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\DOI{10.5802/crgeos.238}
\datereceived{2023-04-28}
\daterevised{2023-09-14}
\dateaccepted{2023-09-18}
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\dateposted{2023-11-21}
\begin{document}

\begin{noXML}


\makeatletter
\def\TITREspecial{\relax}
\def\cdr@specialtitle@english{Geodynamics of continents and oceans - A tribute to Jean Aubouin}
%\def\cdr@specialtitle@french{G\'eodynamique des continents et des Oc\'eans - Hommage \`a Jean Aubouin}
\makeatother

\shortrunauthors

\title{From subduction to collision and subduction again, the drivers
of crustal-scale deformation in the Hellenides-Aegean region}

\alttitle{De la subduction \`{a} la collision et \`{a} la subduction
\`{a} nouveau, les moteurs de la d\'{e}formation \`{a} l'\'{e}chelle
crustale dans la r\'{e}gion des Hell\'{e}nides et de la mer
\'{E}g\'{e}e}

\author{\firstname{Jacky} \lastname{Ferriere}}
\address{Universit\'{e} de Lille, ULCO, CNRS, IRD, UMR 8187, LOG, 
Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F59000 Lille, France}
\email[J. Ferriere]{jacky.ferriere@univ-lille.fr}

\author{\firstname{Laurent} \lastname{Jolivet}\CDRorcid{0000-0002-2596-2017}}
\address{Sorbonne Universit\'{e}, UMR 7193 CNRS-UPMC, 
Institut des Sciences de la Terre, 75005 Paris, France}
\email[L. Jolivet]{laurent.jolivet@sorbonne-universite.fr}

\author{\firstname{Frank} \lastname{Chanier}\CDRorcid{0000-0001-6030-6982}\IsCorresp}
\addressSameAs{1}{Universit\'{e} de Lille, ULCO, CNRS, IRD, UMR 8187, LOG, 
Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F59000 Lille, France}
\email[F. Chanier]{frank.chanier@univ-lille.fr}

\keywords{\kwd{Hellenides}
\kwd{Aegean sea}
\kwd{Subduction}
\kwd{Obduction}
\kwd{Collision}
\kwd{Detachments}}

\altkeywords{\kwd{Hell\'{e}nides}
\kwd{Mer \'{E}g\'{e}e}
\kwd{Subduction}
\kwd{Obduction}
\kwd{Collision}
\kwd{D\'{e}tachements}}

\begin{abstract} 
In 
the 60s, the Hellenic belt was taken as the ``Geosyncline model''.
Here we take a modern look at this Hellenic/Aegean region which as a
concentrate encompasses all the main geodynamic processes known in
orogens. This region documents the fundamental characteristics of the
geodynamic processes related to mountain building, namely continental
rifting, oceanic spreading, oceanic then continental subduction,
collision i.e., closure of oceanic basins. In particular, the Mesozoic
obduction events in northern Hellenides, and the effects of a still
ongoing major extension linked to the active subduction under the
Aegean domain.
\end{abstract}

\begin{altabstract} 
Dans les ann\'{e}es 60, la cha\^{i}ne hell\'{e}nique \'{e}tait
consid\'{e}r\'{e}e comme le
\guillemotleft{}~mod\`{e}le g\'{e}osynclinal~{}\guillemotright. Nous
jetons ici un regard moderne sur cette r\'{e}gion
hell\'{e}nique/\'{e}g\'{e}enne qui englobe tous les principaux
processus g\'{e}odynamiques connus dans les orog\`{e}nes. Cette
r\'{e}gion pr\'{e}sente les caract\'{e}ristiques fondamentales des
processus g\'{e}odynamiques li\'{e}s \`{a} l'\'{e}dification des
cha\^{i}nes de montagnes, \`{a} savoir : le rifting continental,
l'expansion oc\'{e}anique, la subduction oc\'{e}anique puis
continentale, la collision, c'est-\`{a}-dire la fermeture des bassins
oc\'{e}aniques. En particulier, les \'{e}v\'{e}nements d'obduction
m\'{e}sozo\"{i}ques dans les Hell\'{e}nides du Nord, et les effets
d'une extension majeure encore en cours li\'{e}e \`{a} la subduction
active sous le domaine \'{e}g\'{e}en.
\end{altabstract} 

\maketitle

\vspace*{4pt}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1} 
Tethyan mountain belts such as the Alps and the Hellenides served as
key-sites for defining new tectonic concepts such as the notion of
nappes derived from isopic zones, defined as domains with coherent
stratigraphy and paleogeography. They were reinterpreted in the
framework of  ``new global tectonics'' in the early 70s. 
Presently,\vadjust{\pagebreak}
they are now the\unskip\break 
center of attention of a large community working on
the interaction between subduction dynamics and crustal deformation.
The evolution of Mediterranean back-arc basins, such as Alboran,
Tyrrhenian, and Aegean seas is nowadays explained by new concepts of
subduction dynamics that have flourished since the development of
seismic tomography, seismic anisotropy, analog and numerical modelling,
but also the improvement of field tectonic and metamorphic studies. The
Aegean Sea is emblematic of this evolution and several schools have
developed evolutionary models based on new concepts and\unskip\break 
observations. 

Strangely, the Hellenic mountain belt has not been given the same
attention in the recent period. On the other hand, the early part of
the tectonic history of the Aegean region, before the change in
subduction dynamics leading to back-arc extension some 30--35~Ma ago,
is rather poorly documented despite the emblematic character of this
large alpine-type Hellenic belt, including the Aegean back-arc.

Jean Aubouin was a prominent figure among those who developed the early
concepts on the Hellenides, since his thesis  \citep{Aubouin1959},
followed by many collaborative works  [references in 
\citealp{Jacobshagen1986} and \citealp{FerriereFaure2023}]. 
Jean Aubouin's school unraveled a large part of the External Hellenides
while Jan Brunn and his students were mainly working in the Internal
Zones. Both schools worked in detail on the Pelagonian domain located
in the most external part of the Internal Zones, particularly
well-known for the occurrence of huge volumes of ophiolitic material.
Consequently, important ophiolitic complexes from the Eastern
Hellenides have been the subject of multiple studies. One of the first
ophiolite models corresponding to a massive submarine magmatic outflow 
(``Ophiolitic balloon model'') has been proposed by the studies carried
out in northern Greece  \citep{Brunn1956,Aubouin1959}. 

The development of the concept of obduction developed in the early 70s 
\citep{Coleman1971,DeweyBird1971}
reinforced the interest in the study
of Hellenic ophiolites that served as new models such as the Vourinos,
North-Pindos and Othris ophiolitic units 
\citep[e.g.][]{Smithetal1975,Beccaluvaetal1984}.

A large part of the current knowledge was established during this
initial period  (60s--70s), when field geologists mapped the Greek
territory as well as the Dinarides in former Yugoslavia. Yet, although
it may be less visible than the many projects developed in the back-arc
region, the geological knowledge has since considerably improved in the
continental part of the Hellenides but the consequences on the recent
evolution have not been fully explored. One symptomatic observation of
a certain lack of attention drawn to the Hellenides is the absence of
crustal-scale cross-sections similar to those proposed in the Alps or
the Betic-Rif. Nevertheless, one can mention recent work providing
detailed kinematic reconstructions  \citep{vanHinsbergenetal2020} or
correlations between tectonic units  \citep{Schmidetal2020} published
on the evolution of the Alps and the Balkans/Hellenides. A different
approach was used by  \citet{Menantetal2016a,Menantetal2016b} 
who reconstructed the
magmatic and metallogenic evolution of the Hellenides and Taurides from
the Late Cretaceous to the Present and discussed the implications in
terms of subduction dynamics.

In this contribution we summarise the most recent findings, mainly
stemming from the Internal Hellenides, and show their implications for
the geodynamic evolution of this wide region. Moreover, we show how the
Hellenides can be used as a general model for Mediterranean mountain
belts.

\section{Geological setting}\label{sec2}   
The Hellenides and the Aegean domain cover the entire Greece and
extends eastward onto the Anatolian side of the Aegean Sea in the
Taurides and the Menderes massif. The western Hellenides form a rather
continuous massif at high altitude resting on a ca. 40--45~km thick
continental crust  \citep{Makrisetal2013}. 
In the eastern part of the belt,
NW--SE striking reliefs, including Mount Olympos  (2918~m) alternate
with recent sedimentary basins. Further east, the Aegean back-arc basin
is characterised by the Cyclades, Dodecanese, Eastern Aegean Islands,
or Crete. The continental crust is progressively thinned toward the
Aegean domain. 

This spatial organisation results from the superimposition of several
orogenic events:  (i)~the Jurassic obduction of the Tethyan  (Maliac)
oceanic lithosphere 
\citep[e.g.][]{BernoulliLaubscher1972,Dercourt1970,Ferriere1972,Ferriere1982,Hynesetal1972,Jacobshagen1986,Smithetal1975};
(ii)~an Eocene collision between the Adria (Apulia) to the SW and the
European domain to the NE  (Figures~\ref{fig1} and~\ref{fig2}) 
\citep{Aubouin1959,Jacobshagen1986,Schmidetal2020}; (iii)~A more
recent, mainly Miocene extension with a stretching amount increasing
toward the east, reworking the structural pattern inherited from the
collision. 

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Tectonic map of the Hellenides and of the Aegean
region  \citep[modified after][references within]{Jolivetetal2021}.
(A--D)~location of cross-sections  Figure~\ref{fig2};  FF$'$: upper
cross-section Figure~\ref{fig6}.  The light colours at the top of the
cartouches represent the recent sedimentary levels deposited on the
concerned basement in darker colours.} 
\vspace*{4pt}
\end{figure*}

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Cross-sections across the Hellenic belt. 
(A--D)~Location  Figure~\ref{fig1}.
(A)~In northern Greece from Rhodope to Pindos-Ionian Zones through the
Vardar and Pelagonian Internal Zones;
(B)~cross-section in the Pelagonian domain and Plio-Quaternary basins
within the Internal Zones with the Ossa window showing External units
underneath;
(C)~cross-section in the Internal Zones showing the Jurassic
syn-obduction Maliac units; 
(D)~cross-section across the Pelagonian and Parnassus Zones to the
current subduction zone of the eastern Mediterranean Sea.}
\end{figure*}

This late tectonic episode results from the change in subduction
dynamics and the beginning of slab retreat in the Late Eocene 
\citep{JolivetFaccenna2000}. The western Hellenides, corresponding to
External Zones of the belt, were tectonized only during the Tertiary
while the eastern Hellenides, where the obduction episode is recorded,
form the Internal Zones  \citep{Brunn1956}. 

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Stratigraphic logs related to the Hellenic isopic
zones. (1--6)~Limestones (lmst);  (1)~benthic (platform),  (2)~oolithic
(down) to conglomerates (up);  (3)~fine-grained (down) and siliceous
(up); (4)~Ammonitico Rosso  (Peonias);  (5)~alternating calcarenites
(up, triangles) and siliceous lmst (down, dashes);  (6)~siliceous
fine-grained lmst and calcarenites (up), marls and sandstones (down); 
(7)~radiolarites (down: pelitic);  (8)~argillaceous beds (down),
argillaceous sandstones (up);  (9)~sandstones with siliceous
conglomerates (triangles, down); (10)~conglomerates (MHB); 
(11)~flyschs;  (12)~ophiolitic m\'{e}langes; \mbox{(13--16)}~volcanism; 
(13)~rhyolites;  (14)~basaltes;  (15)~andesites;  (16)~Triassc syn-rift
pillow-lavas (violet);  (17--19)~ophiolites (green); 
(17)~pillow-lavas;  (18)~gabbros, dolerites;  (19)~peridotites; 
(20)~metamorphic basements;  (21)~main unconformities;  (22)~Thrusts.}
\end{figure*}

\subsection{Regarding the Hellenides}\label{sec2.1}
\textit{The western Hellenides or External Zones} are made up of a
stack of nappes derived from the eastern margin of the Adria block.
This apparent simplicity made this sector ideal for the first important
tectonic syntheses. The main isopic zones and nappes were deciphered as
early as in the 50--60s, notably by  \citet{Aubouin1959}
(Figures~\ref{fig1} and~\ref{fig3}). Going from west to east, we first
find the pre-Apulian Zone, a carbonate platform followed by the Ionian
Zone comprising subsiding basin since the Lias-Dogger, and the
Gavrovo-Tripolitza Zone, a platform until the Middle-Late Eocene.
Further east, the Pindos Zone represents a deep basin from the Triassic
onward, including syn-rift lavas and finally an Eocene flysch 
\citep{Brunn1956,Aubouin1959} (Figure~\ref{fig3}). Later on, numerous
stratigraphic and structural features were recognised 
\citep{DegnanRobertson1998,Doutsosetal2006,Faupletal1998,Fleury1980,
IGRS-IFP1966,KonstantopoulosZelilidis2012,Pe-PiperPiper1984,Thiebault1982}.
Uncertainties persist on the nature of the Pindos Zone, whether a
continental crust, more or less thinned 
\citep{Bortolottietal2013,vanHinsbergenetal2020,Schmidetal2020,Thiebault1982}, or an oceanic crust 
\citep{Bonneau1982,Menantetal2016a,Menantetal2016b} interpreted as the root of the
supra-Pelagonian ophiolites 
\citep{Dercourt1970,Dileketal2007,RassiosSmith2000,JonesRobertson1991,SaccaniPhotiades2004,Smithetal1975}.


