\makeatletter
\@ifundefined{HCode}
{\documentclass[screen, CRGEOS, Unicode, Thematic]{cedram}
%\usepackage[sort&compress,square,comma,authoryear]{natbib}
\usepackage[square,comma,authoryear]{natbib}
\newenvironment{Table}{\begin{table}}{\end{table}}
\renewcommand\bibsection{\section*{\refname}}
\newenvironment{noXML}{}{}
\def\thead{\noalign{\relax}\hline}
\def\endthead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\botline{\\\hline}
\RequirePackage{etoolbox}
\usepackage{upgreek}
\usepackage[T1]{fontenc} 
\def\jobid{crgeos20221210}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\let\Lbreak\newline
\def\ubreak{\newline}
\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{}
\def\back#1{}
\csdef{Seqnsplit}{\\}
\DOI{10.5802/crgeos.215}
\datereceived{2022-12-09}
\daterevised{2023-04-27}
\dateaccepted{2023-04-28}
\ItHasTeXPublished
}
{
\PassOptionsToPackage{authoryear}{natbib}
\documentclass[crgeos]{article}
\usepackage[T1]{fontenc} 
\def\CDRdoi{10.5802/crgeos.215}
\let\newline\relax
\let\ubreak\relax
\makeatletter
%\def\CDRsupplementaryTwotypes#1#2{}
 \def\selectlanguage#1{} 
}
\makeatother

\dateposted{2023-05-31}
\begin{document}

\begin{noXML}

%\TopicEF{Geodynamics of continents and oceans - A tribute to Jean
%Aubouin}{G\'eodynamique des continents et des Oc\'eans - Hommage \`a
%Jean Aubouin}

\title{Alpine, Variscan, eo-Variscan belts: comparison between hot and
cold orogens from the examples of French segments}

\author{\firstname{Michel} \lastname{Faure}\CDRorcid{0000-0003-1880-8115}}
\address{Institut des Sciences de la Terre d'Orl\'{e}ans (ISTO), UMR
7327-CNRS/Universit\'{e} d'Orl\'{e}ans/BRGM,  Campus G\'{e}osciences,
1A Rue de la F\'{e}rollerie, 45071 Orl\'{e}ans Cedex~2, France} 
\email{michel.faure@univ-orleans.fr}     

\begin{abstract}
The Cenozoic Alpine, and Paleozoic Variscan and eo-Variscan collisional
belts are compared in the framework of the Wilson cycle considering
differences between cold and hot orogens. The W. Alps result of the
opening and closure of the Liguro-Piemonte ocean, whereas the Paleozoic
Eo-variscan and Variscan orogenies document multiple ocean openings and
collisions in space and a polyorogenic history in time. Jurassic or
Early Ordovician break-up of Pangea or Pannotia megacontinents led to
the formation of passive continental margins, and the opening of
Liguro-Piemonte, or Rheic, Tepla-Le Conquet, and Medio-European oceans,
respectively. In Paleozoic or Mesozoic, microcontinents such as Apulia
and Sesia or Armorica and Saxo-Thuringia were individualized. The
oceanic convergence stage was associated with the development of arcs
and back-arc basins in the Variscan belt but magmatic arcs are missing
in the W. Alps, and inferred in the Eo-variscan one. Though the nappe
stack is mainly developed in the subducted European or Gondwana crust
in the western Alps and Eo-variscan cases, the Moldanubian nappes
formed in the upper plate in the Variscan case. The Alpine and Variscan
metamorphic evolutions occurred under ca. 8~\textdegree C/km and
30~\textdegree C/km gradients, respectively. During the late- to
post-orogenic stages, all belts experienced ``unthickening''
accommodated by extensional tectonics, metamorphic retrogression, and
intramontane basin opening. The importance of crustal melting,
represented by migmatites, granites, and hydrothermal circulations in
the Variscan and Eo-Variscan belts is the major difference with the W.
Alpine one. The presence, or absence, of a previous Variscan or
Cadomian continental basement might have also influenced the
rheological behavior of the crust. \vspace*{-2pt}
\end{abstract}

\keywords{\kwd{Hot and cold orogens}
\kwd{Alpine}
\kwd{Variscan}
\kwd{Eo-variscan}
\kwd{Oceanic convergence}
\kwd{P--T paths}
\kwd{Crustal melting}}

%\datepublished{1999-12-31}
\makeatletter
\def\cdr@preabstracthook{\par}
\makeatother

\maketitle

\vspace*{-10.5pt}

\twocolumngrid

\end{noXML}

\section{Introduction}

Since the early times of plate tectonics, mountain building is viewed
as a consequence of lithospheric evolution at converging plate
boundaries, though intracontinental orogens may also develop in places
where lithosphere has been previously thinned. The scenario formalized
by the Wilson cycle \citep{Wilson1966,DeweyBird1970,BurkeDewey1974} is
divided into several stages, namely: (i)~\textit{pre-orogenic}
lithospheric divergence, characterized by ocean basin opening, between
two continents,\unskip\break (ii) \textit{orogenic} lithospheric convergence with
successively oceanic subduction, continental subduction, and final
collision when two continents contact, leaving a few relics of oceanic
lithosphere as an ophiolitic suture, (iii) \textit{late to post-orogenic
stage} intracontinental deformation characterized by the disappearance
of relief accommodated both by erosion and tectonics. Note that
continental subduction is not a relevant criterion for collision, since
this process may also occur in intracontinental belts and obduction
chains in which the upper plate consists of oceanic lithosphere. This
simple Wilson cycle model does not reflect the variety of collisional
processes as plate velocities, lithosphere convergence vectors, slab
dip, and crust internal rheology are not considered. The ophiolitic
suture separating two collided continents may be erased by late
tectonics, particularly by strike-slip movements. It became popular to
distinguish ``hot'' and ``cold'' orogens due to their different thermal
state, and crustal rheology   \citep[e.g.][and references
therein]{Vanderhaeghe2012,JamiesonBeaumont2013}. Such a distinction was
already recognized as ``hercynotype'' and ``alpinotype'' orogens before
the onset of plate tectonics \citep{Zwart1967}.\looseness=-1

In Europe, the Cenozoic Alpine and Paleozoic Variscan belts are
examples of collision orogens. However, in spite of similar
architectural designs, each belt exhibits also its own peculiarities.
This paper explores in which way the features of cold-narrow and
large-hot orogens can be recognized in the Alpine and Variscan belts.
Emphasis is placed on the tectonic, metamorphic and magmatic events
occurring during each stage of the Wilson cycle. Due to space
limitations, this comparison is restricted to the French segments of
the two belts. A detailed description of the Alpine and Variscan belts
is beyond the scope of this article  [for details see
\citealp{Matte1986,LagabrielleLemoine1997,Lemoineetal2000,Agardetal2002,Schmidetal2004,AgardLemoine2005,Faureetal2005,Faureetal2009,Ballevreetal2009,Beltrandoetal2010,Lardeaux2014a,Lardeaux2014b,Lardeauxetal2014}; 
Table~\ref{tab1}].

