Some of the largest and most significant Miocene porphyry copper systems in China are within the Gangdese metallogenic belt on the southern Tibetan Plateau. It has been recognized that the crustal architecture and rheology, derived from regional tectonic events, has direct implications for the evolution and transport of fluids and magmas, and thus for the metallogenesis and prospectivity. Using data from a magnetetolluric array, which intersects the Lhasa–Mozugongka district of the Gangdese metal belt, a 3-D electrical resistivity model was generated, with the goal of investigating the tectonic and rheological controls on the magmatic mineral system. The lithospheric temperature distribution was estimated by applying the Arrhenius equation to conductivity profiles generated from 1-D Monte-Carlo models of long-period magnetotelluric data. The conductivity of partial melts in the lower and middle crust (30–60 km depth) was estimated for local conditions by applying the experimentally-derived equation of X. Guo et al. (2018). Subsequently, we estimated the melt fraction required to explain the observed bulk resistivity in each part of the study area. Variations in the effective viscosity of the lower and middle crust were constrained by the electrical resistivity model by applying the empirical relation of Liu and Hasterock (2016). Beneath the Miocene Cu–Mo deposits in the Lhasa terrane, conductive features in the lower and middle crust are attributed to partial melt fractions of more than 5% and viscosity reductions of 1–2 orders of magnitude. These conductive features may represent the signatures of (ore-controlling) melt/fluid migration channels and deeper melt/fluid source regions in the form of extensive crustal reservoirs of partial melt. Based on the interpretations of the geophysical model, and other available geological and geochemical evidence, a model of the metallogenic dynamics of the Miocene porphyry Cu–Mo deposits is proposed. Overall, the study highlights the applicability of electromagnetic geophysical methods to reliably link resistivity structures to melt/fluid transport channels and sources within a mineral system and supports the hypothesis that crustal rheology exerts a major control on the distribution of ore deposits.@en

Whole-lithosphere structure has direct implications for both the genesis of minerals and the locations of mineral emplacement; thus knowledge of the deep structural framework of the lithosphere can advance understanding of the development and evolution of mineral systems (e.g., Huston et al., 2016; Groves et al., 2018; Davies et al., 2020). Transient tectonic and geodynamic processes — occurring at various spatial and temporal scales — control the structure of the lithosphere. In turn, this structure influences the transportation of fluids (including ore-forming fluids) through the crust, largely by controlling permeability (e.g., Huston et al., 2016).@en

Intraplate processes, such as continental surface uplift and intraplate volcanism, are enigmatic and the underlying mechanisms responsible are not fully understood. Central Mongolia is an ideal natural laboratory for studying such processes because of its location in the continental interior far from tectonic plate boundaries, its high-elevation plateau, and its widespread, low-volume, basaltic volcanism. The processes responsible for developing this region remain largely unexplained —due in part to a lack of high-resolution geophysical studies —and thus are open questions. @en

Joint inversion of complementary datasets is an important tool to gather new insights and aid interpretation, especially in regions which show structural complexity. Using the joint inversion framework jif3D [1] with a newly developed coupling for density and resistivity, based on a variation of information approach which is a machine-learning method that constructs a possible relationship between the properties [2], we combine satellite gravity measurements with electromagnetic data, from broadband and long-period magnetotellurics [3,4,5,6]. Central Mongolia is located in the continental interior, far from tectonic plate boundaries, yet has a high-elevation plateau and enigmatic widespread low-volume basaltic volcanism [7,8,9]. The processes responsible for developing this region remain unexplained and there are questions about its tectonic evolution. A recent project employed thermo-mechanical numerical modeling [10] to simulate the temporal evolution of various tectonic scenarios, offering an opportunity to test hypotheses and determine which are physically plausible mechanisms. Constraints on lithospheric properties, e.g., density distribution, are important for evaluating the geodynamic models. Furthermore, they can help shed light on questions regarding the nature of lower crustal electrical conductors [11], which may be related to tectonically-significant low-viscosity zones. We will present preliminary results that provide new constraints on the 3-D structure of the lithosphere beneath Central Mongolia, as well as a roadmap for moving towards integrating geophysical results into geodynamic modeling to better understand the evolution of the lithosphere.@en

