Luminescence Chronology of Quaternary Landforms in Assam- Bhutan Himalayan Foothills, Kokrajhar: Implications to Climate and Tectonics

Narzary, Belligraham (2026) Luminescence Chronology of Quaternary Landforms in Assam- Bhutan Himalayan Foothills, Kokrajhar: Implications to Climate and Tectonics. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Belligraham Narzary (17RS035)) 17RS0355.pdf - Submitted Version Restricted to Repository staff only Download (7MB)

Abstract

The Himalayan foreland basin preserves extensive geomorphic archives in the form of fluvial terraces, alluvial fans, and associated depositional systems, which record the interplay between tectonic uplift and climatic variability. In the eastern Himalaya, particularly the Assam–Bhutan foothills, these archives provide critical insights into active deformation along major thrust systems, yet remain poorly constrained chronologically. This study integrates geomorphic, sedimentological, and chronological investigations from two key regions: (i) the Sankosh River valley, and (ii) the Sarpang re-entrant in Assam. The Sankosh River, a glacial-fed system originating in northern Bhutan, traverses the Main Central Thrust (MCT), Main Boundary Thrust (MBT), and MFT before joining the Brahmaputra River. Active deformation at the MFT has resulted in channel deflection and the development of four levels of strath terraces (T4–T1), recording progressive river incision and uplift. Optically Stimulated Luminescence (OSL) dating of quartz and feldspar minerals was employed to establish a chronological framework. Due to low quartz sensitivity, fading-corrected Infrared Stimulated Luminescence (IRSL) dating of feldspar grains was used for Sankosh River terraces. The obtained ages indicate terrace formation during ~143–77 ka (T4), ~65–36 ka (T2), and ~35–14 ka (T1), while T3 remains unsampled due to limited exposure. These ages suggest aggradation and incision linked to glacial–interglacial climatic fluctuations. In the Sarpang re-entrant, geomorphological and sedimentological analyses indicate the presence of an active ~30 km long east–west trending Frontal Back Thrust (FBT), which forms a characteristic pop-up structure marked by prominent tectonic scarps, linear lakes, and structurally controlled drainage networks. The uplifted landscape is segmented into discrete tectonic blocks separated by antecedent river systems. Late Quaternary alluvial fan deposits and ponded lacustrine sediments within these blocks preserve evidence of deformation associated with fault-propagation folding and thrusting. Luminescence chronology constrains the earliest recorded fluvial damming event to ~75 ka, represented by a ~1 m thick lacustrine clay unit in the hanging wall of the Pinkhua block. The obtained luminescence ages exhibit an eastward younging pattern across the uplifted blocks, which is interpreted as the surface expression of progressive deformation and fault-tip propagation along the Frontal Back Thrust system. This indicates episodic tectonic uplift and repeated seismic activity along the FBT during the Late Quaternary.Overall, this study highlights that fluvial terraces and alluvial fan–lacustrine systems in the eastern Himalayan foreland preserve valuable records of neotectonic deformation in thrust-front and back-thrust settings. The integration of geomorphic mapping, sedimentological analysis, and luminescence chronology provides improved constraints on the timing and style of deformation in the region, and offers insights into the interplay between tectonic activity and climatic variability in shaping active mountain-front landscapes.

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