# Timing of syn-orogenic extension in the Western Alps
revealed by calcite U-Pb and hematite (U-Th)/He
dating

Antonin Bilau, Yann Rolland, Stéphane Schwartz, Cécile Gautheron, Thierry Dumont, Dorian Bienveignant, Benjamin Brigaud, Nicolas Godeau, Abel Guihou, Pierre Deschamps, et al.

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To cite this version:

Antonin Bilau, Yann Rolland, Stéphane Schwartz, Cécile Gautheron, Thierry Dumont, et al. Timing of syn-orogenic extension in the Western Alps revealed by calcite U-Pb and hematite (U-Th)/He dating. Geoscience Frontiers, 2025, 16 (2), pp.101969. 10.1016/j.gsf.2024.101969. hal-04941533

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Research Paper

# Timing of syn-orogenic extension in the Western Alps revealed by calcite U-Pb and hematite (U-Th)/He dating

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Antonin Bilau<sup>a,b,*</sup>, Yann Rolland<sup>a,b</sup>, Stéphane Schwartz<sup>b</sup>, Cécile Gautheron<sup>b,c</sup>, Thierry Dumont<sup>b</sup>, Dorian Bienveignant<sup>b</sup>, Benjamin Brigaud<sup>c</sup>, Nicolas Godeau<sup>d</sup>, Abel Guihou<sup>d</sup>, Pierre Deschamps<sup>d</sup>, Xavier Mangenot<sup>d,e</sup>, Marianna Corre<sup>b</sup>, Rosella Pinna-Jamme<sup>c</sup>, Nathaniel Findling<sup>b</sup>

## ARTICLE INFO

**Article history:**

Received 13 March 2024

Revised 28 September 2024

Accepted 14 November 2024

Available online 27 November 2024

Handling Editor: S. Glorie

**Keywords:**

Western Alps

Briançonnais zone

Hematite (U-Th)/He

U-Pb calcite

Clumped isotopes

Syn-orogenic extension

## ABSTRACT

Understanding fault activity over time provides valuable insights for reconstructing the tectonic history of an orogen, assessing seismological risks and understanding mineralization processes. In the Western Alps, one of the main controversies in existing tectonic models is the understanding of syn-orogenic extension. Seismological evidence shows widespread extensional deformation related to the reactivation of major lithospheric structures, such as the Penninic Frontal Thrust (PFT). However, the onset age and origin of extension are still debated due to the lack of suitable geochronological data. Fault hematite and calcite geochronology as well as clumped isotope data can be used to relate fluid regimes to fault activity. The analysis of calcite brecciae from extensional faults above the PFT shows that two distinct fluid regimes were present. The first regime, occurring before 2 Ma is associated with upwelling of deep fluids and is recorded by fault calcite at a temperature > 110°C. The second fluid regime is characterized by a meteoric signature and temperatures around 36 °C, representing crystallization since 2 Ma. This study presents a new model for the Miocene tectonic history of the Western Alps that combines (U-Th)/He and U-Pb geochronology on fault hematite (13.3 ± 0.8 to < 0.8 Ma) and calcite (5.3 ± 0.6 Ma). Results demonstrate a progression of extensional fault activity from east to west, from the Middle Miocene (ca. 13 Ma) to the Quaternary. The onset of extension in the inner part of the belt coincides with the development of the fold and thrust belt in the western Alpine foreland. Our new model proposes that extension occurs in the hanging wall of a large top-to-the-west thrust, known as the Alpine Frontal Thrust. This thrust, located to the west of the External Crystalline Massifs gives rise to their uplifting and extension at the rear.

© 2024 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. on behalf of China University of Geosciences (Beijing). This is an open access article under the CC BY license ([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)).

## 1. Introduction

The tectonic evolution of fault networks can be constrained by using geochronological methods on minerals formed during fault activity. Among these, calcite and hematite are classically observed on surfaces of tectonic structures (fault veins, fibers, striae etc.) and can be dated by U-Pb on calcite (Roberts et al., 2017) or by (U-Th)/He dating on hematite (e.g., Wernicke and Lippolt, 1994;

Evenson et al., 2014; McDermott et al., 2017, 2021; Ault, 2020). These methods can give crystallization ages related to the development of brittle deformation during fault activity (e.g., Roberts and Walker, 2016; Beaudoin et al., 2018; Parizot et al., 2021; Roberts and Holdsworth, 2022; Bilau et al., 2023a, b; Lacombe and Beaudoin, 2024). Additionally, $\delta^{18}$O, $\delta^{13}$C isotopes and $\Delta_{47}$ clumped isotopes on calcite provide information about the composition and temperature of the precipitating fluids (e.g., Bonifacie et al., 2017; Pagel et al., 2018).

In this study, we combine geochronological methods with stable isotope analyses to document the relationship between fluid and fault history. This study focusses on the High-Durance Fault

<footnote><sup>a</sup>EDYTEM, Université Savoie Mont Blanc, CNRS, UMR 5204, Le Bourget du Lac, France</footnote>

<footnote><sup>b</sup>ISTerre, Université Grenoble Alpes, USMB, CNRS, IRD, UGE, Grenoble, France</footnote>

<footnote><sup>c</sup>GEOPS, CNRS, Université Paris-Saclay, 91405 Orsay, France</footnote>

<footnote><sup>d</sup>Aix-Marseille Université, CNRS, IRD, INRAE, CEREGE Collège de France, Aix-en-Provence, France</footnote>

<footnote><sup>e</sup>Caltech, Geological and Planetary Sciences, Pasadena, CA, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: antonin.bilau@kit.edu (A. Bilau), antonin.bilau@kit.edu (A.</footnote>
System, a significant transtensional fault complex in the Briançonnais zone of the Western Alps (Fig. 1), which is currently active (Sue and Tricart, 2003; Sue et al., 2007). The extensional faults of this complex cross-cut and partly reactivate some of the previous crustal-scale Alpine structures like the Penninic Frontal Thrust (PFT). Extensional activity is confined to the core of the Alpine belt, which is still undergoing uplift, raising questions about the timing of the extensional deformation in a collisional context (e.g., Tricart et al., 2006). Some authors have proposed that the presence of extension is explained by an extensional collapse after a phase of collisional thickening (e.g., Selverstone, 2005). However, in the case of the Western Alps, the current strain field is dominated by a strike-slip context with a strong partitioning of deformation highlighted by compression on their forelands (Malusà et al., 2017, 2021; Mathey et al., 2021; Schwartz et al., 2024). Extension appears to be syn-orogenic and contemporaneous with strike-slip and compression tectonics in the Alpine foreland. However, the onset of extension remains temporally unconstrained, which leads to controversial interpretations (Larroque et al., 2009; Bauve et al., 2014). Further, driving forces for syn-orogenic extension are still a matter of debate (Sternai et al., 2019). The driver of the extensional component of the strain field could result from various processes such as a decrease in compressive stress (Selverstone, 2005), slab breakoff (von Blanckenburg and Davies, 1995), crustal overcompensation or isostatic rebound due to erosion or glacial melting (e.g., Champagnac et al., 2007; Sternai et al., 2019), or mantle indentation of the orogenic wedge and Adria counterclockwise rotation (Schwartz et al., 2024).

