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\DOI{10.5802/crgeos.280}
\datereceived{2024-07-17}
\daterevised{2024-11-05}
\dateaccepted{2024-11-19}
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\shortrunauthors

\title{First in situ calcite U--Pb dating revealing the past episodes of
the 2019 Le Teil Earthquake (France)}

\alttitle{Premi\`{e}re datation U--Pb in situ sur ciments de calcite
r\'{e}v\'{e}lant les \'{e}pisodes ant\'{e}rieurs au s\'{e}isme du Teil
de 2019 (France)}

\author{\firstname{Aude} \lastname{G\'{e}belin}\CDRorcid{0000-0003-4765-8611}\IsCorresp}
\address{Universit\'{e} de Lorraine, UMR CNRS 7359, GeoRessources, F-54506 
Vand\oe{}uvre-l\`{e}s-Nancy, France}
\email[A. G\'{e}belin]{aude.gebelin@univ-lorraine.fr}

\author{\firstname{Christophe} \lastname{Matonti}\CDRorcid{0000-0001-8105-9642}}
\address{G\'{e}oazur, Univ. C\^{o}te d'Azur, CNRS, 
Observatoire de la C\^{o}te d'Azur, IRD, Valbonne, France}

\author{\firstname{Christophe} \lastname{Larroque}\CDRorcid{0000-0002-0866-9441}}
\addressSameAs{2}{G\'{e}oazur, Univ. C\^{o}te d'Azur, CNRS, 
Observatoire de la C\^{o}te d'Azur, IRD, Valbonne, France}
\address{Universit\'{e} de Reims Champagne-Ardenne, 
GEGENA UR3795, Reims, France}

\author{\firstname{Patrick} \lastname{Carr}\CDRorcid{0000-0001-7076-1196}}
\addressSameAs{1}{Universit\'{e} de Lorraine, UMR CNRS 7359, 
GeoRessources, F-54506 Vand\oe{}uvre-l\`{e}s-Nancy, France}

\author{\firstname{Jean-Paul} \lastname{Ampuero}\CDRorcid{0000-0002-4827-7987}}
\addressSameAs{2}{G\'{e}oazur, Univ. C\^{o}te d'Azur, CNRS, 
Observatoire de la C\^{o}te d'Azur, IRD, Valbonne, France}

\keywords{\kwd{Le Teil earthquake}
\kwd{La Rouvi\`{e}re fault}
\kwd{U--Pb geochronology}
\kwd{Calcite veins}
\kwd{Pyrite crystals}}

\altkeywords{\kwd{S\'{e}isme du Teil}
\kwd{Faille de La Rouvi\`{e}re}
\kwd{G\'{e}ochronologie U--Pb}
\kwd{Veines de calcite}
\kwd{Cristaux de pyrite}}

\thanks{Observatoire de la C\^{o}te d'Azur,
CNRS/INSU project Tellus FREMTEIL}

\begin{abstract} 
In the Rh\^{o}ne Valley near the city of Mont\'{e}limar, a Mw 4.9
surface-rupturing earthquake occurring on the 11th of November 2019 was
linked to the reactivation of the NE--SW trending normal La Rouvi\`{e}re
fault as a reverse fault mechanism. Here, we present the first in situ
calcite U--Pb data acquired on well-characterized microstructures from
core samples from a first exploratory drilling conducted in the NE part
of the La Rouvi\`{e}re fault. In agreement with relative chronology
data based on microstructural observations, our results allow us to
define four main tectonic events: A first event from \textasciitilde90 to 75 
interpreted to reflect a thermal subsidence at the regional scale 
(Ard\`{e}che---South-East basin of France), a second event bracketed
between \textasciitilde60 to 50 Ma related to the N--S trending shortening of
the South-East basin, a third tectonic event dated from \textasciitilde40 to
37 Ma that we interpret to reflect the opening of the Gulf of Lion and
European Cenozoic Rift System (ECRIS), and a fourth and final event at
\textasciitilde25 Ma that very likely represents a middle to late stage of the
ECRIS. No more recent event has been identified on this fault.
\end{abstract}

\begin{altabstract} 
Dans la vall\'{e}e du Rh\^{o}ne, pr\`{e}s de la ville de
Mont\'{e}limar, un s\'{e}isme de magnitude Mw 4.9 li\'{e} \`{a} la
r\'{e}activation en faille inverse de la faille normale NE--SW de La
Rouvi\`{e}re est survenu le 11 novembre 2019. Sont ici
pr\'{e}sent\'{e}es les premi\`{e}res donn\'{e}es U--Pb in situ obtenues
sur des ciments fins de calcite issus d'\'{e}chantillons de carottes
pr\'{e}lev\'{e}es lors d'un premier forage exploratoire r\'{e}alis\'{e}
dans la partie NE de la faille de La Rouvi\`{e}re. En accord avec les
donn\'{e}es de chronologie relative d\'{e}duites des observations
microstructurales, nos r\'{e}sultats nous permettent de mettre en
\'{e}vidence quatre \'{e}v\'{e}nements tectoniques majeurs : Un premier
\'{e}v\'{e}nement pr\'{e}coce entre \textasciitilde90 et 75 Ma
interpr\'{e}t\'{e} comme le r\'{e}sultat de la subsidence thermique
reconnue d'\'{e}chelle r\'{e}gionale (Ard\`{e}che --- bassin sud-est de
la France), un deuxi\`{e}me \'{e}v\'{e}nement entre \textasciitilde60 et 50 Ma
reli\'{e} au raccourcissement du bassin du sud-est lors des stades
tardifs de la compression pyr\'{e}n\'{e}enne, un troisi\`{e}me
\'{e}v\'{e}nement tectonique dat\'{e} de 40 \`{a} 37 Ma associ\'{e}
\`{a} l'ouverture du Golfe du Lion et \`{a} l'activation du Rift
C\'{e}nozo\"{i}que ouest Europ\'{e}en (ECRIS), et enfin un
quatri\`{e}me \'{e}v\'{e}nement \`{a} \textasciitilde25 Ma interpr\'{e}t\'{e}
comme l'expression du stade tardif de l'ECRIS.
\end{altabstract} 

