Low-temperature deformation of limestone, Isola Palmaria, northern Apennine, Italy – The role of primary textures, precursory veins and intracrystalline deformation in localization

Abstract

Fine-grained limestones of the Tuscan nappe from Isola Palmaria (northern Apennine, Italy) contain sporadically developed, bedding-parallel shear zones, some of which exhibit well-developed calcite mylonites. The presence of shear zones that formed at temperatures less than 250 °C allows the study of limestone deformation mechanisms active under low-temperature, upper-crustal conditions. Two materially distinct types of shear zones developed during the syn-nappe stage of deformation and have been characterized by integrated field, strain, microstructural (optical microscopy, cathodoluminescence, transmission electron microscopy) and EBSD fabric studies. Type A shear zones occur within decametre-thick packages accommodating heterogeneously distributed strain in very fine-grained limestone (micrite). Type B shear zones are localized within calcite veins that develop into recrystallized calc-mylonites. Both types form at the contact with dolomite, although this does not play an important role in strain localization. In both cases, low-temperature ductility occurs by extensive dislocation glide sustained by athermal network-accommodated recovery. Although, the end-state microstructures of both shear zone types cannot be differentiated optically, differences do exist in the crystallographic preferred orientations (CPO) and the grain boundary structure of deformed micrite and vein calcite. Deformed micrite grain boundaries are rich in voids consistent with extensive fluid flux that can enhance material transport and independent grain displacements, while suppressing CPO development. The crystallographically defined grain boundaries in recrystallized vein calcite suggest that lower grain boundary diffusivity enables more typical dislocation-mediated CPO formation.