Interactions between tectonics, climate and surface processes over the last 200,000 years in the Naryn Basin (Kyrgyz Tien Shan).

Geomorphological markers such as alluvial fans, fluvial or lacustrine terraces, and landslides reflect the interaction among tectonics, climate, and surface processes in mountain belts and basins. To understand the evolution of the Tien Shan Range, which is located in Central Asia, the study of intramontane basins may provide insights on the rate of the deformation as active faults and folds commonly deform both Cenozoic and Quaternary deposits. In this active orogen, the intramontane Naryn Basin is the right place to study tectonic deformations recorded by fluvial deposits as well as changes in fluvial networks since the Pliocene-Pleistocene. Based on analysis of high-resolution topographic data and field investigations, we propose a revised mapping of the western Naryn Basin and a new evolutionary model for the chronology of its Quaternary deposits. Indeed, the use of several Quaternary dating methods (luminescence, cosmogenic nuclides, radiocarbon) enables us to better constrain the evolution of the landscape at different time scales, highlighting drastic changes over the last 200,000 years.

First, the western Naryn Basin was characterised by fluvial dynamics with deep fluvial incisions, aggradation of large alluvial fans and terraces, likely controlled by glacial/interglacial cycles. These deposits were deformed by several deep-seated, relatively steeply dipping thrust faults during the Pliocene-Pleistocene. During the late Pleistocene, a major earthquake probably triggered the giant Beshkiol landslide which blocked the Naryn River, having a major impact on the sedimentary dynamics of the entire upstream basin. A large lake more than 80 km long and lake sedimentation lasted around 36,000 years, changing the base level of the Naryn river and reshaping the surrounding paleotopography. This lake was probably emptied during a cataclysmic event (dam breach) with evidence found both in the Naryn and the downstream Kazarman basins. The external factors that led to the dam's failure are still debated, but they are contemporary with the Bølling-Allerød interstade. Fluvial conditions prevailed for a short period of time before a second damming of the Naryn basin by the Beshkiol landslide, and the subsequent restoration of lake conditions for a period of ~7 600 years. This second lake was gradually emptied and formed large geomorphological flats, which have long been wrongly interpreted as fluvial top surfaces. In the late Holocene, the rapid incision of the Naryn river to restore its base level has strongly reshaped the Naryn basin, erased large volumes of sediments and new fluvial terraces are emplaced. Weak evidence of tectonic deformation is noted along the thrust faults, which raises the question of possible inhibition of tectonic activity by the long residence time of the two lakes.