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\DOI{10.5802/crgeos.278}
\datereceived{2024-01-10}
\daterevised{2024-10-20}
\dateaccepted{2024-10-31}
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\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\shortrunauthors

\title{Fluvial terrace formation in a mountainous area (2): influence
of eustatism, tectonics and altitudinal distribution of watersheds 
based on an allostratigraphic study (Albania)}

\alttitle{Formation de terrasses fluviales dans une r\'{e}gion
montagneuse (2) : influence de l'eustatisme, de la tectonique et de la
distribution altitudinale des bassins versants sur la base d'une
\'{e}tude allostratigraphique (Albanie)}

\author{\firstname{Jean-Louis} \lastname{Mugnier}\CDRorcid{0000-0002-3403-7747}\IsCorresp}
\address{ISTerre, Universit\'{e} Grenoble Alpes, 
Universit\'{e} Savoie Mont Blanc, CNRS, IRD, Le Bourget du Lac, France}
\email[J.-L. Mugnier]{jemug@univ-smb.fr}

\author{\firstname{Oswaldo} \lastname{Guzm\'{a}n}\CDRorcid{0000-0002-7803-4513}}
\addressSameAs{1}{ISTerre, Universit\'{e} Grenoble Alpes, 
Universit\'{e} Savoie Mont Blanc, CNRS, IRD, Le Bourget du Lac, France}
\address{Grupo de Investigaci\'{o}n en Ciencias de La Tierra y Clima, 
Universidad Regional Amaz\'{o}nica Ikiam, Muyuna, 
Ecuador} 

\author{\firstname{Riccardo} \lastname{Vassallo}\CDRorcid{0000-0003-2785-8835}}
\addressSameAs{1}{ISTerre, Universit\'{e} Grenoble Alpes, 
Universit\'{e} Savoie Mont Blanc, CNRS, IRD, Le Bourget du Lac, France}

\author{\firstname{Kristina} \lastname{~Matraku}\CDRorcid{0000-0002-2280-8938}}
\addressSameAs{1}{ISTerre, Universit\'{e} Grenoble Alpes, 
Universit\'{e} Savoie Mont Blanc, CNRS, IRD, Le Bourget du Lac, France}
\address{Institute of Geosciences of the Polytechnical University of Tirana, 
Albania}

\author{\firstname{Fran\c{c}ois} \lastname{Jouanne}\CDRorcid{0000-0001-9374-7880}}
\addressSameAs{1}{ISTerre, Universit\'{e} Grenoble Alpes, 
Universit\'{e} Savoie Mont Blanc, CNRS, IRD, Le Bourget du Lac, France}

\keywords{\kwd{Strath}
\kwd{Fill-terraces}
\kwd{Strath terraces}
\kwd{Climatic and vegetation controls}
\kwd{Late glacial stage}
\kwd{Paraglacial}}

\altkeywords{\kwd{Strath}
\kwd{Terrasses de remplissage}
\kwd{Terrasses d'abrasion}
\kwd{Contr\^{o}les climatiques et par la v\'{e}g\'{e}tation}
\kwd{Dernier stade glaciaire}
\kwd{Paraglaciaire}}

\thanks{NATO SFP 977993, Science for Peace team,
Simon Bolivar University, IRD-DPF}

\begin{abstract} 
Terraces are highly developed along the Albanian rivers and eleven
levels of terraces are recognized in the area, nine occurring during
the last glacial cycle (MIS 5d to end of MIS 2). An allostratigraphy
study of the fluvial terraces shows a large variety of the geometry of
the sedimentary units beneath this set of terraces. This variety is
controlled by the respective amount of the three parameters: lateral
erosion, aggradation and difference between incision and aggradation.

Active faults offset the paleoriver profiles at throw rates locally
greater than 1 mm\textperiodcentered{yr}\textsuperscript{-1} and the tectonic
uplift influences the long term incision: Nested strath terraces or
nested filled terraces with substratum risers occur in zones of high
uplift rate (greater than 0.4 mm\textperiodcentered{yr}\textsuperscript{-1}),
superposed units and nested fill terraces with hidden substratum risers
occur in the weakly uplifted zones of the intra-mountainous graben-like
structures.

