Spatial and temporal distribution of inland fans/terminal fans between the Ghaghara and Kosi rivers indicate eastward shift of neotectonic activities along the Himalayan front. A study from parts of the upper and middle Gangetic plains, India

Neotectonic deformation in the Indo-Gangetic plains is primarily manifested by surface ruptures caused by both normal faults and thrust splays. The topographic breaks along these faults provide suitable sites for the formation of inland fans/terminal fans on the down-thrown blocks. As these depositional features invariably form along the faults contemporaneously or soon after faulting, they have widely been used as potential geomorphic markers in mapping faults in the flat Gangetic plains. Based on this association, infrared stimulated luminescence (IRSL) dating of the fans has successfully been used for dating the faulting episodes in the Gangetic plains. Thus the spatial and temporal distribution of inland fans/terminal fans suggests migration of faulting activities from west to east with time and successive faulting events occur over periods of several hundreds of years to 103 years.In the present study, the correlation of eighteen dated inland fans/terminal fans aligned along the faults and using the surficial and buried soil profiles suggests four deformation episodes (7.89–7.30 Ka, 5.84–3.99 Ka, 3.18–2.73 Ka, and 1.41–0.75 Ka) in the region between the Ghaghara and Kosi rivers in the Gangetic plains. These deformation episodes were witnessed by the development and reactivation of the extensional normal faults transverse to the Himalayan Frontal Thrust (HFT) in the Ghaghara–Rapti interfluve and the thrust splays parallel/sub-parallel to the HFT in the Gandak–Kosi interfan regions. The Rapti-Gandak region in the middle serves as a transition zone in-between the interfluve and the interfan region and shows both types of fault development at different times.Six inland fans/terminal fans in the Ghaghara–Rapti interfluve were formed between 7.89 and 2.73 Ka in three deformation/faulting episodes (two in each phase). The broad age difference between those and strongly developed soils suggests the faulting activities to be less frequent in this region. The fan building episodes in the Gandak–Kosi interfan region started at 4.92 Ka by the initiation of thrusting activities. Out of eight inland fans/terminal fans, five of them were formed during 1.41–0.75 Ka in association with various thrust splays. These depositional features are characterised by a narrow age gap and weak to no surface soils which indicate early burial by high sediment supply from frequent thrusting activities.Segments of the plains having a high/low population of inland fans/terminal fans of narrow/broad age gaps and with relatively weakly/strongly developed soils lie close to or opposite to the segments having the highest/lowest rate of convergence and upliftment of the Himalayas along the HFT.