GEOMORPHOLOGICAL AND SEISMOTECTONIC EVALUATION OF THE REGION AROUND AGRA

Abstract

The Ganga basin is bounded by the Himalaya in the north and the Peninsular shield in the south. The northward thrusting of the Indian Plate has caused complex processes of intraplate deformation and reactivation of fault and fractures. The major objective of this scientific pursuit is to understand the geomorphological pattern and seismotectonics of a region governed by the underlying deformation of the earth's crust. The study area is characterised by the interfacing of the Gangetic alluvium of Recent age on one hand, and the Precambrian group of rocks on the other. The river Yamuna originating from the Himalayas and flowing southward, takes a sharp rectangular curve to follow the "margin" of the Precambrian shield region. The study has been based on the following data : remote sensing data (including Landsat TM, SPOT-HRV and IRS-LISS-II ), topographic maps, geological maps, groundwater well log data, geophysical data, data on earthquake occurrences and selected field observations. Digital image processing of the remote sensing data (CCT's) has been carried out to enhance geomorphological and lineament features in the area. Integrated interpretation have been made using various remote sensing data, geological maps, topographic maps, groundwater well log data and geophysical data. The geological formations in the study area comprise alluvium of Pleistocene to Recent age and meta-sedimentary rocks of Precambrian age. The alluvium covers the northern parts of the area, and is the result of aggradational processes of Yamuna-Ganga rivers. The southern parts of the area are characterised by the meta-sedimentary rocks of the Delhi Supergroup and the Vindhyan Supergroup comprising phyllites, schists, quartzites, shales and sandstones. The zone of Rajasthan Great Boundary Fault (RGBF) trending NNE-SSW to NE-SW, which runs approximately over a distance of 400 km, forms a major dislocation zone in the area. The RGBF zone separates the Aravalli-Delhi Supergroup from the Vindhyan Supergroup of rocks. The extension of this fault zone is traceable northwards of Agra in the Ganga plains. A detailed geomorphological map of the study area has been prepared with the help of Landsat TM, SPOT and IRS (LISS-II) satellite images and processed digital data. In the alluvial terrain a number of landforms have been mapped. Four broad geomorphic divisions have been identified in the area, viz. younger flood plains, older flood plains, upland plains and hilly tract. An attempt has been made to deduce the geomorphic evolution of the alluvial terrain which appears to be governed by the migration of rivers. The RGBF is a major tectonic element in this region on the either side of which there are distinct and notable differences in the hydrological regime and river meander movement directions. A detailed lineament-tectonic map of the adjoining Peninsular shield terrain has h' *n prepared based on studies of Landsat TM, IRS (LISS-II), SPOT images and processed digital data. Various types of lineaments have been mapped and the statistical analysis has been carried out. A microlineament map of the northern part of Precambrian shield terrain has been prepared based on the IRS (LISS-II) images. It is inferred that the region pertaining to the Delhi Supergroup is more intensely folded and exhibits several fold closures, ill T indicating strong deformation, on the other hand, the rocks of Vindhyan Supergroup are less deformed. A map showing regional segments and lineaments of Precambrian shield terrain has been prepared based on hydrogeomorphology of the area. It is inferred that between the RGBF and the Chambal lineament, there occur two distinct zones of compressed meanders. These indicate tectonic activities along the pre-existing zones of weaknesses, which must have contributed to the migration of the river Yamuna. A detailed review of the previous work dealing with the structure of the Ganga basin has been given. Based on geophysical data, a number of subsurface deep seated faults and structures such as Moradabad fault, Lucknow fault, Etah fault, Sarda depression, Faizabad ridge, Gandak depression and Monghyr-Saharsa ridge are known to occur in the Ganga basin. During the course of this study several major tectonic features in the Ganga basin have been inferred, collectively based on various data sets, viz. remote sensing data, hydrogeomorphological segmentation, aeromagnetic data, topographic profile data and sub-surface well log data. It has been observed that a few lineaments in the Ganga basin follow the extensions of major tectonic features of the Precambrian shield terrain, into the Ganga basin. Some of the major tectonic features inferred are Jalesar-Etah-Ujhani lineament, Jaswantnagar-Etah-Bareilly lineament, Moradabad fault and Meerut-Najibabad ridge. The RGBF is the most important feature and represents a 15-20 km wide zone. A number of lineaments parallel to sub-parallel and en-echelon to the RGBF are observed. It is inferred that some of these lineaments are extending as subsurface features into the Indo-Gangetic plains. The identification of lineaments based oh remote sensing data, geology and geohydromorphological

data and geophysical data in the alluvial terrain (Ganga basin) indicates that the RGBF is still undergoing movements in the area. The Ganga basin is noted to be composed of crustal blocks bounded by major dislocations and faults. The boundaries of these blocks have shown seismic activity in the past. A regional seismotectonic map of the area has been prepared. It is inferred that the regional lineaments, viz. the RGBF, DHR, Mathura lineament, Meerut-Najibabad ridge and Moradabad fault show a fairly good correspondence with the earthquake epicenters in the region. The study has interrelated the various aspects such as the geomorphological evolution, river migration pattern, lineament-tectonic pattern in the Peninsular shield terrain, sub-surface features in the Gangetic plains (as deciphered from remote sensing data, aeromagnetic data, topographic data, groundwater well log data), and seismotectonism of the area.