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\DOI{10.5802/crgeos.255}
\datereceived{2023-04-28}
\daterevised{2023-11-29}
\datererevised{2024-01-11}
\dateaccepted{2024-02-12}
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\dateposted{2024-03-26}
\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

\title{The Main Pelagonian Detachment (MPD): extensional reactivation
of the frontal thrust of the Internal Zones of the Hellenides (Greece)}

\alttitle{Le D\'{e}tachement P\'{e}lagonien (MPD) : r\'{e}activation en
extension du chevauchement frontal des zones internes des
Hell\'{e}nides (Gr\`{e}ce)}

\author{\firstname{Frank} \lastname{Chanier}\CDRorcid{0000-0001-6030-6982}\IsCorresp}
\address{Univ. Lille, CNRS, ULCO, IRD, UMR 8187, LOG - Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F-59000 Lille, France} 
\email[F. Chanier]{frank.chanier@univ-lille.fr}

\author{\firstname{Jacky} \lastname{Ferri\`ere}}
\address[1]{Univ. Lille, CNRS, ULCO, IRD, UMR 8187, LOG - Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F-59000 Lille, France} 

\author{\firstname{Olivier} \lastname{Averbuch}\CDRorcid{0000-0002-8480-1176}}
\address[1]{Univ. Lille, CNRS, ULCO, IRD, UMR 8187, LOG - Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F-59000 Lille, France} 

\author{\firstname{Fabien} \lastname{Graveleau}\CDRorcid{0000-0002-9225-9364}}
\address[1]{Univ. Lille, CNRS, ULCO, IRD, UMR 8187, LOG - Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F-59000 Lille, France} 

\author{\firstname{Fabien}\nobreakauthor\lastname{Caroir}\CDRorcid{0000-0002-6897-5272}}
\address[1]{Univ. Lille, CNRS, ULCO, IRD, UMR 8187, LOG - Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F-59000 Lille, France} 

\author{\firstname{Virginie} \lastname{Gaullier}\CDRorcid{0000-0003-3980-3508}}
\address[1]{Univ. Lille, CNRS, ULCO, IRD, UMR 8187, LOG - Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F-59000 Lille, France} 

\author{\firstname{Louise} \lastname{Watremez}\CDRorcid{0000-0001-7350-4945}}
\address[1]{Univ. Lille, CNRS, ULCO, IRD, UMR 8187, LOG - Laboratoire d'Oc\'{e}anologie et de G\'{e}osciences, F-59000 Lille, France} 

\shortrunauthors

\keywords{\kwd{Tectonic inversion}
\kwd{Inherited structures} 
\kwd{Detachments} 
\kwd{Pliocene-Quaternary extensional basins} 
\kwd{Central Greece}}

\altkeywords{\kwd{Inversion tectonique}
\kwd{Structures h\'{e}rit\'{e}es} 
\kwd{D\'{e}tachements} 
\kwd{Bassins en extension plioc\`{e}ne-quaternaire} 
\kwd{Gr\`{e}ce centrale}}

\begin{abstract} 
During the orogenic development of the Hellenides, the ``Frontal Thrust
of Internal Zones'' (FTIZ) localised tectonic superposition of the
Internal Zones over the External Zones. The belt was subsequently
subject to a major extensional deformation responsible for a negative
tectonic inversion of the FTIZ, along a major detachment called here
``Main Pelagonian Detachment'' (MPD). The along-strike changes in the
geometry and the kinematics of the MPD suggest different tectonic
configurations largely inherited from the flat--ramp--flat initial
geometry of the thrust. The distribution of the recent basins, mainly
located within the Internal Zones, illustrates the major role of the
FTIZ reactivation in the overall collapse of Internal Hellenides.
\end{abstract}

\begin{altabstract} 
Au cours du d\'{e}veloppement orog\'{e}nique des Hell\'{e}nides, le
\guillemotleft{}~Frontal Thrust of Internal
Zones~\guillemotright{}~(FTIZ) a entrain\'{e} la superposition
tectonique des zones internes sur les zones externes. La cha\^{i}ne des
Hellenides a ensuite \'{e}t\'{e} soumise \`{a} une importante
d\'{e}formation en extension responsable d'une inversion tectonique
n\'{e}gative du FTIZ, formant ainsi un d\'{e}tachement majeur
appel\'{e} ici \guillemotleft{}~d\'{e}tachement p\'{e}lagonien
principal~\guillemotright{}~(MPD). Les changements de la
g\'{e}om\'{e}trie et de la cin\'{e}matique du MPD le long du
chevauchement sugg\`{e}rent diff\'{e}rentes configurations tectoniques
largement h\'{e}rit\'{e}es de la g\'{e}om\'{e}trie initiale
plat-rampe-plat du chevauchement. La distribution des bassins
r\'{e}cents, principalement situ\'{e}s dans les zones internes,
illustre le r\^{o}le majeur de la r\'{e}activation de la FTIZ dans
l'effondrement global des Hell\'{e}nides internes.
\end{altabstract}

\thanks{Tellus Program of CNRS-INSU (SYSTER action), Research team
``TIMES'' from the \textit{Laboratoire d'Oc\'{e}anologie et
Geosciences} (LOG)}

\maketitle

\vspace*{.2pc}

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\end{noXML}


\section{Introduction}

The Hellenides mountain belt in Greece is a part of the alpine Tethyan
orogen developed in the Mediterranean domain  
\citep[e.g.,][]{Aubouin1959,Ricouetal1986,Dewey1988,JolivetFaccenna2000}.
It is limited to the south and southwest by the subduction front of the
African plate (Adria) beneath the Anatolian--Aegean plate
(Figure~\ref{fig1}).

\begin{figure*}[t]
{\vspace*{1pt}}
\includegraphics{fig01}
\caption{\label{fig1}Simplified geologic map of the Aegean Domain. NAF:
North Anatolian Fault; NAT: North Anatolian Trough 
\citep[modified from][]{Brunetal2016}.}
\end{figure*}


