Structural relationships between ultramylonite, pseudotachylyte and cataclasite in the East Pernambuco shear zone (Borborema Province, NE Brazil)

Abstract

Fabrics of ultramylonite, pseudotachylyte and cataclasite were studied via microstructural and Electron Backscatter Diffraction (EBSD) analyses in order to investigate their deformation mechanisms within the East Pernambuco shear zone (EPSZ). Pseudotachylyte veins occur at the margins of ultramylonite bands that crosscut the foliation of the Bezerros granodiorite. Cataclasite reworks the host granodiorite and the ultramylonite. The microstructure of all rock types shows brittle-ductile fabrics observed as fractured feldspar porphyroclasts in the ultramylonite, cataclastic damage zones in pseudotachylyte and broken ultramylonite fragments in the cataclasite. Quartz crystallographic fabrics in the ultramylonite and in the central portion of the pseudotachylyte vein are similar and suggest recrystallization via dislocation creep. Crystallographic fabrics of feldspars are mostly random in all rock types, and amphibole shows the activation of the (100)[001] slip system in the ultramylonite and the less common (010)[001] slip system in the pseudotachylyte. Crystallographic fabrics in the cataclasite are mostly random. These observations suggest that pseudotachylyte was developed through seismic ruptures nucleated along discrete planes at the boundaries between the host granodiorite and the ultramylonite. Such stress accumulations may be linked to reactivation of Precambrian structures along the EPSZ that are associated with present-day seismicity recorded in northeast Brazil.