Micromechanics of brittle faulting and cataclastic flow in Tavel limestone

Abstract

Previous studies reveal that while compact carbonate rocks display exclusively dilatancy under compressive deformation, compaction may be observed in their more porous counterparts. In their compactive behavior the porous carbonate rocks are more akin to e.g., sandstone. Whereas the micromechanics of brittle faulting and cataclastic flow in sandstone has been studied extensively, little is known about these processes in a porous limestone. To investigate both failure modes we deformed samples of Tavel limestone with porosity 10–14% to various stages of deformation in conventional triaxial configuration at confining pressures corresponding to brittle faulting and cataclastic flow and described the microstructures associated with the damage evolution using optical and electron microscopy. In this porous micritic limestone cataclasis is the dominant mechanism of deformation. The microcracks initiate as pore-emanated and while all pores contribute to microcrack initiation, it is the large pores that drive crack propagation and coalescence leading to failure. In brittle faulting, dilatancy arises from microcracks growing parallel to maximum principal stress, with their coalescence leading to shear localization. In cataclastic flow microcracks do not have preferred orientation. Interplay between pore collapse and formation of new microcracks determines the compactive versus dilatant character of the cataclastic flow.