Concealed thrusts in the Middle Gangetic plain, India – A ground penetrating radar study proves the truth against the geomorphic features supporting normal faulting

Terminal fans and the Ganga plain tectonism: A study of neotectonism and segmentation episodes of the Indo-Gangetic foreland basin, India

“…Unlike the terminal fans in the upper Gangetic plain, the same in the middle Gangetic plain do not show pure distributary network of channels from the trunk river as described by Miall (1992) and Leier et al (2005) and complete termination of associated streams as described by North and Warwick (2007). Hence, Pati et al (2011b) introduced the term "inland fan" for describing such features and classified the fans as "braided stream terminal fan" and "meandering stream terminal fan" depending upon the type of stream (meandering or braided nature of streams on the fan) involvement. In the upper Gangetic plain, many of the terminal fans are formed by the involvement of the whole stream networks.…”

“…This vast stretch of alluvium chiefly consists of coarse to fine grained sand and mud deposited by the Ganga River and its tributaries. Alternate gravel and sand sequences are reported in the northern part of the region i.e., in the piedmont zone (Pati et al, 2011b) and at places gravel progradation by thrusting is also observed in the GandakKosi interfan region (Pati et al, 2011a). Due to unconsolidated nature of the sediments, fluvial reworking is frequent which promotes frequent fan building events on the down thrown blocks of faults during or soon after the faulting activities.…”

“…Acharya et al (2012) confirmed the subsurface presence of the Markanda fault by ground penetrating radar (GPR) and observed that instead of the whole Markanda River, a splay off the main river diverted at the Markanda fault, was responsible for the formation of the terminal fan. In the mean time, many terminal fans associated with normal faults have also been reported from the upper Gangetic plain Bhosle et al, 2008;Pati et al, 2011b) and in the middle Gangetic plain between the Gandak-Kosi interfan region associated with reverse faults (Pati et al, 2011a). So it is not necessary to have arid or semiarid climate and ephemeral channels to form terminal fans in fact it is the relief difference across the stream course which is vital for it.…”

“…Pati et al (2012) correlated the population of terminal fans, the degree of soil development with the faulting events in parts of the upper and middle Gangetic plains which in turn with the convergence and upliftment rates along the Himalayan front. Sediment characteristics and geomorphology of the terminal fans in the upper and the middle Gangetic plains were studied by Pati et al (2011b). The above workers observed that those depositional features in the upper Gangetic plain are sand rich and show salt efflorescence and calcrete intervening layers.…”

“…As the plain is coupled and segmented with the Himalaya (Parkash et al, 2011), its geomorphic processes and tectonism of respective segments are mostly controlled by the Himalayan tectonics . Neotectonic deformation in the plains are mainly manifested by the surface ruptures as several transverse normal and longitudinal reverse faults which create minor relief differences in the monotonous flat plain (Pati et al, 2011b). The relief difference across the faults varies from 4 to 13 m. However, this relief difference is attributed to different tectonic events by the concerned fault or it may be by a single event depending upon the number of tectonic events to which the concern fault has responded.…”