Intracontinental deformation and intraplate volcanism, which occur far from tectonic plate boundaries, are not fully understood. Their origin and evolution are linked by crust-mantle interactions and mantle convection dynamics. Mongolia is an ideal natural laboratory for studying such processes because it is located far into the continental interior, several thousand kilometres from major tectonic margins. To the north is the Siberian craton, which is relatively stable, to the northeast is an extensional regime near the Baikal rift zone, which stretches for more than one thousand kilometres, and to the south are the North China and Tarim cratons, which have northward-directed motion creating a compressional regime. Central Mongolia, which contains a high plateau (with indications of vertical motion), is characterized by a shallow lithosphere-asthenosphere boundary that deepens at the edges, notably northwards towards the Siberian Craton. Continental intraplate basaltic volcanism of Cenozoic age exists across central and northern Mongolia, with several large concentrations within the Hangai region. As part of an ongoing project, we are investigating the lithospheric properties and lithospheric architecture beneath this region with magnetotelluric measurements and three-dimensional models of electrical resistivity. In addition, thermo-mechanical numerical modelling, with geophysically-guided constraints, is being used to provide valuable insight by testing different hypotheses for the temporal evolution and dynamic processes -- such as whether an upwelling asthenosphere and/or lithospheric removal could realistically be a consequence of delamination, edge-driven convection mechanisms from a lithospheric step, or some combination. Towards these goals, geophysical models that image the transition from thin lithosphere to thick lithosphere (and its geometry), believed to occur beneath northern Mongolia, are beneficial. There exists a wealth of recent geophysical data across central Mongolia, in addition to petrological data. This includes a temporary broadband seismic array that covers the Gobi, Hangai, and Hovsgol regions. In this presentation, we will report on 79 new magnetotelluric measurements acquired in 2022 in northern Mongolia across the Hovsgol and Darhad regions, as well as 77 new measurements acquired from 2020-2022 in central-east Mongolia (Bulgan, Arvaikheer). The acquired data are very good quality with low noise, a clear benefit of the remote location. Recordings were carried out at each location for approximately 1-5 days. The data typically had reliable periods up to 1,000 - 8,000 s. The new data will, ultimately, be integrated into the previously collected dataset across central Mongolia (Hangai, Bayankhongor, and Gobi-Altai), which consists of 328 measurement locations (thus approximately 500 total), which covers a total area of, currently, approximately 1000 km by 800 km. This is a notably large area, within the realm of several large regional and national magnetotelluric (and seismic) surveys. Furthermore, the data across northern Mongolia fill the last gap in a remarkable transect of existing magnetotelluric data that extends approximately 4,000 km from across the Siberian Craton to across the Tibetan Plateau.@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. @en

Central Mongolia is a prominent region of intracontinental surface deformation and intraplate volcanism. To study these processes, which are poorly understood, we collected magnetotelluric (MT) data in the Hangai and Gobi-Altai region in central Mongolia and derived the first 3-D resistivity model of the crustal and upper mantle structure in this region. The geological and tectonic history of this region is complex, resulting in features over a wide range of spatial scales, which that are coupled through a variety of geodynamic processes. Many Earth properties that are critical for the understanding of these processes, such as temperature as well as fluid and melt properties, affect the electrical conductivity in the subsurface. 3-D imaging using MT can resolve the distribution of electrical conductivity within the Earth at scales ranging from tens of metres to hundreds of kilometres, thereby providing constraints on possible geodynamic scenarios. We present an approach to survey design, data acquisition, and inversion that aims to bridge various spatial scales while keeping the required field work and computational cost of the subsequent 3-D inversion feasible. MT transfer functions were estimated for a 650 × 400 km2 grid, which included measurements on an array with regular 50 × 50 km2 spacing and along several profiles with a denser 5–15 km spacing. The use of telluric-only data loggers on these profiles allowed for an efficient data acquisition with a high spatial resolution. A 3-D finite element forward modelling and inversion code was used to obtain the resistivity model. Locally refined unstructured hexahedral meshes allow for a multiscale model parametrization and accurate topography representation. The inversion process was carried out over four stages, whereby the result from each stage was used as input for the following stage that included a finer model parametrization and/or additional data (i.e. more stations, wider frequency range). The final model reveals a detailed resistivity structure and fits the observed data well, across all periods and site locations, offering new insights into the subsurface structure of central Mongolia. A prominent feature is a large low-resistivity zone detected in the upper mantle. This feature suggests a non-uniform lithosphere-asthenosphere boundary that contains localized upwellings that shallow to a depth of 70 km, consistent with previous studies. The 3-D model reveals the complex geometry of the feature, which appears rooted below the Eastern Hangai Dome with a second smaller feature slightly south of the Hangai Dome. Within the highly resistive upper crust, several conductive anomalies are observed. These may be explained by late Cenozoic volcanic zones and modern geothermal areas, which appear linked to mantle structures, as well as by major fault systems, which mark terrane boundaries and mineralized zones. Well resolved, heterogeneous low-resistivity zones that permeate the lower crust may be explained by fluid-rich domains.@en

