Shear zone behavior is mainly controlled by deformation regime (brittle versus ductile), deformation temperature, strain rate and magnitude, and rheology of the deformed rocks. If a gradient of strain is established across a shear zone, softening phenomena can produce progressive localization of deformation in its core, resulting in the shear zone maintaining a constant thickness with increasing strain. In contrast, strain hardening processes may induce migration of deformation into the wall-rocks, causing an increase in shear zone thickness. In the Western Alps we have studied a NW-SE striking steeply dipping km-scale shear zone, the Ferriere-Mollières Shear Zone (FMSZ), that cross-cuts Variscan migmatites in the Argentera External Crystalline Massif. The shear zone is characterized by a deformation gradient, with strain increasing toward the center of the shear zone which we interpret to be associated with strain softening during Variscan retrograde metamorphism.

In this study, by combining structural and microstructural analyses with quartz fabric analysis, quartz palaeopiezometry and petrochronology, we have identified three main stages of shear zone development, with each stage characterized by specific age, temperature and deformation regime. Stage I occurred between ~340 Ma and ~330 Ma under a temperature range of ~610 - 590 °C with a prevalent (76%–65%) component of pure shear deformation; stage II occurred between ~ 330 and 320 Ma at temperatures between ~ 530 and 480 °C with a decrease in the component of pure shear (73%–49%); stage III developed from ~320 to 300 Ma under temperature conditions between ~ 500 and 420 °C with a prevalent component of simple shear (pure shear of 39%–31%).

The FMSZ is a new example of a strain-softening and long-lasting regional-scale shear zone, that may prove to be a useful natural study area for future research on processes operating in large-scale shear zones. We argue that the integration of multiple analytical techniques is essential in the study of such regional-scale shear zones and should be regarded as a standard approach.

剪切带的行为主要受变形状态（脆性与韧性）、变形温度、应变速率和幅度以及变形岩石的流变学控制。如果在剪切带上建立了应变梯度，软化现象会在其核心产生渐进的局部变形，导致剪切带随着应变的增加保持恒定的厚度。相反，应变硬化过程可能会导致变形迁移到围岩中，导致剪切带厚度增加。在西阿尔卑斯山，我们研究了 NW-SE 显着的陡倾公里级剪切带，即 Ferriere-Mollières 剪切带 (FMSZ)，它与 ​​Argentera 外部结晶地块中的 Variscan 混合岩横切。剪切带的特点是变形梯度，

在这项研究中，通过将结构和显微结构分析与石英组构分析、石英古压力测定法和岩石年代学相结合，我们确定了剪切带发展的三个主要阶段，每个阶段都具有特定的年龄、温度和变形机制。第一阶段发生在 ~340 Ma 和 ~330 Ma 之间，温度范围为 ~610 - 590 °C，纯剪切变形的主要成分 (76%–65%)；第二阶段发生在 ~ 330 到 320 Ma 之间，温度在 ~ 530 到 480 °C 之间，纯剪切分量减少（73%–49%）；在~500 到 420 °C 的温度条件下，阶段 III 从~320 Ma 发展到 300 Ma，主要成分为简单剪切（纯剪切为 39%~31%）。

FMSZ 是应变软化和持久的区域尺度剪切带的一个新例子，它可能被证明是一个有用的自然研究区域，可用于未来研究在大尺度剪切带中运行的过程。我们认为，多种分析技术的整合在此类区域尺度剪切带的研究中是必不可少的，应该被视为一种标准方法。