In the Western Alps, different shear zones acting at different depths have been investigated for explaining multistage exhumation of (U)HP units, and several exhumation models have been proposed for explaining present-day stacking of different tectonometamorphic units. This study aims to reconstruct the tectonic evolution of the Susa Shear Zone (SSZ), a polyphasic first-order shear zone, outcropping in the Susa Valley. The SSZ consists of a thick mylonitic zone, along which units characterized by different Alpine metamorphic P–T peaks are coupled. In the study area, the footwall of the SSZ mostly consists of oceanic units (i.e., Internal Piedmont Zone), which record eclogitic conditions, whereas the hanging wall consists of oceanic units (i.e., External Piedmont Zone), which record blueschist-facies conditions. These tectonic units were deformed during subduction- and exhumation-related Alpine history, throughout four main regional deformation phases (from D1 to D4), and were coupled along the SSZ, wherein two shearing events have been distinguished (T1 and T2). T1 occurred during early exhumation and was characterized by “apparent reverse” Top-to-E kinematics, whereas T2 occurred during late exhumation and was characterized by Top-to-W kinematics. Detailed fieldwork and structural analysis allowed us to describe the main features of the different deformation stages and define the deformation relative timing. As final result, we propose a four-step geodynamic model, focused on the different stages developed along the SSZ, from pre-T1 to syn-T2, showing the geometrical relationships between the tectonic units involved in the exhumation. The model aims at explaining the role of the SSZ in the axial sector of the Western Alps.

中文翻译：

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苏萨剪切带上的结构演化：一阶剪切带在变蛇绿岩单元（西阿尔卑斯山）的发掘中的作用

在西部阿尔卑斯山，已经研究了作用在不同深度的不同剪切带，以解释（U）HP单元的多阶段掘尸，并提出了几种掘尸模型来解释当今不同构造地貌单元的堆叠。这项研究旨在重建苏萨山谷露头的多相一阶剪切带苏萨剪切带（SSZ）的构造演化。SSZ由厚厚的麦粒状带组成，沿着该带，以不同的高山变质PT峰为特征的单元耦合。在研究区域中，SSZ的底盘主要由洋洋单元（即内皮埃蒙特区）组成，记录了渐近条件，而悬壁由洋洋单元（即外皮埃蒙特区）组成，其中记录了蓝片岩相条件。这些构造单元在俯冲和回火相关的高山历史中经历了四个主要区域变形阶段（从D1到D4），并沿SSZ耦合，其中已经区分了两个剪切事件（T1和T2）。T1发生在早期发掘期间，其特征是“表观反向” Top-to-E运动学，而T2发生在发掘晚期，并具有Top-to-W运动学。详细的现场工作和结构分析使我们能够描述不同变形阶段的主要特征，并定义变形的相对时间。作为最后的结果，我们提出了一个四步的地球动力学模型，重点研究了沿SSZ从T1到S2到T2的不同阶段，显示了掘尸过程中构造单元之间的几何关系。