Late Holocene debris flows and valley floor development in the northern Zailiiskiy Alatau, Tien Shan mountains, Kazakhstan

摘要

This study uses geomorphological, dendrochronological, and archival research to investigate the pattern, chronology, and sedimentology of debris-flow deposits in two reaches of the Zailiiskiy Alatau range of the Tien Shan mountains, Kazakhstan. Steep and narrow low-order tributary valleys in this environment promote rapid coarse sediment transfer to trunk streams and, in wider trunk valley reaches, locally result in development of debris-now assemblages and terraced sequences of coarse-grained fluvial sediments. Since the mid-19th century the region has experienced 23 documented large-scale debris flows, including 14 in the study area, and these coincide with a period of climate warming. The majority of recorded events are attributed to the failure of moraine-dammed lakes, while the remainder were triggered by intense summer rainstorms. Landform-sediment assemblages investigated here have extended the documentary record by identifying at least 6 major debris-flow assemblages dating respectively from sometime before the early 17th century, ca. 1607-1633. ca. 1702-1728, ca. 1725-1751. ca. 1769-1795. and the mid-late 18th century. The geomorphological record of debris flows spanning the 17th to 19th centuries indicates therefore that high-magnitude events occurred also during the cooler climatic conditions of the Little Ice Age, although it is suggested that these events may have coincided with short-lived phases of glacier retreat. Debris flows in this environment may be considered as an important component of the paraglacial response to glacier recession, and this has clear implications for future patterns of valley floor development and its interaction with human activity.