Rocks of the 3570--3080 Ma Barberton greenstone belt record a geological history of at least 490 million years (m.y.), and were affected by regional thermal perturbations for over 1500 m.y. Small tectonic wedges of 3570--3511 Ma schistose tonalite are the oldest rocks yet recognized, but little is known about their tectonothermal history. The earliest, well-recognized tectonothermal events (3490--3450 Ma) represent mid-ocean ridge-like processes. These were followed by two periods of arc-related and trench-related processes, separated by ~160 m.y.; the first (3445--3416 Ma) recording an intraoceanic suprasubduction-like environment and the second, (3260--3225 Ma) intra-arc and interarc-like processes, culminating in arc amalgamation. Accretion-like convergent processes dominated between ~3230 Ma and 3080 Ma, overlapping with the assembly of the Kaapvaal craton. At 3080 Ma the Barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system. Soon afterward, transtension became distributed across the entire Kaapvaal craton, heralding the onset of formation of the Witwatersrand basin by ~3070 Ma. ¿ American Geophysical Union 1994