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

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

In late 2022, 79 magnetotelluric (MT) measurements were acquired across the Hovsgol and Darhad region, in northern Mongolia, consisting of an array (200 km by 200 km) and several denser profiles (~10 km site spacing). Currently, little is known about the subsurface structure of the Hovsgol and Darhad region. However, it is an important region because it represents the transition from the thin lithosphere, thick crust, and high plateau of central Mongolia to the south to the thick lithosphere of the Siberian Craton to the north. The region contains three parallel, seismically-active, rift valleys, oriented approximately north-south only ~100 km south of the Siberian Craton and ~200 km west of the Baikal rift zone. This change in the style of crustal deformation from compression, as compared to central Mongolia, is very intriguing. Meanwhile, petrological studies indicate that Cenozoic magmatic activity in the region may possibly be related to that in the Hangai. Further MT measurements have been acquired across the Siberian craton west of Lake Baikal (i.e., to the north). The data will provide constraints for geodynamic modelling on the lithospheric architecture of the region, with respect to, for example, edge-driven convection in the mantle due to the edge of the Siberian Craton. In this presentation, we report on a new 3-D electrical resistivity model of the Hovsgol and Darhad region, northern Mongolia.@en

Three-dimensional (3-D) modelling of magnetotelluric (MT) data is standard practice nowadays, with various 3D inverse solvers being available for commercial and scientific usage. Three approaches are commonly used to numerically solve Maxwell’s equations in practice: finite-differences, finite elements, and volume integral equations. Most standard forward and inverse MT solvers/approaches have been benchmarked against each other and tested on several synthetic data/models. However, there has been few comparisons of the electrical conductivity models recovered by different solvers from real datasets.

In this presentation, we tackle this issue by generating inverse models from MT impedances taken from a subset of a large regional array in Central Mongolia using different codes: MODEM, based on finite differences; GEMMIE, based on integral equations; and FEMALY, a solver based on finite elements. In addition, we compare the recovered models with a published model, which was obtained by the finite elements code GOFEM. We will discuss the obtained models considering the underlying fundamentals of each method, the different inversion strategies, and the corresponding inversion parameters used, such as mesh discretization and regularization.
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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