Within the Ulaanbaatar region, Mongolia, 120 earthquake events were recorded with a magnitude between 3.4 and 5.6 and 978 earthquake events had a magnitude between 2.5 and 3.4, for the period from 1994 to 2016. Several of these have been strongly felt by residents of Ulaanbaatar. Historical records, since 1905, show that Mongolia as a whole has experienced four major earthquakes with magnitudes larger than 8 and many moderate earthquakes with magnitudes larger than 5.5. The seismicity in Mongolia is concentrated along the Mongolian-Altai and Gobi-Altai, the Bulnay (north of the Khangai mountains), and around Mogod (east of the Khangai mountains).

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

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

We aim to give an integrated interpretation of the electrical conductivity structure of the subsurface with geomorphological and geological knowledge, geodetic measurements, paleo-seismic trenching, and near-surface ground-penetrating radar surveys. We also aim to discuss the relation with fault mechanical models and local fault damage zones, and the relevance of the low slip rate. Understanding the subsurface structure of the region and characterizing the active faults is an important step for assessing the seismic hazard.
@en

Mongolia is a region of major scientific relevancy because it is a prime example of continental intraplate surface deformation, which is poorly studied and not well understood. There are open questions regarding the tectonic evolution of the region, including the closure of the Mongol-Okhotsk Ocean and subsequent re-arrangement, the development of the Central Asian Orogenic Belt, and the genesis of the Khangai Dome and Mongolian Plateau.

Previous magnetotelluric (MT) field campaigns (2016-2018: 328 MT sites) across the Khangai Dome (Central Mongolia) imaged a localized asthenospheric upwelling with a corresponding thin lithosphere and fluid-rich domains within the lower crust. In this study, we report on new MT data consisting of 378 MT sites installed across Mongolia, west and east of Central Mongolia, from 2020 to 2023. This extended survey area now includes approximately 700 magnetotelluric measurements collected over a total area of approximately 1000 km by more than 1150 km, similar in scope to other national survey programs.

We use MT responses (impedances) estimated from both the previous and new measurements to generate a new, regional-scale, 3-D electrical conductivity model of more than half of Mongolia, using an open-access forward and inverse solver (GEMMIE), based on an integral equation approach. The new data were processed by employing, in particular, a multi-taper approach to improve the estimated MT responses at long periods.

The new 3-D model reveals lithospheric high-conductivity anomalies consistent with the main geological and tectonic features of Mongolia and indicates that the lithospheric anomalies previously imaged below Central Mongolia extend further westward but are bounded to the east by the Mogod fault system. It also reveals significant lithospheric-scale boundaries separating the northern and southern regions within Eastern and Western Mongolia. Furthermore, it establishes links between high-conductivity anomalies in the lower lithosphere with features of scientific and economic interest, such as fault or suture systems, important mineral zones, and intraplate volcanism.@en

We are investigating the lithospheric properties and lithospheric architecture beneath Mongolia with three-dimensional models of the electrical resistivity generated from magnetotelluric measurements. In addition, thermo-mechanical numerical modelling, with geophysically-guided constraints, is being used to provide valuable insights by testing the mechanical viability of different hypotheses for the temporal evolution and dynamic processes within this region. Mongolia is located between the relatively stable Siberian craton and the extensional regime near the Baikal rift zone to the north and to the south the North China and Tarim cratons that have a northward-directed compressional regime. Due to its location, it is an excellent region to study intracontinental deformation. Furthermore, enigmatic continental intraplate basaltic volcanism of the Cenozoic age exists across Mongolia. In addition, this region contains economically important mineral zones (copper and gold), with the origin and evolution of the mineral systems linked to the whole-lithosphere architecture, crust-mantle interactions, and mantle convection dynamics. Magnetotelluric data has been collected across Western, Central, and Eastern Mongolia. Three field campaigns in 2016, 2017, and 2018 collected more than 328 sites on an array (50 km spacing) and along three dense profiles (3-15 km spacing) that focused on the Hangai Dome (plateau) and Gobi-Altai (Arkhangai, Bayankhongor) over an area of approximately 800 km (north-south) by 400 km (east-west). Between 2020 and 2022, the array was extended to the east with 77 sites collected across central-east Mongolia (Bulgan, Selenge, Tuv, Uvurkhangai, Dundgovi; 400 by 200 km), including 34 sites along an 810 km long north-south profile crossing the Mongol-Okhotsk suture zone. In late 2022, 79 measurements were acquired in northern Mongolia across the Hovsgol region and Darhad (200 by 200 km) with an array and several profiles, which connect to data west of Lake Baikal. In early 2023, 38 sites were collected in central-east Mongolia (Umnugovi; 200 by 200 km), completing the eastern array. Later in 2023, a major field campaign was launched that successfully collected 150 measurements in western Mongolia (Zavkhan, Uvs, Govi-Altai, Khovd) over an area of approximately 500 by 400 km. This included an array (50 km spacing) and three dense profiles (5-10 km spacing). This gives approximately 700 magnetotelluric measurements collected over a total area of approximately 1000 km (north-south) by more than 1150 km (east-west). This is a large area that approaches the scope of several other regional and national magnetotelluric survey programs. What’s more, this dataset fills an important gap between the existing magnetotelluric data across China and the Tibetan Plateau with several profiles across the Siberian Craton, in principle completing a remarkable transect of 4000 km across a variety of tectonic domains. In this presentation, we will report on the new measurements. They will be integrated into the previously collected dataset, and new models will be generated that incorporate all data. We will also present new models of western, central and eastern Mongolia that provide insights on the properties, structure, and evolution of the Hangai Dome, the Mongol-Okhotsk suture and the Central Asian Orogenic Belt. @en

The 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

In the framework of a mineral system approach, a combination of components is required to develop a mineral system. This includes the whole-lithosphere architecture, which controls the transport of ore-forming fluids, and favorable tectonic and geodynamic processes, occurring at various spatial and temporal scales, that influence the genesis and evolution of ore-forming fluids (Huston et al., 2016; Groves et al., 2018; Davies et al., 2020). Knowledge of the deep structural framework can advance the understanding of the development of a mineral system and the emplacement of mineral deposits. Deep geophysical exploration carried out with this aim is increasingly important for targeting new ore deposits in unexplored and underexplored regions (Dentith et al., 2018; Dentith, 2019). We analyze data and electrical resistivity models generated from magnetotelluric measurements acquired across Mongolia, part of the Central Asian Orogenic Belt, as part of a regional array (Käufl et al., 2020; Rigaud et al., 2023a, b; Comeau et al., 2024; see Fig. 1) and focus on several metallogenic zones. These zones contain significant resources of copper and gold, as well as rare earth elements. We interpret the results, with the help of geological and geochemical data, in addition to seismic velocity data, and discuss fluid transport pathways and links to the surface expressions of mineral deposits.@en

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

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

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 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