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

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

Mineral systems can be thought of as a combination of several critical elements, including the whole-lithosphere architecture, favorable geodynamic/tectonic events, and fertility. 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 in central Mongolia. We explore 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. In the upper-middle crust, the 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 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, particularly those related to refertilization and metasomatism of the lithospheric mantle by long-lived subduction at the craton margin, and discuss several possibilities.@en

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

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

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

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

The Songliao Block is located in the eastern part of the Central Asian Orogenic Belt. Its creation and evolution are believed to be related to the closure of the Paleo-Asian and Mongol-Okhotsk oceans, and to the subduction of the Paleo-Pacific Ocean. The deep seismic reflection profiles showed that there are sloping mantle reflections below the Songliao Block, which are suspected to be the result of the convergence of three tectonic domains. However, it is still not clear the current structural form of the Songliao Block is caused by the direct action or not of the tectonic systems. This work used 138 broadband magnetotelluric stations to obtain a three-dimensional electrical structural model of the northern Songliao Block. The results showed there are orthogonal network fault systems, faulted basins, igneous rocks. And the Lindian fault depression is the center of the asthenospheric upwelling, the shallowest up to 45 km. Combined with evidence from seismic studies, we proposed that the superposition of tectonic systems may have produced weak tectonic zones. These zones provided channels for the later upward movement of fluids and melt, likely due to hydrous upwellings caused by the subduction of the Paleo-Pacific system.@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

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

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 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

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