Prolonged sediment aggradation in an internal Himalayan valley due to out-of-sequence lateral fault growth

Abstract

Despite high rock uplift and surface runoff, most Himalayan river valleys feature tens of meters thick fill terraces, a legacy of aggradational episodes. Climate oscillations during the late Quaternary are commonly invoked to explain shifts from river incision to aggradation. While tectonic processes are known to cause transient aggradation, identifying their signatures and possible causes remains challenging. This study comprehensively analyzes ∼120-m thick valley-fill deposits exposed above the modern channel of the upper Beas River in the northwestern Indian Himalayan state of Himachal Pradesh. Luminescence dating of the valley fill reveals an aggradation period exceeding 100 kyr during the late Pleistocene, which likely commenced considerably earlier, based on an estimated >400-m thick fill buried beneath the present valley floor. The prolonged aggradation, encompassing several glacial-interglacial cycles, and the absence of systematic changes in

Be-derived paleo-erosion rates or

Sr/

Sr and

Nd/

Nd isotope ratio-derived sediment provenance over time rule out late Quaternary climate oscillations as the sole driver of aggradation. Instead, we explain the longevity and extent of valley aggradation as the fluvial system's response to localized higher rock uplift rates downstream, as substantiated by the southern range flank morphology and the river network topology. We suggest that along-strike variations in thrust kinematics in this sector of the Himalaya likely caused the out-of-sequence lateral range growth and, consequently, triggered aggradation in the upstream valley. The implied temporal changes in stress and strain are critical for seismic hazard assessment.