This paper presents an updated synthesis of morphotectonic studies that quantify active tectonics along the Gurvan Bogd mountain range in the Mongolian Gobi-Altay, the site of one of the strongest historic intracontinental earthquakes (Mw 8.1) in 1957. Our goal was to determine the slip rate along the constituent fault segments and to estimate the return period of such large events. Along each segment, cumulative offsets were estimated from topographic surveys, and the ages of the offset markers were determined using cosmic-ray exposure dating. In this review, we reevaluate 10 Be data reported in previous publications using a chi-square inversion analysis of depth profiles and an updated scaling model for spatial production rate variations. We also discuss sampling strategies for dating alluvial fans in arid settings. This study confirms the low horizontal and vertical slip rates within the massifs of the Gurvan Bogd mountain range for the Late Pleistocene–Holocene period, suggests that episodes of aggradation occurred near the times of major glacial-interglacial terminations (at ca. 15–20 ka and ca. 100–130 ka), and provides evidence for another much earlier aggradational episode, occurring before 400 ka. The Bogd fault has a maximum horizontal left-lateral slip rate of ∼1.5 mm/yr, while reverse fault segments along the Gurvan Bogd fault system have vertical slip rates between 0.1 and 0.2 mm/yr. Characteristic dislocations observed along the Bogd fault suggest return periods of earthquakes similar to 1957 between 3000 and 4000 yr.

Abstract Restraining bend mountain ranges are fundamental orogenic elements in the Altai, Gobi Altai and eastern Tien Shan. In this paper, 12 separate restraining bends are reviewed to identify common structural and topographic characteristics. The 12 restraining bends occur in one of three different tectonic settings: (1) strike-slip fault termination zones; (2) at a major strike-slip fault bend where the individual strike-slip fault can be traced continuously from one end of the range to the other; and (3) where two separate strike-slip fault segments converge and overlap. Fault maps of the 12 separate bends reveal that they are all flower or half-flower structures in cross-section, but there is considerable architectural diversity and all have unique individual topographic, structural and dimensional characteristics. Many factors account for the architectural diversity of the restraining bend mountains, especially stepover width, total amounts of strike-slip displacement, reactivation of older structures, tectonic setting, and the angular relation between fault trace and maximum horizontal stress. The stepover sense for regionally important strike-slip faults is controlled by pre-existing basement heterogeneities and is dominantly contractional. Therefore, releasing bends and transtensional basins are largely absent. Throughout the region there is a continuum of mountain range types, from purely contractional ridges to isolated restraining bends along strike-slip-dominated zones. Nucleation, topographic uplift, along- and across-strike growth of the bend, and restraining bend coalescence with adjacent ranges appears to be an important mountain-building process in the Altai, Gobi Altai and eastern Tien Shan; similar processes are likely in other intracontinental transpressional orogens.

Abstract The Gobi Altai is an intraplate, intracontinental transpressional orogen in southern Mongolia that formed in the Late Cenozoic as a distant response to the Indo-Eurasia collision. The modern range formed within crust constructed by successive terrane accretion and ocean suturing events and widespread granite plutonism throughout the Palaeozoic. Modern reactivation of the Gobi Altai crust and the kinematics of Quaternary faults are fundamentally controlled by Palaeozoic basement structural trends, the location of rigid Precambrian blocks, orientation of SH max and possible thermal weakening of the lower crust as a result of an extensive history of Mesozoic–Cenozoic basaltic volcanism in the region, and the presence of thermally elevated asthenosphere under the Hangay Dome to the north. Modern mountain building processes in the Gobi Altai typically involve reactivation of NW–SE-striking basement structures in thrust mode and development of linking east–west left-lateral strike-slip faults that crosscut basement structures within an overall left-lateral transpressional regime. Restraining bends, other transpressional ridges and thrust basement blocks are the main range type, but are discontinuously distributed and separated by internally drained basins filling with modern alluvial deposits. Unlike a contractional thrust belt, there is no orogenic foreland or hinterland, and thrusts are both NE and SW directed with no evidence for a basal décollement. Normal faults related to widespread Cretaceous rifting in the region appear to be unfavourably oriented for Late Cenozoic reactivation despite widespread topographic inversion of Cretaceous basin sequences. Because the Gobi Altai is an actively developing youthful mountain range in an arid region with low erosion rates, it provides an excellent opportunity to study the way in which a continental interior reactivates as a result a distant continental collision. In addition, it offers important insights into how other more advanced intracontinental transpressional orogens may have developed during earlier stages of their evolution.

In this century, western Mongolia and the area adjacent to it in China have been one of the most seismic intracontinental regions of the world. Four earthquakes with magnitudes (M) ≥ 8 have occurred. Average displacements of several meters along ruptures more than 100 km long characterize all of them. The dominant style of faulting for each was strike-slip: left-lateral on easterly trending planes in the 1905 Bulnay and Tsetserleg earthquakes and in the 1957 Gobi Altay earthquake and right-lateral on a north-northwesterly trending Fu-yun fault in 1931. The ruptures associated with these earthquakes, with most other, smaller earthquakes, and with one older great earthquake suggest that western Mongolia is undergoing conjugate strike-slip deformation. Equivalently, the region undergoes northeast-southwest shortening and northwest-southeast extension. The component of shortening can be seen as a manifestation of the convergence between India and Siberia. The component of extension seems to mark a transition from an area of largely crustal shortening in China to another, the Baikal Rift system, where crustal extension is dominant. The average rate of seismic deformation in western Mongolia in this century consists of 49 (± 15) mm/a of northeast-southwest shortening and 40 (± 12) mm/a of northwest-southeast extension. Such high rates imply strongly that the twentieth-century seismicity has been abnormally high. Moreover, crude estimates of average recurrence intervals for great earthquakes on the Bulnay fault and in the Gobi Altay region are about 1,000 yr. Approximate Holocene or late Quaternary average slip rates on these faults are a few millimeters per year, suggesting that Mongolia is being sheared left-laterally with respect to Siberia at about 10 mm/a (between 5 and 20 mm/a). A similarly crude estimate for right-lateral slip along the northwest-trending Mongolian Altay is also 10 mm/a. We suspect that this right-lateral shear is a manifestation of the left-lateral regional shear parallel to east-west planes and of counterclockwise rotation of both the Mongolian Altay and the strike-slip faults within the range. Correspondingly, the eastward translation of western Mongolia with respect to Siberia manifests itself as crustal extension and rifting along the Hövsgöl and Baikal rift zones. In the Hangay, the broad upland in the interior of western Mongolia, scattered minor normal faulting with no obvious preferred orientation appears to be the common style of deformation. This area seems to be underlain by the same relatively hot upper mantle that underlies the Baikal rift system. Thus, as others have suggested, the collision between India and Eurasia does not appear to be the only cause of the active tectonics of western Mongolia. The perturbations to the stress field in the crust resulting from the emplacement (and upwelling) of hot material beneath the Baikal area and the Hangay and Hövsgöl uplands also play a key role in the active tectonics. Finally, the active deformation in western Mongolia appears to be young. Some surface faulting bears no obvious relation to the present topography. The very flat summit of Ih Bogd, the highest peak in the Gobi Altay, may have risen from the surrounding lowlands since only 1 Ma. Thus, the rapid deformation in western Mongolia seems to have begun tens of millions of years after India collided with Eurasia, perhaps as recently as a few million years ago.