The Hellenides present a contrasted morphology (Figure~\ref{fig2}A)
with a western mountainous external part displaying high relief
(elevations up to ${\sim}$2600~m) and a general topographical continuity
from the NW to the SE, and an eastern, more segmented, internal part
characterised by alternating ranges and troughs. The troughs correspond
to post-orogenic \mbox{intermontane} rift basins. Such a ``basin-and-range''
type morphology, as well as the development of the Aegean Sea in the
core of the arcuate Hellenides orogenic domain, basically results from
the extensional collapse of the internal zones of the belt. This
collapse is interpreted as the result of the southward retreat of the
underlying subducted slab of the Africa plate margin and of the
associated upwelling of asthenospheric mantle into the subduction
\mbox{corner}
\citep{Brunetal2016,VanHinsbergenetal2005,JolivetBrun2010,Jolivetetal2021,Menantetal2016}.
%\looseness=-1


\begin{figure*}[p]
\includegraphics{fig02}
\caption{\label{fig2}\fontsize{9.8}{12}\selectfont 
Main characteristics of the Hellenides in
Continental Greece. (A)~Topography from Digital Elevation
Model (SRTM-DEM) showing the contrasting elevations from both the
External Zones and the Internal Zones. The latter clearly shows
alternating ridges and basins. Topography from Shuttle Radar Topography
Mission \citep[SRTM,][]{Farretal2007} and bathymetry from
\citet{SmithSandwell1997}. (B)~Simplified geological map of
continental Greece outlining the locations of detailed maps, sections
across the Hellenic belt (A, B, C, Figure~\ref{fig8}) and the occurrence of the
main recent basins; e.g., Trikala, Larissa, Kifissos (Quaternary) and
Reginio (Neogene) basins. The Frontal Thrust of Internal Zones (FTIZ)
has been widely re-activated along a main extensional faulted structure
(MPD: red line). Abbr. b: basin; z: Zone.}
\end{figure*}

The transition zone between the two morphological parts of the Hellenic
belt is narrow (bold dotted line on Figure~\ref{fig2}A) and broadly
follows a major compressional structure of the mountain belt, the
Frontal Thrust of the Internal Zones (FTIZ), which separates the
internal and external tectonic units (Figure~\ref{fig2}B). Such a
first-order geometrical pattern suggests that the FTIZ has been
reactivated during the extensional collapse phase thereby representing
a major detachment system within the Aegean rifted domain. However, the
way this topographic collapse and the negative tectonic inversion
process have operated remains poorly documented. It was only described
at a local scale, at the transition between the Parnassos and
Pelagonian domains in the Amphiklia area (Figure~\ref{fig3}), where a
low-angle contact between the uplifted unit (Parnassos mountains) and
the collapsed unit (Pelagonian internal units) is interpreted as an
extensional detachment, which reactivated the former thrust between the
two units \citep{KranisandPapanikolaou2001}.

\begin{figure*}[p]
\includegraphics{fig03}
\caption{\label{fig3}(A)~Schematic geological map of the recent basins
from the area of Othris and Parnassos mountains. (B): SW--NE
cross-section from Parnassos unit to Maliac-North Evia Gulf across
Amphiklia area (Location on A). Abbr. FTIZ: Frontal Thrust of
Internal Zones; MPD: Main Pelagonian Detachment; KAF:
Kallidromon-Atalanti Fault; KVAFS: Kamena Vourla-Arkitsa Fault System;
1: Eocene thrust; 2: Plio-Quaternary extensional deformation.}
\end{figure*}


In this study, we propose to address this inversion tectonics mechanism
and the related structural geometries by reassessing the geometry and
kinematics of the FTIZ in the Hellenides north of the Corinth Gulf.
Our study considers firstly, regional scale \mbox{mapping} and
cross-sections to analyse spatial relationships between compressional
and subsequent extensional structures and, secondly, outcrop
description to better constrain the kinematics of deformation. As the
observed rifted structures appear as highly segmented laterally, we
examinate the variation in structural geometries along strike of the
FTIZ trace and document the 3D heterogeneity of the structural pattern
of the inversion process. We investigate in detail the Dio 
Vouna--Amphiklia area (east of Mount Oiti, Figure~\ref{fig3}A)
where the complex geometry of the FTIZ provides a unique opportunity to
decipher the tectonic inversion mechanisms along this major tectonic
boundary.

\section{Geological setting}\label{sec2}

The Hellenic alpine belt is the result of the early Tertiary
convergence between Africa--Adria and Eurasia plates. The major
collisional phase started by the Eocene but the involved rifted
continental margins were affected by earlier compressional events.
Indeed, during the Middle--Late Jurassic, the distal parts of the
Adria--Africa continental margin (Pelagonia) underwent an obduction
event implying ophiolitic nappes emplacement from the Maliac (Vardar)
oceanic seaway \citep[e.g.,][]{Ferriereetal2012,Ferriereetal2015,Ferriereetal2016}. 
The Internal Zones (IZ) correspond to the areas that were covered by the
ophiolitic nappes during the Jurassic obduction \citep{Brunn1956},
while the External Zones (EZ) were deformed only during Tertiary times
(Figure~\ref{fig2}B). The boundary between these two sectors is the
Frontal Thrust of the Internal Zones (FTIZ), whose importance is
demonstrated by the occurrence of tectonic windows showing external
series underthrust below the internal zones 
\citep[e.g., Olympus window,][]{Godfriaux1968}.
By the end of the collisional period, a large
basin, the Mesohellenic Basin (Figure~\ref{fig2}B), developed in a
piggy-back position on top of the main internal unit (Pelagonian
basement) from early Oligocene to Middle Miocene times
\citep{Brunn1956,Ferriereetal2004,Ferriereetal2013}. This basin is slightly
folded and elongated parallel to the fold-and-thrust belt.

During the Neogene, most of the Hellenides were subject to the
extension that led to the development of deep intra-mountainous
sedimentary basins. In continental Greece, these extensional basins are
\mbox{observed} in the IZ (e.g., Trikala--Karditsa Basin; Larissa--Ptolemais
Basin; Figures~\ref{fig2}B and~\ref{fig3}A), between the elevated
Pindos fold-and-thrust belt (part of the EZ) to the west, and the
Aegean Sea domain to the east. The Gulf of Corinth is the main
extensional basin developed across both the IZ and EZ
\citep{Armijoetal1996,Jolivetetal2010a}, but the whole Aegean Sea
Domain is also affected by widespread brittle extensional deformation
from Late Miocene times to present-day
\citep{GoldsworthyJackson2001,JolivetFaccenna2000,Jolivetetal2013,Brunetal2016}.

