We describe a previously unrecognized major dextral strike-slip fault system in the South Carpathians, hereafter referred to as the Transcarpathian fault system. The master fault has been active since the mid-Cretaceous and has a total offset of ∼150 km, of which only !35 km are post-Oligocene. The fault acted as a subduction-transform edge propagator (STEP) fault during the mid-Cretaceous subduction of the Ceahlău-Severin ocean system and separated an area to the north where the subduction system was accretionary (the East Carpathians) from an area to the south (the western half of the South Carpathians) where the subduction system was erosive. In the South Carpathians, the oceanic basin closed during the mid-Cretaceous after commencement of higher convergence rates and subduction erosion of the trench, leading to tectonic underplating and continental collision between the Dacia and Moesian microplates. The results bolster the idea that STEP-type strike-slip faults are critical in the development of highly curved orogens and that accretionary versus erosional trench segments lead to very different structural configurations along the same subduction/collisional system.