\begin{table*}
\caption{\label{tab1}Main litho-tectonic elements of the W. Alps,
Eo-variscan, and Variscan belts\vspace*{-2pt}}
\footnotesize\begin{tabular}{llll}
\thead
& {Western Alps} & {Devonian Eo-Variscan belt} & {Carboniferous
Variscan belt} \\ 
 \endthead
{\textit{Ophiolites}} & \parbox[t]{3.8cm}{Liguro-Piemonte ocean
\newline With (Monviso) or without  \newline (Chenaillet) metamorphism}
& \parbox[t]{3.3cm}{Medio European ocean \newline (Drain unit,
Audierne)} & \parbox[t]{3.5cm}{Rheic \& Tepla oceans \newline Lizard
klippe \newline Le Conquet ophiolites}\vspace*{5pt}\\ 

{\textit{Subduction complex}} & \parbox[t]{3.8cm}{Helminthoid flysch
\newline Schistes lustr\'{e}s}  & Ile de Groix  & Not exposed in
France\vspace*{5pt} \\ 

{\textit{Magmatic arc}} & Absent & \parbox[t]{3.3cm}{Ligerian arc
(inferred)}  & \parbox[t]{3.6cm}{NE Morvan arc (Somme series) and
Limousin tonalite line}\vspace*{2pt}  \\ 

{\textit{Back-arc basin}} &  & \parbox[t]{3.3cm}{Back-arc basin:
\newline (St-Georges/Loire unit)} & \parbox[t]{3.8cm}{Back-arc:
Br\'{e}venne ophiolites}\vspace*{5pt} \\ 

{\textit{Basement nappe}} & Austroalpine nappe & Mauges nappe &
\parbox[t]{3.5cm}{Not documented in France \newline Concealed below
Paris basin}\vspace*{5pt} \\ 

\parbox[t]{3.3cm}{{\textit{Ductile syn-{\ubreak}metamorphic
nappes}}} & \parbox[t]{3.8cm}{Schistes lustr\'{e}s nappe \Lbreak
Brian\c{c}onnais nappes \Lbreak Penninic recumbent folds} &
\parbox[t]{3.3cm}{Upper Gneiss Unit \Lbreak Lower Gneiss Unit} &
\parbox[t]{3.8cm}{Para-autochthonous Unit \newline thrust
sheets}\vspace*{5pt} \\ 

{\textit{HP Metamorphism}} & \parbox[t]{3.8cm}{Eclogite, Blue schist.
\Lbreak Gradient\ 8~\textdegree C/km} & \parbox[t]{3.3cm}{Eclogites, HT
Granulites \newline Blue schist. Gradient \newline  8--10~\textdegree
C/km} & \parbox[t]{3.8cm}{Not documented in France}\vspace*{5pt} \\ 

{\textit{MP/MT metamorphism}} & Amphibolite facies  &
\parbox[t]{3.3cm}{Barrovian metamorphism. \newline 30~\textdegree C/km}
& \parbox[t]{3.8cm}{Barrovian metamorphism. \newline 30~\textdegree
C/km}\vspace*{5pt} \\ 

\parbox[t]{3.3cm}{{\textit{Non- or \newline weakly
metamorphic \newline nappes}}} & \parbox[t]{3.8cm}{Dauphinois
Fold-and-Thrust \newline belt\Lbreak Helvetic recumbent folds} &
\parbox[t]{3.3cm}{Not documented in France} & \parbox[t]{3.8cm}{North
Fold-and-Thrust belt \newline (Ardennes, SW England) \Lbreak South
Fold-and-Thrust belt \newline (Montagne Noire, Pyrenees)}\vspace*{5pt}
\\ 

\parbox[t]{3.3cm}{\textit{Foreland basin}}   &
\parbox[t]{3.5cm}{Helvetic molasse \Lbreak Po plain molasse} & Absent
& \parbox[t]{3.8cm}{North foreland basin \Lbreak South Foreland
basin}\vspace*{5pt} \\ 

{\textit{Intramontane basin}} & Some alpine valleys & & Intramontane
coal basins\vspace*{5pt}\\

{\textit{Pre-orogenic magmatism}} & \parbox[t]{3.8cm}{Triassic basalts,
no Jurassic \newline magmatism} & \parbox[t]{3.3cm}{Felsic and mafic
volcanites \newline Alkaline plutons} & \parbox[t]{3.8cm}{Br\'{e}venne
back-arc basin \newline Vosges klippen line}\vspace*{5pt} \\ 

{\textit{Syn-orogenic magmatism}} & Absent & Absent &
\parbox[t]{3.8cm}{\raggedright Gu\'{e}ret biotite--cordierite granitic
massif}\vspace*{5pt} \\ 

\parbox[t]{3.3cm}{{\textit{Late to post orogenic \newline
magmatism}}} & \parbox[t]{3.5cm}{Rare (Biella, Adamello,
Bergell \newline in Central Alps)} & Absent & \parbox[t]{3.8cm}{Visean
``Tufs Anthracif\`{e}res'' \Lbreak Monzogranites \Lbreak Two-mica
granites \Lbreak Mg--K granites}\vspace*{5pt} \\ 

{\textit{Crustal melting}} & Rare (Lepontine dome) & Devonian Migmatite
& Carboniferous Migmatite 

\botline
\end{tabular}
\end{table*}

\section{Tectonic zonation}

At the scale of the peri-Mediterranean belts, the Alpine system
(Figure~\ref{fig1}) is related to the lithospheric convergence between
Europe and Africa through the opening and closure of several
intervening oceanic basins. The Variscan belt, including parts overlain
by Meso-Cenozoic basins, and  the Alpine basement, forms the substratum
of Medio-Europa. The Alpine chain \textit{stricto sensu} is
geographically subdivided into the Western (French--Italian), Central
(Swiss) and Eastern (Austrian) parts (Figures~\ref{fig2},~\ref{fig3}).
In the following, due to space constraints, only the Western Alps will
be considered. 