Nappe stacking in the Hellenides exhibits quite a regular NW--SE strike
that can be followed northward up to the Dinarides 
\citep{Aubouin1959}. This simple pattern is locally perturbed by the
presence of the Parnassus Zone, an additional platform sandwiched
between the Pindos and the Internal Zones, observed between the Corinth
and Sperchios rifts  (Figures~\ref{fig1} and~\ref{fig2}D) 
\citep{Celet1962}. Similarly, this Hellenic strike is disturbed in
Crete and Peloponnese due to the presence of tectonic windows revealing
the substratum made of the Gavrovo-Tripolitza Nappe 
\citep{Jolivetetal1996,JolivetBrun2010,Jolivetetal2010b,Seideletal1982,
TheyeSeidel1991,Theyeetal1992,Thiebault1982}.
These windows are metamorphic core complexes formed during syn-orogenic
exhumation and post-orogenic extension, accommodated by the
north-dipping Cretan Detachment. Crete and the Peloponnese are
underthrust by the subducting oceanic lithosphere of the Eastern
Mediterranean located south of the Adria block. Further to the NW, in
the Hellenides, continental\unskip\break 
collision is still active. The transition
from oceanic subduction to continental collision is accommodated by the
dextral Kephalonia transfer fault  \citep{Ozbakiretal2020}
(Figure~\ref{fig1}).

To the NE, the Serbo-Macedonian and Rhodope massifs belong to the
European continental plate. The area used to be referred to as the 
``Zwischengebirge'' 
\citep{Kossmat1924}. 

At present, the Rhodope massif is a large metamorphic core complex 
(MCC) reworking a nappe stack emplaced during the Tertiary Alpine
orogeny with a HT-LP metamorphism locally reaching melting conditions 
\citep{Burg2011,Burgetal1996,Gautieretal2017,Wawrzenitzetal2015}. Units
showing HP-LT metamorphism are present 
\citep{LiatiMposkos1990,LiatiSeidel1996}, 
even remnants of UHP-LT
metamorphism with the preservation of coesite 
\citep{Kostopoulosetal2000}. Several low-angle detachments have been
found within the Rhodope such as the Nestos shear zone 
\citep{BrunSokoutis2007,Burg2011}. In contrast, the same structure has
been interpreted as a  thrust by 
\citet{Gautieretal2010,Gautieretal2017}. The MCC was exhumed underneath
several SW-dipping detachments, including the Strymon Valley Detachment
\citep{DinterRoyden1993} and the Kerdilion Detachment further to the
west, active from the Eocene  (42~Ma) to the Late Oligocene (24~Ma) 
\citep{BrunSokoutis2007}. For a more exhaustive description of
metamorphic units in the Rhodope, see 
\citep{BrunSokoutis2007,Burg2011,Kounovetal2015,Krennetal2010,Kydonakisetal2014,LiatiSeidel1996,Schmidetal2020}.

\textit{The Internal Zones} of Pelagonian and Vardar domains develop
between the Pindos Zone and the Rhodopes  (Figure~\ref{fig1}). The
Pelagonian is a fundamental unit in the Hellenides, bounded by major
Tertiary thrusts. It extends northward across Albania and to
ex-Yugoslavia  \citep{Aubouinetal1970}. This domain is rich in
metamorphic rocks assumed of Paleozoic or older ages 
\citep{Andersetal2006,Schenkeretal2014,Schenkeretal2015}. The
Pelagonian calcareous cover, paleontologically dated from Triassic to
Jurassic  [\citealp{Brunn1956,CeletFerriere1978} and references
therein;  \citealp{Ferriere1982}], is overthrusted by ophiolitic nappes
among which the well-known Vourinos unit is unconformably overlain by
Late Jurassic--Cretaceous deposits  (Figure~\ref{fig3}). As an isopic
zone, the Pelagonian is only defined for the Triassic--Mid-Jurassic,\unskip\break
before obduction  \citep{CeletFerriere1978}. This domain bordering the
Pindos Zone was initially considered in  \citeauthor{Aubouin1959}'s 
[\citeyear{Aubouin1959}] Geosyncline model as the  ``eugeoanticline'',
an undeformed tectonic unit during the Alpine orogenic cycle.

A first revolution came with the discovery of the Olympos tectonic
window within the Pelagonian domain 
\citep{Godfriaux1962,Godfriaux1968} witnessing a major Alpine thrust
above a platform sequence attributed to the Gavrovo-Tripolitza (GT)
Zone  \citep{FleuryGodfriaux1974}. Similar structures were described
further south such as the Ossa window  \citep{DeryckeGodfriaux1978} and
the Almyropotamos one in Evia 
\citep{DuboisBignot1978,Katsikatsosetal1976}. The Olympos window shows
additional tectonic units between the Upper Pelagonian nappe and the
lower GT limestones, the Infra-Pierian Unit, locally containing
ophiolites, and just below, the blueschist-facies Ambelakia unit
attributed to the Pindos Zone partly because of its structural position
\citep{Hinshawetal2023,Schmitt1983,Schermer1990}. The analyses of
metamorphic terranes present in the Olympos window have revealed
consistent Eocene ages of the blueschists as well as in the Pelagonian,
corresponding to the period of subduction and stacking of these nappes 
\citep{Lipsetal1998,Schermer1990,Schermer1993,Schermeretal1990}. Other
blueschist-facies units have been described in an equivalent structural
position below the Pelagonian, the Makrinitsa Unit in the Pelion 
\citep{Ferriere1982} and the Styra--Ochi Unit in Evia 
\citep{Katsikatsosetal1976}. 

These HP-LT units are the lateral equivalent of the Cycladic
Blueschists  \citep{Jolivetetal2004a,Jolivetetal2004b}. 
Outcrops of platform deposits
similar to the GT series in the Cyclades  (Tinos, Samos, Naxos),
sometimes attributed to the so-called Basal Unit 
\citep[e.g.,][]{Ringetal2010,Ringetal2011} are probably the equivalent
of these tectonic windows in the Internal Zones of the Hellenides 
\citep[references in][]{Jacobshagen1986,
AvigadGarfunkel1989,Jolivetetal2004b,Schmidetal2020}. 

A large sedimentary basin, the Mesohellenic molassic basin (MHB)
developed in a piggy-back mode on top of the Pelagonian--Ophiolitic
domain  \citep{Ferriereetal2004} from the Late Eocene to the Middle
Miocene 
\citep{Brunn1956,Aubouin1959,Doutsosetal1994,Zelilidisetal2002,Ferriereetal1998,Ferriereetal2004,Ferriereetal2013}. 
The subduction of the Pindos and GT zones underneath the Pelagonian was
coeval with the successive sedimentary sequences of the MHB.\unskip\break 
After the
Miocene, the Pelagonian was reworked by a major phase of extension
observed on the continent  (i.e., Trikala and Larissa basins) or in the
Aegean domain where extension scattered the Upper Cycladic Nappe from
the Northern Cyclades to Crete  \citep{Bonneau1984,
Jolivetetal2004b}.

The concept of obduction  \citep{Coleman1971}, a second revolution,
changed the interpretation of the Pelagonian domain. It led to consider
the ophiolites as nappes remnants of oceanic lithosphere instead of
mere intruding magmatic bodies later overthrust as postulated by the
Geosyncline model. 

\citet{Brunn1956} and  \citet{Moores1969} published pioneer
observations of the petrology of these ophiolites in the northern
Pindos and the Vourinos massifs with a complete succession from basal
harzburgites, then gabbros and dolerite dykes, to lavas and finally
pelagic sediments. 

Before the onset of Plate tectonics,  \citet{Brunn1959} compared these
ophiolites with an ocean floor. He further noticed the presence of a
reverse metamorphic gradient with lenses of amphibolites on top of
greenschists underneath the ophiolites.

The Othris ophiolites  (Figures~\ref{fig1} and~\ref{fig2}) allow
reconstructing the Maliac Ocean, a major part of the Tethys Ocean
located between the Pelagonian and the Rhodope continental domains 
\citep{Ferriere1976b}. Between the ophiolitic nappes and the
parautochtonous Pelagonian domain, several syn-obduction units
represent the Triassic--Mid Jurassic western passive margin of the
Maliac Ocean 
[\citealp{Ferriere1972,Ferriere1974,Ferriere1982,Ferriere1985,Ferriereetal2016} 
and references therein,  \citealp{Hynesetal1972,Smithetal1975}],
including the proximal margin, also present in Argolid 
\citep{Ferriere1974,Baumgartner1985,Ferriereetal2016} and mostly the
distal margin is not observed in other sectors of the Hellenides 
\citep{Ferriere1974,Ferriere1982} (Figure~\ref{fig3}). Tithonian--Early
Cretaceous Beotian Flysch  \citep{Celetetal1976a,Celetetal1976b,Nirtaetal2018},
deposited to the west of the ophiolite units, is interpreted as a
syn-obduction foreland basin, supporting the rooting of the ophiolites
to the east of the Pelagonian, i.e., in the Maliac Ocean, instead of
postulating a Pindos origin as formally proposed   \citep[see
discussion in][and references
therein]{Ferriereetal2012,Ferriereetal2016}. 

The Vardar Zone is a complex domain, that underthrusts the
Serbo-Macedonian domain to the east and overthrusts the Pelagonian to
the west\unskip\break 
(Figures~\ref{fig1} and~\ref{fig2}).  \citet{Mercier1968}
distinguished several isopic/tectonic zones, namely from SW to NE: the
Almopias and Peonias units, deep basins separated by the Paikon
platform. An additional pre-Peonian unit, containing the Guevgueli
ophiolites, is sandwiched between the Paikon and Peonias units 
(Figure~\ref{fig1}).  \citet{Kauffmannetal1976} elaborated the content
of the Peonias series and  \citet{Kockel1986} replaced the Pre-Peonias
and Peonias by the  ``Guevgueli-Stip Axios ophiolites'' respectively;
and defined the ``Circum Rhodope Belt''.  \citet{Mercier1968} soon
pointed out to the imprint of Tertiary tectonics in the Vardar domain.
Various structural interpretations of the Paikon  (cf. infra
Section~\ref{sec4.5.2}) have been proposed 
\citep{BrownRobertson2003,Ferriereetal2001,Mercier1968,VergelyMercier2000}. 

The interpretation of the Vardar domain and its ophiolites benefited
from the obduction model, too. Indeed,  \citet{BernoulliLaubscher1972}
located the origin of the supra-Pelagonian ophiolites in the Central
Tethys to the east that's to say from the Vardar domain. They were then
joined by  \citet{Dercourt1972}, first in favour of a Pindos origin 
\citep{Dercourt1970}, then by numerous authors 
\citep{Baumgartner1985,Bortolottietal2013,Ferriere1982,Ferriereetal2012,Ferriereetal2016,Schmidetal2008} with the notable exception of most Anglo-Saxon authors. The Tethyan (Maliac) suture, where these ophiolites come from, would thus be located in the Almopias domain with its highly diversified series 
(Figure~\ref{fig3}) 
\citep{MercierVergely1984,MigirosGaleos1990,Saccanietal2015,SharpRobertson2006,Staisetal1990}. Triassic pre-obduction radiolarites, witnessing a deep basin 
(probably oceanic) have indeed been observed in the Almopias sector 
[Unit\'{e} de Vryssi,  \citealp{Staisetal1990,StaisFerriere1991}]. An
important issue remains debated: the width of the oceanic domain left
after obduction in this domain. Some authors draw a wide residual ocean
in this region  \citep[e.g.,][]{Menantetal2016a,Menantetal2016b,StampfliBorel2002}. 

