• B RAVI KUMAR

      Articles written in Journal of Earth System Science

    • Tectonic divisions and accretionary model within Dharwar Craton: New insights from gravity surveys on status of Chitradurga Schist Belt

      J V RAMA RAO B RAVI KUMAR BALAKRISHNA B VEERAIAH B

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      The eastern margin of the Chitradurga Schist Belt is widely believed as the dividing line of Dharwar Craton (DC) into Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). This study aims at re-defining the division of DC based on pronounced gravity high encompassing Chitradurga Schist Belt of WDC and Closepet granite to Ramgiri–Penackacherla Schist Belt of EDC. The newly acquired high-resolution gravity data by the Geological Survey of India covering the Chitradurga Schist Belt and adjoining areas coupled with the available regional gravity data gives ample opportunity to understand divisions and opens up the possibility of grouping Chitradurga Schist Belt with the transitory block between WDC and EDC. This new data clearly suggests that there is a crustal sub-block between WDC and EDC and the boundaries are characterized with significant gravity signatures. Further, this data indicates that the first division should be at the end of Shimoga–Bababudan Schist Belts up to the arms of Chitradurga Schist Belt, with a significant and corroborative observation of deep crustal rocks in the form of the Sargur group of rocks. From this division to a narrow corridor of gravity lows bounded with high gravity gradients, representing a possible suture zone near Ramagiri–Penackacherla Schist Belt, is the suggested geographic disposition of Central Dharwar Craton (CDC). Similarly, a corridor of gravity lows bounded with gravity high gradients is identified as a possible subducting zone separating CDC and EDC between Ramagiri–Penackacherla Schist Belt and Kolar Schist belt. The above postulation has evidence from the five long profiles of 330 km (75$^{\circ}$–78$^{\circ}$E), separated with 30$^{\prime}$ interval, from the gridded new data. It is clearly seen that the major changes in the crustal architecture are around the intervening portions of Shimoga–Chitradurga Schist belts and end of RPSB, characterized with sharp paired anomalies. In comparison, only small changes were highlighting the eastern margin of the Chitradurga Schist Belt. Processed maps suggested the possibility of hitherto unknown ancient suture zone east of Ramagiri–Penackacherla Schist Belt in the form of a narrow zone of gravity lows, not related to surface geology. Accordingly, gravity models are proposed after fixing the regional field from the 5th order polynomial. A comparative study is made with the published seismic studies. Accretionary models of DC along 14$^{\circ}$N and 14$^{\circ}$30$^{\prime}$N are proposed from the gravity signatures.

    • Tectonic evolution of Nellore Schist Belt along the southeastern margin of Eastern Dharwar Craton, South India: A critical study with a geological and geophysical approach

      M BURHANUDDIN B RAVI KUMAR

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      The Nellore Schist Belt (NSB) along the southeastern margin of Eastern Dharwar Craton (EDC) is similar to all other Late Archaean greenstone belts of EDC. The NSB of EDC is bounded by Proterozoic Cuddapah sediments including the deformed and metamorphosed Mesoproterozoic Nallamalai Fold Belt(NFB) in the west; and high-grade Palaeo- to Mesoproterozoic Eastern Ghat Mobile Belt (EGMB) towards further east. Two parallel Western and Eastern arms of different environmental facies of NSB found to be evolved in foreland and back-arc/continental-arc basinal setups, respectively. An initial Late Archaean subducting coupled with the east to west convergence mechanism with regards to evolution of the NSB is discussed in detail. Based on gravity and seismic studies, an elliptical gravity high over the Eastern arm and with its gradual decrease in intensity towards the Western arm indicates the presence of a high-density layer over the Eastern arm of NSB with its continuity and wedging out below the Western arm defines the subducting and convergence mechanism from east to west in concurrence with several established studies. Accordingly, the NSB is differentiated as one continuous single segment, i.e., Late Archaean in contrary to earlier views. The steep gravity gradient in the west defines the Cuddapah Eastern Marginal Thrust, which separates the NFB in the west and NSB in the east. A similar gravity gradient is differentiated as low-angle Malakondasatram Thrust corroborating with the geological features in the eastern part, which defines the eastern limit of EDC. In the NSB domain of Kanigiri–Pamuru area, an isolated structurally controlled Mesoproterozoic metasedimentary unit is established unconformably overlying both Western and Eastern arms of the NSB. This unit is transformed as high-grade kyanite schist in the Vinjamuru domain of extreme eastern part at the EGMB front. The occurrence of Mesoproterozoic high-grade schist over the low-grade schist of NSB is interpreted as a postdated overprint with composition and layer specific metamorphism in response to the Proterozoic tectono-thermal events of Easternghat orogeny.

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