The cyclic interplay of seismic and aseismic deformation in the lower continental crust

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Deformation and metamorphism of the lower continental crust causes significant changes of its physical and mechanical properties that have an important impact on the geodynamic evolution of mountain belts and the earthquake cycle. Classical crustal strength considerations suggest that the lower crust should flow rather than break. Yet, some examples of exhumed lower crustal rocks feature large volumes of pseudotachylytes, which are evidence of paleo-seismicity within the rock record. Additionally, geophysical studies show that earthquakes occur in the crust well below the seismogenic zone. The proposed project targets this contradiction with a crustal scale assessment of brittle and ductile deformation. While previous studies have demonstrated convincingly that locally brittle failure and deformation by viscous creep can be coeval, a crustal scale assessment of this interplay is still missing. It is imperative to thoroughly evaluate the tectonic and kinematic setting of exposed analogues to understand the mechanisms that cause present-day seismicity. To address these fundamental questions the proposed project will target an ideal exposed analogue located in the Lofoten archipelago (Norway). By carefully evaluating the preserved structural relationships and kinematic reference frame it will be possible to delineate a succession of deformation within the lower continental crust. Further, the timing of deformation will be constrained using Lu-Hf and U-Pb dating of garnets and apatite from pseudotachylytes and ductile shear zones. These results in combination with the detailed structural constraints will show whether cyclical switches between brittle (seismic) and ductile (aseismic) deformation indeed occur on a crustal scale, or if they are successive with brittle deformation paving the way for infiltrating fluids and the initiation of deformation by viscous mechanisms. Finally, these results will be combined with the determination of H2O in nominally anhydrous minerals, which will allow to evaluate the magnitude of hydration and the spatial distribution of varying fluid contents. Taken together the results of the proposed project will significantly advance our understanding of crustal scale rheology and how the deformation of the lower crust is linked to the earthquake cycle.