\maketitle

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\section{Introduction}\label{sec1}

\begin{figure*}
\includegraphics{fig01}
\vspace*{2pt}
\caption{\label{fig1}The northern C\'{e}vennes Faults system at the
boundary between the Massif Central and the South-East basin of France.
(A)~Epicenter of the 2019-11-11 Le Teil earthquake 
\citep{Delouisetal2021},
(B)~instrumental seismicity (1962--2018,
\href{https://renass.unistra.fr/instrumental-seismicity-in-mainland-france/}{https://renass.unistra.fr/instrumental-seismicity-in-mainland-{\break}france/}),
(C)~Historical seismicity  [\url{https://www.sisfrance.net/}, 
\citealp{Jomardetal2021}], (D)~faults 
(\url{http://infoterre.brgm.fr/page/telechargement-cartes-geologiques}),
in red: La Rouvi\`{e}re fault (LRF).  Inset: MC: Massif Central, SE:
South-East basin.  The yellow circle corresponds to the drilling
location.} 
\vspace*{2pt}
\end{figure*}

The Mw 4.9 earthquake that occurred on the 11th of November 2019 near
Mont\'{e}limar, at the western side of the South-East basin of France,
with a focal depth in the range of 1.0--1.3~km, produced
a 4.5-km-long surface rupture  (Figure~\ref{fig1}) and strong ground
motions in the epicentral area that were unexpected in such a low
deformation rate plate interior  \citep{Ritzetal2020,Causseetal2021}.
This earthquake, damaging many buildings and injuring people in Le Teil
village, reinforces the idea that the NE--SW trending La Rouvi\`{e}re
fault (LRF), belonging to the C\'{e}vennes faults system, is still
active. Following this event, many geophysical research projects have
been undertaken 
\citep[e.g.][]{Ritzetal2020,Delouisetal2021,Causseetal2021} to
characterize and determine the epicenter and depth of the rupture.
However, none of them have focused on rock mechanics, past deformation,
fluid-rock-deformation interactions, or physical and chemical
conditions. Our long-term project is to drill at hypocentral depth with
the goal of  (1)~examine in detail the above aspects of this very
shallow seismogenic zone, and  (2)~correlate the structure and
mineralogy of the fault zone with their seismological behavior 
\citep[e.g.][]{Boullier2011}, based on samples collected from the
rupture depth initiation up to the surface  (Hyperdrill project, PI
J.~P.~Ampuero, pers. Comm.). To argue the feasibility and interest of
such a large-scale project, our first step was to carry out an
exploratory drilling in the NE part of the fault  (Figure~\ref{fig1}
and Supplementary Figure~1) which was located at ${\sim}$10~m
away from the surface rupture  n\textdegree~1  (44.5306\textdegree\ 
N--04.6690\textdegree\ E) observed by  \citet{Ritzetal2020}. The drill
crossed the fault at 20~m depth and core samples were taken from the
surface down to 25.50~m depth. However, due to technical conditions the
cores were not continuous, and it was clearly not possible to identify
the principal slip zone of the 2019 rupture. Nevertheless, in the
northeastern part of the rupture, the coseismic displacement is
estimated at ${\sim}$10~cm  \citep{Ritzetal2020}. We have identified
large and localized water loss in the breccia zone within the drill
core which could reflect a recent destructuration along a slip zone.
However, we cannot rule out the possibility that this water loss is
related to pre-existing karstic\unskip\break 
fracture.

Here, we focus on the LRF zone that is characterized by a
${\sim}$3~m-thick calcareous breccia showing numerous recrystallized
calcite veins and to a lesser extent some pyrite-filled fractures.\ 
Based on the \mbox{identification} of key carbonate recrystallizations and
microstructural observations, this paper presents the first U--Pb in
situ calcite geochronological data that constrains the timing of
tectonic episodes along the northern C\'{e}vennes faults system and the
structural heritage of the active LRF.

\section{Geological and seismotectonic context of the La Rouvi\`{e}re
fault}\label{sec2} 
Field, seismological and geodetical data attest that Le Teil earthquake
occurred on the LRF 
\citep{Ritzetal2020,DeNovellisetal2020,Vallageetal2021,Marconatoetal2022}. 
This fault belongs to the northeastern tip of the NE--SW trending
C\'{e}vennes faults system (CFS) that can be followed over more than
100~km from Montpellier to Valence, and separates the Massif Central
Paleozoic crystalline basement to the west from the Meso-Cenozoic
sedimentary Provence Basin to the east  [Figure~\ref{fig1};
\citealp{Elmietal1996}]. This foreland basin, bounded by the Rh\^{o}ne
Valley on its western side, is located west of the Alpine belt
including the sub-alpine front and Penninic frontal thrust, located at 
${\sim}$50 and ${\sim}$150~km from the CFS, respectively. Reverse
faulting and folding have been reported in the late Miocene molasses of
the Rh\^{o}ne Valley and are considered to be the westernmost evidences
of E--W compressive stress related to the Alpine orogenesis in the area 
\citep{BlesGros1991,Roureetal1994}. Regarding the CFS to which the LRF
belongs, it underwent several tectonic episodes since the end of the
Paleozoic  \citep{Roureetal1992,Bonijolyetal1996}. The 10-km-long LRF
extends from the SW of the Escoutay Valley to the NE  (Rh\^{o}ne
Valley), and strikes N45E ${\pm}$ 5\textdegree\ with a dip of 
55\textdegree\ SE ${\pm}$ 5\textdegree\ (Figure~\ref{fig1}). It was
activated by extensional tectonic during the Oligocene as a result of
the Mediterranean sea opening  \citep{Seranne1999}.
This major
event is well identified in seismic reflexion profiles acquired along
the Escoutay Valley where a normal offset of up to 200~m can be
measured  \citep{Thomassetetal2024}, but also in the field, where the
LRF scarp marks the\unskip\break  
geomorphology.

