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Título

THE STRUCTURAL SETTING OF THE GONGO SOCO BASIN, QUADRILÁTERO FERRÍFERO: EVIDENCES OF RESURGENT TECTONICS

Texto do resumo

The Cenozoic record of the Quadrilátero Ferrífero has long been acknowledged for its scattered deposits, reflecting a complex history of sedimentary deposition and structural evolution. The Gandarela Syncline (GS) is renowned for hosting Cenozoic basins, with Fazenda Gandarela Formation recognized as the most comprehensive Cenozoic stratigraphic section in southeastern Brazil, spanning from the Eocene to the Miocene records. This study updates the understanding of the geological and structural processes affecting the sediments of the Gongo Soco Formation (GSF) through detailed structural analysis conducted in the Gongo Soco mine (GSM). Methodologies employed include systematic geological-structural mapping, stereographic analyses, morphostructural lineament analysis, and interpretation of drilling cores. Several mapping efforts were made, analyzing over 5,600 planar and linear measurements in all lithotypes within the mine. The GSF is hosted in the central portion of the GS's normal flank, with approximately 1.540.000 m² area, maximum elongation E-W and a minimum observed depth of 120 meters. The GSF overlies the Itabira Group, primarily the dolomitic itabirites and dolomitic marbles from the Cauê and Gandarela formations, comprising predominantly reddish and ochre-colored silty clay to sandy silt sediments, containing quartz grains and millimeter to centimeter-sized lithic fragments dispersed in the matrix. Additionally, is observed intermittent metric gravel layers and fine-grained sand, interbedded with two distinguished coal layers, potentially organic fossil-bearing. The GSF is generated and affected by the deformational history of its basement (Itabira Group), which had been affected by at least by two distinct tectonic events, E1 and E2. E1, predominantly ductile, generated the Sn foliation and Le lineation within a dextral shear zone context, transitioning progressively to a ductile-brittle until a brittle regime, forming assimetric folds, shear zones, S-C structures and four fracture families. E2, characterized by brittleness reology, generated the recent structures such as low-angle fractures and five fault families, exploiting E1 boundary conditions. Geological structures are associated with four preferential directions (E-W, N-S, NW-SE, and NE-SW), which significantly influence the local geodynamics. The E-W direction is correlated to the main shear zones planes, Sn foliation (180/42), Fr4 fracture family (180/30) and Fl3 fault family (172/60). In the N-S direction, structures such Fr1 (271/78) fracture family (fracture cleavage), Fl1 (279/70) and Fl4 (77/78) fault families are observed. NW-SE normal faults are notably recognized, these faults can be linked to the reactivation of pre-existing fractures in the bedrock, mainly Fr2 (37/75) fracture family, which is interpretated as tension fractures. The NE-SW discontinuities are associated with positive lineaments and the Fr3 (315/85) fracture family and reverse faults Fl5 (313/57). As result, a structural model was created, based on Riedel's non-coaxial deformation, within a dextral strike slip fault system, orientated nearly E-W with tensors σ1 and σ3 oriented NW-SE and NE-SW, respectively, the σ2 is vertical. This dextral strike-slip system would have acted at different times (E1 and E2), and its tectonic resurgence (Neotectonics) can clearly be responsible for the development of the fault systems observed in the GSF.

Palavras Chave

Neotectonics; Strike slip fault; Cenozoic basin; Structural geology