The extension direction varied from NE--SW during the Pliocene and early
Quaternary period to NNE--SSW and N--S during the late Quaternary
\citep{Caputo1995,CaputoPavlides1993,DoutsosKokkalas2001,Faucheretal2021,Kiliasetal2008,Kranis2002,Kranis2007,Mercieretal1979}.
Most focal mechanisms consistently reveal a N--S direction of extension
\citep{Kiliasetal2008,Papazachosetal1998}, which is confirmed by GPS
geodetic studies \citep[e.g.,][]{Mulleretal2013}. From both
seismological and GPS data, the N--S extension appears to be the
prevailing active deformation style in the studied area, especially in
western Thessaly and in the northern Gulf of Evia. In the Aegean Domain,
active deformation is locally dominated by strike-slip tectonics along
the North Anatolian Fault (Figure~\ref{fig1}) and its westward
extension into the northern Aegean Sea
\citep{Hatzfeldetal1999,Jackson1994,SakellariouTsampouraki-Kraounaki2019}.

Among the extensional basins in the studied region, the Reginio Basin
(or Lokris Basin) is a narrow uplifted and emerged basin located
between the Kallidromon ridge and the North Evia Gulf
(Figure~\ref{fig3}A). It is one of the oldest basins associated to the
post-orogenic extension in this domain as it developed from the latest
Miocene to early Pliocene times
\citep{AndrePavlopoulos2004,CeletDelcourt1960,IoakimRondoyanni1988,Kranis2007}. 
Its sedimentary filling is mainly composed of lacustrine and fluvial
deposits \citep{DermitzakisPapanikolaou1981,Kranis2007} that are
controlled by the development of large normal fault systems on both
sides: the Kallidromon--Atalanti Fault to the south (KAF,
Figure~\ref{fig3}) and the Kamena Vourla--Arkitsa Fault System to the
north (KVAFS, Figure~\ref{fig3}). Both fault systems express an
Uppermost Miocene to Early Pliocene extensional regime along the
NNE--SSW direction  \citep{Caroiretal0000,Kranis2007}.

Further west, the Sperchios Basin forms an asymmetric half-graben
controlled by north-dipping normal faults belonging to the Sperchios
Fault System \citep{Kiliasetal2008} (Figure~\ref{fig3}A). These major
normal faults on the southern border of the Sperchios Basin also
coincide with the FTIZ, between the Parnassos external units to the
south and the internal units to the north
\citep{Apostolopoulos2005,Caroiretal2021,Caroiretal0000,Celet1962,Chanieretal2017,ElietGawthorpe1995,Ferriere1982,Kiliasetal2008,Marinosetal1967}.
Extension probably initiated during the Pliocene but the basin is
mainly filled by Pleistocene and Holocene sediments
\citep{Apostolopoulos2005,Pechlivanidouetal2014,Pechlivanidouetal2018}. The
Sperchios Basin connects to the North Evia Gulf toward the east, which
also corresponds to a major rift system characterised by a succession
of Pliocene and Quaternary subbasins that subsided during the Pliocene
and the Quaternary \citep{Caroiretal2021,Caroiretal0000,Chanieretal2021,Sakellariouetal2007}.

North of the Sperchios rift, the FTIZ corresponds to the western
boundary of ophiolitic units detached from their Triassic--Jurassic
Internal Zones basement during the Tertiary compressional episodes
\citep[e.g.,][]{Aubouin1959,Ferriere1982,Ferriereetal2024}. This is
particularly the case in the northernmost Hellenic domain, where
important ophiolitic units (N-Pindos ophiolites) are present over the
Pindos Eocene flysch (Figure~\ref{fig2}B). Further south, from the
Koziakas area and southeastward, these units are present at much lower
elevation, below the Trikala plain (Figure~\ref{fig2}B). 

South of the Sperchios rift, the allochthonous units belonging to the
Internal Zones are represented by Pelagonian units, locally covered by
ophiolites. These units are thrust over the Beotian and Parnassos units
from the External Zones [Figures~\ref{fig3}A and~B;
\citealp{Celet1962,Celetetal1976,Nirtaetal2018,Papastamatiouetal1962}].


At the scale of the Hellenides, the FTIZ is well recognised as the
crustal thrust of the Pelagonian Domain over the EZ 
\citep[e.g.,][and references therein]{Ferriereetal2024}. 
It was reactivated during the
Miocene exhumation of the Cycladic metamorphic core complexes
\citep[][and references therein]{Jolivetetal2010b,Jolivetetal2021}.
Such an extensional reactivation occurred as early as ca.\ 35~Ma, during
early exhumation, as evidenced in the Sporades Islands 
\citep{Porkolabetal2020}. However, few studies investigated in \mbox{detail}
the Plio-Quaternary reactivation of major thrust faults and their
relationship with recent extensional features. In the literature
\citep{Kranis2002,Kranis2007,KranisandPapanikolaou2001}, extensional
structures associated with the reactivation as a detachment fault of
the main thrust at the base of the Pelagonian nappe were evidenced near
Amphiklia (Figure~\ref{fig4}A). 

\begin{figure*}[p]
\includegraphics{fig04}
\caption{\label{fig4}(A)~Schematic geological map of the
Amphiklia area. Purple lines 1, 2 and 3: location of cross-sections 
on (B); Tr--Jur: Triassic--Jurassic; Cret: Cretaceous.
(B)~Cross-sections 1 to 3 in the Amphiklia area located on
(A). 1: Quaternary (Q); 2: Neogene-Quaternary (N.Q.); 3 to 5:
Parnassos zone; 3: Flysch Paleocene--Eocene; 4: Cretaceous (K) and
Jurassic (J) massive limestones; 5: Triassic (T) dolomitic limestones;
6 to 8, Pelagonian zone, 6: (K) Cretaceous limestones; 7: Jurassic
limestones locally with ophiolites (v: section 1); 8: Triassic (T)
limestones; 9: arrows with (1): Tertiary Thrust SW verging; 10: arrows
with (2): tectonic inversion on the previous thrust during recent
extensional deformation. Abbr. T: Triassic; Jur: Jurassic, K:
Cretaceous; Pg: Pelagonian; MPD and FTIZ: Main Pelagonian Detachment
and Frontal Thrust of the Internal Zones.}
\end{figure*}

{\vspace*{-2pt}}

\section{Geometry and kinematics of the FTIZ/MPD at the boundary
between Pelagonian and Parnassos units}\label{sec3}

{\vspace*{-2pt}}

In both the Dio Vouna--Oiti and Amphiklia
areas, the observed tectonic pattern shows the superposition of
Pelagonian nappes (IZ) over the units of the Parnassos zone (EZ) with
locally the occurrence of some Beotian thrust sheets (EZ) in between
(Figures~\ref{fig3}B and~\ref{fig4}A). The main lineament between EZ and IZ (FTIZ) is
overprinted by several extensional faults and the structural geometries
along this inverted contact are different in the Amphiklia and Dio
Vouna--Oiti areas, as described below.

