LAUSR

Abstract The Gandak megafan formed by episodic tilting of tectonic block followed by an eastward shift of the Gandak River. The study area has been identified with 5 distinct depositional units, which were chronologically named as oldest Gandak plain (OSGP), old Gandak plain (OLGP), Gandak terminal fan (GTFn)-I, -II and -III. An integrated approach including morphometry, pedology, GPR study and OSL dating was adopted to study morphotectonics of the megafan. DEM of the megafan and anomalies in drainage of the area helped to delineate NE-SW trending normal faults, which divide the megafan block into 3 tectonic sub-blocks. Present-day channels reciprocate the fault movements through anomalies in drainage and change in sinuosity around the faults. Sinuosity of the Gandak, Chhoti Gandak and other small streams (Daha, Gandaki and Rohini) varies from 1.6, 4.5 and 3.5, respectively, which suddenly increases across faults. The concurred variations in longitudinal river profiles and SL index appear to be attributable to the tectonic controls. The development of these faults and subsequent tilting of the sub-blocks influenced the sedimentary architecture and fluvial geomorphology of the megafan. A linear arrangement of natural waterlogs and paleochannels observed in satellite images imply lateral shifting of the Gandak River. Older depositional units, OSGP and OLGP, show well-developed thick solum formation (~2.2 m and 1.9 m, respectively). Comparatively, the younger units i.e. terminal fans show poor to no development of soil with ~1.2 m to 0.8 m thick successions. OSL dates indicate that older and younger units were deposited between ~10.7 and 8.5 Ka, and ~5.7 and 1.4 Ka, respectively. The tectonically stable period of ~3 Ka is also exhibited by non-depositional surfaces in soil successions.