\makeatletter
\@ifundefined{HCode}
{\documentclass[screen,CRGEOS,Unicode,biblatex]{cedram}
\addbibresource{crgeos20250636.bib}
\newenvironment{noXML}{}{}
\let\citep\parencite
\let\citet\textcite
\def\xcitealp#1#2{\citeauthor{#1}, \citelink{#1}{#2}}
\def\citealt#1#2{\citeauthor{#1}, \citelink{#1}{#2}}
\newcommand*{\citelink}[2]{\hyperlink{cite.\therefsection @#1}{#2}}
\def\defcitealias#1#2{}
\let\citetalias\textcite
\let\citepalias\parencite
\def\thead{\noalign{\relax}\hline}
\def\endthead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}}
\def\tabnote#1{\vskip4pt\parbox{.96\linewidth}{#1}}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\ndash{\text{--}}
\def\hyphen{\text{-}}
\def\og{\guillemotleft}
\def\fg{\guillemotright}
\def\0{\phantom{0}}
\def\bDelta{\boldsymbol{\Delta}}
\def\bPhi{\boldsymbol{\Phi}}
\def\bbeta{\boldsymbol{\beta}}
\def\btau{\boldsymbol{\tau}}
\def\balpha{\boldsymbol{\alpha}}
\def\bgamma{\boldsymbol{\gamma}}
\def\mathbi#1{\text{\textbf{\textit{#1}}}}
\makeatletter
\g@addto@macro{\UrlBreaks}{\UrlOrds}
\gappto{\UrlBreaks}{\UrlOrds}
\RequirePackage{etoolbox}
\usepackage{upgreek}
\usepackage{amssymb}
\def\inlinefig#1{\includegraphics{#1}}
\def\jobid{crgeos20250636}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\def\xmorerows#1#2{#2}
\newcounter{runlevel}
\usepackage{multirow}
\def\nrow#1{\@tempcnta #1\relax%
\advance\@tempcnta by 1\relax%
\xdef\lenrow{\the\@tempcnta}}
\def\morerows#1#2{\nrow{#1}\multirow{\lenrow}{*}{#2}}
\let\MakeYrStrItalic\relax
\def\refinput#1{}
\csdef{Seqnsplit}{\\}
\def\botline{\\\hline}
\def\back#1{}
\newcommand*{\hyperlinkcite}[1]{\hyper@link{cite}{cite.#1}}
\DOI{10.5802/crgeos.326}
\datereceived{2025-10-09}
\daterevised{2026-02-02}
\dateaccepted{2026-02-02}
\ItHasTeXPublished
}
{
\PassOptionsToPackage{authoryear}{natbib}
\documentclass[crgeos]{article}
\usepackage[T1]{fontenc}
\def\CDRdoi{10.5802/crgeos.326}
\def\citelink#1#2{\citeyear{#1}}
\def\xcitealp#1#2{\citealp{#1}}
\def\href#1#2{\url[#1]{#2}}
\makeatletter
\def\CDRsupplementaryTwotypes[#1]#2#3{}
\def\hyperlinkcite#1#2{\citeyear{#1}}
}
\makeatother

\usepackage{upgreek}

\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\TopicFR{Tectonique, tectonophysique, g\'eodynamique}
\TopicEN{Tectonics, tectonophysics, geodynamics}

\title{Morphostructural analysis of the Lake Chambon Basin (Eastern
Monts Dore, Massif Central, France)}

\alttitle{Analyse morphostructurale et n\'eotectonique du bassin du lac
Chambon (Est des Monts Dore, Massif central, France)}

\author{\firstname{Ludovic} \lastname{Chender}\CDRorcid{0009-0009-1014-3703}\IsCorresp}
\address{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France}
\email[L. Chender]{ludovic.chender@doctorant.uca.fr}

\author{\firstname{R\'egis} \lastname{Thi\'ery}\CDRorcid{0000-0002-2984-5758}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France}
\email[R. Thi\'ery]{regis.thiery@uca.fr}

\keywords{\kwd{Structural geology}\kwd{Active
tectonics}\kwd{Lineament analysis}\kwd{Clustering}\kwd{GIS}
\kwd{Seismic hazards}}

\altkeywords{\kwd{G\'eologie
structurale}\kwd{Tectonique
active}\kwd{Analyse des lin\'eaments}\kwd{Classification}
\kwd{SIG}\kwd{Al\'eas sismiques}}

\begin{abstract}
The Lake Chambon area, located between the Col de la Croix-Morand and
Murol (Massif Central, France), consists of a Hercynian crystalline
basement partially overlain by Cenozoic formations, largely composed of
volcanic products related to the Mont-Dore stratovolcano. The
present-day topography, sedimentation patterns, and drainage network
are strongly controlled by a complex fault system. A detailed
morphostructural analysis identified more than 500 lineaments from a
high-resolution digital elevation model (DEM), which were digitized and
analyzed in a GIS environment using QGIS. A directional classification
combining expert-based interpretation with a semi-supervised
machine-learning approach (k-means clustering) revealed seven major
fault families, grouped into clusters consistent with a regional
dextral shear regime. An interpretive tectonic model is proposed,
consistent with the current stress field ($\sigma_1$ trending between
N160{\textdegree}E and N170{\textdegree}E). Faults of the F1 family are
interpreted as dextral shear zones related to the South Armorican Shear
Zone--Cholet--Poitiers Fault--Southern Border Fault of the Limagne
graben system, associated with secondary Riedel-type structures. The
influence of the sinistral Sillon Houiller Fault is expressed by the
F\tralicstex{6′′}{$6^{\prime\prime}$} family (N20{\textdegree}E) and by the F\tralicstex{2′}{$2^\prime$}
family, whose orientation is comparable to that of the
Tauves--Aigueperse fault system (N50{\textdegree}E). The F4 family
corresponds to extensional faults, locally reactivated within this
broader strike-slip tectonic framework. The proposed neotectonic
framework allows for the interpretation of several key geomorphological
features. The Lake Chambon Basin may correspond to a transtensional
pull-apart structure. In contrast, the slow-moving landslide at
Chambon-sur-Lac, located between the transtensional zones of the
Rochers de Pousseterre to the west and Lake Chambon to the east,
appears to be controlled by the structural inheritance and kinematics
of faults F4, F\tralicstex{6′′}{$6^{\prime\prime}$} and F\tralicstex{2′}{$2^\prime$}, which locally
accommodate oblique deformation within a transpressive regime. Finally,
the study suggests that deep hydrothermal activity at Chambon-sur-Lac
may be linked to regional seismicity associated with the F1 fault
system.
\vspace*{2pt}
\end{abstract}

\begin{altabstract}
La r\'egion du lac Chambon, situ\'ee entre le col de la Croix-Morand et
Murol (Massif central, France), est constitu\'ee dun socle cristallin
hercynien partiellement recouvert par des formations c\'enozo\"{i}ques,
majoritairement compos\'ees de produits volcaniques li\'es au
stratovolcan du Mont-Dore. La topographie actuelle, les modalit\'es de
la s\'edimentation et lorganisation du r\'eseau hydrographique sont
fortement contr\^ol\'ees par un syst\`eme de failles complexe. Une
analyse morphostructurale d\'etaill\'ee a permis didentifier plus de
500 lin\'eaments \`a partir dun mod\`ele num\'erique de terrain (MNT)
\`a haute r\'esolution, num\'eris\'es et analys\'es dans un
environnement SIG \`a laide de QGIS. Une classification directionnelle, combinant une interpr\'etation
experte et une approche dapprentissage automatique semi-supervis\'ee
(classification k-means), met en \'evidence sept grandes familles de
failles, regroup\'ees en clusters compatibles avec un r\'egime
r\'egional de cisaillement dextre. Un mod\`ele tectonique interpr\'etatif est propos\'e, en accord avec le
champ de contraintes actuel ($\sigma_1$ orient\'ee entre
N160{\textdegree}E et N170{\textdegree}E). Les failles de la famille F1
sont interpr\'et\'ees comme des zones de cisaillement dextre en lien
avec le Cisaillement Sud-Armoricain --- faille de Cholet--Poitiers ---
faille de la bordure sud du graben de la Limagne, associ\'ees \`a des
structures secondaires de type Riedel. Linfluence de la faille
d\'ecrochante s\'enestre du Sillon Houiller est exprim\'ee par la
famille F\tralicstex{6′′}{$6^{\prime\prime}$} (N20{\textdegree}E) et par la famille
F\tralicstex{2′}{$2^\prime$}, dont lorientation est comparable \`a celle du syst\`eme de
failles de Tauves--Aigueperse (N50{\textdegree}E). La famille F4
correspond \`a des failles extensives, localement r\'eactiv\'ees dans
ce cadre tectonique d\'ecrochant plus large.

Le cadre n\'eotectonique propos\'e permet dinterpr\'eter plusieurs
\'el\'ements g\'eomorphologiques majeurs. Le bassin du lac Chambon
pourrait correspondre \`a une structure de pull-apart transtensive. \`A
linverse, le glissement lent de Chambon-sur-Lac, situ\'e entre les
zones transtentives des Rochers de Pousseterre \`a louest et du lac
Chambon \`a lest, semble contr\^ol\'e par lh\'eritage structural et la
cin\'ematique des failles F4, F\tralicstex{6′′}{$6^{\prime\prime}$} et F\tralicstex{2′}{$2^\prime$}, qui
accommodent localement une d\'eformation oblique dans un r\'egime
transpressif. Enfin, l\'etude sugg\`ere que lactivit\'e hydrothermale
profonde \`a Chambon-sur-Lac pourrait \^etre li\'ee \`a une sismicit\'e
r\'egionale associ\'ee au syst\`eme de failles F1.
\end{altabstract}

%\input{CR-pagedemetas}

\maketitle

\vspace*{1pt}

\twocolumngrid

\end{noXML}

\defcitealias{chevremont22}{ibid.}
\defcitealias{chapron22}{ibid.}
\defcitealias{CRAIG2025}{ibid.}
\defcitealias{macaire97}{ibid.}
\defcitealias{gailler25}{ibid.}

