Evolution of mantle peridotite rocks - structures generated in a transition from the ductile-brittle regime in the Equatorial Atlantic

The exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile-brittle deformations until the complete emplacement in the oceanic lithosphere. São Pedro and São Paulo Archipelago, is located at 1° N latitude, 1000km from the Brazilian mainlad. Ten isles compose the archipelago with a total exposed area of 17 km². Those isles record the deformational products of ductile, brittle and the rocks/fluid interaction generating specific structures in each domain. The deformational stages are related to the transpressional and transtensional geodynamics of São Paulo Transform Fault (SPTF). The ductile-brittle fabrics were observed in a multiscale context, using drone images, geological mapping, fault analysis, and microstructural studies. Using all these tools to define the tectonic tensions and structures associated with a transition between ductile to the brittle deformational settings. Firstly during the transpressional context, the exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700° - 800°C. Leading to olivine and orthopyroxene recrystallization forming such as well-marked mylonitic foliation and rotated porphyroclast with left-lateral kinematic. The interaction with fluids initially originated from the mantle, generates fragmented crystals of amphibole and oxide-rich levels, marking the transition to semi-brittle deformation. The continuous and rapid uplift led to the superposition of deformation mechanisms, with reactivation of pre-existing structures and predominance of brittle deformation mechanisms. The tectonics associated with an NW-SE shortening in the transpressional tectonics context led to greater availability of hydrothermal fluids. Consequently, the formation of four serpentinization episodes, which are associated with semi-brittle to brittle transition, with temperatures between 300 - 400° C. The presence of serpentine marks the transition between semi-brittle to brittle regimes, whose dextral kinematics is marked by the domino faults, microfaults and gash veins. The kinematics at the brittle moment is compatible with the current movement of the SPTF. Finally, the complete exhumation and establishment of brittle mechanisms led to the carbonatation phase near the surface, with temperatures ranging from 150 - 300°C. The active NW-SE tectonic stress generated an E-W strike-slip faults that filled by carbonates, symbolizing the final exhumation stage.