This study presents unexpected microstructures in carbonate fault rocks that formed in the Salzach-Ennstal-Mariazell-Puchberg fault (Austria), which accommodated lateral extrusion of the Eastern Alps in the Oligocene and Miocene. The investigated eastern part of the sinistral strike-slip fault exposes a shallow crustal, brittle segment. Deformation mechanisms in the carbonate fault rocks involve cataclasis, fracturing, pressure solution–assisted creep, and intracrystalline creep. Besides deformation twinning, intracrystalline deformation of calcite includes dislocation activity present as undulose extinction and development of low-angle grain boundaries. Intracrystalline deformation under these conditions proves the existence of very low-temperature (<150 °C) ductile deformation in calcite. Because intracrystalline deformation in calcite is restricted to centimeter-scale secondary shear zones, and evidence for both internal and external heat sources is lacking, we speculate that intracrystalline deformation typical of higher metamorphic shear zones in marbles can operate under temperature conditions <150 °C.