Progressive strain localization in a major strike-slip fault exhumed from midseismogenic depths: Structural observations from the Salzach-Ennstal-Mariazell-Puchberg fault system, Austria

Analysis of a strike-slip fault exhumed from midseismogenic depths reveals that the fault experienced progressive strain localization toward a high-strain fault core. We focus on the Ennstal segment of the 400-km-long Salzach-Ennstal-Mariazell-Puchberg (SEMP) strike-slip fault system in the Eastern Alps, which accommodated ∼60 km of left lateral displacement during Oligo-Miocene time. Macroscopic and microscopic observations reveal a zoned fault featuring a high-strain core at least 10 m wide within a fault zone that is at least 150 m wide. Grain-size distribution analysis shows how the Ennstal segment of the SEMP evolved. Our data reveal a 10-m-wide high-strain fault core (characterized by a power law relationship of grain sizes, D₂ ≈ 2.0) bordered by a 54-m-wide "transition zone" where the largest and smallest grains are characterized by two power law relationships (D₂ ≈ 2.0 and 1.6, respectively). This zone is in turn bordered by a region with grain sizes that show a single low-strain power law relationship of D₂ ≈ 1.6. We interpret these relationships to be the result of concentrated shear overprinting an initial low-strain, power law grain-size distribution before strain localized to the core. This is consistent with the theory that faults mature by smoothing geometrical complexities, forming a highly localized, high-strain fault core. The data do not support the idea that damage forms primarily in response to dynamic stresses during seismic rupture, although they do suggest that this mechanism may operate within tens of meters of the fault once it has developed its zoned structure.