{\vspace*{-2pt}}

\subsection{The Amphiklia area}\label{sec31}

{\vspace*{-2pt}}

The structural analysis of the Amphiklia area is based on the
construction of three cross-sections to document the present
relationships between the Pelagonian and the Parnassos units, and the
southwestern boundary of the Kifissos Basin (Figure~\ref{fig4}). These
cross-sections are constrained by geological maps and field
investigations. To our knowledge, neither deep boreholes nor seismic
data exist to constrain the subsurface structures.

Near Amphiklia locality (Figures~\ref{fig4}A, B-2), the
Triassic--Jurassic limestones of the Pelagonian nappe are thrust over
the Eocene flysch, lying itself over the Parnassos limestones. In map
view, the major boundary between the two units corresponds to the SW
border of the recent Kifissos Basin (Figure~\ref{fig4}A). Close to the
Amphiklia locality (Figure~\ref{fig4}A), some klippes of Pelagonian
series forming the high-elevated landforms
\citep{Celet1962,Papastamatiouetal1962} are still connected to the main
Pelagonian unit on the SW border of the Kifissos Basin. West of
Amphiklia, in the Jerolekas unit (Figure~\ref{fig4}B-1), the Pelagonian
Cretaceous series directly overlie the Eocene flysch of the Parnassos
unit. 

{\advance\baselineskip by -.05pt
At the transition from the Parnassos mountains and the Kifissos plain,
the Parnassos series and the overlying tectonic contact with the
Pelagonian unit (Figure~\ref{fig4}B) are affected by a large bulge, and
locally by a significant NE-facing flexure parallel to the Kifissos
basin (Figures~\ref{fig4}A, B-1 and B-2). Consequently, the basal
thrust of the Pelagonian klippes was slightly steepened on the SW
border of the Kifissos Basin, particularly SE of Gravia and SE of
Amphiklia  (Figures~\ref{fig4}A and~B).

On the SW border of the Kifissos Basin, i.e., SE of Amphiklia
(Figures~\ref{fig4}A and B-3), the Parnassos unit forms a wide
anticline (Panayas anticline), whose NE--SW-directed axis is not
parallel to the Kifissos Basin and cannot therefore be associated to
the flexural deformation described above. The deformations of these
Parnassos levels along the SW boundary of the Kifissos basin show that
the Panayas anticline is older than the flexural event
(Figure~\ref{fig4}A). Moreover, some high-angle NW--SE-directed normal
faults also cross-cut the SW border of the Kifissos Basin as it can be
seen SE of Amphiklia  (Figures~\ref{fig4}B and~C). 

In the area of Agia Marina
(Figure~\ref{fig4}A), there are no preserved Pelagonian klippes over
the Parnassos series. A major normal fault offsets downward the
Pelagonian allochthonous units relative to the Parnassos series. Unlike
the Amphiklia sector, this steeply dipping normal fault does not
coincide in its location with the major thrust contact between
Pelagonian and Parnassos series (Figures~\ref{fig4}A \unskip\break and B-3).

In summary, from our mapping and structural analysis in the Amphiklia
area, we show that the extensional reactivation of the FTIZ is
localised on the low-dipping thrust thereby resulting in a major
basin-bounding low-angle normal fault, i.e., the Main Pelagonian
Detachment. This tectonic pattern is well expressed along the Gravia
and Amphiklia segments (Figure~\ref{fig4}A). The reactivation did not
generate any modifications on the initial superposition of the internal
units over the External Zones.
}

{\vspace*{-2pt}}

\subsection{The Dio Vouna area}\label{sec32} 

{\vspace*{-2pt}}

In the Oiti mountains (Figures~\ref{fig2} and~\ref{fig5}), the basal
contact of the IZ klippes upon the Parnassos units (FTIZ) is exposed at
a high elevation (ca.\ 1000 to 2000~m-high, Figure~\ref{fig5}B), while the
crustal-rooted thrust in front of the Internal Zones is exposed next to
Dio Vouna locality at an elevation of only 500~m. In this area of
Frantzi--Dio Vouna (Figure~\ref{fig5}A), the tectonic contact between
the EZ and IZ units displays the most recent series from the IZ
(Cretaceous or Late Jurassic in age) over the older series of the
Parnassos domain, Jurassic in age (Figures~\ref{fig5}A and~B).  The
allochthonous units are mostly typical Pelagonian series, but some
Cretaceous formations could locally belong to Beotian sedimentary
units, forming tectonic slices transported at the base of the FTIZ
\citep{Nirtaetal2018}. The tectonic contact between the EZ and IZ units
is sub-horizontal at the base of the klippes and roots northeastward
below the IZ (and the overlying Neogene-Quaternary basins) along a
low-angle surface, which is generally dipping about 30\textdegree NE
(Figures~\ref{fig5}B and~\mbox{\ref{fig6}A--D).}\looseness=1


{\def\ffigcaption{\label{fig5} Continued on next page.}
\xsubfig[p]{05}{}
\figcnt
\begin{figure*}[t]
\xincludegraphics{fig05a}
\xcaption{\label{fig5a}\ffigcaption}{(A)~Schematic geological map of the 
Dio Vouna--Oiti
area (location on Figure~\ref{fig2}B). 1,~2 and 3: location of cross-sections
from~B. Abbr: J: Jurassic; K: Cretaceous; N.Q.:
Neogene-Quaternary; N: North; S: South. Tectonic units: Pa: Parnassos;
Pg: Pelagonian. (B)~Cross-sections 1, 2 and 3 from the Dio
Vouna--Oiti area (location on A). 1:~massive limestones (K:
Cretaceous, J:~Jurassic) with bauxites (bx) from Parnassos zone (Pa);
2:~Early Tertiary flysch from Parnassos zone; 3:~Early Tertiary flysch,
Late Cretaceous pelagic limestones and detrital Cretaceous formations
from Beotian unit; 4:~Jurassic limestones~(Pelagonian Zone, Pg); 5:
m\'{e}lange with blocks and ophiolitic units (Pg); 6: Late Cretaceous
limestones and detrital terrigenous beds (Pg); 7: Quaternary (Q); 8:
Tertiary Thrust SW-verging; 9: negative tectonic inversion on the
pre-existing thrust.}
{\vspace*{-2pt}}
\end{figure*}}

