Abstract
This paper presents 40Ar/39Ar 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.
Highlights
Extensional structures have been described in several regions closely associated, temporally and/or spatially, with compressive tectonics (Coney and Harms 1984, Royden and Burchfield 1987, Ratschbacher et al 1991a, b, Wheeler and Butler 1993, Braathen et al 2002)
Several well-preserved examples of extensional structures have been described in different segments of the São Francisco craton, presenting east/southeast-verging extension (Fig. 1), such as in the eastern portion of the Quadrilátero Ferrífero (Endo 1997), in the Araçuaí fold belt (Nalini 1997, Machado et al 2001, Alkmim et al 2002) and in the Costeiro Granite Belt (Basei et al 2000), which crops out as a distinct unit within the Ribeira belt
In the Cajati region, kinematic indicators related to ductile deformation, such as S-C foliation planes, asymmetric porphyroclasts and boudins, and asymmetrical extensional shear bands are quite consistent with oblique, E to SE extension towards east-to south, with important sinistral strike-slip motion (Dehler et al 2000). These structures are related to a mid- to deep-crustal transtensive shear zone which developed after the main crustal thickening event under compressive tectonic regime of the Ribeira belt
Summary
Extensional structures have been described in several regions closely associated, temporally and/or spatially, with compressive tectonics (Coney and Harms 1984, Royden and Burchfield 1987, Ratschbacher et al 1991a, b, Wheeler and Butler 1993, Braathen et al 2002). In the Cajati region, kinematic indicators related to ductile deformation, such as S-C foliation planes, asymmetric porphyroclasts and boudins, and asymmetrical extensional shear bands are quite consistent with oblique, E to SE extension towards east-to south, with important sinistral strike-slip motion (Dehler et al 2000) These structures are related to a mid- to deep-crustal transtensive shear zone which developed after the main crustal thickening event under compressive tectonic regime of the Ribeira belt. Recent kinematic studies have described ductile sinistral transtensional tectonics with top-downto ESE for the same region (Dehler et al 2000) associated with the Serra do Azeite shear zone These authors described mylonites with foliation dipping to southeast, associated with a well-developed SE-SSE plunging stretching lineation. A K/Ar investigation of mylonites in this area yielded cooling ages of 565 ± 39 Ma in hornblende, 521 ± 26 Ma in biotite, and 587 ± 21 Ma in phlogopite (Campagnoli 1996)
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