Structural analysis of the Tocantins river graben system, western border of the Parnaíba Basin (states of Tocantins, Pará and Maranhão - Brazil)

Abstract

The western border of the Parnaíba Basin displays structures that differentiate it from those in other regions of the basin. Two graben structures can be highlighted (Araguaína and Muricizal), which compose the graben system in this study, the first one inside the basin, and the second implanted directly on the crystalline basement (Araguaia Belt). Along the borders of the main graben (Araguaína), the contacts between the Motuca and Pedra de Fogo formations and older units are defined by normal and sinistral normal-oblique faults. Further south, in the Palmas-Miracema do Tocantins region, the contacts between the Serra Grande Group, the Pimenteiras Formation and the crystalline basement follow normal faults. The westerly secondary graben also hosts the Motuca Formation and shows the same N-S orientation. The analysis of SRTM images allowed to map the photolineaments, the graben borders and, associated with field data, supports the proposition of three deformational events (D₁, D₂ and D₃). The N-S fault orientation involves a 20º to 30º of variation to E or W, which is controlled by anisotropies of the crystalline basement related to the Araguaia Belt. Along the main graben, normal and normal-sinistral faults point to a NE‑trending extension (D₁ event). The minimum age for D1 is estimated to be in the Permo‑Triassic interval, based on structural and stratigraphic relationships. To the north, E-W/ENE photolineaments controlled the emplacement of the Eojurassic basalts and diabases of the Mosquito Suite (D₂ event), overprinting the N-S structures of the grabens. NNE structures in the Mosquito Suite and normal or transcurrent faults recording NW distension (D₃ event) are younger and tentatively correlated to the opening of the Central and South Atlantic, respectively.