\section{Introduction}

The northern and eastern slopes of the Monts Dore Massif constitute a
particularly complex structural zone. This area lies at or near the
intersection of two major tectonic systems of metropolitan France: the
South Armorican Shear Zone (SASZ) and the Sillon Houiller Fault (SHF).
This geographical position gives the region a key role in understanding
the recent tectonic structuring of the Massif Central and the
geodynamic evolution of the French territory.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig01}
{\vspace*{-2pt}}
\caption{\label{fig1}Simplified map based on data from the Database of
Potentially Active Faults (BDFA, \citep{irsn_bdfa}). Faults are shown
in black, without distinction of age or activity. The main locations
indicated are: \textbf{(1)}~Cholet--Poitiers Fault (CPF), forming one
of the eastern branches of the South Armorican Shear Zone (SASZ);
\textbf{(2)}~the Southern Border Fault of the Limagne Graben (SBLF)
between Issoire and Brioude, extending eastward toward Le Puy-en-Velay
and the Proven\c{c}al Basin; \textbf{(3)}~the Sillon Houiller Fault (SHF);
\textbf{(4)}~the Villefranche-de-Rouergue Fault (VRF); \textbf{(5)}~the
Loire Fault (LF); \textbf{(6)}~the Sillon Houiller Fault between Bert
and Le Creusot (BLCF) and its eastward continuation; \textbf{(7)}~the
Limagne Fault along its western border between Issoire and Volvic
(WBLF); \textbf{(8)}~the Tauves--Aigueperse Fault (TAF); and
\textbf{(o)}~the study area (red frame), located along the inferred
missing fault segment between the CPF and the SBLF.}
{\vspace*{-2pt}}
\end{figure*}

The South Armorican Shear Zone (SASZ) is a wide strike-slip zone
inherited from the Variscan orogeny, trending
generally NW--SE, with some branches still considered active
\citep{palumbo13}. One of its eastern extensions, supported by the
Cholet--Poitiers Fault (CPF), appears to continue toward the Southern
Border Fault of the Limagne Graben (SBLF), between Issoire and Brioude.
This fault then continues eastward toward the Puy-en-Velay Basin, and
further toward the Proven\c{c}al Basin (Figure~\ref{fig1}).

The Sillon Houiller Fault (SHF), on the other hand, follows a
N20{\textdegree}E trend and is characterized by an associated network
of faults, including the Loire Fault (LF) to the north and the
Tauves--Aigueperse Fault (TAF) to the northeast, which continues toward
the Bert--Le Creusot Fault (BLCF). Thus, the Sillon Houiller Fault also
constitutes a major tectonic structure, forming a vast seismogenic
lineament extending from the Villefranche-de-Rouergue Fault (VRF) to
the Paris Basin. The interactions between these two major tectonic
systems---the South Armorican Shear Zone (SASZ) and the Sillon Houiller
Fault (SHF)---remain poorly understood, particularly due to the
extensive cover of volcanic formations in the Massif Central.
\looseness=-1

Nevertheless, this structural context may have played a determining
role in the emplacement and evolution of regional volcanism, notably by
controlling magma ascent and the location of volcanic edifices.
Analyzing this zone of tectonic convergence is therefore essential for
refining our understanding of the recent geodynamic history of the
Massif Central.

The objective of this article is to highlight the respective influence
of the SASZ and the SHF on the present-day structuring of the eastern
slope of the Monts Dore. To this end, we present a detailed
morphostructural study of an area located between the Col de la
Croix-Morand and the town of Murol, around Lake Chambon. This analysis
is based on the integration of field observations with data derived
from aerial photographs, satellite imagery, and digital elevation
models. It leads to a preliminary interpretive model of tectonic
structuring in this transitional zone, along with an assessment of
associated seismic hazards.

\section{Geological context}\label{sec2}
\subsection{Previous work}\label{sec2.1}

The study area is located on the eastern slope of the Monts Dore
Massif. This zone has been the subject of several significant
geological studies, which today allow a better understanding of its
main characteristics. Among the notable contributions:
\begin{itemize}
\item As early as the late 1950s, Louis Glangeaud proposed a regional
geodynamic framework to explain volcanism within the Massif Central, in
connection with a network of faults and dykes oriented
N0{\textdegree}E, N60{\textdegree}E, and N120{\textdegree}E
\citep{glangeaud58, glangeaud65}.
\item Following the oil shocks of the 1970s, research programs were
initiated to assess the geothermal potential of the Massif Central. In
this context, the exploration work led by \citet{varet77, varet80}
provided a benchmark synthesis of the state of hydrological and
geophysical knowledge of the Monts Dore massif at that time, rightly
emphasizing that ``a profound lack of understanding of the Tertiary and
Quaternary tectonics\,\ldots\ at the scale of this massif still
remains'' \citep{varet77}.
\item Jean-Claude Besson \citep{besson78} specifically studied the
Mont-Dore volcanic formations, providing detailed insights into the
stratigraphy and nature of eruptive products.
\item In the 1990s, Professor Jean-Jacques Macaire's team (University
of Tours) \mbox{conducted} several in-depth studies of Lake Chambon.\ These
works focused mainly on lacustrine sediments, their volume and depth,
as well as sedimentation and erosion rates in the catchment during the
Late Glacial and Holocene periods \citep{macaire92, dupis96, macaire97,
gay98, gay99}.\looseness=-1
\item Around the same time, the Laboratoire Magmas et Volcans
(University of Clermont-Ferrand II) published studies on debris
avalanche deposits in the area \citep{cantagrel90, vidal96, bernard09}.
\item More recently, the BRGM released a preliminary harmonized vector
version of the geological map for the area (Veyre-Monton sheet),
accessible via the InfoTerre platform (Figure~\ref{fig2}).
The 1:50\,000 scale map has not yet been officially published, but a
provisional version of its explanatory booklet is available in open
access on the BRGM publications website \citep{chevremont22} at
\mbox{\href{https://www.geolfrance.brgm.fr}{geolfrance.brgm.fr}.}
\end{itemize}

\begin{figure*}[p!]
\includegraphics{fig02}
\caption{\label{fig2}Screenshot of the harmonized 1:50\,000 geological
map extracted from the BRGM Infoterre website
(\url{https://infoterre.brgm.fr}). A semi-transparent digital elevation
model is superimposed. \textbf{I}.~Surficial deposits and processes.
\textbf{F}:~alluvium. \textbf{FG}:~fluvioglacial deposits.
\textbf{G}:~moraines. \textbf{T}:~peat bogs. \textbf{E}:~volcanic rock
scree deposits. \textbf{C}:~volcanic rock colluvium.
\textbf{GT}:~rockfalls and landslides. \textbf{II}.~Recent Quaternary
volcanism. $\bbeta _{\mathbi{q}}$: leucobasalts (including those of
Tartaret). \textbf{III}.~Lower to Middle Pleistocene volcanism of the
Sancy massif. $\bDelta_2$:~Sancy debris-avalanche deposits. 
$\mathbf{S}_b$:~volcanoclastic formations of the Sancy.
\textbf{S}:~Lower to Middle Pleistocene sancyites of the Sancy massif.
$\btau\balpha_2$: Lower to Middle Pleistocene trachybasalts and
trachyandesites of the Sancy. \textbf{IV}.~Upper Pliocene to Lower
Pleistocene volcanism of the Monts Dore. $\bDelta_1$:~Monts Dore
debris-avalanche deposits. $\bPhi$:~phonolites.
$\btau\balpha_1$:~trachyandesites (broad sense). \textbf{D}:~mugearites
and dor\'eites. \textbf{O}:~ordanchites. $\btau\bbeta$:~Bressouleille
trachybasalts. $\bbeta_{o-p}$:~demi-deuil basalts, basalts with olivine
or pyroxene phenocrysts. \textbf{V}.~Pliocene volcanism of the D\^omes
Plateau. $\mathbf{S}_{p}$:~scoria and scoriaceous ejecta.
$\bbeta_{p}$:~undifferentiated basalts. 
\textbf{t-vs}:~undifferentiated Plio-Quaternary volcanism (tephra and
volcano-sedimentary deposits). $\bbeta_{m}$:~Miocene volcanism of the
D\^omes Plateau (ankaramites and basalts). \textbf{VI}.~Oligocene.
$\mathbf{g}_{1-2}$:~quartz--feldspathic clays and sands.
\textbf{VII}.~Hercynian basement. $\bgamma^3$:~two-mica granite of
Chambon-sur-Lac. $\bgamma^2$:~St-Nectaire granite. \textbf{Pg}:~Upper
Gneiss Unit (paraderived migmatites). \textbf{Lp}:~Upper Gneiss Unit
(leptynitic gneisses). Basement faults are shown in black.
\textbf{(a)}~Quancouve stream fault (QF); \textbf{(b)}~Jassat fault
(JF); and \textbf{(c)}~Saint-Victor-la-Rivi\`ere fault (SVRF). Yellow
stars indicate eruptive vents, whose alignments suggest probable magma
ascent pathways. Arabic numerals and red circles indicate major
landslide areas potentially related to recent seismogenic faults:
\textbf{1}:~Dent du Marais landslide; \textbf{2}:~Champsiaux--Berlaire
(or Chambon-sur-Lac) landslide; \textbf{3}:~Montagne de la Plate
\mbox{landslides}; \textbf{4}:~Jonas landslide; \textbf{5}:~La Borie
landslide; \textbf{6}:~Murol landslide. Black arrows indicate the
direction of debris-avalanche movement.}
\end{figure*}

\vspace*{-6pt}

\subsection{Geological framework}\label{sec2.2}

An initial analysis of the harmonized 1:50\,000-scale geological map
from the BRGM \citepalias{chevremont22} reveals the superposition of recent
volcanic formations over an older granito-metamorphic basement. The
main geological units identified in the study area are as
\mbox{follows}:
\begin{itemize}
\item \textbf{The two-mica granite of Chambon-sur-Lac} constitutes the
main plutonic unit of the region. Its precise boundaries remain
uncertain, largely obscured by the overlying volcanic formations. It is
nevertheless noteworthy that this pluton is cut by several normal
faults striking N140--150{\textdegree}E in the area west of Lake
Chambon.
\item \textbf{The metamorphic formations of the Upper Gneiss Unit
(UGU)} appear to the east of the Chambon granite. These are leptynitic
gneisses that form the metamorphic country rock of the granite pluton.
The contact between these two units is tectonic, marked by the
\textbf{Quancouve stream fault} (QF), oriented N150{\textdegree}E. This
fault appears to extend along two segments:
\begin{enumerate}[(1)]
\item Southward, along the northern flank of the Suc du Coq and near
the hamlet of \textbf{Jassat}, where it is referred to as the
\textbf{Jassat Fault (JF)}.
\item Further south, near \textbf{Saint-Victor-la-Rivi\`ere}, where it
is referred to as the \textbf{Saint-Victor-la-Rivi\`ere Fault (SVRF)}.
\end{enumerate}
\end{itemize}

