New structures in the Argyllshire Highlands

Abstract

The district described extends from Spean Bridge in the north to Loch Creagan in the south, and from Loch Linnhe in the west to the margins of the Rannoch Moor and Cruachan granites in the east. The main tectonic feature of this region is the occurrence of two recumbent folds, closing towards the south-east, which have an amplitude of more than a dozen miles apiece. The lower limbs of these folds are in part replaced by very important fold-faults or “ slips,” which, from analogy with the North-West Highlands and the Alps, it is natural to regard as thrust planes. If these slips are in reality thrusts, then it follows that the folds, the lower limbs of which they replace, must be recumbent anticlines; and this, of course, would indicate a movement of the first order of magnitude directed <i>from</i> the north-west. Although this deduction seems well founded, it is not quite secure. In fact, examples of minor slips affecting the <i>upper</i> limbs of these same folds have also been detected, showing that, in disturbances involving regional metamorphism, slipping may occur in either limb of a recumbent fold. The point remains, however, that these minor slips do not approach in magnitude the major slips referred to above as the probable correlatives of the well-known thrust planes. The earlier recumbent folds and attendant slip planes are themselves often folded isoclinally during the later stages of the movement, and this has materially added to the complication of the district.