Post-Devonian transpressional reactivation of a Proterozoic ductile shear zone in Ceará, NE Brazil

Abstract

Reactivation of Proterozoic ductile shear zones is often difficult to document because of the lack of conclusive evidence. However, the reactivation of a Proterozoic age ductile shear zone along the northeastern extremity of the Transbrazilian Lineament in northwest Ceará can be well documented by the intense deformation of an early Devonian sandstone body, about 30 km long and 4 km wide. This lineament, which extends across Brazil for more than 2700 km, is well exposed in Ceará as a dextral ductile shear zone with late Proterozoic age subvertical mylonites striking approximately 040°. Post-Devonian reactivation of the shear zone, now by dextral transpression, gave rise to brittle faults that limit the sandstone body. Near the faults, the sandstone beds are steep to subvertical, passing into gently dipping or subhorizontal away from the shear zone. Conjugate Riedel shear fractures within the sandstone body suggest a compressive stress field in which the maximum principal compressive stress (σ1) trends about 100°, with the intermediate principal stress (σ2) subvertical. The age of the post-Devonian reactivation in Ceará is uncertain, but structures in mid-Cretaceous sediments of the Parnaiba Basin, and Aptian to early Cenomanian sediments of the offshore Ceará Basin, both situated along the Transbrasilian Lineament, suggest a later mid-Cretaceous age.