Large-scale rheological and tectonic effects of localized garnet breakdown: a case from the Pannonian Basin

The accurate representation of continental lower crust rheology is critical for modelling plate tectonic processes. However, limited observations and highly heterogenous composition make it difficult to describe its large-scale behaviour. Here we aim to link local heterogeneities to large-scale behaviour through a case study from the Pannonian Basin. Available samples of the lower crust are dominantly strong and dry, garnet-rich mafic granulites. In contrast, inferences from large-scale tectonics suggest a weak rheology for the lower crust. We combine observations and numerical models to show that networks of weak zones related to fluid percolation and garnet breakdown surround the strong domains. Strain localization in the weak zones significantly decreases overall long-term stress magnitudes. Consequently, lower crust domains that mainly consist of strong lithologies may still behave as weak layers in the lithosphere due to reaction-induced long-term weakening. Strength increases significantly when weak zones are scarce or discontinuous.