Holocene tectono-geomorphic evolution of parts of the Upper and Middle Gangetic plains, India

Abstract

Using multispectral scanning (MSS) images and digital elevation models (DEMs), 26 terrain morphological units were delineated in the study area between the Rapti and Kosi Rivers in the Gangetic plains. Based on the degree of soil development and the infrared stimulated luminescence chronology, these terrain morphological units have been grouped into six members of a morphostratigraphic sequence with ages ≤ 1.65, 1.65–5.3, 5.3–7.5, 7.5–8.5, 8.5–10, and > 10 Ka. Integrated use of geomorphic markers including different drainage patterns identified from MSS images, significant breaks in topographic profiles, terminal fans, and artifact terrain features (e.g., ‘cliffs’ and ‘significant breaks in slopes’) in DEMs/digital terrain models (DTMs) with highly exaggerated vertical dimensions were made to infer and map 20 nascent faults in the region between the Ghaghara and Kosi Rivers in the flat Gangetic plains. The three major tectonic blocks are; the upland Ghaghara–Rapti block of the Upper Gangetic Plain, and the Rapti–Gandak and Gandak–Kosi blocks of the Middle Gangetic Plain, marked by high rates of subsidence and sedimentation. The Ghaghara–Rapti block bounded by the incised Ghaghara and Rapti rivers is an upland region and is primarily overlain by > 10 Ka old soils. Subsequent activity along the NE-SW trending transverse normal faults led to deposition of young terminal fans. Southward tilt of the Ghaghara–Rapti block generated two E–W trending terraces on the left bank of the Ghaghara River. Southwestward tilting of the upper part of the Ghaghara–Rapti block led to development of the widest Old Ghaghara Plain and Rapti Floodplain in the SW direction. Northeastward tilting of the Rapti–Gandak block shifted the Gandak River from west to east over a distance of 80 km during the period from 10 to 8.5 Ka. Further compression from the SW caused development of four extensional normal faults. The activity of three faults between 5.7 and 1.0 Ka generated four terminal alluvial fans on this block. The Gandak–Kosi block is marked by a set of seven thrust splays off the Himalayan Frontal Thrust (HFT) in the footwall. Activity of these thrust splays in the foothill region caused progradation of the gravel piedmont and thus developed the widest piedmont zone (124 km) in the Gangetic plains. This is the first report of gravel piedmont progradation from a modern environment and role of thrust splays in such a progradation. In the southernmost part of this block, the continuous occurrence of terminal fans over a length of 140 km south of the South Muzaffarpur Fault forms a terminal fan piedmont/bajada. Orientation of the faults was used to interpret compression from the SW in the Ghaghara–Rapti Interfluve Plain in the west since ~ 10 Ka and compression from the south in the Gandak–Kosi interfan region in the east after 4.5 Ka. The Rapti–Gandak region in the central part shows the effects of mainly compression from the SW and compression from the south at different times. In addition to terminal fans formed because of low relief terrain features created by the activity of faults by involvement of the entire discharge of small meandering/braided streams, a new type of fan formed by splays from a large river on the downthrown sides of faults, called ‘splay terminal fans’ is observed from the Rapti–Gandak block.