Laboratory studies of frictional properties of rocks at slip velocities approaching the seismic range (similar to 0.1-1 m s(-1)), and at moderate normal stresses (1-10 MPa), have revealed a complex evolution of the dynamic shear strength, with at least two phases of weakening separated by strengthening at the onset of wholesale melting(1-4). The second post-melting weakening phase is governed by viscous properties of the melt layer and is reasonably well understood(5,6). The initial phase of extreme weakening, however, remains a subject of much debate. Here we show that the initial weakening of gabbro is associated with the formation of hotspots and macroscopic streaks of melt ('melt welts'), which partially unload the rest of the slip interface. Melt welts begin to form when the average rate of frictional heating exceeds 0.1-0.4 MW m(-2), while the average temperature of the shear zone is well below the solidus (250-450 degrees C). Similar heterogeneities in stress and temperature are likely to occur on natural fault surfaces during rapid slip, and to be important for earthquake rupture dynamics.