Rheological and tectonic consequences of garnet breakdown in the lower crust: 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 the large-scale behavior of the lower crust. We aim to link local heterogeneities to large-scale behavior through a case study from the Pannonian Basin. Most of the available samples of the lower crust are rheologically very strong, dry, garnet-rich mafic granulites. In contrast, inferences from large-scale tectonics, such as the widespread extension, the formation of detachment systems, or the lack of lower crustal earthquakes, suggest a generally weak rheology for the Pannonian lower crust. Observations show that zones of garnet breakdown related to decompression and fluid percolation surround the intact, strong domains. Based on these observations, we designed visco-elastic numerical simulations to demonstrate that 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.   

Acknowledgements

This study was supported by the MTA FI FluidsByDepth Lendület (Momentum) project, provided by the Hungarian Academy of Sciences (LP2022-2/2022). K.P. was supported by the National Research, Development and Innovation Fund, Hungary (PD143377) and the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.