Interpretation of a vertical field magnetic survey in North-East England

Abstract

Summary A vertical field magnetic survey of Durham and South Northumberland supplements the gravity survey described in the previous paper. The field survey and its reduction are described and some of the problems of interpretation are mentioned. A negative magnetic anomaly over the Alston Block is interpreted as another manifestation of a concealed granite under Weardale. Similar anomalies occur over exposed mineralized granites elsewhere, and arise from granite with low polarization existing in country rocks with higher polarization. The magnetic minima are displaced to the south-east of the corresponding gravity minima, and it is thought that this is caused by permanent polarization directed westwards in the country rocks. A violent positive anomaly lies immediately to the north at Blanchland and is tentatively interpreted as a marginal feature of the assumed granite. A magnetic anomaly of Caledonian strike in Northumberland probably marks the southern limit of low-polarization basement rocks. This may well represent the south-eastern limit of the sub-Carboniferous outcrop of the Southern Uplands Upper Ordovician and Silurian rocks.