In situ Rb–Sr geochronology of slickensides reveals reactivation of cratonic margins post-Gondwana assembly

Abstract

Assessing the time-strain evolution of orogenic structures is a crucial but difficult task, especially for upper-crust structures where the deformation conditions are often below the closing temperature of the main geochronometers. Conventional Rb–Sr and total fusion ⁴⁰Ar/³⁹Ar techniques are insufficient to spatially-resolve distinct microstructures, such as inherited and recrystallized micas associated with fault zones developed under low to medium-temperature conditions. Here we present in situ Rb–Sr geochronology of fine-grained micas from fault-related rocks of the São Francisco Craton margin, in the Southern Espinhaço Range (SE Brazil), to investigate the timing and effects of fault reactivation at the dawn of Gondwana assembly. Fine-grained muscovite and biotite from slickensides yield Rb–Sr isochron ages spanning 500–450 Ma associated to thrust faults reactivation. This tectonic reactivation is coeval with brittle-ductile deformation of marbles in the footwall of the main regional thrust zones. These novel mica Rb–Sr ages indicate that thrust reactivation was 30–80 Ma younger than the last regional metamorphic event related to collisional deformation in the Araçuaí Orogen, coinciding with the timing of post-collisional magmatism and a fluid flow event in the cratonic margin. We envisage that fault reactivation in the São Francisco Craton margin might be associated with thermal post–orogenic relaxation due to gravitational collapse of the Araçuaí Orogen, with gravity-driven mass transport from the orogenic core causing compressive reactivation in the external fold-thrust-belt. These new results indicate that the Brasiliano thrust fronts remained active throughout the main interval of biogeochemical development in the Ediacaran–Cambrian transition post-Gondwana assembly.