A gravity survey on the southern and central parts of the Isle of man

Abstract

The Bouguer anomalies on the Isle of Man have been calculated from a network of 365 stations. The results ofrock density measurements made in the laboratory are stated. The main features shown by the gravity survey of the southern and central parts of the island are two elongated negative anomalies, centered at Foxdale and Dhoon. They are shown to be shallow in origin and it is suggested that they are due to the prensence of two large granite bodies lower in density than the surrounding Manx Slates and only represented at the surface by small outcrops. These two granites are apparently steep sided and lie on a north-east to south-west 'axis' but are not connected. The negative anomaly seen in part in the extreme South-west of the island is probably due to arise on the Poxdale granite. The elongated shape of the granites revealed by the survey can be correlated with the zones of metamorphism, the distribution of the mineral veins and the "anticline of cleavage". Comparing the mass of land above sea level with the mass deficiency due to the granites the southern and central parts of the Isle of Man are found to be under-compensated. A residual gravity map of the Peel area was prepared and it revealed a negative anomaly which is probably due to the baain of lower density Peel Sandstones. The louguer anomalies are found generally to be high on the Isle of Man. This emphasises the fact that the Irish Sea seems to be an area of large positive anomalies for gradients have also been observed on the mainland. No definite conclusion has been reached as to the cause of this Irish Sea anomaly or, indeed, that it has only one cause.