\setcounter{figure}{5}

\begin{figure*}[p]
\includegraphics{fig06}
\caption{\label{fig6}Normal faulting from the Dio Vouna area. (A)~Map
extract of the Dio Vouna area with the location of photographs from
this (B, C, E, F, H). (B)~General view from the~south towards
the low-angle MPD in the Dio Vouna area. (C)~Closer view of the MPD
dipping 30\textdegree\ towards Dio Vouna locality. (D)~Stereoplot
(Schmidt, lower hemisphere) showing faults and striations along the MPD
in the Dio Vouna area. Solid lines are projections of fault planes with
slicken-slide lineations as dots with arrows (outward for normal
motion). (E)~Downward view on a Late Quaternary normal fault near
Frantzi with the Pelagonian zone on the hanging wall. (F)~Closer view
on the fault surface. (G)~Stereoplot (Schmidt, lower hemisphere)
showing faults and striations of the fault zone in Kostalexis--Frantzi
area. (H)~Normal fault west of Dio Vouna, and related stereoplot. (I)
Next to Ano Vardates village.}
\end{figure*}

In the klippe area on top of the Parnassos mountains (i.e., the Oiti
massif; Figures~\ref{fig5}A and~B), the Pelagonian thrust sheets lay
sub-horizontally onto the Eocene flysch. The klippes show some complex
internal structures with multiple repetitions of some stratigraphic
units, which appear locally inverted (Figure~\ref{fig5}B-3). 

In contrast, along the NE-dipping rooting zone of the FTIZ 
(Dio Vouna--Frantzi--Kostalexis 
area on Figure~\ref{fig5}A) stratigraphic
surfaces on both sides of the main tectonic contact are not parallel to
it. The bedding planes are dipping about 20\textdegree SW in the
Parnassos limestones and are almost horizontal next to the fault
surface within the internal units (Figure~\ref{fig5}B). The mean
direction of the MPD is about N150\textdegree E with striations on the
surface indicating a general top to the NE normal slip
(Figures~\ref{fig5}A and~\ref{fig6}A--D). Here, the overall direction
is slightly different from the other main fault contacts between the IZ
and EZ, mainly N125\textdegree E to N135\textdegree E, for instance
in the Amphiklia region (Figures~\ref{fig3}A and~\ref{fig4}A). However, this
moderate change of direction is not significant at the scale of the
Hellenic range and has no impact in the style of deformation that
depends at first order on the changes in dip\break angles.

In the Dio Vouna area, the most recent normal faults, such as those
exposed in the Ano-Vardates area, are E--W directed with striations
indicating a N--S extension (Figures~\ref{fig6}E--I). The \mbox{variations}
in the strike of the fault surfaces and in the directions of movement
are likely to represent the complex accommodation of the evolving
Aegean extension, from NE--SW extension during the Pliocene and early
Quaternary period to N--S extension during the Late Quaternary, by a
non-cylindrical initial shape of the FTIZ thrust \mbox{complex.}\looseness=1

\subsection{The inverted FTIZ in central Greece: the main Pelagonian
detachment}\label{sec33}

Field mapping and structural data from the southern side of the
Sperchios Rift suggest that the Late Miocene to Quaternary extensional
reactivation of the FTIZ gave rise to a new extensional structure: the
Main Pelagonian Detachment (MPD). 

From a geomechanical point of view, inherited crustal thrust zones,
such as the FTIZ, are largely documented as weak zones likely to be
reactivated in an extensional setting, provided they display
substantial dip and relative low strength
\citep{Ivinsetal1990,Lecomteetal2011,LePourhietetal2004,LePourhietetal2006,Mattionietal2006}.
Considering the classical flat--ramp--flat geometry of thrust structures
within the crust, extensional reactivation rarely affects the entire
thrust surface. The footwall ramp, that exhibits the highest dip along
the thrust, is a preferential site for reactivation 
\citep[e.g.,][]{Averbuchetal1992,MohapatraJohnson1998,Ouzgai2010,PowellWilliams1989,SmithBruhn1984,Tarietal2023,Williamsetal1989}. 
These mechanical constraints lead to post- or late orogenic surface
normal faults short-cutting the footwall flats and rooting down in the
thrust ramps at depth
\citep{DAgostinoetal1998,Legrandetal1991,Minguelyetal2010,MohapatraJohnson1998,Roureetal1994,SteinBlundell1990,Tarietal2023,Tavarnelli1999}. 
Hence, exhumation of the deep geometry of the reactivating fault system
(the initial thrust ramps) may lead to the observation of anomalously
low angle normal faults \citep[e.g.,][]{Ratcliffeetal1986,Morley2009}
like in the Corinth Gulf region
\citep{Flotteetal2005,Jolivetetal2010a,Lecomteetal2012,PapanikolaouRoyden2007,Papanikolaouetal2009,Sorel2000}.
\looseness=-1

In the studied region, the extensional reactivation of the initial
crustal-scale thrust (FTIZ) developed in two types of configurations,
in two separate areas (Amphiklia and Dio Vouna areas).

\subsubsection{Amphiklia area}\label{sec331}
In the Amphiklia area, the reactivation of the Tertiary Pelagonian
thrust (FTIZ) was previously considered as a ``detachment fault'' from
the analysis of the thrust surface nearby this locality
\citep{KranisandPapanikolaou2001}. Thus, from Gravia to the SE of
Amphiklia (Figure~\ref{fig4}), the observed ``negative tectonic
inversion'' structure shows the following characteristics. Firstly, the
basal contact of the Pelagonian nappe is always over the Eocene
Parnassos flysch, which corresponds to the most recent stratigraphic
series from the underlying unit. Secondly, the series of the Pelagonian
nappe overriding this Eocene flysch are among the oldest series of the
Pelagonian platform, Triassic or Jurassic in age (SW Amphiklia), or
Cretaceous in some klippes further west (e.g., Mount Jerolekas, south
of Gravia locality, Figure~\ref{fig4}). Such a geometry implies an upper
flat configuration for this particular FTIZ segment.


