Structure and tectonics of Kachchh region of western India: An appraisal

Structure and tectonics of Kachchh region of western India: An appraisal

R. V. Karanth
Department of Geology, Faculty of Science, M. S. University of Baroda
Vaodara - 390002

Introduction
The Kachchh region is an excellent example of a tectonically controlled landscape whose physiographic features are the manifestation of the earth movements along the tectonic lineaments of the Pre-Mesozoic basinal configuration, that was produced by the primordial fault pattern in the Precambrian basement (Biswas, 1971; 1974). Major E-W structural orientations have played pivotal role in its geomorphic evolution. This region of western Indian subcontinent has a unique history of continued tectonism since the advent of Mesozoic, and its landscape, comprising an assemblage of various tectonogenic geomorphic features is the reflection of movements along the major E-W longitudinal faults viz. Katrol Hill Fault (KHF), Kachchh Mainland Fault (KMF), Banni Fault (BF), Island Belt Fault (IBF), Allah Band Fault (ABF) and Nagar Parkar Fault (NPF) (Fig 1).

Fig 1. Structural Map of Kachchh (after, Biswas, 1987)

These faults were reactivated from time to time (Biswas, 1970, 1980, 1982,1987 and Karanth, 2000). Apart from the clear evidences in the form of diversity of landforms, Paleoseismicity record and several seismic events point to a continued tectonic activity. As a result the Kachchh has experienced several episodes of earth movements due to successive unidirectional tectonic upheavals along these lineaments all throughout the Cenozoic. These have not only contributed to the evolution of a youthful topography, but have further accentuated the structural pattern (Biswas, 1971; Kar, 1988). We have observed that numerous oblique cutting subordinate faults developed during various tectonic events trending N-S, NNE-SSW, ENE-WSW and WNW-ESE have controlled the trends of major drainage network both in the rocky upland as well as in the alluvial tract.

The Kachchh landscape comprises an array of tectonogenic geomorphic elements and is a manifestation of uplifts and residual depressions. Elevated landforms are the areas occupied by Mesozoic and Tertiary rocks, whereas the residual depressions or low-lying regions between the uplifts consist of Quaternary sediment successions marked alluvial river terraces in the rocky mainland and the mud-flats and salt pans in the Great and Little Ranns and Banni Plains. The general forms of the uplifts are marked by domes and asymmetric anticlines, which are confined to the south of the major faults (Fig. 1). All major uplifts are bounded, at least on one side, by a fault or a sharp monoclinal flexure, and on the other side by gently dipping peripheral plains, the strata (Tertiary) in which dip gently into the surrounding residual depression (Biswas, 1980). It is observed that the uplifts exposing folded Mesozoic rocks (Middle Jurassic-Lower Cretaceous), however, are not simple broad-topped upwarps with a faulted margin. They are complicated by flexures along the bounding faults and by secondary uplifts along faults with accompanying flexures within them.

Geomorphologically, Kachchh can be categorized into four major E-W trending zones (Fig. 2):

Coastal Zone - demarcating the southern fringe
Kachchh Mainland - divided into the central portion comprising rocky upland, northern hill range and coastal plains, 3) Banni Plains (less than 5m MSL)-marked by raised fluviomarine sediments, mud flats and salt pans and 4) the two Ranns Great Rann (~ 2m MSL) in the north and Little Rann in the east comprising vast saline wasteland. The boundaries of these main geomorphic zones are bounded by the major E-W trending faults.

Fig 2 Geomorphic Map of Kachchh

The Kachchh region abounds in evidences of continued tectonism since Mesozoic times and its physiography is a clear manifestation of active tectonism. It provides a good example of a terrain where neotectonic activity has influenced evolution of landforms and fluvial regime. Obviously, the changes are represented by (1) periodic reactivation of movements along the older faults, (2) development of new fractures (faults, joints) (3) development of related fault propagation folding during uplifts along fault blocks and (4) modification of older drainage and development of new fractured controlled streams.

Various major stream trends, events of terracing and channel entrenchment, all point to the response towards reactivation along the fault planes. The stream courses not only follow tectonic trends (faults and joints) but their valleys have preserved good evidences of flexure related uplifts also. Streams of varying dimensions belonging to several generations exhibit lineament control and the channel trends and their morphology provide valuable information on nature and sequence of tectonic events of the Quaternary period. Occurrence of paired strath and fluvial terraces along the various river valleys, incised channels and recent gullies along the margins of the Banni Plains-Great Rann reveal close relationship between the drainage development and active tectonic movements. These are envisaged to have taken place during Quaternary period and continuing even upto the present [(Biswas, 1974; Kar, 1993b; Malik et al. 1999; Sohoni et al. 1999; Malik et al. (In press)].

