y
Evenson et al., 2014; McDermott et al., 2017, 2021; Ault, 2020
).
These methods can give crystallization ages related to the develop-
ment 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,
d
18
O,
d
13
C isotopes and
D
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).
⇑
Corresponding author.
Geoscience Frontiers 16 (2025) 101969
Contents lists available at
ScienceDirect
Geoscience Frontiers
journal homepage:
www.else
vier.com/locate/gsf
Research Paper
Timing of syn-orogenic extension in the Western Alps revealed by calcite
U-Pb and hematite (U-Th)/He dating
Antonin Bilau
a,b ,
⇑
,
a
EDYTEM, Université Savoie Mont Blanc, CNRS, UMR 5204, Le Bourget du Lac, France
b
ISTerre, Université Grenoble Alpes, USMB, CNRS, IRD, UGE, Grenoble, France
Yann Rolland
a,b
, Stéphane Schwartz
b
, Cécile Gautheron
b,c
,
c
GEOPS, CNRS, Université Paris-Saclay, 91405 Orsay, France
Thierry Dumont
b
,
Dorian Bienveignant
b
,
d
Aix-Marseille Université, CNRS, IRD, INRAE, CEREGE Collège de France, Aix-en-Provence, France
Benjamin Brigaud
c
, Nicolas Godeau
d
, Abel Guihou
d
, Pierre Deschamps
d
,
Xavier Mangenot
d,e
,
e
Caltech, Geological and Planetary Sciences, Pasadena, CA, USA
Marianna Corre
b
, Rosella Pinna-Jamme
c
, Nathaniel Findling
b
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 uplift-
ing 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/
).
1. Introduction
The tectonic evolution of fault networks can be constrained by
using geochronologic
al 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
)orb
(U-Th)/He dating
on hematite (e.g.,
Wernicke and Lippolt, 1994;
E-mail address:
antonin.bilau@kit.edu
(A. Bilau),
antonin.bilau@kit.edu
(A.
https://doi.org/10.1016/j.gsf.2024.101969
1674-9871/
©
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/
).
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
System, a significant transtensional fault complex in the Briançon-
nais 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 overcom-
pensation
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
).
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
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
).
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.)
2
The objective of this study is to provide definitive temporal con-
straints 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 propaga-
tion of extension within the High-Durance Fault System. In addi-
tion, clumped isotope analysis performed on calcite from the
same dated sites give insights into the related regional fluid circu-
lation 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. Geologica
l setting
The western Alpine
belt resulted from the convergence and col-
lision of the European and Adriatic plates (e.g.,
Tricart, 1984
) since
Cretaceo
us time. The internal zones (Fig.
1) are characterized by
High-Pre
ssure – Low-Temperature
(HP-LT) metamorphic condi-
tions 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
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).
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
The external zones comprise the sub-greenschist facies (e.g.
Frey and Ferreiro Mählmann, 1999
) European Mesozoic sedimen-
tary 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) high-
lights 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 charac-
terized 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 con-
trolled by the displacements imposed by the current NW/SE con-
vergence 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
whithin the
High Durance Fault System and mostly clusters at
shallow depths of 3 to 8 km, where the faults are structurally con-
nected 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 deforma-
tion is
highlighted by observed GPS motions (Walpersdorf et al.,
2018; Mathey et al., 2021, 2022). The analysis of GPS velocity pro-
files 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 compo-
nent correspond
s to vertical motions of 0.5 to 3.0 mm per year.
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.)
3
The onset of compressive tectonic activity along the PFT is con-
strained 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
circulating along the HDZ, with temperatures exceeding 80
°
C
(Fig. 2
, Tricart et al., 2007
).
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
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 foot-
wall (
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
D
47
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
Supple-
mentary Data 2 and 3
.
3.1.1. Clarée site
At the Clarée site, the CFZ (Fig. 3
a) 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 high-
lighted by a thick reddish zone of > 100 m length (Fig. 3
a). Bedding
is
tilted by
40
°
to the west (Fig. 3
b) 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 nor-
mal slip and affects local karst fillings that are located in the core of
a syncline. Motions along the CFZ fault show overprinting relation-
ships with alternating pure
strike-slip and extensional stress
regimes (
Fig. 3
b). The fault zone runs along the valley for several
kilometers
and has a width of several tens of meters, with
numer-
ous striated planes and tectonic breccias. It cuts through paleokarst
fillings and has remobilized some of the iron-rich material. Hema-
tite and calcite mineralization has been observed and both sam-
pled 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 system-
atically 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.
Table 1
Sample coordinates and information.