Middle and Late Jurassic rhyolites and andesites in the Paikon zone led
several authors to consider the existence of a volcanic arc related to
the eastward subduction of the Tethys Ocean  (Maliac) 
\citep{Bortolottietal2013,FerriereStais1995,Ferriereetal2012,Ferriereetal2015,Ferriereetal2016,Mercieretal1975,Robertson2012}.
Hence, these authors conclude that the Middle and Late Jurassic
Guevgueli ophiolites  \citep{Danelianetal1996,Kukocetal2015}, east of
the Paikon, originated as a back-arc basin. Geochemical data confirm
this attribution  \citep{Saccanietal2008}. Additional studies
integrating data obtained from zircons sampled in the metamorphic
sectors further to the SE, led authors  \citep{Bonevetal2015} to
propose the involvement of several subduction zones in the evolution of
the Paikon arc, the Guevgueli ophiolites and their equivalents. The
Peonias series have been described in detail, including those of the
Chalkidiki Peninsula 
\citep{Bonevetal2015,FerriereStais1995,Kauffmannetal1976,Mercier1968,StaisFerriere1991}.
They represent the Triassic--Jurassic eastern margin of the Maliac
ocean, inverted during Cenzoic tectonics 
\citep{FerriereStais1995,Ferriereetal2016}. Within the Peonias series,
Triassic lavas and facies transitions witness the syn-rift period 
\citep{AsvestaDimitriadis1992,FerriereStais1995} coeval with the
syn-rift of the west-Maliac margin, one argument in favour of these two
domains being the\unskip\break 
conjugate margins of that ocean 
\citep[cf.][]{Ferriereetal2016}. 
\vspace*{-4pt}

\subsection{Aegean domain}\label{sec2.2}  
The first-order structure of the Aegean domain results from a change in
subduction dynamics some 30--35~Ma ago when slab retreat started as a
result of the Africa--Eurasia collision  \citep{JolivetFaccenna2000}.
The consequence was the opening of several back-arc basins, among them
the Tyrrhenian Sea and the Aegean Sea 
\citep[e.g.,][]{Jolivetetal2013,LePichonAngelier1979,LePichonAngelier1981}.

The first observations in this last region showed some similarities
with the Internal Zones of the Hellenides and postulated possible
correlations  \citep{Bonneau1984,Jacobshagen1986}. The definition
of the Cycladic Blueschists Nappe  \citep{Blakeetal1981} was an
important milestone to integrate this region into the regional tectonic
framework, showing tectonic units exhumed from the depth of the
subduction interface 
\citep{Altherretal1979,Bonneau1982,Bonneau1984,BonneauKienast1982,Bonneauetal1980,Hecht1984,Okruschetal1978} 
but finding a regional logic was difficult. A synthetic account of
available observations at that time in the Attic-Cycladic massif can be
found in  \citet{Bonneau1984} and  
\citet{Jacobshagen1986}. The
same pile of nappes observed in the continental Hellenides is
recognised at this period with the Pelagonian Unit at the top 
\citep[e.g.,][]{Laskarietal2022} with basement lithologies and
ophiolites, locally
named the Upper Cycladic Unit, devoid of any
evidence of Eocene HP-LT\unskip\break 
metamorphism. 

This unit overlies the Cycladic Blueschists\unskip\break  
(Figures~\ref{fig4}--\ref{fig6}), 
equivalent to the Ambelakia Unit observed
near Mount Olympos, and the basal platform series similar to the
External Zones  (Gavrovo-Tripolitza), with either HP-LT metamorphic
parageneses reaching the blueschist-facies or the eclogite facies 
(Syros, Sifnos, Tinos, Samos) or HT-LP ones locally reaching anatexis 
(Paros, Naxos, Mykonos) 
\citep{Altherretal1979,Altherretal1982,Andriessenetal1979,Blakeetal1981,
BonneauKienast1982,Hecht1984,JansenSchuilling1976,Okruschetal1978,Papanikolaou1977,Papanikolaou1980}.

\begin{figure*}
\includegraphics{fig04}
\vspace*{2pt}
\caption{\label{fig4}Metamorphic core complexes in the Aegean region
from the Rhodope to Crete and the Menderes. The main detachments
accommodating their exhumation are represented with different colours
depending upon their timing of activity. Red arrows represent the
kinematics of ductile and brittle deformation in the shear zones
observed in the footwalls. NCDS: North Cycladic Detachment System,
NPFS: Naxos--Paros Fault System, WCDS: West Cycladic Detachment System,
SCD: South Cycladic Detachment, CR D: Cretan Detachment, UCSB: Upper
Cycladic Blueschists, LCSB: Lower Cycladic Blueschists.}
\vspace*{4pt}
\end{figure*}

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig05}
\vspace*{3pt}
\caption{\label{fig5}Detailed map of the Cycladic MCC and the Menderes
Massif. NCDS: North Cycladic Detachment System, NPFS: Naxos--Paros Fault
System, WCDS: West Cycladic Detachment System, SCD: South Cycladic
Detachment, CR D: Cretan Detachment, UCSB: Upper Cycladic Blueschists,
LCSB: Lower Cycladic Blueschists. IBTZ: Izmir-Bornova Transfer Zone,
Ev: Evia, An: Andros, Ti: Tinos, My: Mykonos--Delos--Rhenia, IK: Ikaria,
Sa: Samos, Sy: Syros, Pa: Paros, Na: Naxos, Ke: Kea, Ky: Kythnos, Se:
Serifos, Si: Sifnos, Fol: Folegandros, Si: Sikinos, Io: Ios, Fo:
Fourni, Ar: Arki: Li: Lipsi, Le: Leros, Ka: Kalimnos, Ko: Kos, Mi:
Milos, Sa: Santorini, An: Anafi, As: Astipalea. AA$'$, BB$'$ and CC$'$:
cross-sections  Figure~\ref{fig6}.}
\vspace*{5pt}
\end{figure*}

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig06}
\vspace*{3pt}
\caption{\label{fig6}Cross-sections through the Aegean region and zoom
on the Cycladic MCC. Upper: a simplified N--S section through the
entire back-arc region after  \citet{JolivetBrun2010}; location on
Figure~\ref{fig1} (FF$'$); the orange domain represents the part of the
crust partially molten during extension. Lower: AA$'$, BB$'$, CC$'$,
three sections through the main Aegean MCC and corresponding
detachments after  \citet{Jolivetetal2015}; location on
Figure~\ref{fig5}.}
\vspace*{5pt}
\end{figure*}

The structural position of the Cycladic Blueschists and their Eocene
metamorphic ages  (50--35~Ma) make them lateral equivalents of the
Pindos Nappe  \citep{Bonneau1984,Jolivetetal2004a,Jolivetetal2004b}. 

Later works were devoted to the detailed description of the HP-LT
parageneses in the Cycladic Blueschists and associated deformation and
time constraints 
\citep{BrockerEnders1999,Brockeretal2013,Laurentetal2016,Parraetal2002,Trotetetal2001a,Trotetetal2001b,Uunketal2022,WijbransMcDougall1988,Wijbransetal1993}.
Recent studies in the HP-LT units also evidenced the existence of two
main units with distinct $P$--$T$ evolution. The eclogite-bearing Cycladic
Blueschists of Syros, Tinos and Sifnos belong to the Upper Cycladic
Blueschists Nappe, while the blueschists of Kea or Kythnos, that never
reached the eclogite facies belong to the Lower Cycladic Blueschists
Nappe, the contact going across the basement of Milos 
\citep{Grasemannetal2017,Rocheetal2019}
(Figure~\ref{fig5}).

The seminal paper by  \citet{Listeretal1984} changed the game. In
Naxos, these authors recognised a cordilleran-type metamorphic core
complex (MCC) that they related to the extension leading to the
formation of the Aegean Sea. This was the first of a long series of
studies systematically describing the Aegean MCCs and their
detachments. These low-angle normal faults and the associated ductile
and brittle deformation in the footwalls were studied in great detail
by several teams. The main detachments  (Figures~\ref{fig4}
and~\ref{fig5}) are the NE-dipping North Cycladic Detachment System
(NCDS)  \citep{AvigadGarfunkel1989,Jolivetetal2010a} fringing 
the\unskip\break
northern Cyclades and Eastern Aegean Islands  (Andros, Tinos, Mykonos,
Ikaria) and extending to the NW all the way to offshore Olympos and to
NE into the Simav Detachment, the SW-dipping West Cycladic Detachment
System (WCDS) observed on the SW Cyclades  (Kea, Kythnos, Serifos) 
\citep{Grasemannetal2012}, the S-dipping South Cycladic Detachment or
Santorini Detachment mainly observed on Santorini 
\citep{Ringetal2011,Schneideretal2018} and the N-dipping Paros--Naxos
Fault System (NPFS)           
\citep{Bargnesietal2013,Gautieretal1993,Buick1991,Uraietal1990}.

The kinematics of these detachments and of the ductile-to-brittle
deformation in the footwall show a simple pattern with N--S extension
in the south changing to NE--SW toward the NE 
\citep{FaureBonneau1988,Faureetal1991,GautierBrun1994,Gautieretal1993,Grasemannetal2012,Jolivetetal2004a,Jolivetetal2004b,Jolivetetal2013,Rocheetal2019}. 
These major detachments are the main structures accommodating crustal
thinning in the Aegean Sea in the Oligocene and Miocene, with typical
offsets reaching several tens of kilometers or even 70--100 for the
NCDS \citep{Brichauetal2006, Brichauetal2007, Brichauetal2008,
Jolivetetal2010a,Jolivetetal2010b,Menantetal2016a,Menantetal2016b}. 
\looseness=1

The Aegean Sea is one of the key regions where the activity of
detachments formed at a low-angle was demonstrated 
\citep{Jolivetetal2010a,Lacombeetal2013,Mehletal2005}. The late stages
of extension were accompanied by the intrusion of granitic plutons or
dykes in Tinos, Mykonos, Delos, Rhenia, Ikaria, Samos, Paros, Naxos,
Serifos and in Lavrion on south Attica. These plutons interacted with
the activity of the detachments 
\citep{FaureBonneau1988,Faureetal1991,LeeLister1992}. 
Detailed studies of
these interactions showed that the internal geometry of these plutons,
from the flow of magma to the sub-solidus deformation in the vicinity
of the detachments, were controlled by the detachment and the
associated stress regime  \citep{Rabillardetal2018,Jolivetetal2021}.
The detachments and the MCCs are also observed outside the Aegean Sea,
in the Rhodope massif to the north  \citep{BrunSokoutis2007,Burg2011}
and in the Menderes massif to the east 
\citep{BozkurtPark1994,BozkurtOberhansli2001,BozkurtSatir2000,Gessneretal2001,Hetzeletal1995,
Ringetal1999}
showing the imprint of back-arc extension over a wide region 
(Figures~\ref{fig1} and~\ref{fig4}). Extension was recorded as far
south as the island of Crete where the exhumation of the HP-LT
Phyllites--Quartzites and Plattenkalk\unskip\break (Ionian nappes) was accommodated
by syn-orogenic detachments  (the Cretan Detachment) obeying to the
same simple pattern of N--S extension during~the Miocene as further
north in the Cyclades 
\citep{Jolivetetal1994a,Jolivetetal1994b,Jolivetetal1996,
Fassoulasetal2004,Seideletal2007,Grasemannetal2019}.

Synthetic models were proposed at various stages of the progressive
understanding of the geological context.  \citet{BonneauKienast1982}
described the progressive formation of the Cycladic Blueschists; 
\citet{Jolivetetal2003},  \citet{vanHinsbergenetal2005a,vanHinsbergenetal2005b} 
and \citet{Grasemannetal2017} showed the progressive stacking of nappes
during the northward subduction of Apulia and then the eastern
Mediterranean oceanic lithosphere.  \citet{JolivetBrun2010},
\citet{Ringetal2010} and  \citet{Grasemannetal2012} described the
progressive subduction and exhumation of the Cycladic Blueschists from
underneath the detachments;  \citet{Menantetal2016a,Menantetal2016b} did the same but in
3D. At a large scale, the Hellenides and the Aegean region were
integrated in paleogeographic reconstructions of the Tethys Ocean from
the Triassic to the Present 
\citep{LaubscherBernoulli1977,Biju-Duvaletal1977,Dercourtetal1986,vanHinsbergenetal2020,StampfliBorel2002}. 

\section{Mediterranean period}\label{sec3}    
Since this paper is mainly a synthesis of the evolution of the
Hellenides and its geodynamic drivers, we only focused on the main
observations that constrain the origin of forces driving back-arc
extension in the Aegean region. As presented above, the Aegean
extension and resulting crustal thinning were coeval with subduction
and formation of HP-LT metamorphic parageneses in subducted units
during the Oligocene and Miocene, followed by their exhumation in the
plates interface of the Hellenic subduction after 35~Ma 
\citep{vanHinsbergenetal2005a,vanHinsbergenetal2005b,JolivetBrun2010,Jolivetetal1996,Jolivetetal2010b,Seideletal1982,
TheyeSeidel1991,Theyeetal1992}. The HP-LT units of the Phyllite--Quartzite nappe 
(\textit{Nappe des Phyllades} in French) and the underlying Plattenkalk
Nappe  (\textit{Ionian Nappe} in French) were parts of the southern
margin of the Adria  (Apulia) block, which also includes the Upper and
Lower Cycladic Blueschists Units subducted and accreted earlier during
the Eocene. They were exhumed underneath the Cretan Detachment at the
same time as the Aegean detachments were active 
\citep{Jolivetetal1996,Jolivetetal2013} during the southward retreat of
the African slab. This behavior is typical of the Neogene Mediterranean
subduction and collision zones, from the Gibraltar arc, the Apennines
all the way to the Hellenic subduction. The same Cretan Detachment has
been recognised in the Peloponnese  (Figures~\ref{fig1}
and~\ref{fig4}), as far north as the Zarouchla window just south of the
Corinth Rift  \citep{Jolivetetal2010b,Trotetetal2006}. The $P$--$T$
conditions and paths recorded by the Phyllite--Quartzite Nappe in Crete
and the Peloponnese show a gradient from east to west, with a colder
$P$/$T$ gradient in the east and a warmer one in the west, with a
difference of about  100~\textdegree C for the pressure peak. This has
been attributed to differences in kinematic boundary conditions of the
subduction zone by  \citet{Jolivetetal2010b}. Crete, where the $P$/$T$
gradient is the coldest, is located along the transect of the Aegean
domain that has seen the largest and fastest extension, because of fast
slab retreat. This would have created a more opened subduction channel
where crustal rocks are first subducted and then exhumed. This, in turn
facilitated the two-way circulation of rock material and thus avoided
substantial heating. This mechanism was probably active also during the
Tethyan period, before 35~Ma, when the Cycladic Blueschists were
subducted and exhumed further north. Slab retreat was slower during
this period  \citep{JolivetBrun2010} and the metamorphic parageneses
show warmer conditions than in Crete with a differential of about 
100~\textdegree C.

