Morphotectonic evidence for widespread active faulting in southern Mongolia

Four M~8 earthquakes in the 20^th century reflect active deformation in western Mongolia as a result of far-field stresses related to the India-Eurasia collision. Historic seismicity indicates that deformation localises around the relatively rigid Hangay dome in central Mongolia, however, tectonic lineaments in the surrounding Valley of Lakes basins suggest more widespread and diffuse deformation. In southern Mongolia, seismicity clusters around the Bogd fault, which ruptured during the 1957 Mw 8.1 Gobi Altai earthquake. To determine whether the kinematics interpreted from this earthquake are regionally representative, especially in consideration of the heterogeneity of intraplate tectonics, we expand the spatial scale of tectonic studies to range between the Gobi Altai and Hangay massifs. We do this by combining observations from regional and local digital elevation models, ground-penetrating radar analyses, geological and geomorphological field data, and seismic reflection data. Additionally, we increase the temporal scale of palaeoseismic studies up until the Middle Pleistocene through OSL and surface exposure dating, to compare the effects of tectonic processes to those of Quaternary climate variations on landscape evolution. We show that reverse and oblique strands of the Bogd fault accommodate <0.3 mm/yr vertical slip rates along the northern margin of the transpressive Gobi Altai massif. Four ~E-W striking faults in the seismically quiescent Valley of Gobi Lakes each have the potential for M~7 earthquakes and they are likely part of a left-lateral strike-slip system rooted at depth. Although cumulatively, the Valley of Gobi Lakes faults are deforming at a regionally representative ~0.3 mm/yr vertical slip rate, recurrence intervals of major earthquakes are much longer than those determined along the Bogd fault (~5-80 ka vs. 3-5 ka). Overall, we interpret the Valley of Gobi Lakes faults to have played a large role in drainage reorganisation and Middle Pleistocene to modern landscape evolution. Sub-surface faults interpreted from seismic reflection data and associated geomorphological irregularities in the Orog Nuur Basin indicate two NW-SE striking lineaments that may connect the Valley of Gobi Lakes fault system to the Bogd fault system. Our observations suggest a more complex and extensive fault system in southern Mongolia than previously expected and the geometry and potential connectivity of faults indicates a continuing northward progression of transpressive deformation from the Gobi Altai towards the Hangay. The obscurity of active deformation in the Valley of Gobi Lakes is likely due to faster erosion and deposition rates and this highlights the importance of understanding the interplay between tectonic, climatic and geomorphological processes and their effects on the landscape system. We suggest that, especially in slowly deforming, intraplate regions, an increase of spatial and temporal scales of active tectonic research is necessary to improve interpretations of tectonically altered landforms, palaeo-environmental reconstructions, and seismic hazard assessments.