Evidence for the Lower Palaeozoic age of the Tay Nappe: the timing and nature of Grampian events in the Scottish Highland sector of the Laurentian Margin

Abstract

Stratigraphical and structural evidence for latest Proterozoic–Phanerozoic deposition of the Dalradian Supergroup in the Scottish Highlands sector of the Laurentian margin is incompatible with geochronological data suggesting c. 750 Ma (Knoydartian) and c. 600 Ma (Grampian) orogenesis. The age of the 590 ± 2 Ma Ben Vuirich Granite is critical to the dating of Grampian orogenesis. Fabric evidence indicates that the granite predates D2 while its relationship to D1 is unclear. In this paper the D1–D2 overprint during the evolution of the Tay Nappe, a regional Grampian structure, is examined. Documentation of the distribution, style and orientation of D1 and D2 structures from well exposed sections near the Highland Border has allowed the development of a kinematic model for the D1/D2 deformation. In this model early crustal shortening resulted in D1 upright structures that were subsequently overturned in a D2, SE‐directed shear regime associated with underthrusting of a southern landmass. The model is best explained in terms of a single continental collision event with no evidence of a time gap between D1 and D2. This analysis is consistent with pre‐regional D1 intrusion of the Ben Vuirich Granite and an Early Palaeozoic Grampian Orogeny.