In the Briançonnais zone, the extensional domain is localized to the east of the Pelvoux Massif (Tricart et al., 2001; Lardeaux et al., 2006). This extension could result from a local accommodation at the back of a crustal-scale thrust related to the propagation of the belt towards its foreland in a thick-skinned mode (Bilau et al., 2023a). It would therefore be the result of a deep tectonic

process rather than a surface re-adjustment related to erosional processes (e.g., Vernant et al., 2013). However, it is still difficult to discuss the potential causality of thick-skinned tectonics trigger and the development of syn-orogenic extension in the Western Alps as the age of extension initiation is still unconstrained. The timing of brittle deformation stages is primarily constrained by indirect dating techniques, such as fission track thermochronology (e.g., Tricart et al., 2007).

The objective of this study is to provide definitive temporal constraints on the initiation of extensional processes that have occurred during the tectonic reactivation of the PFT. Additionally, the aim is to examine the chronological relationship between these processes and the development of the frontal Alpine fold and thrust belt and the exhumation of the External Crystalline Massifs (ECM). In this paper, we provide geochronological constraints based on U-Pb calcite and (U-Th)/He hematite data within the Briançonnais zone, which allow to discuss the onset and propagation of extension within the High-Durance Fault System. In addition, clumped isotope analysis performed on calcite from the same dated sites give insights into the related regional fluid circulation context (e.g., Smeraglia et al., 2022; Bilau et al., 2023a). The new data are further discussed in the structural context of the Western Alps and its European foreland.

## 2. Geological setting

The western Alpine belt resulted from the convergence and collision of the European and Adriatic plates (e.g., Tricart, 1984) since Cretaceous time. The internal zones (Fig. 1) are characterized by High-Pressure – Low-Temperature (HP-LT) metamorphic conditions related to the subduction of the paleo-distal European margin of the Briançonnais zone and oceanic-derived units of the Piedmont zone (Schwartz et al., 2009; Dumont et al., 2022). The Briançonnais zone is composed of Mesozoic and Paleozoic

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** (a) Location of the Western European Alps. (b) Geological map coupled to a DEM of the Western Alps of the study area (red rectangle, for closer map, see Fig. 2). The main faults structuring the foreland and the inner part of the belt are indicated. The Briançonnais zone is crosscut by the High-Durance Fault System (HDFS), which develops along the Penninic Frontal Thrust. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
sedimentary units derived from the thinned European continental margin (Manatschal, 2004). This zone corresponds to a stack of exhumed and folded metamorphic nappes with opposite vergence on either side (Dumont et al., 2022).

The external zones comprise the sub-greenschist facies (e.g. Frey and Ferreiro Mâhlmann, 1999) European Mesozoic sedimentary cover and its Paleozoic basement corresponding to the ECMs. Collisional dynamics led to a major lithospheric-scale tectonic structure, the PFT (e.g., Tardy et al., 1990), which was active in Late Eocene to Oligocene times (Dumont et al., 2012, 2022; Bellahsen et al., 2014). The PFT accommodated the westward thrusting of the internal over the external zones at 34–31 Ma (Schmid and Kissling, 2000; Ceriani et al., 2001; Ceriani and Schmid, 2004; Lardeaux et al., 2006; Simon-Labric et al., 2009). Ambient noise Vs tomography (Nouibat et al., 2022; Schwartz et al., 2024) highlights the deep structure of the chain. To the East of the PFT, in the internal zones representing a subduction wedge, the crustal geometry shows the presence of a European continental slab that was locally subducted to more than 80 km beneath the Adria plate in the SW part of the Alpine arc. The Moho morphology is characterized by two mantle indenters located above the subducted European plate at different depths, which appear to control the locus of active deformation. The rigid nature of the Adriatic mantle explains the localization of brittle deformation that is transferred towards the upper crust. The strain field partitioning results in a combination of strike-slip with either shortening or extension controlled by the displacements imposed by the current NW/SE convergence associated with the anticlockwise rotation of Adria (see Schwartz et al., 2024 for details). In this context, the High Durance Fault System corresponds to a cluster of normal faults reactivating

the PFT and partly cross-cutting it at depth (Sue et al., 2007). In detail, this fault cluster is bounded by two main branches on both sides of the Briançonnais zone. To the east, the Clarée Fault Zone (CFZ) and to the west, along the PFT, the High Durance fault Zone (HDZ). Near Tournoux (Fig. 2), the HDZ branch on the PFT, while more to the south, at Plan de Phazy, the HDZ crosscuts the PFT (Sue and Tricart, 2003; Thouvenot and Fréchet, 2006; Sue et al., 2007). Present-day seismicity is distributed along lineaments within the High Durance Fault System and mostly clusters at shallow depths of 3 to 8 km, where the faults are structurally connected to the PFT (Sue and Tricart, 2003; Thouvenot and Fréchet, 2006; Sue et al., 2007). Focal mechanisms show a combination of strike-slip and extensional components consistent with a regional transtensional tectonic regime (Sue et al., 2007). Ongoing deformation is highlighted by observed GPS motions (Walpersdorf et al., 2018; Mathey et al., 2021, 2022). The analysis of GPS velocity profiles highlights zones of extension in the center of the belt localized in the Briançonnais zone, and shortening in the foreland (Walpersdorf et al., 2018). In the ECMs, the main observed component corresponds to vertical motions of 0.5 to 3.0 mm per year.

The onset of compressive tectonic activity along the PFT is constrained to 34 Ma based on syn-kinematic Ar-Ar ages on phengite, U-Pb ages on allanite and on reset zircon (U-Th)/He ages in the PFT fault zone (Simon-Labric et al., 2009; Cenki-Tok et al., 2014; Bellanger et al., 2015; Maino et al., 2015). Apatite fission track (AFT) ages from the Briançonnais zone range from 31 Ma to 22 Ma (Fig. 2; Tricart et al., 2001, 2007) and suggest that the exhumation of the Briançonnais zone is related to reverse faulting along the PFT. The younger AFT ages in the Briançonnais zone between 10 and 15 Ma are related to the circulation of hot fluids

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Geological map coupled to a DEM of the study area with location of the High-Durance Fault System and the Penninic Frontal Thrust (PFT), modified after Tricart (2004), with its eastern (Clarée Fault Zone, CFZ) and western branch (High-Durance Zone, HDZ). The orange stars indicate calcite U-Pb dating sites and red stars indicate hematite (U-Th)/He dating sites. The main regional hot springs are indicated by grey circles. Numbers in white retangles correspond to AFT ages (in Ma) from Tricart et al. (2007) and Seward et al. (1999). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
circulating along the HDZ, with temperatures exceeding 80 °C (Fig. 2, Tricart et al., 2007).

The onset of extension in the Briançonnais zone is still poorly constrained. Several authors proposed that the transition from compression to extension took place in the age range of ca. 15 Ma to 5 Ma, on the basis of the AFT ages obtained in the PFT footwall (Fig. 2) related to the ECMs exhumation (Tricart et al., 2001, 2006, 2007; Schwartz et al., 2007). Calcite U-Pb dating on the HDZ highlights several brittle events between 3.6 ± 0.4 Ma and 2. 3 ± 0.2 Ma, and provides a minimum age for the onset of extension within the High-Durance Fault System (Bilau et al., 2021).

