Rock avalanches and the pace of late Quaternary development of river valleys in the Karakoram Himalaya

Abstract

We discuss the implications of a set of terrestrial cosmogenic nuclide (TCN) ages on blocky, cross-valley deposits of large rock avalanches along upper Indus streams. The dated deposits are key to understanding late Quaternary events that play a major role in landscape evolution in the Karakoram Himalaya. The landslides occurred between 3 and 8 ka ago, challenging existing chronologies of events along Indus streams. The TCN ages may support a mid-Holocene climatic role in preparing slopes for failure, but the balance of evidence suggests that large earthquakes triggered the landslides. Each landslide dammed the Indus or a major tributary and controlled base level and sedimentation for millennia. They produced landforms long regarded as characteristic of the region, including extensive lacustrine deposits, flights of river terraces, epigenetic gorges, and sediment fans. Until the 1990s, most of the landslides were interpreted as moraines; related lacustrine and other sediments continue to be attributed to glacial damming, and stream terraces to tectonic processes. Generally they were seen to originate tens of thousands to hundreds of thousands of years earlier than the new ages require. Instead we argue that they record interactions among different geomorphic processes in landslide-fragmented valleys during the Holocene. Rather than being geomorphic markers of tectonic and climatic events, the landslides have buffered or redirected climatic and tectonic forcing. In such an active orogen, millennia-long episodes of zero net bedrock incision at each site are surprising. However, rates of sedimentation above landslide barriers and erosion controlled by their breaching are close to today9s high measured rates for geomorphic activity. We propose that landslide-fragmented rivers may, in fact, characterize interglaciations and future patterns of upper Indus landscape evolution at time scales of 10 3 to 10 4 years.