Quaternary alluvial fans in the Gobi of southern Mongolia: evidence for neotectonics and climate change

Alluvial fans in southern Monglia occur along a group of narrow discontinuous mountain ranges which formed as transpressional uplifts along a series of strike-slip faults. They provide information on the nature of neotectonic activity in the eastern Gobi Altai range and on palaeoclimate change. Alluvial fan formation was dominated by various geomorphological processes largely controlled by climatic changes related to an increase in aridity throughout late Quaternary times. Their sedimentology shows that initially they experienced humid conditions, when the sedimentary environments were dominated by perennial streams, followed by a period of increasing aridity, during which coarse fanglomerates were deposited in alluvial fans by ephemerial streams and active-layer structures were produced by permafrost within the alluvial fan sediments. With climatic amelioration during early Holocene times, the permafrost degraded and fan incision and entrenchment dominated. Sedimentation was then confined to the upper reaches of the fans, adjacent to steep mountain slopes, and within the entrenched channels. The alluvial fans have been neotectonically deformed, faulted and their surface warped by small thrust faults that propagate from the mountain fronts into their forelands. Localised uplift rates are in the order of 0.1 to 1 m Ka−1.  1997 John Wiley & Sons, Ltd. alluvial fans; neotectonics; Gobi; Mongolia; cryoturbation. palaeoclimatologically, because variations in the intensity of Introduction the Mongolian High Pressure System (MHPS), which develops over the region and has a strong control on the regional climate of central Asia, have occurred throughout Impressive alluvial fans in the Gobi cover an extensive area that stretches from the Hangay Dome southwards to the Quaternary times. Changes in the MHPS are recorded in the Chinese loess sequence because the majority of loessic silt Qilian Shan, and eastwards from Bogda Shan to the eastern border of Mongolia (Fig. 1). These alluvial fans prograde on the Loess Plateau in China is derived from the Gobi and carried by western winds produced by the MHPS. Yet, from a series of narrow mountain ranges, which include the the Gobi Altai Mountains and the Gurvan Sayhan Nuruu. despite the importance of this pressure system for understanding the climate of central Asia, little is known of the The geomorphology of the alluvial fans in the Gobi was first described by Berkey and Morris (1927). Although they nature of climate change within Mongolia itself. This paper will examine the alluvial fans and their associated landforms recognised a series of phases of fan development, little attention was paid to their mode of formation with respect and sediments in the Gobi in order to determine the nature of their formation and to help reconstruct the palaeoclimatic to tectonics or climate change. This paper will, therefore, examine the geomorphology, sedimentology and the neotecconditions in which they formed. The alluvial fan record and the inferred palaeoclimatic conditions are compared tonic setting of the alluvial fans in order to provide a model for their formation and to aid in the elucidation of the with the existing palaeoclimatic data base in central China to aid providing a chronological control on their formation. tectonic history of the Gobi Altai Mountains in southern Mongolia. In addition, southern Mongolia is important