\begin{figure*}
\includegraphics{fig01}
\caption{Distribution of the Alpine (yellow) and Variscan (brown)
orogenic systems. The continuity of the Alpine belt is disrupted by the
opening of the Neogene back-arc basins. The Variscan belt forms the
basement of the Alpine one. The Neoproterozoic Cadomian belt is hidden
by younger formations or reworked in the Variscan belt. Ar: Ardenne,
Co: Corsica, Ma: Maures, AM: Armorican Massif, EA: Eastern Alps, MC:
Massif Central, Mi: Minorca, Ro: Rhodope, S: Schwarzwald, Sa:
Sardinia, SCa: South Carpathians, SM: Serbo-Macedonian, St: Strandja,
TESZ: Trans-European Suture Zone, V: Vosges, WA: Western Alps, WCa:
Western Carpathians. \label{fig1}}
\vspace*{7.5pt}
\end{figure*}

\begin{figure*}
\vspace*{3pt}
\includegraphics{fig02}
\caption{Tectonic map of the European Alps showing the European
External, Internal, and Apulian domains, the foreland and hinterland
molassic basins, and the Eocene--Oligocene plutons.\label{fig2}}
\vspace*{4pt}
\end{figure*}

\begin{figure}
{\vspace*{5pt}}
\includegraphics{fig03}
{\vspace*{3pt}}
\caption{(A) Tectonic map of the Western Alps with the sense of
synmetamorphic ductile shearing. MB: Mont Blanc, B: Belledonne, P:
Pelvoux, A: Argentera, GP: Gran Paradiso, DM: Dora Maira, DB: Dent
Blanche, MR: Monte Rosa, PF: Penninic Front, MJ: Mont Jovet  
\citep[modified from][]{Lemoineetal2000, AgardLemoine2005}. (B)
Schematic crustal-scale cross section (located in the map) of the
Western Alps \citep[modified from][]{Roureetal1989}. The Internal
klippes of Prealps, Mont-Jovet, and Dent Blanche, exposed in the
northern part of the Western Alps are projected on the line of
section.\label{fig3}}
\end{figure}

The W. Alpine belt results from the succession of the following 
events: (i) Pangea break-up, (ii) opening of the Liguro-Piemonte (LP)
ocean, (iii) disappearance of the LP ocean by subduction below the
Apulian margin, (iv) subduction of the European-derived
Brian\c{c}onnais continental ribbon below the Apulian margin, and (v)
collision of the European continental margin with Apulia. Collision led
to crustal thickening, nappe stacking, topographic rise, foreland and
intramontane terrigenous basin formation, and exhumation of the deeply
buried oceanic and continental crust.

Unlike this simple pattern of a single oceanic lithosphere jammed
between two continental crusts, the Variscan belt exhibits a more
complex tectonic framework. In the following, we deal with the Variscan
segment that extends from SW England and Belgium to S. France. From
North to South, it is subdivided into several lithotectonic domains,
namely: (i) Northern foreland in Laurussia, (ii)~Rheno-Hercynian, (iii)
Saxo-Thuringian, (iv) Armorica microcontinent, (v) Moldanubian, and (vi)
Southern foreland in Gondwana (Figure~\ref{fig4}). In terms of plate
tectonics, the domain boundaries correspond to ophiolitic sutures, even
if ophiolites are not always well preserved due to subsequent
tectonics. The Rheic, Tepla-Le Conquet, and Eo-variscan sutures
separate continental blocks: Laurussia-Avalonia, Saxothuringia,
Armorica, and N. Gondwana margin. The Western Alps result from a
{\textit{single cycle}} of lithosphere divergence and
convergence from Late Triassic (ca. 225 Ma) to Miocene (ca.~10~Ma),
during 215~My. In contrast, the Variscan orogen was produced by
Eo-variscan and Variscan Paleozoic cycles of rifting and rewelding of
Saxo-Thuringia, and Armorica microcontinents between Laurussia and
Gondwana  \citep[e.g.][]{Pin1990,Faureetal2005}. Consequently, the
Variscan orogeny can be considered as the result of a
{\textit{polycyclic}} and {\textit{multi-collisional}}
process that developed during 260 Ma from {Ediacaran}  (ca. 550
Ma) to Late \mbox{Carboniferous} (290 Ma), \mbox{involving} three oceanic
basins, diachronously closed along three subduction zones. 

\begin{figure*}
\includegraphics{fig04}
\caption{Structural map and crustal-scale cross section (located in the
map) of the Variscan belt of Western Europe  
\citep[modified from][]{Matte1986, FaureFerriere2022}. \label{fig4}}
\end{figure*}


\section{The pre-orogenic stage}

\subsection{Liguro-Piemonte ocean: Pangea breakup}

After the completion of the Variscan orogeny, a Permian-Triassic
peneplain developed upon the eroded chain. A Carnian (ca. 225 Ma) early
rifting episode occurred in the Brian\c{c}onnais zone
\citep{Lemoineetal2000}, and the Late Triassic alkaline magmatism
recognized in the Helvetic zone is also ascribed to this rifting stage.
In the External Crystalline massifs, east-facing normal faults coeval
with syntectonic deposits in their hangingwalls document a middle to
late Jurassic age for the main rifting stage, but contemporaneous
magmatism is absent. 

The Callovian--Oxfordian (ca. 165--155 Ma) opening age of the
Liguro-Piemonte ocean is \mbox{paleontologically} and radiometrically
constrained \citep{CordeyBailly2007,Lietal2013}. Sedimentological and
structural observations suggest that the opening of the Liguro-Piemonte
ocean was accommodated by east-dipping detachment faults
\citep{Lemoineetal1987,LagabrielleLemoine1997}. At the Pangea scale,
the Liguro-Piemonte ocean corresponds to a left-lateral pull-apart
basin linking Central Atlantic and Tethys. Furthermore, in Central
Alps, the Cretaceous Valaisan basin and the Bio Unit
(Figure~\ref{fig3}), similar to the Schistes Lustr\'{e}s, can be also
considered as other oceanic basins that isolated the Brian\c{c}onnais
and Sesia continental ribbons with respect to Europa and Apulia,\break
respectively. 

\subsection{Variscan rifting: Pannotia break-up}\label{sec3.2}

In the Variscan belt, two rifting events occurred during the
Ediacaran--Cambrian (ca. 550--540 Ma) and early Ordovician (480--465 Ma).
The early one is represented by alkaline felsic magmatism exposed in
Montagne Noire or in Normandy. This aborted rifting episode did not
result in continental break-up. The main Variscan rifting took place in
Early Ordovician. Alkaline or calcalkaline plutonism is widespread in
the Moldanubian, Armorican and Saxothuringian domains  
\citep[e.g.][]{Ballevreetal2012} Ordovician felsic volcanic-sedimentary
``porphyro\"{i}d'' formations are recognized in S. Brittany,
Vend\'{e}e, S. Limousin, Albigeois, Rouergue, C\'{e}vennes
\citep{Poucletetal2017,Cousinieetal2022}. Alkaline basalts, dolerite,
and gabbro are also locally exposed \citep{PinMarini1993}. The
Ordovician rifting is responsible for the development of
``leptyno-amphibolite complexes''  
\citep[e.g.][and references therein]{Lardeaux2014a,Lardeaux2014b}. This
peculiar formation consists of cm- to m-scale alternations of rhyolitic
lavas, tuffs, mafic lavas, dolerite, gabbro, and subordinate
ultramafics. Such a bimodal magmatism commonly develops in areas where
intense crustal thinning triggers mantle upwelling, and partial
melting. The leptyno-amphibolite complex, exposed in both Upper and
Lower Gneiss units but with different metamorphic grades, represents
the ocean-continent transition. The Early Ordovician turbidites in
Montagne Noire and Pyrenees are also ascribed to the Early Paleozoic
rifting event.