Instrumental seismicity was sparse in the Le Teil area before the 2019
rupture. However, within a radius of 20 km around Le Teil village, only
39 earthquakes with a local magnitude lower than 2.9 have been detected
between 1962 and 2018  (from ReNaSS and LDG catalogs, 
Figure~\ref{fig1}). Also, several historical events occurred nearby
with macroseismic intensities up to VII MSK in 1773, 1873 and 1934 for
instance  \citep{Manchueletal2017,Larroqueetal2021}.  Therefore, the
unexpected 2019 Le Teil earthquake occurred on the unsuspected active
LRF \citep{Jomardetal2017} and remains the strongest event known in the
area. Also, it is the first time in
metropolitan France that an earthquake is clearly related to a known
fault from surface ruptures observations. The moment magnitude  (4.9)
was moderate but large ground accelerations  [${>}$1~g in some places; 
\citealp{Causseetal2021}] were suspected, making the scientists
convinced that the focal depth was particularly shallow  [1.0--1.3~km; 
\citealp{Delouisetal2021}]. The focal mechanism was purely reverse with
nodal planes parallel to the trace of LRF and to the alignment of
surface ruptures, making the Le Teil earthquake unique as no evidence
of cumulative tectonic inversion related to previous reverse
earthquakes has been found so far \citep{Ritzetal2020,Marconatoetal2022}.

Currently, no horizontal motion is detected from the Global Navigation
Satellite Systems  (GNSS) at the boundary between the Massif Central
and the South-East basin. Nevertheless from a precise analysis of 20
years of continuous geodetic data,  \citet{Massonetal2019} evidenced a
NW-SE compressional strain rate of 
$0.7\times10^{-9}~\mathrm{yr}^{-1}$,  typical of a very low intraplate
deformation rate. The 2019 reverse faulting earthquake is interpreted
to reflect a tectonic inversion of the La Rouvi\`{e}re normal fault 
\citep{Ritzetal2020,Marconatoetal2022}. 

\section{Samples description}\label{sec3} 

\subsection{Macroscale structures}\label{sec3.1} 
The lithologies encountered in this drill hole correspond to a
succession of sedimentary layers of early Cretaceous age including
sands, clays and marly limestones followed by a grey calcareous breccia
occurring within the 20.80~m 
and 23.00~m depth intervals that lies on
top of massive beige limestone  (Supplementary Figures~1
and~2). At 21~m depth, the breccia is cut by a gouge of less 
than 1~cm thick. 

Here, we focus on the ${\sim}$4~m thick LRF zone that could be divided
into two domains: (1)~a damage zone where rocks are affected by multiple
generations of open fractures and veins and, to a lesser extent, by
pyrite-filled fractures  (see below EC2 sample), and (2)~a 
${\sim}$3~m thick fault core characterized by a
\mbox{calcareous} breccia, and a 1~cm thick gouge band in
its central part  (Supplementary Figures~1 and~2).

Most fractures and veins are crosscut by numerous tectonic stylolithes
of variable dip  (e.g.\ EC3 sample in  Supplementary Figure~2).
The matrix alternates from a dark initial rock texture to a more beige
colored texture giving the rock a macroscopically flecked aspect  
(e.g.\ EC5 sample in  Supplementary Figure~2). These heterogeneities
emphasize the polyphase feature of this sampled fault breccia. The
first phase of brecciation consists of centimeter-to-decimeter scale
angular clasts embedded in a matrix made of fine dark grey calcitic
material  (EC1 and EC2 samples in  Supplementary Figure~2).
This breccia  (from herein referred to as breccia 1) characterises the
upper part of the core and is then reworked as clasts during a second
fluid-assisted brecciation event well observed going down the borehole
and especially at the 22.5~m depth interval where a typical jigsaw
geometry characterizes the core samples  (``breccia 2'', EC3 sample in 
Supplementary Figure~2). Here, a light brownish matrix is well
represented and fragments are well individualized. This chaotic  
\citep[sensu][]{MortWoodcock2008} matrix supported breccia is even more
pronounced going down section embedding dark grey fragments from
breccia 1  (EC5 sample in  Supplementary Figure~2). Although
the majority of fragments are angular, some of them display rounded
corners that could be due to a combination of pressure fluctuation,
chemical and physical processes  \citep{Jebrak1997}. The light brown
breccia matrix is affected by numerous stylolithes, and consists of a
mixture of finely crushed breccia 1 and non-cohesive sedimentary
material percolating through the porous space created from fracture
opening and dilatancy processes.

\subsection{Microscale structures}\label{sec3.2}  
We focused our attention on 5 representative samples EC1 (21~m), EC2 
(22~m), EC3  (22,20~m), EC4  (22,50~m) and EC5  (24~m) homogeneously
distributed over the  ${\sim}$3~m of core drilling  (Supplementary
Figures~1 and~2). To characterize the sedimentary facies and
the paragenesis of the deformation markers and their crosscutting
relationships, thin sections were made for each sample and studied
using an optical microscope. In addition, 
{optical} \mbox{cathodoluminescence}
(CL; NewTec Scientific Cathodyne\tsup{\text{\textcopyright}}) was used
to precisely identify the successive phases of calcite
(re)crystallization  (with possible calcite zoning) and cements in
fractures/veins as well as in interclast spaces of brecciated samples.

