Tectonic and Climatic Controls on Erosion and Sediment Aggradation in the Himalaya

Chauhan, Vaishanavi (2026) Tectonic and Climatic Controls on Erosion and Sediment Aggradation in the Himalaya. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Vaishanavi Chauhan (19RS086)) 19RS086.pdf - Submitted Version Restricted to Repository staff only Download (19MB)

Abstract

The Himalaya, Earth’s highest and most dynamic mountain range, continues to rise as the Indian subcontinent collides with Asia, a process ongoing for nearly 54 million years. While glaciers sculpt the high peaks and monsoon-fed rivers carve deep bedrock gorges, many valleys paradoxically preserve thick sedimentary fills that record prolonged intervals of aggradation rather than incision. Understanding why and when this valley aggradation occurs is crucial for deciphering how mountain landscapes evolve and respond to both tectonic forces and climate variability. This thesis investigates how these processes together govern erosion and sediment accumulation across the northwestern Himalaya through three complementary case studies spanning distinct temporal and spatial scales. In the Beas Valley, a ∼120 m-thick valley fill accumulated over more than 100 kyr during the late Pleistocene, with subsurface modeling revealing an even thicker (>400 m) succession buried beneath the present valley floor. Luminescence dating, cosmogenic ¹⁰Be-derived paleo-erosion rates and Sr–Nd isotopic data indicate prolonged aggradation independent of Quaternary climate oscillations, instead reflecting localized downstream uplift and out-of-sequence thrust activity that created upstream accommodation. In contrast, the structurally confined Pinjore Basin contains a ∼55 m alluvial fan that aggraded over ∼39 kyr between ~52 and 13 ka during weak monsoon phases. Persistently low catchment-scale erosion rates and stable sediment provenance suggest that reduced fluvial transport capacity, rather than increased sediment supply, drove valley filling, while coarsening after ~26 ka likely reflects slope destabilization linked to vegetation changes associated with glacial cooling. Renewed fan incision after ~13 ka coincides with monsoon strengthening, marking a threshold-driven shift to a transport-dominated regime. Extending to million-year scales, a ~3 Myr record from uplifted late Pliocene-Pleistocene Siwalik foreland basin strata in the Ghaggar catchment reconstructs erosion histories from cosmogenic ¹⁰Be data, revealing cyclic increases and declines in erosion rates interpreted as pulses of thrust propagation and mountain growth. Enhanced subsidence and uplift between ~2.5 and 1.5 Ma led to accelerated erosion and sedimentation, followed by stabilization to near-modern rates. Together, these investigations reveal that Himalayan sediment routing systems operate as complex, multi-scale networks where the relative importance of tectonic and climatic forcing differentially regulate sediment production, temporary storage within intramontane valleys, and downstream sediment export across space and time. At local spatial scales and over intermediate timescales (10⁵-10⁶ years), morphotectonic processes such as fault growth can dominate, creating long-lasting valley aggradation independent of climate variability. At regional spatial scales and over shorter timescales (10³-10⁴ years), monsoon variability exerts primary control through its influence on runoff, vegetation, and fluvial transport capacity, while over geological timescales (>10⁶ years), both mechanisms operate synergistically to regulate landscape evolution trajectories. The threshold-sensitive nature of these systems means that small changes in external forcing can produce disproportionately large geomorphic responses, with profound implications for understanding landscape evolution in active mountain belts and for anticipating future sediment-routing behavior under ongoing climate change and continued tectonic activity. These findings advance fundamental understanding of Earth surface processes while providing practical insights for seismic hazard assessment, flood risk evaluation, and landscape management in one of the world’s most tectonically active and climatically sensitive mountain regions.

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