In spite of the intense ductile deformation and metamorphism
experienced by these formations, it is possible to unravel the passive
continental margin from a proximal sediment-dominated part to a distal
magmatic-dominated part (Figure~\ref{fig5}). The true oceanic basin
with ultramafics, gabbro, diabase, and oceanic sediments are only
recognized along the Eo-Variscan suture in S. Brittany
\citep{Faureetal2005,Faureetal2008,Ballevreetal2009}. In the French
Massif Central, Early Paleozoic ophiolites are not exposed.\unskip\break The
km-sized masses of serpentinite displayed in\unskip\break Decazeville, Central
Limousin, or in Cantal are interpreted as pieces of infra-continental
mantle. Deep oceanic sedimentary rocks (i.e. siliceous mudstone,
radiolarian chert) are missing. 

\begin{figure*}
\includegraphics{fig05}
\caption{Simplified reconstruction of the Gondwana passive continental
margin in Early Ordovician. PAU: Para-autochthonous Unit, LGU: Lower
Gneiss Unit, UGU: Upper Gneiss Unit. The UGU will experience a high to
ultra-high pressure metamorphism during continental subduction.
\label{fig5}}
\end{figure*}

In Pyrenees, the Late Ordovician unconformity upon Early Ordovician
rocks was regarded as an\unskip\break evidence for an Early Paleozoic Caledonian
orogeny. However, it is now interpreted as a post-rift onlap upon the
Early Ordovician rifting 
\citep{LaumonierWiazemsky-Donzeau2014,Pudduetal2019}. In Central
Brittany, the Early Ordovician syn-rift terrigenous deposits are
unconformably covered by the Arenig Armorican sandstone post-rift
deposit\unskip\break \citep{Ballardetal1986}. In Ardenne, the Early Devonian
unconformity upon Cambrian--Ordovician turbidites, previously considered
as an evidence for a Caledonian event, is now interpreted as the mark
of the post-rift event \citep{Sintubinetal2009}. 

In summary, the Variscan pre-orogenic stage led to the development of
three continental stripes, from South to North in the present
coordinates: Armorica, Saxo-Thuringia, and Avalonia. These
microcontinents were separated by oceanic basins, called
Medio-European, Le Conquet-Tepla, and Rheic oceans. They drifted from
the N. Gondwana margin, represented by the S. part of the Armorican
Massif, Massif Central, Pyrenees, and S. Vosges. An important
difference with the W. Alps already appears at this stage, the Gondwana
passive continental margin was the place of a widespread magmatism
whereas syn-rift magmatism is absent in the Alpine one.

\section{The oceanic convergence stage}

\subsection{Western Alps: Liguro-Piemonte Ocean closure}

The Alpine orogeny started with the closure of the Liguro-Piemonte
ocean, accommodated by eastward subduction below Apulia
(Figure~\ref{fig6}A). The Alpine oceanic convergence is at variance
with the Wilson scheme since the magmatic arc is lacking, and even
arc-sourced detrital Cretaceous zircons are absent in the flysch
deposits  \citep[e.g.][]{Chuetal2016}. The Late Cretaceous Helminthoid
flysch is considered as a trench-fill deposit that escaped deep-seated
subduction but was thrust in Middle Eocene upon the Brian\c{c}onnais
zone. In contrast, the Schistes Lustr\'{e}s-with-ophiolites nappe is
interpreted as the metamorphic part of the subducted material. The
HP/LT metamorphism is well documented
[\citealp{Agardetal2002,Lardeaux2014a}; Figure~\ref{fig7}].
Two types of
ophiolites are distinguished: the weakly deformed ones (e.g.
Chenaillet) that probably belong to the upper plate, and the ophiolite
thrust sheets that record a HP/LT metamorphism (e.g. Monviso) attached
to the subducting plate. 

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig06}
\vspace*{2pt}
\caption{Compared geodynamic cross sections during the Alpine (A),
Eo-variscan (B), and Variscan (C) convergence stage showing the
diversity of subduction with forearc ophiolites (FAO), magmatic arc,
back-arc basin. In the Alps, the Bio unit might be also an oceanic
basin that separated the Sesia block from Apulia. Ch: Chenaillet, HF:
Helminthoid flysch. PF: future Penninic front. To the North, the Valais
ocean separated the Brian\c{c}onnais block from Europe. In the Variscan
belt, the Carboniferous tectonic, metamorphic, and magmatic features
developed in the Moldanubian Domain are located in the upper plate 
\citep[from][]{Lemoineetal2000,Faureetal2005,Faureetal2008}.
\label{fig6}}
\vspace*{1pt}
\end{figure*}

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig07}
\caption{Compared PT paths for the Western Alps, Eo-variscan and
Variscan Moldanubian domains  
\citep[modified from][]{AgardLemoine2005,Bosseetal2005,Faureetal2009,Lardeaux2014a,Lardeaux2014b}.
Note the contrast between cold (8~\textdegree C/km) and hot
(30~\textdegree C/km) thermal gradients in the Alpine and Variscan
belts, respectively.\label{fig7}}
\vspace*{-4pt}
\end{figure*}

\subsection{The Eo-Variscan belt: Medio-European ocean closure}

In France, Eo-Variscan ophiolites formed in the Medio-European ocean
are exposed only in S. Brittany, along the Nort-sur-Erdre fault, and in
the Audierne bay. In the former area, the mafic-ultramafic association
and sedimentary rocks, devoid of HP metamorphism, are ascribed to the
upper plate (Figure~\ref{fig6}). They overthrust gneiss and migmatites
of the Champtoceaux complex, correlated to the Massif Central Upper
Gneiss Unit. In France, blueschists are rare, the largest exposures are
found in S. Brittany (Ile de Groix) where metapelites and metabasites
are interpreted as formed in an accretionary complex
[\citealp{Bosseetal2005,Ballevreetal2009}; Figure~\ref{fig7}]. 
The architecture of
the Eo-variscan belt documents a stack of nappes similar to the Alpine
one (Figure~\ref{fig4}). The ophiolitic nappe is tectonically overlain
by the Mauges nappe with Neoproterozoic metamorphic rocks already
deformed during the Cadomian orogeny, and unconformably covered by
Early Paleozoic sedimentary rocks. The disappearance of the
Medio-European ocean was accommodated by a northward subduction below
Armorica. The oceanic convergence setting has to take into account also
the St-Georges-sur-Loire Unit located in the Armorica
block immediately north of the Eo-variscan suture. This block-in-matrix
unit is interpreted as a back-arc basin, north of a possible magmatic
arc subducted during the Armorica--Gondwana convergence  [Figure~\ref{fig6}B;
\citealp{CartierFaure2004,Faureetal2008,Ducassouetal2011}].

