Structural and geomorphic aspects of mountain-front evolution in an active continental collision zone: The NW Pamir, Southern Kyrgyzstan

M. R. Strecker, G. E. Hilley, J. R. Arrowsmith

Research output: Contribution to journalArticle

Abstract

The segmented thrust front (eastern, central and western segments) and compartmentalized drainage areas along the Main Pamir Thrust of the trans Alai range, Kyrgyzstan, are characterized by ongoing tectonic activity. Along strike, the mountain-front segments have accommodated varying degrees of approachment of the Pamir orogen with respect to the southern mountain front of the Tien Shan range to the north. This has resulted in a westward progression of closure of the intramontane Alai/Surkhob valley, which separates the Tien Shan and Trans Alai ranges (Pavlis et al., 1997; Arrowsmith and Strecker, 1999). The character of mountain-fronts can thus be examined in sectors ranging from incipient, advanced, and complete collision of the range; this environment thus provides the opportunity to evaluate the role of differential surficial and tectonic processes in range-front development in regions with a common structural history. 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 regraded, smooth alluvial fans whose topography is controlled by the baselevel of the axial Kyzilsu river which drains the Alai valley to the west. The sinuous western segment is defined by dextrally oblique thrusting in a zone of deformation in Tertiary sandstone, conglomerate, and siltstone, 300-500 m wide. Faulting has obliterated nearly all sedimentary characteristics of these rocks and created a zone of highly erodible fault gauge that abuts a narrow piedmont that is sometimes truncated by the Kyzilsu river. Due to the different lithologic and structural conditions in this segment, 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 of the former or 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 form 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 river. Unlike in the western segment, accomodation space for sediments still exists in the central segment, and the terraced steep mountain front is still preserved. In the west, the sediment accomodation space does not exist anymore and sediment is continuously transported out of the system similar to areas farther west (Pavlis et al., 1997). The large drainage basin area of the Kyzilsu river and constant runoff guarantee that the effective interplay between tectonic uplift and erosion is maintained. In contrast, in the more arid eastern sector of the Alai valley the limited erosional forces could not prevent a complete closure between The Trans Alai and the Tien Shan. In contrast to the other segments, active deformation has stepped back into the orogen and is now concentrated along the Markansu Fault, which is characterized by dextrally oblique thrusting and strike-slip faulting. Therefore, the geomorphically different mountain-front segments highlight the relations between tectonic uplift, geomorphic processes (which are in turn controlled by the lithology and current topographic conditions), and history of sediment routing throughout the landscape.

Original languageEnglish (US)
Number of pages1
JournalJournal of Asian Earth Sciences
Volume19
Issue number3A
StatePublished - May 12 2001
Externally publishedYes

ASJC Scopus subject areas

  • Geology
  • Earth-Surface Processes

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