Modelling results
The 2-D resistivity model derived from the MT data reveals several interesting features, described and discussed below. Variations in resistivity can be due to multiple causes (including different rock types, the presence of fluids, or partial melts), and therefore any interpretation of model features benefits from additional information (e.g., Unsworth and Rondenay 2012).
The near-surface layer (< 1.5 km) has a highly variable resistivity (3–3000 Ωm; see Fig. 4). It features two highly conductive regions (< 30 Ωm), in the Valley of Lakes (near Bogd), which is an internally drained basin between the Hangai and Gobi–Altai mountains (e.g., Cunningham 2001), and north of the Nemegtiyn mountain range (near Gurvantes), which is a Cenozoic alluvial basin (Badarch et al. 2002) (see Fig. 1 for locations). These conductive features may be caused by porous sediments in these regions or by increased salinity due to evaporite minerals. Another shallow (< 5 km) conductive feature (~ 100 Ωm; G1) is seen at the northern end of the profile. It is located below the Tsagaan Tsahir Uul gold deposit, which is an area of significant mineralization, containing important sources of gold and copper (e.g., Buchan et al. 2001; Dejidmaa and Badarch 1999). The observed electrical signature may be associated with emplaced mineralization in the upper crust, or past hydrothermal fluid alteration (see Comeau et al. 2018b).
The upper crust (< 20 km) is generally resistive (1000–30,000 Ωm). This can be explained by the Neoproterozoic basement rocks and Paleozoic ultra-mafic rocks believed to comprise this region (e.g., Guy et al. 2015; Badarch et al. 2002). Within the upper crust, MT data detect two prominent anomalously conductive features (30–100 Ωm; F1 and F2), imaged as elongated structures stretching nearly vertically through the upper crust to depths of 20+ km and dipping slightly southwards. The feature F1 was previously imaged by Comeau et al. (2018a) and has a consistent geometry and resistivity, despite differences in data preparation and inversion methodology. Both features F1 and F2 are well resolved by the dense MT site spacing. Faults and suture zones are regions of fractured and weakened crust that may contain circulating fluids or hydrothermally altered zones that significantly lower the electrical resistivity (e.g., Unsworth and Rondenay 2012). Therefore, these features are interpreted to represent suture zones.
In fact, these anomalous features are spatially associated with the surface expressions of known fault zones and inferred terrane boundaries (Badarch et al. 2002), which are expected to be deep-reaching, possibly lithospheric-scale (Calais et al. 2003). The feature F1 is coincident with the location of the large, seismically active, left-lateral strike–slip and transpressional Bogd fault zone at the base of Ih Bogdin and the Gobi–Altai mountain range (Walker et al. 2007; Calais et al. 2003). Moreover, it is known to be a major lithological boundary (Badarch et al. 2002), and is thought to be an ancient suture zone (e.g., Xiao et al. 2015). The epicentre of a large rupture event in 1957, which had a strike–slip mechanism and a moment magnitude larger than 8, is located less than 100 km to the west, near the Bayantsagaani mountain range (e.g., Calais et al. 2003, and references therein; Rizza et al. 2015). Furthermore, the locations of recently measured earthquakes (see Meltzer et al. 2019) align on each side of the feature F1, at depths of 0–17 km. The feature F2 is coincident with the major Gobi–Tienshan fault as it passes near the Nemegtiyn mountain range, which is a left-lateral strike–slip fault that stretches westward into China and can be continuously traced for more than 1000 km (e.g., Styron 2018; Walker et al. 2007).
Both features appear to dip southwards, approximately 70–80° to the horizontal. This is consistent with geological estimates of fault dips in this region (e.g., Rizza et al. 2015). Furthermore, these fault zones each consist of two surface traces with a broad zone between. Because a combination of strike–slip and transpressional faulting (along restraining bends) is observed here, we speculate that this may indicate a subsurface flower-structure, with the fault segments joining into a single strand in the basement. The fault extent at greater depths is speculative, but because these faults represent terrane sutures they are believed to be lithospheric-scale (e.g., Calais et al. 2003), that is, they are expected to pass through the crust into the upper mantle.
