Thrust-related dome structures in the Karibib district and the origin of orthogonal fabric domains in the south Central Zone of the Pan-African Damara belt, Namibia

Abstract

The high-grade metamorphic south Central Zone (sCZ) exposes an oblique crustal section through the internal parts of the Pan-African Damara belt in Namibia. The structural pattern of the south Central Zone is characterized by kilometer-scale, northeast-trending dome structures for which a number of different origins have been proposed. Detailed structural mapping of the Karibib and Usakos domes in the Karibib district indicates an origin of the two domes as large tip-line folds located above blind thrusts. The two domes are overridden from the southeast by a crystalline thrust sheet along the Mon Repos thrust zone and the thrusts and associated folds form part of a deeply-eroded, foreland- (northwest-) vergent fold-and-thrust belt. Intrusive relationships of syn- and post-tectonic granitoids constrain the timing of thrusting to between 550 and 540 Ma, corresponding to the main phase of collisional tectonics in the Damara belt. On a regional scale, the south Central Zone is underlain by two orthogonal fabric domains. Northwest-vergent tectonic transport, documented in this study, dominates the lower amphibolite-facies northeastern parts of the south Central Zone. In contrast, a southwest-directed orogen-parallel extrusion is widely documented for the lower structural levels of the upper-amphibolite- to granulite-facies southwestern parts of the south Central Zone that contain voluminous synkinematic granites and widespread migmatization. Fabric development in the two orthogonal domains can be shown to be contemporaneous and related to the main collision between the Congo and Kalahari cratons at ca. 540–550 Ma. We suggest that the formation of the two orthogonal fabric domains represents the different crustal response of the rheologically layered crust to the late-Proterozoic crustal shortening. At shallower crustal levels, shortening of sufficiently strong crust in the Karibib district resulted in vertical thickening and orogen-normal folding and thrust transport. Shortening of the deeper levels of the sCZ resulted in pure shear deformation of the rheologically weakened and partially molten crust and the lateral, orogen-parallel displacement of rocks.