transpressional uplift in the Gobi Altai, southern Mongolia The landscape evolution of Nemegt Uul: a late Cenozoic Geological Society, London, Special Publications

The geomorphology and structural geology ofNemegt Uul, Southern Mongolia, is examined as an example of a mountain range that has formed within a restraining bend along a major intracontinental strike-slip fault system, the Gobi-Tien Shan fault system. Structural and geomorphological analysis demonstrates that the mountain belt is young and has been differentially tilted and eroded. A geomorphological model is developed showing that uplift and erosion have resulted in the formation of deeply incised mountains, alluvial fans, badlands, desert pavements and dunes. The Gobi Altai Mountains of southern Mongolia comprise a series of discontinuous ranges that trend approximately east-west into northwest China (Fig. 1). The Gobi Altai rise from an extensive desert surface, which has elevations of between 1000 and 2000 m, to a maximum of 3957m in Ih Bodg. Cunningham et al. (1996, 1997) and Cunningham & Windley (1995) have shown that these mountains are transpressional uplifts that formed during the Cenozoic along a series of strike-slip faults that define a corridor of left-lateral transpressional deformation from southern Mongolia to NW China. The longest fault system within this corridor is the Gob i Tien Shan fault system which can be traced for over 800 km from the easternmost Tien Shan to the Gurvan Sayhan Ranges of the SE Gobi Altai. Cunningham et al. (1996) proposed that this corridor of left-lateral transpressional deformation represents a distant response to the continued northeastward indentation of India into Asia since approximately 50 Ma. Understanding the evolution of the Gobi-Tien Shan fault system, as well as other strike-slip fault systems, is important for understanding the overall Cenozoic deformation field of Central Asia (cf. Baljinnyam et al. 1993; Davy & Cobbold 1988; Tapponnier & Molnar 1979). The geomorphology of the Gobi Altai directly reflects the late Cenozoic active tectonics of southern Mongolia and provides important constraints on the geometry, style and timing of uplift in the region. This paper describes the tectonic and geomorphological characteristics of one of the main mountain ranges, Nemegt Uul, situated in southernmost Mongolia (Fig. 1). In it, a model for the landscape evolution of the region is presented that may be applied to the other transpressional mountain ranges in Mongolia and in similar mountain regions elsewhere in the world. Methodology Geomorphological and geological field mapping was undertaken at a variety of scales ranging from 1 : 5000 to 1:20 000 following established procedures using field survey techniques aided by Global Positioning Systems, barometric altimetry and levelling in selected regions of Nemegt Uul (cf. Owen et al. 1997). Mapping was assisted by the use of Landsat MSS images (Fig. 2) and aerial photographs. Transects into the mountain range were made to study the structural geology, geomorphology and sedimentology. Topographic maps (1:100000 scale) were used to examine the regional geomorphology of Nemegt Uul. A geographical information system (GIS, using ARC/INFO) was used for digital terrain analysis and to calculate geomorphological indices for part of central Nemegt Uul (Fig. 2B). Using the GIS, hypsometric curves were constructed using the methods of Strahler (1952) for the whole of the Nemegt Uul study area, north and south of the main drainage divide, and for individual catchment areas. An index was then derived to assess the skewness of elevations (catchment area skewness) within individual catchment areas as an aid to assessing their relative geomorphological maturity. In addition, geomorphological indices From: SMITH, B. J., WHALLEY, W. B. & WARKE, P. A. (eds) 1999. Uplift, Erosion and Stability." Perspectives on Longterm Landscape Development. Geological Society, London, Special Publications, 162, 201-218.1-86239-030-4/99/ $15.00 9 The Geological Society of London 1999. 202 L.A. OWEN E T AL . Fig. I. (A) The geographical setting of Nemegt Uul in southern Mongolia within the Gobi Altai Mountains. IB, lh Bogd; BB, Baga Bogd; AB, Arsta Bogd; SU, Severy Uul; UB, Ulaan Baatar; U, Urumchi. (B) Structural setting of Nemegt Uul along the Gobi Tien Shan fault system (after Cunningham et al. 1996). were used to compare regions throughout Nemegt Uul. These included mountain front sinuosity, using the methods of Bull & McFadden (1977), and a transverse topographic symmetry factor using Cox's (1994) method. Tectonic setting Two dominant left-lateral strike-slip fault systems are present in southern Mongolia and northwest China: the North Gobi Altai and the Gobi-Tien Shan fault systems (Cunningham et al. 1996). These fault systems occupy the central part of a wide corridor of left-lateral crustal movement between the Hangay Dome to the north and the Qilian Shan on the north side of the Tibetan Plateau. The North Gobi Altai fault system continues for over 300km from the eastern Gobi Altai to the western Altai mountains. To the south, the Gobi-Tien Shan fault system can be traced for over 800 km and terminates in the east in a series of ranges (Gurvan Sayhan Nuruu) that have the geometry of a horse-tail splay (Fig. 1B), while in the west it passes into the easternmost Tien Shan terminating in the sigmoidal-shaped restraining bends of Barkol Tagh and Bogda Shan. The study area, Nemegt Uul, is near the eastern end of the Gobi-Tien Shan fault system and constitutes a broad sigmoidal-shaped restraining bend (Figs 1B and 2B). Rock types and structural geology Unpublished mapping by the Mongolian Geological Survey coupled with field observations LANDSCAPE EVOLUTION: SOUTHERN MONGOLIA 203