Deformation bands in porous carbonate grainstones: Field and laboratory observations

摘要

Recent field-based studies documented deformation bands in porous carbonates; these structures accommodate volumetric and/or shear strain by means of pore collapse, grain rotation and/or sliding. Microstructural observations of natural deformation bands in carbonates showed that, at advanced stages of deformation, pressure solution helps to reduce the grain size, enhancing comminuted flow and forming narrow cataclastic zones within the bands. In contrast, laboratory studies on the mechanics of deformation bands in limestones identified grain crushing, pore collapse and mechanical twinning as the micromechanisms leading to strain localization. Here, we present a multidisciplinary field and laboratory study performed on a Cretaceous carbonate grainstone to investigate the microprocesses associated to deformation banding in this rock. A quantitative microstructural analysis, carried out on natural deformation bands aimed at defining the spatial distribution of pressure solutions, was accompanied by a force chain orientation study. Two sets of triaxial experiments were performed under wet conditions on selected host rock samples. The deformed samples often displayed a shear-enhanced compaction behavior and strain hardening, associated with various patterns of strain localization. We constrained the pressure conditions at which natural deformation bands developed by reproducing in laboratory both low and high angle to the major principal stress axis deformation bands. The comparison among natural and laboratory-formed structures, allowed us to gain new insights into the role, and the relative predominance, of different microprocesses (i.e. microcracking, twinning and pressure solution) in nature and laboratory.