Brittle to semibrittle transition in quartz sandstone: Energetics and crack interaction

Using quantitative microscopy, we estimated the energy partitioning of different deformation mechanisms in the brittle faulting and semibrittle faulting regimes in quartz sandstone deformed at Pe to 175 MPa and T to 900°C. (1) Our results show that the energy input is fully accounted by a sum of fracture surface energy (US) and frictional energy along shear bands at all PT tested. This indicates that, for both regimes, the deformation away from the macroscopic fault is accommodated primarily by grain-scale brittle mechanisms with little contribution from dislocation mechanisms, supporting the findings from previous studies. (2) Our analysis shows that US is much greater in the semibrittle regime than estimated in the brittle regime; however, the relative importance of different mechanisms (intragranular tensile fracture, intergranular shear band, and grain crushing) remains similar between the two regimes. This relationship, together with the temperature dependence of yield strength observed, indicates that the increase in US with increasing PT results from the operation of subcritical microcracking over a longer experiment time (strain) in these mechanisms. These results suggest that the growth of brittle damage (Us) is influenced by the timescale over which thermally-activated cataclastic mechanisms occur. (3) Our results suggest that shear band is the primary process governing the shape of stress-strain curves through frictionally dissipating 95% of plastic energy. Our findings illuminate an important role of shear bands on the constitutive behavior of granular rocks, in addition to influencing fluid transport properties. On the basis of mapping microfractures over a range of length scales and comparing with models of crack interaction, we determined a critical geometry (the ratio of fracture spacing to length) for crack interaction leading to shear localization. Our results suggest that the interaction of mm-scale shear fractures is responsible for the formation of the macroscopic fault in both the brittle and semibrittle faulting regimes. The findings from our granular rock samples are distinct from the models of fault formation by tensile crack interaction proposed for brittle faulting in non-porous rocks.