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：里奥帕尔多突出构造（Rio Pardo salient）是由阿拉瓜伊褶皱冲断带沿圣弗朗西斯科克拉通（São Francisco craton）东南边缘形成的大型反倾向曲度构造，是巴西利亚造山带阿拉瓜伊-西刚果造山系统（Brasiliano Araçuaí-West Congo orogenic system，简称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，该突出构造主要由新元古代马卡巴斯群（Neoproterozoic Macaúbas Group）与萨利纳斯组（Salinas Formation）的变沉积岩构成。其西翼占据埃斯皮尼亚苏岭（Espinhaço ridge）的一部分，阿拉瓜伊带的南北向（NS-trending）构造逐渐向北东、东向弯曲，进而在米纳斯吉拉斯-巴伊亚边界的Serra Geral沿线形成枢纽带。东翼呈北西向展布，以大型剪切带为标志性特征。 在通过新元古代马卡巴斯盆地闭合形成AWCO的相关构造模型中，该大型曲度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。该曲度构造究竟如何起源？又有哪些因素控制了其形成？我们在阿拉瓜伊造山带北部开展的野外研究刻画了该突出构造的运动学特征，并提出了可解答上述问题的构造模型。 研究结果表明，里奥帕尔多突出构造的形成经历了四期变形作用。收缩性的D1、D2期变形为共轴变形，驱动了沿该突出构造外弧指向克拉通的构造运移，同时伴随内弧整体向南的运动，由此形成了极为复杂的运动学图景。此外，D1/D2期的构造在该突出构造的前缘带形成了交替出现北东向与北西向段的锯齿状格局：在北东向段，变沉积岩以向北西向逆冲的方式覆盖于克拉通基底之上；而在北西向段，表壳岩沿位于基底-盖层接触面的右旋至逆冲右旋走滑挤压剪切带发生位移。 发育于枢纽带内的D3期构造记录了最终的西西南-东东北向收缩作用，该作用使得先期组构要素绕北西北向轴发生旋转，并强化了该突出构造的曲率。D4期为伸展期变形，其记录包括两处大型构造：沙帕达阿考昂（Chapada Acauã）与廷吉（Tingui）正剪切带，以及作为该突出构造东翼标志性构造的伊塔佩比走滑剪切带（Itapebi strike-slip shear zone）的正走滑左旋复活。 我们认为，里奥帕尔多突出构造的启动发生于碰撞期的5.65~5.75亿年前的D1/D2期，本质上属于原生弧构造，其形成主要受控于马卡巴斯先驱盆地的几何形态。里奥帕尔多突出构造的增厚内部区域在约5.30亿年前受到伸展构造作用的影响，该作用由D4期变形所记录，目前该期变形被归因于阿拉瓜伊-西刚果造山系统的伸展垮塌。