Observation of the geological map (Figure~\ref{fig2}) suggests an
apparent offset between segments QF, JF, and SVRF, which can be
initially interpreted as the result of sinistral strike-slip movement
along masked faults oriented N80--90{\textdegree}E. However, this
interpretation remains hypothetical as long as the vertical
displacement and the dip direction (eastward or westward) of the faults
are not better constrained.
\begin{itemize}
\item \textbf{Paragneisses of the Upper Gneiss Unit (UGU)} (Pg) are
also present south of the Tartaret volcano and in the southeastern
corner of Lake Chambon.
\item \textbf{Trachyandesites} ($\tau\alpha_1$), of Upper Pliocene to
Lower Pleistocene age, form domes or dome--lava flow complexes, well
developed between Puy de la Tache and Puy de l'Angle within the
Adventive Massif west of Lake Chambon.
\item \textbf{Trachybasalts} ($\tau\beta$) crop out west of
Bressouleille, to the northwest of Lake Chambon.
\item \textbf{Undifferentiated basalts} ($\beta_p$) are mapped around
Beaune-le-Froid (north of Lake Chambon) and as basement units beneath
Puy de Bessolles (east of Lake Chambon and the Tartaret volcano).
\item Finally, north of Lake Chambon, \textbf{debris-avalanche
deposits} ($\Delta_2$), referred to as the Fontenille deposits, are
clearly identifiable. These deposits have been dated at approximately
1~Ma \citep{cantagrel90, vidal96, bernard09}.
\end{itemize}

The excerpt from the harmonized 1:50\,000-scale geological map provides
a synthetic representation of surface formations and structures, but it
does not allow for a direct inference of the current stress state or
ongoing deformation. The studied area, located within the crystalline
basement of the \mbox{Massif} \mbox{Central,} is characterized by predominantly
brittle deformation at the surface, expressed through dislocations,
joints, and faults. These structures \mbox{influence} the organization
of the topography, particularly the course of the drainage network,
which responds sensitively to fracturing of the basement. In addition,
the alignment and distribution of volcanic edifices provide indirect
clues about the organization of deeper fault structures within the
crust. Thus, morphostructural interpretation can offer complementary
insights to those provided by conventional geological data
\citep{hauchard02}.

\vspace*{-2pt}

\subsection{Recent Quaternary geomorphological{\hfill\break} evolution}
\label{sec2.3}

\vspace*{-2pt}

The morphostructural evolution of the Lake Chambon area results from a
succession of major events, the consideration of which is essential for
a proper understanding and interpretation of the geomorphological and
seismotectonic data:

\begin{itemize}
\item The Chaudefour Valley, the Couze Surains Valley, as well as the
downstream part of the Couze Chambon Valley as far as Sapchat,
correspond to a former glacial valley exhibiting characteristic glacial
landforms, including upstream glacial cirques, U-shaped valleys, and
associated till deposits \citep{varlet76, GoerDeHerve1992,
chevremont22}.
\item Glacial retreat promoted the development of postglacial
gravitational deposits, including rockfall deposits, landslides, and
debris flows, reflecting a phase of slope destabilization following
deglaciation.
\item In particular, 1.6~km south of Monneaux lies the large
amphitheater of the Montagne de la Plate, developed on the southern
flank of the Chaudefour Valley, 5~km upstream from Lake Chambon. This
amphitheater was formed by two successive rock-slope failures
\citep{GoerDeHerve1992}, respectively referred to as the
``Pal\'eo--Plate'' and the ``N\'eo--Plate''. The N\'eo--Plate landslide
gave rise to a former landslide-dammed lake, known as Lake Monneaux,
which is thought to have persisted during the period 8500--8000~BP
\citep{macaire94}.\looseness=-1
\item \looseness=-1 The construction of the strombolian volcanic edifice of Tartaret,
dated to \mbox{approximately} 12\,500--12\,600~years BP \citep{macaire92,
vidal96, macaire97}, led to the formation of a volcanic-dammed lake,
referred to as Lake Tartaret. This lake flooded the downstream part of
the Chaudefour Valley as well as the present extent of Lake Chambon,
before disappearing around 8500~years BP \citep{macaire97} or
9500~years BP \citep{chapron22}.
\item Lake Chambon formed following the collapse of the Dent du Marais,
which occurred around 2600~years BP \citep{vidal96} or around
2730~years BP ${\pm}$ 30~years \citep{chapron22}. Following this event,
the lake initially reached a much larger extent, corresponding to
approximately twice its present-day surface area.
\item A second landslide of the Dent du Marais, identified by
\citetalias{chapron22}, is thought to have occurred around A.D.~1250 ${\pm}$
30 years, contributing to the morphological reorganization of the basin
in its northeastern part (Lake Lacassou).
\end{itemize}

\subsection{Neotectonic framework} \label{sec2.4}

The site of Chambon-sur-Lac lies at the intersection of multiple
tectonic influences:
\begin{itemize}
\item to the southwest, the continental collision between the Eurasian
plate and the Iberian plate, which generates the Pyrenean mountain
range;
\item to the south, the push of the African plate toward the European
plate in the Mediterranean region;
\item to the southeast, the collision between the Eurasian plate and
the Apulian plate, which gives rise to the Alpine range and possibly
contributes to asthenospheric upwellings responsible for the rifting
and volcanism of the Massif Central \citep{michon00};
\item and to the west and northwest, the uplift related to the
Icelandic hotspot and the oceanic accretion along the North Atlantic
Ridge.\looseness=-1
\end{itemize}

A major synthesis of the stress state, crustal deformation, and
seismicity in metropolitan France was published by \citet{mazotti20}.
Figure~\ref{fig1} shows the potential seismogenic faults in
metropolitan France in relation to the observed seismicity. While
regions such as the Alps, the Pyrenees, Brittany, or the Rhine Graben
show seismicity that can be easily linked to regional fault systems,
this is not the case for the Massif Central. For instance, it may be
tempting to extrapolate one of the branches of the South Armorican
Shear Zone southeastward across the Massif Central via the
Murol--Brioude--Le Puy fault.

The currently active stress field in France, particularly in the Massif
Central, is fairly well known (Figure~\ref{fig3}), and some general
trends emerge regarding the preferential directions of active faults
\citep{godefroy84, dorel95, chassiot16}:\looseness=-1

\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig03}
\vspace*{-4pt}
\caption{\label{fig3}Map of the stress state in metropolitan France.
Red lines indicate the directions of maximum horizontal stress
\citep{mazotti20}.}
\vspace*{-4pt}
\end{figure*}

\begin{itemize}
\item the \textbf{north--south trend} corresponds to that of the
Western Border Limagne Fault (WBLF), which was mainly active during the
Oligocene but whose activity continues to the present day. This N--S
trend also characterizes the faults that likely facilitated magma
ascent within the Monts Dore Massif, in relation to the E--W extension
of the massif and the likewise N--S arrangement of the two superposed
calderas above former magma chambers.
\item the N--S trend is close to that of the Sillon Houiller Fault
(SHF), located approximately 25~km west of Chambon-sur-Lac, whose
present-day kinematics are interpreted as sinistral strike-slip along a
N20{\textdegree}E direction.
\item Associated with the Sillon Houiller Fault is the
Tauves--Aigueperse Fault (TAF), striking N50{\textdegree}E, of
sinistral strike-slip type, and located only about 15~km west of the
study area.\looseness=-1
\item the \textbf{NW--SE trend} extends the major South Armorican Shear
Zones (SASZ), which for the most part currently operate as dextral
shear zones \citep{palumbo13}.
\end{itemize}

To date, the structural geology of the area remains poorly documented
in detail. The forthcoming explanatory note of the 1:50\,000 geological
map of the Veyre--Monton sheet \citep{chevremont22} does not include a
chapter specifically devoted to tectonic structures. Only a limited
number of faults have been mapped in this sector (Figure~\ref{fig2}),
and many of them are concealed by volcanic or sedimentary
\mbox{formations}.

A more comprehensive compilation of structural data (Figure~\ref{fig5})
was carried out using the OGC WFS services provided by the BRGM. When
visualized in QGIS, these data reveal a clear predominance of
NW--SE--oriented faults, striking between N120{\textdegree}E and
N130{\textdegree}E. N--S--trending faults, such as the Western Border
Limagne Fault, are less well represented.

\begin{figure*}
\includegraphics{fig04}
\vspace*{-1pt}
\caption{\label{fig5}Geological map displayed in QGIS over the digital
elevation model (DEM) provided by the French National Geographic
Institute (IGN). The map covers an area extending from the Monts Dore
Massif to Brioude. Black lines represent faults, either ancient or
active, catalogued by the WFS service ``FR BRGM 1:1M Surface Geologic
Faults'', and geological units are provided by the WFS service ``FR
BRGM 1:1M Surface Geologic Units Age''. The red box outlines the study
area addressed in this article. \textbf{1}:~Lake Chambon.
\textbf{2}:~Haute--Dordogne caldera. \textbf{3}:~Sancy caldera.
\textbf{4}:~Cha\^{\i}ne des Puys. \textbf{5}:~Limagne Graben.
\textbf{6}:~Lake Aydat. \textbf{7}:~Lake Pavin. \textbf{8}:~Lake
Chauvet. \textbf{9}:~Lake Montcineyre. \textbf{10}:~Lake d'En Haut (La
Godivelle). \textbf{11}:~Brioude. \textbf{SBLF}: Southern Border Fault
of the Limagne Graben. \textbf{WBLF}: Western Border Fault of the
Limagne Graben. \textbf{SHF}: Sillon Houiller Fault. \textbf{TAF}:
Tauves--Aigueperse Fault.}
\vspace*{-1pt}
\end{figure*}

Accordingly, the Chambon-sur-Lac region appears to be part of a NW--SE
fault system, with a preferred orientation ranging from
N110{\textdegree}E to N130{\textdegree}E, extending between the Monts
Dore and Brioude and trending toward Le Puy--en--Velay. This trend is
corroborated by the map in Figure~\ref{fig6}, which locates the
epicenters of several historical earthquakes. These seismic foci
clearly align along a NW--SE direction. Epicentral intensities are
reported in Table~\ref{tab1}, which lists eight earthquakes over a
relatively short period of approximately 90~years (from 1833 to 1921),
to which a more recent event, occurring in 1980 at the Croix--Morand
Pass, can be added.

