Multiscale structural analysis of the Palmital shear zone: A possible suture between Luís Alves Microplate and Paranaguá Terrane, Southern Brazil

Abstract

The Palmital Shear Zone, located in the southern Ribeira Orogen, is a NW-SE oriented crustal-scale shear zone that marks the tectonic boundary between two contrasting geological terranes: the Luís Alves Microplate (LAM) to the west and the Paranaguá Terrane (PT) to the east. This study aims to characterize the Palmital SZ through multi-scale structural analysis and geophysical data, to elucidate its tectonic evolution during the amalgamation of the southwestern Gondwana continent. The Palmital SZ is identified by contrasting anomalies between the adjacent geological terranes. The PT is characterized by negative magnetic anomalies, while the LAM exhibits positive magnetic anomalies. Towards the offshore, this magnetic signature contrast extends into the shallow portion of the Brazilian continental margin. The study area presents mafic granulites and type-A granites from the LAM, and ortho- and paragneisses basement intruded by syn-collisional granites belonging to the PT. The tectonic fabric of the PT is characterized by a gneissic foliation (S₁) trending NE-SW and a mylonitic foliation (S₂). The S₂ exhibits proto-to ultra-mylonitic fabrics, trending NNW-SSE with subvertical dip and two stretching lineation orientations: the southern domain displays sub-horizontal plunge, while the central-northern domain, low to moderate plunge to NNE. Microstructural analysis of the mylonitic foliation (S₂) indicates deformation conditions of low temperature (≤450 °C), equivalent to greenschist facies, transitioning from ductile to ductile-brittle regime. Kinematic criteria such as S-C shear bands, quartz oblique foliation, asymmetric feldspar porphyroclasts and mica fish, document sinistral kinematics of the Palmital SZ. Structural, geological and geophysical evidences suggest that the Palmital SZ originated and evolved as a suture juxtaposing latterally the PT Neoproterozoic units with the Paleoproterozoic units of the LAM during the Brasiliano Orogeny. The Palmital SZ progressed from a transpressional deformation to transtensional deformation phase, registering an oblique continental collision.