Very low temperature, natural deformation of fine grained limestone: a case study from the Lucania region, southern Apennines, Italy

The deformation behavior of fine grained limestones from the Monte Sirino a rea (Lucania region) of the southern Apennines has been analysed by constra ining microstructural observations and crystallographic fabrics with data o n the metamorphic conditions of deformation. X-ray and infrared analysis of clay minerals, together with illite 'crystallinity' data, suggest that the studied rocks underwent very low grade metamorphism in the deep diagenetic zone. The limestones consist of very fine grained (< 10 mum) aggregates of micrite. Elliptically-shaped radiolarians, preserved as moulds with coarse r (> 20 mum) crystalline fillings, provide common strain markers. Optical m icrostructures and strain analysis indicate heterogeneous intracrystalline strain in the coarser (> 50 mum) calcite. On the other hand, SEM and TEM ob servations, and crystallographic fabrics determined by X-ray texture goniom etry, indicate a deformation involving not only intracrystalline slip, but also an important component of grain boundary sliding in the fine grained m atrix. The inferred microscopic deformation mechanisms are compared with co nstitutive flow laws derived from experimental studies. For the maximum inf erred temperature of deformation of 250 degreesC and geologic strain rates of 10(-13)-10(-15) s(-1), deformation mechanism maps for calcite suggest tw inning and other glide mechanisms to be active in grains larger than about 5-10 mum . Smaller grains would be mostly deformed by grain size sensitive creep mechanisms, which include both diffusion mass transfer processes and grain boundary sliding. Deformation features observed in the study limeston es are compatible with the prediction of such temperature-dependent mechani sm maps. (C) 2001 Editions scientifiques et medicales Elsevier SAS.