Polyphase deformation and metamorphism of the Cuiabá group in the Poconé region (MT), Paraguay Fold and Thrust Belt: kinematic and tectonic implications

Several deformation models have been proposed for the Paraguay Belt, which primarily differ in the number of phases of deformation, direction of vergence and tectonic style. Structural features presented in this work indicate that the tectonics was dominated by low dip thrust sheets in an initial phase, followed by two progressive deformation phases. The first phase of deformation is characterized by a slate cleavage and axial plane of isoclinal recumbent folds with a NE axial direction, with a recrystallization of the minerals in the greenschist facies associated with horizontal shear zones with a top-to-the-SE sense of movement. The second stage shows vergence towards the NW, characterized by crenulation cleavage axial plane to F2 open folds over S0 and S1, locally associated with reverse faults. The third phase of deformation is characterized by subvertical faults and fractures with a NW direction showing sinistral movement, which are commonly filled by quartz veins. The collection of tectonic structures and metamorphic paragenesis described indicate that the most intense deformation at the deeper crustal level, greenschistfacies, occurred during F1, which accommodated significant crustal shortening through isoclinal recumbent folds and shear zones with low dip angles and hangwall movement to the SE, in a thin-skinned tectonic regime. The F2 deformation phase was less intense and had a brittle to ductile behavior that accommodated a slight shortening through normal open subvertical folds, and reverse faults developed in shallower crustal level, with vergence towards the Amazonian Craton. The third phase was less pervasive, and the shortening was accommodated by relief subvertical sinistral faults.

针对巴拉圭构造带（Paraguay Belt），学界已提出多种变形模型，其核心差异主要体现在变形期次、构造倒向与构造样式三个维度。本研究获取的构造形迹表明，该区域构造演化的初始阶段以低倾角逆冲岩席为主导，随后经历两期递进变形作用。第一期变形以板状劈理、轴迹北东向的等斜平卧褶皱轴面为典型特征，伴随绿片岩相（greenschist facies）矿物重结晶作用，相关构造还包括顶向SE运动的水平剪切带。第二期变形的构造倒向指向NW，表现为以S0与S1面理之上的F2开阔褶皱为轴面的褶劈理，局部伴随逆冲断层发育。第三期变形以北西向近直立断层及裂隙为特征，兼具左旋运动属性，此类构造通常被石英脉充填。
本次研究梳理的构造形迹与变质共生组合表明，绿片岩相条件下的地壳深部层次最强变形作用发生于F1期，该阶段通过等斜平卧褶皱、低倾角剪切带以及上盘向SE的运动，实现了显著的地壳缩短，整体隶属于薄皮构造体制（thin-skinned tectonic regime）。F2变形期强度相对较弱，呈现脆-韧性变形行为，通过近直立轴面的开阔褶皱与浅部地壳层次发育的逆冲断层实现轻微地壳缩短，其构造倒向指向亚马孙克拉通（Amazonian Craton）。第三期变形分布范围相对有限，地壳缩短主要通过起伏状近直立左旋断层得以调节。