# 3. Methods

## 3.1. Sampled sites along the High-Durance Fault System

Three sites were sampled from east to west in the Briançonnais zone along the High-Durance Fault System in order to collect hematite and calcite formed during fault activity: the Clarée, the Puy-Chalvin and the Tournoux sites. Those samples complement the U-Pb and Δ₄₇ data from the same area by Bilau et al. (2021). Sample names, mineralogical characteristics and locations are reported in Table 1. Structural measurements are displayed in the Supplementary Data 1. Additional mineralogical and major-trace element analyses of the samples are reported in the Supplementary Data 2 and 3.

### 3.1.1. Clarée site

At the Clarée site, the CFZ (Fig. 3a) can be traced at the surface across Bathonian-Callovian bioclastic limestone locally karstified and filled by iron-rich red micrite. The sample fault zone is highlighted by a thick reddish zone of > 100 m length (Fig. 3a). Bedding is tilted by ~ 40° to the west (Fig. 3b) and the paleokarst formation occurred after folding. The fault system runs NNW-SSE with a dip of 80° – 50° to the southwest. The fault exhibits strike-slip to normal slip and affects local karst fillings that are located in the core of a syncline. Motions along the CFZ fault show overprinting relationships with alternating pure strike-slip and extensional stress regimes (Fig. 3b). The fault zone runs along the valley for several kilometers and has a width of several tens of meters, with numerous striated planes and tectonic breccias. It cuts through paleokarst fillings and has remobilized some of the iron-rich material. Hematite and calcite mineralization has been observed and both sampled in two habitus (veins and striated planes). The vertical veins branch into the fault planes, and are thus kinematically coherent to the same extensional fault motions. In the veins, calcite systematically crystallizes before hematite. Three hematite samples have been selected for hematite (U-Th)/He (H/He) analysis. The calcite samples taken from fault planes and veins proved unsuitable for dating.

### 3.1.2. Puy-Chalvin site

The samples were collected along an extensional fault affecting Lower Triassic quartzites in the central part of the Briançonnais zone, located on the eastern edge of the HDZ (Fig. 2, Puy-Chalvin site). Fault planes are vertical to steeply dipping to the SE. The fault strike is oriented SW-NE, with striations showing normal and partly strike-slip motions (down dip to 45° towards NE, Fig. 4). These fault geometry and kinematics correspond to the commonly observed features of extensional faults within the High-Durance Fault System (e.g., Mathey et al., 2020). In this site, the present study has not yielded any results that would allow for the dating of hematite. However, the dating of calcite has been successful.

### 3.1.3. Tournoux site

On the western edge of the Briançonnais zone, along the HDZ, in the hanging wall of the PFT (internal Alps), the Tournoux site (Figs. 2, 5a) was investigated as a complement to previous work by Bilau et al. (2021) from the same location (in outcrop 1, Fig. 5). Calcite-filling fault samples were selected for Δ₄₇ analysis in relation to the previous samples from Bilau et al. (2021) and the obtained U-Pb ages of 3.6 ± 0.4 Ma and 2.3 ± 0.2 Ma (Table 1). To the east of Tournoux site (outcrop 2), close to the Col de la Pousterle and 400 m away from Tournoux outcrop 1, samples were retrieved from an outcrop of a normal fault, which roots down into the PFT (Fig. 5a). The fault offset is about 300 m and juxtaposes brecciated whitish Triassic quartzites next to Upper Cretaceous calcschists (Sue and Tricart, 1999, Fig. 5b). The fault zone contains a breccia with clasts of stretched Triassic dolomitic limestones cemented by a first generation of calcite (Fig. 5c). This breccia is cut by hematite veins several millimetres wide. The hematite was striated by later fault plane activity and underwent recrystallization. Twenty samples of striated fault planes (centimetre-long) were collected, composed of layers of hematite interlayered with phyllosilicates (Fig. 5d). A first relative chronology at the scale of the outcrop can be established (Fig. 5c) with (1) calcite precipitation, (2) crosscut by hematite veins, and (3) striated hematite on the fault plane. Similar to the Clarée site, hematite is found to be the most recent precipitation product.

## 3.2. Scanning electron microscope imaging

Hematite samples of cm-scale were photographed under a binocular microscope and mounted for scanning electron microscope (SEM) analysis. For most samples, observations perpendicular to fault striation were made using the SEM Vega 3 Tescan of ISTERre (University Grenoble Alpes, France). BSE and EDS where used to identify the most Fe-rich zones and then a detailed image was made to characterize the crystallite sizes. Acquisition parameters were optimized on each picture in order to reach a ~ 20 nm resolution. Crystallite sizes of hematite were measured

<table><caption>**Table 1**
Sample coordinates and information.</caption><thead><tr><th>Briançonnais zone</th><th>Name</th><th>Latitude (°N)</th><th>Longitude (°E)</th><th>Elevation (m)</th><th>Sample type</th><th>Analyses done</th><th>Reference</th></tr></thead><tbody><tr><td>East</td><td>Clarée</td><td>44°55′27.52″</td><td>6°39′44.34″</td><td>1815</td><td>hematite fault and vein</td><td>mineralogy, geochemistry, (U-Th)/He</td><td>this study</td></tr><tr><td>Central</td><td>Puy-Chalvin</td><td>44°52′36.23″</td><td>6°34′44.17″</td><td>1546</td><td>calcite fault</td><td>calcite U-Pb,</td><td>this study</td></tr><tr><td rowspan="2">West</td><td>Tournoux outcrop 1</td><td>44°48′29.75″</td><td>6°30′31.38″</td><td>1910</td><td>calcite fault</td><td>Δ<sub>47</sub></td><td>this study</td></tr><tr><td>Tournoux outcrop 2</td><td>44°47′55.70″</td><td>6°30′53.30″</td><td>1641</td><td>hematite fault and vein</td><td>calcite U-Pb<br>mineralogy, geochemistry, (U-Th)/He</td><td>Bilau et al. (2021)<br>this study</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td>calcite fault</td><td>Δ<sub>47</sub></td><td>this study</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** (a) General view to the western side of the Clarée Valley. Blue lines correspond to the main bedding highlighting a syncline within the Upper Trias (Ts) and Middle Jurassic (jm) rocks. White lines correspond to normal faults. These faults (in black) and the average bedding (in blue) are displayed in the Schmidt stereogram (lower hemisphere). (b) Outcrop within the karstified limestones crosscut by normal faults related to the CFZ. The faults and the corresponding striae are shown in the Schmidt stereogram. Fault plane data are available in the Supplementary Data 1. (c) Close-up of a sampled hematite striated fault plane. (d) Views of calcite and hematite veins showing that hematite crystallized after calcite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

on more than twenty images and mean values are reported in **Supplementary Data 4**.

