The structural framework of the north-central Guiana Shield: a fossil fold-and-thrust belt in northeastern Roraima state, Brazil

Abstract

The northeastern Roraima state (Brazil), located in the north-central portion of Guiana Shield, shows shallow-granitoids and associated volcanic rocks of the Orocaima Igneous Belt (2.00-1.96 Ga) overlaid by the Arai Formation sedimentary succession (1.96-1.87 Ga). Previous authors suggested a compressional event affecting the region. Meanwhile, recent papers interpreted that transpressional deformation, related to a distal effect of a Late Mesoproterozoic tectonics (K'Mudku Event), reactivated some basement structures and generated drag folds in the supracrustal successions. This paper presents a multi-scale approach to the best-exposed sector of this geological framework, gathering and analyzing new data from the regional photointerpretation to the thin section scales. The structural data reveal the fractal character and the rheological control on the E-W-and ESE-WNW-trending compressional structures. The southern domain presents sub-vertical shear zones with down-dip to oblique lineation, which are related to the uplift of the shallow-granitoids in relation to the coeval volcanic successions. The central domain, where the volcanic succession crops out, exhibits ubiquitous E-W-to ESE-WNWtrending sub-vertical foliation parallel to the axial plane of the folds. The southern border of the northern domain (Arai Formation) shows spaced cleavage and gentle folds, besides sub-horizontal thrust faults. The structures reveal a fold-and-thrust belt that presents a transition from thick-to thin-skinned tectonics with mass transport to the north. These geometric relations do not match with the K'Mudku lineaments, usually following a NE-SW direction in the north-central Guiana Shield. However, the southward Middle Orosirian structural framework (Uatumã-Anauá Domain) shows similar geometric relations with nearly E-W-trending lineaments and parallel sub-vertical foliation.