\subsection{The Variscan belt: Rheic and Tepla oceans closure}

The Carboniferous Variscan orogeny corresponds to the closure of the
Rheic and Tepla-Le Conquet oceans with ophiolitic sutures located in
the English Channel and in NW Brittany, respectively (Figures~\ref{fig4},~\ref{fig6}C). In
the L\'{e}on (i.e. Saxo-Thuringian microcontinent), the
top-to-the-North synmetamorphic ductile shearing argues for a
S-directed subduction  \citep{Roletetal1994,Faureetal2010}. Another
evidence for southward subduction is found in the NE Massif Central
(Morvan area). There, basalt, andesite, volcanic-sedimentary rocks and
massive sulfide deposits support a Late Devonian magmatic arc 
\citep{Delfour1989,Faureetal2005}. The Br\'{e}venne ophiolites
represent the back-arc basin opened during the southward subduction of
the Tepla ocean. Between the Ardennes and Vosges, geophysical data also
document south dipping Rheic and Tepla sutures 
\citep{EdelSchulmann2009}. The Devonian diorite-granodiorite massifs of
the Limousin ``tonalite line''  \citep{DidierLameyre1969,Peiffer1986}
are also interpreted as an evidence for arc plutonism. 

\section{The collisional stage}

\vspace*{-1pt}

\subsection{Western Alps: suturing of the Liguro-Piemonte ocean}

\vspace*{-1pt}

The contact between two continents, i.e. collision, was preceded by
continental subduction as demonstrated by the HP and UHP metamorphism
recognized in the Internal Crystalline Massifs (Dora Maira, Gran
Paradiso) and Brian\c{c}onnais zone 
[\citealp{Chopin1984,Agardetal2002,Beltrandoetal2010,Lardeaux2014b};
Figure~\ref{fig7}]. The nappe stacking from the SE to the NW that developed
during this stage is documented by the shift of radiometric ages from
Paleocene (ca. 50 Ma) to Miocene (ca. 10 Ma) 
\citep{MoniePhilippot1989,Fordetal2006,Bonnetetal2022}. The sedimentary
record also shows a NW-ward migration of the terrigenous deposits. The
erosion of the early reliefs in the Internal zone supplied the material
for the Middle Eocene (ca.~40~Ma) Brian\c{c}onnais black flysch, then in
the Outer zone, the Priabonian (35 Ma) gr\`{e}s d'Annot-Aiguilles
d'Arves flysch, and the Rupelian (ca. 30 Ma) Dauphinois \mbox{flysch.}\break
The~syn-tectonic molassic sedimentation in the foreland basin continued
until the Miocene (10 Ma). 

Although the structural style and thermo-barometric conditions may
change along the strike of the belt, the general architecture of the
Western Alps is acknowledged from top to bottom with: (i)~an uppermost
Austroalpine continental crust domain overlying (ii) the Liguro-Piemonte
oceanic sedimentary and magmatic rocks, in turn overlying (iii) the
Brian\c{c}onnais sedimentary nappe stack that overthrusts along the
Penninic thrust iv) the less deformed and metamorphosed
Dauphino-Helvetic domain (Figure~\ref{fig3}). Microtectonics document a
general top-to-the-NW ductile shearing. However, during the Miocene, a
SW-ward nappe displacement developed in the S. part of the Dauphinois
zone (Figure~\ref{fig3}). 

{\vspace*{-1pt}}

\subsection{The Eo-Variscan Gondwana--Armorica: suturing of the Medio-European ocean}

{\vspace*{-1pt}}

HP to UHP eclogites and granulites are exposed in the Moldanubian
domain  [S. Brittany, Vend\'{e}e, Limousin, Marvejols, Lyonnais areas;
e.g. \citealp{Faureetal2005,Lardeaux2014b}; Figure~\ref{fig7}]. As mentioned
above, the protoliths of these rocks are not ophiolites but mafic and
felsic dykes and sills emplaced during the early Ordovician rifting
within a thinned continental crust. {The UHP assemblages argue for ca
100 km deep continental subduction 
\citep{Lardeauxetal2001,Bergeretal2010}.} The age of the Eo-variscan HP
metamorphism is still disputed: either around 415--400 Ma 
\citep{PinLancelot1982,DoCoutoetal2016} or 370--360 Ma 
\citep{Bosseetal2000,Paquetteetal2017,Lotoutetal2018}. At the
lithosphere scale, the collision between Gondwana and Armorica was
responsible for a nappe stack, from top-to-bottom: (i) the Mauges nappe
derived from Armorica, (ii) the ophiolitic units derived from the
Medio-European Ocean, (iii) the (U)HP metamorphic units, referred to as
the Upper Gneiss Unit, (iv) the Lower Gneiss unit. The last two units
derived from stretched, or hyperextended, Gondwana continental crust.
It is worth to note that the lowermost units, Para-autochthonous and
\mbox{Fold-and-Thrust} belt, did not experienced the Eo-variscan
tectono-metamorphic events.

Moreover, an important difference between the Alpine and Eo-variscan
orogens is the development in the latter of a pervasive crustal melting
coeval with the exhumation of the HP rocks (Figure~\ref{fig7}).
Migmatites are widespread in the paragneiss of the Upper and Lower
Gneiss units. After the continental subduction, the UGU underwent an
adiabatic decompression that retrogressed the eclogites into garnet
amphibolites. In the same time, the Al-rich metapelites and felsic
orthogneiss were melted to produce the metatexites observed in the
Champtoceaux, Limousin, Sioule, Haut Allier, Lyonnais, and Rouergue
areas. The migmatites in the UGU and LGU yield zircon U/Pb ages at
385--380 Ma and 380--375~Ma, respectively  \citep{Faureetal2008}. They
exhibit a NE--SW striking mineral lineation coeval with top-to-the-SW
shearing that suggest syn-convergence exhumation. In Morvan, Devonian
migmatites that include eclogites and retrogressed garnet amphibolites 
\citep{Godard1990} are older than Frasnian (ca. 383--372~Ma) sedimentary
rock \citep[for details see][]{Leloixetal1999,Faureetal2005}. 

