Micromechanics of the Brittle-Ductile Transition in Indiana and Tavel Limestones

To elucidate the micromechanics of dilatant and compactant failure, Indiana and Tavel limestone samples with porosities of 18% and 10%, respectively were deformed at several different confining pressures and detailed microscopy observations were conducted on the failed samples. The two limestones show similar phenomenology of failure: dilatancy and shear localization developed under low confinement, while strain hardening and shear-enhanced compaction were observed at elevated pressures. Notwithstanding these similarities, significant differences were observed in the micromechanics of failure, which seem to be controlled by geometric complexities of the pore space. The Tavel limestone is a white micritic limestone with relatively large clasts and equant pores embedded in a fine-grained matrix. At a pressure of 30 MPa, stress-induced cracks was observed to emanate from pre- existing pores, and brittle failure developed by the coalescence of such pore-emanated wing cracks preferentially aligned with the maximum compression direction. At a pressure of 150 MPa, stress-induced cracks were also observed to emanate from the pores, and the distributed cracking provided the freedom for broken grains to move and collapse into the pores, leading to shear-enhanced compaction. In contrast, the Indiana limestone is a gray fossiliferous limestone, with allochemical components made up of relatively large ooids, peloids and bioclasts. The matrix between the allochems is a mixture of microcrystalline and sparry calcite, and the porosity derives from relatively large pores between the allochems and micropores embedded within the oolitic coatings. At a pressure of 5 MPa, dilatancy was observed to be induced by grain boundary cracking, relative movement among grains and intragranular cracking at impinging grains. Brittle faulting resulted from the coalescence of such wing cracks. At a pressure of 20 MPa distributed cracking developed by similar mechanisms led to pore collapse and shear-enhanced compaction. In both limestones mechanical twinning was observed to accompany these cataclastic processes. The extent and intensity of twinning increased with increasing pressure and strain, and its coupling to microcracking can be very complicated, especially in the Indiana limestone with the more complex pore structure. Such mechanical coupling and pore space complexity should be accounted for in a micromechanical model of the brittle-ductile transition in porous carbonate rocks.