Further north, in the Cyclades, in the Internal Zones of the
Hellenides, in the southern part of the Rhodope massif and in the
Menderes Massif\unskip\break 
(Figures~\ref{fig1} and~\ref{fig4}), extension was
accommodated by the activity of the detachments reported above and the
exhumation of MCCs \citep{GautierBrun1994,JolivetPatriat1999,
Jolivetetal2004a,Jolivetetal2004b,RingLayer2003,
Bargnesietal2013,Grasemannetal2012,Rocheetal2019}. 
Two types of MCCs\unskip\break
(Figures~\ref{fig5} and~\ref{fig6}) are recognised, depending on the
$P$--$T$ conditions of deformation  \citep{Jolivetetal2004a,Jolivetetal2004b}. Cold MCC are
those where the Eocene HP-LT parageneses of the Cycladic Blueschists
are best preserved, Andros, Tinos, Syros, Sifnos, Kea, Kythnos,
Amorgos, Samos. Hot MCC are those where the Eocene HP-LT parageneses
have been almost totally erased by later high-temperature imprint,
Naxos, Paros, Mykonos--Delos--Rhenia, Ikaria. The shapes of $P$--$T$ paths are
drastically different with a clear excursion toward high temperature
along the retrograde path in hot MCC, reaching the conditions of
anatexis. The two extreme behaviors can be illustrated by the examples
of Syros  (or Sifnos) on the cold side and Naxos on the hot side. In
the best-preserved units of Syros, the retrograde path traveled along a
cold gradient, parallel to the prograde path, without any loop toward
high temperature. Intermediate shapes of $P$--$T$ paths have been retrieved
from the MCCs of Andros and Tinos, as well as from the deepest units of
Syros, that show some strong retrogression in the greenschist-facies.
The retrograde path starts along a cold gradient during the Eocene
until the pressure of 9--10 kbar reached at approximately 37~Ma. An
isothermal heating is then observed until  ${\sim}$35~Ma and exhumation
resumes along a warmer path. These two different gradients in fact
correspond to the succession of the Tethyan and Mediterranean periods.
In the Eocene, the Cycladic Blueschists were exhumed within the
subduction channel. This process is referred to syn-orogenic
exhumation, just like it occurred in Crete later on, in the Oligocene
and Miocene. From the Late Eocene to the Miocene, exhumation was
accommodated by post-orogenic detachments, responsible for crustal
thinning in the back-arc regions associated with a higher heat flow.
Syn-orogenic exhumation had then migrated southward in Crete and the
Peloponnese, following slab retreat 
\citep{Jolivetetal1994b,JolivetBrun2010}. 

Recent studies by \citet{Lamontetal2020} and \citet{SearleLamont2020}
present a different interpretation where post-orogenic extension is
more recent and is mostly a low-temperature process. In their
interpretation these authors consider that most of the deformation in
the deep parts of the Naxos MCC are related to the shortening period,
before the Aegean extension that would not have started before 15 Ma.
In that sense they join the interpretation of  \citet{Ringetal2010}
dating the inception of extension at  ${\sim}$21~Ma, which is the age
of the first marine sediments deposited in the Aegean Sea. Answering
this debate would necessitate more detailed studies, but we can already
state that the 35 Ma age we propose for the transition from
syn-orogenic to post-orogenic tectonics is based upon the fast
southward migration of the magmatic arc, which imposes a component of
back-arc extension at that period. It is also based upon the ages of
the greenschist-facies shearing deformation underneath the NCDS in
Tinos and Andros  \citep{Laurentetal2017}. The study of the deep core
of the Mykonos--Delos--Rhenia MCC shows that the migmatites observed
there were molten and deformed at the same time as the Middle Miocene
granite plutons intruded  \citep{Jolivetetal2021}. The Naxos pluton
intruded the high-temperature dome once it had already been exhumed and
cooled from the depth of anatexis 
\citep{JansenSchuilling1976,Uraietal1990,Gautieretal1993,
Bessiereetal2018}
by the activity of the top-to-north Paros--Naxos detachment, which was
thus active before, while the core rocks were partially molten and
sheared toward the north. One can observe in all these MCC a continuum
of shearing and exhumation, either top-north or top-south 
\citep{Jolivetetal2013,Grasemannetal2012,Rocheetal2019}. 

Slab retreat is thus independently documented by the southward
migration of magmatic products from the Late Eocene to the present 
\citep{Fytikasetal1984,Jolivetetal2004b}. Another trend of migration is
observed regarding Miocene granitic intrusions. The oldest Aegean
plutons are found in Ikaria  [16~Ma, 
\citealp{Bolharetal2010,Bolharetal2012}] 
associated with migmatites 
\citep{Beaudoinetal2015}. During about 8~Ma, younger plutons were
emplaced further to the southwest and south, the youngest being found
in the Lavrion MCC some 8~Ma ago  \citep[and references
therein]{Jolivetetal2015}. This migration has been interpreted as a
consequence of a slab tear below the eastern Aegean and western
Anatolia evidenced by seismic tomography 
\citep{Biryoletal2011,deBoorderetal1998,Salaunetal2012}. The slab tear
is thought to have occurred between 15 and 8~Ma by 
\citet{vanHinsbergenetal2005a,vanHinsbergenetal2005b} based on paleomagnetic data showing a
fast clockwise rotation of continental Greece at that time  [see also 
\citealp{KisselLaj1988} for the first evidence of this rotation]. 

3D numerical modelling indeed shows the migration of magmatism and
rotations in the crust during the propagation of the tear 
\citep{Sternaietal2014,Menantetal2016a,Menantetal2016b}. 

The progresses of seismic studies led to the publication of maps of
seismic anisotropy underneath the Aegean region 
\citep{Hatzfeldetal2001,Endrunetal2008,Pauletal2014}. The comparison of
these maps with the long-term extensional strain observed in the crust
of the overriding plate 
\citep{Jolivetetal2009,Kreemeretal2004}
led to the idea that
the asthenospheric flow produced by slab retreat was one of the main
drivers of extension observed in MCC. 

3-D numerical models have also shown that slab tearing and retreat are
essential ingredients of the present kinematics observed in the
Anatolian region. Elaborating on the original ideas that slab retreat
and progressive narrowing by tearing concentrate the slab-pull forces
controlling slab retreat and associated mantle flow, 3-D fully coupled
thermo-mechanical models with original setups were tested on the
eastern Mediterranean contexts 
\citep{Capitanio2014,Funicielloetal2003,GoversWortel2005,Piromalloetal2006,Worteletal2009}.
These studies showed that the lateral evolution from continental
collision in the east  (Arabia--Eurasia collision) and subduction with
slab retreat in the west (Hellenic subduction) lead to a rotational
extrusion of the overriding continental lithosphere and that slab
retreat is an essential component. Mapping the instantaneous strain
field in the models  \citep{Sternaietal2014} shows the formation of a
dextral shear zone similar to the North Anatolian Fault and an
extensional domain similar to the Aegean. The modelled velocity field
is very similar to that measured with GPS satellites 
\citep{LePichonetal1995,McCluskyetal2000,Reilingeretal1995,Reilingeretal2006}.

The most recent part of the Mediterranean period is thus characterized
by the progressive establishment of the current dynamics and the
localisation of deformation along a few major structures, the North
Anatolian Fault  \citep{Armijoetal1999,LePichonetal2003} and the
Kephalonia Fault  \citep{Ozbakiretal2020} as dextral strike-slip
faults, active extension in the Corinth Rift 
\citep{Armijoetal1996,Brioleetal2000,Jacksonetal1982,Kingetal1985,Lyon-Caenetal2004} and the Menderes Massif 
\citep{Aktugetal2009,Gessneretal2001}. Active extension is also
observed in the southern Aegean in the Amorgos graben and toward
Santorini  \citep{Nomikouetal2018}, in Crete and the Peloponnese 
\citep{Armijoetal1999,Lyon-Caenetal1988}. This late localisation of
deformation has left the Cyclades almost undeformed since the Late
Miocene.

\begin{figure*}
\vspace*{-1pt}
\includegraphics{fig07}
\vspace*{-2pt}
\caption{\label{fig7}(A--C)~The Rifting period.  (A)~Early Triassic to
Early Anisian (stretching): platform limestones;  (B)~Middle Anisian
(thinning): volcanism, pelagic facies on the proximal  (and distal ?)
margin;  (C)~Late Anisian: oldest Maliac oceanic crust; beginning of
the post-rift period but the  ``syn-rift'' volcanism reaches the Late
Ladinian.} 
\vspace*{-3pt}
\end{figure*}

\section{Tethyan period}\label{sec4}
The Tethyan period is characterized by the involvement of several
basins belonging to the Tethyan realm  (i.e. Maliac Ocean, Pindos
Basin). These paleogeographic elements that appear in the Mid Triassic 
(rifting) were stacked during the Tertiary 
collision\unskip\break 
and were partly
affected by the Jurassic obduction  (Maliac). Later extension belongs
to the Mediterranean period described above. For the Tethyan period, we
have chosen to focus on the regions situated north of the Corinth Rift
and more specifically to the north of the Sperchios. The
characteristics of the main isopic zones  (paleogeographic domains) are
summarised on Figure~\ref{fig3}.

\subsection{Initial rifting (duration ca. 10~Ma)}\label{sec4.1}
The Triassic rifting episode is recorded in a large portion of the
Hellenic zones. One example of such a syn-rift evolution, the western
margin of the Maliac Ocean, is proposed on  Figure~\ref{fig7}. 

The pre-rift Mesozoic series, well known in the Internal Zones, are
generally represented by a Lower Triassic-Anisian carbonate platform,
resting on detrital metamorphic and non-metamorphic Palaeozoic
formations. The rifting period is characterized in the Mid-Triassic by
volcanism and changing facies in the Internal and Pindos Zones 
(Figure~\ref{fig3}). This period lasts for approximately 10~Ma  (ca.
247--237~Ma). In Othris, in the Maliac proximal margin, the Anisian
benthic limestones are overlain by a syn-rift succession of calcareous
breccias, radiolarites, black pelites and yellowish sandstones,
associated with pillow-lavas and dolerites. In the distal Maliac
margin, Late Ladinian and Carnian radiolarites directly overlain the
lavas  \citep{Ferriere1982}. 
 
Magmatism is well developed in the Pelagonian and the deep basins 
(Maliac Ocean and Pindos Zone). It is represented by basaltic
pillow-lavas, dolerites and rare alkaline trachytes, like in Othris, or
by porphyric andesites like in the Pindos or in Evia 
\citep{Bortolottietal2008,Celetetal1976a,Celetetal1976b,Celetetal1980,Ferriere1982,Lefevreetal1993,Monjoieetal2008,Pe-Piper1998,Pe-PiperPiper2002}.
In Othris, the upper part of the distal margin pillow-lavas is dated as
late Ladinian by microfossils present in inter-pillow sediments 
\citep{Ferriere1982,Ferriereetal2015}. This volcanic formation is
truncated at its bottom by a tectonic contact making an Anisian age
possible for the initiation of the syn-rift volcanism, as demonstrated
further north, in the Dinarides  \citep{Celetetal1976a,Celetetal1976b,Sudaretal2013}.

