The structure of continental intraplate volcanic systems — which occur far from tectonic boundaries, unlike the majority of Earth's volcanism — is enigmatic and not fully understood, as are the underlying mechanisms responsible, due in part to a lack of high-resolution geophysical data. Central Mongolia contains Quaternary–Neogene aged alkaline basalt flows and volcanic cones, thousands of kilometres from active tectonic margins, in addition to an abundance of geochemical and petrological data — making this a natural laboratory to study intraplate volcanism. Using a recently collected, high-resolution, multi-scale, magnetotelluric dataset acquired across central Mongolia, we generate and analyze electrical resistivity models of the structure beneath the Tariat and Chuluut volcanic zones with the goal of imaging the volcanic system from surface to mantle source. The models reveal narrow, subvertical, lower resistivity anomalies in the middle-upper crust that are conspicuously located beneath surface expressions of volcanism. The lower crust (depths of 25–50 km) is characterized by the widespread distribution of isolated low-resistivity zones. A local low-resistivity zone is imaged in the mantle (depths of 60–90 km) above a broad, homogenous, doming low-resistivity feature. Considering the available evidence, we propose that the low-resistivity anomalies in the middle-upper crust are the remnant signatures of past transient magma pathways (or collection of pathways), caused by metasomatic alteration during the ascent of hot magmatic fluids. The lower crustal anomalies are interpreted to be domains of saline fluids in a thermally perturbed lower crust. In the mantle, the low-resistivity structure is explained by a broad mantle upwelling and thermal anomaly with a local zone of low-percent partial melt — the source for intraplate volcanism. The geophysical images are consistent with geochemical and petrological evidence from erupted lavas that indicates a single common mantle source region, limited crustal contamination, and rapid direct ascent, making crustal magma storage unlikely. Thus the geophysical models show remarkable and unique translithospheric images of a continental intraplate volcanic system, from surface to mantle source, with the results relevant to other continental regions@en

Electric currents induced in conductive planetary interiors by time-varying magnetospheric and ionospheric current systems have a significant effect on electromagnetic (EM) field observations. Complete characterization of EM induction effects is difficult owing to nonlinear interactions between the three-dimensional electrical structure of a planet and spatial complexity of inducing current systems. We present, a general framework for time-domain modeling of three-dimensional EM induction effects in heterogeneous conducting planets. Our approach does not assume that the magnetic field is potential, allows for an arbitrary distribution of electrical conductivity within a planet, and can deal with spatially complex time-varying current systems. The method is applicable to both data measured at stationary observation sites and satellite platforms, and enables the calculation of three-dimensional EM induction effects in near real-time settings.

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We investigate how a conceptual hydrodynamic model consisting of fluid localization and stagnation by thermally activated compaction can explain low-resistivity anomalies observed in the lower crust (>20 km depth). Electrical resistivity models, derived from magnetotelluric data collected across the intracontinental Bulnay region, a subset of a larger regional array across central Mongolia, are generated. They reveal low-resistivity (3 - 30 Ωm) domains with a width of ~25 km and a vertical extent of <10 km in the lower crust, with their tops ~5 km below the brittle-ductile transition zone. In 3-D these features appear as laterally extended (tube-like) structures, 300 km long, rather than disconnected ellipsoids. The features are oriented parallel to the adjacent Bulnay fault zone segments and perpendicular to the far-field compressive tectonic stress (i.e., northward motion from China and Tibet). These low-resistivity domains are consistent with the presence of saline metamorphic fluids. Deeper features imaged with the data include a large upper mantle conductor that we attribute to an asthenospheric upwelling, and thin lithosphere, related to intraplate surface uplift and volcanism, in agreement with recent geodynamic modelling of lithospheric removal in this region.

Based on the observed thermal structure of the crust, and assuming the mean stress at the brittle-ductile transition is twice the vertical load, the hydrodynamic model predicts that fluids would collect in zones <9 km below the brittle-ductile transition zone, and the zones would have a vertical extent of ~9 km, both in agreement with the resistivity models across the Bulnay region. The hydrodynamic model also gives plausible values for the activation energy for viscous creep (270 - 360 kJ/mol), suggesting that the mechanism is dislocation creep.

From the electrical resistivity models, the lower crustal viscous compaction-length is constrained to be ~25 km - in this region. Within the conceptual model, this length-scale is entirely consistent with independent estimates for the specific hydraulic and rheological properties of this region. In fact, this can be used to independently constrain acceptable ranges for the lower crustal effective viscosity, which is found to be low (on the order of 10^18 Pas). Accordingly, the results indicate that low-salinity fluids (likely 1 - 0.01 wt% NaCl), and correspondingly low porosities (likely 5 - 0.1 vol%), are the most plausible. These key findings suggest partial melts are not favoured to explain the anomalies. Overall, the results of this contribution imply that it is tectonic and compaction processes that control lower crustal fluid flow, rather than lithological or structural heterogeneity.@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