Briançonnais zone
Name
Latitude (
°
N)
Longitude (
°
E)
Elevation
(m)
Sample type
Analyses done
Reference
East
Clarée
44
°
55
27.52
6
°
39 44.34
1815
hematite fault and vein
mineralogy, geochemistry,
(U-Th)/He
this study
Central
Puy-Chalvin
44
°
52 36.23
6
°
3
4
44.17
1546
calcite fault
calcite U-Pb,
this study
West
Tournoux outcrop 1
44
°
48 29.75
6
°
30 31.38
1910
calcite fault
D
47
this study
calcite U-Pb
Bilau et al. (2021)
Tournoux outcrop 2
44
°
47
55.70
6
°
30 53.30
1641
hematite fault and vein
mineralogy, geochemistry,
(U-Th)/He
this study
calcite fault
D
47
this study
4
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, 5
a) 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
D
47
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 Pous-
terle 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. 5
a). The fault offset is about 300 m and juxtaposes
brecciated
whitish Triassic quartzites next to Upper Cretaceous
calcschists (
Sue and Tricart, 1999
, Fig. 5
b). The fault zone contains
a
breccia with clasts of stretched Triassic dolomitic limestones
cemented by a first generation of calcite (Fig. 5
c). This breccia is
cut
by hematite veins several millimetres wide. The hematite
was striated by later fault plane activity and underwent recrystal-
lization. Twenty samples of striated fault planes (centimetre-long)
were collected, composed of layers of hematite interlayered with
phyllosilicates (Fig. 5
d). A first relative chronology at the scale of
the
outcrop can be established (Fig. 5
c) with (1) calcite precipita-
tion,
(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 micro-
scope (SEM) analysis. For most samples, observations perpendicu-
lar 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 param-
eters were optimized on each picture in order to reach
a
20 nm resolution. Crystallite sizes of hematite were measured
on more than twenty images and mean values are reported in
Sup-
plementary Data 4
.
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
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 whithin 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 Supplemetary 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.)
3.3. Hematite (U-Th)/He geochronology
H/He dating was undertaken at the GEOPS laboratory (Univer-
sity
Paris Saclay, France). Centimetre-scale hematite vein and stri-
ated 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 sam-
ples were fragmented in
500
l
m size aliquots in order to com-
pensate 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 ultra-
sonic 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
l
m
with weights ranging from 150 to 1020
l
g 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 ali-
quote
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
.
5
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
l
m and firing for 20 s
lead to a 30
l
m downhole (see
Bilau et al., 2023a
for futher
details).
A NIST-614 glass standard was used as primary reference
material for drift and Pb isotopes; WC-1 carbonate served as a ref-
erence material for matrix matching of
206
Pb/
238
U and B6 as sec-
ondary material (Roberts et al., 2017; Pagel et al., 2018; Brigaud
et al., 2021). For data processing, raw intensities and baseline cor-
rection
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
206
Pb/
238
U are cal-
culated using an in-house Python code. Tera-Wasserburg plots, cal-
culation of intercept ages and initial Pb compositions were made
using IsoplotR (model-1) (Vermeesch, 2018
). Ages are quoted with
2
r
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 uncertain-
ties included the age uncertainty of reference material. Data are
reported in
Supplementary Data 5
.
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
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.)
6
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
(
D
47
) analyses (
Table 2
). Thirty milligrams of pure calcite was
retrieved
using a Dremel on each of the three samples in order to
extract and analyse CO
2
at CalTech (USA). Each
D
47
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 CO
2
was collected
via a U-shaped trap at a temperature of
196
°
C. The CO
2
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 CO
2
were measured with a Thermo Scientific MAT 253 Mass
Spectrometer. The
d
13
C and
d
18
O 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-
laboratory reference frame,
D
47-CDES25
(Dennis et al., 2011;
Bonifacie et al., 2017). During the analysis sessions, 8 samples of
Carrara marble
showed
D
47-CDES25
results of 0.394
‰
± 0.012
‰
(1
r
), and 10 samples of carbonate TV04 showed
D
47-CDES25
results
of 0.653
‰
± 0.017
‰
(1
r
). These results correspond to accepted
values for these standards:
D
47-CDES25
= 0.655
‰
for TV04 and
D
47-CDES25
= 0.405
‰
for Carrara (
Mangenot et al., 2024). The error
related to
the standards was used (± 0.014
‰
,1
r
) for samples with-
out 3 replicates. Finally, the corrected
D
47
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
d
18
O signature of the calcite mineralizing
fluid is calculated using the
Kim and O’Neil (1997)
equation
(Table 3
).
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
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 = 2
r
), 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; Tl: 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.)
7
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
Table 2
Clumped isotope results on calcite from Tournoux site tectonic breccia and calculated temperatures.
D
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)
.