\begin{figure*}
\includegraphics{fig02} 
\vspace*{4pt}
\bcaption{\label{fig2}}{1}{Microphotographs of thin sections from samples of
the La Rouvi\`{e}re Fault Damage zone. Left panels column are views in
polarized light, right column are the same areas observed in
cathodoluminescence (CL).  (A)~Fr1 fracture displaying their complex
infilling originating from 2 opening events. (insert) Close-up on the
anisopachous vadose calcitic infilling,  (B)~CL image showing the
dull-luminescence of the infillings of Fr1,  (C)~EC5B thin section
illustrating areas of recrystallized micrite, and the partially
dissolved and sheared Fr1;  (D)~similar to C but in CL, 
(E)~Crosscutting relationship between Fr1 and Fr2 which is
characterized by 2 different infillings;  (F)~CL image showing the
bright luminescence of Fr2 fractures compared to Fr1; 
(G)~Crosscutting relationship between Fr1 fracture and a stylolithe
plane; (H)~interpretative scheme of the G panel. Note that U--Pb ages
obtained in this study have been added on photographs for clarity.}
\vspace*{8pt}
\end{figure*} 

Using optical microscopy and CL, several phases of deformation and
cementation, part of the so-called structural diagenesis 
\citep{Laubachetal2010}, can be identified in both the damage zone and
the fault core. In the damage zone, we distinguish two main vein
families. We first identify a tens-to-hundreds of microns wide fracture
family filled with multiple anisopachous fringes/bands of small
sparitic to micritic material  (Fr1, Figure~\ref{fig2}A) that we
interpret to have formed from speletothem-like processes in vadose
environment  (lower arrow, Figure~\ref{fig2}A and inset) 
\citep[e.g.][]{ScholleUlmer-Scholle2003}. These fractures are sometimes
reactivated in opening mode, allowing for larger  (${>}$50~$\rmmu$m)
sparite cement to fill the newly opened fracture space  (upper arrow,
Figure~\ref{fig2}A). Both of these Fr1 calcite fillings display a dull
blue to orange luminescence in CL  (Figure~\ref{fig2}B). We note that
the associated sedimentary micritic matrix is affected by partial
recrystallization occuring over millimeter-to-centimeter scale areas 
(upper left arrow,  Figure~\ref{fig2}C). In some places, 
\mbox{micrite}
crystals are replaced by a large mosaic sparite fabric highlighted by
contrasted luminescence in CL, with colours from dark blue to bright
orange\unskip\break 
(Figure~\ref{fig2}D). 

This first Fr1 fracture family is crosscut by a second Fr2 fracture
family whose veins are linear and of variable thicknesses 
(Figure~\ref{fig2}E). In CL, these Fr2 fractures/veins are easily
recognizable by their orange luminescence  (Figure~\ref{fig2}F). Two
successive infilling materials characterized these Fr2 fractures: 
(1)~fine granular material, likely originating from the breccia 2
matrix, is commonly observed on the fracture wall  (arrows in
Figures~\ref{fig2}E and~\ref{fig2}F), and  (2)~large sparite crystals
packing the remaining fracture space correspond to the main infilling
material of these fractures  (Fr2 on the top right of
Figures~\ref{fig2}E and~\ref{fig2}F).

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig03} 
\vspace*{-4pt}
\caption{\label{fig3}\fontsize{9.45}{11.4}\selectfont
Microphotographs of thin sections from samples of
the La Rouvi\`{e}re Fault Core.  (A)~Scan of the sample EC1A entire
thin section showing the crosscutting relationships between fracture
families and breccia phases. The left part of the image is a clast from
breccia 1,  (B)~Close-up CL image showing the highly bright orange
luminescence of Fr3 and crosscutting relationships with previous
fractures and breccias,  (C)~EC2 sample showing main sigmoidal shape
fracture filled by calcite and pyrite crystals at the scale of the hand
specimen,  (D)~EC2 fracture filled with calcite and pyrite observed in
reflected light. Note the pyrite crystals fragmentation at the bottom
right, (E)~Main EC2 fracture in cathodoluminescence image (CL), 
(F)~Calcite growing episodes of main EC2 vein under cross polarized
light, (G)~Main and second phase of pyrite crystallization in reflected
light. Note that U--Pb ages obtained in this study have been added on
photographs for clarity.}
\end{figure*}

Fr1 and Fr2 are very often crosscut by pervasive stylolites planes 
(Figures~\ref{fig2}G and~\ref{fig2}H). This deformation phase is also
evidenced by the occurrence of sheared stylolites  (Supplementary
Figure~3) and sigmoidal en-echelon calcite vein arrays that
formed through pressure solution at the restraining bends of
pre-existing fractures  (Figure~\ref{fig2}C and  Supplementary 
Figure~3).

In the fault core, the two main phases of brecciation are well observed
at the microscopic scale  (Samples EC1, EC2 and partially EC3, 
Figures~\ref{fig3}A and~\ref{fig3}B). As suspected at the macroscopic
scale, the proportion of angular clasts is higher in breccia 1  
(e.g.\ EC1 and EC2 samples in  Supplementary Figures~2 and~4) than in
breccia 2 (EC5 in  Supplementary Figure~2). In addition to the
presence of more rounded clasts, the matrix in breccia 2 is
characterized by pervasive micro-stylolithes, concavo-convex clast
contact  (Supplementary Figure~3), as well as compaction
and/or shear bands that developed as a consequence of pressure solution
(Figure~\ref{fig3}A and EC1 sample in Supplementary Figure~2
and~3). This matrix is also characterized
by the presence of iron and possibly clay minerals 
(Figure~\ref{fig3}A), whose presence could be associated with pressure
solution processes, increasing the ratio of non-soluble over initial
calcitic material. This breccia 2 matrix is also observed in
non-brecciated areas, injected into re-opened fractures and dilatant
zones  (EC3 sample in  Supplementary Figure~2, and arrows in 
Supplementary\break Figure~3).

At the ${\sim}$22~m depth interval, the breccia 1 type core sample
(EC2) is affected by a main fracture with a  (sigmoidal shape) opening
space that seems to crosscut both breccias 1 and 2 
(Figures~\ref{fig3}C,  \ref{fig3}D and~\ref{fig3}E and  Supplementary
Figure~2). We interpret this fracture to be syn-to
post-breccia 2  (see above and Figure~\ref{fig2}C). This EC2 fracture
displays a millimeter-sized opening filled with calcite crystals
nucleating on the fracture wall  (Figure~\ref{fig3}C). Observations in
CL  (Figure~\ref{fig3}E) and under cross-polarized light  (Figure~\ref{fig3}F)
highlight successive growing episodes of this syntaxial vein. From the
wallrock to the median line, we can see a fringe of small calcite
crystals  $(20~\rmmu\mathrm{m}<\mathrm{size}<50~\rmmu\mathrm{m})$
overlain by larger  (${\sim}$100~$\rmmu$m) tooth shape calcite crystals
(``Dog-tooth core'' in Figures~\ref{fig3}E and~\ref{fig3}F). In CL,
both crystal types are characterized by a dull orange luminescence
suggesting that they formed at the same time  (Figure~\ref{fig3}E). In
addition, a fringe of bright orange calcite overlies each of the large
dog-tooth shape crystals  (``Dog-tooth rim'' in Figures~\ref{fig3}E
and~\ref{fig3}F). This new growing episode forms a continuous rim that
can reach up to  200~$\rmmu$m in thickness  (Figure~\ref{fig3}E). Also,
the presence of pyrite grains growing on the top of this zoning calcite
area  (Figures~\ref{fig3}D and~\ref{fig3}G) indicates that chemically
different fluids have circulated through this EC2 sample. In addition
to the zoned calcite minerals, different observations made from the
pyrite grains analysis allows us to confirm the presence of different
signatures of fluids in the fault system: (1)~zonation areas revealed
in pyrite grains using reflected light, and electron microprobe
analyses  (Supplementary Figures~4 and~5 and Supplementary
Table~2), (2)~local fine fragmentation of pyrite crystals
possibly related to fluid overpressure during faulting 
(Figure~\ref{fig3}D), and (3)~reprecipitation of tiny pyrite grains
along calcite veins  (Figures~\ref{fig3}D and~\ref{fig3}G) forming an
angle of  ${\sim}$25\textdegree--30\textdegree\ with the main fracture surface 
(Figure~\ref{fig3}F), and/or at the boundary between the host breccia
rock and the first calcite generation  (Figure~\ref{fig3}G). As
suspected from previous observations and cross-cutting relationships,
this sample clearly reveals a polyphase history of the LRF zone.

Following up these two brecciation events, a main last phase of
fracturing Fr3, cross-cutting all the above described structures, is
well observed in the fault core samples  (Figure~\ref{fig3}A). These
Fr3 features are distinguished in CL where they appear as a bright
orange colour  (Figure~\ref{fig3}B). This third event is also
recognizable by the presence of more or less tortuous fractures 
(Figure~\ref{fig3}B) filled with altered subhedral 
\mbox{micro} to macro
sparite suggesting to some degree that a phase of dissolution occurred.
Also, these Fr3 fractures are not systematically cemented as they can
occasionally contain pore
spaces.

\section{U--Pb geochronology}\label{sec4} 

\subsection{U--Pb isotopes analytical methods}\label{sec4.1} 
U--Pb isotopes of carbonate minerals were determined by laser ablation
inductively coupled mass spectrometry  (LA-ICP-MS) with an ESI
NWR193UC laser coupled to a Nu AttoM ES sector field ICP-MS at
GeoRessources, Universit\'{e} de Lorraine. Key instrument parameters
are included in the  Supplementary Table~1  (Table~1
SX). Key laser operating conditions include a laser spot size of
between 80--150~$\rmmu$m with 8~Hz repetition rate and between 
$2\ndash4~\mathrm{J}{\cdot}\mathrm{cm}^{-2}$ energy density at the
sample surface. The following masses were monitored with the respective
dwell time  (in microseconds) in parentheses: 202 (10), 204 (10), 206
(10), 207 (50), 208 (10), 232 (10), 235 (10), 238 (10). This
configuration gives a total integration time of 0.1668 seconds for all
masses. Raw data were processed in Iolite  (v.\ 4) 
\citep{Patonetal2011} using the UPb Combine DRS 
\citep{PetrusKamber2012}. The WC1 calcite was used as the primary
reference material for U/Pb ratios  [assuming an age of 
$254.4\pm6.4$~Ma and $\mbox{}^{207}$Pb/$\mbox{}^{206}$Pb$_{i}$ of 0.85
${\pm}$ 0.04;  \citealp{Robertsetal2017}], and the NIST SRM614 for
Pb/Pb ratios  [assuming  $\mbox{}^{207}$Pb/$\mbox{}^{206}$Pb of 
$0.8710\pm0.008$; \citealp{WoodheadHergt2001}]. The Duff Brown Tank
limestone  [U--Pb age of  $64.07\pm0.67$;  \citealp{Hilletal2016}] and
the B6 calcite breccia  [U--Pb age of  $43.0\pm1.0$~Ma; 
\citealp{Pageletal2018}] were used to validate our reported ages. For
all sessions  (Supplementary Table~1 SY) we obtained a pooled
lower intercept age of  $63.42\pm1.19$~Ma  (2s,  $n=62$, MSWD ${=}$ 1.5)
for the Duff Tank Limestone, and  $40.67\pm0.55$~Ma  (2s,  $n= 55$,
MSWD ${=}$ 0.94) for the B6 calcite breccia. We report uncertainty at two
levels for all samples in the form age ${\pm}x(y)$ Ma whereby  $x$ is
the measurement uncertainty, and $y$ includes an excess variance of 2.5\%
\citep[estimated after][]{Guillongetal2020} added to  $x$ in
quadrature. The concentration Pb  (total of all isotopes), Th, and U
were obtained using the Trace Elements DRS with the NIST614 
\citep{Jochumetal2011} as the primary reference
material.

\subsection{U--Pb isotopes results}\label{sec4.2}
U--Th--Pb concentrations and U--Pb isotope ratios are included in 
the Supplementary Table~1 (Table~SZ). Rejected analyses are marked
by an ``$x$'' in the data tables, and represented by an unfilled
ellipse in  Figure~\ref{fig4}. These values were rejected for low
concentrations leading to large measurement uncertainties. 

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig4}$\mbox{}^{238}$U/$\mbox{}^{206}$Pb  vs.
$\mbox{}^{207}$Pb/$\mbox{}^{206}$Pb  (a.k.a Terra-Wasserburg) diagrams
for all analysed sampled.}
\end{figure*}

\begin{table*}
\caption{\label{tab1}Summary of U--Pb results\vspace*{-1pt}}
\begin{tabular}{cccccccc}  
\thead
Sample & $n$ & Age & 95\% conf. & 95\% conf. $+$\ $\varepsilon$ & 
$\mbox{}^{207}$Pb$\mbox{}^{206}$Pb$_c$ & 95\% conf. & MSWD \\ 
\endthead
EC1A---Fr3 & 24 & 25.1 & 1.7 & 1.8 & 0.755 & 0.013 & 0.99 \\ 
EC2A---Dog-tooth rim & 21 & 37.3 & 0.7 & 1.2 & 0.810 & 0.250 & 1 \\ 
EC2A---Dog-tooth core & 23 & 39.9 & 1.1 & 1.5 & 0.733 & 0.022 & 0.79 \\ 
EC4A---Fr2 (vein 2) & 26 & 50.7 & 2.4 & 2.7 & 0.711 & 0.089 & 0.93 \\ 
EC4A---Fr2 (vein 1) & 26 & 53.0 & 8.0 & 8.1 & 0.793 & 0.087 & 1.2 \\ 
EC5B---Fr1 & 31 & 78.1 & 2.2 & 2.9 & 0.741 & 0.052 & 1.2 \\ 
EC4C---Fr1 & 33 & 81.5 & 1.1 & 2.3 & 1.050 & 0.370 & 1.3 \\ 
EC1A---Fr1 & 11 & 82.5 & 9.2 & 9.4 & 1.140 & 0.530 & 1.1 \\ 
EC5B-Recrystal. micrite & 22 & 87.7 & 3.8 & 4.4 & 0.796 & 0.073 & 0.68 
\botline 
\end{tabular}
\end{table*}

All calcite minerals in this study have low and variable U between
0.013 and 13.50~ppm, and Pb$_{\mathrm{total}}$ between 0.008 and
1.15~ppm.  Thorium concentrations are typically below or at the limit
of detection. However some samples show relative enrichment of up to 
1.4~ppm. The range in U and  Pb$_{\mathrm{total}}$ of these carbonate
minerals is like concentrations found in carbonates from a range of
geological environments elsewhere including veins, diagenetic, biogenic
and speleothem  \citep[e.g.,][]{Robertsetal2020}. 

All carbonate samples contain non-radiogenic Pb leading to discordant
analyses on a  $\mbox{}^{238}$U/$\mbox{}^{206}$Pb vs.
$\mbox{}^{207}$Pb/$\mbox{}^{206}$Pb  (a.k.a Terra-Wasserburg) diagram 
(Figure~\ref{fig4}). Lower intercept ages and their uncertainties are
included in the  Supplementary Table~1 (Table Summary of ages)
with the upper intercept initial Pb composition estimate, and the
mean-square-weighted-deviation  (MSWD) of the Discordia fit. Ages range
from  $87.7\pm3.8$ (4.4) Ma to  $25.1\pm1.7$ (1.8) Ma with MSWD 1.3 or
lower  (Figure~\ref{fig5}; Table~\ref{tab1}). Upper intercept
$\mbox{}^{207}$Pb/$\mbox{}^{206}$Pb$_{c}$ estimates are within the
range of the terrestrial bulk Earth for their age 
\citep[e.g.][]{StaceyKramers1975} except for EC1A-Fr3, EC2A-Dog-tooth
core, EC4A-Fr2  (vein 2) and EC5B-Fr1 (sigmoid) with values closer to
{ca.}\ 0.75 and indicative of a more radiogenic common Pb isotopic
composition.

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Summary of U--Pb ages with uncertainties.}
\end{figure}

\section{Discussion}\label{sec5}
The core samples analysed in this paper were taken from the first hole
drilled through the LRF which was reactivated during the 2019 Mw 4.9 Le
Teil earthquake in France. This first drilling is part of the
``HYPERDRILL'' project aiming at reaching the focal depth of this recent
earthquake to study rock mechanics and the impact of fluids on the
nucleation and propagation of seismic ruptures 
\citep[e.g.][]{Paglialungaetal2023}. Here, our first goal was to
reconstruct the long-term history of this deformation zone by applying
LA-ICP-MS in situ calcite U--Pb geochronology on distinct generations
of calcite-filled veins.

\subsection{Relative versus absolute dating 
(Figure~\myRef{fig6})}\label{sec5.1} 
Microstructural and relative chronology data from carbonate rocks of
the LRF zone highlight four main tectonic events that have been
bracketed by U--Pb geochronology between ${\sim}$25~Ma and
${\sim}$90~Ma  (Figures~\ref{fig4} and~\ref{fig5}; Table~\ref{tab1}). 