Southern Mongolia is part of the Central Asian Orogenic Belt, the origin and evolution of which is not fully known and is often debated. It is composed of several east–west trending lithostratigraphic domains that are attributed to an assemblage of accreted terranes or tectonic zones. This is in contrast to Central Mongolia, which is dominated by a cratonic block in the Hangai region. Terranes are typically bounded by suture zones that are expected to be deep-reaching, but may be difficult to identify based on observable surface fault traces alone. Thus, attempts to match lithostratigraphic domains to surface faulting have revealed some disagreements in the positions of suspected terranes. Furthermore, the subsurface structure of this region remains relatively unknown. Therefore, high-resolution geophysical data are required to determine the locations of terrane boundaries. Magnetotelluric data and telluric-only data were acquired across Southern Mongolia on a profile along a longitude of approximately 100.5° E. The profile extends ~ 350 km from the Hangai Mountains, across the Gobi–Altai Mountains, to the China–Mongolia border. The data were used to generate an electrical resistivity model of the crust and upper mantle, presented here, that can contribute to the understanding of the structure of this region, and of the evolution of the Central Asian Orogenic Belt. The resistivity model shows a generally resistive upper crust (0–20 km) with several anomalously conductive features that are believed to indicate suture zones and the boundaries of tectonic zones. Moreover, their spatial distribution is coincident with known surface fault segments and active seismicity. The lower crust (30–45 km) becomes generally less resistive, but contains an anomalously conductive feature below the Gobi–Altai zone. This potentially agrees with studies that have argued for an allochthonous lower crust below this region that has been relaminated and metamorphosed. Furthermore, there is a large contrast in the electrical properties between identified tectonic zones, due to their unique tectonic histories. Although penetration to greater depths is limited, the magnetotelluric data indicate a thick lithosphere below Southern Mongolia, in contrast to the previously reported thin lithosphere below Central Mongolia.[Figure not available: see fulltext.].

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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

We present electrical resistivity models, derived from magnetotelluric data, of the crust beneath the Bulnay region, Mongolia. They reveal that the lower crust contains a pattern of discrete zones (width of ~25 km) of low resistivity (<30 Ωm). Such features may be an effect of unaccounted-for electrical anisotropy. However, when anisotropy is considered in the modeling, the features remain. We investigate an alternative explanation, based on a conceptual model of fluid localization and stagnation by thermally activated compaction, and demonstrate it is compatible with the observed low-resistivity zones. The model explains the location, shape, and size of the zones, with plausible values of the activation energy for lower crustal creep (270–360 kJ/mol), and a viscous compaction length on the order of 10 km. The results imply tectonic deformation and compaction processes, rather than lithological-structural heterogeneity, control the regional lower crustal fluid flow.

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Central Mongolia is a prominent region of intra-plate volcanism and deformation. To study these processes, many of which are poorly understood, magnetotelluric data was collected in the Hangai and Gobi-Altai region in central Mongolia. The geologic history of this region exhibits features over a wide range of spatial scales, which are coupled through a variety of geodynamic processes. Three-dimensional imaging using magnetotellurics can resolve the distribution of electrical resistivity within the Earth at scales ranging from tens of metres to hundreds of kilometres. However, designing a survey which can probe various scales and running subsequent three-dimensional inversions requires that multiple constraints imposed by the data acquisition cost, logistical efforts and computational complexity are satisfied.

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 costs feasible. This approach was applied to obtain the first 3-D multi-scale resistivity model of the Hangai and Gobi-Altai mountains. Magnetotelluric transfer functions were estimated for a 650 x 400 km2 grid, which included measurements on an array with regular 50 x 50 km2 spacing and along several profiles with a denser 5-15 km spacing for higher crustal resolution. A 3-D finite element forward modelling and inversion code was used in a four stage inversion process to obtain a resistivity model.

The final model reveals a complex resistivity structure and fits the observed data well across all periods and site locations. It resolves shallow structures in the upper crust, linked to surface observables (faults, volcanic provinces, hot springs), together with anomalous lithospheric conductors as well as a large-scale asthenospheric upwelling. This model provides crucial information to constrain numerical modelling of geodynamic processes related to the formation of the Hangai Mountains, as well as intra-continental deformation and volcanism.@en

