The late Mesoproterozoic-early Neoproterozoic tectonostratigraphic evolution of NW Scotland: the Torridonian revisited

The Torridonian succession of NW Scotland comprises three groups, deposited during late Mesoproterozoic to early Neoproterozoic time, the Stoer, Sleat and Torridon. Previous workers have interred that each was formed in a rift basin and that each is internally conformable. New fieldwork and detrital zircon age data indicate that this model is incorrect. Our main findings are as follows: (1) the facies characteristics and detrital zircon data for the Sleat Group indicate that it is genetically unrelated to the Torridon Group; (2) the Sleat and Steer Groups contain features suggestive of deposition in extension-related basins that predate the c. 1.0 Ga Grenville Orogeny; (3) the base of the Applecross-Aultbea succession of the Torridon Group is an unconformity (4) the Applecross-Aultbea succession is most objectively interpreted as a non-marine molasse. The significance of these data is that they can be used as a constraint to test and define tectonic models for the deposition of the Torridonian succession and geological evolution of the Scottish Highlands. The view that the Torridonian rocks record deposition in a suite of long-lived rifts whereas the rest of the consanguineous Laurentian margin experienced collisional and orogenic episodes becomes equivocal and in need of reassessment, if not outright abandonment.