Pressure-temperature-time deformation of an Ediacaran-Cambrian foreland fold-and-thrust belt: the Southern Espinhaço Front, Araçuaí Orogen

Abstract

To reconstruct the tectonic evolution of a foreland fold-and-thrust belt, a multi-method approach was carried out involving field-based structural observations, strain quantification, analyses of quartz crystallographic textures, fluid inclusions and illite crystallinity (IC), and in situ muscovite Rb-Sr dating and K-Ar dating of fine fractions of white mica. The studied area encompasses the Southern Espinhaço Front in Serra do Cipó, Brazil, marking the boundary between the Foreland Domain (FLD) of the São Francisco Craton (west) and the Fold-and-Thrust Domain (FTD) of the Araçuaí-West Congo Orogen (east). In this area, the Mesoproterozoic Espinhaço Supergroup was thrust over the Ediacaran-Cambrian Bambuí Group during the Brasiliano-Pan-African orogeny. All structural features indicate west-verging tectonics, with oblate strain ellipsoids, suggesting flattening strain and volume loss. Quartz crystallographic textures indicate primarily rhomb <a> dislocation glide for the larger granulometric fractions (>100 μm), with finer grains experiencing dissolution-precipitation creep. Primary, aqueous fluid inclusions hosted in syn-tectonic veins indicate local intra-basinal fluid circulation. Deformation conditions ranges were 300–410 °C and 1.7–6.0 kbar in the FLD, and 385–450 °C and 2.8–8.0 kbar in the FTD. Rb-Sr muscovite dating suggests a metamorphic peak at 549 ± 17 Ma. K-Ar ages for <2 μm mica were 510–496 Ma in the FLD and 476–456 Ma in the FTD, and <0.2 μm mica dated at 432.6 ± 7.5 Ma. These results indicate progressive crustal wedge thickening under a critical taper model, driving deformation and basal detachment propagation into the foreland accompanied by exhumation.