Geometria e cinemática da aba sul da estrutura divergente do Rio Paraíba do sul ao longo da Seção Areal-Três Rios, Rio de Janeiro

Abstract

The Rio Paraiba do Sul structural divergence is a regional scale structure in which the mylonitic foliation delineate a fan geometry, those of the northern limb dipping to SE, and those of the southern limb dipping to NW. On the core of the structure occurs a large dextral ductile transpressional shear belt. The present work deals with the geometry and kinematics of structures in a section cross-cutting the southern limb of this structure. At the base of section, occurs a high-grade banded mylonite granitoids, with mylonitic foliation dipping moderately to NW and stretching lineation plunging to NNE. Northwards, the same geometry of ductile structures is seen in mylonitic S-type granitoids and paragneisses. In this region, however, the foliation steepens and the lineation increases in plunge with a slightly rotation towards the NE. Flattening strains leads to the formation of S and SL high-grade tectonites. At the base of section shear-sense indicators (S-C foliations, s -porphyroclasts and shear-bands) suggest a dextral component in horizontal planes and down-dip normal motion in a monoclinic shear-zone model. Towards the top of section, however, the asymmetry of the structures is not clear, but one oblique shearing with top-to-SW is also present. To the north the foliation have slow dips, and the stretching lineation plunges shallower to NE. In this domain the shearsense indicators suggest top-to-SW sense of shear parallel to the orogenic trend. Field relations suggests that ganitoids sheets were intruded lit-par-lit during this stage of the tectonic movements. In this domain also occur open to tight folds with axis sub-parallel to the stretching lineation in the same outcrop and axial surfaces with high dips preferentially to NW. This folding is interpreted as resulted from a shortening component at high angle to the regional shear-zones during transpressional strain, coeval with transpressive dextral shear-zones and thrust (ductile/ruptile) with top-to-SE sense of shear. The structural and field data suggests that the early movements were parallel and oblique to the belt in this region, and occurred near the peak temperatures of regional metamorphism accompanied by widespread crustal anatexis. This early stage of motion would be interpreted as horizontal and vertical extrusion of a partially molten crust during unconfined transpression.