Delamination of the lower crust or lithospheric mantle is one explanation for the surface uplift observed in areas of mountain building. This process describes the removal of the lower part of the tectonic plate and can occur in various ways. Different styles of delamination typically have in common that the upper material (e.g., lowermost crust or lithospheric mantle) is denser than the underlying material (e.g., asthenosphere) and therefore sinks. It has been proposed that the higher density can be caused by the formation of eclogite. In this study we apply a thermomechanical model featuring a density increase within the lithosphere by a phase transition. The model setup is designed to investigate surface uplift and mountain building in an intracontinental setting. Specifically, the model is arranged to closely resemble central Mongolia. The models give insights into the dynamically evolving flow field with respect to the style of removal, therefore the general outcome is also applicable to other orogenic regions. In addition to a systematic study on the phase transition, we also investigate the influence of convergent motion and of the rheology of the crust. Our results reveal that for the absence of a dense (eclogite) layer, delamination initially occurs as a stationary Rayleigh-Taylor instability which appears as a late and short-lived event. In comparison, for a strong density contrast an early, long-lived peeling-off removal style with a stationary slab results. The subsequent asthenospheric upwelling causes further peeling-off events for all density contrasts. For this removal style a retreating slab is observed that occasionally breaks off giving way to a periodic behaviour. The findings confirm that a strong convergence and low viscosity of the crust promote delamination. In addition, the asthenospheric upwelling yields a wide and flat surface uplift. Such dome-like features are observed to be more pronounced for high density contrasts (i.e., strong eclogitisation).@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

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.@en

Thermo-mechanical numerical modeling can provide valuable insights by simulating the temporal evolution of dynamic processes. The simulation model can be evaluated against the available observational evidence and physically plausible mechanisms can be further explored. To better understand the evolution of the lithosphere, multi-disciplinary results can be integrated into the geodynamic modeling, creating more realistic and reliable models. What’s more, such modeling offers an excellent opportunity to test various hypotheses and to constrain the possible ranges of parameters. By systematically varying physical parameters, their influence and control on dynamic tectonic processes can be tested. Here we present work that uses Central Mongolia as a case study. This location is an ideal natural laboratory for studying surface deformation and intraplate uplift because of its high-elevation plateau in a location in the continental interior — far from tectonic plate boundaries. Intracontinental surface deformation is enigmatic, and the underlying mechanisms responsible are not fully understood. However, because deformation solely by means of tectonic plate motion is not possible in an intraplate setting, crust-mantle interactions (e.g., driven by mantle convection) are likely required to explain the origin and evolution of intracontinental deformation. Explanations for surface uplift in the continental interior include: 1. hot, buoyant, deep-rooted mantle plumes; 2. crustal thickening from mafic magmatic underplating; 3. broad-scale mantleflow and thermal convection processes that produce dynamic topography; and 4. small-scale asthenospheric upwelling prompted by isolated lithospheric removal. We use self-consistent thermo-mechanical numerical modeling to investigate a subset of the latter explanation: lithospheric removal by delamination or a Rayleigh-Taylor instability. We explore the conditions under which delamination can occur and investigate the timing and amplitude of the consequent surface deformation.@en

