Regional magnetotelluric study across Mongolia: Constraining lithospheric properties and architecture

Abstract

Abstract Mongolia is a region of major scientific interest because it is a prime example of continental intraplate surface deformation and volcanism, both of which are poorly studied and understood. It is located in the Central Asian Orogenic Belt between the stable Siberian Craton and the North China and Tarim Cratons, being subjected to northward compression transitioning to an eastward extrusion with the Hangai Dome, an intracontinental plateau with widespread Cenozoic volcanism, being located in this transition zone. Early explanations for the intraplate processes in Mongolia, such as vertical surface deformation, were associated with a deep-rooted mantle plume. However, modern geophysical and geochemical evidence is inconsistent with such a hypothesis. Much of the knowledge of the deep Earth comes from seismic geophysical methods. However, the analysis of seismic velocities alone can not unambiguously constrain the thermal and compositional structure of the mantle. The only other geophysical technique that can provide information about the subsurface down to mantle depths is the electromagnetic (EM) geophysical method, which includes magnetotellurics (MT) to probe the crust and upper mantle and geomagnetic depth sounding (GDS) to probe the mid and lower mantle. They are an appealing choice to complement the analysis of seismic data since electrical conductivity is sensitive to the connectivity and composition of fluid and partial melt within the subsurface, as well as temperature. These EM induction techniques rely on time-varying electric and/or magnetic fields originated by external sources of natural origin that propagate inside the Earth. Transfer functions (TFs) can be obobtained from the inducing and induced electric and/or magnetic fields, from which the Earth’s crust and mantle conductivity can be estimated. Subsurface properties such as temperature, viscosity, or fluid and melt content can then be interpreted and constrained in terms of electrical conductivity variations. Several MT field campaigns were performed in Mongolia over the last eight years, which ultimately resulted in a three-dimensional (3-D) electrical resistivity model of Central Mongolia/ Hangai Dome that imaged a localized asthenospheric upwelling with a correspondingly thin lithosphere and fluid-rich domains within the lower crust, indicating a mechanically weak region and providing evidence for lithospheric delamination as the main process driving the intercontinental surface deformation in Mongolia. However, the previous MT measurements were restricted to the Hangai Dome, and it is currently unknown if and how the previously identified features extend to other regions in Mongolia, which are of scientific and economic interest since they host mineral deposits and geothermal sources. Taking this into account, the main objective of this PhD project was to acquire, process, and subsequently invert new MT data across Mongolia to generate a new, regionalscale, 3-D electrical resistivity model. In total, 378 MT sites were installed between 2020 and 2023, from which we took 226 sites composing a grid with approximately 50 km average spacing and covering a 1248 × 896 m2. Furthermore, we used 34 sites composing an N-S oriented ∼ 810 km long profile crossing Eastern Mongolia to obtain a high-resolution 2-D model of this region. We processed the data employing a novel multitaper approach to improve the estimated MT impedances at long periods. Our models image significantly distinct subsurface electrical resistivity distributions to the west and east of the Hangai Dome, a major lithospheric-scale boundary separating Northern and Southern Mongolia, and links between upper mantle conductive anomalies and surface expressions of fault zones and mineral deposits.