The Rio Preto and Riacho do Pontal Belts

Abstract

Together, the Rio Preto and Riacho do Pontal belts form a 600 km-long orogenic system developed along the northwestern and northern margins of the São Francisco craton during the Neoproterozoic Brasiliano orogeny. Involving the Paleoproterozoic (*1.9 Ga) Formosa Formation (schist, quartzite, greenschist and amphibolite) and the Neoproterozoic (900–600 Ma) Canabravinha Formation (metadiamictite, metawacke, metaturbidite), the Rio Preto fold belt, exposed in Bahia and Piauí states, borders the craton to the northwest. Neoproterozoic deformation between 600 and 540 Ma originated a complex, asymmetrical and double-verging thrust wedge, whose southern branch propagated for over 100 km into the craton interior in form of a thin-skinned deformation front. The Rio Preto belt probably represents an inverted Neoproterozoic hemi-graben developed along the northern margin of the craton. The Riacho do Pontal fold belt occupies the northern margin of the craton. Its external zone is made up of a south-verging thin-skinned nappe system detached along the basement-cover contact. Ages of syn- to late-collisional granitic intrusions suggest that the main deformation phase in the Riacho do Pontal belt occurred between 667 and 555 Ma. The Barra Bonita Formation (quartzite, schist and marble), a correlative of the Una Group in craton interior (Paramirim aulacogen), represents a platformal unit, deposited on the northern São Francisco passive margin. The Monte Orebe metabasalts, exposed further north in the central sector of the belt, might represent remnants of a Neoproterozoic oceanic crust.