Two main geochemical trends are observed:  (i)~alkaline volcanism rich
in pillow-lavas and basaltic flows evoking generalised extension. This
is the case of the Triassic volcanism in Othris 
\citep{BarthGluhak2009,Bortolottietal2008,Ferriere1982,Hynes1974} 
although
more diversified affinities have been described with some
supra-Subduction Zone (SSZ) lavas  \citep{Monjoieetal2008};  (ii)~SSZ
volcanism rich in basaltic lavas and andesites erupted in the context
of active or extinct subductions with deep slabs inherited from the
Paleotethys subduction. The consumption of the Paleotethys by these
subduction zones would then be the cause of the opening of the
Neotethys as a back-arc basin 
\citep{vanHinsbergenetal2020,MaffionevanHinsbergen2018,StampfliBorel2002}. 
An alternative interpretation excluding subduction and involving
instead the partial melting of peculiar mantle compositions have been
proposed  \citep{Pe-Piper1998,Saccanietal2015}.  \citet{Bonevetal2019}
describe Triassic lavas of MORB and OIB type linked with the Volvi rift
complex in the Serbo-Macedonian massif, then the margin of the
Neotethys. Upper Mid-Triassic  (U--Pb ages on zircon) metagranitoids
have also been reported in association with the Cycladic Blueschists
such as in Evia where they are possibly related to an anorogenic rift
setting  \citep{Chatzarasetal2013}. Some older metagranitoids,
Carboniferous in age  (U--Pb on zircon) in the Cycladic Basement in Ios
island have been attributed to subduction processes 
\citep{Flansburgetal2019}.

Syn-rift tectonic structures are almost unknown because of later
deformation, early normal faults being reworked as syn-obduction nappe
contacts during the Jurassic  (Figures~\ref{fig7} and~\ref{fig8}; and 
Section~\ref{sec4.3.3}) 
\citep{Ferriereetal2016,FerriereChanier2020,FerriereChanier2021}.

\begin{figure*}
\includegraphics{fig08}
\vspace*{2pt}
\caption{\label{fig8}(D--F)~The post-Rift period: oceanic drifting 
(Maliac ocean):  (D)~Carnian: subsidence (thick black arrows),
siliceous limestones and radiolarites (thick blackdashes) on the
margin; (E)~Early Jurassic: deep-sea fan with siliceous fine-grained
limestones and calcarenites, calciturbidites (black triangles); 
(F)~Middle Jurassic: convergence giving rise to the
subduction--obduction processes. The Triassic syn-rift normal faults
guide the Jurassic syn-obduction tectonic contacts.}
\vspace*{4pt}
\end{figure*}

The data summarised above indicate that the Triassic rifting creating
the main paleogeographic domains corresponds to the Anisian and
Ladinian periods. Syn-rift volcanism was locally active until the Late
Ladinian and continued after the formation of the Maliac Oceanic crust 
(late Anisian) considered as the beginning of the Post-Rift period. 

\subsection{Oceanic spreading: the post-rift period\newline  (duration $\sim$
70~Ma)}\label{sec4.2}
This period that sees the development of the main Hellenic
paleogeographic domains, especially the Tethyan Maliac Ocean 
\citep{Ferriere1976b} lasted approximately 70~Ma, from 240  (Middle
Triassic) to 170~Ma (Dogger pp)  (Figure~\ref{fig8}). Lavas of the
ophiolitic units are dated from the Dogger in the Vourinos or Mega
Isoma in Othris  \citep{Chiarietal2003,Ferriereetal2015,Liatietal2004}.
On the other hand, the Fourka ophiolitic unit made of MORB-type
pillow-lavas underlying the peridotitic units, is dated from the
Triassic  \citep{Bortolottietal2013}, and locally more precisely from
the Late Anisian with radiolarites in direct contact with the lavas 
\citep{Ferriereetal2015}, the oldest age for the Maliac oceanic crust. 

The series deposited on the oceanic crust and on the margins record a
thermal subsidence from the Middle Triassic to the Middle Jurassic. The
margins of the main basins, notably the Maliac and Pindos Zones, are
rich in calcarenites, sometimes turbiditic, with elements supplied by
the bordering platforms where subsidence was compensated by an intense
carbonate productivity. The distal margin of the Maliac Ocean, well
characterized in Othris, is rich in lavas, radiolarites and pelites,
making these units difficult to distinguish from the Middle Jurassic of
the syn-obduction underlying units  \citep{Ferriere1974,Ferriere1982}.
The most distal formations are rich in Early Jurassic siliceous
pelites, but, as in the entire Hellenides, the absence of Liassic
determinable radiolarian does not permit to date precisely this period 
\citep[cf.][]{Chiarietal2013}. The transition from the post-rift
divergence to the first convergence  (Dogger subduction and obduction)
is marked by a fast subsidence in some of the Internal Zones 
(Figure~\ref{fig8}F). 

\subsection{Early convergence: Jurassic subduction(s) and
obduction(s)}\label{sec4.3}

\subsubsection{The Hellenic ophiolites}\label{sec4.3.1}   
Obduction characteristics are nicely illustrated in the Hellenides,
whether for the petrology of ophiolites  (e.g., Vourinos and N-Pindos)
or the syn-obduction tectonic units originated from the Maliac Ocean
and its margins in Argolid and Othris. Recent synthetic publications
develop various hypotheses for this obduction event.
Most geologists consider that the concerned ocean was located on the
eastern side of the Pelagonian continental domain  
\citep[e.g.][therein references]{Bortolottietal2013,FerriereChanier2020,
Schmidetal2020,Ferriereetal2012,Ferriereetal2015,Ferriereetal2016}. 

Three main ophiolitic nappes coming from the Maliac Ocean are present
in the Pelagonian domain  \citep[e.g.,][]{Brunn1956,Ferriereetal2012}.
Two thick nappes with complete succession from peridotites to lavas:
the Mega Isoma Unit in W-Othris and the N-Pindos Unit  (N-Hellenides)
mainly with MORB lavas and the more eastern Metalleion Unit in W-Othris
and the Vourinos Unit  (N-Hellenides) with SSZ lavas 
\citep[e.g.,][]{Brunn1956,Beccaluvaetal1984,Ferriere1982}. The lavas of
the Vourinos and Mega Isoma nappes are dated by radiolarian as Late
Bajocian and Bathonian  \citep{Chiarietal2003,Ferriereetal2015}. The
third supra-Pelagonian ophiolitic nappe  [Fourka unit, 
\citealp{Celetetal1980}], located just below the main ones is very
different. It consists of pillow-lavas of MORB affinities dated as
Middle Triassic  \citep{Bortolottietal2008}, locally Late Anisian
immediately above the lavas  \citep{Ferriereetal2015}. 

In the Internal Zones, the Guevgueli ophiolites are considered as
back-arc ophiolites on the eastern side of the Jurassic Paikon volcanic
arc 
\citep{Bebien1983,FerriereStais1995,Mercier1968,Mercieretal1975,
Saccanietal2008}. The origin of the Rhodope ophiolites is discussed 
\citep{Froitzheimetal2014}.

\subsubsection{The first evidence of convergence: Jurassic
subduction(s)}\label{sec4.3.2}
Magmatic witnesses are the main arguments concerning the existence of
one or two Jurassic subduction(s) in the Maliac Ocean: (i)~In the
supra-Pelagonian Vourinos ophiolites, the characteristics of the
peridotites, mainly harzburgites and of the overlying Middle-Jurassic
lavas indicate a supra-Subduction Zone (SSZ) environment 
\citep{Beccaluvaetal1984}. Boninitic dykes and SSZ-type lavas are also
reported above MORB-type lavas from the N-Pindos ophiolites 
\citep{SaccaniPhotiades2004,Saccanietal2004} (Figure~\ref{fig9}).
Evidence of subduction extends in Albania where one can distinguish the
same framework with a western unit rich in MORB lavas and an eastern
one with SSZ andesites and rhyolites 
\citep{Dileketal2008,DilekFurnes2009};  (ii)~The interpretation of the
Paikon as a volcanic arc associated with the Guevgueli back-arc basin
is also a convincing argument for the existence of a Jurassic
subduction on the Maliac eastern margin 
\citep{Bortolottietal2013,Danelianetal1996,FerriereStais1995,Ferriereetal2012,Ferriereetal2015,Ferriereetal2016,Kukocetal2015,Mercieretal1975,
Robertson2012,Saccanietal2008}.

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}Jurassic Obduction. 
(A--E)~The obduction process and its consequences  (Beotian Basin). 
(A)~Initial state;  (B,C)~west-verging Maliac obduction toward the
Pelagonian domain;  (C1,C2)~two hypotheses concerning the
subduction(s),  (C1)~the supra-Pelagonian SSZ ophiolites  (Vourinos,
Metalleion) and the Paikon arc to the east, are linked to the same
subduction;  (C2)~two different subductions are responsible for the
western SSZ ophiolites and for the Paikon arc and Guevgueli back-arc
basin;  (D)~East Maliac obduction toward the East;  (E)~Foreland
Boeotian sedimentary domain.}
\end{figure*}

Metamorphic data also document the subduction and obduction history in
the Hellenides. 

High-temperature amphibolite-facies units resting on top of lower
temperature  (greenschist-facies) lenses are present below the
ophiolitic nappes in the Hellenides  \citet{Brunn1956,Sprayetal1984}.
In the Albanides, the thick metamorphic sole associated with the
Mirdita ophiolites gave thermobarometric constraints:
800--860~\textdegree C and 1~GPa  \citep{Dimo-Lahitteetal2001}, but the
main amphibolite units show temperatures ranges from 624  $\pm$
9\textdegree\ to 796 $\pm$ 50~\textdegree C with pressures always lower
than 0.7~GPa  
\citep{Gaggeroetal2009}
(Figure~\ref{fig9}B). These
metamorphic soles, made of metabasites and pelagic sediments are
classically interpreted as metamorphosed and deformed at the onset of
the subduction and dragged at the base of the ophiolitic nappes while
obduction advances 
\citep{Agardetal2016,Agardetal2020,Plunderetal2016}. 

Taking into account the location of the Tethyan Maliac Ocean on the
eastern side of the Pelagonian block, an eastward dipping intra-oceanic
subduction has to be considered to explain the emplacement to the west
of the Jurassic supra-Pelagonian\unskip\break  
ophiolites.

Two possibilities arise for the number of subductions: In the first
hypothesis  (Figure~\ref{fig9}, C1) the subduction below the Maliac
eastern margin with the Paikon arc is also responsible for the
development of the Vourinos SSZ Unit, and partly of the N-Pindos one.
The Bathonian ages of some radiolarites deposited on lavas of the
Guevgueli back-arc basin  \citep{Kukocetal2015} similar or close to the
Middle Jurassic ages of the Vourinos SSZ lavas\unskip\break
(Figure~\ref{fig3}) are
not opposed to such a single subduction in the Maliac ocean. In the
second hypothesis, more likely  (Figure~\ref{fig9}, C2), a second
subduction zone, further west, would be responsible for the Vourinos
SSZ oceanic lithosphere. The subduction under the Paikon arc would be
due to blocking during the\unskip\break 
obduction of the ophiolites onto the
Pelagonian continental domain. In the Hellenides, the similarities of
the obduction history from Greece to former
Yugoslavia imply
localisation within a zone that ran parallel to the earlier
paleogeographic domains. The Mid-Jurassic ages of the MORB-type
ophiolitic lavas of the N-Pindos and those of the Mega Isoma lavas 
(Othris), very close to the age of the obduction events, indicate that
they were formed near the active Maliac Mid-Oceanic ridge 
(Figure~\ref{fig9}), thus far from the active eastern margin of the
70~Ma old Triassic--Jurassic Maliac ocean.  \citet{Maffioneetal2015} and
\citet{vanHinsbergenetal2015} partly based on numerical modelling,
propose an initiation of subduction near the mid-ocean ridge by the
reactivation of a detachment fault similar to those observed in slow
accretion contexts. Assuming the existence of such an east-dipping
detachment, on the western side of the Maliac ridge, this hypothesis
allows the evolution of the former ridge in a forearc domain with a
thin lithosphere and its magmatic consequences. The origin of the
Middle Jurassic Vourinos lithosphere could be the result of such a
geodynamic context. The more western N-Pindos ophiolites corresponding
to a MORB-type oceanic crust would have been trapped in the upper plate
of the subduction zone, then covered or crossed by SSZ pillow-lavas and
dykes  \citep[e.g.,][]{SaccaniPhotiades2004}. 

By contrast with the main peridotitic ophiolite units, the Triassic
Fourka oceanic unit, located near the continental-oceanic transition 
(between Pelagonian distal margin and the early Maliac oceanic crust),
was part of the lower plate of the intra-oceanic subduction before the
continental subduction of the Pelagonian domain  (see infra
Section~\ref{sec4.3.3}).

\subsubsection{Development of obduction on the West-Maliac continental
margin}\label{sec4.3.3}
The evolution from a young intra-oceanic subduction to obduction
corresponds to the overthrusting of the oceanic lithosphere on the
continental crust that starts to subduct; the case of the Hellenides is
rich in examples of this process 
\citep{Bortolottietal2013,Ferriere1985,Ferriereetal2012,Ferriereetal2015,Ferriereetal2016,Papanikolaou2009,RassiosMoores2006,Robertson2012,Saccanietal2011,SmithRassios2003}. 

