The Cabo Frio Thrust: A folded suture zone, Ribeira belt, SE Brazil

Abstract

Kinematic evolution of thrust systems in which vergence changes along strike result in divergent interpretations regarding the subduction polarity. The Ribeira belt, located in southeastern Brazil, resulted from progressive amalgamation of several terranes against the southeastern margin of the São Francisco Craton (SFC) between ca. 620 and 580 Ma. This study brings new structural analysis of the Cabo Frio Thrust (CFT) located in the Ribeira belt, that displays strike variations and apparently double thrusting vergence, generating current debate on subduction vergence for the region. Four major stages of deformation (Dn-1, D1cf, D2cf and D3cf) have been identified in the southwestern segment of the CFT. The Dn-1 affected only the metasedimentary rocks of the Oriental Terrane previously the collision. The D1 phase is associated with the thrusting of orthogneisses of the Cabo Frio Terrane onto the metasedimentary rocks of the Oriental Terrane, with the development of a penetrative S1 foliation and L1 stretching lineation. The D2 deformation phase produced a recumbent synform in the early/syn-D2 stages that overturned the CFT. This resulted in F2 isoclinal folds and S2 axial plane schistosity with F2 axis plunges towards W-SW, parallel to the L1 stretching and L2 intersection lineations. During the late-D2 stage, the recumbent F2 fold was sheared and refolded, leading to horizontal rotation of linear and planar elements, changing the CFT strike from W-SW to N–S. A series of irregular upright folds were developed in the D3 deformation phase. The double vergence and strike variations of the CFT are a result of the major F2 fold geometry, influenced by the granitic batholith (Tinguí Complex) that acted as an indenter during continuous crustal shortening towards northwest.