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig06}
{\vspace*{-3pt}}
\caption{\label{fig6}Chronological timeline linking the main identified
(micro) structures to the structural diagenesis events. Note that U--Pb
ages bracketing the main identified tectonic events have been
mentioned.}
{\vspace*{-3pt}}
\end{figure*}

Microstructural analysis of representative calcite veins allows us to
identify a first opening-mode fracture  (Fr1) closely associated with
angular clasts breccia 1 that we interpret to reflect extensional
tectonics. These  (micro)structures are superimposed by a second
episode characterized by brecciated rocks  (breccia 2) and fractures
(Fr2) as well as shear deformation markers such as sheared stylolites
and sigmoidal sheared-cracks that could be associated with a
strike-slip fault kinematics on the LRF plane 
\citep[e.g.][]{Tondietal2006}. Fracture filled by dog-tooth shape
calcite crystals characterizing the EC2 sample developed after these
two episodes. Finally, a last event is marked by open fractures Fr3 not
systematically cemented that clearly cross-cut the previous structures.

When comparing these four key markers with in situ calcite U--Pb
geochronology, we conclude that the LRF zone recorded the main tectonic
events identified in the South-East basin of France:  (1)~both the Fr1
cements and sparite from recrystallized micrite have recorded U--Pb
ages from ${\sim}$90 to 75~Ma  (Figures~\ref{fig2}, \ref{fig4}
and~\ref{fig5};  Table~\ref{tab1}) that could be related to local
extensional tectonics during the late Cretaceous where sedimentation
was controlled by thermal subsidence in the Ard\`{e}che area inducing
the migration of hanging wall of listric normal fault system to the
east  \citep{Bonijolyetal1996},  (2)~Fr2 fracture-fill calcite cement
yielded an age of ${\sim}$50~Ma which is compatible with the later
phase of the Pyrenean-Provence tectonic compression phase 
\citep[e.g.][]{Thomassetetal2024},  (3)~ages of ${\sim}$40 and
${\sim}$37~Ma from the EC2 sample obtained on dog-tooth core and rim,
respectively, clearly indicate two stages of calcite crystallization
within the EC2 fracture that may have opened during the early stage of
the ESE--WNW extensional tectonics associated with the opening of Lion
Gulf  \citep{Seranneetal2002} and to a larger extent with the European
Cenozoic Rift System  (ECRIS)  \citep[e.g.][]{Dezesetal2004},  (4)~the
Fr3 fracture yielded an age of ${\sim}$25~Ma that we interpret to
reflect the ECRIS  \citep[e.g.][]{Dezesetal2004}.\looseness=1

\subsection{The La Rouvi\`{e}re fault zone, a critical interface for
fluid flow}\label{sec5.2} 
Cathodoluminescence images highlight contrasting carbonate families for
each of the deformation markers. In calcite, luminescence is mainly
controlled by the trace presence of Fe$^{2+}$ and Mn$^{2+}$ ions
providing antagonistic effects 
\citep[e.g.][]{ScholleUlmer-Scholle2003} which in turn deliver
information on the fluid chemistry from which the calcite formed. As a
first approximation, early marine eogenetic cements generally display
dull luminescence, whereas bright luminescence is often associated with
deeper mesogenetic fluid or telogenetic meteoric diagenesis 
\citep[e.g.][]{Buddetal2000}. 
Based on these contrasted luminescence, we
can state that the different calcite rich veins from the LRF zone have
formed in the presence of different water composition, from sea water
and/or meteoric to connate water. In addition, the crystallization of
pyrite on the top of the \mbox{${\sim}$40--37~Ma} calcite minerals of the EC2
sample indicates that a fluid of different origin than marine and/or
surface fluids invaded the LRF core after the main phase of dog-teeth
calcite core and rim crystallizations. Based on the EC2 pyrite chemical
composition, three different origins of fluids could be considered to
explain pyrite precipitation. First, a local iron and sulfur enriched
fluid  (from intense pressure solution processes) that could have been
stocked during the second phase of brecciation  (breccia 2) and
released in response to the Oligocene extensional 
\mbox{tectonics.} Second, a
fluid sourced away from the fault core, which obtained the necessary
iron and sulfur contents to precipitate pyrite crystals in the EC2 main
fracture from the underlying organic matter-rich shale Valanginian
formation  \citep[e.g.][]{Berner1984}. Third, an allochthonous fluid
that has become enriched with sulfur by passing through underlying
Triassic evaporites  \citep[e.g.][]{Laurentetal2021}. As demonstrated
by the zoning of pyrite grains from EC2 revealed by electron microprobe
analyses  (Supplementary Figures~4 and~5 and  Supplementary
Table~2), a mixing of these different fluid sources could have
been involved. Also, the zoned dog-tooth calcite minerals from the EC2
sample yielding two different ages  (39.9 ${\pm}$1.1 (1.5) Ma (core)
and 37.3 ${\pm}$0.7 (1.2) Ma (rim)) provide information on the kinetics
of fracture cementation and sealing processes which, although being
thought to last from centuries  \citep{Richardetal2015} to millions of
years  \citep{Beckeretal2010}, have not been so far quantified. Here,
our U--Pb results indicate that these processes can remain active
during at least ${\sim}$1~Ma.

Determining the fluids source that circulated in the La Rouvi\`{e}re
deformation zone is beyond the scope of this paper. However, our study
emphasizes the polyphase nature of the tectonic history recorded in the
Le Teil area and points out that each tectonic episode has led to the
formation of new structures and/or re-opening of fracture networks that
enhanced porosity and permeability providing pathways for fluids to
migrate and precipitate carbonate and/or pyrite. 