The aim of this work is to develop a geodynamic model of our measured area in Mongolia using the knowledge from our magnetotelluric model and other fields, such as geology. In this geodynamically active region, where we observe volcanism and uplift, a thin lithosphere is determined by the magnetotelluric data. One possible process for lithosphere thinning is delamination, which is coupled to convective processes. It has been shown that features in magnetotelluric data can be linked to viscosity structures, and these can be investigated with geodynamic models. The combination of magnetotellurics and geodynamics is a new approach which we use to bring further insights into the geological history of Mongolia.@en

We present electrical resistivity models of the crust and upper mantle estimated from 2D inversions of broadband magnetotellurics (MT) data acquired from two profiles in the western desert of Egypt, which can contribute to the understanding of the structural setup of this region. The first profile data are collected from 14 stations along a 250 km profile, in EW direction profile runs along latitude ~25.5°N from Kharga oasis to Dakhla oasis. The second profile comprises 19 stations measured along a 130 km profile in NS direction centered at longitude 28°E and crossing the Farafra. The acquisition for both profiles continued for 1 to 3 days at each station, which allowed for the calculation of impedances for periods from 0.01 sec up to 4096 sec at some sites. The wide frequency band corresponds to a maximal skin depths of up to 150 km that can provide penetration to the top of the asthenosphere. The inversion models display high-conductivity sediments cover at the near surface (<1-2 km), which can be associated with the Nubian aquifer. Along the EW-profile from Kaharge to Dhakla, the crustal basement is overly highly resistive and homogeneous und underlain by a more conductive lithospheric mantle below depths of 30-40 km. Along the N-S profile across Farafra, only the southern portion exhibits a highly resistive crust, whereas beneath Farafra northwards, moderate crustal conductivities are encountered. A comparison has been made between the resultant resistivity models with the 1° tessellated updated crust and lithospheric model of the Earth (LITHO1.0) which was developed by Pasyanos, 2014 on the basis of seismic velocity data. The obtained results show a remarkable consistency between the resistivity models and the calculated crustal boundaries. Especially at the Kharga-Dakhla profile a clear matching can be noticed at the upper and lower boundaries of a characteristic anomaly with the Moho and LAB boundaries respectively.@en

Deformation in the continental interior, far from tectonic plate boundaries, is not fully understood. Due to its location, Mongolia is a prime natural laboratory for studying effects such as intracontinental deformation and intraplate volcanism. A previous regional magnetotelluric (MT) study, including three (2016-2018) field campaigns, identified a localized asthenospheric upwelling with a correspondingly thin lithosphere underneath the Hangai Dome (Central Mongolia). Compared with Central Mongolia, Eastern Mongolia is less studied with geophysical methods; consequently, its underlying lithospheric and asthenospheric properties are less constrained. At the same time, this region is of economic and scientific interest, as it hosts several mineral zones and relevant geological features, such as the Mongolia-Okhotsk suture zone. Furthermore, it is unknown to what extent the identified electrical conductivity anomalies in Central Mongolia extend to the east and how the crust and mantle differ in this region.This work presents the first results of a new MT field study covering Central-Eastern Mongolia. 64 broadband MT stations were deployed between 2020 and 2022 along five profiles east of the Hangai. The data were processed using a two-step multi-taper processing approach, simultaneously improving the quality at short and long periods, providing credible MT responses up to 2048 seconds. The previously acquired and new data were jointly inverted regarding 3-D conductivity variations. Furthermore, data from an 880 km long 2-D profile extending from the Selenga Basin to Gobi Desert were inverted independently. This study is part of a broader project aiming to constrain the electrical conductivity of entire Mongolia.@en