The Hangai is an intra-continental mountain range in central Mongolia with unknown orogenesis. Previous seismic and gravitational studies revealed a low velocity/low density anomaly, but the understanding of the uplift process remains vague. Instead, detailed 2-D and 3-D conductivity models beneath the Hangai and surrounding areas can facilitate the understanding of the Hangai Mountain uplift. To obtain such models, we conducted a magnetotelluric survey in the Hangai region. During three field campaigns (2016-2018) a total of 328 stations were installed on a regular 50 x 50 km grid and along several profiles with a finer spacing. The grid covers a total area of 360 x 700 km, including the Hangai Dome, its surroundings and a part of the Gobi-Altai mountain range. The estimated transfer functions (impedance, tipper, and phase tensor, as well as intersite impedance/phase tensor) cover a wide frequency range (from 0.008s to 3000s at most and up to 16000s at some stations) and are of high quality due to low electromagnetic noise, although affected by galvanic distortions. We employ a 3-D FEM code (GoFEM) to obtain an image of the conductivity structure below the survey area. Locally refined unstructured meshes are used to ensure numerical accuracy with a sufficiently fine discretisation of the inversion domain, while keeping the computational cost feasible, and also allow for static shift correction. Models obtained by an inversion of the impedance tensor resolve the subsurface down to the Lithosphere-Asthenosphere boundary (LAB). The models show a strong subdivision, the northern part with the Hangai Dome is characterized by a predominantly layered structure and a shallow LAB, whereas the Gobi-Altai in the south is laterally more heterogeneous with a deeper LAB. The transition is rather abrupt and follows the previously known South Hangai Fault Zone. Additionally, we imaged two large anomalous vertical conductors in the lithosphere, which are likely related to the uplift process.@en

The Bayankhongor Metal Belt passes through the Valley of the Lakes, south-central Mongolia, between the Hangai Dome and the Gobi-Altai Mountains. It is an important region because it is associated with significant mineral occurences, including important sources of gold and copper. Adjacent to this is the South Hangai suture zone and the Bayankhongor Ophiolite Belt. This is an ancient suture zone and terrane boundary that resulted from the closure of a paleo-ocean, marked by obducted ophiolites, and is possibly the longest continuous ophiolite in the world. Magnetotelluric data were used to generate 3-D electrical resistivity models of the upper crustal structure, which was previously poorly understood. The cratonic upper crust is highly resistive and thus the low-resistivity fault/suture system is easily detected. It is revealed to be a major crustal-scale structure. A clear transition in crustal electrical properties was observed across the zone and may reflect both the rheological and petrological differences across accreted terranes. Furthermore, other anomalous low-resistivity zones are spatially associated with the surface expressions of known mineral occurences and resource extraction projects. By combining electrical resistivity results with geological and petrological data we attempt to gain insights into the potential mineral resources of this unique region and their origin.@en

The Valley of the Lakes, south central Mongolia is located between the uplifted Hangai Dome and the Gobi Altai Mountains , within the Central Asian Orogenic Belt . It includes many interesting features, including t he South Hangai f ault system that represents an ancient suture zone and terrane boundary . This zone is possibly an extension of the Mongol Okhotsk suture that resulted from the closur e of the Mongol Okhotsk Ocean The adjacent obducted Bayankhongor Ophiolite Belt is possibly the longest continuous ophiolite belt in the world. This region is important because it is associated with the Bayankhongor Meta l logenic Belt that is an economic ally significant zone for ore extraction in Mongolia , including important sources of gold and copper.

Electrical resistivity is a key parameter for mineral exploration. Because faults and suture zones are regions of fractured, weakened crust they often have circulating fluids that act to i ncrease their electrical conduc tivity . Additionally, econom ic mineralization is commonly associated with a conductive signature from associated sulfide minerology. We present magnetotelluric data acquired in a n array across central Mongolia (Comeau et al., 2018; K äu fl et al., 2018; Becken et al., 2018; this abstra ct volume) volume). The magnetotelluric data we re used to generate 3 D electrical resistivity models of the shallow crustal structure, which was previously poorly understood. Because the cratonic upper crust is highly resistive (>1000 ohm m) the low res istivity (<30 ohm m) South Hangai f ault system is easily detected . It is revealed to be a major crustal scale structure. A clear transi tion in crustal electrical properties wa s observed across the suture zone and may reflect both the rheological and petrological d iff erences across accreted terranes Furthermore, a nomalous, low resistivity zones in the crust are spatially associated with the surface expressions of known mineralization and resource extraction projects. By combining our electrical resistivity results with other geological and petrological data we attempt to gain insights into the potential mineral resources of this unique region and their origin@en

The Hangai Dome, Mongolia, is an unusual high-elevation, intra-continental plateau characterized by dispersed, low-volume, intraplate volcanism. Its subsurface structure and its origin remains unexplained, due in part to a lack of high-resolution geophysical data. Magnetotelluric data along a ∼610 km profile crossing the Hangai Dome were used to generate electrical resistivity models of the crust and upper mantle. The crust is found to be unexpectedly heterogeneous. The upper crust is highly resistive but contains several features interpreted as ancient fluid pathways and fault zones, including the South Hangai fault system and ophiolite belt that is revealed to be a major crustal boundary. South of the Hangai Dome a clear transition in crustal properties is observed which reflects the rheological differences across accreted terranes. The lower crust contains discrete zones of low-resistivity material that indicate the presence of fluids and a weakened lower crust. The upper mantle contains a large low-resistivity zone that is consistent with the presence of partial melt within an asthenospheric upwelling, believed to be driving intraplate volcanism and supporting uplift.@en