40Ar/39Ar ages (600-570 Ma) of the Serra do Azeite transtensional shear zone: evidence for syncontractional extension in the Cajati area, southern Ribeira belt

Abstract

This paper presents 40 Ar/ 39 Ar ages of the rocks from the Serra do Azeite transtensional shear zone in the southern part of the Ribeira belt, between the States of São Paulo and Paraná, and also discusses the regional correlations and the tectonic implications for other parts of the belt. The geochronological data suggest that transtensional deformation was active between 600 and 580 Ma (hornblende and muscovite apparent ages, respectively). This time span is considerably older than previous proposals for the period of activity of these structures (520-480 Ma) in the northern segment of this belt and in the Araçuaí belt. Kinematic analysis of the dated mylonites shows extensional structures with top-down movement to ESE compatible with structures found in other tectonic segments in the eastern portion of the Quadrilátero Ferrífero and in the Rio Doce Valley region. Our ages are situated in the same time interval defined for the alkaline magmatism of the Serra do Mar suite. We suggest that the regional tectonic framework was developed during continental-scale extension. This process has been coeval with convergent strain in the adjacent Neoproterozoic shear zones of the Apiaí/Ribeira and Araçuaí belts, which make up significant segments of these belts. The available data show that these structures may not be simply related to post-orogenic gravitational collapse, but must involve a more complex process probably related to dynamic balance between crustal thickening and thinning during tectonic convergence, basin formation and exhumation processes.

Deformational, crystallographic and petrochronological analyses of the mylonites from Taxaquara shear zone in Ribeira Belt (São Paulo)

“…16B) can be explained by partition of deformation (Dewey et al, 1998;Ebert and Hasui, 1998;Hackspacher and Godoy, 1998;Egydio-Silva et al, 2005), an important characteristic of transpressional tectonics during oblique collision at high angle of plate boundaries with dominant simple-shear component (Tikoff and Teyssier, 1994). As suggested in the literature, it also can represent episodic localized transtension regime (Dehler et al, 2007) or superposition of thrusts and lateral displacements under a transpressional regime (Faleiros, 2003).…”

“…Authors have suggested generic transpression model as the main tectonic setting for this major transcurrent shear zone system developed in the Ribeira Belt during final stages of Southwestern Gondwana assembly. This was based on field-scale observations, emplacement of synkinematic granitic plutons and a few numerical models (Ebert and Hasui, 1992;Ebert et al, 1996;Hackspacher and Godoy, 1998;Egydio-Silva et al, 2002Dehler et al, 2007). However, little has been done in order to quantify finite strain directly, which could support this model or not.…”

“…Single spot U-Pb and trace elements analyses from apatite, titanite and zircon from granitic mylonite and undeformed granite samples were obtained through laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS) at Isotopia Laboratory, School of Earth Atmosphere and Environment, Monash University, Australia. These results, coupled with fish-shaped muscovite 40 Ar/ 39 Ar ages from granitic mylonite samples and closure temperature estimates for apatite and muscovite based on Pb and Ar diffusion, respectively, improved the understanding of i) how to date shear zones; ii) that apatite U-Pb ages record ductile deformation in mid-crustal shear zones at ~500°C, and iii) a new perspective regarding the tectonic importance of the shear zones in the RB in comparison to the published 'deformational' ages of other shear zones (Faleiros et al, 2011;Machado et al, 2007;Passarelli et al, , 2008Passarelli et al, , 2007.…”