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SUMMARY We carried out morphotectonic studies along the left-lateral strike-slip Bogd Fault, the principal structure involved in the Gobi-Altay earthquake of 1957 December 4 (published magnitudes range from 7.8 to 8.3). The Bogd Fault is 260 km long and can be subdivided into five main geometric segments, based on variation in strike direction. West to East these segments are, respectively: the West Ih Bogd (WIB), The North Ih Bogd (NIB), the West Ih Bogd (WIB), the West Baga Bogd (WBB) and the East Baga Bogd (EBB) segments. Morphological analysis of offset streams, ridges and alluvial fans-particularly well preserved in the arid environment of the Gobi region-allows evaluation of late Quaternary slip rates along the different faults segments. In this paper, we measure slip rates over the past 200 ka at four sites distributed across the three western segments of the Bogd Fault. Our results show that the left-lateral slip rate is ∼1 mm yr-1 along the WIB and EIB segments and ∼0.5 mm yr-1 along the NIB segment. These variations are consistent with the restraining bend geometry of the Bogd Fault. Our study also provides additional estimates of the horizontal offset associated with the 1957 earthquake along the western part of the Bogd rupture, complementing previously published studies. We show that the mean horizontal offset associated with the 1957 earthquake decreases progressively from 5.2 m in the west to 2.0 m in the east, reflecting the progressive change of kinematic style from pure left-lateral strike-slip faulting to left-lateral-reverse faulting. Along the three western segments, we measure cumulative displacements that are multiples of the 1957 coseismic offset, which may be consistent with a characteristic slip. Moreover, using these data, we re-estimate the moment magnitude of the Gobi-Altay earthquake at Mw 7.78-7.95. Combining our slip rate estimates and the slip distribution per event we also determined a mean recurrence interval of ∼2500-5200 yr for past earthquakes along the different segments of the western Bogd Fault. This suggests that the three western segments of the Bogd Fault and the Gurvan Bulag thrust fault (a reverse fault bounding the southern side of the Ih Bogd range that ruptured during the 1957 earthquake) have similar average recurrence times, and therefore may have ruptured together in previous earthquakes as they did in 1957. These results suggest that the western part of the Bogd Fault system, including the Gurvan Bulag thrust fault, usually behaves in a 'characteristic earthquake' mode. [ABSTRACT FROM AUTHOR]

We investigate mountain building in the Altai range of western Mongolia, focusing on Baatar Hyarhan, a NW-trending massif bounded by active thrust faults. Our primary aims are to describe how thrusting has evolved over time, to calculate late Quaternary slip rates by dating offset alluvial markers with optically stimulated luminescence (OSL) and to compare these late Quaternary rates with measurements of deformation on decadal and geological timescales. Patterns of topography and drainage suggest that Baatar Hyarhan has grown in length and has propagated laterally from the SE towards the NW over time. On the NE side of the massif, the range-bounding Zereg fault appears active only along younger parts of Baatar Hyarhan; next to the oldest, SE part of the massif faulting has migrated into the adjacent Zereg Basin, where it has uplifted low, linear ridges of folded sediment, known locally as forebergs. On the SW side of the massif, only the range-bounding Tsetseg fault appears active. Using OSL, we establish ages of ∼15, ∼20 and ∼85 kyr for alluvial deposits cut by these faults. These ages are close to those of alluvial markers in the separate Gobi Altai range, suggesting that periods of fan and terrace formation may correlate over wide tracts of Mongolia, presumably under the primary control of climate. Combining our OSL ages with offsets measured with differential GPS, we calculate Late Quaternary slip rates across forebergs in the Zereg Basin and across the range-bounding Zereg and Tsetseg faults. Uncertainties in fault dip (due to lack of clear fault exposures) and burial ages (due to incomplete resetting of the luminescence clock) mean that the exact slip rates are poorly constrained. Nevertheless, the vertical displacement rates we calculate across the Zereg and Tsetseg range-front faults—0.2–0.6 and 0.1–0.4 mm yr−1, respectively—are at the lower end of long-term (∼5 Myr) estimates of 0.4–0.8 and 0.3–0.7 mm yr−1, respectively. Vertical rates of deformation may, therefore, have remained constant over the past ∼5 Myr, but equally the late Quaternary rates might be lower than the geological ones. This possible discrepancy could be accounted for if some of the shortening has shifted away from the range-front faults onto other nearby structures. The forebergs in the eastern Zereg Basin are an obvious candidate, but they show at least 10 km cumulative shortening (which would take a few Ma to accumulate at late Quaternary rates) and cannot simply be regarded as the latest stage of outward mountain growth. The total Late Quaternary shortening rate across all three areas of faulting is 0.7–2.4 mm yr−1, making up between one tenth and one third of the ∼7 mm yr−1 convergence across the whole Altai range. [ABSTRACT FROM AUTHOR]

