Hydrothermal silicification in the intraplate Samambaia seismogenic fault, Brazil: Implications for fault loss of cohesion and healing in rifted crust

This study investigates the Samambaia seismogenic fault at the border of the Potiguar rift Basin, Brazil, to evaluate the relationships among hydrothermal processes, seismicity, and preexisting tectonic fabric. The fault is a 27 km-long structure composed of three left-bend, en echelon segments spaced 1.0–1.5 km apart. The segments strike N31°E – N35°E, dip 70° NW and have focal depths between 1 and 9 km. The seismogenic fault coincides with a swarm of silica-rich veins that range in size from a few centimeters wide and 1–2 m long to 20–50 m wide and hundreds of m long. These structures encompass a V1 vein set composed of quartz veins parallel to the Proterozoic-Archean basement metamorphic foliation and a V2 vein set composed of chalcedony, opal, cryptocrystalline silica, and quartz geodes that cut across Cretaceous siliciclastic and carbonate sedimentary units in the basin and the basement. Fluid inclusions from the V1 vein set yield trapping temperatures of 91 °C–336 °C, whereas the V2 vein set yields temperatures of 147 °C–388 °C. The fluid inclusions and oxygen isotope analyses indicate a magmatic fluid source, although contamination by lateritic soil–derived fluids can be recognized in the V2 vein set. We conclude that in the Samambaia fault, fluid pressure reduces the effective stress until the fault experiences failure, contributing to fault weakening and seismicity. Subsequent cooling of hydrothermal fluids leads to fault healing after coseismic slip and precipitation of Si-rich cements. This process was associated with variation from hydrostatic to lithostatic pressure during cementation, reducing permeability along the fault. Then, the fault locks and fluid pressures begin to rise again. We interpret the superposed V1 and V2 vein sets as the expression of fault-valve behavior caused by repeated fluid-driven reactivation through time along preexisting structures in an extended, rifted crust.