Abstract

In Early Palaeozoic time northern Scotland occupied a 120° promontory between the Appalachian and Greenland Iapetus-facing margins of Laurentia. It is proposed that this originated at a triple rift (RRR) jundion between Laurentia and two other continents, possibly Baltica and West Gondwana. The third arm is marked by trans-Caledonoid extensional lineaments that controlled Vendian sedimentation and MORB-type volcanicity in the Grampians. The model accounts for the huge scale of the SE-prograding Moine-Dalradian clastic wedge as compared with Neoproterozoic sequences elsewhere on the E Laurentian margin where an extensional regime persisted throughout the Grenville-Caledonian interval. Evidence currently cited for Precambrian orogeny in the Highlands needs to be reappraised from that viewpoint. Major stretching events are inferred at c. 750 and 600 Ma; anorogenic ‘older granite’ magmatism was associated with the latter, which culminated in the opening of Iapetus.

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