Most of the Albanian terraces are located above straths (nearly flat
erosional surfaces) linked to phases of lateral beveling. The thickness
of the sedimentary units above the strath surfaces is mostly influenced
by the altitudinal distribution of the river catchments: thin strath
terraces are found in the low elevation catchments, while thick fill
terraces are found in large catchments and we suggest here that the
deposition of fill terrace sediments occurred very rapidly at the
cold-warm transitions when the high elevation areas of the large
catchments were not protected by vegetation and heavily affected by
hillslope processes that delivered a large volume of sediments. The
thick Holocene valley fill, locally affected by fill-cut terraces,
extends several tens kilometer within the mountain valleys and is
probably linked to the mid-Holocene sea-level highstand.
\end{abstract}

\begin{altabstract} 
Les terrasses sont tr\`{e}s d\'{e}velopp\'{e}es le long des
rivi\`{e}res albanaises et onze niveaux de terrasses sont observ\'{e}s
dans la r\'{e}gion, neuf s'\'{e}tant form\'{e}s au cours du dernier
cycle glaciaire (MIS 5d \`{a} la fin du MIS 2). Une \'{e}tude
allostratigraphique de ces niveaux de terrasses montre une grande
vari\'{e}t\'{e} dans la g\'{e}om\'{e}trie des unit\'{e}s
s\'{e}dimentaires situ\'{e}es sous les surfaces des terrasses. Cette
vari\'{e}t\'{e} est contr\^{o}l\'{e}e par l'importance respective des
trois param\`{e}tres : l'\'{e}rosion lat\'{e}rale, l'aggradation et la
diff\'{e}rence entre l'incision et l'aggradation. 

Des failles actives d\'{e}calent les profils des pal\'{e}orivi\`{e}res
\`{a} des vitesses localement sup\'{e}rieures \`{a} 
1 mm\textperiodcentered{an}\textsuperscript{-1} et le soul\`{e}vement tectonique
influence l'incision \`{a} long terme : Des terrasses d'abrasion
imbriqu\'{e}es ou des terrasses de remplissage imbriqu\'{e}es avec des
escarpements de substratum visibles se produisent dans les zones de
fort taux de soul\`{e}vement (sup\'{e}rieur \`{a} 
0,4 mm\textperiodcentered{an}\textsuperscript{-1}), des unit\'{e}s
s\'{e}dimentaires superpos\'{e}es et des terrasses de remplissage
imbriqu\'{e}es avec des escarpements de substratum cach\'{e}s se
produisent dans les zones faiblement soulev\'{e}es des structures
intra-montagneuses de type graben. La plupart des terrasses albanaises
sont situ\'{e}es au-dessus de straths (surfaces d'abrasion presque
plates) li\'{e}s \`{a} des phases d'\'{e}rosion lat\'{e}rale.
L'\'{e}paisseur des unit\'{e}s s\'{e}dimentaires au-dessus des surfaces
de straths est principalement influenc\'{e}e par la distribution
altitudinale des bassins versants : les terrasses de straths minces se
trouvent dans les bassins versants de faible altitude, tandis que les
terrasses de remplissage \'{e}paisses se trouvent dans les grands
bassins versants et nous sugg\'{e}rons ici que le d\'{e}p\^{o}t des
s\'{e}diments des terrasses de remplissage s'est produit tr\`{e}s
rapidement lors des transitions froid-chaud lorsque les zones de haute
altitude des grands bassins versants n'\'{e}taient pas
prot\'{e}g\'{e}es par la v\'{e}g\'{e}tation et fortement affect\'{e}es
par des processus de pente. L'\'{e}pais remplissage holoc\`{e}ne des
vall\'{e}es, localement recoup\'{e} par des terrasses d'abrasion,
s'\'{e}tend sur plusieurs dizaines de kilom\`{e}tres \`{a}
l'int\'{e}rieur des vall\'{e}es montagneuses et est probablement
li\'{e} \`{a} l'\'{e}l\'{e}vation du niveau de la mer au milieu de
l'Holoc\`{e}ne.
\end{altabstract} 

\maketitle

\vspace*{18pt}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}
A huge body of published works  [e.g.\ bibliography in 
\citealp{Cordieretal2017}] suggests that the formation of river
terraces is affected by the climate. Numerous studies show that
terraces are not a simple climatic proxy 
\citep{Merrittsetal1994,Schanzetal2018,Pazzaglia2022} and tectonics,
eustatism and climate interact together in terrace formation 
\citep{Starkel1994,Vandenberghe2015}. Furthermore, the pathway between
climate and terrace development involves several steps: the climate
includes both temperature and precipitation, but also controls the type
of vegetation. All together, these factors control the sedimentary
supply to the river and the water flux 
\citep[e.g.][]{Vandenberghe2008}; these hydro-sedimentary conditions
control the balance between sediment supply and transport capacity 
\citep{Bull1991}. This balance is expressed through several
geomorphologic processes  [i.e.\ aggradation in the river channel,
deposition on the overbank during flooding, sediment reworking,
vertical incision and lateral erosion linked to lateral migration of
the channel;  \citealp{Schumm1977}] and lastly, the succession of these
processes controls the characteristics of the sediment and the geometry
of the sedimentary units and terrace landforms. 

