Abstract. In the Western Alps, the Penninic Frontal Thrust (PFT) is the main crustal-scale tectonic structure of the belt. This thrust transported the high-pressure metamorphosed internal units over the un-metamorphosed European margin during the Oligocene (34–29 Ma). The PFT was later reactivated as an extensional detachment associated with the development of the High-Durance extensional fault system. This inversion of tectonic motions on a major tectonic structure has been widely emphasized as an example of extensional collapse of a thickened collisional orogen. However, the inception age of the extensional inversion remains unconstrained. Here, for the first time, we provide chronological constraints on the extensional motion of an exhumed zone of the PFT by applying U-Pb dating on secondary calcites from a fault zone cataclasite. The calcite cement of the cataclasite, formed after the main fault slip event, is dated at 3.4 ± 1.5 Ma. Cross-cutting calcite veins featuring the last fault motions are dated at 2.6 ± 0.3 and 2.3 ± 0.3 Ma. δ¹³C and δ¹⁸O fluid signatures derived from these secondary calcites suggest fluid percolation from deep-seated reservoir at the scale of the Western Alps. Our data give evidence that the PFT extensional reactivation is active since at least ~ 3 Ma, at the crustal scale and results from the westward propagation of the compressional deformation related to the exhumation of External Crystalline Massifs. In this context, the exhumation of the dated extensional fault is associated with the eastward translation of the seismogenic zone controlled by the deep rooting of the High-Durance fault system.

中文翻译：

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断层带加长石中方解石上的U-Pb定年证明了3 Ma以前的Pennicic额叶逆冲推张作用的扩展活化。

摘要。在西部阿尔卑斯山，Penninic额叶逆冲推力（PFT）是该带的主要地壳尺度构造结构。在渐新世（34-29 Ma）期间，该推力将高压变质的内部单元输送到了未变质的欧洲边缘。PFT后来作为与高耐用性延伸断层系统发展相关的延伸脱离而被重新激活。构造运动在主要构造结构上的反转已被广泛强调为增厚的碰撞造山带伸展塌陷的一个例子。但是，延伸倒置的起始年龄仍然不受限制。在这里，我们首次通过对断层带加成岩次生方解石施加U-Pb年代，对PFT掘出带的伸展运动提供了时间约束。在主要的断层滑动事件之后形成的该催化裂隙的方解石胶结物的日期为3.4±1.5 Ma。具有最后断层运动的横切方解石脉日期为2.6±0.3和2.3±0.3 Ma。δ¹³ C和δ ¹⁸ ö从这些二次方解石衍生流体签名建议在西方阿尔卑斯山的规模从深层储层流体渗滤。我们的数据表明，自地壳尺度起至少〜3 Ma以来，PFT伸展复活作用是活跃的，这是由于与外部结晶地块掘出有关的压缩形变向西传播的结果。在这种情况下，过时的伸展断层的发掘与高耐久性断层系统的深层根源控制的发震带的东移有关。