Extensional reactivation of the Penninic frontal thrust 3 Myr ago as evidenced by U–Pb dating on calcite in fault zone cataclasite

Abstract

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 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.

How sensitive are intraplate inherited structures? Insight from the Cévennes Fault System (Languedoc, SE France)

“…Intraplate deformation may be studied through the definition of the stress state from structural measurements (Constantin et al 2002) or from geophysical measurements such as velocity wave anisotropy (Fagereng et al 2010). More recent methods of quantifying the amplitude of movements from low-temperature thermochronology (Leprêtre et al 2017) or the age of movements (U-Pb dating of syn-faulting calcites (Beaudoin et al 2018;Roberts et al 2020;Bilau et al 2021) allow us to go further and to propose a precise timing of intraplate deformation, in order to determine how and why older faults are reactivated, and what the impact of those faults is.…”

Glacial isostatic adjustment strain rate – stress paradox in the Western Alps, impact on active faults and seismicity

“…Results of the full CFS computation (local background stress + GIA perturbation) at 5 km depth for the northern Briançonnais extension system are summarized in Figure 6. Assuming that the seismicity is associated with the reactivation of the Penninic Frontal Thrust (Bilau et al, 2021;, the variations of obliquity of the stress tensor to the fault geometry results in negative CFS from ca. -50 MPa to -10 MPa for a standard friction µ' = 0.6 (Fig.…”

Glacial-isostatic-adjustment strain rate–stress paradox in the Western Alps and impact on active faults and seismicity

“…6. Assuming that the seismicity is associated with the reactivation of the Penninic Frontal Thrust (Bilau et al, 2021;, the variations in obliquity of the stress tensor to the fault geometry result in negative CFS from ca. −50 to −10 MPa for a standard friction µ = 0.6 (Fig.…”