Shear zones in Brasiliano-Pan-African belts and their role in the amalgamation and break-Up of Southwest Gondwana

Abstract

Crustal-scale shear zones are ubiquitous in most Brasiliano–Pan-African belts of southwestern Gondwana and they resulted from the assembly of the Río de la Plata, Congo and Kalahari cratons. In the Dom Feliciano Belt, the Sierra Ballena-Dorsal do Canguçu-Major Gercino shear zone system and Sarandí del Yí Shear Zone are the most prominent structures, and they share a common history with shear zones of the Kaoko Belt, such as the Purros and Three Palms Mylonite Zones. The Purros Mylonite Zone, in turn, can be traced further south in the Damara Belt, where it is correlated with the Ogden Mylonite Zone. All these orogen-parallel shear zones underwent ductile deformation mostly at c. 630–580 Ma. However, further shearing is recorded in both the Kaoko and Dom Feliciano Belts at c. 550 Ma, thus being coeval with shearing along the Colenso Fault of the Saldania Belt. Though the Brasiliano–Pan-African Orogeny led to a relative stabilization of the South American and African continental crust by the early Paleozoic, shear zones were subsequently reactivated under brittle conditions during the Phanerozoic. These fault zones were particularly active during the opening of the South Atlantic Ocean in the Cretaceous, controlling magmatism emplacement, basin development and crustal exhumation. Shear zones thus played a major role not only during the Neoproterozoic assembly but also during the subsequent break-up of Gondwana.