\begin{figure*}
{\vspace*{.2pc}}
\includegraphics{fig05}
{\vspace*{.2pc}}
\caption{\label{fig6}Map of historical earthquakes since 1833.
Epicentral intensity, expressed according to the MSK scale, is
represented by the color and size of the symbols (source: BRGM,
InfoTerre portal). The shaded bands delineate the main regional
seismogenic corridors: a dominant corridor trending N110{\textdegree}E,
associated with the Croix--Morand--Murol Fault (CMF) and the southern
border fault of the Limagne Graben (SBLF), and a secondary corridor
following the Couze Pavin and Couze Chambon Valleys, oriented
approximately E--W (N90{\textdegree}E), interpreted as structurally
connected to the main seismogenic system.\ Yellow arrows indicate the
sense of relative displacement of the fault blocks along the main
dextral strike--slip structures.\ WBLF corresponds to the{\break}
western border fault of the Limagne Graben. Fault traces are extracted
from the BRGM database \textit{``FR BRGM 1:1M Surface Geologic
Faults''}.}
{\vspace*{.2pc}}
\end{figure*}

\begin{table}[t!]%tab1
\tabcolsep=8pt
\caption{\label{tab1}Historical earthquakes in the study area
{\vspace*{.2pc}}}
\begin{tabular}{ccc}
\thead
Location & Date--time & I\\
\endthead
Issoire & 10/15/1833 & IV\\
(21) & 23:00 &\\
Puy de Monne & 01/01/1844 & IV\\
(6.1) & 10:00 &\\
Chambon-sur-Lac & 01/03/1844 & V--VI\\
& 23:00 &\\
La Grande Cascade & 09/18/1863 & V\\
(8) & 18:20 &\\
La Grande Cascade & 09/18/1863 & V\\
(8) & 19:15 &\\
La Banne d'Ordanche & 10/11/1885 & V\\
(12.5) & 14:06 &\\
Boudes & 08/26/1892 & VII\\
(24) & 10:10 &\\
Lac du Gu\'ery & 05/24/1921 & IV--V\\
(9.2) & 03:52 &\\
Col de la Croix--Morand & 04/01/1980 & IV\\
(6.5) & 01:24 &
\botline
\end{tabular}
\tabnote{$I$ denotes the epicentral intensity (MSK). The number in
parentheses indicates the distance in kilometers between the epicenter
and Lake\break Chambon.}
{\vspace*{.2pc}}
\end{table}

In addition to the main NW--SE seismogenic corridor, the Couze Pavin
and Couze Chambon valleys may also represent a secondary seismogenic
zone, as suggested by the widespread occurrence of rockfalls and
landslides along these valleys, although not all can be unequivocally
attributed to seismic {triggering} \citep{vidal96}. While fluvial
reorganizations were strongly influenced by debris avalanches
\citep{pastre97}, their recurrence within a structurally controlled
corridor suggests that seismic \mbox{activity} may have acted as a
long-term preconditioning factor.\looseness=1

This historical seismicity fits within the broader framework of active
deformation in the Massif Central, where Quaternary seismites in the
Limagne basin record major seismic crises during the Late Boreal and
the Middle Ages \citep{vernet13,chassiot16,chapron22}. The high density
of mantle-derived CO$_2$ emissions along this seismogenic corridor
\citep{varet77,ricci24} further supports the presence of a major
N110\textdegree--130\textdegree E crustal fault zone east of the Monts
Dore massif.

\vspace*{-2pt}

\section{Methodology} \label{sec3}

\vspace*{-1pt}

\subsection{The digital elevation model} \label{sec3.1}

\vspace*{-1pt}

The first step of the work consisted in selecting a Digital Elevation
Model (DEM) with optimal resolution. For this purpose, open-access data
provided by the {Centre R\'egional Auvergne-Rh\^one-Alpes de
l'Information G\'eographique (CRAIG)} were used \citep{CRAIG2025}.

Initially, the ``Puy-de-D\^ome'' DEM, with a 10-meter resolution,
produced by the TopoGeodis \mbox{company} for CRAIG, was selected. This
dataset, available in the form of 3978 tiles via the CRAIG platform
\citepalias{CRAIG2025}, was assembled using the QGIS software and tools from
the GDAL library, including the ``gdal\_merge'' and ``gdaldem''
commands, following the methodology described in \citet{thiery22}.

A raster layer entitled ``Shaded Relief of Auvergne,'' also available
from the CRAIG platform, was used due to its high visual quality
\citep{CRAIG2025}. Although its resolution is officially stated as 10~m
per pixel, the fine detail visible in the image suggests a higher
effective resolution. Verifications confirmed that this layer was
produced from the RGE Alti 3D digital \mbox{elevation} model of the French
National Geographic Institute (IGN), which has a resolution of 5~m.

All data were processed using the IGNF LAMB93 coordinate reference
system. The geographical accuracy of the DEM was checked by overlaying
it with topographic landmarks visible on IGN map \mbox{backgrounds}.

\subsection{Lineament identification} \label{sec3.2}

In order to interpret tectonic structures from the topography, a
mapping of the visible lineaments on the Digital Elevation Model (DEM)
was carried out by manually digitizing segments in the Geographic
Information System QGIS. This mapping is based on a set of
morphostructural criteria that we systematically applied to identify
the lineaments:
\begin{itemize}
\item traces of present-day valleys, especially when the hydrographic
network includes straight segments interrupted by abrupt changes in
orientation;
\item presence of well-expressed linear escarpments, resulting from
differential erosion probably marking a tectonic contact between
lithological units of contrasting \mbox{resistance};
\item alignment of volcanic eruptive centers that are close to each
other;
\item direction of volcanic lava flows, insofar as these generally
follow structural valleys often guided by faults;
\item presence of dykes recognizable at the \mbox{surface}.
\end{itemize}

The 527 identified lineaments were vectorized as segments and recorded
in a georeferenced GeoPackage layer. The associated attribute table,
including metadata and descriptive information, is provided as
Supplementary Material.

\section{Lineaments} \label{sec4}

The lineament map is shown in Figure~\ref{fig7}.

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig7}Map of digitized lineaments (in red) superimposed
on the digital elevation model (DEM) of the study area. Colored dots
correspond to the epicenters of recent historical earthquakes, as
presented in Figure~\ref{fig6}. Numbers refer to the locations of the
sites listed in Table~\ref{tab5}.}
\end{figure*}

\subsection{Data analysis} \label{sec4.1}

The \textbf{Length} and \textbf{Azimuth} attributes of the attribute
table were subjected to statistical analysis using data processing
methods. Only the results concerning the azimuth are presented here;
those related to length are available in the Supplementary Material.

The azimuth histogram is shown in Figure~\ref{fig8}. The corresponding
frequency table is provided in Table~\ref{tab2}. Additionally, a rose
diagram (Figure~\ref{fig9}) was produced using the \textit{Line
Direction Histogram} plugin developed for QGIS by \citet{tveite15}.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig8}Histogram of the azimuths of lineaments.}
\end{figure}

\begin{figure}
\includegraphics{fig08}
\caption{\label{fig9}Rose diagram of the directions of lineaments.}
\end{figure}

\begin{table}[t!]%tab2
\caption{\label{tab2}Distribution of azimuths in classes $A_i$ with
absolute frequencies $n_i$ and relative frequencies $f_i$ in~\%
{\vspace*{.2pc}}}
\begin{tabular}{cccc}
\thead
$A$ & $n_i$ & $f_i$ & Family\\
\endthead
[0, 10[ & 28 & 5.31 & F6\\
{}[10, 20[ & 32 & 6.07 & F6\\
{}[20, 30[ & 14 & 2.66 &\\
{}[30, 40[ & 25 & 4.74 &\\
{}[40, 50[ & 34 & 6.45 &\\
{}[50, 60[ & 37 & 7.02 &\\
{}[60, 70[ & 61 & 11.57\0 & F2\\
{}[70, 80[ & 44 & 8.35 &\\
{}[80, 90[ & 52 & 9.87 & F3\\
{}[90, 100[ & 38 & 7.21 &\\
{}[100, 110[ & 55 & 10.44\0 & F1\\
{}[110, 120[ & 25 & 4.74 & F5\\
{}[120, 130[ & \04 & 0.76 &\\
{}[130, 140[ & \04 & 0.76 &\\
{}[140, 150[ & \05 & 0.95 &\\
{}[150, 160[ & 18 & 3.42 &\\
{}[160, 170[ & 23 & 4.36 & F4\\
{}[170, 180[ & 28 & 5.31 & F4\vspace*{5pt}\\
Total & 527\0 & 100 &
\botline
\end{tabular}
{\vspace*{.2pc}}
\end{table}

The analysis of the lineament azimuths is of particular interest, as it
provides insight into the \mbox{orientation} of the stress field and the
nature of brittle deformation (Figure~\ref{fig9}).

