Lithofacies distribution and geological context of Mekong rapids in the Chiang Khan area, Loei Province, Thailand: evidence from petrography, geochemistry, and structural analysis

Geological and tectonic characteristics of the Mekong River Rapids in Chiang Khan District, Northern Loei Province, are investigated within the structural context of the Loei-Phetchabun Foldbelt. A multidisciplinary approach – including detailed fieldwork, petrographic analysis, whole-rock geochemistry (major, trace, and rare earth elements), and structural interpretation – reveals a complex lithological assemblage spanning the Middle Devonian to Jurassic periods. Volcanic sequences, including rhyolite, rhyodacite, dacite, and andesite, exhibit calc-alkaline arc signatures. Associated intrusive rocks (aplite and microgranodiorite) are enriched in LREEs and exhibit elevated LaN/YbN (10.06–15.89) and LILE/HFSE ratios, indicating subduction-related magmatism. Sedimentary formations reflect fore-arc basin settings, with Carboniferous strata dominating the east and Permian units in the west. Contact metamorphism has produced marble, garnet-pyroxene skarn, and spotted phyllite. Structural evidence documents multiphase deformation, linked to Indosinian compression followed by post-orogenic extension. The differential persistence of rapids is governed by lithological variability, tectonic uplift, intense monsoonal regimes, and pervasive silica-rich hydrothermal alteration that enhances bedrock competence through silicification. These factors collectively increase resistance to fluvial incision despite pronounced seasonal erosional forces. The findings underscore the complex interplay between lithology, structural deformation, and climatic dynamics in controlling the long-term stability and geomorphic evolution of rapids in tectonically active fluvial landscapes.