AbstractWe present the results of hydromechanical changes across and along the evolving rupture surface of carbonate rocks during direct shear experiments. Direct shear experiments were performed on a laminated travertine of continental, microbial origin with calcite content of 99 wt%, chosen as a lithological analogue for Aptian presalt oil reservoir rocks found in South Atlantic presalt basins. Medical X‐ray CT images show that the porosity (~9–13%) is mainly composed of subplanar pores and vugs. Permeability is high along the laminations (~50–200 mD), controlled by interconnected pores and fractures, and extremely low across the laminations (≪1 mD). Six intact samples of travertine were sheared across the bedding direction to short displacement (20 mm), and a further three samples were sheared to a much longer displacement (120 mm). A constant effective vertical stresses of either 35, 40 or 45 MPa was applied throughout the tests. Fluid flow response across and along the fault zone was monitored continuously during both shear deformation (dynamic transmissibility) and hold periods (static transmissibility), while keeping a constant pore pressure throughout the measurement. While the samples show some microstructural variability, the set of samples sheared to 20‐mm displacement followed the same early kinematics and show very similar mechanical response to that seen in the same period of the samples where shear was continued on to 120‐mm displacement. After 20‐mm displacement, the microstructures are dominated by fractures in a zone 10–20 mm wide resembling a typical fault damage zone, with only thin and patchy development of gouge on the principal displacement plane. In all of the samples sheared to a high displacement of 120 mm, a continuous layer of gouge (6.2–13 mm thick) was formed, defining a distinct core of the shear zone. The dynamic transmissibility across the fault decreases progressively in all the sheared samples regardless of the applied effective stress. In general, the volume decreases as well, though at a tapering rate, throughout shear. A small vertical dilation occurred near the peak strength for travertine sheared under 35 MPa vertical stress, but this was not accompanied by an increase in transmissibility. From the results, we conclude that once the gouge material is developed in the core of the fault zone, the dynamic transmissibility across the fault is permanently decreased.