Six major fracture directions can be distinguished (Tables~\ref{tab2}
and~\ref{tab3}). In decreasing order of importance, they are
distributed as follows:

\begin{table}[t!]%tab3
\tabcolsep=16pt
\caption{\label{tab3}Summary of fault families and their interpretation
within the Riedel shear model framework, for \mbox{$\sigma_1 =
\mathrm{N}$170{\textdegree}E}}
\begin{tabular}{ccc}
\thead
a & b & c\\
\endthead
F1 & 100--110{\textdegree} & Main shear\\
F2 & 60--70{\textdegree} & Inherited faults\\
F3 & 80--90{\textdegree} & P\\
F4 & 170--180{\textdegree} & T\\
F5 & 110--120{\textdegree} & R\\
F6 & 0--20{\textdegree} & R$'$
\botline
\end{tabular}
\tabnote{Column~(a): fault family; column~(b): orientation (azimuth) in
{\textdegree}; column~(c): interpretation (Riedel model).}
\end{table}

\begin{enumerate}[(1)]
\item A direction ranging from N100{\textdegree}E to N110{\textdegree},
corresponding to a WNW--ESE orientation, well illustrated by the axis
of the Dordogne Valley between Le Mont-Dore and
\mbox{Saint-Sauves-d'Auvergne}.\ This family of faults is designated
as~\textbf{F1}.
\item A direction from N60{\textdegree}E to N70{\textdegree}E
(WSW--ENE), notably represented by the orientation of the upper part of
the Couze de Chaudefour.\ These structures are grouped into
\mbox{family}~\textbf{F2}.
\item A direction close to N80{\textdegree}E to N90{\textdegree}E,
i.e., an E--W orientation, corresponding to fault
\mbox{family}~\textbf{F3}.
\item A direction between N160{\textdegree}E and N180{\textdegree},
that is, an almost north--south orientation, illustrated for instance by
the fractures bordering the Monts Dore calderas. These faults belong to
family~\textbf{F4}.
\item A direction from N110{\textdegree}E to N120{\textdegree}, which
is distinguished from family F1 by its dispersion and geometric
relationships with other systems; it constitutes family~\textbf{F5}.
\item Finally, a direction from N0{\textdegree}E to N20{\textdegree},
corresponding to fault family~\textbf{F6}.
\end{enumerate}

\subsection{Discussion} \label{sec4.2}

The identification and analysis of lineaments may be affected by
several sources of bias:
\begin{itemize}
\item \textbf{Non-tectonic origin of lineaments}---Some lineaments
may result from anthropogenic features (e.g., road cuttings) rather
than tectonic structures. However, the impact of this bias is
considered negligible. If necessary, it can be reduced by verification
using high-resolution aerial imagery.
\item \textbf{Overrepresentation of lineaments well individualized by
erosion}---Lineaments highlighted by erosion processes may be
overrepresented in the database. This situation is particularly
plausible on the eastern slope of the Monts Dore, where the west--east
oriented slope aligns with volcanic flow directions and gravitational,
glacial, and fluvial erosion dynamics.
\item \textbf{Influence of geological substrate on lineament
density}---The apparent density of lineaments may vary according to
outcropping lithologies. Volcanic flows may mask underlying basement
structures, whereas granitic formations provide excellent tracers of
brittle deformation. This bias is however mitigated by the fact that
volcanic flows themselves are often guided by pre-existing tectonic
structures in the basement.
\item \textbf{Lack of weighting by lineament length}---In the
statistical processing, each lineament is taken into account with equal
weight regardless of its length. A more rigorous approach would be to
weight the data according to lineament length, which could be
implemented in future analyses. Nevertheless, it is likely that this
simplification has limited impact on the results, as the directions of
short structures generally represent those of the major regional
faults.
\end{itemize}

\subsection{Interpretation according to the riedel shear model}
\label{sec4.3}

Given the current state of knowledge regarding the stress regime
affecting metropolitan France (Figure~\ref{fig3}), it is possible to
propose a neotectonic model capable of explaining the distribution of
lineament orientations observed in the eastern part of the Monts Dore
Massif. The principal stress~$\sigma_1$, oriented approximately
N160{\textdegree}E--170{\textdegree}E \citep{mazotti20}, suggests that
faults with the same orientation (F4~family) correspond to faults
currently undergoing extension.\ This interpretation is further
\mbox{supported} by the observation that most eruptive centers of Monts
Dore volcanism, as well as subsequent volcanic activity, are aligned
along these structures (e.g., the Monts Dore calderas, the Dent du
Marais volcano, the Tartaret volcano, etc.). The region would therefore
be subjected to a WNW--ESE--oriented extensional tectonic regime
\citep{dorel95}.

Nevertheless, a model based solely on pure flattening does not suffice
to explain the presence of fault families F1, F2, F3, and F5, whose
orientations are oblique with respect to the main stress
axis~$\sigma_1$. These orientations suggest the superposition of a
simple shear regime, which must be considered in order to account for
the full range of observed fault \mbox{directions}.

The proposed model is illustrated in Figure~\ref{fig10}. It consists of
a system of dextral shear bands oriented according to the direction of
the main F1 faults, which correspond to an eastern continuation of the
Cholet--Poitiers Fault within the South Armorican Shear Zone, with
associated secondary faults consistent with the classical Riedel shear
model \citep{tchalenko70}.

\begin{figure}
\includegraphics{fig09}
\caption{\label{fig10}Interpretative attempt of the lineament
directions. (a)~The different fault families superimposed on the rose
diagram of lineament orientations. (b)~The simple dextral shear model.}
\end{figure}

\begin{itemize}
\item Faults of the F3 family could be interpreted as \textbf{P}
shears, i.e., secondary dextral strike--slip faults, arranged at a high
angle relative to the main dextral shear direction~F1, with an average
orientation on the order of N85{\textdegree}E.
\item Faults of the F5 family would correspond to synthetic Riedel
\textbf{R} fractures, also exhibiting dextral strike--slip motion,
arranged at a low angle relative to the main dextral shear direction
F1, with azimuths typically ranging between N110{\textdegree}E and
N120{\textdegree}E.
\item Faults of the F6 family, oriented approximately N10{\textdegree}E
to N20{\textdegree}E, could in turn \mbox{correspond} to antithetic Riedel
$\mathbf{R}'$ fractures, i.e., secondary sinistral strike--slip faults.
\item Finally, F4 faults, previously interpreted as normal faults
related to extension under the principal stress~$\sigma_1$, fit into
this system as tension fractures denoted \textbf{T} in the Riedel
model. They typically form an angle of about 30{\textdegree} with the R
and R$'$ fractures (respectively F5 and F6) and are oriented at
approximately 45{\textdegree} to the main F1 faults.
\end{itemize}

Within the proposed interpretative model, only the F2 faults, oriented
N60{\textdegree}E--N70{\textdegree}E, do not find a direct explanation
in the set of main shear bands F1. This orientation is particularly
well developed in the upstream part of the Chaudefour Valley, located
southwest of Lake Chambon.

\section{Semi-supervised directional classification using k-means}
\label{sec5}

To better understand the origin and structural significance of the
fault families, the full dataset of lineament azimuths was analyzed
using a semi-supervised clustering approach. This procedure was
informed by the initial visual classification presented in the previous
section, which tentatively assigned the lineament orientations to a
Riedel-type shear model. Building on this conceptual framework, we
applied a clustering algorithm to refine the grouping of orientations
and to reveal statistically consistent families of structures.

The classification method employed is based on the \textbf{k-means}
algorithm, a well-established semi-supervised learning technique from
the field of \mbox{artificial} intelligence, commonly used to detect
natural patterns in complex multidimensional datasets. In order to
account for the circular nature of azimuthal data, raw azimuths
(expressed in degrees) were first converted to radians, then
transformed into unit vectors in polar coordinates ($\cos\theta,
\sin\theta$), allowing for a geometry-consistent clustering.

The number of classes was set to seven ($k=7$), taking into account the
apparent angular diversity of the structures and based on the
silhouette score analysis (obtained value: 0.534), an indicator of the
internal consistency of the formed groups. For each cluster, a mean
direction was determined from the vector mean of the azimuths, and the
associated angular dispersion was calculated using the circular
standard deviation. These results were then represented as a rose
diagram (Figure~\ref{fig11}), where each group is visualized with its
dominant direction and an angular uncertainty sector of $\pm 1\sigma$
(Table~\ref{tab4}). This approach allows for a more systematic
identification of dominant directional families in the spatial
organization of the lineaments, independently of visual interpretation
biases.

\begin{figure}
\includegraphics{fig10}
\caption{\label{fig11}Rose diagram of measured directions, grouped
according to the clusters identified by a semi-supervised
classification approach, guided by a Riedel shear model. Each cluster
is represented by a distinct color. The shaded sectors indicate the
angular dispersion (${\pm}1\sigma$) around the mean direction of each
cluster.}
\end{figure}

\begin{table}[t!]%tab4
\caption{\label{tab4}Characteristics of clusters $i$ obtained by
classification: number of elements ($n$) and mean direction ($\alpha$)
with uncertainty (${\pm}1\sigma$){\vspace*{-.3pc}}}
\begin{tabular}{cccc}
\thead
$i$ & $n$ & $\alpha$ & Type\\
\endthead
1 & 92 & \017.6 ${\pm}$ 10.1 & F$6^{\prime\prime}$ (SHF)\\
2 & 125\0 & 54.6 ${\pm}$ 8.6 & F$2^{\prime}$ (TAF)\\
3 & 97 & 77.5 ${\pm}$ 5.9 & F3 (P)\\
4 & 78 & 96.5 ${\pm}$ 5.3 & F1 (SASZ)\\
5 & 59 & 112.0 ${\pm}$ 6.1\0 & F5 (R)\\
6 & 41 & 156.1 ${\pm}$ 7.3\0 & F4 (T)\\
7 & 35 & 173.9 ${\pm}$ 4.3\0 & F$6^{\prime}$ (R$'$)
\botline
\end{tabular}
{\vspace*{-.5pc}}
\end{table}

The automatic classification performed on the directions of the
lineaments generally confirms the previously proposed structural
interpretation, while providing important nuances.

The lineaments initially assigned to group F2 are now divided into two
distinct subsets.
\begin{itemize}
\item A first subgroup, labeled F$2^{\prime}$, with a
N50{\textdegree}E--N60{\textdegree}E orientation, appears to correspond
to the direction of the Tauves--Aigueperse Fault (TAF), a secondary
structure affiliated with the Sillon Houiller Fault (SHF) Zone,
operating as a left-lateral strike-slip fault.
\item The remaining lineaments previously associated with F2 are now
included in group F3, which is interpreted as secondary P-type faults,
right-lateral strike-slip, related to the main F1 faults.
\end{itemize}

Similarly, the lineaments from the former F6 group are now distributed
into two sets:
\begin{itemize}
\item A group F$6^{\prime}$, oriented
N170{\textdegree}E--N180{\textdegree}E, identified as R$^{\prime}$
faults (in continuity with~F6);
\item A group F$6^{\prime\prime}$, with a
N10{\textdegree}E--N20{\textdegree}E orientation, corresponding to the
orientation of the Sillon Houiller Fault (SHF).
\vspace*{-.5pc}
\end{itemize}

\section{The structural framework of Lake Chambon} \label{sec6}

Based on these elements, it is possible to propose a reinterpretation
of the relief of Lake Chambon and its surrounding area.
\vspace*{-.6pc}

\subsection{Structural control} \label{sec6.1}

Indeed, until now, previous studies \citep{macaire92, macaire97, gay98,
gay99} have \mbox{interpreted} the Lake Chambon area as a dammed lake,
resulting mainly from external geodynamic processes (see
Section~\ref{sec2.3}). These processes include glacial erosion typical
of glacial valleys, as well as valley blockage by the emplacement of
the Tartaret volcanic deposits or by gravitational processes such as
the collapses of the Dent du Marais.