\subsection{The Variscan collisions: suturing of the Rheic and Tepla-Le Conquet ocean}

 The Variscan orogeny \textit{stricto sensu} corresponds to the closure
of the Rheic and Tepla-Le Conquet oceans through S-directed subductions
(Figure~\ref{fig6}). The collision of Laurussia with Saxo-Thuringia
gave rise to N-displaced Lizard ophiolitic klippe in SW England. The
collision of Saxo-Thuringia with Armorica was responsible for the
N-directed syn-metamorphic nappes exposed in the L\'{e}on block. In
other places, similar structures are concealed beneath the sedimentary
rocks of the Paris basin, as shown by the ECORS seismic profile 
\citep{Cazesetal1985}. The Variscan collisions reworked the Eo-Variscan
structures of the Moldanubian domain in the Massif Central and S.
Armorican massif. It is worth to note that this domain belongs to the
upper plate. A top-to-the NW ductile shearing, coeval with a MP/MT
metamorphism, developed in the Famennian--Tournaisian  [ca. 360--350 Ma;
Figure~\ref{fig6}; \citealp{Faureetal2009,DoCoutoetal2016}]. This event, called
``Bretonian phase'' was recognized in Central Brittany by the erosional
gap of Late and Middle \mbox{Devonian} formations, and Tournaisian
unconformity  \citep{Cogne1965,Parisetal1982,Rolet1982,Faureetal2017}.
In NE Massif central, the northwestward shearing was responsible for
emplacement of the Br\'{e}venne back-arc ophiolitic rocks on top of the
retrogressed Eo-Variscan gneiss  \citep{Leloixetal1999,Faureetal2005}.

The final S-directed nappe stacking, coeval with greenschist facies
conditions, occurred in Visean-Bashkirian (ca. 340--320 Ma) in the
hinterland basin of Southern Massif Central, Pyrenees and Vend\'{e}e.
In Montagne Noire and Pyrenees, thrusting was a syn-sedimentary event
with a turbiditic sedimentation 
\citep{Engeletal1978,Delvolveetal1998}. A similar pattern is recognized
in the northern foreland basin: Ardenne, SW England 
[Figure~\ref{fig4}; e.g. \citealp{FielitzMansy1999}].\looseness=-1

\section{The intracontinental syn- to late convergence} 

\subsection{Western Alps}

In the Inner zone, the exhumation of the metamorphosed oceanic and
continental units was accommodated by ductile normal faults, such as
the Monviso one  \citep{Ballevreetal1990}. Although not exposed in the
Western Alps, a HT/LP (metamorphism developed during the Oligocene
(28--21 Ma) in the Lepontine dome  [Figure~\ref{fig2}, \citealp{Bergeretal2020}].
The sillimanite and K-feldspar isogrades, oblique to the thrust
contacts, define an elliptical domal shape. The Bergell granodiorite
emplaced along the Tonale line. The Miocene exhumation of the External
Crystalline Massifs accommodated by thrusting  
\citep[e.g.][]{Leloupetal2005,Eglietal2017} was coeval with belt parallel
extension  \citep{Sueetal2007}. In the External zone, intracontinental
shortening was responsible for the NW- and SW-ward thrusting of the
Dauphino-Helvetic Mesozoic series upon the molassic deposits
(Figure~\ref{fig2}). Sismotectonics, stress tensor analyses and
geodetic data reveal a complex tectonic pattern with a small
displacement rate (ca. 1--2 mm/yr) accommodated by thrusting in Jura and
Po plain, and belt-transverse extension in the Internal zone. GPS data
and paleomagnetism argue for a counterclockwise rotation of Apulia.
Thus, the origin of the present tectonics results from both body forces
due to gravitational re-equilibration of the thickened crust, and
far-field boundary forces related to Apulia indentation.

\subsection{Variscan belt}

Intracontinental tectonics likely related to the exhumation of the
Eo-Variscan rocks are not documented. On the contrary, the Variscan
late to post-orogenic stage is recorded by magmatic, tectonic, and
sedimentary processes. The Variscan orogen displays a large amount of
magmatic rocks emplaced from Late Tournaisian (biotite--cordierite
Gu\'{e}ret massif) to Gzhelian. Carboniferous migmatites are widespread
from the L\'{e}on block to the Pyrenees, through South Armorica,
Vend\'{e}e, Massif Central, and Central Vosges. The migmatites, formed
at the expense of metasediments and orthogneiss, are dated between 330
and 325 Ma, with younger ages around 315--305 Ma close to
post-migmatitic intrusions 
\citep{BeMezemeetal2006,Turrillotetal2011,Augieretal2015,Trapetal2017,Vanderhaegheetal2020}. 
The tectonic setting of the domes, namely, diapiric, compressional,
extensional, or transcurrent, will not be discussed here 
\citep[for details
see][]{VanDenDriesscheBrun1992,EchtlerMalavieille1990,Faure1995,Frankeetal2011,Deneleetal2014}. 
\looseness=-1

The Late Visean magmatism is also represented by the ``Tufs
anthracif\`{e}res'' volcanic-sedimentary series exposed in the NE part
of the Massif Central and S. Vosges. The emplacement of this series
that consists of undeformed and unmetamorphosed felsic and
intermediate-type lava flows, pyroclastites, sandstone, siltstones, and
coal measures, was controlled by a NW--SE stretching during the onset of
the late orogenic extension in the northern Moldanubian Domain, whereas
the thickening event was still active in the S. Massif central, and
Pyrenees. In the Northern Massif central, the ``red granites'' and
microgranites of the Montagne Bourbonnaise represent the deep part of
this late orogenic magmatic suite. 

Due to different crustal sources, per-aluminous two-mica granites and
porphyritic monzogranites of Serpukhovian to Bashkirian age (ca. 325--310
Ma) are distinguished  \citep{DidierLameyre1969}. The former group is
well represented in S. Brittany, Vend\'{e}e, and Limousin, and the
latter is mainly exposed in NE and SE Massif central. Both pluton types
are syn-tectonic bodies characterized by a NW--SE stretching recorded in
the plutons, contact aureole, and country rocks. Ductile normal faults
coeval with pluton emplacement support an extensional setting. In the
Armorican Massif, the syn- to late-orogenic plutonism was associated
with dextral strike-slip faults of the S. Armorican and N. Armorican
shear zones  \citep{Jegouzo1980}.

