Sub-surface structure of a craton–mobile belt interface: Evidence from geological and gravity studies across the Rengali Province–Eastern Ghats Belt boundary, eastern India

Abstract

Bouguer gravity anomalies characteristically vary from negative to positive values across craton–mobile belt boundaries in Precambrian shields. This transition is also documented in eastern India, where Proterozoic granulites of the Eastern Ghats Belt (EGB) form part of a mobile belt to the south of the late Archaean, amphibolite facies Rengali Province. The northern margin of the EGB with the Rengali Province is a sub-vertical Cambro-Ordovician strike-slip shear zone, on which the Talchir Gondwana sedimentary basin was deposited during late Palaeozoic extension. This extension also led to partial uplift of the lower crust below the terrane boundary. Closely coordinated geological and gravity studies were conducted through the basin, across the craton–mobile belt contact. Modeling of the Bouguer anomaly using 2-D and 3-D compact inversion schemes, along with 2-D forward modeling, indicates significant differences in the density configurations of the uppermost crusts of the EGB and the Rengali Province, as indicated by the surface geology. However, both inverse and forward models consistently predict that below ~7km, density configurations across the contact are similar and have low upper crustal values, suggesting that typically cratonic crust lies below both terranes. Since it is highly unlikely that lower crust can remain unaffected during granulite facies metamorphism, it is inferred that the present sub-surface EGB crust could not have experienced the high grade event. Rather, the EGB rocks were probably overthrust onto the craton significantly after granulite metamorphism. The top of the thrust sheet may have been eroded prior to or post-dating the overthrusting, leaving the present EGB granulites stranded on low density cratonic crust below.