Structural analysis of the Rio Preto fold belt
                    (northwestern Bahia / southern Piauí), a doubly-vergent
                    asymmetric fan developed during the Brasiliano Orogeny

The Rio Preto fold belt borders the northwestern São Francisco craton and shows an exquisite kilometric doubly-vergent asymmetric fan structure, of polyphasic structural evolution attributed exclusively to the Brasiliano Orogeny (∼600-540 Ma). The fold belt can be subdivided into three structural compartments: The Northern and Southern compartments showing a general NE-SW trend, separated by the Central Compartment which shows a roughly E-W trend. The change of dip of S₂, a tight crenulation foliation which is the main structure of the fold belt, between the three compartments, characterizes the fan structure. The Central Compartment is characterized by sub-vertical mylonitic quartzites, which materialize a system of low-T strike slip shear zones (Malhadinha – Rio Preto Shear Zone) crosscutting the central portion of the fold belt. In comparison to published analog models, we consider that the unique structure of the Rio Preto fold belt was generated by the oblique, dextral-sense interaction between the Cristalândia do Piauí block to the north and the São Francisco craton to the south.

Brasiliano Orogeny; doubly-vergent fan; Rio Preto fold belt; São Francisco craton; structural analysis

Authorship

FABRÍCIO A. CAXITO

Departamento de Geologia, Instituto de Geociências, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, 31270-901 Belo Horizonte, MG, BrasilUniversidade Federal de Minas GeraisBrazilBelo Horizonte, MG, BrazilDepartamento de Geologia, Instituto de Geociências, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, 31270-901 Belo Horizonte, MG, Brasil

ALEXANDRE UHLEIN

LUIZ F.G. MORALES

Géosciences Montpellier, UMR 5243 - CC 60, Université Montpellier 2, Place E. Bataillon, 34095 Montpellier cedex 5, FranceUniversité Montpellier 2FranceFranceGéosciences Montpellier, UMR 5243 - CC 60, Université Montpellier 2, Place E. Bataillon, 34095 Montpellier cedex 5, France

MARCOS EGYDIO-SILVA

Departamento de Mineralogia e Geotectônica, Instituto de Geociências/ USP, Rua do Lago, 562, 05508-080 São Paulo, SP, BrasilInstituto de Geociências/ USPBrazilSão Paulo, SP, BrazilDepartamento de Mineralogia e Geotectônica, Instituto de Geociências/ USP, Rua do Lago, 562, 05508-080 São Paulo, SP, Brasil

JULIO C.D. SANGLARD

TATIANA G. DIAS

MONICA C.O. MENDES

Correspondence to: Fabrício de Andrade Caxito E-mail: boni@ufmg.br

SCIMAGO INSTITUTIONS RANKINGS

Figures

Figures (7)

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Figure 1 -
The São Francisco craton and surrounding fold belts. The rectangle indicates the location of Fig. 2. Modified from Alkmim (2004)Alkmim FF. 2004. O que faz de um cráton um cráton? O Cráton do São Francisco e as revelações almeidianas ao delimita-lo. In: Mantesso-Neto V, Bartorelli A, Carneiro CDR and Brito-Neves BB (Orgs), Geologia do Continente Sul-Americano: evolução da obra de Fernando Flávio Marques de Almeida, Beca, São Paulo, p. 17-35..

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Figure 2 -
Geological map and cross section of the Rio Preto fold belt.

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Figure 3 -
Photolineament map and structural compartmentation of the Rio Preto fold belt, with isoareal contour stereograms for the poles of S₂ and scattered points for L₂ for a) Northern, b) Central and c) Southern Compartment. The attitudes in each stereogram represent the modal maximum for S₂ and L₂ (the latter in italics). Positive lineaments correspond to positive relief in the field, often coincident with bedding or metamorphic foliation, while negative lineaments correspond to negative relief, often coincident with fractures, faults or shear zones.

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Figure 4 -
Structural features of the Rio Preto fold belt. a) Photomicrography showing the microscopic relationships between the three deformational foliations, in fold hinge zones of a Canabravinha Formation phyllite, Southern Compartment; b) tight fold of the Canabravinha Formation, Southern Compartment; c) sub-vertical mylonitic quartzite of the Canabravinha Formation, Central Compartment, with development of S-C pairs; d) Photomicrography showing the microscopic aspect of mylonitic quartzite of the Central Compartment, with dextral shear sense indicator; e) ramp and platform geometry of thrust fronts in Formosa Formation schists, Northern Compartment; f) development of NW-verging thrust faults parallel to S₃, Central Compartment. a) and d) under crossed polarizers.

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Figure 5 -
Isoareal stereograms for structures of the Rio Preto fold belt.

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Figure 6 -
Isoareal stereograms for structures in a) Rio Preto Shear Zone; b) central portion of the Central Compartment; c) Malhadinha Shear Zone. In d) and e), structural schemes of the Central Compartment, in plane and section view, respectively.

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Figure 7 -
Isoareal pole figures for the crystallographic axis orientation (LPO) of quartz grains for samples of the Central Compartment.

Figure 1 - The São Francisco craton and surrounding fold belts. The rectangle indicates the location of Fig. 2. Modified from Alkmim (2004)Alkmim FF. 2004. O que faz de um cráton um cráton? O Cráton do São Francisco e as revelações almeidianas ao delimita-lo. In: Mantesso-Neto V, Bartorelli A, Carneiro CDR and Brito-Neves BB (Orgs), Geologia do Continente Sul-Americano: evolução da obra de Fernando Flávio Marques de Almeida, Beca, São Paulo, p. 17-35..

Figure 2 - Geological map and cross section of the Rio Preto fold belt.

Figure 3 - Photolineament map and structural compartmentation of the Rio Preto fold belt, with isoareal contour stereograms for the poles of S₂ and scattered points for L₂ for a) Northern, b) Central and c) Southern Compartment. The attitudes in each stereogram represent the modal maximum for S₂ and L₂ (the latter in italics). Positive lineaments correspond to positive relief in the field, often coincident with bedding or metamorphic foliation, while negative lineaments correspond to negative relief, often coincident with fractures, faults or shear zones.

Figure 4 - Structural features of the Rio Preto fold belt. a) Photomicrography showing the microscopic relationships between the three deformational foliations, in fold hinge zones of a Canabravinha Formation phyllite, Southern Compartment; b) tight fold of the Canabravinha Formation, Southern Compartment; c) sub-vertical mylonitic quartzite of the Canabravinha Formation, Central Compartment, with development of S-C pairs; d) Photomicrography showing the microscopic aspect of mylonitic quartzite of the Central Compartment, with dextral shear sense indicator; e) ramp and platform geometry of thrust fronts in Formosa Formation schists, Northern Compartment; f) development of NW-verging thrust faults parallel to S₃, Central Compartment. a) and d) under crossed polarizers.

Figure 5 - Isoareal stereograms for structures of the Rio Preto fold belt.

Figure 6 - Isoareal stereograms for structures in a) Rio Preto Shear Zone; b) central portion of the Central Compartment; c) Malhadinha Shear Zone. In d) and e), structural schemes of the Central Compartment, in plane and section view, respectively.

Figure 7 - Isoareal pole figures for the crystallographic axis orientation (LPO) of quartz grains for samples of the Central Compartment.

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Métricas

Structural analysis of the Rio Preto fold belt (northwestern Bahia / southern Piauí), a doubly-vergent asymmetric fan developed during the Brasiliano Orogeny

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[1] Correspondence to: Fabrício de Andrade Caxito E-mail: boni@ufmg.br