The first signs of convergence are characterized by a massive
subsidence of the Pelagonian\unskip\break 
continental domain and its margin.
Radiolarites are deposited on the different domains, including the
benthic limestones of the Pelagonian Zone  (Figure~\ref{fig10}D). The
Tourla-Trilofon (TT) Unit, locally showing Triassic benthic limestone
blocks, makes the transition between the Maliac distal margin and the
ophiolitic nappes  (Figures~\ref{fig7} and~\ref{fig10}). 

\def\ffigcaption{\label{fig10} Caption continued on next page.}
\xsubfig[ph!]{10}{}
\figcnt
\begin{figure*}
%\xincludegraphics{fig10}
\xcaption{\label{fig10a}\ffigcaption}{Jurassic 
Obduction. This figure exposes:  (i)~the
thrusting of the Triassic Fourka ophiolitic unit located on the lower
plate (A--D) and,  (ii)~the obduction development taking into account
the different ophiolitic m\'{e}langes (D--G). 
(A)~Pre-convergence initial state; 
(B)~Mid-Jurassic intra-oceanic Subduction development; 
(C)~Obduction upon the distal margin of the Fourka unit (oceanic
pillow-lavas unit) located on the oceanic lower Plate; 
(D)~obduction of the main peridotitic ophiolitic nappes coming from the
oceanic upper Plate and fast underthrusting of the thin continental
crust distal margin (ca. no m\'{e}langes); 
(E,F)~development of m\'{e}langes in areas with thick continental
crust  (Proximal margin and Pelagonian zone; low underthrusting) in
front of the upper syn-obduction nappes  (ophiolitic and distal margin
units); (G) Boeotian foreland basin development.} 
\vspace*{4pt}
\end{figure*}

\def\figcnt{\def\thefigure{11}}

\figcnt

This intensely deformed TT Unit is the first domain of the margin
overthrust by the ophiolitic nappes, in this case the Triassic Fourka
ophiolites  \citep{Ferriere1982}. The expected fate of such an oceanic
lithosphere of the Continent--Ocean Transition, furthermore, belonging
to the oceanic lithosphere of the downgoing plate of the oceanic
subduction zone, is that it underthrusts the continental lithosphere,
while the opposite is observed here\unskip\break
(Figures~\ref{fig9}
and~\ref{fig10}).

Two additional processes accompany the overthrusting of the Fourka unit
(Figure~\ref{fig10}C):  (i)~the subsidence of the continental margin,
attested by the deposition of radiolarites on top of all the margin
series; (ii)~the progression of the thick intra-oceanic nappes of
peridotites inducing a tilting of the oceanic crust of the Fourka Unit
and then a decollement of its upper part allowed by the presence of
serpentinites.

The nature and distribution of m\'{e}langes at the front of the
syn-obduction Maliac units provide insights on the emplacement of these
nappes in Greece or in Albania 
\citep{Celetetal1977,Ferriere1982,FerriereChanier2020,Gawlicketal2008}\unskip\break 
(Figure~\ref{fig10}D--F). For instance, in Othris, the thin distal
margin crust soon underthrusts the ophiolites, preventing the massive
formation of syn-obduction m\'{e}langes on the top of these units,
while m\'{e}langes are widely observed on the proximal margin units
because of their thicker crust, the underthrusting of which creates
relief and erosion during a long period. The Pelagonian platform
behaves the same way with m\'{e}langes fed by the ophiolites and distal
margin units while the proximal Maliac margin units are overrun and
dragged under the overriding nappes  (Figure~\ref{fig10}F).

Possible inheritance of early faults exists in syn-obduction nappes.
The reconstitution of the West-Maliac margin shows fast sedimentation
jumps between units supposedly close to each other on the margin. We
have attributed these fast changes to normal faults, within the margin 
\citep{Ferriere1982,FerriereChanier2020}. The reactivation of such
faults, mostly dipping oceanward, have facilitated the formation of
nappes from the margin. During the progression of these nappes, a
decollement localized within or underneath the Middle Triassic syn-rift
lavas  (distal margin) or deeper in the series, mainly the base of the
Early Triassic Ansian limestones  (proximal margin). In Othris, in the
proximal margin units, tectonic structures are organised with
pluri-kilometric fault-bend folds overturned toward the west.
Cretaceous deposits, locally unconformable on one of these
syn-obduction major structures, support the hypothesis of a Maliac
Ocean east of the Pelagonian  \citep{Ferriere1982}.

Concerning syn-obduction metamorphism, different cases have to be
distinguished. The metamorphic transformations observed within the
ophiolites are linked to their evolution within the ocean. The case of
the metamorphic soles with amphibolites and greenschists was discussed
above  (Section~\ref{sec4.3.1}). In the Hellenides, in Argolid and
Othris, syn-obduction sedimentary units do not show any metamorphic
overprint of that age. In other sectors, further east or northeast 
[e.g., Pelion,
\citealp{Ferriere1976b,Ferriere1982}], Tertiary
metamorphisms mask possible earlier metamorphic events. In a few
sectors, however, Late Jurassic--Early Cretaceous metamorphic ages have
been obtained in the basement 
\citep{Kiliasetal2010,Most2003,Mposkosetal2010}. For instance, 
\citet{Kiliasetal2010} describe within the Pelagonian basement in the
northern Hellenides, a succession of tectonic events with local
evidence of HP-LT metamorphism attributed to the obduction at
160--150~Ma, locally preserved in a greenschist to amphibolite-facies
event at 150--130~Ma. Anyhow, such syn-obduction HP-LT metamorphic
events coeval with Jurassic obduction seem to be rare in the
Hellenides. 

Some sediments allow us to precise the duration of the obduction
processes in Argolid  \citep{Baumgartner1985,Ferriereetal2016}. There,
spinels reworked in sediments deposited at the Callovian--Oxfordian
limit  (ca. 163~Ma) on the West Maliac margin, are the first witnesses
of the proximity of the ophiolitic nappe. The ages of the syn-obduction
m\'{e}langes and clastic formations deposited on the Pelagonian domain
are latest Oxfordian p.p.--Kimmeridgian  (ca. 158--154~Ma) and Late
Kimmeridgien--Early Tithonian  (ca. 153--150~Ma). The Tithonian deposits
rest unconformably on the syn-obduction units, implying a duration of
the order of 10~Ma. 

\subsubsection{Obduction on the East-Maliac margin}\label{sec4.3.4}   
Another obduction event recorded on the East-Maliac margin is
temporally close to the west-Maliac ophiolites emplacement  (Middle
Late Jurassic). As described above, the East-Maliac margin is
associated with a back-arc basin with a Middle--Upper Jurassic oceanic
crust  (Guevgueli domain) between the Paikon and Peonias Units in the
west and east, respectively  (Figure~\ref{fig9}). When restoring the
situation before the Cenozoic deformations, one observes that the
back-arc ophiolites were overthrusted on top of the Peonias margin
units, i.e., eastward, a situation opposite to that of the
supra-Pelagonian ophiolites 
\citep{FerriereStais1995,Ferriereetal2012,Ferriereetal2016}. Obduction
on the East-Maliac margin dates back to the Jurassic as shown by the
Bathonian to Oxfordian ages of deformed radiolarites deposited on the
Guevgueli ophiolitic lavas  (cf. Section~\ref{sec4.3.1}) and the
Tithonian age of unconformable conglomerates  \citep{Mercier1968}. A
set of granites and migmatites dated around 150~Ma intruded the
ophiolitic complex  \citep{Mercier1968}, confirming 
this conclusion. 

\subsection{The post-obduction and pre-collision\newline 
period: the Latest
Jurassic and Cretaceous}\label{sec4.4}
Well-dated series spanning the Upper Jurassic and the Early Cretaceous
are locally observed on the ophiolites~in Theopetra, close to Meteora, 
\citep{Ferriere1982,Surmontetal1991} or further north, in the Vourinos
area  \citep{Mavridisetal1993,FazzuoliCarras2007}. Unfortunately, these
series are not observed resting on the thrust contacts between the
ophiolites and the Pelagonian substratum, the question is whether these
layers, or part of them, were deposited during or after the obduction.
The end of the obduction period is characterized by a large reworking
of the paleogeography of the Internal Zones and their external rim
where the Beotian basin developed  (Figures~\ref{fig9} and~\ref{fig10})
as a flexural foreland basin in the front of the obduction chain,
hosting a flysch rich in ophiolitic elements as soon as the Tithonian 
\citep{Celetetal1976a,Celetetal1976b}. South of Sperchios  (Figure~\ref{fig1}), this
important Beotian basin develops on top of a single large platform
domain, at the boundary between the Pelagonian platform covered with
ophiolitic nappes in the east and the Parnassus platform in the west,
untouched by the obduction  (Figures~\ref{fig1},  \ref{fig9}
and~\ref{fig10}). 

Several Cretaceous reconstructions show a remnant post-obduction ocean
in the Almopias area, between Paikon and Pelagonian, sometimes named
``Vardar Ocean'' 
\citep{vanHinsbergenetal2020,Menantetal2016a,Menantetal2016b,StampfliBorel2002}. In this
Almopias sector, along the western border of the Paikon, the
Kranies--Mavrolakkos series, made of MORB pillow-lavas and rare depleted
lavas covered with Late Jurassic--Early Cretaceous radiolarites and a
flysch-type detrital formation  (Figure~\ref{fig3}, kr) could testify
for a deep oceanic environment 
\citep{Bechon1981,FerriereStais1995,Mercier1968,Saccanietal2015,SharpRobertson2006,Staisetal1990}. 

Two interpretations can be envisaged: either the remains of the eastern
part of the Maliac oceanic crust obducted toward the west during the
Middle-Late Jurassic, or the neoformation of an oceanic basin during
the late stages of obduction or just after. The imprecise age of the
radiolarites renders the choice between these two hypotheses difficult.

The period that can undoubtedly be considered as post-obduction, starts
in the Barremian--Aptian  (ca. 125~Ma). Due to the Albian--Turonian
eustatic transgression, marine formations develop on the already
emerged Internal Zones. Calcareous fine-grained\unskip\break 
formations are
deposited in the deep regions such as the Pindos and the Beotian basins
(Figure~\ref{fig3}). A mid-Cretaceous formation called  ``premier
flysch du Pinde''  \citep{Aubouin1959,Fleury1980} testifies for
vertical movements along the rims of that basin. Chaotic formations in
the Almopias domain have been attributed to Early Cretaceous
strike-slip movements  \citep{Vergely1984,MercierVergely1972}.
According to  \citet{Schenkeretal2014}, an Early Cretaceous blueschist
facies event in the Pelagonian was followed by an amphibolite-facies
overprint  (116 $\pm$ 8~Ma).  \citet{Lipsetal1998} proposed HP-LT
metamorphic ages ranging from 100--85~Ma to about 54~Ma in the
Pelagonian of the Ossa Massif, thus characterizing an  ``early--middle
Alpine cycle''. Cretaceous ages were also obtained in the Cycladic
Blueschists  \citep{Altherretal1994,Brockeretal2014}. Foliated
amphibolites interpreted as the sole of an ophiolitic nappe were dated
from the Late Cretaceous  (70--75~Ma) in Syros  
\citep{Maluskietal1987} or Tinos \citep{Patzaketal1994,
SearleLamont2022}. A Late Cretaceous
obduction is widely recognised along the Izmir--Ankara suture zone
further east  
\citep{OkayTuysuz1999}.
A first-order question then
arises about the relationships between the Late Jurassic obduction on
the Pelagonian zone and the Late Cretaceous obduction observed in the
Cyclades and further east. Finally, at the Cretaceous--Tertiary
boundary, the deposition of the flysch deposits in the Internal Zones
is the first evidence of the Hellenic collision  (Figure~\ref{fig3}) 
\citep{Aubouin1959}.

\subsection{Tertiary Collision}\label{sec4.5}
The notion of collision is partly debatable. Does it correspond to the
moment when the last oceanic space has been consumed by subduction or
to the period of strong mechanical coupling between the two continental
crusts and consequential shortening of the downgoing one? In the Alps,
a clear distinction is usually made between the two periods, the first
one being described as a continental subduction and the second one,
after 32~Ma, as real collision  \citep{Bellahsenetal2014}.

In the Hellenides, the disappearance of the last Tethyan oceanic space
between Apulia  (Adria) and Eurasia (Rhodope s.l.) is dated to
70--65~Ma \citep{Menantetal2016a,Menantetal2016b}, even to 40--35~Ma
considering the Pindos zone uncertainty  (Figures~\ref{fig1}
and~\ref{fig11}). 