Location
Name
U-Pb age
d
18
O
calcite
(SMOW)
d
13
C
calcite
(VPDB)
D
47 CDES90
1
r
D
47
D
47 min
D
47 max
T
(
°
C)
T
min
(
°
C)
T
max
(
°
C)
Tournoux outcrop 1
FP18-2B
3.4 ± 1.4 Ma
8.70
0.09
0.41
8.32
0.10
0.38
8.51
0.10
0.39
0.014
0.379
0.407
125
115
135
FP18-3A
2.3 ± 0.2 Ma
9.10
1.46
0.39
9.11
1.44
0.38
9.11
1.45
0.38
0.007
0.376
0.389
133
128
138
calcite
cement
Tournoux outcrop 2
no data
13.75
1.18
0.57
13.77
1.20
0.56
13.86
1.26
0.57
13.79
1.22
0.57
0.003
0.564
0.570
36
35
37
Table 3
Calculated
d
18
O signatures of the calcite mineralizing fluid.
Location
Name
d
18
O
fluid
‰
(SMOW) (
Kim and O’Neil, 1997
)
1000ln(a)
a
d
18
O
fluid
‰
(SMOW)
d
18
O
fluid
min
d
18
O
fluid
max
Tournoux outcrop 1
FP18-2B
12.92
1.013
4.44
5.58
3.27
FP18-3A
11.99
1.012
2.92
3.49
2.35
Tournoux outcrop 2
calcite cement
25.86
1.026
12.08
12.26
11.91
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, whith 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
l
m in diam-
eter (
Fig. 6
). The mean crystallite diameter is 40 nm (± 10 nm at
1
r
, 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) (Supple-
mentary 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 sam-
ples 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.
8
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 uncer-
tainty 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. 7
b). 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. 8
a). The first corresponds to dolo-
mite
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 cal-
cite (cal 2 in
Fig. 8
). In the cal 2 patches, crystals are small (<10
l
m)
and appear dark in cathodoluminescence. Clear growth zonings are
observable in both small and large crystals under cathodolumines-
cence. 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 remo-
bilized dolomite clast cemented by the cal 2 generation. A third
generation (cal 3 in
Fig. 8
) is a clear sparitic calcite in plain polar-
ized
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. 8
b). This
age
corresponds to the breccia cementation, which is thus poste-
rior to the fault movement.
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
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.
4.4. Stable isotope results
A significant difference in
D
47
temperature was obtained
between samples from the western Tournoux outcrop 1 ( 130
°
C) and eastern
Tournoux outcrop 2 ( 36
°
C, Table 2
). Assumin
g
that
these temperatures correspond to the crystallization temper-
ature, 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
(
d
18
O
fluid
= 2.4
‰
to 5.6
‰
in outcrop 1) and eastern (
d
18
O
fluid
=
11.9
‰
to 12.3
‰
in outcrop 2) parts
of Tournoux site (
Table 3
).
9
5. Discussion
5.1. Relationships
between fluid and fault activity
Several studies have
traced the evolution of near-surface condi-
tions 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
extension
al 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
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
d
13
C
calcite
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) indi-
cates that the temperature since that time was below
90
°
C, cor-
responding 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
D
47
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
d
18
O gradient with elevation observed for meteoric water in
the area (
Fig. 10
).
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
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.
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
d
18
O 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-
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.
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
Fig. 8.
U-Pb age data and microscopic images of calcite from the cataclasite 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,
v
2
) related to the number of spots (
n
).
Fig. 9.
Calculated crystallization temperatures based on
D
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.)
11
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.
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.
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
Fig. 10.
Isotopic
d
18
O fluid compositions from the studied fault zones (red and orange stars, calculated from the
D
47
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.)
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. 6
f–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
Danišík et al. (2013)
or to U-Th or
Sm
concentrations that evolved during the precipitation of hema-
tite in the vein and striated hematite.
12
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 faut planes are
characterized by low eU (1.5–2 ppm), while the youngest hematite
s
amples show an increase of the eU content up to 5 ppm at 0.2–
0.4 Ma (
Fig. 7
c). This increase in eU and Sm contents is interpreted
as
reflecting the evolution of the parent fluid.
A. Bilau, Y. Rolland, Stéphane Schwartz et al.
Geoscience Frontiers 16 (2025) 101969
Despite a similar exhumation context (based on similar AFT
ages,
Seward
et
al., 1999; Tricart et al., 2007
) for the two sites stud-
ied
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 exhuma-
tion 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 sup-
ported
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 proba-
bly occurred much earlier. Our study suggests a westward propa-
gation 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
exhumati
on 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 desig-
nated
the Adria Seismic Body (ASB,
Fig. 12
) is responsible for the
shortenin
g 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 highlight
ed
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).
Fig. 11.
Tectonic cross-section of the Briançonnais zone with the ages of fault activity (modified from
Bilau et al., 2021
). Yellow line correspond 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 2
r
error while (U-Th)/He ages are given with 1
r
error. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
13
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 com-
pressive regime in the external zones (ECMs and European fore-
land,
Bilau et
al., 2023a). This
compression is suggested to be
repsonsible 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 pro-
cess, 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