metacraton, experienced a complex and long history of tectonic events. These include cratonization, which resulted in thickening of the lithosphere and formation of stable cratons, and decratonization, which occurred as a result of the remobilization and reactivation of the tectonic domains through subsequent orogenies, or destruction of the cratonic root during extensional events. One outstanding question is the present-day architecture of the lithosphere across this region, including the location of important tectonic boundaries. Several geophysical investigations have been conducted to study the lithosphere, including density and velocity modeling; however, they have mainly focused on the hydrocarbon-rich areas offshore and onshore close to the western coast of the Gulf of Suez, in addition to some small regional-scale studies. We present a tectonic model of the Arabian-Nubian Shield and Saharan Metacraton derived, in part, from a 3D electrical resistivity model generated from magnetotelluric measurements acquired along a 700 km long profile across the central part of Egypt. The profile, roughly west-east, consists of 57 measurements, a subset of a larger dataset acquired in the region. The profile crosses the main tectonic boundaries in Egypt: the Arabian Nubian Shield (ANS) in the eastern part, the Nile River in the central part, and the Saharan Metacraton (SMC), in addition to its cratonic remnants (Al-Kufra), in the western part. The profile runs approximately along a line from Dahkla to Kharga, across to Qena, and towards Hurghada on the coast. On average, the measurement spacing is approximately 10 km, although it is denser in some regions (e.g., near ANS) and sparser in others (e.g., near Qena) due to local conditions. The data were acquired in campaigns carried out in autumn 2019, spring 2020, spring 2021, and spring 2022. The measurements used Metronix data loggers (ADU07e) and Metronix induction coils along with locally developed copper-copper sulphate electrodes to measure the electric field. Most sites were recorded for 2-5 days. The sampling rate used was 512 Hz. Periods up to 1,000 – 5,000 s were recorded. The data are generally considered to be of good quality and had low noise; this is primarily due to the lack of urban electrical noise in most of the survey area. Dimensionality analyses suggest a 3D character for long-period data, particularly in the ANS area, that requires the use of full 3D inversion to properly describe all aspects of the data. Several sensitivity tests were carried out to validate the robustness of the features in the 3D electrical resistivity model. A comparison of the resistivity model with other geophysical models in this region (including density and velocity models) shows a good correlation for the location of the cratonic boundary, which has a clear resistive electrical signature.@en