The role of the Cruachan Lineament during Dalradian evolution

Abstract

Synopsis Geological and geophysical evidence is reviewed for the existence of a trans-Caledonoid (NW–SE) lineament, the Cruachan Lineament, in the Scottish Caledonides, separating a high-density SW Highland block dominated by abundant late Precambrian Dalradian mafic rocks in which the Moine is absent from a low density Moine plus Dalradian Central Highland block poor in mafic rocks. Contrasted metamorphic P-T-time histories are predicted on opposite sides of the lineament. An alternative model of early Dalradian evolution is proposed in which basaltic volcanism and sedimentation in the SW Scottish Highlands occurred in a quasi-oceanic trans-Caledonoid pull-apart basin bounded along its NE side by a major fault now identified as the Cruachan Lineament. The Dalradian basin(s) may have experienced right-lateral translation relative to the crustal block to the SE along a dextral transcurrent fault zone possibly in the vicinity of the Highland Boundary Fault during late Precambrian time. In this model, the Dalradian terrain may provide no evidence regarding the early history of the NW side of the lapetus Ocean.