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Structure and tectonics of Kachchh region of western India: An appraisal
Structure and Tectonics
Tectonically, the Kachchh is situated at the area where northwestern margin of the Indian continental shield meets the geosynclinal belt of the Sindh-Baluchistan. Thus, this marginal portion of the Indian shield forms a mobile zone characterised by block faulting and consequent folding. Block faults are typical of a basement made up of old rocks already consolidated by Pre-cambrian orogenic movements. This marginal mobile belt of the Indian shield extends along the shelf zone with its coastal counterpart of the west coast from Bombay to Kachchh through Surat and Broach coasts and Saurashtra peninsula, reaching its maximum intensity near Kachchh where it borders the geosyncline of Sindh-Baluchistan. Regionally, the structure of the Kachchh is characterised by a series of uplifts along master faults (upthrusts) and along the Delhi tectonic trend that were reactivated during geologic time. The faulted margins of the uplifts are marked by narrow linear zones of folding; a string of asymmetric domes and brachy anticlines occurs along these tectonised zones. Igneous intrusions such as laccoliths, plugs, sills and dyke swarms are localized in these zones. The Mesozoic strata are folded, faulted and intruded by igneous rocks. The Tertiary strata which warp around the Mesozoic highs in contrast, show very gentle dips and lie over the eroded Mesozoic folds in some places, bearing testimony of a major pre-Tertiary tectonic movement. Biswas (1982) has invoked a unique feature in the Kachchh, a meridianal high across the basin occurring along the hinge zone (Fig. 1). Evidence of sediment thickness and the facies present indicate that this high came into existence in late Oxfordian time. The Kachchh Mainland is bounded by following major regional faults which are responsible for its existing configuration. These bounding faults are (1) Kachchh Mainland Fault striking ESE -WNW to E-W forming the northern limit of the Mainland. This fault has also caused southward tilting of the Mainland and thereby has greatly influenced the deposition of Mesozoic and Tertiary sequences, (2) Offshore West Coast Fault striking NW-SE and marking the western limit. It is probably the extension of the fault which is believed to have caused the almost straight west coastline of India, and (3) Gulf of Kachchh Fault and the Little Rann of Kachchh Fault System bounding the southern and eastern limits of the Kachchh Mainland respectively (Biswas, 1980, 1982). In addition, a number of faults related to one or the other of the major faults are encountered within the Kachchh Mainland which developed during various stages of the repeated reactivation of the major bounding faults at later dates. These faults have significantly influenced the Cenozoic landscape of the Kachchh. Along the Kachchh coast, Cenozoic sediments typically show evidences of the differential movements of faulted blocks. The folding of strata is essentially a reflection of the fault movements of the various blocks under the sediment cover. The fault uplifts and subsidences occurred more or less along the old lines of weaknesses. The control exercised by the pre-existing Trap basement topography over which strata of the Tertiary and Quaternary ages were draped can also not be ruled out. There are ample evidences to suggest that various parts of the Kachchh Mainland have undergone differential movements even during Quaternary period, and even the coastline's geological and geomorphological evolution is controlled by the periodic movements along tectonic lineaments. The sea level changes of the Quaternary period together with the various faults have contributed to the evolution of the coastal landforms and sediment diversity. The main structural elements recorded by various workers (Srivastava, 1964; Biswas, 1980, 1982 and Biswas and Deshpande, 1970; Sharma, 1990), which played significant role in the post-Mesozoic geological and geomorphological evolution of Kachchh Mainland are,1. Katrol Hill Fault, 2. Vigodi Fault, 3. Little Rann of Kachchh Fault system, 4. Naira River Fault, 5. Bhujpur Fault, 6. Vinjhan Fault, Vinjhan Anticlinal Nose and Kothara Embayment, 7. Gulf of Kachchh Embayment, 8. Narayan Sarovar Anticlinal Nose, 9. Bhachau Anticlinal Nose. Seismicity It is now more or less established that in Kachchh, seismicity has been a manifestation of continued tectonism, mostly along pre-existing planes of weaknesses (faults, major joints etc). Fig 4 illustrates possible active faults recorded in the region of Kachchh (Karanth, 2000; Sohoni, 2001). These faults are as follows; a) Nagar Parkar Fault, b) Allahbund Fault, c) Kachchh Mainland Fault, d) Katrol Hill Fault, e) Naira River Fault, f) Bhujpur Fault and g) Wagad Fault. The region that continues to be tectonically an active area and in future also earthquakes will remain an important geological activity. It is well known that Kachchh falls under seismically active zone V of the Indian Sub-continent. With the increasing socio-economic development of the Kachchh region, it is therefore most essential to reliably estimate the local earthquake risk not only in the main earthquake zones but also in the regions, which have scatterred seismicity. A unique information on active tectonics can be sought by studying seismicity, including the spatio-temporal distribution of earthquakes and the orientation, amount and extent of faulting during particular earthquake. Seismicity in a way, also provides insitu information on the nature and spatial distribution of seismotectonic processes at depths that are not easily recognized otherwise. In turn, the range of geodynamic observations is better understood by investigating the seismic aspects of a region.  Fig 4 Map showing Active Faults in Kachchh region |