Faulting in the Early Cretaceous Rio do Peixe basin (NE Brazil) and its significance for the opening of the Atlantic

Abstract

The intracratonic Rio do Peixe basin (NE Brazil) developed during the Early Cretaceous. Ductile shear zones in the Precambrian basement became reactivated as brittle master faults, bounding the Brejo das Freiras, Sousa and Pombal sub-basins. Sediments are well-dated as Berriasian to Barremian. Large growth folds are distributed en échelon along the master faults, which are dominantly strike-slip. We have measured the attitudes of 300 minor faults and striations at 11 localities in basin sediments. For 160 faults, we found the sense of slip to be unequivocal. Most common are right-lateral strike-slip faults, striking NNE; then normal faults, striking ENE; then reverse faults, striking ESE; and finally, left-lateral strike-slip faults, striking ENE. We have used a kinematic contouring method to estimate regional strain and the method of Etchecopar to estimate stresses. At eight localities, the principal direction of compression is nearly horizontal and strikes ENE; at three localities it is nearly vertical. At eight localities, the principal direction of extension is nearly horizontal and strikes SSE; at three localities, it is nearly vertical. Thus the regional pattern is one of wrenching, with local deviations towards either transpression or transtension. Stratigraphic constraints on the timing of fault motions are sparse. Master faults were active throughout the entire history of sedimentation. In the Pombal sub-basin, transtensional conditions prevailed. In the Brejo das Freiras sub-basin, transpression replaced transtension some time after the Berriasian. From stress trajectories and other data, we infer that left-lateral wrenching along the E-W Patos shear zone dominated over right-lateral wrenching along smaller faults striking NE-SW. This is confirmed by palaeomagnetic results, indicating small block rotations about vertical axes, counterclockwise in the northwest and clockwise in the southeast. Early Cretaccous wrenching along the reactivated Patos shear zone appears to correlate with that reported for the Benue trough in Africa. Intracontinental deformation was probably associated with a northward-propagating rift zone, prior to opening of the South Atlantic.