A case study of superposed structures in the tectonic interference zone between the Southern Brasília and Ribeira orogens, southeastern Brazil

Abstract

The superposition of the Ribeira Orogen (590-560 Ma) on the southern segment of the Southern Brasília Orogen (630-605 Ma) generated a region known as the tectono-metamorphic interference zone. This zone is characterized by the almost orthogonal superposition of structures and metamorphic isograds attributed to the Ribeira Orogen on older structures and metamorphism belonging to the Southern Brasília Orogen.A flat-lying nappe stack characterizes the Southern Brasília Orogen, and it presents top to the east tectonic transport with intense ductile deformation inside the nappes, visible by the development of a strong E-W stretching lineation and related sheath folds. In the Ribeira Orogen more open upright folds, with axial planes dipping steeply to SE predominate, often with a down dip stretching lineation.In the study area, located in this interference zone, descriptive criteria permit grouping of three sets of structures, attributed to three deformation phases, Dn, Dn+1 and Dn+2, here understood as either separated in time or parts of a progressive deformation. Dn produced the main schistosity, Sn, dipping steeply to the south/southeast; associated tight folds and a strong stretching lineation (Ln), plunging shallowly to E/NE, are also ascribed to this phase. Dn+1 generated open to tight regional folds with NE-SW axes and axial planes dipping moderate to steeply to SE. Gentle to open Dn+2 folds with subvertical ENE-WSW trending axial planes refolded older structures locally. Superposed structures between these phases, of type I (Dome and Basin), type II (Mushroom) and type III (Walking Stick) were recognized. The attitude of the structures of these three phases and the type of interference patterns allow to interpret Dn as related to the northeastward tectonic transport in the southern part of the Southern Brasília Orogen and Dn+1 and Dn+2 as reflecting the NW-SE shortening typical for the Ribeira Orogen deformation. The folds and the interference pattern between Dn and Dn+1 are predominantly coaxial. Our understanding of this coaxiallity is that the strong NE-SW linear anisotropy related to thrusting to NE during the Southern Brasília Orogen (Dn) was used as fold axes by the NW-SE shortening related to the Ribeira Orogen.