Influence of neotectonism on geomorphology and depositional architecture of the Gandak megafan, middle Ganga plain, India Pitambar Pati a , Aditya K. Verma a, ⁎, Chinmay Dash a , Narendra K. Patel a , Ankit Gupta a , Vijay Sharma a , Rajendra P. Jakhmola b , B. Parkash a , A.K. Awasthi a , A.K. Saraf a a Department of Earth Sciences, Indian Institute of Technology Roorkee, 247667, Uttarakhand, India b Oil and Natural Gas Corporation, New Delhi, India abstract article info Article history: Received 16 April 2018 Received in revised form 21 November 2018 Accepted 27 November 2018 Available online 29 November 2018 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 nor- mal 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 ap- pear 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 arrange- ment 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. © 2018 Elsevier B.V. All rights reserved. Keywords: Gandak megafan Neotectonics Middle Ganga plain Terminal fan 1. Introduction Since the development of the Indo-Gangetic foreland basin at about 60 Ma by the collision of the Indian and Eurasian plates (Dewey and Bird, 1970; Patriat and Achache, 1984; Besse and Courtillot, 1988; Klootwijk et al., 1992; Beck et al., 1995), it remained tectonically active. Major sedimentation in the basin is taking place in the form of megafans formed by large rivers, including the Gandak, debouching into the plains from the Himalayas (Geddes, 1960). The basin has been continuously affected by active tectonism (Parkash and Kumar, 1991) and active faults have locally influenced river gradient, sinuosity and planform (Jain and Sinha, 2005). The basin has been divided into numerous fault-bounded tectonic blocks of varying dimensions. Due to compres- sion between the Himalaya and the peninsula, independent tilting and sagging of these blocks and sub-blocks have resulted in interconnected and spatially arranged different fluvial sedimentary entities. Therefore, mapping of these sedimentary entities helps in interpreting the spatial and temporal fluvial changes and particularly the influence of neotectonism in the basin, which strongly controls the fluvial architec- ture and depositional processes. According to the classification by Srivastava et al. (1994) and Thomas et al. (2002), the Gandak megafan is confined within the middle Ganga plain, which is characterized by relatively high-rate of sedimen- tation and subsidence. The megafan has a typical radial drainage pattern and convex form. The upper part of the megafan has an average slope of 38 cm/km with dichotomic slope pattern (Howard, 1967) and the lower portion has more gentle slopes (average 20 cm/km). Mohindra et al. (1992) studied the neotectonic activities in the region by using soil chronoassociation. The study revealed that tilting of the Gandak tec- tonic block caused eastward shifting of the Gandak River by ~80 km, which consequently formed older fluvial plains from east to west. However, little emphasis was given on recognizing and explaining the younger geomorphic units and their depositional processes that might have important implications for understanding the post tilting episodes of surface faulting and related sedimentation in the rapidly filling and Geomorphology 327 (2019) 489–503 ⁎ Corresponding author. E-mail address: aditya.verma.geo@gmail.com (A.K. Verma). https://doi.org/10.1016/j.geomorph.2018.11.029 0169-555X/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph