Análise dos petrotramas de eixos-C de quartzo: Zona de Cisalhamento Major Gercino (SC)

Abstract

The analysis of quartz c-axis improved the kinematic characterization of the Major Gercino Shear Zone (MGSZ) in the Canelinha - Garcia area in central-eastern Santa Catarina State. This shear zone is one of the major lineaments that affect ali southern Brazilian and Uruguaian precambrian terrains. In Santa Catarina the MGSZ separates supracrustal rocks of the Brusque Group (northern part) from the Granitoid-Migmatitic Complex to the south. This zone is characterized by a regional NE trend, a dextral sense of movement, and predominantly ductile-brittle structures. The MGSZ is composed of two mylonitic belts to the northwest and southeast, respectively, separated by granitoid rocks probably associated with the development of the shear zone. Both shear zones have cataclastic to ultramylonitc rocks, but mylonites and protomylonites predominate. Most of the mylonitic rocks were produced under greenschist facies metamorphic conditions and a high strain rate. Although sinistral displacement is often observed, most of the mesoscopic structures and kinematic indicators show a predominance of dextral movement with an important oblique component. The results of c-axis analyses in quartz-rich rocks are in good agreement with this interpretation and also indicate a coaxial deformation produced by pure shear strain as can be seen in the symmetric pattern of the prefered orientations of the c-axis in many of the diagrams obtained in the MGSZ, as well as indicated by the presence of local sinistrai shear movements. The mylonitic rocks of the MGSZ have crystallographic orientations developed at relatively low temperatures, as confirmed by the low-grade metamorphic conditions. In spite of the low temperatures, the orientation developed under a high strain rate as shown by the deformed and stretched minerais and the commom parallelism of S and C surfaces.