### 3.3. Hematite (U-Th)/He geochronology

H/He dating was undertaken at the GEOPS laboratory (University Paris Saclay, France). Centimetre-scale hematite vein and striated hematite plane samples (Clarée: 2 samples, Tournoux: 17 samples) were selected for (U-Th)/He dating based on color and luster as a proxy for the highest amount of hematite. These samples were fragmented in ~500 µm size aliquots in order to compensate the He ejection distance (Ketcham et al., 2011). Six aliquots from Clarée and 53 for Tournoux site were analysed. Whenever possible, several aliquots from the sample sampling site were retrieved and a distance from the fault striated plane was estimated. Hematite grains were cleaned with ethanol in an ultrasonic bath and by additional analyses on the same sample it was checked that no He loss was caused by the treatment. Aliquots were weighed using a Mettler Toledo balance and their sizes were measured under a binocular microscope. Aliquots of 200–500 µm with weights ranging from 150 to 1020 µg were selected and He was extracted at a temperature < 900 °C for 30 min to avoid volatilization of U during degassing (Danišík et al., 2013; Hofmann et al., 2020). The He, U, Th and Sm contents were obtained using protocols described in Allard et al. (2018) and Gautheron et al. (2021), and concentration obtained using the aliquote measured weight. Error on He is currently < 2% but could get up to 10% depending on He content. U, Th and Sm contents are < 2%, and error on uncorrected H/He age is ranging from 6%

to 10%. Accuracy of the procedure is monitored by the analysis of Durango apatite and an internal goethite standard samples. A 5% He loss correction was applied to the (U-Th)/He age with an 10% error added to the analytical error to include He loss associated with crystallite size heterogeneity and phase mixing within the samples (Heller et al., 2022). All data are listed in **Supplementary Data 4**.

### 3.4. Calcite U-Pb geochronology

In-situ U-Pb calcite analyses were carried out at the CEREGE (Centre Européen de Recherche et d'Enseignement des Géosciences de l'Environnement, Aix-en-Provence, France) using a Thermo Fisher Scientific, Element XR, and a 193 nm laser wavelength, 0.8 to 1.4 J·cm⁻² fluence and 10 Hz repetition rate. The carrier gas was composed of 100% He 5.0 – 0.9 L·min⁻¹, Ar make-up gas ~ 1 L·min⁻¹ combined using a Y-piece ~ 50 cm before the connection to the injector. The laser spot size was 150 µm and firing for 20 s lead to a ~ 30 µm downhole (see Bilau et al., 2023a for further details). A NIST-614 glass standard was used as primary reference material for drift and Pb isotopes; WC-1 carbonate served as a reference material for matrix matching of ²⁰⁶Pb/²³⁸U and B6 as secondary material (Roberts et al., 2017; Pagel et al., 2018; Brigaud et al., 2021). For data processing, raw intensities and baseline correction were made with the Iolite 3 baseline DRS (Paton et al., 2011). Instrumental drift based on NIST-614 analyses (Woodhead and Hergt, 2001), Pb isotopes composition and ²⁰⁶Pb/²³⁸U are calculated using an in-house Python code. Tera-Wasserburg plots, calculation of intercept ages and initial Pb compositions were made
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Photographs and structural data of the calcite fault breccia dated by U-Pb on calcite at the Puy-Chalvin site. (a) Photograph of normal faults cutting through the Triassic quartzites. The schistosity orientation (S1) is shown by the cyan line and the sense of shear are indicated by the white arrows. Structural data (schistosity S1 in blue and faults in black) are plotted on a Schmidt stereogram (lower hemisphere). (b) Example of the sampled fault zone showing a cataclasite with calcite cement (corresponding to the dated sample in Fig. 8) and slickensides. (c) Striated fault plane mineralized by calcite and showing tectoglyphs indicative of superimposed normal and partly transcurrent slip. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

using IsoplotR (model-1) (Vermeesch, 2018). Ages are quoted with 2σ absolute error with propagation of WC-1 2.51% age error by quadratic addition (Roberts et al., 2017). Excess variance within the reference material was considered and systematic uncertainties included the age uncertainty of reference material. Data are reported in Supplementary Data 5.

### 3.5. *Stable isotope analysis*

Two brecciated calcite samples from the Tournoux outcrop 1 and one sample from outcrop 2 were selected for clumped isotopes ($\Delta_{47}$) analyses (Table 2). Thirty milligrams of pure calcite was retrieved using a Dremel on each of the three samples in order to
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** (a) General view of the Tournoux 1 and 2 sampling sites in the hanging-wall of the Penninic Frontal Thrust. The orange star to the left refers to the older faults dated on breccia calcite (Tournoux outcrop 1, 3.6 ± 0.4 Ma to 2.3 ± 0.2 Ma, errors = 2σ), while the red star to the right refers to Tournoux outcrop 2 with the younger fault (<2 Ma) passing across the Pousterle Pass. Cs: Late Cretaceous calcschists; Tm: Middle Triassic dolomitic limestones; TI: Lower Triassic sandstones. White lines correspond to High-Durance Zone normal faults. (b) Outcrop view of the Tournoux outcrop 2, the normal fault is highlighted by the tectonic contact of Upper Cretaceous calc-schists on top of Triassic quartzites. The orientation of the fault planes and related striae are plotted in the Schmidt stereogram (lower hemisphere). Measured faults show conjugate orientations with a consistent normal (and minor strike-slip) motion deduced from striae and slickenside relationships. The orientation of the Upper Cretaceous schistosity is shown in blue, and is close to horizontal (S1 parallel to S0). (c) Fault breccia showing relative cross-cutting relationships of a first generation of calcite cementing darker clasts of the host rock, intersected by a hematite vein and then striated by the activity of the fault plane coated with hematite. (d) Cross-section view (CSV) of the hematite-bearing fault plane sample showing multiple bands of hematite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

extract and analyse CO₂ at CalTech (USA). Each Δ₄₇ analysis was replicated. Eight milligrams of calcite powder were placed in a sil- ver capsule and dissolved under static vacuum in a bath of phos- phoric acid for 20 min at 90°C. The produced CO₂ was collected via a U-shaped trap at a temperature of –196 °C. The CO₂ was then transported with a helium flux through a Porapak Q 120/80 mesh separation column at –20°C. After purification, the 44 – 49 masses of CO₂ were measured with a Thermo Scientific MAT 253 Mass Spectrometer. The δ¹³C and δ¹⁸O isotope ratios were also measured via gas standards analysis and calibrated against PDB standard. The raw data were corrected for fractionation by acid dissolution at 90 °C with a fractionation factor of 1.00811 (Swart et al., 1991) as well as instrumental drift, and all results are expressed in the inter-

Bonifacie et al., 2017). During the analysis sessions, 8 samples of Carrara marble showed Δ₄₇⁻CDE$_{S25}$ results of 0.394‰ ± 0.012‰ (1σ), and 10 samples of carbonate TV04 showed Δ₄₇⁻CDE$_{S25}$ results of 0.653‰ ± 0.017‰ (1σ). These results correspond to accepted values for these standards: Δ₄₇⁻CDE$_{S25}$ = 0.655‰ for TV04 and Δ₄₇⁻CDE$_{S25}$ = 0.405‰ for Carrara (Mangenot et al., 2024). The error related to the standards was used (± 0.014‰, 1σ) for samples with- out 3 replicates. Finally, the corrected Δ₄₇ measurements were converted to temperature using the equation of Bonifacie et al. (2017) and results are reported in Table 3. Assuming that this tem- perature is a crystallization temperature and that the system remained closed, the δ¹⁸O signature of the calcite mineralizing fluid is calculated using the Kim and O'Neil (1997) equation (Table 3).
<caption>**Table 2**</caption>

Clumped isotope results on calcite from Tournoux site tectonic breccia and calculated temperatures. $\Delta_{47-CDES90}$ correspond to Carbon Dioxide Equilibrium Scale with acid digestion at 90°C, SMOW = Standard Mean Ocean Water, VPDB = Vienna Pee Dee Belemnite. U-Pb calcite ages for FP18-2B and FP18-3A are from Bilau et al. (2021).

