Electrical resistivity image of a basement ridge in the foreland central Ganga basin

Abstract

The foreland Ganga basin is traversed by several transverse ridges and faults, buried beneath the thick alluvial cover, that are considered to be underthrusting the Himalaya. The Faizabad Ridge, assumed to be the NE-to-NNE extension of the Bundelkhand craton, is one such prominent transverse tectonic feature in the central Ganga basin whose exact extent up to the Himalayan front is not known. In order to delineate its extent beneath the sediments, we have carried out magnetotelluric (MT) study along a 335-km-long profile, starting from the Vindhyans in the south and reaching up to the Himalayan front in the north. MT data were acquired at 32 sites with average inter-station spacing of 10 km. The impedance tensors in the period band of 0.001–25 s were analyzed for distortion and decomposition yielding a dominant strike direction of N105°E, coinciding with the general strike of the Ganga basin in this region. The distortion corrected TE- and TM-mode decomposed data were inverted using a two-dimensional inversion algorithm. The geoelectric structure reveals a gradual increase in the depth to the top of the Ridge. At Faizabad, the basement depth is about 3.4 km, but it rapidly increases thereafter and reaches to more than 8 km at the northernmost part of the profile where the Ridge signatures are not seen. We infer that the Ridge does not continue to the Himalayan Foothills in this region. The results also suggest the presence of a near-vertical weak zone of moderate resistivity in the vicinity of the Yamuna River. This region is also the contact between the buried Faizabad Ride and the Vindhyan Supergroup.