Patterns of Silurian deformation and magmatism during sinistral oblique convergence, northern Scottish Caledonides

Regional ductile thrusting and syn-kinematic granitic magmatism within the Caledonides of northern Scotland occurred within a sinistrally-oblique convergent tectonic setting during the Silurian closure of the Iapetus Ocean. The highest thrust nappes are dominated by structures of probable Grampian (Ordovician) age, and Scandian (Silurian) deformation dominates the underlying thrust nappes. Deformation was overall foreland-propagating but the nappe stack was modified by out-of-sequence thrusting and probable synchronous development of thrusts at different structural levels. Localised dextrally-transpressive deformation is related to an inferred lateral ramp located offshore. New U-Pb (CA-IDTIMS) zircon ages from syn-tectonic granites indicate that the internal Naver Thrust was active between c. 432 Ma and c. 426 Ma. This is consistent with other data sets that indicate that contractional deformation and high-grade metamorphism, and by implication displacements in the Moine Thrust Zone, may have lasted until c. 420-415 Ma. The synchroneity of thrusting and strike-slip movements along the Great Glen Fault implies that partitioning of transpressional strain occurred above a regional basal decollement. The short duration of the Scandian orogen in Scotland (c. 437-415 Ma?) is consistent with only moderate crustal thickening and a location on the periphery of the main Laurentia-Baltica collision further north.