<table><caption>**Table 3**</caption><thead><tr><th>Location</th><th>Name</th><th>U-Pb age</th><th>δ<sup>18</sup>O calcite (SMOW)</th><th>δ<sup>13</sup>C calcite (VPDB)</th><th>Δ<sub>47</sub> CDES90</th><th>1σ Δ<sub>47</sub></th><th>Δ<sub>47</sub> min</th><th>Δ<sub>47</sub> max</th><th>T(°C)</th><th>T<sub>min</sub>(°C)</th><th>T<sub>max</sub>(°C)</th></tr></thead><tbody><tr><td rowspan="4">Tournoux outcrop 1</td><td rowspan="2">FP18-2B</td><td rowspan="2">3.4 ± 1.4 Ma</td><td>8.70</td><td>-0.09</td><td>0.41</td><td rowspan="4">0.014</td><td rowspan="4">0.379</td><td rowspan="4">0.407</td><td rowspan="4">125</td><td rowspan="4">115</td><td rowspan="4">135</td></tr><tr><td>8.32</td><td>-0.10</td><td>0.38</td></tr><tr><td rowspan="2">FP18-3A</td><td rowspan="2">2.3 ± 0.2 Ma</td><td><strong>8.51</strong></td><td><strong>-0.10</strong></td><td><strong>0.39</strong></td></tr><tr><td>9.10</td><td>-1.46</td><td>0.39</td></tr><tr><td rowspan="4">Tournoux outcrop 2</td><td rowspan="4">calcite cement</td><td rowspan="4">no data</td><td>9.11</td><td>-1.44</td><td>0.38</td><td rowspan="4">0.007</td><td rowspan="4">0.376</td><td rowspan="4">0.389</td><td rowspan="4">133</td><td rowspan="4">128</td><td rowspan="4">138</td></tr><tr><td><strong>9.11</strong></td><td><strong>-1.45</strong></td><td><strong>0.38</strong></td></tr><tr><td>13.75</td><td>-1.18</td><td>0.57</td></tr><tr><td>13.77</td><td>-1.20</td><td>0.56</td></tr><tr><td rowspan="4">Tournoux outcrop 2</td><td rowspan="4">calcite cement</td><td rowspan="4">no data</td><td>13.86</td><td>-1.26</td><td>0.57</td><td rowspan="4">0.003</td><td rowspan="4">0.564</td><td rowspan="4">0.570</td><td rowspan="4">36</td><td rowspan="4">35</td><td rowspan="4">37</td></tr><tr><td><strong>13.79</strong></td><td><strong>-1.22</strong></td><td><strong>0.57</strong></td></tr><tr><td>13.77</td><td>-1.20</td><td>0.56</td></tr><tr><td>13.86</td><td>-1.26</td><td>0.57</td></tr></tbody></table>

Calculated δ¹⁸O signatures of the calcite mineralizing fluid.

<table><thead><tr><th rowspan="2">Location</th><th rowspan="2">Name</th><th colspan="5">δ<sup>18</sup>O<sub>fluid</sub> %e (SMOW) (Kim and O'Neil, 1997)</th></tr><tr><th>1000ln(a)</th><th>a</th><th>δ<sup>18</sup>O<sub>fluid</sub> %e (SMOW)</th><th>δ<sup>18</sup>O<sub>fluid</sub> min</th><th>δ<sup>18</sup>O<sub>fluid</sub> max</th></tr></thead><tbody><tr><td rowspan="2">Tournoux outcrop 1</td><td>FP18-2B</td><td>12.92</td><td>1.013</td><td><strong>-4.44</strong></td><td>-5.58</td><td>-3.27</td></tr><tr><td>FP18-3A</td><td>11.99</td><td>1.012</td><td><strong>-2.92</strong></td><td>-3.49</td><td>-2.35</td></tr><tr><td>Tournoux outcrop 2</td><td>calcite cement</td><td>25.86</td><td>1.026</td><td><strong>-12.08</strong></td><td>-12.26</td><td>-11.91</td></tr></tbody></table>

# 4. Results

## 4.1. Petrology and microscopical analysis

The petrological microcrystalline structure of the hematite-rich samples from the Clarée and Tournoux sites were observed by optical and SEM imagery. Fig. 6 shows optical and SEM images of veins and striated hematite planes at both macro and micro scales in representative samples. The vein and fault plane samples taken from the Clarée site exhibit botryoidal aggregates and randomly oriented platy texture, with a mean crystallite diameter of approximately 70 nm (from 40 to 110 nm). At the Tournoux site, the hematite samples from the fault exhibit clusters of hematite that are more or less botryoidal and approximately 2 µm in diameter (Fig. 6). The mean crystallite diameter is ~ 40 nm (± 10 nm at 1σ, Supplementary Data 4). No variation in crystallite size was observed with increasing distance from the shear plane. Hematites observed on the shear plane are striated (Fig. 6). The hematite vein samples also exhibit a botryoidal texture and planar flakes without any preferential orientation. The mean hematite crystallite size in these samples is ~ 68 nm, which is significantly larger than that of the fault hematite (Fig. 6).

## 4.2. Hematite (U-Th)/He data

In this study, 59 hematite (U-Th)/He measurements were obtained, 6 on sample aliquots from the Clarée site (three from veins and three from a striated fault plane) and 53 on the Tournoux site samples (12 from veins and 41 from the fault plane) (Supplementary Data 4). Fig. 7 shows H/He ages corrected for He loss as a function of mean crystallite size, Sm and effective uranium (eU) concentration (eU = [U] + 0.238 × [Th] + 0.0012 × [Sm]; Gastil et al., 1967). In the Tournoux site, hematites show a broad linear correlation between mean crystallite size and H/He age (Fig. 7). In addition, the Sm and eU concentrations (Supplementary Data 3) in the dated aliquots are in good agreement with the major and trace analyses (Fig. 7, Supplementary Data 2). The Clarée samples show H/He ages older than the Tournoux site samples and have lower Sm and eU values (Fig. 7, Supplementary Data 2).

In detail, hematite vein and striated fault plan samples from the Clarée site show (U-Th)/He ages ranging from 13.3 ± 0.8 Ma to 8.9 ± 0.4 Ma and Sm and eU concentrations of 0.2–1.6 ppm and 1.0–2.2 ppm, respectively.

For the Tournoux site, vein H/He ages range from 1.9 ± 0.1 Ma to 1.0 ± 0.1 Ma, with Sm and eU concentrations of ~ 1–2 and ~ 3–5 ppm, respectively (Fig. 7). It is observed that for certain sub-samples, aliquots exhibit homogeneous ages within an age uncertainty of 5% (Supplementary Data 4). The hematites found on the striated vein and fault plane exhibit younger ages compared to the veins. The age values range from 1.6 ± 0.2 Ma to 0.2 ± 0.1 Ma (except for one age at 1.6 ± 0.2 Ma, which seems to be an outlier, Fig. 7b). Additionally, they have higher Sm content (~2–5 ppm) but similar eU concentration (~2–5 ppm). Interestingly, the H/He age and Sm are negatively correlated for all Tournoux site hematite samples.

