Abstract The NE–SW-trending Grampian Fold Belt has anomalous features that set it apart from other belts in the Caledonides. It consists mainly of polyphase-deformed (D1–D4) Neoproterozoic–early Ordovician rocks (Dalradian Supergroup), deposited in a basin that formed behind a microcontinent detached from Laurentia during Iapetus opening. Detailed cross-strike structural traverses through the D1/D2 Tay Nappe in the SW Highlands show that part of the Ligurian-type basin floor (Highland Border Complex) was obducted on to the Laurentian margin prior to orogenesis. On the top limb of the nappe, δ-porphyroclasts give a top-to-the-SE shear sense for the D1 stretching lineation, whereas D2 microlithons within a zone of non-coaxial strain on the lower limb indicate top-to-the-east. Subduction was initially orthogonal to the Belt but became oblique during D2. Synchroneity at c. 470 Ma of deformation, regional metamorphism, magmatism, termination of sedimentation and formation of the Buchan Block was probably linked to subduction of the spreading ridge. The driving force was NW-directed Andean-type subduction of part of the basin floor, and later of the microcontinental margin. These events caused progressive, diachronous non-coaxial D1–D3 deformation of the hanging wall (Dalradian). Crustal thickening on the SE margin was balanced by thrusting on to the foreland.

Abstract The Dalradian Supergroup, comprising the Grampian (oldest), Appin, Argyll and Southern Highland (youngest) groups (Fig. 13) consists of lithologically diverse metasediments and typically mafic metavolcanic rocks. The supergroup is probably entirely late Precambrian in age and underlies an area of about 48000 km 2 in north and west Ireland, Scotland and Shetland. Neither the base nor the top of the Dalradian is seen but, taking the maximum thickness preserved for every group and making no allowance for tectonic thickening, it is calculated to have a total cumulative thickness of c. 25.5 km.