{\advance\baselineskip by -.33pt
Our interpretation of this configuration is that the inversion of the
FTIZ gave rise to the MPD where the upper flat of the FTIZ displays the
highest dip. This increase in dip could be due to an anticline flexure
of the footwall (Parnassos border) and hanging wall of the thrust
(Figures~\ref{fig7}A and~C), subparallel to the Kifissos basin and to
the general orogenic strike in the area. The extensional reactivation
of such a thrust upper flat is relatively uncommon because reactivation
generally requires substantial dips to be reactivated  
\citep[e.g.,][]{LePourhietetal2004,LePourhietetal2006,Mattionietal2006,Ouzgai2010,Lecomteetal2011,Lecomteetal2012}.
However, the local tilt of the thrust associated with this flexure is
likely to form an initial instability prone to control the inversion
process. 

\begin{figure*}[p]
\includegraphics{fig07}
\caption{\label{fig7}Early Tertiary to Quaternary tectonic evolution of
the boundary between external (Parnassos) and internal zones
(Pelagonian): (A)~in Amphiklia area, and (B)~in Dio Vouna--Oiti
area. See text for detailed explanations. (C)~Sketch cross-section
illustrating the two different mechanisms giving rise to the negative
tectonic inversion in both areas. (1): Paleocene--Eocene compressional
phase; (2) Neogene-Quaternary extensional phase. Abbr. J: Jurassic; K:
Cretaceous; Q: Quaternary; Bt: Beotian zone; Pa:~Parnassos zone;
Pg: Pelagonian zone.}
\end{figure*}

Thus, the crustal rooting of the FTIZ in this specific area does not
occur in relation to a major ramp as usually observed in orogenic
systems, but is due to the backward flexure. Some secondary normal faults
(Figure~\ref{fig4}B-2) developed along the flank of the flexure in the
footwall layers. The offsets along these second-order normal faults
account for the geometry and the variability of the age of the
Parnassos series on the southwestern border of the Kifissos Basin
between Gravia, Amphiklia and Tithorea localities (Figure~\ref{fig4}A).
It is sometimes difficult to differentiate, within this sector, the
recent normal fault planes from the inverted thrust contact, which
displays also some extension markers. \looseness=-1

A common origin of the inversion of the FTIZ (MPD) and the flexure of
the footwall with associated normal faults during the same
Pliocene-Quaternary extensional tectonic event is conceivable. However,
as it probably has some direct control on the reactivation of the FTIZ,
it is more likely that the flexure appeared earlier, by the end of the
Tertiary compressional tectonic episode. In this hypothesis, some
locking of the motion along the main thrust surface (FTIZ) could have
triggered the development of incipient back thrusts of the Parnassos
series northeastwards (Figure~\ref{fig7}A---stage ``d''). Despite these
different second-order deformations, the tectonic contact between the
Pelagonian nappe and the Parnassos unit generally remains parallel to
the layers involved in the thrusting events in the Amphiklia area
(Figure~\ref{fig4}B).\looseness=-1
}

\subsubsection{Dio Vouna--Oiti area}\label{sec332}

Similarly to what was observed in the Amphiklia area, the Dio Vouna--Oiti 
structural pattern (Figure~\ref{fig5}) is characterised by (i), the
contact of the Pelagonian units over the Parnassos series, with local
slivers of Beotian series, and (ii), the inversion of the structural
topography acquired during the orogenic development, resulting in the
hanging wall Pelagonian units to be at a much lower elevation than the
underthrust Parnassos unit. 

However, the tectonic inversion pattern observed in the Dio Vouna--Oiti
area differs from that of the Amphiklia area, on several important
points (Figure~\ref{fig7}B). Firstly, the major tectonic contact
involved in the negative tectonic inversion is a low-angle surface,
dipping 30\textdegree~or less, which is significantly less than the
usual dips of most normal faults within the entire Central Greece
region. Secondly, the series at the footwall and hanging wall of the
FTIZ are truncated by the tectonic contact, so that the Jurassic
Parnassos series and the Jurassic--Cretaceous Pelagonian units are often
dipping opposite to the contact. Thirdly, the Pelagonian klippes
exposed west of the main MPD in the Oiti mountains are affected by a
complex deformation (tight folds, reverse series), much different from
the regular stratal dips observed within the klippes near Amphiklia.

These observations lead us to consider that, in the Dio Vouna area, the
inverted FTIZ here defined as the MPD corresponds to a large normal
fault zone dissecting the upper flat of the FTIZ and reactivating at
depth the thrust along a major footwall ramp (Figures~\ref{fig7}B and~C). 
This ramp, striking approximately N150\textdegree E, is almost
perpendicular to the main Tertiary thrust movement of the IZ unit, and
is dipping at about 30\textdegree, that is a classical dip for such
type of structure. The peculiar geometry of the Pelagonian strata on
the FTIZ hanging wall implies that these series were tilted after the
thrust activation, i.e., during the negative tectonic inversion of the
thrust and the coeval MPD development. Such cut-off relationships along
the surface clearly indicate a roll-over type of folding associated
with the normal fault displacement along the MPD (Figure~\ref{fig5}B). 

The complex internal structures in the klippes overriding the thick
Parnassos platform series could be possibly the result of lithological
weaknesses and particularly high basal shear conditions along the ramp
and the upper flat of the FTIZ during nappes emplacements in this area.

In summary, the Dio Vouna--Oiti structural geometry is consistent with
the classical views suggesting that, due to their weakness and optimal
dip, the footwall ramps are the preferential zones to localise
\mbox{extensional} reactivation along pre-existing thrusts 
\citep[e.g.,][]{Averbuchetal1992,MohapatraJohnson1998,Minguelyetal2010,Ouzgai2010,PowellWilliams1989,SmithBruhn1984,Williamsetal1989}. 

In the Amphiklia area, where the upper flat of the thrust has been
inverted, the structural pattern is different. It has been driven by
the late tilting of the thrust upper flat in response to the
development of an anticline that folded the FTIZ thrust structure after
its original emplacement (Figure~\ref{fig7}A, Stage ``d''). 

There is no clear evidence for the precise location of a major ramp of
the FTIZ at depth across the Amphiklia cross-section. As shown in
Figure~\ref{fig7}A, the major normal fault zone bounding to the SW 
the large Pliocene Reginio basin (i.e., the Kallidromon Fault) could be
the superficial expression of such a deeply seated ramp of the FTIZ,
reactivated with normal motion during the Late Neogene.


