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.