Reactivated shear zones: A case study in a tectonic superposition zone between the Southern Brasília and Ribeira orogens, southeastern Brazil

Abstract

New data are presented related to the evolution of major shear zones in the Neoproterozoic Southern Brasília and Ribeira orogens, considering their attitude, kinematics and metamorphic conditions. The integration and analysis of geological maps, aerogeophysical and new geochronological U-Pb data, allowed to unravel the kinematics and reactivation of these shear zones. The first major event related to the collision of the São Francisco and Paranapanema paleocontinents is represented by the Alterosa Suture Zone (ASZ), interpreted as the suture of the Southern Brasília Orogen, which is a low to medium angle shear zone with mylonitic fabric indicative of granulite facies conditions. Geochronological analyzes in zircon from this zone provided three age peaks of 630 Ma, 610 Ma and 600 Ma associated with the collision of this orogen. A second regional event related to the Ribeira Orogen was responsible for the formation of steep NE-SW shear zones of low to high metamorphic grade and the local reactivation of the ASZ. Metamorphic zircon from one of these shear zones yielded an age of 574 ± 2 Ma, with minor rim growth and/or lead loss at 553 ± 2 Ma and 524 ± 4 Ma. The age of 574 ± 2 Ma fits well in the peak activity period of the Ribeira Orogen (600-560 Ma). The ages of 553 ± 2 Ma and 524 ± 4 Ma probably reflect reactivation. Several of these shear zones show evidence of original sinistral and later dextral kinematics. The paper also presents a new structure resulting from this dextral reactivation of former sinistral shear zones, which is named here “arrow tip structure”.