The Highland Boundary Fault and the Highland Border Complex

Abstract

Synopsis Stratigraphic evidence, petrography and way up criteria are presented to show that the bulk of the Highland Border Complex in Scotland gets younger to the NW, towards the largely Neoproterozoic, Dalradian block. An early ophiolite has fed younger sediments with detritus, which, in the absence of reliable fossils, has provided a control on the stratigraphical order, placing the ‘Margie Series’ younger than the dated Dounans limestone. Tanner & Sutherland (2007) see the complex as divided into two groups one of which overlies, and is a continuation of the Dalradian sequence, and therefore took part in its folding. This view is false. The Highland Border Complex is a sequence of rocks, disposed in phacoids, beginning with an ophiolite in the early Ordovician followed by younger rocks which include shales, pillow lavas and sandstones. This sequence was deposited whilst the Dalradian highlands rose and shed large volumes of sediment, which have yet to be identified south of the Highland Boundary Fault. The boundary between the Dalradian and the Highland Border Complex, the Highland Boundary Fault, is therefore a major fracture bringing unrelated (but not exotic) Highland Border Complex rocks adjacent to the Dalradian block. In terms of its structural control, the terrestrial Old Red Sandstone is a continuation of the Highland Border Complex, being progressively less deformed as time went on. It was probably laid down in transcurrent fault-generated basins which were gradually converged upon by the Dalradian block and in so doing, uplifted some of the more northerly basins, possibly resting on Dalradian sediments, to yield sediment to the younger basins further to the south. The Highland Border Complex forms the basement to the Old Red Sandstone and, in places, forms a plane of detachment upon which the overlying basins formed. This may account for the extreme heterogeneous nature of its deformation from extensively sheared to totally unsheared rocks.