Abstract The Itapirapua Shear Zone is a major NE-trending dextral transcurrent zone from the southern Ribeira Belt (Brazil), which was active during the Neoproterozoic Brasiliano-Pan African Orogeny, related with West Gondwana assembly. Macroscopic sigmoid-shaped blocks in its northernmost splay termination present a NE-trending subvertical tectonic foliation and subvertical stretching lineation suggesting a transpressional tectonics. This paper investigates this splay termination using integration of data from geological mapping, thermodynamic modeling, microtectonics, quartz c-axis fabrics, kinematic vorticity and finite strain analysis. Distinct rock assemblages are placed in tectonic contacts along the splay termination, including Statherian sillimanite-biotite paragneiss and granitic orthogneiss (Apiai-Mirim Complex), Statherian phyllites (Bairro dos Prestes Formation), Calymmian chlorite-epidote-actinolite metabasites (Fazenda Velha Suite), Ediacaran phyllites, metalimestone and quartzite (Itaiacoca Group) and Ediacaran granite (Tres Corregos Suite). Isochemical phase diagrams constrain synkinematic metamorphic conditions of 490–510 °C and 11–12.5 kbar (chlorite-biotite-phengite phyllite), 520–560 °C and 9–10 kbar (biotite-phengite phyllite) and 680–690 °C and 5–6 kbar (sillimanite-biotite paragneiss), with isothermal decompression paths. Microstructural evidence suggests that increase in metamorphic temperatures were accompanied by changes in the dominant deformation mechanism in quartz aggregates from subgrain rotation to grain boundary migration recrystallization. Significant contribution of dissolution-precipitation creep indicates water-abundant deformational conditions. Strain ellipsoids are oblate-shaped and present NE-trending subvertical XY planes and subvertical X-axes in agreement with field tectonic fabric. Quartz c-axis fabrics in XZ sections of the strain ellipsoid are dominated by symmetrical crossed girdles, while the fabrics present dextral asymmetry in horizontal YZ sections, consistent with macroscopic structural deflection, indicating a subvertical vorticity axis. Vorticity results indicate 48–59% of pure shear contribution. The data indicate that strain in the Itapirapua Shear Zone northern termination was accommodated by a pure shear-dominated transpression with vertical extrusion at mid-crustal conditions, which contrasts with a transtensional deformation in the southeastern block.

中文翻译：

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巴西里贝拉带 Itapirapuã 剪切带走滑断层张开中纯剪切主导的挤压和垂直挤压

摘要 Itapirapua剪切带是南里贝拉带（巴西）的一个主要的NE向右旋横流带，在新元古代巴西-泛非造山运动期间活跃，与西冈瓦纳大陆组装有关。宏观 S 形块体在其最北端的张开末端呈现出 NE 向的近垂直构造叶理和近垂直伸展线状结构，表明存在挤压构造。本文使用来自地质绘图、热力学建模、微构造、石英 c 轴结构、运动涡度和有限应变分析的数据的集成来研究这种张开终止。不同的岩石组合位于沿着张开终点的构造接触中，包括 Statherian 硅线石-黑云母副片麻岩和花岗岩正片麻岩（Apiai-Mirim Complex），Statherian 千枚岩（Bairro dos Prestes Formation）、Calymmian 绿泥石-绿帘石-阳起石偏铝质岩（Fazenda Velha Suite）、Ediacaran 千枚岩、metalimestone 和石英岩（Itaiacoca Group）和 Ediacaran 花岗岩（Tres Corregos Suite）。等化相图限制了 490-510 °C 和 11-12.5 kbar（绿泥石-黑云母-千枚岩）、520-560 °C 和 9-10 kbar（黑云母-六星石千枚岩）和 680-690 °C 的同运动变质条件和5-6 kbar（硅线石-黑云母片麻岩），具有等温减压路径。微观结构证据表明，变质温度的升高伴随着石英聚集体从亚晶旋转到晶界迁移再结晶的主要变形机制的变化。溶解-沉淀蠕变的显着贡献表明水丰富的变形条件。应变椭球体呈扁圆状，呈现 NE 向的垂直 XY 平面和垂直 X 轴，与现场构造结构一致。应变椭球 XZ 截面中石英 c 轴织物以对称交叉腰围为主，而水平 YZ 截面中织物呈现右向不对称，与宏观结构偏转一致，表明存在近垂直涡度轴。涡度结果表明 48-59% 的纯剪切贡献。数据表明，Itapirapua 剪切带北端的应变由在地壳中部条件下具有垂直挤压的纯剪切主导的挤压调节，这与东南块的拉伸变形形成对比。