Stress variability in the Parnaíba Basin, Brazil, during Cretaceous rifting

Abstract

The Cretaceous section of the Parnaíba Basin, designated as Grajaú Basin, represents an intracontinental half-graben formed during the Early Cretaceous due to the separation between the South American and African continents during the final dispersal of Western Gondwana. Here, through a synergetic approach between fluid inclusion planes (FIPs), outcropping geologic structures, borehole breakouts and remote sensing data, we elucidate in different scales the main structural features and their kinematic indicators. Normal faults strike mainly NNE–SSW and WNW–ESE, while deformation bands and extension joints trend to NW–SE and NE–SW, and FIPs to NE–SW and WNW–ESE. In addition, normal fault-generated scarps border geomorphological units and constitute dense zones of deformation bands and fluidization. Microthermometric FIP analyses suggest this fracturing event occurred at shallow basin levels, at temperatures below 50°C. Furthermore, joints, bands and FIPs present mutually cross-cutting relationships, thus indicating contemporaneity. The numerical inversions applied to striated faults, non-striated faults, joints, deformation bands and FIPs suggest the occurrence of an extensional event characterized by variable direction of extension (σ3) trending from NW–SE to WNW–ESE or NE–SE to NNE-SSW. This event probably started in the Albian during the final Gondwana fragmentation stages. The quasi-perpendicular σ3 trend could be caused by one or all of the following phenomena: a) stress ratio R values obtained (<0.4) indicate the possibility of radial extension (σ2≈σ3), perhaps in response to the deformation partitioning due to kinematic simple-shearing constraints along the Equatorial Atlantic transform margin; b) along the WNW-oriented release normal faults, the local extension (NNE–SSW) switch positions in relation to the regional extension (WNW–ESE) by rotating 90°; c) influence of the pre-existing structures on the regional stress field. Thus, the spatial and temporal relationships between Cretaceous fault activity, stress field and the development of the geomorphological features in the Grajaú Basin contribute to understanding of the Brazilian Equatorial margin geodynamics.