Evidence for terrane boundaries and suture zones across Southern Mongolia detected with a 2-dimensional magnetotelluric transect

Abstract

Southern Mongolia is part of the Central Asian Orogenic Belt, the origin and evolution of which is not fully known and is often debated. It is composed of several east–west trending lithostratigraphic domains that are attributed to an assemblage of accreted terranes or tectonic zones. This is in contrast to Central Mongolia, which is dominated by a cratonic block in the Hangai region. Terranes are typically bounded by suture zones that are expected to be deep-reaching, but may be difficult to identify based on observable surface fault traces alone. Thus, attempts to match lithostratigraphic domains to surface faulting have revealed some disagreements in the positions of suspected terranes. Furthermore, the subsurface structure of this region remains relatively unknown. Therefore, high-resolution geophysical data are required to determine the locations of terrane boundaries. Magnetotelluric data and telluric-only data were acquired across Southern Mongolia on a profile along a longitude of approximately 100.5° E. The profile extends ~ 350 km from the Hangai Mountains, across the Gobi–Altai Mountains, to the China–Mongolia border. The data were used to generate an electrical resistivity model of the crust and upper mantle, presented here, that can contribute to the understanding of the structure of this region, and of the evolution of the Central Asian Orogenic Belt. The resistivity model shows a generally resistive upper crust (0–20 km) with several anomalously conductive features that are believed to indicate suture zones and the boundaries of tectonic zones. Moreover, their spatial distribution is coincident with known surface fault segments and active seismicity. The lower crust (30–45 km) becomes generally less resistive, but contains an anomalously conductive feature below the Gobi–Altai zone. This potentially agrees with studies that have argued for an allochthonous lower crust below this region that has been relaminated and metamorphosed. Furthermore, there is a large contrast in the electrical properties between identified tectonic zones, due to their unique tectonic histories. Although penetration to greater depths is limited, the magnetotelluric data indicate a thick lithosphere below Southern Mongolia, in contrast to the previously reported thin lithosphere below Central Mongolia.

Magnetotelluric multiscale 3-D inversion reveals crustal and upper mantle structure beneath the Hangai and Gobi-Altai region in Mongolia

“…see Figs 19 and 22). This is consistent with previous 2-D models (Comeau et al, 2018c;Comeau et al, 2019b). However, what is unique in the 3-D model presented here is that the non-uniformness and lateral complexity of the feature is imaged.…”

“…The Gobi-Altai fault zone runs along the south of the Gobi-Altai mountain plateau (Calais et al 2003;Walker et al 2007). The contrasting properties observed across this zone may reflect the rheological differences between accreted terranes of different origins (see Guy et al, 2015 and references therein;Comeau et al, 2019b). Again, the location of the low-resistivity anomalies is coincident with these fault zones, and they offer a potential explanation for these features.…”

Compaction‐Driven Fluid Localization as an Explanation for Lower Crustal Electrical Conductors in an Intracontinental Setting

“…We analyze magnetotelluric (MT) data in the Bulnay region collected as part of a large regional array (Comeau et al, 2018, 2019, 2020; Käufl et al, 2020). Measurement sites have a spacing of <10 km along ~200 km long parallel profile segments, separated by ~50 and ~150 km (Figure 1).…”

Geodynamic Modeling of Lithospheric Removal and Surface Deformation: Application to Intraplate Uplift in Central Mongolia

“…South of the Hangai Dome, an ancient (Cambrian) collisional suture zone is found (e.g., Buchan et al, 2001), which is crustal-scale or possibly lithospheric-scale, that is today associated with a re-activated fault system, the South Hangai fault system (Walker et al, 2007). The structure appears as a pronounced feature in electrical resistivity models and is interpreted to be a weak zone between rheologically-distinct tectonic blocks (Comeau et al, 2021(Comeau et al, , 2020a(Comeau et al, , 2018Käufl et al, 2020). Comeau et al (2018) established…”