However, the very angular geometry of the Lake Chambon Basin and of the
Chaudefour Valley suggests that tectonic processes may also play a
significant role in shaping the relief.

The publication by \citet{macaire97} provides a detailed description of
the boreholes and core drillings carried out in the Chaudefour Valley
and in Lake Chambon. Their Figures~5 and~7 present cross-sections
through the deposits of the Tartaret paleolake and the present-day Lake
Chambon. The associated raw data, including digitized borehole logs,
are available through the BSS (Banque du Sous-Sol) database, managed by
the BRGM and accessible via Infoterre.

Analysis of these documents shows that, contrary to what the relatively
flat surface morphology suggests, the shallow sedimentary deposits of
the Chaudefour Valley and the Lake Chambon basin rest on a granitic
substratum characterized by a highly irregular buried surface at depth.

An abrupt deepening of the granitic substratum is observed
(Table~\ref{tab5}) along the southern continuation of the Quancouve
stream fault (Figure~\ref{fig7}) between four neighboring borehole
sites (sites~1--4). East of this continuation, the upper surface of the
granite shows a deepening of nearly 30--40~m (sites~2 and~4) compared
to sites~1 and~3 located farther west (Table~\ref{tab5},
Figures~\ref{fig7} and~\ref{fig12}). In this area, the continuation of
this fault can therefore be interpreted as an east-dipping normal
fault.

\begin{figure*}
\includegraphics{fig11}
{\vspace*{-.1pc}}
\caption{\label{fig12}Structural sketch of the Lake Chambon area.
Shaded areas indicate subsiding zones created by transtension, located
westward near the Rochers de Pousseterre and eastward beneath Lake
Chambon, framing a central transpressive zone corresponding to the
slow-moving Champsiaux--Berlaire landslide. The fault labeled (CF)
corresponds to the Chambon Fault, interpreted at depth as a
west-dipping normal fault \citep{gailler25}. Fault FQ corresponds to
the Quancouve stream fault, whose subsurface continuation beneath Lake
Chambon appears to exert a significant local control on the deepening
of the granitic bedrock by nearly 40~m at points~(2) and~(4) relative
to points~(1) and~(3). Areas of destabilized slopes (the slow-moving
Champsiaux--Berlaire landslide to the west and the Dent du Marais
rockslide to the east, together with their associated deposits in the
Lacassou Lake area) are shown in semi-transparent orange, with their
head scarps highlighted by barbed lines.}
{\vspace*{-.2pc}}
\end{figure*}

\begin{table}[t!]%tab5
\caption{\label{tab5}Characteristics of the investigated borehole
sites}
\tabcolsep3pt
\begin{tabular}{cccccc}
\thead
Site no. & $n$
& id & $E$ (m) & $C$ (m)& $d$ (m)\\
\endthead
1 & BSS01UKLM & LV3 & 26 & 851 & \05\\
2 & BSS01UKLN & LV4 & ${>}$48\0 & $<$828\0 & ${>}$28\0\\
3 & BSS01UKLR & LV7 & 25 & 856 & \00\\
4 & BSS01UKLX & LVC4 & ${>}$58\0 & 817 & ${>}$39\0\vspace*{4pt}\\
5 & BSS01UKLP & LV5 & 21 & 864 & \00\\
6 & BSS01UKLQ & LV6 & 21 & 857 & \07\vspace*{4pt}\\
7 & BSS01UKLS & PV1 & ${>}$34\0 & 864 & 34\\
8 & BSS01UKLT & PV2 & \06 & 898 & \00\\
9 & BSS01UKLU & PV3 & 14 & 895 & \03
\botline
\end{tabular}
\tabnote{$n$~corresponds to the BSS record number (replace \texttt{n}
with its value in the URL
\url{http://ficheinfoterre.brgm.fr/InfoterreFiche/ficheBss.action?id=n}),
id is the borehole identifier as used by \citet{macaire97}, $E$~is the
thickness of the Tartaret or Chambon lake deposits, $C$~represents the
elevation of the basement--sediment interface, and $d$ is the relative
vertical deepening calculated with respect to the neighboring site
showing the highest basement elevation, for which $d$ is set to 0~m.}
\end{table}

Similarly, near the village of Chambon-sur-Lac, a deepening of
approximately 8~m of the basement--sediment interface is observed at
site~6 compared to site~5 farther west (Table~\ref{tab5}), also
suggesting the presence of an east-dipping normal fault.

In the same way, farther upstream in the Chaudefour Valley, between
Voissi\`ere and Chambon-sur-Lac, the boreholes carried out by
\citetalias{macaire97} (sites~7--9) suggest a local tectonic uplift of the
basement in the vicinity of faults trending in \mbox{several}
directions (Figures~\ref{fig7} and~\ref{fig12}); however, the maximum
depth reached by the boreholes did not allow the granitic substratum to
be directly intersected, although the vertical displacement of the
basement exceeds 30~m (Table~\ref{tab5}).

It is therefore difficult to attribute these abrupt variations in the
depth of the granitic substratum solely to glacial erosion,
particularly in the form of glacial thresholds, since the lithology is
identical on either side of the observed discontinuities. A tectonic
origin therefore appears more likely.
{\vspace*{-.2pc}}

\subsection{Proposed model}\label{sec6.2}

Lake Chambon is thus interpreted as a narrow subsiding basin of
parallelepipedal shape.\ Its \mbox{geometric} form is only partially
obscured in its northeastern part (Lacassou Lake area) by the rockfall
deposits derived from the Dent du Marais \citep{vidal96, chapron22}.
This transtensional basin most likely formed as a pull-apart structure,
resulting from a right-stepping relay between two offset branches of
the main dextral strike-slip fault system F1, within the fault zone
extending from the Col de la Croix-Morand to Murol
(Figure~\ref{fig12}).

The basin is bounded by a set of border faults dominated by the
regional dextral shear faults F1, together with secondary faults F3
(P-type), F5 (R-type), F$6'$ (R$'$-type), F$2'$ (faults with
orientations comparable to that of the Tauves--Aigueperse Fault), and
F$6''$ (with orientations close to that of the Sillon Houiller Fault),
which frame this extensional relay zone. At depth, the basin displays a
negative flower structure \citep[see, for example,][]{muksin14},
characteristic of transtensional settings, with internally subsiding
blocks mainly bounded by F4 faults\break (T-type).

Within this structural framework, the F$6''$ faults can be interpreted
as sinistral antithetic faults associated with the regional dextral
shear accommodated by the F1 faults, in continuity with the
Cholet--Poitiers Fault of the South Armorican Shear Zone (SASZ).
Furthermore, the R$'$ faults (corresponding here to the F$6'$
structures) likely underwent progressive clockwise rotation during
deformation, in accordance with Riedel shear reorientation models
described by \citet{tchalenko70} and \citet{atmaoui2005}. This
conceptual model thus accounts for the geometric arrangement of the
observed faults, the localization of subsidence zones, and the coupling
between transtensional tectonics and volcanism (Dent du Marais and
Tartaret).

Furthermore, the same explanatory model could be applied at a broader
scale, that of the Monts Dore Massif (Figure~\ref{fig14}), where
volcanism may also have developed within a crustal transtensional
context favoring magma ascent. As shown in Figure~\ref{fig14}, the two
calderas of the Monts Dore Massif appear to be structured along the
orientations of the F4 faults, suggesting that these faults played a
major role in the localization and organization of volcanic centers.

\begin{figure*}
{\vspace*{-.1pc}}
\includegraphics{fig12}
{\vspace*{-.1pc}}
\caption{\label{fig14}Simplified structural sketch of the Monts Dore
Massif superimposed on its DEM. (1)~Lake Gu\'ery; (2)~Puy de Sancy;
(3)~Lake Pavin; (4)~Lake Chambon; (5)~Saint--Nectaire;
(6)~Besse--et--Saint--Anastaise; (7)~Murat--le--Quaire; (8)~Mont--Dore;
(9)~Couze Pavin at Lomprat. (a)~Murol--La Croix--Morand Fault;
(b)~Croix--Morand--Banne d'Ordanche Fault; (c)~La Bourboule Fault;
(d)~Chaudefour faults. F1: main dextral shear faults; F3: secondary
dextral P-type strike--slip faults; F4: extensional T-type faults;
F$2'$: faults related to the Tauves--Aigueperse Fault. Dotted lines
represent the inferred boundaries of the two calderas
\citep{cantagrel90}, that of the Gu\'ery volcano to the north and that
of the Sancy to the south. The shaded area indicates the subsiding
transtensional basin of Lake Chambon. $\star$:~Dent du Marais
rockslide. $\diamond$:~Montagne de la Plate rockslide.
$\triangle$:~Champsiaux--Berlaire (or Chambon) landslide.}
{\vspace*{-.1pc}}
\end{figure*}

The main regional dextral shear directions (F1), previously described
in the context of the Lake Chambon area, are also observed in the
Dordogne Valley, between Murat--le--Quaire and the town of Mont--Dore,
indicating a regional continuity of strike--slip deformation. In
addition, SSW--NNE--trending fault directions (F$2'$), similar to the
orientation of the Tauves--Aigueperse Fault, are recognized in the
Chaudefour Valley as well as in the Couze Pavin Valley near Besse.

Furthermore, the work of \citet{cantagrel90} shows that the area
located northwest of the Croix--Morand Pass, corresponding today to the
Lake Gu\'ery sector, may represent the core of a former major volcanic
edifice, the Gu\'ery stratovolcano. This edifice is thought to have
undergone a preferential collapse of its northeastern quarter,
generating large lahar and debris-avalanche deposits, notably the
so-called Perrier debris-avalanche deposits and lahars. Such a mode of
flank destabilization is consistent with the model proposed by
\citet{lagmay00}, according to which a volcanic cone built above a
strike--slip fault preferentially develops flank instabilities within
the sector of the cone located between the secondary R and P faults
associated with the active shear fault 
\citep[see, for example,][]{lagmay06,lagmay12}. In the case of the former
Gu\'ery stratovolcano, the \mbox{orientation} of debris avalanches toward the
NE and E sectors is thus compatible with a structural control exerted
by a regional dextral shear oriented N110{\textdegree}E. In this
context, the destabilization of the Gu\'ery stratovolcano is
interpreted as being related to the same system of regional dextral
strike--slip F1 faults oriented N110{\textdegree}E as that recognized
throughout the rest of the study area.

