The Northern Margin of the Eastern Ghats Mobile Belt: Evidence for Strike-Slip Tectonics Along a Craton-Mobile Belt Boundary

The Eastern Ghats Mobile Belt (EGMB) lies to the east of the Archaean Bastar and Dharwar Cratons, and to the southeast of the Singbhum Craton in the Indian shield. Along its western boundary, the EGMB granulites have been thrust westward as a nappe over the Bastar Craton along a mylonitic contact zone. Earlier studies considered the northern boundary of the EGMB with the Singhbhum Craton to be a thrust, although this interface is geometrically parallel to the west-directed transport direction of the granulitic nappe. Detailed geological studies along this northern margin reveal that the c. 1.0 Ga granulites of the EGMB do not share a direct contact with the Archaean granite-greenstone terrane of Singhbhum, but are actually juxtaposed against a Late Archaean (2.8-2.5 Ga) high grade terrane referred to as the Rengali Province. Structural studies reveal that the EGMB-Rengali Province contact has a WNW-ESE strike with sub-vertical dip, with prominent asymmetric markers indicating dextral strike-slip shearing along a horizontal transport vector. Microstructural studies indicate that fabric formation during strike-slip deformation is controlled by plastic deformation of quartz, while other minerals remained passive or deformed in a brittle manner. Electron Back-Scatter Diffraction (EBSD) studies on selected samples from the contact zone indicate that quartz deformed mostly by prism < a >, rhomb < a > and basal < a > slip, with asymmetry indicating dextral simple shearing. The Rengali Province samples show that earlier shortening (pure shear) microstructures and quartz CPO patterns were also sheared dextrally by this later deformation. These results confirm that the northern boundary of the EGMB is not a thrust, but a strike-slip shear zone that operated well after granulite metamorphism, and at lower temperatures characteristic of the greenschist facies. Shortening structures in the Rengali Province and the Singhbhum Craton are related to an older deformation event unrelated to emplacement of the EGMB. Gravity studies across the contact confirm that the shear zone continues vertically to a depth of at least 25 km. Thus, integrated geological and geophysical studies confirm the strike-slip nature of the craton-mobile belt boundary.