Slow earthquakes, including low-frequency earthquakes and tremor, occur in the brittle-ductile (plastic) transition zone on plate boundary faults. To understand how transitions in deformation mechanisms can influence seismic behavior, we conducted stick-slip experiments on halite gouge at normal stresses (σn) of 8 to 113 MPa. In the brittle regime (σn ≤ 18 MPa), the halite gouge showed fast and regular stick-slip associated with velocity-weakening behavior. In contrast, increasing the normal stress within the semibrittle regime (σn ≥ 27 MPa) led to a significant decrease in slip velocity and stress drop, which was associated with a transition from velocity-weakening to velocity-strengthening behavior. Local stress measurements along the simulated faults, made using strain gauges, revealed that the critical nucleation length (Lc) increased with increasing normal stress. Macrostructural and microstructural observations showed that the transition from fast dynamic slip to slow quasi-static slip could be attributed to an increasing contribution from semibrittle/plastic deformation occurring within domains that are heterogeneously distributed within the gouge layer. Our findings suggest that brittle-plastic fault heterogeneity, which may be generated by spatial variations in pore fluid pressure and/or lithology, contributes to the emergence of slow earthquakes.