T R K Chetty
T Vijaya Kumar
National Geophysical Research Institute,
B V V Suresh
International Centre for Science and High Technology,
Geographic Information System (GIS) has greatly improved the efficiency and expanded the possibilities of evaluation, manipulation and combination of multiple data sets. In view of variability of geological features in space and time geologists have been rather slow in making use of such a powerful tool as GIS but are now embracing this technology for a variety of applications. GIS not only saves digital data sources of spatial data but also effectively acts as visualization tool and transformation aid. Visualization is a critically important function of GIS. Earlier, by looking at maps and map patterns geoscientists used to analyze and postulate geologic structure. GIS has made a tremendous impact in manipulation and analysis of individual layers of spatial data. This can be carried out by visualizing the display of different geological objects with specific combinations which effectively create new maps. Such maps can often lead to a better understanding and meaningful interpretations.
In this contribution, we apply GIS to understand the tectonic history of the Eastern Ghats Mobile Belt, a well-known Precambrian orogenic belt of global importance. We attempt integration and superimposition of different geological features such as lithologies, structural trends, fold patterns and shear zone systems etc. from different sources of multiple scales through permutation and combination within the platform of GIS and evaluate their interpretations. We also demonstrate how a GIS approach eases data archiving and map generation for better insights and interpretational possibilities, which were not available with common traditional mapping procedures.
Resource Data and Digitization
The Eastern Ghats Mobile Belt (EGMB), which occurs along the east coast of India, has been subjected to Proterozoic collisional processes (Chetty and Murthy, 1994). The EGMB has received much attention because of its mineral wealth and its role in the reconstruction of Gondwana supercontinent. The major lithologies are charnockites, khondalite group of rocks, migmatites including other granitoids, which have been metamorphosed to granulite facies metamorphism around 3000Ma. These are subsequently invaded by alkaline and anorthositic rocks (1450-850 Ma), preferably emplaced along the shear zones at the western margins. Proterozoic metasedimentary basins of varied size and shape occur in the craton in the vicinity of the western margin of the EGMB. The EGMB is transected by across by two major Precambrian rift structures namely Godavari rift in the south and Mahanadi rift in the north. Details of geologic history of the EGMB are provided in recent reviews (eg. Ramakrishnan et al., 1998; Chetty, 1995, 2001; Chetty and Murthy, 1994, 1998; Rickers et al., 2001).
Fig.1 Network of major ductile shear zones in the EGMB.
We have considered structural interpretation of satellite image, (1:1 million scale), and Geological map of GSI (1:5 million scale) for a regional and comprehensive tectonic analysis of the EGMB.
Structural Interpretation of Satellite Data
We have carried out structural interpretation from a mosaic of Landsat thematic mapper data on a scale of 1:1,000,000. Subsequently, our field traverses reveal that the EGMB constitutes a network of major shear zones (Chetty, 1995) along the margins as well as in the interior (Fig.1). Field observations show that these shear zones are characterized by strong LS-fabrics often associated with a spectrum of mylonites. The shear zones vary in their width from a few meters to a few kilometers. They are oriented in NE-SW in the southern part and nearly east west in the northern part. In the central part, they trend nearly N-S orthogonal to the regional structural grain. Despite their diversity in nature and geometry, it is believed that the shear zones are genetically related to one another and interlinked together. The western boundary of the EGMB is marked by a crustal scale ductile shear zone, termed as the Sileru Shear Zone (SSZ). Alkaline magmatism with protracted history (1400-850 Ma) is centered around several places preferentially along the SSZ suggesting its repeated reactivation. The Northern Boundary Shear Zone (NBSZ) separates the Singhbhum craton and the EGMB in the north. Another prominent shear zone is the Mahanadi Shear Zone (MSZ), following the east flowing Mahanadi river course. The presence of small Gondwana basins along the MSZ points to the uncertainty of the southern margin of the Mahanadi rift. The shear zones in the central part are distinctly marked by Nagavali and Vamsadhara river courses described here as Nagavali Shear Zone (NSZ) to the west and Vamsadhara Shear Zone (VSZ) to the east. Extensive granitic magmatism (900-800 Ma) is distinct along these shear zones. Both these shear zones coalesce together at both the ends and abruptly end against the Bay of Bengal in the south. Chetty (1995) favoured the extension of these shear zones into the Enderby Land coinciding with the interface of the Archaean Napier complex and reworked Proterozoic Rayner complex. This hypothesis strongly supports the theory of juxtaposition of the EGMB and the Enderby Land during the Meso-to Neoproterozoic times. For detailed structural interpretations and their implications, the reader is referred to a series of publications (eg. Chetty, 1995, 2001, Chetty and Murthy, 1998).
Geological map of GSI (1993)
We have made use of the geological map of GSI 1:5,000,000 scale published in 1993. Geological mapping of the EGMB is handicapped by thick weathered crust, limited accessibility, dense vegetation, absence of marker horizons or unconformities and above all, the conventional mapping techniques adopted during early years. With the advent of powerful tools such as Remote Sensing there has been significant advances in mapping techniques. The emergence of modern conceptual models based on the work carried out at similar terranes elsewhere in the world offers general guidance in mapping. The major lithologies digitized from the geological map are: granulite facies rocks constituting charnockites, khondalites, migmatites, Proterozoic metasediments in the vicinity and Gondwana sediments restricted to major rift structures.