Geomorphology
The variety of the geomorphic facets of the Kachchh peninsula such as the present surface configuration, its landforms, the drainage characteristics and the relief pattern reveal a complex interplay of tectonism, sea level changes, lithology and the Cenozoic processes of erosion and deposition. Interestingly, within the limits of the Kachchh peninsula, one comes across conspicuously high hills and extensive low plains. The uplands comprise rugged hilly terrains exposing folded Mesozoic rocks (Middle Jurassic-Lower Cretaceous) bordered by thin strips of gently dipping Cenozoic rocks (Paleocene to Pleistocene) which form the coastal plains. The highlands are the areas of uplift whereas the plains of low lands represent structural basins between the uplifts and are made up of alluvium, mud and salt flats (Ranns).

Topographically, the Kachchh region is made up of east-west trending hill ranges i.e. the Island belt, the Kachchh Mainland and the Wagad. The hill ranges in each of these areas are separated by large tracts of low ground. All hill ranges and the intervening low ground run almost parallel, a characteristic feature indicating that the topography has been controlled to a large extent by the geological factors of folding, faulting and lithology. The highest peak in Kachchh is that of Kaladungar (D 465 m) in the Pachcham island. On the Kachchh Mainland there are several peaks, the Nanadungar showing the maximum altitude of 430 m.

The landscape comprises rocky highlands standing out like "islands" amidst the vast plains of the Great and Little Ranns of Kachchh. Whereas the Ranns and Banni are the depositional plains of Recent times, the highland areas bear evidences of multiple erosional cycles Biswas (1987). According to Biswas (1974) the five denudational cycles are correlatable with major periods of tectonic movements in the region.

Physiographic Divisions
Taking into consideration the factors of altitude, slope and ruggedness of relief, Kachchh can be divided into four main physiographic units from north to south (Fig.2), viz; (1) The Ranns, (2) The Low Lying Banni Plain, (3) The Hilly Region, and (4) The Southern Coastal Plains.

The above four units show considerable diversity within each of them, depending on the rock types, their mode of occurrences and fault patterns.

The Rann is the most remarkable and unique feature of the Kachchh region occupying its northern and eastern parts, forming more than half of the areal extent of the Kachchh. It comprises a flat geomorphic terrain rising hardly upto 3 to 4 m above M.S.L, and is divisible into two parts, viz. the Great Rann occupying the northern part and the Little Rann forming the eastern and southeastern parts of the Kachchh. Within the Great Rann occur a chain of islands comprising Pachham, Khadir, Bela and Chorar, rising above the saline wasteland. The Rann area mostly remains dry except for the rainy season when it is covered by saline water. During summer and winter seasons, practically the whole region is covered with a fairly hard salt encrustation. Different aspects of the Rann have been described by a number of workers (Frere, 1879; Ghosh, 1981; Glennie and Evans, 1976 ; Roy and Merh, 1977 and Gupta, 1975).

The Plains of Banni represent an embayment between the Kachchh Mainland in the south, the uplifts of Pachham in the north, and the Wagad and the Bela uplifts in the east, and cover a wide area. These rise a little higher than the surrounding Rann and are covered with green grass and other shrubs. No outcrop is seen within these featureless plains. These receive water from the Mainland and the islands from the north and east respectively during rainy seasons.

The Hilly Region of the Kachchh consists of following three components:

Island Belt: It consists of four islands, viz. Pachham, Khadir, Bela and Chorar from west to east.
Mainland: The area lying to the south of Banni plains and extending upto the Gulf of Kachchh in south is called Mainland.
Wagad: This region lies to the NE of the Mainland and forms an isolated landmass.

