ABSTRACT We defined the timing of surface abandonment for 10 alluvial and debris-flow fans across contrasting climatic settings in the NW Himalaya of northern India using cosmogenic 10 Be surface exposure dating. Debris-flow fans in the Garhwal, Kullu, and Lahul-Spiti regions of the monsoon-influenced Greater Himalaya were largely abandoned during the Mid- to Late Holocene. Large alluvial fans and smaller debris-flow fans in the semiarid Ladakh region of the Greater and Tethyan Himalaya have surface ages that extend throughout the last glacial. Regional events of landform abandonment and incision were defined for the monsoon-influenced western Himalaya ranges and the semiarid western Himalaya ranges over the past ~120 k.y. In the monsoon-influenced and semiarid western Himalaya ranges, these regional events were limited to the Holocene and from ca. 40 ka, respectively. The timing of fan surface abandonment and regional landform abandonment events coincided with periods of weakening monsoon strength and cooling, and local and regional glacier advances. Regional incision events from the monsoon-influenced and semiarid western Himalaya regions were recognized across various climatic conditions due to the ubiquitous nature of erosion in mountain settings. This study showed that climate-driven processes and glaciation were important drivers in fan sedimentation, catchment sediment flux, and the topographic evolution of the NW Himalaya during the late Quaternary.

Abstract Successful sustainable development and geohazard mitigation in the Himalaya requires an understanding of the nature and dynamics of Earth surface processes and landscape evolution. In recent years, geoscience studies of Himalayan environments have been increasing due to better accessibility, modern technologies and the understanding that there is a necessity to determine the nature and predict likely environmental changes that are occurring due to natural and human influences. The Himalaya is one of the most dynamically active tectonic and geomorphic regions on our planet, and it is the most glaciated mountain area outside of the polar realms. The high mountains and deep valleys are a consequence of the continued collision of the Indian and Eurasian continental plates, rapid uplift and intense denudation by glacial, fluvial, landsliding, aeolian and weathering processes. These processes change over time, influenced by topographic development, climate change and humans. Defining the rates and magnitudes of these processes and their interactions is fundamental in developing a framework to quantify, model and predict future changes for geohazard mitigation and sustainable development.