Testing the importance of sagduction: insights from the Lewisian Gneiss Complex of northwest Scotland

Abstract

Archean cratons often contain rock units that are interpreted as having been juxtaposed from different structural levels. A range of uniformitarian and non-uniformitarian processes have been invoked to explain these occurrences, prominent amongst which is the density-driven ‘sagduction’ of high-density upper-crustal lithologies into the underly ing dominantly felsic mid-crust. In this paper we test the geological importance of sagduction, using a combination of petrology, phase equilibria modelling, and mechanical modelling. We use the Lewisian Gneiss Complex of northwest Scotland as a test case, analysing the range of observed subordinate felsic–ultramafic bodies within the dominantly felsic crust, but our approach is applicable to Archean terranes globally owing to their analogous lithological ranges. We find that for our thermodynamically-estimated densities of the lithologies present in the Lewisian Gneiss Com plex, unrealistically hot temperatures are required for sagduction to be important, given the observed body sizes and available constraints on event durations, geotherms, crustal rheology and emplacement depths. These results suggest sagduction is not responsible for emplacement of the observed subordinate lithologies within the Lewisian felsic mid-crust, and instead support uniformitarian tectonic interpretations. Additionally, our results cast doubt on the importance of sagduction in the structural evolution of granite-greenstone belts. Overall, our study indicates that sagduction was not an important Archean tectonic process.