During the late Carboniferous, the Massif Central and Armorican massif
experienced a second extensional event ascribed to the post-orogenic
stage. The opening of coal-bearing intramontane basins was controlled
by normal or strike-slip faults with a NNE--SSW maximum stretching
direction  \citep{Malavieilleetal1990,Faure1995}. The Velay
granite-migmatite dome formed also during this late Carboniferous event
\citep[e.g.][]{Barbeyetal2015,Moyenetal2017,Laurentetal2017}. Mg--K
magma, widespread in the NE Massif Central and Vosges, formed by the
melting of a mafic lower crust with some\unskip\break mantle input. Lamprophyre
dykes are also interpreted as a consequence of asthenospheric
upwelling. The HT granulite xenoliths in the Neogene lava record a
HT/LP metamorphism similar to the one observed in the Alpine Ivrea zone
or some Pyrenean massifs (Castillon, Agly). These rocks form the
layered lower crust depicted in the ECORS seismic profile 
\citep{Cazesetal1985}.


\section{Conclusive remarks}

This brief review of the main features of the Variscan and Alpine
orogens shows that these belts resulted of crustal thickening due to
nappe stacking after the closure of oceanic domains. The main features
related to the geodynamic evolution stages of the W. Alps, Eo-variscan,
and Variscan belts are listed in Table~\ref{tab2}. Moreover, the Eo-variscan,
Variscan, and Alpine belts exhibit some differences that can be related
to the diversity of the thermal gradients and crustal inheritance. 

\begin{table*}
\caption{\label{tab2}Compared geodynamic evolution stages of the W.
Alps, Eo-variscan, and Variscan belts\vspace*{2pt}}
\footnotesize\begin{tabular}{llll}  
\thead
& {Western Alps} & {Devonian Eo-Variscan belt} & {Carboniferous
Variscan belt} \\ 
\endthead
\parbox[t]{3.1cm}{{\textit{PRE-OROGENIC}} \Lbreak {\textit{Continental
rifting~to ocean opening}}} & \parbox[t]{3.5cm}{Triassic
rifting in Apulia and Brian\c{c}onnais.  \Lbreak Normal
faulting in External \newline Crystalline Massifs,  \newline no
magmatism} & \parbox[t]{3.8cm}{Late Cambrian  \Lbreak Early Ordovician
rifting \Lbreak Leptynite-amphibolite complex \Lbreak
Sardic/Ardennian unconformity \Lbreak Drain ophiolites}  &
\parbox[t]{3.8cm}{Late Cambrian  \Lbreak Early Ordovician rifting
\newline and \Lbreak Le Conquet ophiolites}\vspace*{1.8pt} \\ 

& \parbox[t]{3.5cm}{\textbf{Liguro-Piemonte ocean} \newline (Middle
Jurassic)} & \parbox[t]{3.8cm}{\textbf{Medio European ocean} \newline
(Early Ordovician)}  & \parbox[t]{3.8cm}{\textbf{Rheic \& Tepla oceans}
\newline (Early Ordovician)}\vspace*{6pt} \\ 

{\textit{OROGENIC}} & \parbox[t]{3.5cm}{Helminthoid flysch:
trench \newline fill Schistes Lustr\'es} & \parbox[t]{3.8cm}{Ile de
Groix (subduction \newline melange ?)} & Not exposed in
France\vspace*{1.8pt} \\

{\textit{Oceanic subduction}} & \parbox[t]{3.5cm}{(U)HP
metamorphism\newline (Monviso)}\vspace*{1.8pt} & & \\

\quad{\textit{Magmatic arc}} & Absent & Ligerian arc (inferred) &
\parbox[t]{3cm}{Morvan arc\Lbreak Limousin tonalites}\vspace*{1.8pt}\\

\quad{\textit{Back-arc basin}} & Absent & \parbox[t]{4cm}{St-Georges/Loire
back-arc~basin} & Br\'evenne Back-arc ophiolite\vspace*{6pt}\\

{\textit{Continental subduction}} & \parbox[t]{3.5cm}{Austroalpine
nappe \newline Internal Crystalline Massifs \newline Penninic recumbent
folds} & \parbox[t]{3.8cm}{Mauges nappe\Lbreak~\Lbreak Moldanubian
syn-metamorphic \newline thrusting} & \parbox[t]{3.5cm}{Saxo-Thuringian
(L\'eon): \newline top-to-the N shearing \newline Moldanubian: \newline
Top-to-the NW thrusting}\vspace*{1.8pt}\\

{\textit{Collision}} & Schistes Lustr\'es nappe &
\parbox[t]{3.8cm}{HP-UHP metamorphism \newline (eclogites, HT
granulites)} & \parbox[t]{3.8cm}{Reworking of Eo-Variscan \newline
structures\Lbreak  MP/MT metamorphism}\vspace*{1.8pt}\\

\parbox[t]{2cm}{{\textit{Intracontinental \newline
thrusting}}} & & \parbox[t]{4cm}{MP/MT metamorphism
Upper \newline Gneiss Unit thrusting upon Lower}\vspace*{1.8pt} & \\

& \parbox[t]{3.5cm}{$E\to W$ migration of flysch \newline basins\Lbreak
Helvetic recumbent folds\Lbreak Dauphinois Fold-and-Thrust \newline
belt\Lbreak Brian\c{c}onnais nappes} & Gneiss Unit &
\parbox[t]{3.8cm}{Moldanubian:\newline Para-autochthonous Unit \newline
South-directed Fold-and-Thrust \newline belt (Montagne Noire, \newline
Pyrenees) \newline Rheno-Hercynian: North-\newline directed
Fold-and-Thrust belt \newline (Ardenne, SW England)}\vspace*{6pt} \\

{\textit{LATE TO POST-OROGENIC}} & \parbox[t]{3.5cm}{Central
Alps: \newline Lepontine dome \newline Biella, Bergell, Adamello \newline
plutons} & & \parbox[t]{3.5cm}{Visean marine deposits\Lbreak Tufs
Anthracif\`eres\Lbreak Two-mica granites\Lbreak Monzogranite\Lbreak
Mg--K granites}\vspace*{1.8pt}\\

{\textit{Crustal melting}} & Migmatites & Devonian migmatites &
Carboniferous Migmatite\vspace*{1.8pt}\\

{\textit{Foreland basin}}  & \parbox[t]{3.5cm}{Helvetic molasse\Lbreak
Po plain molasse} & Not documented & \parbox[t]{3.5cm}{North foreland
basin\Lbreak South hinterland basin}\vspace*{1.8pt}\\

{\textit{Intramontane basin}} & Not documented & Not documented &
Gzhelian coal basins

\botline
\end{tabular}
\vspace*{5pt}
\end{table*}

\subsection{Passive continental margins}

As introduced in Section~\ref{sec3.2}, the pre-Variscan Ordovician passive
continental margin resemble the magma-rich margins 
\citep[e.g.][]{Geoffroy2005}. This feature represents a significant
difference with the Jurassic Alpine continental margin in which
syn-rift magmatism is absent  \citep[e.g.][]{Lemoineetal2000}. One
explanation might be that the Ordovician rifting was accommodated by a
higher strain rate than in the Alps, allowing a fast mantle denudation
that enhanced crustal melting. 

