Catastrophic valley fills record large Himalayan earthquakes, Pokhara, Nepal. (1st December 2017)

• Record Type: Journal Article
• Authors: Stolle, Amelie; Bernhardt, Anne; Schwanghart, Wolfgang; Hoelzmann, Philipp; Adhikari, Basanta R.; Fort, Monique; Korup, Oliver
• Is Part Of: Quaternary science reviews. Volume 177(2017)
• Page Start: 88
• Page End: 103
• Publication Date: 2017-12-01
• Subjects: Catastrophic valley infill -- Great Himalayan earthquakes -- Radiocarbon age dating -- Provenance analysis -- Paleoseismology -- Nepal; Geology, Stratigraphic -- Quaternary -- Periodicals; Stratigraphie -- Quaternaire -- Périodiques; 551.79
• DOI: 10.1016/j.quascirev.2017.10.015
• Languages: English
• ISSNs: 0277-3791

Abstract

Abstract: Uncertain timing and magnitudes of past mega-earthquakes continue to confound seismic risk appraisals in the Himalayas. Telltale traces of surface ruptures are rare, while fault trenches document several events at best, so that additional proxies of strong ground motion are needed to complement the paleoseismological record. We study Nepal's Pokhara basin, which has the largest and most extensively dated archive of earthquake-triggered valley fills in the Himalayas. These sediments form a 148-km 2 fan that issues from the steep Seti Khola gorge in the Annapurna Massif, invading and plugging 15 tributary valleys with tens of meters of debris, and impounding several lakes. Nearly a dozen new radiocarbon ages corroborate at least three episodes of catastrophic sedimentation on the fan between ∼700 and ∼1700 AD, coinciding with great earthquakes in ∼1100, 1255, and 1344 AD, and emplacing roughly >5 km 3 of debris that forms the Pokhara Formation. We offer a first systematic sedimentological study of this formation, revealing four lithofacies characterized by thick sequences of mid-fan fluvial conglomerates, debris-flow beds, and fan-marginal slackwater deposits. New geochemical provenance analyses reveal that these upstream dipping deposits of Higher Himalayan origin contain lenses of locally derived river clasts that mark time gaps between at least three major sediment pulses that buried different parts of the fan. The spatial pattern of 14 C dates across the fan and the provenance data are key to distinguishing these individual sediment pulses, as these are not evident from their sedimentology alone. Our study demonstrates how geomorphic and sedimentary evidence of catastrophic valley infill can help to independently verify and augment paleoseismological fault-trench records of great Himalayan earthquakes, while offering unparalleled insights into their long-term geomorphic impacts on major drainage basins. Highlights: We review the largest and most extensively dated archive of earthquake-triggered valley fills in the Lesser Himalayas. Radiocarbon dates indicate at least three pulses of catastrophic sedimentation following great medieval Himalayan earthquakes. Sedimentological features and geochemical analysis independently identify these aggradation events. Evidence for long-runout (70 km) debris flows from High Himalayan Crystalline Sequence.