Abstract
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.
Highlights
Doubly-vergent asymmetric fans are an unusual and interesting feature of many thrust-and-fold belts around the world (e.g. Koons 1990, Willett et al 1993, McClay and Whitehouse 2004, McClay et al 2004)
In Brazil, the Rio Preto fold belt, which border the northwestern margin of the São Francisco craton (Figure 1 and Figure 2; Almeida 1977, Alkmim 2004), is an example of a kilometric doubly-vergent asymmetric fan that serves as an exquisite natural laboratory to study such structures, which, in this scale, are uncommon in the context of the thrust
The lattice preferred orientation (LPO) analysis of two quartzite oriented sections was performed at Université de Montpellier II, France, through the electron backscatter diffraction (EBSD) technique, using a JEOL JSM 5600 scanning electron microscope equipped with an Oxford Instruments/HKL Nordlys EBSD detector and Channel 5® software
Summary
Doubly-vergent asymmetric fans are an unusual and interesting feature of many thrust-and-fold belts around the world (e.g. Koons 1990, Willett et al 1993, McClay and Whitehouse 2004, McClay et al 2004). In Brazil, the Rio Preto fold belt, which border the northwestern margin of the São Francisco craton (Figure 1 and Figure 2; Almeida 1977, Alkmim 2004), is an example of a kilometric doubly-vergent asymmetric fan that serves as an exquisite natural laboratory to study such structures, which, in this scale, are uncommon in the context of the thrust-. The maximum age of deposition of this sequence is around 850 Ma, according to the youngest detrital zircon population dated by the U-Pb LA-ICP-MS method (Caxito et al 2012a, Caxito et al 2014) This unit might correlate with other diamictite-bearing units within and around the São Francisco craton (Caxito et al 2012b)
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