Uplift patterns and orogenic evolution in the Scottish Dalradian

Abstract

Rb-Sr and K-Ar mineral ages from the Dalradian rocks of Angus and Perthshire indicate considerable diachronism of cooling, following the Caledonian regional metamorphism. The oldest mineral ages (~515 Ma) are similar to the independent estimates of peak metamorphic age, and hence indicate very rapid cooling; they occur in the highest grade samples of the Angus region and are associated with localized uplift. Regional rapid uplift events at 460–440 Ma and 410–390 Ma may also be recognized and correlated with structural and igneous activity and metamorphic age dates within the Scottish Highlands. Thermal models show the importance of localized and discontinuous uplift in determining both the timing of metamorphism and the shape of the P–T paths. They suggest that the major diachronous metamorphism (and granite production) in the Scottish Highlands is depth controlled rather than representing major lateral diachronism of thickening or heat sources. Localized uplift may exercise considerable control on mineral facies variation within orogenic belts. The local uplift in Angus introduces hot, high-pressure Tarfside Nappe rocks into higher level Tay Nappe rocks. Thus, the uniform block uplift models frequently used in the interpretation of metamorphic and structural phenomena in orogenic belts are an oversimplification of the real events.