The N-vergent Main Pamir Thrust (MPT) along the northern perimeter of the Trans Alai Range illustrates how ongoing continental collision is absorbed and partitioned into discrete fault segments that record a Holocene slip rate of >6 mm/yr. Detailed structural and tectonic landform mapping, seismicity, and paleoseismicity data along the mountain front between 073°E and 071°E30°N defines thrust dominated central and western segments. The western segment is approximately 50 km long, which is linked to the central segment by a 20-km-wide transfer zone with dextral shear and thrust faults. Although both segments are tectonically active, the structural and geomorphic evolution of their mountain fronts differ significantly.
Along the linear central segment, deformation is confined to a narrow single S-dipping thrust fault zone that juxtaposes Neogene/Pleistocene and Holocene conglomerates. In this sector, the mountain front has numerous Quaternary offsets, including a 4.4 ka terrace uplifted 18 m, and an array of younger offsets. Prevailing structural style and long term deformation in the central segment are underscored by multiple flights of gently sloping pediments and glacigenic terrace surfaces that abruptly terminate at the steep mountain front. The adjacent piedmont, however, is characterized by up to 15-km long re-graded, smooth, alluvial fans whose topography is controlled by the baselevel of the Kyzilsu River which drains the Alai Valley to the west. In contrast, the sinuous western segment is defined by dextrally oblique thrusting in a zone of deformation in Palaeogene sandstone, conglomerate, and siltstone, 300 to 500 m wide. Faulting has obliterated nearly all sedimentary characteristics of these rocks and created a zone of highly erodible fault gouge that abuts a narrow piedmont that is sometimes truncated by the Kyzilsu River. Due to the segment's different lithologic and structural conditions, the principal fault is covered by numerous large landslides and rotational slumps that are rooted in the incompetent fault-gouge rocks. Large landslides in this setting are always associated with different levels of fluvial terraces or the present thalweg of the Kyzilsu River. This suggests a causative relation between lateral fluvial scouring, failure of mountain fronts, and sustained faulting in an environment of ongoing collisional deformation.
Apart from highlighting the principal processes of shaping tectonically active mountain fronts, the two segments also document how differing stages of progressive closure of an intramontane basin may influence feedback mechanisms between surface processes and active faulting at the mountain front. Faults of the western segment nearly impinge upon the basement rocks of the southern Tien Shan Range and annihilate the intramontane basin, replacing it with a narrow valley. In contrast, faults in the wider central segment displace rocks not prone to landsliding at locations far from the immediate erosive influence of the trunk stream. Uplift in this sector provides high sediment yields in tributary streams responsible for a northward migration of the Kyzilsu. Unlike in the western segment, accommodation space for sediments still exists in the central segment, and the terraced steep mountain front is still preserved. Therefore, these two geomorphically different segments highlight the interplay between tectonic uplift, geomorphic processes acting over the landscape (which are in turn controlled by the lithology and current topographic conditions), and history of sediment routing throughout the landscape.