XI.—Regional Gravity Anomalies in the Western Midland Valley of Scotland

Abstract

SynopsisIn the western Midland Valley of Scotland and the neighbouring areas of the south-western Grampians and Southern Uplands, the regional Bouguer anomalies may be resolved into (i) a westerly-rising component similar to that prevailing throughout western Scotland, and (ii) a broad high over the central zone of the graben with marked decreases towards the bordering mountains.When allowance is made for the contribution to the gravity anomalies of the light Upper Palæozoic rocks within the graben, the adjusted values outline an accentuated high over the graben with a drop of 25–30 mgal from the maximum to the northern limit of the area and with a corresponding drop to the south. In a profile normal to the major geological structure, the shape of the high approximates to a parabola with its apex 5 miles north-west of the Inchgotrick Fault.These regional changes may best be explained by a thickening of the Upper Crustal Layer under the Grampians and Southern Uplands, most probably by the addition of thick Lower Palæozoic and Dalradian geosynclinal deposits. Thicknesses of the order of 17,000 ft., additional to those under the Midland Valley, are required to account for the gravity changes. The relative attenuation of the succession in the Midland Valley area, implies that the area of the graben was already a crustal entity in pre-Upper Palæozoic times.The anomalies accord qualitatively with isostatic compensation of topography. The predominant high positive background values, have, however, no significance peculiar to the graben area.