TECTONIC AND CLIMATIC INFLUENCE ON TERMINAL FANS ASSOCIATED WITH BASEMENT FAULTS IN THE GANGA PLAIN

Abstract

The Ganga plain is a tectonically active foreland basin, formed due to the collision of the Indian and Eurasian plates. This vast alluvial plain is drained by the large Himalayan rivers such as the Ghagghar, Yamuna, Ganga, Ghaghara, Rapti, Gandak, and Kosi, as well as by the rivers Betwa, Chambal, Ken, and Sone, which originate from the Indian peninsula. This plain evolved through many stages of tectono-climatic interplays, which have been studied through the corresponding geomorphic markers through space and time. The terminal fan is a geomorphic marker formed by the surface faulting events and subsequently modified by various episodes of tectonoclimatic interplays. In recent decades, the study of neotectonic, geomorphology, and sedimentology in the Ganga plain identified a total of 45 terminal fans. The association of the terminal fans with the basement faults indeed indicate their surface influence. Though numerous research papers have been published on terminal fans recently, but the fundamental questions regarding the growth of terminal fans remain unanswered. Even minor topographic breaks due to faulting provide accommodation space for fangrowth, but forming a fan covering several hundred km2 in an almost flat plain raises several questions regarding these features' growth mechanism. Secondly, the role of tectonics and climate towards the growth of the terminal fans is unclear. This study attempts to address this research gap by detailed sedimentological analysis of the terminal fans associated with the basement faults. Mapping the buried channels and the growth surfaces in the terminal fans and soil properties can add significant scientific data to understand the evolutionary processes. Additionally, the study of basement fault in the Ganga plain and their seismic nature is a century-old problem. Due to their deepseated condition and lack of proper records of past activities, their influence on geomorphology and sedimentation on the associated terminal fans are lacking. The seismic record suggests that the majority of earthquakes on the faults are of moderate size. In this work, early to late Holocene evolutionary history of the terminal fans has been studied using an integrated approach. The geomorphological signatures of the basement faults studied using remote sensing and GIS, available seismic record, and soil chrono-association. Based on the spatial drainage anomaly, such as convergent drainage, offset streams, generation of new streams, change in channel sinuosity, etc. surface projection of possible fault zones of the basement faults demarcated. Ground Penetrating Radar (GPR) study carried out across the demarcated fault zone of three prominent basement faults such as the Mahendragarh-Dehradun Fault (MDF), West Patna Fault (WPF), East Patna Fault (EPF). Displacements detected in the GPR profile at the near-subsurface level confirm these faults' effect on the cover sediments in their respective fault zones. The MDF, WPF, and EPF are normal faults that dip towards the east, west, and east directions, respectively. Several steeply dipping shallow depth normal faults have been identified in the fault zone. Spot-heights and contours from the Survey of India toposheets on a scale of 1:50,000 used to create a digital elevation model (DEM) across a surface projection of the basement faults. The profiles extracted from DEM across the fault zone indicate the surface trace of the basement fault. However, the feeble surficial signature of these basement faults is due to erosion, human settlement, and agricultural activities. Anatomy of the terminal fans carried out using the sedimentary sequence, geophysical study, fieldwork, clay minerals, and soil micromorphology. Modern and paleochannels on the terminal fans studied using remote sensing and GIS, GPR, and field study of the associated facies. The preferred orientation and alignment of the ponds revealed some recently abandoned channels. Some of the ponds show elongated morphology, and their orientation reveals the paleochannels pattern. It is highly likely that these large ponds have resulted from the abandonment of earlier river courses. Numerous ponds indicate a similar hydrological condition, but their size and almost circular shape associate them with the relatively old channel abandonment events. The vertical sections exposed in pits investigated for studying the degree of soil development, and samples collected for clay minerals analysis and soil-micromorphological studies. Different horizons and sub-horizons of the soil profiles were noted. Soils in the older terminal fans are well developed with 2 to 3 m of solum thickness compared to the younger terminal fans, where the solum thickness varies from 1 to 2 m, supported by micromorphological studies.