Erosion and voluminous mass movements during episodes of climate variability: landscape evolution in the southern-central Andes and the NW Himalaya. (Invited)

Landscape morphology and sedimentary archives are recorders of climate change on different time scales. A better understanding of the nature of relatively fast changes in surface processes is becoming increasingly important, particularly in light of global warming and associated changes in geomorphic process rates. Catastrophic mass movements and extreme hydrologic events shape landscapes through a variety of processes that leave distinct sedimentologic and geomorphic signatures. This is certainly true in climatic threshold areas at high elevations that are very sensitive to the effects of climatic variability. Whereas recent low-frequency, high-magnitude hydrological events may be considered as important agents shaping landscapes in these environments, our understanding of the actual range of magnitudes is limited. The monsoonal domain in the NW Himalaya and the southern-central Andes of NW Argentina constitute an excellent natural laboratory to study the effects of climate variability and the effects of changing temperature and precipitation regimes on the surface process system. In the ENSO affected E flanks of the Puna Plateau of Argentina a large range of forcing magnitudes and geomorphic responses exists. Here, large amounts of sediments are transported from the hillslopes through debris flows and landslides and are eventually evacuated to the foreland. Voluminous landslide clusters associated with valley impoundment, the formation of transient lakes, and thick lacustrine sediment sequences during late Pleistocene and Holocene time were coeval with phases of increased precipitation and high lake levels during protracted paleo-ENSOs in the Altiplano-Puna, suggesting a causal relationship. Similarly, in the NW Himalaya increased landsliding activity followed insolation maxima in late Pleistocene and mid Holocene time coupled with intensified summer monsoons. In the Himalaya and the Andes these events also correlate with regionally recognized phases of increased humidity and increased erosion rates. Importantly, both areas show comparable behavior. In both areas landslide deposits typically overlie excavated valley bottoms and virtually never overlie multiple valley fills. Second, landsliding and the formation of intermontane lakes lagged behind the onset of a different climate mode. This suggests that a changeover to different climatic conditions may have been characterized by pronounced erosional processes, during which the trunk streams incised into alluvial fills and sediment was evacuated to the foreland. Subsequently, elevated pore pressures in tectonically overprinted basement rocks, and lateral fluvial scouring destabilized the slopes of the deeply incised ranges, thus increasing the likelihood for slope failure and deep-seated bedrock landslides. Taken together, elevated sediment transport rates during these times and the formation of landslide clusters in these environments emphasize the impact of climate variability on surface processes and landscape evolution and underscore the importance of large landslides in the sculpting of the topography of mountain belts.