A major part of the effort in any GIS project is tied up in assembling and compilation of data in digital form. We digitized the geospatial data of various structural features and different lithological assemblages from different resource maps into “layers” within GIS. We have made use of MAPINFO software and captured the data from different maps by manual digitization.
Storing data within GIS has tremendous advantages in comparison with hard-copy maps or independent computer databases. The access becomes faster once all the data are registered. Further, adding new data is quick and easy with the impact of new information or any given interpretation instantly accessible. It has been recognized that viewing maps on the screen are far more revealing than the paper reproduction. Further, user interaction allows examination of data from any angle with appropriate processing to highlight specific geologic features. Once the data are stored in digital form, the next step is to extract and derive the spatial patterns and mutual relationship of geologic features such as shear zones, lineaments, circular features, different lithologies and geochronological data related to the EGMB.
Spatial analysis and Interpretation
Spatial analysis is one of the key strengths of GIS. In this paper, we have performed a spatial comparison of different geologic features to assess the mutual relationships individually and jointly. Here, we involve the combination and super imposition of various maps one over another to understand the regional tectonics of the EGMB.
It is generally difficult to recognize features which are crucial for the interpretation, particularly in a composite map. It often becomes essential to superimpose features of different geological features one over the other by manipulating the permutation and combinations to have better interpretation. It is particularly difficult in the case of complexly deformed Precambrian orogens such as the Eastern Ghats Mobile Belt. The shear zones are considered the most prominent tectonic features that play a crucial role in understanding the tectonic history of the EGMB and the network of shear zones has been chosen as the fundamental tectonic framework for correlation and interpretation. The significant relationship between the shear zones and the other structural features and different lithological units are examined in the form of a series of maps as described below.
The manifestation of network of shear zones in the EGMB (Fig. 1) is unique in that they represent important deformation markers and play a crucial role in understanding the tectonic history of the EGMB. The shear zones vary from 3-8 km in width and extend for hundreds of kilometers along the strike. The prominent among the shear zones in the EGMB include, (a) Sileru Shear Zone (SSZ) at the junction between the Bastar Craton and the EGMB; (b) Northern Boundary Shear Zone (NBSZ) at the contact between the Singhbhum Craton and the EGMB; (c) Mahanadi Shear Zone (MSZ); (d) Nagavali Shear Zone (NSZ) and (e) Vamsadhara Shear Zone in the central part.
Shear zones and Lineaments
The distribution and the orientation of lineaments both in the Bastar craton and in the EGMB have been analysed. It has been found that the lineaments in the former are dense with dominant NW-SE, and NE-SW trends, which are also common in the Dharwar craton. In the EGMB, the lineaments are less dense and do not show any pattern. However, NW-SE trending lineaments are long, often continuing into the craton and in a way lie sub-parallel to the Precambrian rift structures of the Godavari and Mahanadi rifts. Interestingly, the small isolated Gondwana basin near Ranasthalam, west of Srikakulam, occurs in the proximity of one such NW-SE trending lineament. Such lineaments are also more pronounced in the central part of the EGMB. In summary, it is clear that the lineament patterns are distinct in the contrasting geologic terrains. The presence of shear zone network is strikingly restricted to the EGMB.’
Shear zones and structural trends (fold styles)
Structural trends in the form of folds are common in deformed rocks. They are defined by curvilinear planar features such as bedding, gneissosity, mylonitic foliation etc. They vary from decimeter to a few kilometers in their amplitude in the EGMB (Fig.2). Their development depends on the rheology of rock types pressure-temperature conditions and strain rate resulting in broad spectrum of fold styles. The regional folds are interpreted from Landsat TM data (Fig.2). Similar structural styles on mesoscopic scale are commonly observed in the field. The striking feature is the variation in the orientation of their axial surfaces from near horizontal to subvertical nature. The axial surfaces of these folds lie parallel to the shear zones in their proximity. They also maintain curvilinearity consistent with the trend of the shear zones. Rarely, the fold forms cut across the shear zone suggesting close genetic relationship between the development of different fold styles and the shear zones. This relationship is evidenced by the rotation of axial surfaces of the folds parallel to the shear zones.
Fig.2 Shear zone network and the structural trends (fold styles) in the Eastern Ghats Mobile Belt.