The relative impact of these processes strongly varies from one zone to
another depending on various geomorphological parameters like basin
configuration and catchment size  \citep{Vandenberghe2015}. In the case
of the lowland rivers of northern Europe, where a very low uplift rate
usually leads to the superposition of units, it has been shown that
rivers generally respond simultaneously to climatic changes when the
latter are enhanced by vegetation degradation or growth 
\citep[e.g.][]{Vandenberghe2008}. In the case of mountain belts
affected by tectonics and uplift, the influence of morphological
parameters remains poorly understood. Amongst the numerous questions
that still need to be answered, we will particularly focus on the
following point: the effects of vegetation degradation or growth are
clearly found in lowland environments  \citep{Wohlfarthetal2008} and we
wonder if the same influence of vegetation could be inferred in the
case of nested terraces in a mountainous zone. 

Terrace levels deposited during the last glacial stage are widely
preserved along all the Albanian rivers and their morphology has been
widely \mbox{studied} 
\citep{Melo1961,Prifti1984,Lewinetal1991,Woodwardetal2008,Carcailletetal2009,Kocietal2018}.
Eleven levels of terraces in Albania, nine occurring during the last
glacial stage  (MIS 5d to end of MIS 2) were dated 
\citep{Woodwardetal2008,Carcailletetal2009,Guzmanetal2023}. Albanian
river catchments (Figure~\ref{fig1}a) 
are located in an area where
the large-scale controls  (climate, tectonics or eustatism) can be
considered as homogeneous: the climate is Mediterranean 
\citep{Ozenda1975}, the tectonics is controlled by the Adriatic
subduction beneath South Eastern Europe  \citep{Roureetal2004} and all
of the rivers have a base level linked to the Adriatic Sea 
\citep{LambeckChappell2001}. The catchment characteristics  (size,
altitude distribution) of Albanian rivers vary from ${\sim}$7,000 to
${\sim}$1,000~km$^2$ 
\mbox{(Figure~\ref{fig1}b)}, Hence, their terraces allow
to study how the catchment characteristics modulate the geomorphologic
fluvial response to climate fluctuations in the Mediterranean domain.

\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig01}
\vspace*{-4pt}
\bcaption{\label{fig1}}{0}{The rivers of Albania.
(a)~Altitudinal and neo-tectonic map of the Albania and North-western
Greece  [modified from  \citealp{Aliajetal1996} and 
\citealp{Carcailletetal2009}].  Red circles (a--h)  indicate location
of the sites where the long-term incision rate  has been estimated as
far as possible from the active faults  and in different tectonic
compartments:  (a)~$1~\mathrm{mm}{\cdot}\mathrm{yr}^{-1}$;
(b)~$0.5~\mathrm{mm}{\cdot}\mathrm{yr}^{-1}$; 
(c)~$1.1~\mathrm{mm}{\cdot}\mathrm{yr}^{-1}$; 
(d)~$0.8~\mathrm{mm}{\cdot}\mathrm{yr}^{-1}$; 
(e)~$0.8~\mathrm{mm}{\cdot}\mathrm{yr}^{-1}$; 
(f)~$1.1~\mathrm{mm}{\cdot}\mathrm{yr}^{-1}$  (see Supplementary
Information, Figure~1, Appendix). The overall horizontal component of
current tectonic deformation is represented by black arrows  
\citep[from][]{Jouanneetal2012}. Green  circles  (1) to  (12) indicate sites
where  paleoriver profiles are offset by faults  (see Supplementary
Information, Table~1 Appendix):
(1)~Gornja Klezna Thrust 
$(2.5~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$; (2)~Tirana Back Thrust 
$(0.4~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$;  (3)~Lushnje--Tepelen\"{e}
Thrust  $(0.7\ndash1~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$; 
(4)~Lushnje--Tepelen\"{e} Thrust 
$(0.2~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$;  (5)~Tomorrica Thrust 
$(0.4~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$;  (6)~Tomorrica Thrust 
$(0.3~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$;  (7)~Elbasan Normal Fault 
$({>}1~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$;  (8)~Bulcar Normal Fault
Devoll ($0.3~\mathrm{mm}{\cdot}\mathrm{yr}^{-1}$);
(9)~Kor\c{c}a Normal Fault 
$({>}0.8~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$;
(10)~West Ersek\protect\"e Normal Fault
(1.3~mm${\cdot}$yr$^{-1}$);
(11)~Konista Normal Fault
(${>}$0.4~mm${\cdot}$yr$^{-1}$);
(12)~Papingo fault
(0.6~mm${\cdot}$yr$^{-1}$).
(b)~Altitudinal distribution of the different catchments.  On the
$y$-axis is altitude (m), on the $x$-axis is cumulative area  (km$^2$).
The window on the top right is a zoom of the hypsometry of the high 
areas (see Supplementary Information, Appendix~2).}
{\vspace*{-4pt}}
\end{figure*}