The Mainland comprises a rocky terrain and broadly consists of two subparallel E-W trending hill ranges; viz the central highland and the northern hill range with an intervening low ground, and a southern coastal plain. Lithologically, the central highland is made up of Mesozoic rocks which southward have been separated from the Tertiary rocks and southern coastal plains by the Deccan trap basaltic rocks. Lithology and tectonics have played a very important role in the formation of present geomorphic features of the Kachchh Mainland. The northern hill range is bordered by the Banni plains and the Rann in the north and by the 80-140 m high upland areas in the south (Fig. 2). This hill range forms a chain of domes of Jurassic and Cretaceous rocks, and is flanked to the north by E-W trending Kachchh Mainland Fault (KMF). From west to east, it is marked by a series of domes like Jura, Jumara, Panjal, Keera, Lyari, Chari and Dhar Dongar and anticlines like Jhurio, Habo and Kas. The KMF has significantly controlled the physiography of this part of the terrain. On account of this fault, the northern slopes are steeper whereas the southern slopes coinciding with the dip of the strata are gentler. The southern coastal plains that border the Mainland and overlook the Gulf of Kachchh in the south and the Arabian sea in the west are made up of Tertiary and Quaternary sediments and form a 25-30 km wide belt showing very low gradient. These plains rise gradually from the high waterline to altitude of 80 m beyond which the ground tends to show a rather more conspicuous rise, merging into the central highland with a steeper gradient.

Drainage
The drainage of Kachchh provides an interesting example of a combination of lithologic and tectonic controls along with the influence of sea level fluctuations during Quaternary period. The Central highland forms the main watershed with numerous consequent streams draining the slopes with a radial pattern and pouring their water and sediment load into the Arabian sea, the Gulf of Kachchh and the plains of Banni and the Rann in west, south and north respectively (Fig. 3). The southward flowing streams include Naira, Kankawati, Chok, Sai, Vengdi, Kharod, Rukmawati, Khari, Nagavanti, Phot, Bhuki, Mitti, Sakra and Larekh streams which empty their water into the Gulf of Kachchh and the Arabian sea. The streams originating from the northern slopes of the Central highland, join the streams originated from the Northern hill rang e and pour their water into the Chhari, Bhukhi, Trambo, Kaila, Pur and Kaswali streams which, in turn, debouch into the Rann. In general, the streams are ephemeral (seasonal) and carry water only during good monsoon. Many streams like Kankawati, Kaswali, Kharod, Rukmawati and Bhukhi etc. show very broad channels and vertical cliffy banks in their lower reaches.

The drainage characteristics of the Kachchh Mainland and the relatively well carved valleys which now have only very little flowing water clearly point to the fact that the area had experienced a more wet climatic phase in the past during which the streams carried more water and sediment load and the stream dissection was more effective.

Fig 3. Drainage Map of Kachchh

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

Seismotectonic techniques are a powerful, effective and much economical for studying large scale tectonic problems. As a successful implication of such tools good progress has been made in understanding the large-scale active tectonics of Asia (Molnar and Chen, 1982).

Although, the comparative risk is less than the Himalayan and the other active areas, Kachchh has been an area which has produced during very recent times one of the greatest known earthquakes in the history of mankind, viz. the 1819 Allahband earthquake rated next to 1811-1812 New-Madrid earthquakes (MacMurdo, 1824; Oldham, 1926; Johnston, 1990). A strong earthquake such as the 1819 event in Kachchh region may have disastrous consequences in the present day socio-economic setup. In the present study with the limited available data a primary attempt has been made to underdstand overall seismic pattern of Kachchh region. The study also concerns with the probabilistic seismic hazard assessment.

Modern and historic Earthquake Activity
The modern and historic seismicity of kachchh region has been least investigated except for the few works (e.g. Oldham, 1926; MacMurdo, 1824).

Fig 5. Spatio-Tamporal distribution of earthquakes in Kachchh region (modified after Malik, et al, 1999)

Although, some progress has been made to study seismic aspects of the region in last decade (e.g. Johnston, 1990; Chung, 1995; Rajendran et al, 1998; Sohoni and Malik, 1998), however, most of the work is based on palaeoseismological studies (i.e. studies related to soft sediment deformation). Therefore, it is important to evaluate the spatio-temporal event distribution in the region. Fig 5 shows the spatio-temporal distribution of earthquakes in Kachchh region. Incidentally the earthquake plots fall along the faults illustrated in Fig 4. It should be noted that significant events above M 4 have taken place in the region and hence it becomes important to know the distribution of active faults of the region to assess the seismic hazard possessed by the area.

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Acknowledgements
The author is thankful to Prof. S. S. Merh, Dr. J. N. Malik and Shri. Parag S. Sohoni for their help in various ways in preparing this note. Financial support from Dept. of Science and Technology, New Delhi (Project No. DST/23(98)/ESS/95) is gratefully acknowledged.