Fluid flow and syntectonic veining in an Ediacaran-Cambrian foreland fold–thrust zone, western margin of the São Francisco Craton, Brazil

Abstract

The western margin of the São Francisco Craton, central Brazil presents a 1300 km long foreland fold–thrust belt where Ediacaran-Cambrian (560–520 Ma) metasedimentary rocks from the Bambuí Group were subsequently deformed during post-collisional stages (520–495 Ma) related to Gondwana assembly. This scenario provides an opportunity to quantify fluid flow regimes and fault-related processes that were active in exhumed foreland fold–thrust zones, which were estimated based on structural, microstructural and fluid inclusion studies of syntectonic veins and host rocks. Kaolinite-bearing synkinematic mineral assemblages from metasedimentary rocks, thermodynamic models and grain-scale deformation accommodated by dissolution–precipitation creep and intracrystalline deformation indicate metamorphic and deformational conditions of 250–270°C. Subhorizontal extensional veins formed under subhorizontal shortening and subvertical extension, supporting vein development under a fold–thrust regime that formed regional NW–SE-trending thrust fault zones and megafolds with NW–SE-trending axes. Orientation and growth microstructures indicate that NW–SE-trending subvertical cleavage-parallel veins formed under subhorizontal NE–SW extension, compatible with those inferred to produce mapped kilometre-scale gentle folds with NE–SW-trending traces. Two primary aqueous fluid inclusion assemblages (FIA) are distinguished by salinity variation: 2–21 wt% NaCleq. in subhorizontal veins and 6–0 wt% NaCleq. in cleavage-parallel subvertical veins. Fluid inclusion thermometry and microstructural analysis suggest that veins crystallized between 250 and 270°C under fluid pressure fluctuating within a range of 50–500 MPa (subhorizontal veins) and 80–320 MPa (cleavage-parallel subvertical veins), evidencing fault-valve behaviour. Trends of coupled decreases in salinity and homogenization temperatures in both FIA indicate downward mixing of meteoric fluids, which was more effective in subvertical veins and was in both cases enhanced by fault-valve behaviour. Dominance of moderate salinity and absence of CO2 and CH4 indicate that the fluids are dominated by formation waters. The salinity signature is similar to those of formation waters and metamorphic fluids derived from rocks of shallow marine environments worldwide.Supplementary material: Details of samples and analytical data are available at https://doi.org/10.6084/m9.figshare.c.5275031