Role of granitic intrusions in reducing the density of the crust, and other related problems as illustrated from a gravity study of the Cuddapah Basin, India

Abstract

A gravity ‘low’ with an amplitude of some 70 mgal over the east Cuddapah basin of Precambrian (Proterozoic) age has been mapped which extends for about 400 km paralleling the eastern boundary of the basin with an average width of 60 km. This near parallelism between the boundary of the basin and gravity anomaly contours suggests a strong correlation between gravity and geology of the Cuddapah basin. Direct calculations based on the techniques of Bott & Smith (1958) and Skeels (1963) suggest the anomaly source to be within the crust. In view of these direct calculations and some known granitic intrusions in the northeastern part of the basin, it is suggested that the anomaly is caused by massive granitic intrusions into the heavy rocks of Cuddapah system on the west and Dharwar system on the east. Although granitic intrusions are known only at a few places in the northeast, the extension of the gravity anomaly suggests that the granitic activity may be widespread and may occur all along the eastern Cuddapah basin. An average thickness of 20 km with a density differential of 0.1 g/cm 3 is needed to account for the observed variations in the isostatic anomaly. It is suggested that the granites with which mineralization in the Cuddapah basin is presumed to be associated may be cupolas of this inferred granite mass. For the inferred east Cuddapah basin crustal column to be in isostatic equilibrium, the presence of about 16-5 km material of 3.01 g/cm 3 density between the mantle and the inferred granite or crustal thinning of the order of 6.5 km is required. This leads to a suggestion that in anomalous regions due consideration should be given to the compensating effect of anomalous masses that may lie below the sea level in addition to the usually adopted method of considering only topographic masses in computing isostatic effects. On the basis of the comparison of the east Cuddapah basin crustal column having 20 km of inferred granitic mass with a standard continental crustal column (e. g. that due to Worzel & Shurbet 1955), it is concluded that granitic intrusions have locally reduced the density of an originally heavier upper crust.