\section{Integration within the Hellenides--Aegean domain}\label{sec4}

\subsection{The MPD: a major detachment at the scale of the Hellenides?}\label{sec41}

North of the Sperchios rift basin (Figures~\ref{fig8}A and~B), as there
is no more exposure of the Parnassos units, the FTIZ corresponds to the
contact surface between the Pindos--Beotian external zones and the
ophiolitic units \citep[e.g.,][]{Ferriereetal2012}.
Immediately below the main thrust fault (FTIZ), the Pindos and Beotian
zones are represented by the Eocene flysch that corresponds to their
younger series. 

\begin{figure*}[t]
{\vspace*{3pt}}
\includegraphics{fig08}
\caption{\label{fig8}A, B and C: Cross-sections from north to south
across the Hellenides. Location on Figure~\ref{fig2}B. (A)~From the Ionian and
Pindos external zones to the Olympus window and Vardar Domain (Internal
Zones). (B)~From External Zones bounded by the Koziakas range, the
Trikala and Larissa recent Basins, the Ossa window and up to the
Thermaikos Gulf. (C)~Cross-section further south, exposing the
Parnassos zone against the Pelagonian zone, up to North Evia Gulf and
the Aegean Sea.}
{\vspace*{4pt}}
\end{figure*}


The FTIZ locally includes several IZ klippes preserved over the Pindos
zone, but the main thrust corresponding to the rooting of the FTIZ is
relatively linear between internal and external zones
(Figures~\ref{fig2}A and B). This main deep-seated thrust, locally very
steeply dipping and reactivated in extension, is associated with the
collapse of the internal zones with respect to the external zones.
Accordingly, this thrust can be considered as the prolongation of the
MPD that we define further south. The amount of extensional
reactivation on this NW segment of the MPD seems much reduced compared
to the Dio Vouna--Amphiklia segment, but it can be easily explained
by the fact that the total amount of extensional \mbox{deformation} of the
Aegean domain vanishes significantly westwards (NW of the Sperchios
Basin, see Figure~\ref{fig1}).

In this NW area, north of the Sperchios Basin, two different segments
of the FTIZ show various overall geometries:
\begin{itemize}
\item North of the Kastaniotikos transverse structure
(Figures~\ref{fig2}B and~\ref{fig8}A), important ophiolitic units (e.g., ``North
Pindos nappe'') are preserved west of the relatively linear and
continuous MPD. There, the internal zones appear at a much lower
elevation than the North Pindos nappe that is over the external zones.
The MPD also marks the steeply dipping western boundary of the
Oligo-Miocene Mesohellenic Basin (MHB, Figures~\ref{fig2}B 
and~\ref{fig8}A) that developed originally as a large piggy-back basin
\citep{Ferriereetal2004,Ferriereetal2013}.

\item Between the Kastaniokos transverse structures to the north and
the Sperchios Basin to the south, no main ophiolitic klippes are
preserved over the external zones. In western Othris
(Figure~\ref{fig2}B), the FTIZ is very steep and corresponds to the
western termination of the ophiolitic front. It is there considered as
the MPD as it coincides with the downthrow of the internal zones (to
the east) relative to the external zones exposed on the western side of
the FTIZ. Along this segment, between western Othris and Kastaniotikos
areas, the ophiolitic units are covering the Triassic--Jurassic Koziakas
series at high elevation, the basal contact of the ophiolitic nappe
appearing at 600 m-high (Figure~\ref{fig8}B). On the eastern side of the
MPD, this upper ophiolitic basal contact appears at a much lower
elevation below the Trikala plain. Such an important offset from both
sides of the MPD sub-vertical lineament suggests some significant normal
motion between the Koziakas ophiolitic units and the Trikala plain
(Figure~\ref{fig8}B). 
\end{itemize}

Despite some differences in geometry and amplitude of extension with
the Amphiklia-Dio Vouna area, the flat--ramp--flat thrust model applied
to the northern Hellenides (north of the Sperchios rift) allows us to
propose that the MPD developed on the main ramp of the thrust system
and that the upper flat of the FTIZ is mainly appearing in the
northernmost Hellenides as the N-Pindos ophiolitic nappes. 

South of the Amphiklia area (Central Greece), where the Corinth Rift
cross-cuts the Hellenic structural pattern, the FTIZ is much less
defined and its lateral extension is unclear. Further south, in the
Peloponnese region, the IZ are only represented in the Argolis area,
while the rest is constituted by a complex structural pattern of
external zones, including many detachments intra-EZ \citep[e.g., Itea--Amfissa
detachment;][]{Papanikolaouetal2009,Papanikolaou2021}.
Some deep subsiding basins developed also along this boundary zone
between EZ and IZ such as the Argolic Basin, south of Argolis Peninsula
(Figure~\ref{fig1}). This elongated basin, running parallel to the EZ-IZ
boundary (FTIZ), on top of the internal zones could be very likely the
surface expression of the overall collapse of the IZ relative to the EZ
in this area. This large basement offset along the FTIZ on the eastern
coast of southern Peloponnese (Figure~\ref{fig1}), with a major collapse
of the internal zones, is supporting the reactivation of the FTIZ as
the MPD in this area also. 

In summary, despite variations in amplitude and style of deformation
due to the lateral crustal heterogeneities of the external zones (e.g.,
occurrence or absence of the thick Parnassos series), the MPD shows:
(i) a similar tectonic and geomorphologic pattern along-strike, north
and south of the Sperchios basin (Figure~\ref{fig2}), and (ii) that
this structural development resulted from negative tectonic inversion
of the major thrust zone between Internal and External \unskip\break domains. 


\subsection{The MPD versus other detachments in the Hellenides and
Aegean Sea}\label{sec42}

Many large low-dipping detachments linked to Cenozoic extensions have
been described in the Hellenides and the Aegean Sea 
\citep[see][Figure~11.6]{Papanikolaou2021}.

These detachments notably known in the Rhodope, the Aegean region and
the Peloponnese, affect the EZ and/or the IZ. However, no large
detachments have been listed in the northern continental Hellenides
beyond the Sperchios rift.

We consider that the FTIZ--MPD exposed between the EZ and IZ in the
northern Hellenides represents the lateral continuation of the large
detachment described here in the Dio Vouna--Amphiklia area as: (i)~the
vertical shifts of the Hellenic zones along the MPD in this sector are
similar in nature (collapse of the IZ relative to the EZ;
Figure~\ref{fig2}B); (ii) the major late Neogene to Quaternary
sedimentary basins are widely located above the MPD within the IZ and
northeastward of the MPD.

Within the Hellenides, most detachments affect either a single zone
\citep[i.e., the Parnassos zone for the Itea--Amfissa 
detachment;][]{Papanikolaouetal2009} 
or several zones within the IZ or the EZ
such as in the Peloponnese or in the Aegean Sea
\citep{Papanikolaou2021}. The normal motion along the MPD is not
necessarily larger than other major detachments in the Aegean region,
but the main particularity of the FTIZ--MPD is that it corresponds to
the reactivation of a very important crustal thrust at the scale of the
Hellenides. The initial geometry of this thrust system (flat--ramp
geometry at crustal scale) has significative consequences on the
extensional pattern of the upper crust during the negative inversion.

