Structural evolution of the Paleoproterozoic Rio Itapicuru granite-greenstone belt (Bahia, Brazil): the role of synkinematic plutons in the regional tectonics

Abstract

The Paleoproterozoic structural evolution of the Rio Itapicuru greenstone belt (RIGB, Bahia, Brazil) involves two tectonic events of different natures, both associated with magmatism. The first event (D1) developed in response to NW-SE shortening and was responsible for subhorizontal foliation, NW-SE trending lineation and SE-directed thrusts. D1 was coeval with the emplacement of rare 2130 Ma granodioritic plutons. The D2 tectonic event was characterized by left-lateral strike-slip tectonics, developed on steeply dipping D1 foliation and within numerous N-S elongated ca 2080 Ma plutons. Quartz c-axis analysis within the granites shows that they experienced progressive leftlateral shearing from magmatic to solid-state conditions under decreasing temperatures. This fact, in addition to microstructural arguments, indicates syntectonic emplacement of these plutons. A zone of maximum deformation (the Main Shear Zone) has been identified in the centre of the RIGB on the basis of strain measurements. A tectonic model in which D1 thrusting event initiated basin closure before being replaced by D2 strike-slip motion is proposed. The Main Shear Zone corresponds successively to the main thrust zone and the main wrench fault during D1 and D2 events, respectively. Such a switch from orogen-normal thrusts to orogen-parallel transcurrent movements may represent a common progression during evolution of mountain belts in order to accommodate excess crustal thickness. However, because low-grade metamorphism and low intensity of the deformation demonstrate that excess crustal thickness was not attained, it is argued that granite emplacement and ascent played a significant role in triggering strike-slip tectonics in the RIGB.