The geological interpretation of a gravity survey of the English Lake District and the Vale of Eden

Abstract

The Lake District is dominated by an east-west belt of relatively low Bouguer anomaly which attains individual minima over the exposed Eskdale, Shap and Skiddaw granites. The negative anomaly is attributed to a composite granite batholith which underlies the central and northern parts of the Lake District, connecting the exposed granites at depth. Interpreting on the basis of surface density measurements, the granites appear to extend to a depth of about 7 to 10 km and the contacts with the country rocks generally slope outwards. There are substantial variations in density within the composite granite body. The roof region of the granite includes a series of shallow granite 'ridges,' one of these connecting the Eskdale and Shap granites, and another connecting the Ennerdale, St. John's, Threl-keld and Skiddaw granites. The Shap granite is probably connected to the Weardale granite by a deep granite ridge. Present and past uplift of the Lake District may be attributed to the granite mass deficiency, which is estimated to be 1.1 × 10 18 g and is approximately equal to the present elevation of the Lake District above a 270 ft (82 m) datum. The low gravity values along the Vale of Eden suggest that the Permo-Triassic rocks reach a maximum thickness of at least 1 km northeast of Penrith, and that these rocks formed during contemporaneous subsidence. The detailed gravity interpretation of the structure of the Vale of Eden allows a new assessment to be made of the structural history of the Pennine line which reconciles the Hercynian structures with the occurrence of Whin Sill or dyke pebbles in the Upper Brockram.