THE GEOLOGICAL RECORD OF THE EARTHQUAKE CYCLE IN THE LOWER CRUST: INSIGHTS FROM FIELD STUDIES IN THE MUSGRAVE RANGES (CENTRAL AUSTRALIA) AND IN LOFOTEN (NORWAY)

This contribution discusses the results of field-based geological investigations of exhumed rocks that preserve evidence for deep crustal earthquake sources exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway). These observations establish that deformation of the dry lower continental crust is characterised by a cyclic interplay between viscous creep (mylonitisation) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep which may be already active during the immediate post-seismic deformation along faults initially characterised by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localised stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by the earthquake cycle deformation.