Shear zones and charnockitesA 20-30 km wide zone of charnockites along the western margin, west of SSZ is distinct. Isolated fragments of charnockite massifs and/gneisses also occur conformably with the shear zones throughout the EGMB. For instance, in the northern part these rocks exhibit folded forms conforming to the trends of SSZ and MSZ. These rocks also show north-south elongation in the central part and NE-SW elongation in the southern part consistent with the geometry of major shear zones in the region. It is evident that the distribution of charnockite bodies is undisputedly controlled by geometry of the shear zones in the EGMB. Further, it is possible that the charnockites seen west of SSZ and north of NBSZ may represent upthrusted blocks lying over the cratonic areas from the EGMB (Chetty and Murthy, 1993). In view of reasons cited above, together with widely different geological association and divergence in age, the charnockites cannot be used in a stratigraphic sense. On the other hand, it is likely they were originally of the same stratigraphic position but later chaotically distributed and interleaved with other lithological association by fold-thrust tectonics and dextral strike-slip shearing (Chetty and Murthy, 1998).
Shear zones and khondalites
The khondalitic groups of rocks are pelitic/semipelitic assemblages, which include garnet-sillimanite+ graphite gneisses, quartzites and calc-granulites. The map shows that khondalites are continuous and predominant in the southern part while they are isolated and dismembered in the central and northern parts. The boundaries and the long axes of the khondalite outcrops are broadly conformable with the shear zones. In contrast to charnockites, khondalites are rarely seen on the cratonic areas. The most striking feature in the map is a major regional fold structure displayed by these rocks with its closure to south. This is consistent with a southwesterly and gently plunging regional antiformal structure inferred by detailed structural synthesis in the region (Chetty and Murthy, 1993). The presence of khondalitic rocks in the northeastern part is relatively less, which could be due to extensive migmatization producing garnetiferous quartzo-feldspathic gneisses in the area. The stratigraphic position of these rocks with respect to the charnockites is also not clear.
Shear zones, migmatites and granites
Migmatitic gneisses are wide spread in the northern part of the EGMB. They comprise enclaves of khondalite and charnockitic group of rocks, often with basic granulites and garnetiferous granitoids. Porphyritic hypersthene gneisses with conspicuous megacrysts of euhedral perthite are common rock types inside and in the proximity of major shear zones such as NSZ, VSZ and MSZ. The restriction of these megacrystic gneisses to these shear zones is significant but their extensive occurrence around Bolangir, Berhampur, Angul etc away from shear zones need to be understood. Could they represent flat lying shear zones or movement of the nappe sheets? It is likely that the migmatites and granites must have been formed through melting processes.
Proterozoic, Gondwana sediments and shear zones
The occurrence of Proterozoic sediments of Chattisgarh, Khariar, Indravati and Sukma basins in the close proximity of western margin of the EGMB (Fig.3) suggests the possible linkage between evolution of the sedimentary basins and the Mesoproterozoic collisional processes envisaged for the tectonic evolution of the EGMB (Chetty and Murthy, 1994). The presence of coal-bearing Gondwana sediments along NBSZ is related to basement reactivation tectonics during dextral strike-slip movements (Chetty, 1995b).
Fig.3 Shear zone network in the Eastern Ghats Mobile Belt and the spatial distribution of Proterozoic and Gondwana sediments. Discussion and Conclusions
Examination of the compiled geo-tectonic map combining all geological and structural features in the EGMB (Fig.4) would apparently reveal less information at a first look. A closer and critical look would bring out more additional prominent geological features. But, viewing the features in different combinations and permutation through GIS as described earlier would certainly provide better insights, and help us in arriving at innovative interpretations and tectonic models for the EGMB.
Fig.4 Geo-tectonic map of the Eastern Ghats Mobile Belt showing Litho logical assemblages and structural elements including the network of major shear zones.
As can be seen from the foregoing description the network of shear zones in the EGMB, plays a significant role in influencing the development of remarkable structural patterns and the distribution of lithological units. Our study concludes that the shear zones represent most important tectonic markers in the evolutionary history of the EGMB. They have played a crucial role in fashioning the present outcrop pattern in the region. The shear zones also delineate different litho-tectonic domains in the EGMB, which may represent different thrust nappes as allochthonous sheets and could be described as tectonometamorphic terranes (Chetty 2001). The GIS-generated litho-structural maps presented in the present study demonstrate the significant relationships of shear zones with various geologic features, magmatic and tectonic episodes. Our results also indicate that the shear zones must have been either continuously active or episodically reactivated. The maps also provide better understanding of the structure and tectonics offering new ideas about the structural controls of different mineral deposits such as graphite, tungsten and manganese etc., that are available in the EGMB. GIS is an essential component in defining geologic structural patterns and strengthens our understanding of the regional tectonic history of the EGMB as presented in this paper. GIS facilitates faster, more accurate analysis resulting in better quality and high accessible end products. The maps and models presented here are extremely useful not only for continued and improved understanding of the tectonics but also help in focusing future mineral exploration efforts in the EGMB.
The authors are grateful to Dr V P Dimri, Director, National Geophysical Research Institute for his encouragement and permission to publish this paper. This work has been carried out under the project entitled “Mid crustal shear zones in the Eastern Ghats Mobile Belt, India: possible linkage with east Antarctica.” funded by the Department of Science and Technology (DST), New Delhi. One of the authors (P Vijay) acknowledges the financial assistance provided by CSIR, New Delhi in the form of CSIR research grants.
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