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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

Electrical resistivity is a key parameter for mineral exploration and Mongolia is an important world mineral producer. We present magnetotelluric data acquired in a large regional array across west-central Mongolia. This is the first study of its kind in Mongolia. We will focus on key areas of mineralization, which are of economic importance, namely the South Hangai Fault Zone and the Bayankhongor Ophiolite Belt. The survey area, located between the uplifted Hangai Dome and the Gobi-Altai Mountains, represents an ancient suture zone. The magnetotelluric data are used to generate electrical resistivity models of the crustal structure, which was previously poorly understood.

While the cratonic upper crust is highly resistive, the low-resistivity South Hangai Fault Zone is detected and is revealed to be a major crustal-scale structure. Anomalous, low-resistivity zones in the crust are spatially associated with the surface expressions of the fault zone, known mineralization, and shallow surface resource extraction projects. By combining our electrical resistivity results with other geological and petrological data we attempt to gain insights into the mineral resources of this region. ... Read more

West-central Mongolia is characterized by the Hangai Dome, and intra-continental plateau, and by dispersed, low-volume, intraplate, basaltic volcanism. The results of a recent broadband magnetotelluric study across the young volcanic zones of Tariat and Chuluut are presented. The magnetotelluric data are used to generate high-resolution electrical resistivity models of the crust, and, in combination with petrological data, can give insights into the structure of this region.

Thin, vertically elongated features imaged in the upper crust may represent hydrothermal alteration from ancient and transient conduits of hot magma. These anomalous features are spatially associated with the surface expressions of volcanism (volcanic cones and calderas) and modern-day hydrothermal activity (hot springs). ... Read more

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 ... Read more