There is no near-surface conductive feature spatially aligned with the small Trans-Altai fault, located at the southern edge of the Gobi–Altai plateau and the Gichgeniy mountain range (e.g., Walker et al. 2007; Guy et al. 2014). However, a strong electrical signature is likely absent because deformation along this fault is primarily located to the west, where there is a step in surface topography, as indicated by the end of the mapped surface trace ~ 20 km west of the profile (see Fig. 1; Walker et al. 2007). Although tectonostratigraphic maps show this boundary extends farther east (Kröner et al. 2010). Additionally, this fault has very little seismic activity, historic or current (e.g., Meltzer et al. 2019). However, a deeper (mid-crustal) conductive anomaly (50–100 Ωm; C2) may be related to this fault system (see below). Similarly, no conductive feature is seen further south near the Toson Bumbin mountain range.
In contrast to the upper crust, the lower crust (20–45 km) has a generally moderate resistivity (300–1000 Ωm). Two lower-resistivity zones (50–100 Ωm) are observed beneath the Gobi–Altai zone (C1; 30–50 km depth, it appears smeared downwards) and the Trans-Altai zone (C2; 15–30 km depth). The low-resistivity zones are disconnected and offset, signifying a crustal boundary. According to global models for Curie-point depths (Li et al. 2017), the 550 °C temperature isotherm is at a depth of approximately 20 km in this region, indicating that feature C1 is located below the brittle–ductile transition and feature C2 is near it. The low-resistivity zones suggest that the crustal (lithotectonic) blocks are not homogenous. This may therefore agree with previous studies which have argued that an enigmatic and allochthonous lower crust exists below the Gobi–Altai zone and Trans-Altai zone (Guy et al. 2015; Kröner et al. 2010). One study, based on geochemical constraints and gravity modelling, concluded that the lower crust of the Mongolian CAOB in this region is felsic and juvenile metamorphosed continental crust due to the relamination of the Khantaishir magmatic arc (Guy et al. 2015). Therefore, these low-resistivity zones may be controlled by past deformation episodes. Alternatively, the anomaly C2 may be related to an ancient accretionary wedge associated with the Trans-Altai fault (Badarch et al. 2002; Guy et al. 2015) due to a previous subduction event north of the South-Gobi zone (e.g., Xiao et al. 2015). In contrast, the South-Gobi zone appears to have a homogenous lower crust (> 1000 Ωm), which is consistent with the fact that it is believed to consist of a Precambrian cratonic basement (Guy et al. 2014).
Although resolution at greater depths is limited, the resistivity model images several interesting features within the upper mantle. Below the northern portion of the profile, a conductive feature (~ 100 Ωm; M1) appears below a depth of 70 km. This is believed to be a continuation of the shallow asthenosphere previously detected below Central Mongolia (~ 70 km; Comeau et al. 2018a), which agreed with the architecture of the lithosphere–asthenosphere boundary predicted from a seismic profile across central Mongolia (Petit et al. 2008). For the southern portion of the profile (south of the Gobi–Altai Mountains), the MT data indicate a resistive upper mantle (~ 1000 Ωm). This gives evidence for a thick lithosphere (> 100 km) below Southern Mongolia. Hence, this indicates the presence of a steep lithospheric step. This is corroborated by modern global tomography models (Ho et al. 2016) that predict an increase of ~ 36 km in the thickness of the (thermal) lithosphere across the entire profile (from latitude 45° N to 43° N). In this study, the location of the lithospheric step is clearly imaged beneath the Gobi–Altai mountains, which can possibly be explained by the major suture zone running along the Main Mongolian Lineament (e.g., Xiao et al. 2015). Other studies (Comeau et al. 2018a; Petit et al. 2008) placed the step farther north, and related it to the major suture zone formed by the closure of the Mongol-Okhotsk ocean (e.g., Van der Voo et al. 2015; Sheldrick et al. 2018). In addition, an anomalous feature is imaged in the upper mantle below the South-Gobi zone (M2; 200–300 Ωm). It is unexplained, but may be related to the lithospheric-scale suture of the South-Gobi zone along the Gobi–Tienshan fault. Alternatively, it could be related to hydrated lithosphere, thought to exist in this region from previous subduction events, or it could be a consequence of a very local shallowing asthenosphere (e.g., Sheldrick et al. 2018).