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig11}
\vspace*{-3pt}
\caption{\label{fig11}\fontsize{9.5}{11}\selectfont  Tertiary
Collision; example of the northern Hellenides. (1)~Recent sedimentary
basins (Oligocene to present);  (2)~Ionian units (U.) on the western
Adria continental block;  (3)~Gavrovo-Tripolitza platform and U.; 
(4)~Pindos basin and U.;  (5,6)~Pelagonian basement~(5) and
Triassic--Jurassic limestones (6);  (7)~Maliac margin U.;
(8)~ophiolites;  (9)~unconformable Cretaceous cover  (Pelagonian);
(10)~flyschs;  (11)~western (W) and central (C) Almopias U.; 
(12)~eastern Almopias U. (M: Mavrolakkos; Kr: Kranies);  (13)~Paikon
U.;  (14)~granits through Guevgueli ophiolites;  (15)~Peonias U.; 
(16)~unconformable Tithonian--Lower Cretaceous sedimentary cover above
the Peonias-Guevgueli U.;  (17,18)~Serbo-Macedonian and Rhodopian upper
(17)~and lower  (18)~units. (A)~Late Cretaceous state with several deep
basins  (Peonias-Guevgueli, Almopias, Pindos and Ionian basin on a more
eastern area) between elevated domains;  (B)~beginning of the Eocene
compressive deformation in the eastern domain  (Collision s.l.); 
(C)~Late Eocene: beginning of the underthrusting of the Pindos zone
below the Internal Zone; the compressive tectonic goes on in the
eastern Internal Zones  (from Paikon area with two superposed units); 
(D)~Oligocene: underthrusting of the Gavrovo-Tripolitza domain and
Extensional deformation in the Serbo-Macedonian--Rhodopian domain; 
(E,F)~Early to Late Miocene: deformation of the Ionian and more
external zones and Olympos-Ossa uplift  (black arrow); 
(G)~Pliocene-Quaternary extensional deformation in the eastern
Hellenides and compressive deformation in the western Hellenides.}
\end{figure*}

\subsubsection{Sedimentary markers of collision}\label{sec4.5.1}
In the Hellenides, collision, or continental subduction, is
characterized by the deposition of thick flysch series invading the
Internal Zones and the Pindos Basin, as soon as the Maastrichtian or
Paleocene. The space--time evolution of the ages of these flysch
formations, more recent westward  (Figure~\ref{fig3}), illustrates the
idea of ``orogenic wave''  \citep{Aubouin1959}. The source, of the
sediments, sometimes attributed to islands in the Pelagonian domain is
more likely to be found further east, notably in the
Serbo-Macedonian-Rhodope block, as the Almopias series are also 
onlapped by a flysch as soon as the Maastrichtian pp 
\citep{Mercier1968,SharpRobertson2006}. 

The age and localisation of the piggy-back Mesohellenic Molassic Basin
(MHB)  (Figures~\ref{fig2}B and~\ref{fig3}) have recorded some of the
tectonic events affecting the Hellenides from the Late Eocene to the
Middle Miocene  (ca. 40~Ma to 15--10~Ma) (Figure~\ref{fig2}B) 
\citep{Brunn1956,Doutsosetal1994,Ferriereetal1998,Ferriereetal2004,Ferriereetal2013,Vamvakaetal2006,Zelilidisetal2002}.

In the Late Eocene, the subduction of the Pindos basin led to the
formation of limited basins in the upper unit of the Internal Zones, as
the deep flysch basin of Krania. From the Oligocene, the subduction of
the Gavrovo-Tripolitza platform determines the development of the real
MHB of 300~km $\times$ 30 to 40~km, striking NW--SE, which rests
unconformably on the late Eocene basins but also on the ``Frontal
Thrust of the Internal Zones'' developed on the External Zones 
\citep{Ferriereetal2004,Ferriereetal2013}. Further east, the extension
controlling the formation of the Rhodope MCC may also have started
during the Late Eocene  \citep{BrunSokoutis2007,SokoutisBrun2018}
(Figure~\ref{fig11}), but alternative interpretations  make compressional
tectonics last until the Early Oligocene, at about 33~Ma 
\citep{Gautieretal2010,Gautieretal2017}.

Close to the Oligocene--Miocene boundary, a new major event is attested
by the thick sandstones and conglomerates deposited in shallow marine
environments  (Meteora Gilbert-type deltas) 
\citep{Brunn1956,OriRoveri1987,Ferriereetal2011}. The detrital elements
are linked to the uplift of the eastern Pelagonian sector of the MHB
probably due to the development of ramps and duplexes within the
underthrust Gavrovo-Tripolitza Zone, which also explains the eastward
propagation of depocenters starting at the end of the Oligocene.
Thermochronological studies  (apatite and zircon fission-tracks)
provide some more precision of the timing of the exhumation of the
Pelagonian domain  \citep{Schenkeretal2015}. A fast exhumation in the
Eocene and slower exhumation in the Oligocene and Miocene have been
proposed  \citep{Vamvakaetal2010}. Along a transect running from the
Pelagonian to Mt Olympos, fast exhumational cooling occurred between 12
and 8~Ma at rates of  15--35~\textdegree C/Ma and decreased to 
${<}$3~\textdegree C/Ma by  8--6~Ma  \citep{Coutandetal2014}. Numerous
normal faults exist until the recent Plio-Quaternary extension. 

Once sedimentation stopped in the MHB, near the Miocene--Pliocene
boundary, a new basin developed further east with the same Hellenic
strike, the Larissa-Ptolemais Basin. This process leads to the
morphology in continental Greece, characterized in the Internal Zones
by a succession of ridges and basins, with a wavelength of
${\sim}$30~km  (Figure~\ref{fig2}B).

\subsubsection{Tectonic structures and metamorphic events related to
the Tertiary collision}\label{sec4.5.2}
The structures related to collision are easy to recognise in the
Hellenides External Zones as they have not been affected by the
Jurassic obduction: a Tertiary ``fold-and-thrust belt'' developed,
associated with the formation of major nappes. The NW--SE striking
structures are SW verging 
\citep{Aubouin1959,Doutsosetal1993,Doutsosetal2006,vanHinsbergenetal2005a,vanHinsbergenetal2005b,SkourlisDoutsos2003}. NE-dipping previous syn-rift normal faults 
(i.e., SW margin of Pindos Basin) were inverted to form major thrusts,
while on the opposite where the normal faults dip to the SW  (i.e., NE
Pindos margin), tectonic units are verticalized (i.e. Koziakas and
Vardoussia units,\unskip\break 
Figure~\ref{fig1}). Compared to northern Hellenides,
the large-scale structure seems more complex in the Peloponnese where
major tectonic windows are recognised in the Taygetos and Parnon 
(Figure~\ref{fig1}) showing external units including the Ionian series 
(Palttenkalk), underneath the Pindos and Gavrovo nappes 
\citep{Doutsosetal2006,Jolivetetal1996,Jolivetetal2010a,Jolivetetal2010b,Thiebault1982}.
The importance of the shortening is attested by these tectonic windows
in the Peloponnese but also in the northern Hellenides where such
windows  (e.g., the Olympos window) are known in the Internal Zones
(Figure~\ref{fig1}). 

{Two Tertiary belts of HP-LT metamorphism are observed}:
In the north-eastern Hellenides, Eocene blueschists are found in the
Olympos-Ossa and Almyropotamos tectonic windows below the Pelagonian
units 
\citep[e.g.,][]{Godfriaux1968,Katsikatsosetal1976,Schmidetal2020,Xypoliasetal2012}\unskip\break 
(Figure~\ref{fig1}). 

The Ambelakia unit, attributed to the Pindos zone, shows
blueschist-facies parageneses. The underlying Gavrovo-Tripolitza (GT)
unit locally shows HP-LT parageneses with the occurrence of lawsonite
in the Eocene flysch topping this unit and relict glaucophane and
phengite in some layers  \citep{Shakedetal2000}. These tectonic windows
are also observed in the Cyclades where they are reworked by the
Oligo-Miocene extension. The Cycladic Blueschists are lateral
equivalents of the Ambelakia unit, part of the Pindos Basin 
\citep{Bonneau1984,Jolivetetal2004a,Jolivetetal2004b,Hinshawetal2023}. The underlying
so-called Basal Unit observed on Tinos or Samos is similar to the
Gavrovo-Tripolitza 
\citep{Jolivetetal2004a,Jolivetetal2004b,Ringetal1999}. 
Two compatible solutions can be proposed for the uplift of the Olympos
window: (i)~the uplift of the shoulder of the Thermaikos Gulf rift is
controlled by the large normal faults bounding the Olympos massif to
the east  \citep[e.g.,][]{Schmidetal2020};  (ii)~the uplift is due to
the backthrusting of the GT series against a ramp corresponding to the
Pelagonian/Vardar boundary\unskip\break 
(Figure~\ref{fig11})
\citep{Ferriereetal2004,Ferriereetal2013}. The confirmation by zircon
and apatite fission-tracks  \citep{Vamvakaetal2010,Coutandetal2014} of
the west-to-east migration of uplifts within the Pelagonian domain is
in favour of this second mechanism, but in any case, the uplift was
exacerbated during the recent\unskip\break  
extension. 

The second HP-LT belt observed in the Peloponnese tectonic windows 
(Figures~\ref{fig1} and~\ref{fig4}), in Kythira and in Crete 
\citep{Bonneau1973,Creutzburg1977,Greilling1982} is more recent with
peak-pressure ages ranging from the Oligocene to the Early Miocene 
\citep{Brixetal2002,Jolivetetal1996,Jolivetetal2010b,Seidel1978,
Seideletal1982,Ringetal2001,TheyeSeidel1991,Theyeetal1992,
Thomsonetal1998}. 
There, the external Phyllite--Quartzite Nappe (PQ) and the underlying Plattenkalk (PK) Nappe 
(or Ionian Nappe) show HP-LT parageneses characterized by the
occurrence of Fe--Mg carpholite, lawsonite  $\pm$ chloritoid and garnet
in the metapelites. Fe--Mg carpholite is also observed in 
metabauxites\unskip\break 
found in the Plattenkalk, as well as aragonite in the marbles 
\citep{Jolivetetal1996,Jolivetetal2010a,Jolivetetal2010b,Seideletal1982,TheyeSeidel1991,
Theyeetal1992}. 
The PQ nappe, which shows considerable thickness variations, probably resulting from large-scale boudinage 
\citep{Jolivetetal1996,Jolivetetal2010a,Jolivetetal2010b} is made of a sequence of
metapelites of Triassic age, with also conglomerates, quartzites and
limestones, as well as slices of Paleozoic basement in Kythira and
Crete  \citep{Romanoetal2004,Seideletal2007}. Because of these ages,
the PQ nappe has often been confused with the overlying Tyros Beds, the
lowermost part of the GT stratigraphy, in the Zaroukla--Feneos window,
for instance  \citep[see a discussion in][]{Jolivetetal2010b}.
However, as the GT Nappe does not show any HP-LT overprint, a clear
metamorphic gap is observed between the PQ Nappe and the overlying
Tyros beds. The low-angle contact between this unmetamorphosed GT nappe
and the metamorphic units underneath is a former thrust reactivated as
a major detachment, the Cretan Detachment 
\citep{Fassoulasetal2004,Grasemannetal2019,Jolivetetal1994a,Jolivetetal1994b,Jolivetetal1996,Jolivetetal2010b}.

The presence of HP-LT parageneses in the PQ and PK Nappes shows that
they have been subducted to large depth during the Oligocene and the
Early Miocene, which corresponds to the collision and shortening of the
External Zones in the
Hellenides. 

The subduction and exhumation of these continental units were also
coeval with the extension developing in the back-arc region and the
exhumation of hot and cold metamorphic core complexes 
\citep{Jolivetetal1994a,Jolivetetal1994b,Jolivetetal2010b}. 
These metamorphic external
units  (PQ and PK) do not crop out north of the Corinth Rift. They
probably continue at depth below the Parnassus, Pindos and GT nappes of
the External Hellenides, but the deep geometry there is unknown. Their
exhumation in the Peloponnese and Crete results from the kinematics
imposed by slab retreat and the $P$--$T$ conditions are in part dictated by
the velocity of that retreat, with colder conditions in Crete than in
the Peloponnese  \citep{Jolivetetal2010b}. If the amount of shortening
is about 300 km by the addition of the horizontal displacements of the
Pindos and GT nappes present in the Olympos window, in the Peloponnese,
as the significance of the Phyllite--Quartzite  (PQ) Nappe remains to be
ascertained, the amount of shortening cannot be\unskip\break  
specified. 

Structures observed in the Internal Zones, already deformed during the
Jurassic obduction, are different from those of the External Zones. 

East of the Vardar, Peonias units overthrusting toward the east in the
Late Jurassic obduction, are overturned toward the SW, locally leading
to inverted series  (Figures~\ref{fig2}A and~\ref{fig11})
\citep{FerriereStais1995}. Further west, in the Paikon domain, several
interpretations of the Tertiary structures were proposed: (i)~the Paikon
is a single unit between two west-verging thrusts  \citep{Mercier1968}
or two thrusts with opposite vergence 
\citep{BrownRobertson2003,VergelyMercier2000};  (ii)~one or two windows
may exist, either a window opened on Pelagonian units 
\citep{GodfriauxRicou1991,KatrivanosKilias2013,RicouGodfriaux1995}, or
a window opened on Paikon type-series, below a nappe consisting of the
upper part of the same Paikon type-series  \citep{Ferriereetal2001}.
Finally, in the Almopias area, the previous Maliac Tethys oceanic
suture, numerous thrust units with SW vergence have developed, the
lowermost of which overthrust the Pelagonian domain 
\citep{MercierVergely1984} (Figure~\ref{fig2}A). 

