• J V RAMA RAO

      Articles written in Journal of Earth System Science

    • Gravity–magnetic appraisal at the interface of Cuddapah Basin and Nellore Schist Belt (NSB) for shallow crustal architecture and tectonic settings

      SHUVA SHANKHA GANGULI SANJIT KUMAR PAL J V RAMA RAO B SUNDER RAJ

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      Cuddapah Basin (CB) is an intracontinental, Proterozoic basin Canked by Eastern Dharwar Craton(EDC) in the west, Nellore Schist Belt (NSB) and Eastern Ghat Mobile Belt (EGMB) in the east,represents second largest Proterozoic basin of India. Gravity and magnetic surveys were carried out at the interface of Cuddapah Basin (CB) and Nellore Schist Belt (NSB) covering $\sim 2880 \rm{km}^{2}$ area. Gravity map has brought out some distinct zones. The thrusted contact of NSB and Cuddapah sediments has been well delineated from the gravity map by NE–SW trending steep gradient of contours. Relatively high gravity values are observed over NSB in the southeastern part, moderately high values are observed over Cumbum Formation, but distinct low is observed over Baironkonda Formation. These gravity highs and lows are mainly the manifestation of basement characteristics and intrusives. The magnetic map shows two distinct domains, viz., moderate to low zone in the southern part, and moderate to high zone in the northern part. Regional gravity map suggests a change in basement characteristics from felsic to mafic from NW to SE. Presence of mafic basement may be representing EGMB group of rocks underneath the Cuddapah sediments at the eastern part of the study area. The joint gravity and magnetic modelling reveal varied nature of sedimentary units in terms of density and susceptibility and change in basement characteristic.

    • 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.

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