We present a joint inversion of gravity and teleseismic data to enlighten the lithospheric structures of the Baikal–Mongolia region, an area characterized by high topographic contrasts, sporadic Cenozoic volcanism, extension and large transcurrent faulting in the vicinity of the Baikal Rift, Central Asia. The study uses a 1000 km long seismic transect that cross-cuts the main tectonic structures from north to south (namely, the Siberian platform, Tunka basin, Hangay Dome and Gobi-Altai belt). The Siberian platform depicts a high-velocity lithosphere down to about 150 km. We evidence strong velocity contrasts within the crust below the Hangay Dome and the Tunka depression, interpreted as a thickened crust. A low-velocity/density region is located at various depths below the Hangay Dome. Thanks to the dense spatial coverage of gravity data, we are able to define the 3-D geometry of this particular low-velocity/density anomaly. The Hangay Dome anomalous body extends from 60 to 225 km depth at his largest point and slightly thins to no more than 40 km at its easternmost end. A deep low-velocity zone (below 150 km) with only a weak negative density contrast is observed on the eastern part of the transect. We propose that the late Cenozoic uplift of the whole Mongolian Plateau and associated rifting, magmatism, high heat flow and lithospheric thinning are not externally driven by the India–Asia collision, but results from the interaction of two mantle plumes with the overlying lithosphere. Conversely, recent faulting may be the major expression of the India–Asia collision in this region, as well as N–S striking rift basins that connect and interact with major strike-slip faults, such as the Bolnai fault. The comparison between time residuals of this experiment and the Baikal Rift seismic data shows an undeniable disproportion for the Hangay anomaly. The Hangay time residuals are far more positive than through the Baikal, arguing for an asthenospheric signature that is not seen beneath the Baikal Rift. Furthermore, the southern edge of the thick Siberian cratonic lithosphere may favour the rise of a sublithospheric mantle flow at the contact with the Baikal–Mongolia lithosphere. [ABSTRACT FROM AUTHOR]

We identify and describe a series of east–west left-lateral strike-slip faults (named the Songino-Margats, the Hag Nuur, the Uliastay and the South Hangay fault systems) in the Hangay mountains of central Mongolia: an area that has little in the way of recorded seismicity and which is often considered as a rigid block within the India–Eurasia collision zone. The strike-slip faults of central Mongolia constitute a previously unrecognized hazard in this part of Mongolia. Each of the strike-slip faults show indications of late Quaternary activity in the form of aligned sequences of sag-ponds and pressure-ridges developed in alluvial deposits. Total bed-rock displacements of ∼3 km are measured on both the Songino-Margats and South Hangay fault systems. Bed-rock displacements of 11 km are observed across the Hag Nuur fault. Cumulative offset across the Uliastay fault systems are unknown but are unlikely to be large. We have no quantitative constraint on the age of faulting in the Hangay. The ≤20 km of cumulative slip on the Hangay faults might, at least in part, be inherited from earlier tectonic movements. Our observations show that, despite the almost complete absence of instrumentally recorded seismicity in the Hangay, this part of Mongolia is cut through by numerous distributed strike-slip faults that accommodate regional left-lateral shear between Siberia and China. Central Mongolia is thus an important component of the India–Eurasia collision that would be overlooked in models of the active tectonics based on the distribution of seismicity. We suggest that active faults such as those identified in the Hangay of Mongolia might exist in other, apparently aseismic, regions within continental collision zones. [ABSTRACT FROM AUTHOR]

In this paper, we test the role of vertical axis rotations during transpressional mountain building. To this end, we carried out a palaeomagnetic study in the NE Gobi Altai of southern Mongolia, sampling widely exposed lower Cretaceous lavas allowing comparison of rotation histories of the Ih Bogd, Baga Bogd and Artz Bogd restraining bends at the eastern termination of the Bogd strike-slip zone. We provide new 40Ar/39Ar ages to show that the stratigraphy of mafic lavas and fluvio-lacustrine sediments on the southern flanks of Mt Ih Bogd and Mt Baga Bogd have ages between ∼125 and ∼122 Ma, and a mafic sill that intrudes the sequence has an age of 118.2 ± 0.8 Ma. The lavas are older than previously dated lavas south of Artz Bogd, with ages of 119–115 Ma. Palaeomagnetic results from the 119–115 Ma lavas south of Artz Bogd show a significant steeper inclination than both results from 125 to 122 Ma lavas of Baga Bogd and Ih Bogd, as well as from newly sampled and previously published younger lavas and necks of the 107–92 Ma Tsost Magmatic Field and Shovon and Khurmen Uul basalts. We explain this result by insufficient averaging of secular variation and small errors induced by overcorrection of bedding tilt. We show that individual lavas in the SE Artz Bogd locality represent individual spot readings of the Earth's magnetic field and integrate all results obtained from lower Cretaceous lavas in the Gobi Altai. We present a pole, or rather, an apparent polar wander path without significant plate motion from the reference positions of Eurasia, from ∼125 to 95 Ma, with n= 126, λ= 80.8°, φ= 158.4°, κ= 25.3, A95 = 2.5, palaeolatitude = 48.2 with a scatter Sλ= 16.7 (Sl = 15.3, Su = 17.8) and a regionally consistent direction for the Gobi Altai of D/ I= 11.1°/65.9°, Δ D/Δ I= 3.8°/1.9°. This is one of the best-determined palaeopoles/APWP's for Asia. Formation of the Ih Bogd, Baga Bogd and Artz Bogd restraining bends was thus not associated with vertical axis rotations larger than our error margin of ∼10°. From this we conclude that the Bogd strike-slip zone is a weak fault zone, in which shear is localized. [ABSTRACT FROM AUTHOR]