Morphotectonics of the northern Bogd fault and implications for Middle Pleistocene to modern uplift rates in southern Mongolia

Assessing the activity of strike-slip faults is often facilitated by cumulative offsets of morphological features. The December 4th 1957 Mongolia earthquake, generally referred to as the “Gobi Altai” or “Bogd” earthquake, had MW 8.1 and created long and complex surface ruptures of c. 350 km length along the Bogd fault that cuts across the foothills of the Gurvan Bogd mountains. Several left-laterally and vertically displaced Quaternary alluvial fans and morphogenic markers show cumulative offsets, allowing for an assessment of the effect of repeated earthquakes and active tectonics on landscape evolution. We propose to expand the active tectonic record along this fault up to a Middle Pleistocene time scale that spans regional climatic fluctuations, i.e. repeated glacial-interglacial cycles, and hence varying erosional and depositional rates.Five main drainage areas crosscutting the North Ikh Bogd segment of the Bogd fault were analysed and compared in terms of hypsometry, drainage steepness and alluvial fan slope inclination. Along the mountain front, deposits from all drainages form multiple alluvial fan levels that reflect recurring deposition-incision cycles. Previously published ages, as well as the spatial distribution of these fan levels, suggest that their incision and abandonment is induced by regionally dominant climate fluctuations. However, the increase of fan slope angles over time and the increased intensity of sub-surface folding observed in ground-penetrating radar profiles, suggests that cumulative tectonic uplift also has a strong impact on alluvial fan morphology. Estimates of the increase of fan slope angles as a result of cumulative tectonic uplift imply that Middle Pleistocene to modern vertical slip rates along the North Ikh Bogd segment of the Bogd fault are in the order of 0.9–1 mm/yr. However, much slower vertical slip rates along individual fault segments, ranging between 0.06 ± 0.01 mm/yr and 0.28 ± 0.07 mm/yr, indicate that uplift of the Ikh Bogd plateau occurs as a result of distributed deformation across multiple parallel fault strands within bedrock as well as alluvial fans. Our results emphasise the importance of regional studies along active faults that take into account the complexity of surface ruptures as well as the occurrence of related structures that can accommodate deformation.