SUPERPOSIÇÃO DESTRAL EM AMPLA ZONA DE CISALHAMENTO SINISTRAL: CINTURÃO DE CISALHAMENTO SÃO PAULO, IMEDIAÇÕES DE PIRACAIA - SP

Abstract

The shear zones in question are roughly NE-SW and intersect gneissic-migmatitic terrains of the Piracaia Complex to the northwest and low- to medium-grade metavolcano-sedimentary sequences of the Sao Roque and Serra do Itaberaba Groups to the southeast. The early Sao Bento do Sapucai and Sertaozinho shear zones (SBSSZ and SSZ) comprise a large mylonitic belt that records sinistral lateral displacement that took place under low to medium metamorphic conditions. This movement generated both widespread subsidence toward the SE and transtensile domains in which less metamorphic rocks and granitic bodies were emplaced. This arrangement (including a probable decrease in the metamorphic grade toward the SE) was disrupted by later dextral displacement of the Jundiuvira Shear Zone (JSZ), a narrower zone of mylonites and ultramylonites with a foliation that is distinguished from the former by retrograde minerais. A SSW to ENE inflection of the JSZ produced both transtensile and transpressive domains, the latter being characterised by asymmetric cylindrical, northwest-verging folds in the SBSSZ region. Lattice preferred orientation (LPO) analysis of quartz C-axes suggests distinct deformational histories for two different parts of the area, whose boundaries are related to the JSZ transpressive domain. In the northwest (SBSSZ) the most typical LPO diagrams show small-circle girdles with asymmetric maxima, which are thought to represent non-coaxial deformation followed by flattening (transpression), while in the southeast (SSZ) the patterns are well-defined sinistral single girdles, typical of non-coaxial deformation (simple shear). The general decrease in metamorphic grade observed in the area is also suggested by the quartz C-axes patterns, which display a reduction in the basal slip (indicative of low temperatures) from the border to the centre of the JSZ.