The Rio Pardo salient, northern Araçuaí orogen: an example of a complex basin-controlled fold-thrust belt curve

ABSTRACT: The Rio Pardo salient, the large antitaxial curve described by the Araçuaí fold-and-thrust belt along the southeastern edge of the São Francisco craton, is one of the most prominent and one of the least studied features of the Brasiliano Araçuaí-West Congo orogenic system (AWCO). In addition to the Archean/Paleoproterozoic basement, the salient is comprised of metasedimentary rocks mainly from the Neoproterozoic Macaúbas Group and the Salinas Formation. Its western limb occupies a portion of the Espinhaço ridge, where the NS-trending structures of the Araçuaí belt progressively curve NE and E, thereby defining the hinge zone along the Serra Geral on the Minas-Bahia boundary. The eastern limb is NW-trending and marked by a major shear zone. In models postulated to generate the AWCO through the closure of the Neoproterozoic Macaúbas basin, this large curve plays a critical kinematic role. Yet, in spite of this, its development is still not fully understood. How did this curve originate? Which factors controlled its generation? Our field study performed in the northern Araçuaí orogen characterized the kinematic picture of the salient, and led to a model that addresses these questions. The results we obtained indicate that the Rio Pardo salient developed in response to four deformation phases. The contractional D1 and D2 phases are coaxial and responsible for a craton-directed tectonic transport along the salient’s outer arc, which is coupled with an overall southward motion of the inner arc, thereby giving rise to a rather complex kinematic picture. Furthermore, structures of the D1/D2 phases define a zigzag pattern with alternating NE- and NW-trending segments along the salient’s leading edge. Along the NE-trending segments, the metasedimentary rocks are thrust northwestwards on top of the craton basement, while along the NW-trending segments, the supracrustal rocks are displaced along dextral to reverse-dextral transpressional shear zones located on the basement/cover contact. Structures of the D3 phase, which are well developed in the hinge zone, record a final WSW-ENE contraction, which was responsible for rotation of the preexistent fabric elements around NNW-trending axes and the enhancement of the salient curvature. The D4 phase is extensional and is recorded by two large-scale structures, the Chapada Acauã and Tingui normal shear zones, as well as by the normal-sinistral reactivation of the Itapebi strike-slip shear zone that marks the salient eastern limb. We interpret the initiation of the Rio Pardo salient during the collisional 565-575 Ma D1/D2 phases essentially as a primary arc that is mainly controlled by the geometry of the Macaúbas precursor basin. The thickened internal portion of the Rio Pardo salient was affected by extensional tectonism at c. 530 Ma, and is recorded by the D4 deformation phase, which is currently ascribed to the extensional collapse of the Araçuaí-West Congo orogen.

摘要：位于圣弗朗西斯科克拉通（São Francisco craton）东南缘、由阿拉苏阿伊褶皱冲断带（Araçuaí fold-and-thrust belt）塑造的大型反轴弧形构造——里奥帕尔多突出体（Rio Pardo salient），是巴西利亚造山带阿拉苏阿伊-西刚果造山系统（Brasiliano Araçuaí-West Congo orogenic system，AWCO）中最显著且研究程度最低的构造特征之一。除太古宙/古元古代基底外，该突出体主要由新元古代马卡乌巴斯群（Neoproterozoic Macaúbas Group）与萨利纳斯组（Salinas Formation）的变质沉积岩构成。其西翼占据埃斯皮尼亚苏山脉（Espinhaço ridge）的一部分，阿拉苏阿伊带的南北向构造在此逐渐向东北与东向偏转，由此形成米纳斯-巴伊亚边界处塞拉格拉尔沿线的枢纽带。东翼呈北西走向，以大型剪切带（shear zone）为典型特征。在通过新元古代马卡乌巴斯盆地闭合形成AWCO的相关模型中，该大型弧形构造发挥着关键的运动学作用。尽管如此，其形成演化过程仍未被完全阐明：该弧形构造如何起源？又受何种因素调控其形成？本研究在阿拉苏阿伊造山带北部开展野外调查，厘清了该突出体的运动学特征，并提出了可解答上述问题的构造模型。研究结果表明，里奥帕尔多突出体的形成经历了四期变形作用。收缩变形期D1与D2为同轴变形，驱动突出体外弧发生指向克拉通的构造运移，同时伴随内弧整体向南运动，由此形成极为复杂的运动学图景。此外，D1/D2期构造在突出体前缘形成了北东向与北西向段交替出现的锯齿状构造样式。在北东向构造段，变质沉积岩沿北西向逆冲于克拉通基底之上；而在北西向构造段，表壳岩沿基底-盖层接触带的右行至逆右行压扭剪切带（dextral to reverse-dextral transpressional shear zones）发生位移。枢纽带中发育良好的D3期构造记录了最终的西南西-东北东向收缩作用，该作用使先存构造组构绕北西北向轴发生旋转，并强化了突出体的弧形曲率。D4期为伸展变形期，其构造响应体现为两处大型构造：查帕达阿考昂（Chapada Acauã）与廷吉（Tingui）正剪切带，以及标记突出体东翼的伊塔佩比走滑剪切带（Itapebi strike-slip shear zone）的正左行走滑活化。我们认为，里奥帕尔多突出体在5.65~5.75亿年的碰撞型D1/D2期开始形成，本质上属于受马卡乌巴斯前陆盆地几何形态主导的原生弧形构造。里奥帕尔多突出体的增厚内部区域在约5.30亿年受到伸展构造作用影响，该作用由D4期变形记录，目前该期变形被归因为阿拉苏阿伊-西刚果造山带的伸展垮塌。