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.
Highlights
Dating of major tectonic inversions in orogens is generally achieved by indirect and relative dating, but rarely by the direct dating of fault-related minerals using absolute geochronometers
A Miocene age has been proposed for the onset of the extensional activation of the Penninic frontal thrust (PFT) based on apatite fission tracks (AFTs) dating on both sides of this major fault, i.e. in the Pelvoux external crystalline massif and in the Champsaur sandstones to the west and in the Briançonnais zone to the east (Tricart et al, 2001, 2007; Beucher et al, 2012)
Five U–Pb ages of calcite have been obtained in extensional fault structures connected to the PFT, giving two distinct groups of ages of 3.5 ± 0.5 Ma for the main deformation phase represented by the cataclasite calcite cement, cross-cut by later discrete phases represented by millimetrelarge veins dated from 2.6 ± 0.3 to 2.3 ± 0.3 Ma
Summary
Dating of major tectonic inversions in orogens is generally achieved by indirect and relative dating, but rarely by the direct dating of fault-related minerals using absolute geochronometers. The recent progress in U–Pb dating of carbonate using high-resolution laser ablation analyses (Roberts et al, 2020) allows us to directly date minerals formed during fault activity and to establish the age of tectonic phases by absolute radiometric dates (Ring and Gerdes, 2016; Goodfellow et al, 2017; Beaudoin et al, 2018). This method is especially well suited to disentangle the successive tectonic motions along a given tectonic structure. In the Western Alps, the Penninic frontal thrust, or PFT, represents a major thrust structure at lithospheric scale (e.g. Tardy et al, 1990; Mugnier et al, 1993; Zhao et al, 2015) that accommodated the main collisional phase during the Palaeogene–Neogene
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