Interaction of frictional and plastic deformation mechanisms in a clay-carbonate strike-slip fault (Northern Calcareous Alps, Austria)

Abstract

This study presents microstructural investigations of limestone fault rocks formed in a shallow crustal (∼4 km), compressional segment of the Salzachtal-Ennstal-Mariazell-Puchberg fault in the Northern Calcareous Alps. The investigated fault strand accommodated sinistral slip of several hundreds of meters. Our data show that fault rocks underwent various stages of evolution including the emplacement of fluidized clays at an early frictional stage of faulting overprinted by intense veining and stylolite formation. Increased calcite dissolution along clay-carbonate boundaries is resulting in the development of cleavage domains. Such clay-enriched stylolites and cleavage domains localized further deformation, producing a network of small-scale clay-rich shear zones of up to 1 mm thickness anastomosing around carbonate microlithons, varying from several mm down to some μm in size. Beside pressure-solution processes, calcite microlithons show microstructures typical for crystal-plastic deformation, including deformation twinning, undulose extinction and the development of subgrains. The investigated fault rocks are excellent examples for the interplay of frictional, pressure solution and crystal plastic deformation processes under low-temperature conditions.