\subsection{The FTIZ/MPD and the Neogene basins within the Hellenides} \label{sec43}

The main recent basins of the continental Hellenides, late Miocene to
Quaternary in age, mainly appear over the internal Zones. The E--W
directed Sperchios Basin prolongates slightly westwards within the EZ
but is especially developed in the IZ, while the Trikala and Larissa
basins to the north, and the Kifissos and Reginio basins to the south
are exclusively over the IZ. Moreover, it is clear that the western
boundary of the IZ (the FTIZ/MDP) coincides with the western boundaries
of the Trikala and Kifissos basins. 

The EZ, and specifically the inner EZ, show a thick crust, exceeding 40
km along the Pindos range \citep{Makrisetal2013}. Such a crustal
thickness is related to the important stack of EZ tectonic units during
the Tertiary collision. In contrast, the IZ present a much thinner
crust (approx. 34 to 28 km-thick), gradually thinning further east
towards the Aegean Sea where the crust reaches 22 to 18 km-thick
approximately \citep{Makrisetal2001,Makrisetal2013}. These IZ have a
much more complex structural pattern as they underwent polyphase
development since late Jurassic times, including crustal thickening
with obduction of the Maliac Ocean and underthrusting of EZ such as the
Gavrovo and Pindos zones \citep{Ferriereetal2012,Ferriereetal2015,Ferriereetal2016}. The
existence of underthrust units from the EZ below the Pelagonian crust
is clearly demonstrated by their occurrence at exposure in the Olympus
and Ossa windows (see Figure~\ref{fig8}B). We have therefore to consider that
these IZ, with overthrusted Maliac oceanic crust and underthrust EZ,
had a much thicker crust by the end of the orogenic processes of the
Hellenides belt, before the Tertiary post-orogenic thinning.

Even if the post-orogenic thinning of the Aegean crust is essentially
related to the slab retreat beneath the Aegean Sea, corresponding to
the main driving mechanism for extension 
\citep[e.g.,][]{Jolivetetal2013}, the inception of crustal thinning in
continental Greece appears to be at least partly localised along the
boundary between EZ and IZ, i.e., along the MPD. The development of the
Neogene extensional sedimentary basins, preferentially on top of the
IZ, represents the surface manifestation of crustal thinning in this
part of the Hellenic-Aegean domain. Consequently, we suggest that most
of the major normal faults bordering all these basins are very likely
connected to the MPD surface at depth (Figure~\ref{fig8}). 

\section{Conclusions}\label{sec5}

Our structural analyses in central continental Greece led us to
characterise the tectonic boundary between Internal and External Zones
in the Hellenides mountain belt. Two main geomorphological domains form
the Hellenides in central continental Greece: (1) the western domain,
composed of a high relief fold-and-thrust-belt settled over a
relatively thick crust, and (2), the eastern domain, composed of
alternating elongated ca.\ NW--SE basins and ranges settled over a
thinner crust. The transition between these two domains is not gradual
but localised along a major tectonic structure, the Frontal Thrust of
Internal Zones (FTIZ). The FTIZ corresponds to the thrust contact of
the Pelagonian zone (IZ) over the Pindos and Parnassos zones (EZ). We
describe the structural development along this major tectonic boundary
reactivated as an important detachment during negative tectonic
inversion, and refer to it as the ``Main Pelagonian Detachment'' (MPD).  

In Central Greece and northern Hellenides, the MPD forms a rather
straight and continuous fault zone, with a N130\textdegree E average
direction. It often coincides with the western boundaries of some of
the main Plio-Quaternary basins, such as the Trikala and the Kifissos
Basins.

Structural analysis based on new field mapping, regional
cross-sections, and fault-slip data, allows to characterise the
along-strike geometrical variations of the MPD and to describe the
relationships between this inverted tectonic structure and the
collapsed eastern domain hosting extensional basins. We show that the
diversity of crustal structures that developed in relation to the MPD
development is significantly controlled by the initial geometry of the
FTIZ, i.e., the initial crustal thrust.

In two key areas of central Greece, we document two types of structural
pattern triggered by the development of negative tectonic inversion
along the FTIZ/MPD. 

Firstly, in the Amphiklia area, the oldest series of the Pelagonian
nappe (IZ) are thrust over the youngest formations of Parnassos Zone
(EZ). The FTIZ appears parallel to bedding from both the hanging wall
and the footwall. Reframed in a classical flat--ramp--flat geometry of
crustal thrust emplacement, this configuration suggests that the
structure observed in the Amphiklia region corresponds to an upper flat
tectonic setting subsequently inverted. Its present-day geometry is the
result of extensional reactivation of the gently NE-dipping upper flat
of the FTIZ, presumably tilted in response to backfolding or
backthrusting by the end of the Paleogene compressional episode. 

Secondly, in the Dio Vouna area, the youngest series of the Pelagonian
nappe are adjacent to old Parnassos formations, and the series are
truncated by the major fault responsible for the downward shift of the
Pelagonian units. There, the contact appears as a low-angle normal
fault (${\sim}$30\textdegree~dip) and its direction (N150\textdegree E) 
is slightly different from the average direction of the MPD
throughout the Hellenides (N130\textdegree E). According to the
classical flat--ramp--flat geometry of thrust systems, this configuration
suggests that the main thrust ramp was directly reactivated during
the negative inversion. 

From the analysis of Amphiklia and Dio Vouna areas, we propose that the
thrust system initiated during the early Tertiary collision had
initially a ``flat--ramp'' geometry, that controlled the structural
development during the Late Miocene to Quaternary inversion and the
development of Neogene basins. At the scale of the Hellenides, the
locations and orientations of the Neogene basins highlight the
importance of the MPD in the upper crust deformation of the Internal
Zones. As most of the Neogene extensional basins developed in the IZ
above the MPD, it appears that most of the major normal faults
bordering these basins are likely connected at depth to the MPD.
Consequently, we propose that most of the major normal faults
responsible for the formation of the recent basins, mainly late Miocene
to Quaternary in age, branch onto the MPD, i.e., the former FTIZ, at
the base of the Pelagonian crustal unit.


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

\section*{Funding} 

This work, and especially fundings for field work, was supported by the
Tellus Program of CNRS-INSU (SYSTER action) and by the research team
``TIMES'' from the \textit{Laboratoire d'Oc\'{e}anologie et
Geosciences} (LOG). 

\section*{Acknowledgements} 

We thank Monique Gentric for her great efficiency in administrative
support in the organisation of field work. We also warmly thank the
reviewers Dimitrios Papanikolaou and Olivier Fabbri who provided
constructive comments and suggestions that greatly helped to improve
the earlier drafts of the manuscript.

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