Evolução tectônica da Zona de Cisalhamento Caxambu, MG

The NE-SW trending Caxambu Shear Zone (CSZ) is an example of a group of similar shear zones that crop out in southern Minas Gerais and adjacent Rio de Janeiro states. They represent the final stage of the Neoproterozoic tectonic activity related to the Brasiliano Orogeny in this region. It is a dextral shear zone with predominantly ductile behavior. The CSZ affects metaigneous and metasedimentary rocks of a Paleoproterozoic basement with its Neoproterozoic cover denominated Andrelândia Megasequence (AMS). Six mapping units were recognized: orthogneiss (basement), banded biotite gneiss (AMS), quartzite (AMS), biotite schist (AMS), garnet muscovite schist (AMS) and a plug of alkaline intrusive rocks of probable Cretaceous age. Two groups of structures were recognized in the area. The first group, attributed to deformation phase D B , related to the Brasilia Belt, includes the main foliation and associated lineation and extensive nappe structures with tectonic transport top to the east. The second group, related to the Ribeira Belt and labeled D R , includes the Caxambu Shear Zone. The CSZ is more then 100 km long and about 2.5 km wide; its dextral horizontal displacement in the studied area is estimated at about 14.5 km. Including published estimates from adjacent areas an average value of 17.6 km is obtained, resulting in na approximate average value for γ of simple shear of seven. Associated to the CSZ mylonitic rocks were formed containing the following shear sense indicators: mica fish and other minerals with similar shape and asymmetry, oblique foliation, S-C structures, mantled porphyroclasts and asymmetric microfolds. The metamorphic conditions during mylonitization are estimated at lower amphibolite facies.