Mapping in the Karas Mountains area of southern Namibia has shown the presence of a major lithotectonic boundary at the northern margin of the Namaqua-Natal Metamorphic Province. The area is subdivided into the Northern Volcanic Zone largely comprising mafic, intermediate and felsic volcanic and intrusive rocks metamorphosed to greenschist to amphibolite facies, and the Southern Sedimentary Zone, dominated by pelitic rocks, metamorphosed at amphibolite to granulite facies, and intruded by syntectonic granites. The intervening Lord Hill Boundary Zone is characterized by anastomosing mylonite belts with dips and lineations plunging steeply to the south, within less-deformed metalavas and intrusions similar in character to those of the Northern Volcanic Zone. The intrusive and metavolcanic rocks of the Northern Volcanic and Lord Hill Boundary Zones have geological characteristics of present-day volcanic arcs. Texturally well-preserved felsic metavolcanic rocks of the Lord Hill Boundary Zone have been dated by SHRIMP U-Pb isotope analysis of zircons, and gave an age of 1286.0 ± 7.3 Ma. The age and tectonic setting are very similar to those of the Jannelsepan and Copperton Formations in South Africa, and point to an extensive Mesoproterozoic volcanic arc at the southern edge of the Kalahari Craton.