A gravity survey of a region of the Highland Boundary Fault in Scotland

Abstract

The results of 1591 gravity measurements made in parts of the Midland Valley and the Grampian Highlands of Scotland, flanking the Highland Boundary Fault-zone between Callander and Cowal are described and density units in the Palaeozoic rocks of the area distinguished on the basis of some 800 laboratory measurements. Steep gravity gradients across the Highland Boundary Fault at Callander and Aberfoyle show that its small hade to the north-west, observed at surface, is maintained to a depth of 2400 ft below sea-level, indicating that the fault behaved as a high-angle reversed fault in mid-Devonian times. West of Loch Lomond, the fault continues in general alignment with its course to the east and is inferred to underlie the Carboniferous cover in the Greenock–Inverkip region. The existence of a deep sedimentary basin bounded to the north-west by three step faults and to the south-east by the Highland Boundary Fault is revealed by a gravity low west of Loch Lomond. The basin is probably partly filled by Lower Old Red Sandstone. The maximum thickness of the Lower Old Red Sandstone sediments in the surveyed area is between 5000 and 6000 ft and the formation thins south-eastwards in the region of the Ochil Fault. On the flanks of the Highland Boundary Fault, the survey has provided information of regional importance: the slate-belt probably occupies a downfolded synform within the Dalradian rocks; the Ochil Fault can have only a small magnitude in the surveyed area; the maximum thickness of the Lower Old Red Sandstone sediments in the area is between 5000 and 6000 ft and the formation thins south-eastwards in the region of the Ochil Fault.