VERY LONG-LIVED LOW BEDROCK EROSION RATES AND SLOW SEDIMENT TRANSPORT OUT OF HIGH ELEVATION CATCHMENTS SUSTAIN OROGENIC PLATEAUS: MODERN EXAMPLES FROM THE HIMALAYA OF NORTHERN INDIA

Long-lived, high-elevation orogenic plateaus (altiplanos) are a common feature of modern orogenic belts and have been postulated in ancient orogens. Two key elements underpinning the longevity of altiplanos are low erosion rates and the lack of sediment transport out of high elevation catchments. In the arid Himalaya of northern India, the Ladakh Range and half-grabens in eastern Zanskar, tectonically inactive and active, respectively, mountain landscapes have undergone little change since the Late Miocene based on measurements of ¹⁰Be in bedrock and catchment sediments. In the Ladakh Range, bedrock tors along the range divide between 5600 and 5700m above sea level (asl) in the periglacial domain are eroding at 5.0 ± 0.5 to 13.1 ± 1.2 meters per million years (m/m.y.)., principally by frost shattering. At lower elevation in unglaciated domains, erosion rates for tributary catchments vary between 0.8 ± 0.1 and 2.0 ± 0.3 m/m.y. Bedrock along interfluvial ridge crests between 3900 and 5100m asl that separate these tributary catchments yield erosion rates <0.7 ± 0.1 m/m.y. where the dominant form of bedrock erosion is chemical weathering and grusification (Dietsch et al., 2014, ESPL doi:10.1002/esp.3640). In the Puga and Tso Morari half-grabens where catchments are underlain by quartzo-feldspathic gneissic bedrock, bedrock along catchment divides is also eroding very slowly, about 5m/m.y. and ¹⁰Be concentrations in catchment sediments (irrespective of footwall or hanging wall locations) are ~60–90×10⁵ atoms/g SiO₂ as colluvium accumulates on hillslopes, decoupled from their ephemeral streams. Since 80ka, as glaciers have retreated far up valley at Puga and Tso Morari, the lack of precipitation has prevented sediment derived from glaciation and the weathering of exposed bedrock from being efficiently transported out of the landscape (Dietsch et al., 2020, ESPL doi:10.1002/esp.4954). This transport-limited denudation and the very slow rates of bedrock erosion in both settings continue to be sustained by the arid conditions in the shadow of the Greater Himalaya. The proposed longevity of both modern and paleo-orogenic plateaus, up to 50 m.y., implies that surface processes along the edges of plateaus similar to those described here, and the conditions that sustain them, achieve long-term stasis. Only when large-scale changes in the tectonic regime of an orogen — typically the onset of crustal extension — do the very slow rates of denudation accelerate.