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).
