Shear zone growth by repeated generation of pseudotachylytes: Implications for the seismogenic potential of the lower crust

Networks of coeval pseudotachylytes and mylonites exposed in the lower-crustal section of Nusfjord (Lofoten, Norway) provide a natural laboratory to investigate the cyclic interaction between seismic and aseismic deformation in dry continental lower crust. We combine field observations, structural mapping, photogrammetry, statistical analysis of shear zone geometry, and microstructural observations to investigate the relationships between pseudotachylytes, damaged host rock, and ductile shear zones. The studied structures consist of anastomosing mylonitic shear zones surrounding variably damaged anorthosite blocks that preserve multiple generations of pristine pseudotachylytes. There is an inverse relationship between shear zone thickness and the size of intervening host-rock domains, indicating progressive widening and interaction of adjacent shear zones during network evolution. Field and microstructural relationships suggest that pseudotachylytes nucleated preferentially within fractured anorthosite blocks and propagated toward adjacent shear zones, where they were deflected along the shear zones. Subsequent deformation incorporated and mylonitized the pseudotachylytes and their associated damage zones into the shear zones. This process progressively widened the shear zones while reducing the size of competent anorthosite blocks. With increasing strain and shear zone widening, the capacity of the lower crust to host seismicity decreases. The Nusfjord network records how cyclic brittle-viscous deformation modifies lower-crustal rheology, impacting the distribution and size of continental seismogenic domains through time and space.