The geological interpretation of gravity and magnetic surveys in Devon and Cornwall

Abstract

The main feature of the gravity field of Devon and Cornwall is a belt of large negative Bouguer anomalies which follows the line of the granite outcrops. These low anomalies are shown to originate from a shallow structure and are interpreted as being caused by the relatively low density of the granite itself. They indicate that the exposed granites are cupolas on a single elongated batholith which reaches a depth of at least 8 and possibly 20 km. The magnetic anomalies between Dartmoor and Bodmin Moor support the idea of interconnexion. A more detailed interpretation of the gravity anomalies shows that the contacts between the granite and the enveloping country rocks generally slope outwards. For Dartmoor, at least, the base of the batholith seems to be deeper in the south. A mechanism of emplacement combining forcible intrusion with stoping, in which the magma rose in the south, is consistent with the shape suggested. In marked contrast to the granites of the mainland and Scilly Isles there is only a small drop in the Bouguer anomalies over the Lundy granite, suggesting a laccolithic space form. The present high ground corresponding with the granite exposures of the mainland is shown to be isostatically compensated, or even overcompensated, by mass deficiencies implied by the negative anomalies. As the anomalies seem to be caused by the low density of the granite the compensation is almost certainly caused by the relatively low density of the batholith itself. The distribution of the compensating mass deficiency thus suggested is in closest agreement with Pratt’s hypothesis. The depth of compensation is unusually low at about 1 o or 15 km. There is a marked southward increase of the Bouguer anomalies over the Start and Lizard peninsulas which is thought to be related to the presence of a thrust affecting these regions. Similarly, the northward decrease of the gravity anomalies across Exmoor is attributed to a major overthrust beneath which a great thickness of Carboniferous and Devonian rocks is likely to be present. It is thus possible that Coal Measures are to be found at no great depth beneath the Devonian rocks of the Dunster-Minehead region. Over the Culm synclinorium three east-west ridges of higher Bouguer anomalies are superimposed on a westerly regional gradient and are shown to be of shallow origin. The high magnetic anomalies in this area suggest the presence of magnetic rocks extending to a considerable depth. A smaller gravity ‘low’ coincides with the Crediton trough of Permo—Triassic rocks. A detailed traverse at North Tawton indicates that the trough here is V-shaped and has a maximum depth of 340 m. Depth estimates have also been obtained for the Tertiary lacustrine deposits of Bovey Tracey and at Petrockstow.