DEEP-STRUCTURE OF THE EAST MENDIP HILLS FROM GRAVITY, AEROMAGNETIC AND SEISMIC-REFLECTION DATA

Beacon Hill pericline is the most easterly pericline of the Mendip Hil ls, whose folds and thrusts were formed during the Variscan Orogeny by earth movements directed from the south. The Silurian volcanic rocks making up the core of the pericline are shown to be responsible for th e large, linear magnetic anomalies present over eastern Mendip. Magnet ic anomalies within the Radstock Coalfield to the north, however, are related to volcanic horizons in the Carboniferous Limestone. A three-d imensional model shows the presence of Silurian rocks beneath the spin e of the eastern Mendip Hills. Seismic reflection sections are used to control two-dimensional interpretations of potential field data from Beacon Hill pericline. Although the magnetic anomalies are adequately explained by the seismic model, the gravity anomalies require the pres ence of an additional Low density body beneath the pericline's souther n flank. This body is interpreted as a thrust wedge of Old Red Sandsto ne emplaced in its present position by a previously unrecognized post- Variscan normal fault. The considerable lateral extent of the structur e is demonstrated by the trend of the maxima of the horizontal gradien t of the gravity anomalies.