EPISODIC FLUVIAL INCISION OF RIVERS AND SURFACE UPLIFT IN THE HIMALAYA AND TRANSHIMALAYA

Seventeen strath terraces in Ladakh and Lahul in northern India were dated using ¹⁰Be terrestrial cosmogenic nuclide surface exposure methods to determine Quaternary rates of fluvial incision. ¹⁰Be ages on strath terraces range from ~7 ka to ~735 ka, providing fluvial incision rates of 0.02±0.003 mm/yr to 2.6±1.9 mm/yr. In the Ladakh region, on the northern side of the Ladakh Range, fluvial incision rates based on strath terrace ages are ~1 mm/yr. In contrast, on the northern side of the Zanskar Range, fluvial incision rates are ≤ 0.06±0.005 mm/yr, suggesting that fluvial processes have been essentially ineffective in the Zanskar Range since ~735 ka. New and published fluvial incision rates for Lahul in the southernmost region range from 0.1±0.02 mm/yr to 13.2±6.2 mm/yr, with long-term (>35 ka) rates being ≤ 0.4±0.2 mm/yr. Across the Himalaya and Transhimalaya, new and published Holocene fluvial incision rates range from ~0.02 mm/yr to ~26.0 mm/yr, while Pleistocene strath terraces yield incision rates ≤ 5 mm/yr. Many of the Holocene fluvial incision rates based on strath terraces across the Himalaya and Transhimalaya exceed exhumation rates determined by other methods; yet Pleistocene fluvial incision rates based on strath terraces are comparable to the exhumation rates. The temporal pattern for rates of fluvial incision is likely controlled by episodic fluvial incision linked to significant precipitation changes throughout the Quaternary. These data suggests that strath terraces older than 35 ka are best for assessing average or long-term surface uplift. Moreover, it is necessary to determine ages for multiple samples on flights of strath terraces older than 35 ka to resolve whether episodic fluvial incision is controlling erosion and to enable short-term fluvial incision rates to be extrapolated over long timescales.