Generation of large transient porosity at the brittle to semi-ductile transition

• High porosity develops during semi-brittle and semi-ductile deformation of Carrara marble. • Semi-brittle deformation is marked by localized shear zone formation and distributed microcracking. • Semi-ductile deformation involves diffuse microcracking without shear localization. Fluid circulation within Earth’s crust is contingent on the presence of connected porosity. Yet, at depths near the brittle to ductile transition regimes, where the base of the seismogenic zone is located, high pressure compacts the rock, reducing porosity and inhibiting fluid circulation. At greater depths, plastic mechanisms during ductile deformation further reduce the likelihood of porosity generation. In this study, we experimentally simulated the brittle to semi-ductile transition in Carrara marble by performing triaxial deformation experiments coupled with time-lapse 3D synchrotron X-ray microtomography at five confining pressures, ranging from 5 MPa to 70 MPa at room temperature and tracked the evolution of porosity over time. Contrary to the prevailing belief, our results show that porosity increases with increasing axial stress at stresses corresponding to the brittle to semi-ductile transition, where fluid circulation is thought to be impeded. In both the semi-brittle and semi-ductile regimes, the total porosity (averaged over the entire sample) exhibits similar values, close to 18%, increasing from an initial porosity of approximately 1% at equivalent axial strain. Although the porosity is comparable, the dominant deformation mechanisms differ: localized shear zones and distributed microcracking characterize the semi-brittle regime, whereas distributed microcracking, without localization, becomes more prominent in the semi-ductile regime. Our findings reveal the formation of high porosity within the semi-brittle and semi-ductile regimes and offer new insight into how this porosity, interpreted as transient within the crust, may enable fluid circulation at the base of the seismogenic zone in Earth’s crust.