Assessing the tectonic significance of a large-scale transcurrent shear zone system: the Pernambuco lineament, northeastern Brazil

Abstract

The Pernambuco lineament (Borborema province, northeastern Brazil) is traditionally considered a continuous, 700 km-long dextral, E–W strike-slip shear zone. It is interpreted as a fundamental tectonic element of the Brasiliano/Pan-African orogeny, and proposed to continue into West Africa as the Sanaga or Amadoua faults. Recent structural work reveals that the Pernambuco lineament is segmented into two branches separated by more than 100 km. This study shows that the eastern branch, the East Pernambuco Shear Zone system, consists of two high-temperature (amphibolite facies) and several low-temperature (greenschist facies) mylonitic belts, which are laterally and transversely discontinuous. The main protoliths of the mylonites are magmatic rocks; country rocks proximal to the shear zones are little affected by strike-slip shearing, either at high- or low -T conditions. The high -T mylonitic belts are located at the southern border of a major granitoid batholith, and the low -T belts form a series of overlapping right-stepping shear zones to the south and east of them. These observations strongly indicate that the Pernambuco lineament is not a transcontinental structure, that it was not responsible for significant displacements or for emplacement of large magmatic bodies, and, consequently, that its tectonic role during the Brasiliano event was secondary.