Fig. 1

Map of survey area. a Topographic map of the study area in Southern Mongolia. The profile is marked with a black line (A–A′). Measurement locations are circles; black are magnetotelluric sites and white are telluric-only sites (see text for details); vertical bars indicate measurements from 2016, horizontal bars from 2017, otherwise from 2018. Sites shown in Fig. 6 are marked (2360, 2680, 2690). Blue dashed lines indicate the surface locations of fault zones (black text; Styron 2018; Walker et al. 2007). Names and locations of tectonic zones (grey text) are from Kröner et al. (2010) and Badarch et al. (2002). Local names of mountain ranges are indicated (white text; from Bassa (2012)). Selected villages are indicated for reference (grey squares): Bo Bogd, Ba Bayanlig, Gu Gurvantes. The gold square marks the location of Tsagaan Tsahir Uul (TTU) gold deposit and mining activity. The grey star (labelled BEq) marks the location of the 1957 Bogd rupture, an intraplate earthquake of magnitude 8.4 (Rizza et al. 2015). b Inset map showing location of the survey area in a greater tectonic context. Mongolia is situated between the Siberian craton and the North China and Tarim cratons (dashed grey lines). Southern Mongolia is assembled from accreted terranes amalgamated against the Hangai microcontinent (Badarch et al. 2002). The survey location (grey box), MT profile (red line), Mongolian Altai and Gobi–Altai mountains, political boundary of Mongolia (thin black line), and the capital city, Ulaanbaatar (UB), are indicated