Constraints on tectonic boundaries
Combining geological, geochemical, and geochronological data, Badarch et al. (2002) composed a model of Mongolia consisting of 44 separate lithostratigraphic domains interpreted as terranes. In Southern Mongolia, to first-order, narrow, east–west oriented terranes were amalgamated and accreted about a central nucleus (Badarch et al. 2002), which was separated by a major suture known as the Main Mongolian Lineament (e.g., Kröner et al. 2010; Xiao et al. 2015). This nucleus consisted of the cratonic block of Central Mongolia (Hangai) and microcontinent fragments, including the Baydrag block. Although terrane accretion was spatially and temporally complex, it is believed to have generally occurred north to south with time (e.g., Badarch et al. 2002). Terranes are expected to be fault-bounded fragments, but the lithostratigraphic domains do not consistently align with the position of known surface faults. Kröner et al. (2010), revisiting older models, simplified Southern Mongolia into four principal tectonic zones, or lithotectonic blocks (see Fig. 1). Geophysical potential field data analysed by Guy et al. (2014; gravity and magnetics) revealed minor disagreements in the positions of several suspected terranes, but supported the principal tectonic zones.
The Baydrag block is an early Proterozoic continental fragment overlain by a Cambrian passive margin (Guy et al. 2014; Kröner et al. 2010; Badarch et al. 2002). It is located in the Valley of Lakes, south of the Hangai, and separated by the South Hangai fault zone, and north of the Gobi–Altai (e.g., Badarch et al. 2002). the lake zone is a narrow tectonic zone adjacent to the Bogd fault system, and the Main Mongolian Lineament, that reaches from Baga Bogdin and Ih Bogdin mountain ranges to the Har Argalantin and Bayantsagaani mountain ranges and farther west (Kröner et al. 2010). Guy et al. (2015) describe it as an Early Cambrian accretionary wedge composed of volcanic arcs (possibly island arcs) and accretionary prisms thrust over the basement rocks of the Baydrag block. South of the Gobi–Altai mountains is a plateau, which defines the Gobi–Altai zone, which ends at the Trans-Altai fault. The Gobi–Altai zone consists of thick Cambrian/Ordovician volcano-sedimentary sequences interpreted as an accretionary wedge covered by Silurian/Devonian passive margin and high-grade metamorphic rocks (e.g., Guy et al. 2015). The Trans-Altai zone lies to the south and ends at the Gobi–Tienshan fault. This zone consists of Devonian marine sediments and volcanic rocks, representing a passive margin and a volcanic arc, as well as deeper ultra-mafic rocks that give evidence for the formation of oceanic crust during the Early Devonian, which corresponds with an ocean opening (e.g., Guy et al. 2015). South of this is the South-Gobi zone, which continues to the Mongolia–China border and beyond to another major suture zone in Northern China (Kröner et al. 2010; Xiao et al. 2015). This zone consists of Ordovician/Silurian siliciclastic sediments, typical of continental-margins, and Carboniferous volcanic rocks atop a Precambrian cratonic basement (Guy et al. 2014). In addition, both the Trans-Altai zone and the South-Gobi zone have intrusive Carboniferous and Permian granitoids, found to be related to basement melting from zircon analysis and located along the main tectonic boundaries, coinciding with weakened deformation zones (Guy et al. 2015).
Tectonic zones and terranes, in the classic sense, are fragments and must be bounded by suture zones, which are often deep-reaching. Therefore, by identifying suture zones, the terrane boundaries, we can identify individual terranes. However, ancient suture zones, especially those that have undergone convergent deformation, as is the case in Southern Mongolia, may be obscured and difficult to detect on the surface (e.g., Dewey 1977). Fortunately, faults and suture zones generally contain fluids, giving them an anomalous electrical signature compared to their surroundings, and allowing us to image them (Türkoğlu et al. 2008; Unsworth and Rondenay 2012).
As discussed above, the resistivity model clearly images several major fault zones. Therefore, the model agrees with the simplification of Kröner et al. (2010) for four principal tectonic zones (see Fig. 4). Nevertheless, distinct lithologies may exist within each tectonic zone, although their resistivity differences, due to differences in petrophysical properties and rock types, are not readily distinguishable, but we can conclude that they are not fault bounded. The distinct lithostratigraphic domains of Badarch et al. (2002) can be adjusted slightly so that they fit within the tectonic zones or along the suture zones. It should be noted that, from the resistivity model, we separate the tectonic regions based on the bounding faults only, rather than on other geological, geochronological, or structural constraints, which may cause small discrepancies with the inferred boundaries drawn by others.