GEOPHYSICAL MODELLING OF MAGNETIC ANOMALIES OVER THE BALLANTRAE OPHIOLITE COMPLEX AND SOUTHERN UPLANDS FAULT SYSTEM, SW SCOTLAND

Abstract

The Ballantrae Complex is located in the area north of the Southern Uplands Fault and Southern Uplands accretionary complex of southwest Scotland. It consists of a tectonically emplaced ophiolite dominated by two serpentinised peridotite massifs known as the Southern and Northern Serpentinite Belts. It also contains smaller serpentinite bodies, including a concealed body proved in a BGS borehole at Knocklaugh Lodge. The highly magnetic serpentinites are associated with pronounced magnetic anomalies that form the focus of this study. The project aims to improve understanding of the shape, size and distribution of serpentinites in the subsurface, by integrating detailed ground-level magnetic surveying, analysis of the bulk magnetic properties of rocks present in the area, Fourier-based upward continuation, and 2D and 3D magnetic modelling techniques. 3D modelling of the Knocklaugh Lodge anomaly identifies a NE-SW-trending, wedge-shaped, fault-bounded sliver of serpentinite beneath surficial boulder clay. A 2.8 km long ground-level magnetic profile extending to the NW from this locality to beyond the NW margin of the Northern Serpentinite Belt was upward continued to 100 m elevation to suppress short wavelength near-surface signals. 2D modelling of these data reveals presence of a large, concealed serpentinite body to the SE of the Northern Serpentinite Belt, and suggests that the boundaries of the major serpentinite units are all steeply dipping. 3D modelling at the regional-scale using data from the National Aeromagnetic Survey further constrains the complex 3D subsurface geometry of the Northern and Southern Serpentinite Belts. It also suggests that a separate linear magnetic anomaly extending ~25 km further to the NE along the strike of the major fault systems in the region is caused by highly magnetic sedimentary rocks of the Ordovician Traboyack Formation. Finally, upward continuation of a ground-level magnetic survey over a raised marine platform near Lendalfoot demonstrates presence of a regional magnetic gradient associated with a deep peridotite source combined with a complex pattern of small-scale, local gradients due to shallow sources that probably reflects variations in the paleotopography of a rocky shoreline beneath the terrace.