Evolution Of Damage And Faulting Across The Brittle Ductile Transition

We explore the complex interactions between faults, fault damage and the brittle-ductile transition by the means of rock deformation experiments on Carrara marble. Firstly, we focused on time dependent mechanical closure of microcracks as a source of wave speed recovery around faults. By recording the elastic wave speeds in dry deformed samples at room temperature during hold times at variable confinements, we observed that mechanical crack closure can recover up to 40% of the damage-induced wave speed loss making it one of the most potent wave speed recovery source. Secondly, we studied the interaction between time-dependent creep and crack mechanical closure in dry triaxially loaded samples. We report that time-dependent creep hinders wave speed recovery above a certain threshold strain rate, and coexist with wave speed recovery under this threshold. We interpret this result as a transition between fast brittle and slower plastic creep. Additionally, we report a drop in wave speeds in samples that have undergone a triaxial hold time upon unloading, which we interpret as asperity destruction in reverse-sliding cracks. We then focus on fault reactivation across the brittle-ductile transition. We develop an experimental protocol that allows for the recording of the partitioning of strain between on-fault slip and off-fault ductile deformation in faulted samples. We establish new bounds for the localised-ductile transitional regime where fault slip and ductile deformation coexist and report the existence of an empirical relationship describing the partitioning of strain when both are active. Finally, we test our newly established model against smooth unstable faults by experimenting on saw-cut samples. We report that the localised-ductile transition is still occurring within our established bounds and that it does not appear to impact fault stability.