The Pelagonian, limited on each side by two major Tertiary thrusts, or
more precisely the Pelagonian domain with the Ophiolitic/Maliac
syn-obduction units, is a large Tertiary unit, overthrusting the Pindos
and the Gavrovo  (Figures~\ref{fig1},  \ref{fig2}  and~\ref{fig11}).
This domain may consist of two superposed Pelagonian units, the lower
unit in the east, mainly represented by its crystalline basement and
the upper one further west with the classical Pelagonian Mesozoic
series  \citep[e.g.,][]{Kiliasetal2010}. In some parts of this
Pelagonian domain, for example in Attica  \citep{Clement1983}, Tertiary
thrusts are observed within little or non-metamorphosed Cretaceous
series.  \citet{MercierVergely1977} and \citet{Vergely1984} have
described and mapped several Cretaceous and Tertiary phases in the
Internal Zones: CT1 and CT3 have Dinaric/Hellenic directions  (i.e.,
NW--SE) while CT2 corresponds to transverse structures (NE--SW).

Tertiary syn-metamorphic structures have been observed in the Eastern
Pelagonian domain around the Olympos-Ossa tectonic windows 
\citep{Schmitt1983,Schermer1990,Schermer1993} and in Thessalia 
\citep{Walcott1998}. These syn-metamorphic structures show various
structural directions, associated with greenschist or low
blueschist-facies. Metamorphic Cretaceous layers are also present in
the Pelion and~\mbox{Sporades} 
\citep{Ferriere1976a,Ferriere1976b,Ferriere1982,JacobshagenWallbrecher1984}
locally with blue crossitic amphiboles coeval with folds overturned
toward the SW.  \citet{Schermer1990,Schermer1993} described two phases
of blueschist-facies metamorphism  (61--53~Ma and  42--36~Ma) in the
Pierian (Pelagonian) Units and the Ambelakia (Pindos) Unit coeval with
southwestward thrusting, attributed by  \citet{Godfriauxetal1988} and 
\citet{Schermer1990} to the subduction of the continental crust. These
phases are associated with abundant NE--SW stretching lineations and
mainly top-SW kinematic indicators  \citep{Walcott1998}. However, large
transverse NE--SW, with NW or more often SE-verging structures,
frequently overlooked, also exist in non-metamorphic or schistose
domains, for instance, in northern  (folds and associated schistosity)
and eastern Othris (thrusts)  \citep{Ferriere1982}. In the latter case,
backthrusts are observed with Pelagonian thrust over Maliac Units. A
similar situation is observed in the Olympos-Ossa sector, where
Infra-Pierian units, locally with ophiolites and unconformable
Cretaceous deposits attributed to the Maliac domain, are overthrusted
by the Pelagonian-Pierian units  (Figure~\ref{fig2}B). These
Infra-Pierian Units show abundant, more or less metamorphic N40-N80
striking structures, that could be responsible for the inversion of the
units emplaced during the Jurassic obduction  \citep{Schmitt1983}. This
peculiar structural pattern, clearly demonstrated at the scale of a
large region, should be integrated in regional
interpretations. \looseness=-1

The western limit of the Pelagonian corresponds to the ``Frontal Thrust
of the Internal Zones''  (FTIZ), which is an Eocene feature. This
front, with a general Hellenic strike  (NW--SE), marks the contact
between two drastically different paleogeographic domains, the Internal
Zones that have been obducted during the Jurassic and the External
Zones, only deformed during the Tertiary. North of the Sperchios, the
FTIZ corresponds to the overthrust of ophiolitic units without their
Pelagonian-Maliac substratum, on top of the Pindos eastern margin units
(Koziakas) and/or Pindos flysch. South of the Sperchios, instead, the
FTIZ corresponds to the overthrust of klippes of the Pelagonian
Mesozoic formations on top of the Parnassus zone  (Figure~\ref{fig2}D).
North and south, the FTIZ also rests on top of the Beotian units with
the Malm-Cretaceous flysch resulting from the Jurassic obduction. The
FTIZ is finally reactivated during the Plio-Quaternary extension as a
detachment, the Main Pelagonian Detachment  (MPD) \citep{Chanieretal2023}.
The eastern limit of the Pelagonian, or Vardar Front, is less
clear than the FTIZ. Its present position mostly results from the late
exhumation  (Neogene-Quaternary) of the Olympos-Ossa-Pelion orogenic
axis. The overthrusting of the Pelagonian domain by Vardar units is
significant as the Pelagonian Cretaceous--Eocene units are often
metamorphosed. This metamorphism could however also be partly explained
by the transverse Eocene phase, which involves intra-Pelagonian
thrusting  (cf. supra). On the opposite, overthrusting of the Vardar on
the Pelagonian is underestimated because of the presence of normal
faults and detachments. Such is the case of the eastern border of the
Olympos massif or in the Pelion, where klippen of non-metamorphic
Cretaceous terranes of Vardar origin rest on top of Pelagonian units,
and also directly on top of the Makrinitsa blueschists, normally
situated underneath the Pelagonian nappe 
\citep[e.g.,][]{Ferriere1982}.
\vspace*{-3pt}

\section{Conclusions}\label{sec5}
\vspace*{-1pt}
After a period when the ``Geosyncline model''  \citep{Aubouin1959} was
favoured, the Hellenic belt was soon interpreted in the framework of
plate tectonics and became one natural laboratory ideal to analyse most
of the fundamental geodynamic and orogenic processes governing the
evolution of Alpine-type orogens. The Hellenic chain reveals a fairly
classical evolution of a Tethyan Alpine chain with a period of
divergence  (rifting and oceanic spreading), followed by convergence
from the Middle Jurassic, marked by major tectonic phases 
(subduction/obduction then collision), presenting compressional but
also extensional deformations. Nevertheless, the study of the
Hellenides provides many original data concerning these processes.

Rifting, accompanied by significant volcanism, affected almost the
entire Hellenic domain in the Middle Triassic initiating the major
Tethyan basins, the Maliac Ocean on the eastern side of the Pelagonian
zone and the Pindos Basin on the western side. The geochemistry of some
syn-rift lavas suggests a link with the subduction of the Paleotethys,
but a partial melting of a particular mantle without subduction, is
also conceivable. In Othris, on the West-Maliac, syn-obduction units we
can recognise a very deep ``distal margin'', rich in volcanism and
siliceous beds, very different from the ``proximal margin''. On this
distal margin, the rifting-related volcanism ended in the Late Ladinian
while oceanic accretion began a few million years before in the Late
Anisian. 

The Triassic--Middle Jurassic oceanic spreading lasted some 70~Ma and
was followed by an important convergence period, first marked within
the Maliac Ocean by the development of one or two subduction zones,
attested by the occurrence of typical SSZ lavas in the supra-Pelagonian
ophiolites and also by the presence of the Paikon volcanic arc on the
eastern Maliac margin.

The Middle Jurassic age of the Maliac ophiolites is close to the age of
their obduction on the Pelagonian domain, suggesting that the
subduction zone driving the obduction developed close to and along an
oceanic ridge, either from the central oceanic rift or from an oceanic
detachment fault located on the western side of the ridge.

The main Maliac supra-Pelagonian Ophiolites (SPO) with peridotitic
basement, are thought to originate from the upper plate of an
east-dipping intra-oceanic subduction zone. The western SPO nappes,
i.e., N-Pindos and Mega Isoma in Othris, with lherzolites and MORB-type
lavas overlain by SSZ-type lavas, are ante-subduction oceanic
lithospheric elements trapped in the upper plate of the intra-oceanic
subduction. The SPO units located further east  (Vourinos and
Metalleion in Othris), with harzburgites and SSZ-type lavas, could
correspond to a Middle-Jurassic neo-formation of an oceanic lithosphere
in a sector becoming a forearc domain just after the growth of a
volcanic arc.  

The subduction that affects the East-Maliac margin in the Middle-Late
Jurassic  (Paikon Arc and Guevgueli back-arc basin) corresponds either
to the same subduction forming the SPO, but more probably to a second
subduction located further east. 

The study of the Hellenides provides indications regarding the
emplacement and origin of the obducted units: 
\begin{enumerate}[(i)]
\item 
In Othris, five syn-obduction Middle-Late Jurassic nappes: three of
them corresponding to the proximal West-Maliac margin and two to the
distal margin are present below two major ophiolitic units and above
the Pelagonian continental block; 
\item 
a thick nappe of ophiolitic pillow-lavas of MORB-type (Fourka Unit) of
Middle and Late Triassic age (an age close to the rifting event)
represents the oldest witness of the Maliac Ocean crust. Despite its
initial position in the lower plate during subduction and obduction,
this old Fourka unit was obducted onto the adjacent Pelagonian
continental domain. A mechanism has been proposed to explain this
apparent anomaly  (see text).
\end{enumerate}

During the Cretaceous, between the obduction and collision periods, a
major transgression developed. Radiometric ages retrieved from
metamorphic rocks testify to significant compressional and/or
extensional events during this period, e.g., Early Cretaceous
continuation of obduction in the Pelagonian zone and Late Cretaceous
obduction, ca. 70--75~Ma, attested by amphibolites and ophiolites in the
Aegean domain. These ophiolites emplaced much later than the Vourinos,
coeval with the large-scale obduction observed along the Izmir--Ankara
suture zone further east. The geometrical and kinematic relations
between the Greek and Turkish obduction events are far from clearly
understood.

The Tertiary Collision is at the origin of the main Hellenic belt
presently observed in western Greece. The first stage corresponds to
the disappearance of the main oceanic basins between the continental
Adria and Rhodope blocks, i.e., the Cretaceous Vardar oceanic basin and
the Pindos basin, subducted below the Pelagonian continental domain. A
second stage is the development of nappes in the ``Thrust and Fold
Belt'' of the External Zones.

Large horizontal displacements of the Tertiary nappes are attested by
the existence of the Olympos-Ossa and Almyropotamos tectonic windows,
showing the external Gavrovo-Tripolitza and Pindos series underneath
the Internal Zones. Eocene HP-LT blueschist-facies in the underlying
units further attests for the subduction of the lower units including
the carbonate platform. 

Similar windows also exist further southwest in the Peloponnese and
Crete where Oligocene--Miocene blueschists-facies is observed in more
external units  (Phyllites--Quartzites and Plattenkalk zones) underneath
the non-metamorphosed Gavrovo-Tripolitza nappes. The
underthrusting/subduction of the External Zones under the Internal
ones, was followed in the Miocene by the subduction of the Eastern
Mediterranean lithosphere belonging to the Africa Plate.

This succession of subduction episodes affecting tectonic units
belonging to Adria left a heritage in the subsequent evolution of the
Hellenides. The extensional reactivation of the ``Frontal Thrust of the
Internal Zones'' gave rise to the  `Main Pelagonian Detachment'' as
well as to the development in the Internal Zones of large
Plio-Quaternary extensional basins  (Trikala, Larissa, Kifissos) above
this detachment fault. In the Aegean domain, the effects are even more
spectacular: the structure of the Tertiary nappes pile was profoundly
modified by the formation of major detachment faults at the scale of
the Aegean domain  (e.g., West and North Cycladic Detachment Systems),
the final exhumation of Eocene blueschist units and the development of
metamorphic core-complexes showing Miocene migmatites and granites. 

The lateral transition from the continental Hellenides where the Eocene
nappe stack is best preserved, to the Aegean Sea where it was deeply
reworked by the Oligo-Miocene extension in the back-arc region of the
Hellenic subduction, offers the possibility to read the full Wilson
cycle from the Triassic rifting to the Oligo-Miocene post-orogenic
extension and collapse of the Eocene nappe stack. The quality of the
outcrops makes it an irreplaceable example and natural laboratory for
the study of Alpine-type orogens. It however lacks geophysical studies
focused on the crustal structure of the continental Hellenides. Passive
seismic experiments along seriated long profiles would enlighten the
deep parts of the orogen and its post-orogenic evolution.

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

\section*{Acknowledgements}
This review paper is dedicated to the late Professor Jean Aubouin who
animated and led a large team of young geologists exploring the
Hellenides and the Dinarides in the 60s and 70s, revealing the first
order structures and describing the evolution of an emblematic mountain
belt and subduction zone. The recent evolutions of tectonic and
geodynamic studies in convergence zones would not have been possible
without the patient field work and innumerable observations of these
pioneers. We warmly thank the guest-editor Michel FAURE, and reviewers
Stephan SCHMID and Konstantinos SOUKIS who provided constructive
comments that greatly helped to improve the paper.

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