Abstract
Abstract Shear zones are zones of localized high strain accommodating differential motion in the lithosphere and impacting the crustal rheology and deformational history of orogenic belts. Although terrane bounding shear zones are widely studied, intraterrane shear zones and their tectonic significance, especially in association with supercontinent assembly, is a largely unexplored topic. The Ribeira Belt (SE Brazil), a Neoproterozoic-Cambrian orogenic belt from West Gondwana, is dissected by a crustal-scale NE-trending transcurrent shear zone system that juxtaposes composite terranes. Despite its extensive coverage and complexity, this shear zone system remains poorly investigated. In this paper, we explore the thermal and deformational regimes, and timing of ductile shearing using a multiscale approach combining structural analysis derived from remote sensing and field-based structural data, microstructures, quantitative structural analysis, and multimineral U–Pb geochronology (zircon, titanite, monazite, and xenotime). Our data, combined with previously published data, indicate a transitional northeastward increase in metamorphic conditions from lower greenschist to granulite facies conditions (from 250–300 to 750–800°C), reflecting the different crustal levels that are exposed. Vorticity and finite strain data indicate a complex strain regime with varied contributions of pure and simple shear and oblate-shape ellipsoids in strike-slip shear zones and prolate-shaped ellipsoids in dip-slip reverse shear zones. The strain set suggests that all shear zones were developed under subsimple shear deformational regimes involving thrusting and folding followed by wrench tectonics. The pure shear component of deformation was accommodated in folded domains between shear zones. Geochronological data suggest intermittent ductile shear zone activations from ca. 900–830 to 530 Ma, partially coeval with at least two major episodes of terrane accretion at 850–760 Ma and 610–585 Ma. The spatial and temporal record of shear zones within the Ribeira Belt indicates that some relate to assembly of the belt and represent either terrane bounding structures (e.g., Itapirapuã shear zone) or intraterrane structures (e.g., Ribeira, Figueira, and Agudos Grandes shear zones), whereas others are terrane bounding, postcollisional shear zones (e.g., Taxaquara shear zone) reactivated in an intracontinental setting (560–535 Ma).
Highlights
Shear zones are zones of localized high strain that accommodate differential motion in the Earth’s crust and mantle, exerting a fundamental control on crustal rheology and deformational evolution of orogenic belts [1,2,3]
It has been traditionally interpreted as recording a history evolving from accretionary to collisional with its current architecture controlled by a late-collisional, crustal-scale, transcurrent shear zone system that resulted in the juxtaposition of fault-bounded composite terranes in the EdiacaranCambrian (e.g., [10,11,12,13,14,15])
This paper investigates the role of the terrane boundaries and intraterrane shear zones in the southern Ribeira Belt through a multiscale structural analysis derived from remote sensing and field-based structural analysis, coupled to microstructures, quantitative structural data, and multimineral U–Pb geochronology
Summary
Shear zones are zones of localized high strain that accommodate differential motion in the Earth’s crust and mantle, exerting a fundamental control on crustal rheology and deformational evolution of orogenic belts [1,2,3]. The Ribeira Belt is a Neoproterozoic-Cambrian NEtrending orogen that occupies a central position in West Gondwana reconstructions (Figure 1(a)), associated with the convergence between Archean-Paleoproterozoic rock assemblages from the São Francisco, Paranapanema, Luís Alves, and Congo cratons (Figures 1(a) and 1(b); [9]). It has been traditionally interpreted as recording a history evolving from accretionary to collisional with its current architecture controlled by a late-collisional, crustal-scale, transcurrent shear zone system that resulted in the juxtaposition of fault-bounded composite terranes in the EdiacaranCambrian (e.g., [10,11,12,13,14,15]). Alternative models have proposed an intracontinental setting for the orogen [16,17,18,19], without taking into account the role of terrane bounding shear zones from the southern and central Ribeira Belt
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