Tectonic Zonation using Multi-Criteria Decision-Making (MCDM) Techniques: A case study of Kosi Fan, India Conclusions The integrated study using Remote sensing, Geological/Geophysical data and GIS has revealed that the subsurface structures have played a significant role in shaping the contemporary surface of the Kosi megafan. Direction of faults and lineaments in this region follow the trend of discontinuities in the basement as revealed by the gravity and seismic data. These weaker zones of basement have suffered periodic reactivation. Some of these phases were manifested as the strong earthquakes of 1883, 1934 and 1988 and hotsprings of Monghyr, Bihar. Many smaller earthquakes (less than 5 intensity on Richter scale) have also been recorded in the foothill region. The discharge of the Saptkosi was diverted sequentially in steps into other rivers of Kosi group covering a stretch of around 250 km between Mahanada and present Kosi river. Quaternary faults associated with Begusarai faults, regional E-W tilt of the basement towards the Madhubani depression and subsidence due to the basement faults on their peripheral region helped Kosi river in maintaining its continuous westward migration. Due to continued subsidence of the fault-bounded Madhubani depression in the NW, faults have been rejuvenated and their spatial positions were modified. Misalignment of single and paired channels directly related to amount of fault displacement. Kosi river’s tendency to migrate westward was intensified by the presence of the echelon faults on the various diversion points and flood breaches (figure 3).  Figure 3 Geomorphological map showing shifting courses of Kosi river Kosi flood plain overlies a highly uneven basement formed by the major tectonic features such as Purnea depression, Monghyr-Saharasa (M-S) ridge and Madhubani depression. M-S ridge is traversed by the basement faults such as NNW-SSE trending Kishanganj and Malda basement faults in the east and the Bhawanipur fault in the west. These basement faults such as Motihari, Begusarai, Malda are associated with intricate pattern of Quaternary fault systems that affect the rivers course wherever topographic conditions become favourable. Such channel entrenchment, diversion and shifting have been observed all over the Gangetic Plain. Catchment of Kosi river is covered by metamorphic rocks. Phyllites and schists are present throughout the entire stretch of seismically active Central and Higher Himalayas. Thus, physiography and climatic conditions make phyllites and schists unstable at the higher elevations in Kosi catchment and contribute a lot of sediments. Silt and clay finally choke the Kosi channels in the alluvial plain. The water in these channels tend to avoid the existing arrangement and new passageways are found or created in the form of flood breaches, diversion points and channel spills at suitable locations in its middle course where slope and gradient is sufficiently reduced. These locations are generally found in the areas where fault intersections occur along the Kosi course. Direction of faults and lineaments in this region follow the trend of discontinuities in the basement as revealed by the gravity and seismic data. These weaker zones of basement have suffered periodic reactivation. The discharge of the Saptkosi was diverted sequentially in steps into other rivers of Kosi group covering a stretch of around 250 km between Mahanada and present Kosi river. A Quaternary fault system has been identified in the region. This is an echelon pattern of surface faults associated with Begusarai fault. Within this fault zone, various geomorphic features are found which have their origin in both the lateral and vertical movement of fault-bounded slices, as well as in the persistent strike-slip. In regions where tectonic activity is less pronounced, streams generally flow more or less perpendicular to the adjacent highlands (Figure 4).  Figure 4 Map showing different levels of tectonic influence and shifting of Kosi river Acknowledgements Author is thankful to Dr. S. K. Mukherjee, Vice-Chancellor, Birla Institute of Technology, Mesra, Ranchi for providing facilities to carry out the study. He would also like to thank Dr. H.C. Pandey, VC Emeritus, BIT, Mesra for his constant encouragement. References
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