Palaeomagnetics and igneous activity of the area adjoining the south-westem margin of the Cuddapah basin, India

Summary. Fifty-six orientated samples were collected from 13 sites on five dolerite dykes (between lat.14°23°N, long.77°43'E and lat.14°08'N, long. 77°49'E), which adjoin the south-western margin of the Cuddapah basin in Anantapur district, Andhra Pradesh. After af demagnetization, two dykes (five sites) striking ENE possess similar magnetic directions, (1) D = 57°, 1=-69° (K = 52, α₉₅= 7°) and (2) D = 71°, I = -72° ( K = 260, α₉₅= 5°). Again dykes (3) (three sites), and (4) (two sites) have similar strike (NE) and magnetic directions, D = 64°. I=-7 α( K = 142, α₉₅ = 8°) and D = 53°, I =-8° (K = 142, α₉₅= 6°) and dyke (5) (two sites) striking NW shows D = 320°, I = -34° ( K = 68, α₉₅= 13°). Remanent directions estimated from total field magnetic anomaly data agree well with these results. Synthesis of these data with 10 other published palaeomagnetic studies of Precambrian dolerite dykes on the Indian peninsula, suggest that these three systems of dykes adjoining the Cuddapah basin had been emplaced prior to the basin formation perhaps representing the initial thermal event responsible for the basin development and also that there have been at least three separate periods of dyke emplacement on this shield. The radiometric data, however, are very sparse and these periods cannot be dated with confidence.