Ductile strain partitioning in micritic limestones, Calabria, Italy: the roles and mechanisms of intracrystalline and intercrystalline deformation

Abstract

The Apennine Pollino–Ciagola limestone unit in northern Calabria is characterized by subgreenschist, heterogeneous ductile strain localized along narrow deformation zones at several stratigraphic levels. Paleogene conglomerates and Jurassic calcareous breccias and ooidal packstones have been analyzed with the aim of characterizing the deformation of limestone as a function of the strain recorded by sedimentary markers. Reference sections parallel to principal finite strain planes were prepared at each locality for the study of specific parameters. Image analysis of polished sections by scanning electron microscopy (SEM) was used to obtain the finite strain of calcite grains by Rf/ϕ, harmonic mean and normalized Fry methods. For the range of grain sizes analyzed (1–10 µm), the ellipticity of calcite grains varies as a function of grain size according to a power-law relationship, from which the size of isometric grains is empirically predicted. The finite strain (ellipticity) determined from single calcite grains shows consistently lower values than the corresponding rock strain. For a fixed grain size, grain ellipticity initially increases with rock strain; however for larger strain, scattered ellipticity values are recorded, probably because of dynamic recrystallization. Comparison of bulk strain with grain strain suggests that intercrystalline deformation involving grain boundary sliding contributes 50%–80% of the total strain, for grain sizes in the range of 2–10 µm, increasing to 90% or more for smaller grain sizes. Microstructures (optical, SEM, transmission electron microscopy) are consistent with dominant grain boundary sliding accommodated by dislocation processes. The weakly deformed samples (Rs <4) exhibit straight and subsidiary curved mechanical twins in large grains (d >10 µm), with well-developed glide dislocation substructures in both coarse and micrite grains. In the moderately to highly deformed samples (Rs >4), large grains show curved, thick, and patchy twins, with the development of undulose extinction and subgrains. Subwalls are formed from dislocation networks and relate to subgrain rotation recrystallization in the coarsest grains. Both large and small grains exhibit complex dislocation substructures comprising dislocation networks indicative of concurrent intercrystalline and intracrystalline deformation, whereby grain boundary sliding is accommodated by dislocation processes. Integration of tectonic constraints, field observations, finite strain data, microstructures, and experimental data is consistent with natural deformation at 250 °C, 15–50 MPa, and bulk shear strain rates on the order of 10–13 s–1 to 10–12 s–1.