Climatic, tectonic and landslide-dam signals preserved in alluvial terraces of the Naryn Basin, Kyrgyzstan

Abstract. Fluvial terraces can act as geomorphic markers of past environmental change. Although they are commonly interpreted in the context of a single driver, multiple climatic and/or tectonic processes may occur simultaneously in nature. Here, we test our ability to interpret terrace sequences with potentially complex formation histories by studying the Naryn Basin in the central Tien Shan, where glacial–interglacial cycles affected land-surface processes, numerous active faults accommodate crustal shortening, and a large landslide temporarily dammed a lake. We investigate the relative contribution of these terrace-formation drivers by mapping terrace profiles, providing new constraints on the terrace chronology, and numerically simulating terrace formation in response to climate change and lake drainage. Tributary terrace profiles record local deformation in response to faults and a regional concave-up pattern that could be explained by lacustrine sedimentation and subsequent base-level fall induced by lake drainage. Terrace ages cluster between 15 and 20 ka, correlating with both deglaciation following the last glacial maximum and lake drainage. Our modelling, together with the spatial distribution of modelled terraces, indicates that lake drainage had a strong, but spatially restricted impact on river long-profile evolution, whereas Pleistocene climate triggered regional terrace formation. By integrating numerical modelling of alluvial river long-profile evolution with detailed mapping and geochronology, we present a robust approach to establish direct links between environmental change and the resulting terrace record.