Estruturação tectônica e análise deformacional do segmento central do Cinturão Ribeira na Serra da Bocaina, nos estados de São Paulo e Rio de Janeiro

Abstract

The central segment of the Ribeira belt in southeastern Brazil comprises orthogneisses and paragneisses of medium to high grade metamorphic conditions, beeing dominated by regional NE-SW trending ductile shear zones. This paper presents the results of structural and strain analysis carried out in the Serra da Bocaina range, bordering region between the states of Sao Paulo and Rio de Janeiro, with the aim to better constrain the tectonic regime and the nature of the deformational process associated to the orogenic processes. In that area a pervasive strain is well recorded in the microstructures associated to gneissic (Sn) and mylonitic (Sm) foliations. Samples from 29 gneiss outcrops between the Rio Preto (ZCRP) and the Arcadia-Areal (ZCAA) shear zones were analysed using the Rf/φ method on quartz and feldspar grains in thin sections. On the main kinematic plane, the samples showed typically low to medium strain ratios (2<R xz <3) and some increasing values reaching 5.2 along the shear zones. On the YZ plane, R yz ranges between 1.0 and 3.3. The average ellipsoid estimated from all samples shows X:Y:Z semi-axes dimensions of 1.45:1.1:0.65, which corresponds to a stretching of 45% in X (sub-horizontal NE-SW), 10% in Y (vertical) and a shortening of 35% in Z (NW-SE). The overall finite strain ellipsoid (regional bulk strain) shows an oblate shape (medium K flinn of 0.38) that corresponds to the general flattening field. The highest vorticity values (Wk<0.81) are found along strike-slip shear zones and in the Paraiba do Sul domain, while coaxial deformations (0.15<Wk<0.75) predominate inside the Embu and the Juiz de Fora domains. The integration of shear strain values γ (1.3<γ<8.9) obtained between the ZCRP and ZCAA allows to estimate an overall strike-slip displacement of almost 200 Km. The general tectonic model is represented by two types of deformations: dextral strike-slip and shortening perpendicular to the shear zones, which is visible by symmetrical and asymmetrical folds, as well as mineral aggregates flattening. The partitioning between NW-SE contraction and NE-SW transcurrent structures confirms the transpressional tectonic regime in the central segment of the Ribeira Belt, which is compatible to E-W collision.