The tectonic significance of the Cabo Frio Tectonic Domain in the SE Brazilian margin: A Paleoproterozoic through Cretaceous saga of a reworked continental margin

The Cabo Frio Tectonic Domain is composed of a Paleoproterozoic basement tectonically interleaved with Neoproterozoic supracrustal rocks (Buzios-Palmital successions). It is in contact with the Neoproterozoic-Cambrian Ribeira Orogen along the SE Brazilian coast. The basement was part of at least three continental margins: (a) 1.97 Ga; (b) 0.59 - 0.53 Ga; (c) 0.14 Ga to today. It consists of continental magmatic arc rocks of 1.99 to 1.94 Ga. Zircon cores show a 2.5 - 2.6 Ga inheritance from the ancient margin of the Congo Craton. During the Ediacaran, this domain was thinned and intruded by tholeiitic mafic dykes during the development of an oceanic basin at ca. 0.59 Ma. After the tectonic inversion, these basin deposits reached high P-T metamorphic conditions, by subduction of the oceanic lithosphere, and were later exhumed as nappes over the basement. The Cabo Frio Tectonic Domain collided with the arc domain of the Ribeira Orogen at ca. 0.54 Ga. It is not an exotic block, but the eastern transition between this orogen and the Congo Craton. Almost 400 m.y. later, the South Atlantic rift zone followed roughly this suture, not coincidently. It shows how the Cabo Frio Tectonic Domain was reactivated as a continental margin in successive extensional and convergent events through geological time

Flame foliation: Evidence for a schistosity formed normal to the extension direction

Foliations are normally thought to develop approximately parallel to the XY-plane of the finite strain ellipsoid, i.e., perpendicular to the main shortening direction. We present a new type of schistosity named "flame foliation" that forms orthogonal to the main extension direction, approximately parallel to the YZ-plane of finite strain. Flame foliation consists of anastomosing biotite-rich selvedges overprinting S1 in pelitic layers of metaturbitites in NW Namibia. The biotite crystals in the selvedges are peculiar because they are oriented oblique or orthogonal to the flame foliation itself and parallel to the previous S1 cleavage, a feature no other foliation shows. In some cases, biotite flames flank minor quartz veinlets, implying that the flame foliation developed by infiltration of fluids and biotite growth along extensional fractures perpendicular to the main D1 extension direction. Where overprinting ductile D2 deformation is strong, the flame foliation is transformed into a normal crenulation cleavage by rotation of biotite in the flames. Flame foliation is found preferentially close to syntectonic syenite granite plutons and their formation in metapelitic rocks may be enhanced by fluid overpressure due to devolatilization reactions