Finally, Figure~\ref{fig14} highlights a pattern of crustal deformation
characterized by the presence of numerous dextral strike--slip F1
faults (regional shear faults) and F3 faults (P-type faults),
suggesting a largely diffuse and distributed deformation at the scale
of the massif.\ However, a more pronounced \mbox{structural}
\mbox{corridor} clearly stands out between the Croix--Morand Pass, Lake
Chambon, and Murol, where deformation appears to be concentrated along
a network of more continuous and better-organized faults.

\subsection{Discussion}\label{sec6.3}

The results obtained in this study can be compared with those of
\citet{gailler25}, conducted in the same area. That study is based on
the analysis of focal mechanisms of microseismic events located between
3 and 5~km depth and thus provides direct constraints on the kinematics
of active structures at depth, complementary to surface
morphostructural observations (Figure~\ref{fig50}). Several faults with
well-constrained kinematics are described therein:

\begin{figure*}
{\vspace*{-.2pc}}
\includegraphics{fig13}
{\vspace*{-.2pc}}
\caption{\label{fig50}Kinematic and structural model of the
transtensional relay associated with the Montagne de la Garde, north of
the village of Chambon-sur-Lac. (a)~Fault traces overlaid on a LiDAR
DEM (IGN), obtained from a screenshot of the \textbf{Geoscope} mobile
application \citep{thiery25}. \textbf{CF} corresponds to the Chambon
Fault (N13{\textdegree}E, 68{\textdegree}W dip). \textbf{Fa} and
\textbf{Fb} are two adjacent faults, associated at depth, striking
N173{\textdegree}E and dipping 85{\textdegree}W. \textbf{BBF} is the
Bois de Benne stream fault. The circle with an internal cross indicates
subsidence motion on the Montagne de la Garde plateau. The blue arrow
represents southward ground push at the locality known as ``Rochers de
Pousseterre''. (b)~Kinematic interpretation of faults \textbf{Fa},
\textbf{Fb}, and \textbf{CF} based on a direct application of the
structural model proposed in this study; blue arrows indicate relative
block displacements. $\sigma_h$~represents the maximum horizontal
stress according to \citet{mazotti20}. (c)~Kinematic reinterpretation
incorporating constraints derived from focal mechanisms described by
\citet{gailler25}. Barbed lines represent normal faults, with barbs
indicating the downthrown block, particularly along the Chambon Fault
(\textbf{CF}). The semi-transparent yellow domain corresponds to the
narrow basin, with a triangular to trapezoidal geometry, currently
developing between the \textbf{CF} fault on the one hand and the
\textbf{Fa} and \textbf{Fb} faults on the other. Yellow arrows indicate
the opening direction of the basin. Red dots indicate restraining
points along the \textbf{BBF} fault. (d)~Conceptual WNW--ESE
cross-section through the pull-apart basin, illustrating the geometry
(flower structure) and the combined behavior of the \textbf{Fa},
\textbf{Fb}, and \textbf{CF} faults through the granitic basement.}
{\vspace*{-.2pc}}
\end{figure*}

\begin{itemize}
\item a fault striking N13{\textdegree}E and dipping 68{\textdegree}
westward, corresponding to the Chambon Fault (CF; Figure~\ref{fig12}).
In our structural model, this structure is interpreted at the surface
as a sinistral strike--slip fault, comparable to an R$'$-type fault or
to a structure related to the Sillon Houiller Fault. However,
seismological data indicate that at depth this fault \mbox{behaves} mainly as
a west-dipping normal fault \citepalias{gailler25}.
\item two neighboring faults striking N173{\textdegree}E and dipping
85{\textdegree} westward, referred to as Fa and Fb, located west and
northwest of the Chambon Fault and showing no clear morphostructural
expression at the surface. Although this orientation could initially be
related in our model to a sinistral R$'$-type shear, the focal
mechanisms reveal a dextral strike--slip behavior at depth.
\end{itemize}

At the regional scale, the maximum horizontal stress oriented between
N160{\textdegree}E and N170{\textdegree}E \citep{mazotti20} implies
that the Fa, Fb, and CF faults should operate as sinistral strike--slip
faults, in agreement with their geometry and with the structural model
proposed here (Figure~\ref{fig50}b). However, the focal mechanisms
analyzed by \citet{gailler25} indicate a significantly different
kinematic behavior at depth, with Fa and Fb faults active as dextral
strike--slip faults, whereas the CF fault behaves as a west-dipping
normal fault. This apparent contradiction between the behavior expected
from the regional stress field and the seismological data suggests a
local accommodation of deformation associated with a perturbation of
the stress field.

In such a transtensional setting, even a moderate reorientation of the
principal stresses is sufficient to locally reverse the sense of slip
on faults. We interpret this configuration as a transtensional relay
developed between, to the north, a major F1 fault segment striking
N110{\textdegree}E and accommodating regional dextral strike--slip
motion, and, to the south, an F3 fault segment corresponding to the
Bois de Benne stream fault (BBF), of P-type and also characterized by
dextral kinematics. In this context, a narrow pull-apart basin develops
between the CF, Fa, Fb, F1, and BBF faults (Figure~\ref{fig50}c). The
operation of this basin implies the existence of restraining points
along the BBF fault, particularly in the immediate vicinity of the
village of Chambon-sur-Lac, where several active faults converge. Such
a tectonic configuration may locally promote seismic locking and stress
accumulation, potentially allowing the release of significant seismic
energy in the Chambon-sur-Lac area, without implying any short-term
predictability.\looseness=1

This model is consistent with the deformations observed at the surface,
marked by a southward bulging of the relief at the locality known as
``Rochers de Pousseterre'' and, farther north, by the development of a
depression affecting the summit surface of the Montagne de la Garde
(Figure~\ref{fig50}a). The WNW--ESE cross-section shown in
Figure~\ref{fig50}d illustrates the typical ``flower'' geometry
associated with this type of transtensional basin developed within a
regional shear corridor.

\section{Consequences and natural hazards} \label{sec7}
\subsection{Landslides and rockfalls} \label{sec7.1}

Active fault zones generally leave clear geomorphological evidence of
their recent or ongoing \mbox{activity} along their traces. Such
evidence notably includes fault scarps and other geomorphological
features caused by gravitational movements such as landslides, tilting,
rockfalls, flows, or combinations of these types of processes
\citep{cruden93}.

In the study area, the landslides and rockfalls described below are,
almost systematically, spatially associated with underlying faults.
While these faults naturally contribute to weakening the relief through
rock fracturing, this spatial recurrence is a striking feature,
suggesting that these instabilities may, at least in part, be triggered
or reactivated by seismogenic activity 
\citep[see, for example,][]{song22, song25}.

Such manifestations are clearly visible in the study area, as
illustrated by the major rockfall of the Dent du Marais, dated to
approximately 2600~years BP, which contributed to the partial blockage
of the Couze Chambon and to the development of the present natural dam
lake, Lake Chambon \citep{vidal96, chapron22}. Another significant
event is the sudden drainage of the Tartaret paleolake, which occurred
around 8500~years BP. Although a strictly seismic origin of these
events has not been formally established, the studies of
\citet{chassiot16} and \citet{chapron22} suggest a high probability of
earthquake triggering.

In a similar manner, the collapses of the Montagne de la Plate are
located in an area where analysis of the digital elevation model
highlights the proximity and intersection of F1 and F$2'$ faults
(Figure~\ref{fig14}).

Another major flank landslide, identified as the Chambon (or
Champsiaux--Berlaire) landslide, \mbox{initiated} on the southern slope of the
Montagne de la Garde. Covering an estimated area of ${\sim}$1.5~km$^2$,
the unstable zone extends from the Berlaire farm and the locality of
Champsiaux to the Roches de Pousseterre to the west and the Varennes
locality to the east, and overlooks the village of Chambon-sur-Lac with
a relative relief of approximately 200~m (Figure~\ref{fig15}).

\begin{figure*}[p!] 
\includegraphics{fig14}
\caption{\label{fig15}(a)~Extract from the 1:25,000 IGN topographic
map. (b)~Annotated view of the Champsiaux-Berlaire (or Chambon-sur-Lac)
landslide (Google Maps screenshot). Solid lines indicate the faults
controlling the boundaries of the landslide: B:~Chambon Fault of R$'$
type; C:~F1-type fault; D:~F4 fault (${=}$T-type) of the Quancouve
stream; E:~F4 fault of T-type; F:~F1 fault north of the Berlaire farm.
Lines with barbs indicate landslide scarps of secondary slides
identified by slope breaks on the hillsides. The yellow line,
labeled~A, indicates the head scarp of the slow landslide at the
Berlaire farm. Grey-shaded areas indicate displaced slide panels.
Orange-shaded areas indicate probable mudflows and debris flows.}
\end{figure*}

It is noteworthy (Figure~\ref{fig12}) that this slope abruptly
truncates the northern boundary of the Chaudefour Valley along a
lineament interpreted as an F5 fault of R-type, and then along an F$2'$
fault (with an orientation comparable to that of the Tauves--Aigueperse
Fault) along its southeastern margin. The destabilizing slope is
subsequently clearly compartmentalized by deeply incised gullies, whose
paths are controlled by additional fault structures. Thus, the eastern
part of this slope is dissected by two gullies, including the Quancouve
stream gully, which follows an F4 fault interpreted as a T-type fault
in the Riedel scheme. The western limit of the Champsiaux--Berlaire
landslide is marked by the Chambon Fault (FC), now interpreted as a
west-dipping normal fault. East of the village of Chambon-sur-Lac, the
Bois de Benne stream displays a clear convex southward deflection at
the foot of the landslide, suggesting a progressive push of the slope
toward the valley. Moreover, the large head scarp at the northern part
of the slope shows an orientation consistent with that of the F$2'$ and
F4 faults. These head scarps observed near the Berlaire farm are
expressed by fresh scarps several meters high, indicating ongoing slope
activity, most likely in the form of slow landsliding. The slope
surface is further affected by secondary landslides and associated
gravitational features (depressions, and counter-slopes),
characteristic of moving slopes.

Finally, at the toe of this landslide, mudflows and debris flows have
been identified based on the analysis of aerial photographs and
borehole data \mbox{(presence} of clayey--silty superficial deposits with
blocks in borehole 071175X0059) from the BRGM Subsurface Database,
suggesting recurrent historical flow episodes rather than a single
event.