## 4.3. U-Pb calcite data

A new U-Pb age is presented for calcite from the Puy Chalvin site (see section 3.1.2). At thin-section scale, several generations of carbonates are identified (Fig. 8a). The first corresponds to dolomite crystals of several hundred micrometers diameter (dol 1 in Fig. 8). Although angular in places, this first generation of dolomite was partially replaced by a second generation of dusty-looking calcite (cal 2 in Fig. 8). In the cal 2 patches, crystals are small (<10 µm) and appear dark in cathodoluminescence. Clear growth zonings are observable in both small and large crystals under cathodoluminescence. The chronology of the carbonates is emphasized by the interplay between the growth zonations of dolomite, areas that have been replaced by calcite, and also by the observation of remobilized dolomite clast cemented by the cal 2 generation. A third generation (cal 3 in Fig. 8) is a clear sparitic calcite in plain polarized light, but appears zoned in cathodoluminescence. Only the last generation of calcite (cal 3) showed U-Pb ratios suitable for dating, and an age of 5.3 ± 0.6 Ma was obtained (Fig. 8b). This age corresponds to the breccia cementation, which is thus posterior to the fault movement.
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Representative optical photographs of hematite samples (a, b, plane surface and c, d, perpendicular to the striated plane), of millimetric vein hematite samples (e). (f-i) SEM images showing textural relationships along the striae, where botryoidal crystallization of hematite has occured.</figcaption></figure>

## 4.4. Stable isotope results

A significant difference in $\Delta_{47}$ temperature was obtained between samples from the western Tournoux outcrop 1 (~130 °C) and eastern Tournoux outcrop 2 (~36 °C, Table 2). Assuming that these temperatures correspond to the crystallization temperature, the signature of the mineralizing fluid was calculated using the water-calcite fractionation equations (Kim and O'Neil, 1997). A distinct fluid signature was obtained in calcites from the western ($\delta^{18}$O$_{fluid}$ = -2.4‰ to -5.6‰ in outcrop 1) and eastern ($\delta^{18}$O$_{fluid}$ = -11.9‰ to -12.3‰ in outcrop 2) parts of Tournoux site (Table 3).

## 5. Discussion

### 5.1. Relationships between fluid and fault activity

Several studies have traced the evolution of near-surface conditions in mountain belts through the study of fluid-rock interaction using stable isotopes (Campani et al., 2012; Krsnik et al., 2021; Cardello et al., 2024). At the Tournoux site, the evolution of the extensional fault network resulted in the reactivation of the PFT between 3.6 and 0.5 Ma (based on U-Pb ages from Bilau et al. (2021) and H/He ages from this study). During this extensional
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Relationship of (U-Th)/He ages with respect to mean crystallize size (a), Sm concentration (b) and effective Uranium concentration eU (c) for the striated hematite veins for both the Clarée and the Tournoux outcrop 2. A close-up of the lower part of each figure is presented in a', b' and c' to highlight the relationship within the Tournoux outcrop 2 data.</figcaption></figure>

fault event, two stages of fluid circulation are evidenced. The clumped isotope results show that the 3.4–2.3 Ma calcite breccia (Tournoux outcrop 1, Fig. 5) corresponds to a fluid temperature > 110 °C (115–138 °C, Table 2) and δ¹³Cₗcitzite values in agreement with a highly heated meteoric source (Fig. 9, Bilau et al., 2021). A nearby AFT age of 25 Ma (Tricart et al., 2007) indicates that the temperature since that time was below ~ 90 °C, corresponding to the closure temperature of the AFT (Reiners and Brandon, 2006). This suggests that the 3 Ma old fluids were in dis-equilibrium with the surrounding host rocks. The Δ₄₇ temperature of 115 – 138 °C is interpreted as the crystallization temperature of the calcites during this stage of hot fluid circulation. Given an exhumation rate of 1 mm·yr⁻¹ (Tricart et al., 2007; Girault et al., 2022), and the calcite U-Pb age (Bilau et al., 2021), calcite crystal-lization must have occurred at a depth < 3.5 km. The stable isotopic composition of these > 110 °C Tournoux site fluids (–4‰ SMOW) differs significantly from the signature of Alpine thermal sources (–10‰ to –15‰ SMOW, Figs. 9 and 10), and is inconsistent with

the δ¹⁸O gradient with elevation observed for meteoric water in the area (Fig. 10).

In contrast, the calcite from the fault breccia at Tournoux out-crop 2, crosscut by the ca. 2 Ma hematite, has a lower crystalliza-tion temperature of approximately 36 °C and exhibits a different fluid signature of –12‰ (SMOW). This signature is similar to that obtained for waters from thermal sources in the W-Alps (Poulain, 1977; Blavoux et al., 1982) and corresponds to meteoric fluids that seeped down and interacted to varying degrees with the surround-ing rocks. The δ¹⁸O signatures of the fluids can infer precipitation elevations (Campani et al., 2012) although they also reflect interac-tions with surrounding rocks (Chamberlain and Poage, 2000; Poage and Chamberlain, 2001). According to Poulain (1977), the hydrothermal spring waters within the Briançonnais are likely derived from meteoric waters that were precipitated at an eleva-tion of approximately 1900 m. This elevation corresponds to the average relief of the mountain peaks that surround the Durance valley (Fig. 10). Fluids are therefore interpreted as having a mete-
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** U-Pb age data and microscopic images of calcite from the cataclastite sample of the Puy Chalvin site. (a) Single polarized light (right) and cathodoluminescence (left) images identifying the three generations of carbonates; white circles mark areas where spots were placed during analysis. (b) Tera-Wasserburg diagram, showing the isochron correlation with statistical indicators (Mean Square Weighted Deviation, MSWD and chi-square, $\chi^2$) related to the number of spots (n).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Calculated crystallization temperatures based on $\Delta_{47}$ data as a function of the calculated isotopic fluid signature (Kim and O'Neil, 1997) of the studied High-Durance Fault System compared to data from Western Alps hot springs. Dark grey data correspond to results from thermal spring samples in the internal and external Alps (Poulain 1977; Blavoux et al., 1982). The trends (marked by arrows) are taken from Brigaud et al. (2020) except for the orange arrow, which is based on the data of this study. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

Oric origin coming from an elevation close to ~1700 m, similar to that at which the samples were collected. This indicates that the elevation of the meteoric seepage zones above the sampled active fault was similar to the present day topography during paleofluid circulation 2 Ma ago. The temperature of ~36°C suggests that

the meteoric fluids circulated down to a depth of 1–2 km and ascended back to the valley bottom.

In summary, two main fluid regimes are identified through time and depth within the Tournoux study area, including (1) deep and hot hydrothermal fluids (>110 °C) that flowed upward between ca.
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Isotopic δ¹⁸O fluid compositions from the studied fault zones (red and orange stars, calculated from the Δ₄₇ of the sampled calcites) and hydrothermal (dark grey, taken from springs Poulain, 1977; Blavoux et al., 1982) as well as meteoritic waters in the study area plotted against sampling altitude. The purple curve corresponds to water composition from rainfalls at different altitudes in the Western Alps. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

3.6 Ma and 2 Ma, and (2) shallow fluids associated with the down-
ward infiltration of meteoric water since 2 Ma. The abrupt change
at 2 Ma may have corresponded to the time when glacial erosion
isolated the upper part of the fault due to the lowering of the valley
bottom.

