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 non-metamorphosed European margin during the Oligocene (34–29 Ma). Following the propagation of the compression toward the European foreland, the PFT was later reactivated as an extensional detachment associated with the development of the High Durance extensional fault system (HDFS). This inversion of tectonic displacement along 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 and veins of the cataclasite formed after the main fault slip event, at 3.6 ± 0.4–3.4 ± 0.6 Ma. Cross-cutting calcite veins featuring the last fault activity are dated at 2.6 ± 0.3–2.3 ± 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 provide evidence that the PFT extensional reactivation initiated at least ∼ 3.5 Myr ago with a reactivation phase at ∼ 2.5 Ma. This reactivation may result from the westward propagation of the compressional deformation toward the external Alps, combined with the exhumation of external crystalline massifs. In this context, the exhumation of the dated normal faults is linked to the eastward translation of the HDFS seismogenic zone, in agreement with the present-day seismic activity.

中文翻译：

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

在西部阿尔卑斯山，佩尼尼奇前冲（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。随着压缩向欧洲前陆的传播，PFT随后被重新激活为与高杜兰斯伸展断裂系统（HDFS）的发展相关的伸展脱离。沿主要构造结构的构造位移的这种反演已被广泛强调为增厚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，第一次 我们通过对断层带白云母的次级方解石应用U-Pb年代，对PFT掘出带的伸展运动提供了时间约束。主断层滑动事件发生后，形成了方解石的方解石胶结物和脉状结构，为3.6 ±  0.4–3.4  ±  0.6 Ma。具有最后断层活动的横切方解石脉日期为2.6  ±  0.3–2.3  ±  0.3 Ma。δ ¹³ 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。我们的数据提供的证据表明，PFT伸展重新启动至少〜  3.5秘耳前与在活化期〜 2.5毫安 这种重新活化可能是由于压缩变形向西传播到外部阿尔卑斯山，再加上外部晶体地块的掘出。在这种情况下，与目前的地震活动相一致，过时的正断层的发掘与HDFS地震发生带的东移有关。