The creation and evolution of the Songliao Block, Northeast China, is believed to be related to the closure of the Paleo-Asian and Mongol-Okhotsk oceans, and to the subduction of the Paleo-Pacific Ocean. Geochemical and geophysical data indicate a complex tectonic history that may have left signatures of the superposition of separate tectonic events. However, the relationship between the current lithospheric structure of the Songliao Block and those tectonic events is not clear. In order to shed light on this question, a three-dimensional electrical resistivity model of the crust and mantle was generated from 138 magnetotelluric sites. The results show a heterogenous lithosphere with anomalous low-resistivity zones (∼10 Ω·m). The position of upper-crustal features (<10 km) correlates with a network of fault systems and rift basins. Based on estimates of high crustal temperatures, mid-lower crustal features (15–35 km), some of which appear near seismic reflectors, are attributed to partial melts (∼5%) containing moderate-high water contents (4.5 wt. %). A deep (>45 km) feature is interpreted as an asthenospheric upwelling near the center of the Songliao Block, where seismic evidence indicates a thin lithosphere. It is proposed that the collision, convergence, and suturing of blocks caused by the closure of the Paleo-Asian Ocean and the bidirectional convergence between the Mongol-Okhotsk and Paleo-Pacific Ocean may have produced weak tectonic zones throughout the lithosphere. They provided a structural basis for the later upward movement of fluids and melt, likely due to hydrous upwellings caused by the subduction of the Paleo-Pacific system.@en

The tectonic dynamics of the Lhasa-Gangdese terrane, southern Tibet, are still unclear and open questions persist regarding the structure, physical properties, and rheology of the lithosphere. A three-dimensional electrical resistivity model was generated beneath the Lhasa terrane, 79°−94°E and 28°–33°N, an area of ∼1,500 by ∼600 km, using data from 537 magnetotelluric measurements. To overcome computational challenges, we present an approach in which multiple high-resolution models from overlapping subregions are combined by means of an error-weighted averaging scheme to construct a continuous electrical resistivity model. Based on the electrical structure, the rheology of the mid-lower crust was investigated by constraining the melt fraction, in order to explore controls on dynamic mechanisms from a whole-system perspective. The models shed light on the vertical and lateral migration of crustal materials, magma transportation, and continental deformation in the Lhasa terrane. The model shows resistive features in the south that may represent the Indian plate, and conductive features above this that may represent the migration of materials beneath the Lhasa terrane, both of which vary substantially along the Indus-Yarlung Zangbo suture. In the mid-lower crust, conductive features (with a likely corresponding decrease of the effective viscosity) may be related to underplating, magma reservoirs, mineralization sources, and vertical motion of buoyant materials. A weakened mid-lower crust has consequences for surface deformation and may have contributed to the formation of (north-south) rift zones. Furthermore, the inhomogeneous distribution of weakened areas in the mid-lower crust has implications for the local lateral migration of materials.@en

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

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

Mineral systems can be thought of as a combination of several critical elements, including the whole-lithosphere architecture, favorable geodynamic/tectonic events, and fertility. Because they are driven by processes across various scales, exploration benefits from a scale-integrated approach. There are open questions regarding the source of ore-forming fluids, the depth of genesis, and their transportation through the upper crust to discrete emplacement locations. In this study, we investigate an Au–Cu metal belt located at the margin of an Archean-Paleoproterozoic microcontinent. We explore the geophysical signatures by analyzing three-dimensional models of the electrical resistivity and shear-wave velocity throughout the lithosphere. Directly beneath the metal belt, narrow, vertical, finger-like low-resistivity features are imaged within the resistive upper-middle crust and are connected to a large low-resistivity zone in the lower crust. A broad low-resistivity zone is imaged in the lithospheric mantle, which is well aligned with a zone of low shear-wave velocity, examined with a correlation analysis. In the upper-middle crust, the resistivity signatures give evidence for ancient pathways of fluids, constrained by a structure along a tectonic boundary. In the lower lithosphere, the resistivity and velocity signatures are interpreted to represent a fossil fluid source region. We propose that these signatures were caused by a combination of 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. The whole-lithosphere architecture controls the genesis, evolution, and transport of ore-forming fluids and thus the development of the mineral system.@en