5.2. Significance of hematite ages for the tectonic evolution of the
Briançonnais zone

In both sites dated with the H/He method, hematites precipitate
from low temperature fluids of meteoric origin, which crystallized
during fault activity. For the Tournoux site, fluids have precipitated
at a temperature below 36 ± 1 °C. This temperature is lower than
the He closure temperature of 70–90 °C for hematite crystallites
of 30 to 90 nm (Balout et al., 2017; Farley, 2018). Consequently,
the H/He ages obtained in this study are interpreted as crystalliza-
tion ages. The estimated crystallization depth is of ~ 1 km, based
on a thermal gradient of 30 °C/km. For the Clarée site, the hematite
crystallization temperature is not known, but it is assumed to lie
below the temperature sentivity of the H/He system. This hypoth-
esis is supported by the AFT data (Tricart et al., 2007), which indi-
cate that temperatures have remained below 90 °C since
31 – 22 Ma in this region. These low temperature conditions at
both sites are not sufficient to induce a reopening of the H/He sys-
tem, so these ages can be interpreted as associated with fault activ-
ity (McDermott et al., 2017, 2021; Ault et al., 2019a; Ault, 2020).

For the Clarée site to the east of the Briançonnais zone (see sec-
tion 3.1.1) the youngest obtained age (ca. 8 Ma) is from a hematite

vein without any brittle deformation feature (e.g., striae), which is
unsuggestive of any frictional heating (as defined by Ault et al.,
2019a, b; Calzolari et al., 2020). We thus favor the hypothesis of
a more or less continuous fault activity between ca. 13 Ma and
8 Ma, which led to incremental vein opening and hematite crystal-
lization. In the Tournoux site (Fig. 6f-i) petrological observations
highlight several generations of hematite, which crystallized dur-
ing incremental fault activity. It follows, from our two study sites,
that (U-Th)/He hematite ages should be interpreted as crystalliza-
tion ages in close relationship to fault slip events, during which the
hematite may have crystallized or been reset. The source of the H/
He ages dispersion could be due to overheating even at tempera-
ture of 600 °C as pointed by Danišik et al. (2013) or to U-Th or
Sm concentrations that evolved during the precipitation of hema-
tite in the vein and striated hematite.

The onset of hematite precipitation at the Clarée site could be
older than ca. 13 Ma and could have lasted for some Myrs. Based
on field work observations, it is concluded that the studied pale-
okarst was formed during the Cenozoic after nappe stacking in
the Alps (Mercier, 1977; Barféty et al., 1996). Based on the 13 Ma
minimum age for extensional fault activity in the area, the age of
karst formation must be older than 13 Ma.

To the west of the Briançonnais zone (Tournoux site) the vein
filled with the first hematite generation shows a H/He age range
from ca. 2 Ma to 1 Ma (Fig. 7). H/He ages decrease with increasing
Sm and eU concentrations, which can be interpreted as the chem-
ical evolution of the mineralizing fluid from 2 Ma to 1 Ma (Fig. 7).
In the same location, mm-scale hematite precipitated in the veins
and the hematite fault plane coatings are striated (Fig. 6). The final movements led to the striation of the hematite fault plane and thus the reset of the H/He system at around 1 Ma. The first precipitation of fault plane hematite is recorded at ca. 1.6 Ma, with a Sm content similar to the striated vein that could indicate a similar common mineralizing fluid for the vein and the coated hematite fault plane. However, the first precipitated hematites along the fault planes are characterized by low eU (1.5–2 ppm), while the youngest hematite samples show an increase of the eU content up to 5 ppm at 0.2–0.4 Ma (Fig. 7c). This increase in eU and Sm contents is interpreted as reflecting the evolution of the parent fluid.

Despite a similar exhumation context (based on similar AFT ages, Seward et al., 1999; Tricart et al., 2007) for the two sites studied in the Briançonnais zone, both in the hanging wall of the PFT, the H/He ages obtained here are significantly different and show an east-west diachronism. Ages of 13.3 ± 0.8 Ma to 8.9 ± 0.4 Ma are obtained along the CFZ to the east of the Briançonnais zone, while ages of 1.9 ± 0.1 Ma to < 0.8 Ma are obtained at the Tournoux site, to the west of the Briançonnais zone. Furthermore, while the H/He ages from the CFZ are only slightly younger than the AFT ages from the same unit (Seward et al., 1999; Tricart et al., 2007), the H/He ages from Tournoux site are much younger (<2 Ma), even younger than the U-Pb on calcite ages obtained this site (2–4 Ma). The hematite (U-Th)/He and calcite U-Pb ages from this study (Fig. 11) suggest a westward propagation of transtensional fault activity in the Briançonnais zone, from ca. 13 Ma in the east (CFZ), ca. 5 Ma in the centre (Puy-Chalvin site) to ca. 4 Ma to < 0.8 Ma in the west (HDZ). The ages also indicate that the Briançonnais zone underwent a phase of karstification before 13 Ma, and that this area has undergone little erosion and exhumation since. It follows that the Briançonnais zone remained in a steady state topographic equilibrium for a long time, until the onset of the glaciations in the Quaternary.

### 5.3. Implications for the Western Alps tectonic evolution

The occurrence of extension in the core of the Alpine collisional belt has been interpreted as an example of gravity collapse (e.g., Selverstone, 2005), following a supposedly pure compressional context (Royden and Burchfiel, 1989). This assertion has been supported by the observation of extensional reactivation of major thrusts (e.g., Sue et al., 2007), and relatively young ages obtained for this reactivation (4–2 Ma, Bilau et al., 2021). However, as pointed out in the previous section, the onset of extension probably occurred much earlier. Our study suggests a westward propagation of extensional fault activity in the Briançonnais zone, from 13 Ma to < 2 Ma. Results suggest that extension activity coincided with compression at the front of the chain of the Western Alps (Fig. 12). Westward propagation of the frontal fold and thrust belt into the European foreland is bracketed between 15 Ma and 8 Ma

for the subalpine massifs (Bilau et al., 2023a) and between 14.3 Ma and 4.5 Ma for the Jura mountains (Looser et al., 2021; Smeraglia et al., 2021). Rooting of thrusts below the ECMs allowed their exhumation since around 22 Ma with an acceleration at 10–8 Ma (Beucher et al., 2012; Boutoux et al., 2016; Girault et al., 2022; Lemot et al., 2023). Therefore, we interpret the extension occurring in the Briançonnais zone as a relative down throw motion at the back of the ECMs. Uplift of ECMs is related to thrusting on a crustal-scale ramp, corresponding to the Alpine Frontal Thrust rooting down below the ECMs and propagates westward at the cover-basement interface of the Alpine foreland (Bellahsen et al., 2014; Schwartz et al., 2017, 2024; Rolland et al., 2022, Fig. 12). The combination of recent S-wave tomography (Nouibat et al., 2022) and stress regime inversion (Mathey et al., 2022) suggests that extensional activity in the internal Alps was likely caused by the decoupling of the Adriatic mantle indentor into two main units (Schwartz et al., 2024). The upper part of the rigid mantle designated the Adria Seismic Body (ASB, Fig. 12) is responsible for the shortening of the South Alpine crust and the vertical indentation of the subduction wedge. The uplift of the ASB is indicated by a Moho depth shallower than 10 km. Active extension highlighted in the seismicity is localized at the boundary of the extruding HP wedge of the internal Alps exhumed by the ASB (Schwartz et al., 2024).

