TRANSTENSIONAL EXHUMATION OF MIDDLE AND LOWER CRUSTAL ROCKS IN A TRANSPRESSIONAL OROGEN: THE KAOKO BELT, DAMARA OROGEN, NAMIBIA

Metamorphic petrology and deformation mechanisms of strike-slip mylonite zones, and Pb-Pb and 40Ar/39Ar thermochronology suggest that middle and lower crustal metamorphic rocks in the Kaoko Belt of the Pan-African Damara Orogen (Namibia) were exhumed via transtensional shearing and not vertical extrusion as previously proposed. Mineral assemblages and deformation mechanisms in the major shear zones of the Western Kaoko Zone record pressures of 4-6 kbars and temperatures of ~550 ?C for the Three Palms mylonite zone, 600-650 ?C for the Purros mylonite zone, and ~630-700 ?C for the Village and Khumib mylonite zones. The Three Palms mylonite zone, which separates the accreted Coastal Terrane from the former passive margin of the Congo Craton exhibits progressive deformation through low greenschist facies and into the brittle field, with consistent oblique normal shear indicators. To a lesser degree the Village and Khumib mylonite zones also show significant overprinting by progressively lower temperature structural fabics into the brittle field. The Purros mylonite zone is not significantly retrograded and was for the most part not active at lower temperatures. Thermochronology indicates rapid cooling after peak 575-550 Ma metamorphism of all of the high-grade structural domains in the Western and the Central Kaoko zones between 535 and 525 Ma. These data suggest that oblique extension resulted in tectonic exhumation of the highest-grade structural domains beneath the overriding Coastal Terrane and an intervening medium grade structural panel – the Khumib Domain. Transtensional collapse of the formally transpressional Kaoko Belt was probably caused by a change in plate motion after the final amalgamation of Gondwana.