Structural Framework

Proposed tectonic models for the Ribeira Belt are incompatible with each other, suggesting a southwestern or northwestern subduction, involving collision between the São Francisco-Congo and the Rio de La Plata Cratons. An oblique collision resulted in the diachronous formation of the Brasília and Ribeira Orogenic Belts, whose transpressional tectonics and strain partitioning were identified as significant shortening mechanisms during the Brasiliano Orogeny (Ebert and Hasui, 1998).

Three main deformational events were identified in this segment of the Ribeira Belt (Trouw et al., 2000 and 2003; Heilbron, 1995; Heilbron et al., 2003; Hackspacher et al., 2003). The first (Dn) is characterized by a thrusting system whose main tectonic transport shows top to the NNW. The second (Dn+1) relates to an E-W compressive convergence of colliding plates, responsible for the formation of the anastomosing system of strike slip shear zone trending NE-SW that divides the region into different crustal blocks or domains, such as those defined above (Figure 3). The third event is related to the final stage of Brasiliano Orogeny, characterized by uplift and exhumation of the strike slip shear zones.

The first event took place 660-640 Ma and is associated with the pre-collisional granitic magmatism resulting from subduction of a Neoproterozoic oceanic crust (Hackspacher et al., 2003). The second is linked to the syn- to late-collisional magmatism phase in the Ribeira Belt around 625-590 Ma (Wernick, 2000). The third event characterized by a strong vertical exhumation during post-collisional escape tectonics of the Brasiliano Orogeny occurs in the Ribeira Belt between ca. 590-540 Ma, and is associated with the emplacement of sub-alkaline granites (Hackspacher and Godoy, 1999).

Dn structures developed a Sn regional foliation trending NE-SW with variable dip direction, predominantly SE, and have been described as D1 and D2 structures by others (Heilbron, 1995, Trouw et al., 2000). In the Embu Domain, this foliation is anastomosed in macro- and meso-scales and is indicative of a NW movement with the development of low-angle mylonitic shear zones (Figure 14). The Rio Preto thrust separates the Embu and Andrelândia Domains. Kinematic indicators such as S-C fabric, stretching lineation, and asymmetric mica fish suggest tectonic transport towards NNW.

Figure 14. Ultramylonitic biotite gneiss of the Embu Domain

Ultramylonitic biotite gneiss of the Embu Domain

Ultramylonitic biotite gneiss of the Embu Domain in the Rio Preto thrust with stretching lineation plunging 15 towards 130 (UTM23S-474424/7494730).

Dn+1 structures occur throughout the region and were described as D3 events in several papers (i.e. Trouw et al. 1986; Ribeiro et al. 1990; Heilbron 1993; Heilbron et al., 2003). They are represented by sub-vertical shear zones trending NE-SW (Figure 15) and dextral movement, forming an anastomosed system (i.e. Jundiuvira, Taxaquara, Cubatão, Buquirinha, Alto da Fartura shear zones, and Paraíba do Sul fault; Figure 13). These shear zones are easily identified by their morphology in satellite images and digital terrain model and can be traced for several kilometers (Figure 13). Some strike slip shear zones are 5 km wide and characterized by a strong sub-horizontal or oblique stretching lineation plunging between 30° - 40° to WSW.

Figure 15. Sub-vertical mylonitic foliation development

Sub-vertical mylonitic foliation development

Sub-vertical mylonitic foliation development in the Quebra Cangalha granite defining the Alto da Fartura shear zone. The shear zone was active during emplacement of the Quebra Cangalha granite. (UTM23S -503775/7481955).