\subsection{Summary of tectonic events recorded by the La Rouvi\`{e}re
fault in the frame of the C\'{e}vennes Faults System}\label{sec5.3} 

{\vspace*{-2pt}}

The chronology and the kinematics of tectonic deformations along the
C\'{e}vennes Faults System and the western boundary of the South-East
basin of France have been constrained by geophysical investigations 
(seismic reflection profiles, deep drilling, structural modeling) and
field observations 
\citep[e.g.][]{Roureetal1992,Bonijolyetal1996,Elmietal1996,MartinBergerat1996,Marconatoetal2022,Thomassetetal2024}. 
As mentioned above  (Section~\ref{sec5.1}), our U--Pb results highlight
four main successive tectonic episodes which can be grouped into the
following time intervals:  (1)~from ${\sim}$90 to 75~Ma,  (2)~from
${\sim}$60 to 50~Ma,  (3)~from ${\sim}$40 to  37~Ma, and 
(4)~${\sim}$25~Ma. We interpret these events to reflect  (1)~thermal
subsidence during the late Cretaceous in the Ard\`{e}che region which
is the western margin of the SE basin  \citep{Bonijolyetal1996}, 
(2)~the late-Pyreneo-Proven\c{c}al deformation related to the N--S
trending shortening of the South-East basin 
\citep{Thomassetetal2024},  (3)~the opening of the Gulf of Lion which
is also closely related to the ECRIS 
\citep[e.g.][]{Seranneetal2002,Dezesetal2004}, and  (4)~a middle to
late stage of the ECRIS.

We note that our studied samples did not record the late
Miocene--Pliocene tectonic episodes that have been documented by
previous authors on different regional faults of the CFS 
\citep[e.g.][]{Bonijolyetal1996,MartinBergerat1996}.

As shown by focal mechanisms and InSAR data 
\citep{Ritzetal2020,Marconatoetal2022}, the LRF experienced a reverse
faulting mechanism during the 2019  Le Teil earthquake which attests to
the inversion of this long-lived normal fault system 
\citep{Thomassetetal2024}. However, no morphotectonic evidence of
previous significant compressional tectonic event has been identified
in the field \citep{Ritzetal2020}. This is in agreement with our
microstructural data and related U--Pb results suggesting that the most
recent tectonic event recorded by the LRF is 25.1 ${\pm}$ 1.7 (1.8) Ma
old  (Figures~\ref{fig4} and~\ref{fig5}).\ This last event is very
likely related to the extensional tectonic regime that developed in
response to the ECRIS 
\citep[e.g.][]{Seranne1999,Thomassetetal2024}. 

Also, we note that E--W trending Miocene compressional tectonics related
to Alpine tectonics and documented in different areas within the
Rh\^{o}ne Valley  \citep{Bergerat1982,BlesGros1991} has not been
recorded in our studied samples. However, this does not preclude the
possibility that this event has not affected the Mont\'{e}limar region
as, although any geochronological data have been acquired, 
\citet{MartinBergerat1996} have interpreted structures from the Balazuc
borehole going through the Uzer fault further west and belonging to the
CFS to record Alpine tectonics. The only event suggesting a potential
LRF activity between ${\sim}$25~Ma and 2019 is from a paleoseismology
study from  \citet{Ritzetal2022} mentioning a paleoearthquake at ca.\ 
13.3 ka. This result highlights the very long quiescent period of a
fault that has therefore not been identified as active 
\mbox{\citep{Jomardetal2017}.}

\section{Conclusion}\label{sec6}
U--Pb geochronology on calcite within microstructures of core samples
from the first exploratory drilling cross-cutting the La Rouvi\`{e}re
fault plane  (south-east part of France) highlighted four main tectonic
events between ${\sim}$90 and  25~Ma: (1)~A late stage Cretaceous event
(90--75~Ma) revealed by a first generation of opening fractures that
formed coevally with the development of known regional listric
synsedimentary faults during thermal subsidence of the Ard\`{e}che
area, (2)~An early Eocene event  (${\sim}$50~Ma) marked by a second
fracture group that we interpret to reflect the late stage of the
\mbox{Pyrenean-Provence} tectonic compression/transpression phase, (3)~A mid
to late Eocene episode  (40--37~Ma) recorded by dog-tooth polyphase
calcite crystals from major fractures that very likely occurred during
the opening of the Gulf of Lion, and more widely, represent a
consequence of the European Cenozoic Rift System  (ECRIS) activity, and
(4)~A last event at 25~Ma recorded by narrow fractures cross-cutting all
the above structures that we relate to the late stage of the ECRIS. Our
studied samples did not record any compressional event associated with
Alpine tectonics, neither any reverse fault mechanism from intraplate
processes like that expressed by the Mw 4.9 2019 Le Teil earthquake. 

The La Rouvi\`{e}re fault represents a key object to study the complex
polyphase history of this part of Europe, and through the different
nature of structures and minerals it offers, makes it a natural
laboratory to study in the future the signature of fluids and fluid
flow history.

From a seismic hazard point of view, this study suggests that the La
Rouvi\`{e}re fault has had a very long period of inactivity, close to
25~Ma. Although long recurrence times are to be expected in an
intraplate domain with a very low deformation rate, such a period of
inactivity poses a challenge to the definition of active faults in this
type of environment.

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}
This study was funded through a BQR (Bonus Quality Recherche) project
in 2023 from the Observatoire de la C\^{o}te d'Azur.  The authors thank
the CNRS/INSU project Tellus FREMTEIL for funding the drilling through
La Rouvi\`{e}re fault and the company Sondafor for the drilling of the
borehole. 

\section*{Acknowledgements} 
We appreciated and thank O. Parizot and C. Thomasset for fruitful
discussions in the field. AG acknowledges support from GeoRessources.
CM thanks P.~L\'{e}onide for his access to cathodoluminescence at the
CEREGE laboratory. The authors thank Thin Section Lab for the quality
of polished thin and thick thin sections, and also Idriss Friry and the
TotalEnergies company for scanning the core samples. The authors thank
the editorial team and acknowledge the careful remarks of an anonymous
reviewer.

\back{}

\section*{Supplementary data}
Supporting information for this article is available on the journals
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-280-suppl.pdf}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{Table-S1.xlsx}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{Table-S2.xlsx}}

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