Geometry and emplacement of nappes in the Central Scottish Highlands

Abstract

Summary A deformation history related to polyphase nappe formation is presented for the Central Scottish Highlands as a model for the Dalradian tract. The genesis of a four-fold pattern of tectonic belts recognisable throughout this tract is discussed. The fan structure of the Tummel steep belt is argued to be a deep-seated rotation zone. Movements within this zone spanned the climax of Barrovian metamorphism and were responsible for creating the major belt of inverted rocks—the Loch Tay inversion—which forms the lower limb of the Tay (fold) nappe. The early-formed Tay nappe, nucleated at a relatively high tectonic level, was translated as a result of movements within the deeper rotation zone progressively towards the SE principally by simple shear. The core of the Tay nappe is recognised within the Highland Border steep belt at Aberfoyle, but does not root within the deeper levels of the Central Highlands Dalradian. The Highland Border steep belt at Dunkeld was formed by late orogenic downbending of the previously flat-lying inverted limb of the Tay nappe probably resulting from sub-vertical movements within the basement along the ancestral line of the Highland Boundary fault. A kinematic model explains the nappe geometry by density inversion of the Dalradian succession consequent on the pre-orogenic evolution of the Dalradian (marginal) basin.