The Great Lateral Moraine», Karakoram Himalaya, Inner Asia

Abstract

Large moraines and related ice margin deposits, observed along the ablation zones of Karakoram glaciers, have been grouped together as the Great Lateral Moraine (GLM). It was formerly attributed to the Little Ice Age. Other studies propose a longer sequence beginning with late Pleistocene glaciations. All investigations have assumed the GLM records climate-driven glacier expansions. Evidence presented here challenges this, and the idea of a single origin in time or process. Bualtar Glacier in the Hunza Basin, with a much-discussed GLM, introduces the complexities involved. The glacier is surge-type, its fluctuations affected by large landslides onto the ice. Both have triggered depositional episodes out-ofphase with surrounding glaciers and climate variability. More decisive has been local base-level control by landslides downstream of Bualtar, especially the late Holocene, Baltit-Sumayar landslide. Similar conditions are shown to affect many, if not all, GLMs. No consistent relations were found with glacier size, morphology or known patterns of advance, but many surge-type glaciers and landslides in glacier basins are involved. A pervasive influence has been blocking of the upper Indus streams by large mass movements. To address these complex developments, valley glacier landsystems concepts are employed, especially as applied to debris- covered glaciers. Some distinctive Karakoram variants are identified. The regional environment seems not to produce a unique type, but a complete spectrum of valley glacier landsystems. Recent evidence of glaciers transitioning between landsystem types suggest how GLMs have developed and why interactions of glacial, fluvial, lacustrine and eolian systems, are important. GLMs are distinguished as «transglacial landsystems », developments in which glacial activity is disturbed and reconfigured by non-glacial processes. A paraglacial influence is also present, mainly through glacially induced rock slope instabilities. These lead to large postglacial landslides blocking rivers or descending onto smaller surviving glaciers. The interpretation offered is a challenge for existing views of late Quaternary developments.