\subsection{Crustal melting}

In the Western Alps, evidence for crustal melting is absent. Migmatites
are restricted to the Lepontine dome in Central Alps. The rare
Oligocene plutons represent a few volume compared to the huge mass of
the Variscan ones. It might be argued that Alpine granites are not yet
exposed to the surface. However, as UHP rocks are already exhumed from
ca.~90~km, a large amount of plutons would be expected as well. A
possible cause for the rarity of Alpine melting could be that the
middle and lower crusts are formed by unfertile rocks left after the
Variscan melting. The Paleozoic rocks that already released melts were
unable to produce new magmas in the Cenozoic. The difference in crustal
melting behavior reflects the contrasted thermal gradients recognized
in cold and hot orogens. The Alpine one of ca. 8~\textdegree C/km was
colder than the Eo-variscan and Variscan gradients of ca
20--30~\textdegree C/km (Figure~\ref{fig7}). However, the Eo-variscan
prograde metamorphic gradient is also estimated at 8~\textdegree C/km
before reaching a 30~\textdegree C/km one during exhumation. It has
also been shown that in the northern part of the Massif Central, the
Late Visean thermal imprint was able to reset the $^{40}$Ar/$^{39}$Ar
chronometer  \citep{Faureetal2002}. These observations raise the
question of the origin of heat. On the basis of the Himalayan case, it
is often argued that the increase in radiogenic elements due to crustal
thickening triggered the melting of hydrated rocks to produce
per-aluminous magmas  \citep{Lameyre1984}. Although this mechanism
likely played a role, the widespread distribution of migmatites and
granites throughout the entire belt, even in the weakly thickened
external zone, is hardly explained by an intracrustal heat source.
Alternatively, a mantle source can be envisioned. Mantle convection,
lithosphere mantle delamination or slab breakoff would allow the rise
of hot asthenosphere bringing enough heat to melt the continental
crust. 

\subsection{Fluid circulation and ore deposits}

The Variscan belt is famous for its metallic resources mined since
Celtic times. Au, Sb, W--Sn, Li--Be ore deposits are displayed in the
French Variscan massifs  
\citep[e.g.][]{ChaurisMarcoux1994,Bouchotetal1997,MarignacCuney1999}. These
deposits are linked to the late- or post-orogenic hydrothermal and
magmatic episodes. Massive sulfide deposits are exposed in the S.
Iberian pyrite belt, or in the NE Massif Central where they are
associated either to the Morvan magmatic arc or Br\'{e}venne back-arc
basin developed during the oceanic subduction stage. In contrast, ore
deposits are rare in the Western Alps. As exceptions, the
St-V\'eran, Servette-Chuc and Praborna mines, and
scattered small sized Cu deposits are recognized in the ophiolites of
the Schistes Lustr\'{e}s nappe, but they are related to the
pre-orogenic hydrothermal events. Thus a dry and sterile Alpine crust
is at variance with an hydrated, fertile Variscan one. Furthermore, the
``cold'' temperature gradient was unable to enhance fluid circulations.


\subsection{The basement question}

In the Alps, nappe stacking involves Meso-Cenozoic sedimentary rocks
and Paleozoic metamorphic or magmatic rocks shaped up during the
Variscan orogeny  \citep[e.g.][and references therein]{FaureFerriere2022}.
In the Variscan belt, a Neoproterozoic Cadomian basement
exists in Armorica and Saxo-Thuringia, even not exposed in the L\'{e}on
block. In the Moldanubian domain, Neoproterozoic sedimentary and
magmatic rocks do exist, but they cannot be considered as a basement
since they experienced their first deformation only during the Variscan
orogeny. There, reference to ``Cadomian'' events is therefore
groundless in this domain. The lithological contrast between ``soft''
and ``hard'' rheologies might be also the cause for the different
structural styles between the Moldanubian and Armorican domains.

\subsection{Orogenic time scale}

The ignorance of several parameters such as the precise age and thermal
state of the oceanic lithosphere, or the dip angle of the subducting
slab, does not allow us to accurately depict the geodynamics of the
converging lithospheres. Nevertheless, a rough computation provides a
semi-quantitative estimate of the different stages of the Eo-variscan,
Variscan and Alpine orogenic evolution. From a Callovian--Oxfordian age
(${\sim}$160 Ma) for the opening of the
Liguro-Piemonte ocean and an early Eocene age
(${\sim}$55~Ma) for the continental
subduction or incipient collision, the life-time of the Liguro-Piemonte
ocean was about 105 My. In contrast, the Medio-European ocean was a
short lived one, ca. 80~My, from the Early Ordovician
(${\sim}$480 Ma) break-up of Pannotia to the
Eo-variscan collision at ca. 400 Ma. The Rheic Ocean also opened in
Early Ordovician, but closed in Early Carboniferous
(${\sim}$360 Ma). This 120~My long duration
might account for the development of arc and back-arc basins.

In the Alps, the collisional stage lasted
${\sim}$45 My from Ypresian (55 Ma) to
Tortonian (10 Ma). The duration of the Eo-variscan collision to
post-collision stages is difficult to estimate since Devonian molasse
is not documented. A minimum 30 My time lapse is assumed from
continental subduction (${\sim}$400 Ma) to
migmatisation (${\sim}$370 Ma). From the
Tournaisian (360 Ma) to late Carboniferous  (Gzhelian 300~Ma),
the Variscan collision duration (60~My) was longer than the Alpine and
Eo-variscan orogenic ones. 

The Eo-variscan and Variscan orogenies document multiple collisions in
space and a polycyclic orogenic history in time. With a single cycle,
the case of Western Alps is simpler, but a more complex evolution might
be accepted for the entire Alpine belt if the Valaisan ocean of Central
Alps is considered. Furthermore, it can be argued that the Alpine cycle
is not completed yet since the African plate is still subducting below
Europe (Figure~\ref{fig1}). After the closure of the Tyrrhenian and
Aegean back-arc basins, the Alpine orogen might also present a
bivergent architecture, and a polyorogenic evolution similar to the
Variscan one.

\section*{Conflicts of interest}

The author has no conflict of interest to declare.

\section*{Acknowledgements}

This article benefited of constructive comments and suggestions of
R. Carosi, J. Malavieille, O.~Fabbri, J.~Ferri\`{e}re, and L. Jolivet.

\back{}

\bibliographystyle{crgeos}
\bibliography{crgeos20221210}
\refinput{crgeos20221210-reference.tex}

\end{document}