All these observations suggest that secondary Riedel-type structures,
in connection with the major border faults F1, F$2'$, F4, F$6'$, and
F$6''$, play a decisive role in the segmentation, kinematics, and
gravitational preconditioning of this unstable flank.

The dynamics of this unstable flank justify enhanced geomorphological
and geophysical \mbox{monitoring,} especially as this type of phenomenon is
frequently observed in active fault zones, as reported in other
comparable tectonic settings \citep[e.g.,][]{malamud04, ocakoglu24}.

\subsection{Reactivation of the deep hydrothermal{\hfill\break} system}
\label{sec7.2}

Our structural model also helps to clarify the functioning of the
active deep hydrothermal system located at approximately 3--5~km below
the surface \citep{boudoire25}. This system is evidenced at the surface
by the presence of carbo-gaseous mineral water springs (rich in CO$_2$)
and has recently shown signs of anomalous activity. Between 2021 and
2022, several episodes of microseismicity were recorded in the area,
accompanied by long-period (LP) seismic waves, which are characteristic
of intense hydrothermal activity and, more specifically, of fluid
boiling within subsurface conduits \citep{chouet96, nakano03,
ohminato06}.

The hypothesis proposed here is that these manifestations reflect a
decompression of the hydrothermal system induced by the tectonic
reactivation of deep fault structures. Fractures that were previously
sealed may currently be reopening under the effect of recent tectonic
stresses, allowing fluids to depressurize and migrate toward the
surface. This process can be interpreted as a precursor signal of
tectonic reactivation \citep{chouet96, saade19}. Within the range of
pressures and temperatures expected for a hydrothermal system located
between 3 and 5~km depth (150--300~{\textdegree}C, 300--500~bar under
hydrostatic conditions), the presence of CO$_2$-rich mineral waters may
lead to explosive degassing during fracture opening and associated
decompression. This phase-separation process can generate seismic
signals due to the abrupt release of energy \citep{thiery9a, thiery9b}.

Furthermore, the fact that this hydrothermal system is located within a
transtensional relay provides strong support for our model. In
transtensional relays, the local stress is reduced, which constitutes a
key factor for increased permeability and the development of
hydrothermal circulation, as suggested by the work of \citet{liotta23}.

The association between this hydrothermal microseismicity, field
observations (landslides and surface ruptures), and the geometry of the
faults identified along the southern border fault of the Limagne Graben
(SBLF) suggests that the Croix--Morand--Murol fault (CMF) zone is not
only inherited but still active. This situation argues for enhanced
monitoring, both of the hydrothermal system and of slope stability.
Such systems are known to generate earthquakes related to fluid--fault
interactions, as demonstrated in other volcanic or tectonic settings.

\subsection{Tectono--geomorphological integration} \label{sec7.3}

The Champsiaux--Berlaire slow landslide occupies a key structural
position within the major dextral shear corridor trending
N110{\textdegree}E (WNW--ESE), recognized between the Croix--Morand
Pass and the Murol area. It is also located at the northern
\mbox{termination} of the large Chaudefour Valley structure, oriented
N50{\textdegree}E (SW--NE), a direction comparable to that of the
Tauves--Aigueperse Fault. In this context, the Champsiaux--Berlaire
area appears to be spatially framed by two transtensional domains: to
the west, the small transtensional basin currently forming at the
Rochers de Pousseterre, and to the east, the more developed
transtensional basin of Lake Chambon. This configuration suggests that
the Champsiaux--Berlaire sector corresponds to a transpressive push--up
ridge structure, intercalated between two extensional zones within the
same regional dextral shear corridor. Such a structure promotes
conditions of relative uplift, fracturing, and destabilization of the
substratum, making this area particularly favorable for the development
of a large-scale slow landslide.

The morphostructural continuity observed between the
Champsiaux--Berlaire landslide and the Chaudefour Valley along the same
N50{\textdegree}E axis further supports this interpretation. It
suggests that the Chaudefour Valley may have initially corresponded to
an uplifted structural block, possibly an ancient
N50{\textdegree}E--oriented horst affecting the Chambon-sur-Lac
granitic basement and related to an inherited transpressive structure.
This uplifted block would have been preferentially fractured and
weakened, subsequently favoring enhanced incision by glacial erosion
processes, as well as by rockfalls and gravitational collapses. The
fact that the Chaudefour Valley is not protected by effusive lava
flows, in contrast to the Montagne de la Plate area, supports the idea
that it originally constituted a topographically elevated zone,
dominating paleovalleys that are now infilled by volcanic products of
the Monts Dore Massif. In addition, the Chaudefour Valley clearly
incises Middle Pleistocene volcanic formations, including trachybasalts
(unit $\tau\alpha_2$ in Figure~\ref{fig2}) and the basalts of the
Montagne de la Plate scoria cone, dated at ca.~220~ka \citep{vidal96},
indicating that valley incision initiated no earlier than the Middle
Pleistocene. This implies that the erosion and relief inversion of the
former Chaudefour Valley horst occurred over a relatively short
geological time span. From this perspective, the Chaudefour Valley may
represent a noteworthy example of relief inversion, developed on an
inherited tectonic structure initially expressed as a horst or push-up
ridge. Finally, the northward continuation of this N50{\textdegree}E
axis of basement uplift, in connection with the doming of the D\^omes
Plateau forming the basement of the Cha\^{\i}ne des Puys, suggests a
first-order regional tectonic control on the organization and long-term
evolution of relief at the scale of the Massif Central.

\section{Conclusion} \label{sec8}

This study allowed the identification and mapping of 527 lineaments
based on a detailed digitization of the Digital Elevation Model (DEM)
of the Chambon-sur-Lac region, with a resolution of 5~m per pixel. All
data were integrated into a Geographic Information System (GIS) using
QGIS software, enabling a structured analysis of lineament
orientations.

An initial statistical analysis revealed the existence of six major
fault families oriented along the following directions: F1
(N100{\textdegree}E--N110{\textdegree}E), F2
(N60{\textdegree}E--N70{\textdegree}E), F3
(N80{\textdegree}E--N90{\textdegree}E), F4
(N150{\textdegree}E--N180{\textdegree}E), F5
(N110{\textdegree}E--N130{\textdegree}E), and F6
(N0{\textdegree}E--N20{\textdegree}E).

To objectively support this classification, we also applied a
semi-supervised clustering approach using the k-means algorithm, a
well-known machine learning method.\ This analysis grouped the
lineaments into seven main clusters corresponding to the following
dominant orientations: F$6^{\prime\prime}$
(N10{\textdegree}E--N30{\textdegree}E), F$2^{\prime}$
(N40{\textdegree}E--N60{\textdegree}E), F3
(N70{\textdegree}E--N80{\textdegree}E), F1
(N90{\textdegree}E--N110{\textdegree}E), F5
(N110{\textdegree}E-N120{\textdegree}E), F4
(N150{\textdegree}E--N160{\textdegree}E), and F$6^{\prime}$
(N170{\textdegree}E--N180{\textdegree}E). This classification refines
the initial typology by revealing more precise groupings, indicative of
a complex structural organization potentially related to a general
dextral shear regime.

Based on these observations, an interpretive model of brittle
deformation was proposed, \mbox{consistent} with the current tectonic stress
regime characterized by a principal stress $\sigma_1$ oriented between
N160{\textdegree}E and N180{\textdegree}E \citep{dorel95, mazotti20}.
This model is based on the presence of dextral shear zones associated
with the main F1 faults, part of the SASZ-SBLF fault system,
interpreted within the Riedel shear model framework: F4 corresponding
to extensional T faults, F3 to dextral secondary P-type faults, F5 to R
faults, and F$6^{\prime}$ to sinistral R$^{\prime}$ faults.

The study also highlights the influence of the sinistral shear zone of
the Sillon Houiller Fault (SHF), an antithetic fault to the SASZ-SBLF
system. Indeed, the characteristic orientations of the Sillon Houiller
Fault appear in group F$6^{\prime\prime}$ (interpreted as related to
the SHF), as well as those of the Tauves--Aigueperse Fault (TAF), a
fault associated with the Sillon Houiller Fault in group F$2^{\prime}$.

This neotectonic framework provides an explanation for several
geomorphological and structural features observed in the region. The
Lake Chambon Basin is interpreted as a subsiding pull-apart basin,
formed in a transtensional relay between two F1 fault segments. The
volcanism of Tartaret and Dent du Marais appears to be related to the
extensional F4 faults. This tectonic model can furthermore be
extrapolated to the regional scale of the Monts Dore massif.

The analysis also emphasizes the major tectonic role of the secondary
F4 and F$6^{\prime\prime}$ faults in triggering and driving the large
slow-moving landslide of Champsiaux-Berlaire (Chambon-sur-Lac). 

Finally, this study provides new insights into the understanding of
regional seismic activity. It particularly highlights the structural
importance of the dextral strike-slip fault Croix-Morand--Murol Fault
(CMF). The 2021--2022 seismic crisis may thus be an indicator of the
tectonic activity of this fault system and its underlying hydrothermal
dynamics.

\section*{Acknowledgments}

This work is based on the Master 1 project of Ludovic Chender, carried
out between April and June 2023 and publicly presented on 9 June 2023
at the Observatoire de Physique du Globe de Clermont--Ferrand (OPGC),
within Universit\'e Clermont Auvergne. The manuscript was subsequently
substantially revised and expanded as part of the doctoral thesis of
\mbox{Ludovic}~Chender. The authors also thank the anonymous reviewer for
their constructive comments and suggestions, which significantly
improved the quality of the manuscript. This is contribution
n$^{\mathrm{o}}$ 744 of the ClerVolc program of the International
Research Center for Disaster Sciences and Sustainable Development of
the University of Clermont Auvergne.

\vspace*{-2pt}

\section*{Declaration of interests}

\vspace*{-1pt}

The authors do not work for, advise, hold shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\vspace*{-2pt}

\section*{Supplementary materials}

\vspace*{-1pt}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

It can also be accessed from the following GitHub repository:
\url{https://github.com/regthiery/morphoChambonLake}.

\CDRsupplementaryTwotypes[application/zip]{supplementary-material}{\cdrattach{crgeos-326-suppl.zip}}

\back{}

\vspace*{-2pt}

\printbibliography
\refinput{crgeos20250636-reference.tex}

\end{document}