Based on our new geochronological constraints, we propose that the period between 16 Ma (older ages of the fold-and-thrust belt activity, Bilau et al., 2023a) and 13 Ma (minimum H/He age for the initiation of extension in the High Durance Fault System) was a transitional stage in the evolution of the orogenic wedge of the Western Alps. During this period, there was a transition from compressive to extensive stress regimes in the internal zones (Briançonnais and Piedmont zones) and the development of a compressive regime in the external zones (ECMs and European foreland, Bilau et al., 2023a). This compression is suggested to be responsible for activating the Alpine Frontal Thrust, exhuming the ECMs and developing a fold and thrust belt in the foreland. In this context, the Briançonnais zone, situated behind the PFT exhibits a relative downthrow motion, thereby facilitating the development of the High-Durance Fault System (especially HDZ and CFZ) in conjunction with the extensional reactivation of the PFT. The concomitant occurrence of compressive and extensive regimes, and their parallel westward propagation on both sides of the ECMs, permit the proposition that extension in the Western Alps is not solely based on post-collisional tectonic collapse, or to any enhanced erosional processes (e.g., Cederbom et al., 2004; Champagnac et al., 2007). It can thus be proposed that convergence syn-orogenic extension should be considered as an important process, in response to a global shortening of the chain and linked to anticlockwise rotation of the Adriatic plate. The development of the extension is synchronous with the propagation of the frontal

<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Tectonic cross-section of the Briançonnais zone with the ages of fault activity (modified from Bilau et al., 2021). Yellow line corresponds to the external sedimentary cover, blue line corresponds to the Briançonnais zone, green line corresponds to the Piedmont zone and the grey line corresponds to the Chenaillet unit with the main fault zones, High-Durance Zone (HDZ) and the Clarée Fault Zone (CFZ). U-Pb ages are given with 2σ error while (U-Th)/He ages are given with 1σ error. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 12.** (a) Tectonic map of the Western Alps with position of the cross-sections (same legend as in Fig. 1). (b) Seismic geophysical profile showing interpretations of geological structure boundaries, Alpine Frontal Thrust (AFT), Penninic Frontal Thrust (PFT), Insubric Fault (IF), Adria Seismic Body (ASB), Adria Lithospheric Mantle (ALI), Mantle Wedge (MW) based on the Vs velocity model from Nouibat et al. (2022) and Schwartz et al. (2024). (c) Tectonic cross-section of the Western Alps. The geometry of the Alpine foreland (west of Penninic Frontal Thrust, PFT) is drawn after Bellahsen et al. (2014), Kalifi et al. (2021), and Bilau et al. (2023a). The internal Alps (east of PFT) are taken from Tricart et al. (2006) and Bilau et al. (2021). For lithologies legend, refer to Fig. 1 caption except that External Crystalline Massifs (ECMs) basement is in black. Ages of High-Durance Fault System extensional fault activity (determined by U-Pb calcite and (U-Th)/He on hematite) are shown in red rectangles, while exhumation of ECMs constrained by Apatie Fission Track ages is shown in a black rectangle. Ages of main foreland belt thrust faults indicated by roman letters are U-Pb ages on calcite of fault increments from Bilau et al. (2023a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

thrust structures in the sedimentary cover and to the crustal stack-
ing of the European crust below the ECMs. This tectonic context
remains active, as evidenced by the distribution of the strain field,
in relation to the Adriatic mantle indentor (Schwartz et al., 2024).

**6. Conclusions**

The combination of crystallisation ages obtained through the
use of hematite (U-Th)/He and calcite U-Pb dating, in conjunction
with the isotopic signatures (δ¹³C, δ¹⁸O, and Δ⁴¹) of the fluids
related to calcite allows to pin-point the history of fluid-rock inter-
action in an evolving tectonic and geomorphological framework.
Stable isotope analysis has revealed a transition in the fluid
regimes flowing through the extensional faults over the past three
million years, as recorded in calcite precipitated along the fault.
The first regime, which prevailed before 2 Ma, is associated with
upwelling of deep hydrothermal fluids with T > 110 °C. The second
fluid regime, since 2 Ma, is characterised by a meteoritic signature
and much lower temperatures of around 36 °C. This transition is
interpreted as the result of valley incision in response to the Qua-
ternary glacial phases.

The east-west progression of hematite and calcite ages allows
the formulation of a new model for the comprehension and evolu-
tion of Alpine syn-orogenic extension. The onset of extension, as
determined by helium ages on hematite, is estimated to have
occurred at a minium age of 13 Ma to the east of the Briançonnais
zone (CFZ). The propagation of extension proceeded in a westerly
direction, reaching the center of the Briançonnais zone at ca.
5 Ma and ultimately its western margin (HDZ) at 3 Ma. The exten-
sion observed within the Briançonnais zone is linked to the exten-
sional reactivation of the PFT and is synchronous with the
development of the Alpine foreland, since 16 Ma. The Alpine Fron-
tal Thrust plays a pivotal role in the tectonic evolution of the
region, as its involvement serves to facilitate the exhumation of
the ECMs. The development of the High-Durance Fault System, at
rear back of ECMs, is here considered as the accommodation of

ECMs exhumation. It can thus be concluded that this syn-
convergent orogenic extension, which is associated with the devel-
opment of the collisional structures, is not linked to post-orogenic
processes such as post-collisional tectonic collapse but is con-
trolled by mantle indentation during anticlockwise rotation of
the Adriatic plate.

CRediT authorship contribution statement

**Antonin Bilau:** Writing – review & editing, Writing – original draft, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Yann Rolland:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Project administration, Investigation, Funding acquisition, Conceptualization. **Stéphane Schwartz:** Writing – review & editing, Visualization, Validation, Supervision, Investigation, Funding acquisition, Conceptualization. **Cécile Gautheron:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation. **Thierry Dumont:** Writing – review & editing, Visualization, Validation, Supervision, Investigation, Conceptualization. **Dorian Bienveignant:** Investigation. **Benjamin Brigaud:** Writing – review & editing, Validation, Supervision, Methodology, Data curation. **Pierre Deschamps:** Writing – review & editing, Funding acquisition. **Xavier Mangenot:** Formal analysis, Data curation. **Marianna Corre:** Formal analysis, Data curation. **Rosella Pinna-Jamme:** Formal analysis, Data curation. **Nathaniel Findling:** Formal analysis, Data curation.

**Declaration of competing interest**

The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
## Acknowledgments

This work forms part of the first author's Ph.D. project funded by the French BRGM in the frame of the RGF-Alps program (Référentiel Géologique de la France). The LA-ICP-MS hosted by the ENVITOP platform at CEREGE has received funding from the "Excellence Initiative" of the Aix Marseille University A*MIDEX - project DATCARB, a french "Investissement d'avenir" program. Special thanks to Fayçal Soufi and Pierre-Luigi Canepa for their preparation of polished thin-section. The authors thank the reviewers, Meinert Rahn and Martin Danišík, for their careful reading and insightful comments, and efficient editorial handling by Stijn Glorie (Associate Editor) and M. Santosh (Editorial Advisor), which greatly helped to improve this manuscript.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gsf.2024.101969.

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