Structural framework of the iron district of Itabira, Iron Quadrangle, Minas Gerais

Abstract The Itabira Iron District, located at the northeastern end of the Iron Quadrangle, still presents some uncertainties regarding the tectonic origin of its macrostructure. The Itabira synclinorium is characterized by a large synformal structure in the NE-SW direction, with anticlines and synclines distributed "en echelon". The analysis of the relationship between the primary structures and the plane-axial schistosity, linear and planar structures, allowed the development of a three-dimensional model for the region. The increase in deformation from E to W, observed from the Cauê mine to the Conceição mine, and the rotation of the axes of the macrostructural folds, characterized by the Cauê, Dois Córregos, and Conceição synclines, are explained in this study by a model with six deformation phases. The increase in shortening from the Cauê mine towards the Conceição mine can be explained based on the flanking fold model. The application of this mechanism to explain kilometric-scale folding has been presented for the eastern flank of the Santa Rita Syncline in the Iron Quadrangle.摘要 位于铁四角地区（Iron Quadrangle）东北端的伊塔比拉铁矿区（Itabira Iron District），其宏观构造的成因仍存在诸多不确定性。伊塔比拉复向斜（Itabira synclinorium）以发育北东-南西向大型向状构造为特征，背斜与向斜呈雁列（en echelon）展布。通过分析原生构造与轴面片理、线性及面状构造之间的关联，本研究构建了该区域的三维构造模型。研究中观测到，从考埃矿（Cauê mine）到孔塞桑矿（Conceição mine），变形程度自东向西逐步增强，且以考埃向斜、多伊斯科雷格斯向斜与孔塞桑向斜为代表的宏观褶皱轴发生旋转；本研究通过六期变形模型对上述现象予以解释。考埃矿至孔塞桑矿的构造缩短量递增现象，可借助侧翼褶皱模型（flanking fold model）加以阐释。该构造机制此前已被用于解释铁四角地区圣丽塔向斜（Santa Rita Syncline）东翼的千米级褶皱构造。