Tectonic motions give rise to destructive earthquakes and transient slip events. These movements are often described by friction laws for stick–slip motion on brittle fault surfaces and gouge-filled zones1,2. Yet, many transient slip events, such as slow earthquakes and aseismic creep, occur in rocks that exhibit mixed brittle–ductile rheology, where these friction laws are not clearly applicable3,4. Here we describe the flow and evolution of fractures as observed in a semi-brittle rock analogue exposed to shear stress in laboratory experiments. We find that, depending on the strength of the rock-analogue material, and thus the magnitude of yield stress, the material exhibits either creep-like or stick–slip behaviour. At low yield stress, deformation occurs as constant creep along a main fracture, whereas at high yield stress, the material exhibits stick–slip behaviour. However, the deformation does not involve frictional behaviour; it is instead accommodated by the initiation and growth of a system of tensional and shear fractures. The opening and interplay of such fracture systems could generate tectonic tremor and slow slip. Our laboratory experiments thus support a frictionless alternative mechanism for the development of tectonic strain transients.