Morphological response to Quaternary deformation at an intermontane basin piedmont, the northern Tien Shan, Kyrghyzstan

摘要

The Tien Shan is a most active intracontinental mountain-building range with abundant Quaternary fault-related folding. In order to improve our understanding of Quaternary intermontane basin deformation, we investigated the intermontane Issyk-Kul Lake area, an anticline that was up-warped through the piedmont cover, causing partitioning of the alluvial fan veneer. To follow the morphological scenario during the warping process, we relied on surface-exposed and trenched structures and on alluvial fans and bajadas as reference surfaces. We used air photos and satellite images to analyze the spatial-temporal morphological record and determined the age of near surface sediments by luminescence dating. We demonstrate that the up-warped Ak-Teke hills are a thrust-generated subdued anticline with strong morphological asymmetry which results from the coupling of the competing processes of up-warp and erosional feedback. The active creeks across the up-warped anticline indicate that the antecedent drainage system kept pace with the rate of uplift. The rivers which once sourced the piedmont, like the Toru-Aygyr, Kultor and the Dyuresu, became deeply entrenched and gradually transformed the study area into an abandoned morphological surface. The up-warp caused local lateral drainage diversion in front of the northern backlimb and triggered the formation of a dendritic drainage pattern upfan. Luminescence dating suggest that the period of up-warp and antecedent entrenchment started after 157 ka. The morphologically mature study area demonstrates the response of fluvial systems to growing folds on piedmont areas, induced by a propagating frontal fold at a thrust belt edge, following shortening.