\section{Concepts and methodology}\label{sec2} 

\subsection{Allostratigraphy and geometry of the units}\label{sec2.1} 
An allostratigraphic approach  \citep{Hughes2010} is used here to
analyze the sedimentary units and terrace landforms. Allostratigraphy
is the part of sequence stratigraphy adapted to continental sediments
and is one of the major unifying concepts of the geosciences
indispensable to large scale studies  \citep{McLaughlin2005} as it
provides a framework for understanding the evolution of depositional
systems through time. Allostratigraphy mainly focus on the
determination of the boundaries of sedimentary units whereas
lithostratigraphy and sedimentology focus on the characteristics of the
sediments. We consider that allostratigraphy is the most adapted method
to compare the numerous terraces levels found along the more than
350~km long rivers of Albania and we systematically use the
allostratigraphic approach.

In allostratigraphy, terrace surfaces, that are flat, gently dipping
surfaces, are one of the types of the upper boundary of
allostratigraphic units  (Figure~\ref{fig2}a). The lower boundary of
the allostratigraphic units is frequently an erosional surface  (strath
surface) that affects the substratum or the top of a lower
allostratigraphic unit. The sediment/bedrock relationships or very
specific sediment form other boundaries of the allostratigraphic units.
Paleosols can also form the boundary between two superposed
allostratigraphic sequences and reflect periods of lack of deposition.
Fluvial risers, related to river incision, may laterally limit the
allostatigraphic units and induce steps characteristics of nested
terraces  (Figure~\ref{fig2}b). An allostratigraphic unit may be
decomposed into several sub-units depending on the texture of the
sediments, itself controlled by the mode of deposition. A superposition
of fine-grained, colluvial, eolian, or overflow sub-units on top of a
coarse fluvial sub-unit is frequently found below terrace surfaces 
\citep{Schanzetal2018}.\looseness=-1

\begin{figure*}
\vspace*{5pt}
\includegraphics{fig02}
\vspace*{5pt}
\caption{\label{fig2}Examples of boundaries of allostratigraphic units
in Albania. (a)~Strath terrace bounded at the top by a flat terrace
surface and at the bottom by a flat strath surface beveled in the
substratum; (b)~filled terrace limited laterally by a fluvial riser, at
the top by a flat terrace surface and at the bottom by a flat strath
surface beveled in the substratum; (c)~Superposed units bounded by an
erosional surface; (d)~a~valley fill laterally aggrades along the
valley flank; (e)~geomorphologic processes and sub-units involved
during the formation of the actual flood plain terrace  unit; 
(f)~a~sketch of nested strath terraces; (g)~a~sketch of nested fill terraces
above straths with hidden substratum  riser (between T3$_{(\mathrm{e})}$
and T2$_{(\mathrm{e})}$) or visible riser  (between T2$_{(\mathrm{e})}$
and T1$_{(\mathrm{e})}$),  respectively; (h)~a~sketch of a fill terrace
(T3$_{(\mathrm{e})}$) above deposits constituted of two units aggrading
on the flank of a valley and of a fill-cut terrace
(T2$_{(\mathrm{e})}$). The circles and the dotted lin refer to the
location of another part of this figure.}
\vspace*{5pt}
\end{figure*}

The geomorphology of a mountain river valley is controlled by the
long-term incision, and the allostratigraphic units express momentary
fluctuations in this long-term incision due to others processes.
Lateral erosion, incision and aggradation govern the geometry of the
allostratigraphic units and straths, gorges and basins are respectively
the three end-members geometries associated with these processes  (Full
line squares  of Figure~\ref{fig3}s,  \ref{fig3}g and~\ref{fig3}b).
Depending from the succession and respective amplitude of these three
processes, a large variety of allostratigraphic units' organizations
can develop  (Figure~\ref{fig3}).

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig03}
{\vspace*{-2pt}}
\caption{\label{fig3}(a)~Large scale organization and typology of the
allostratigraphic units. The squares in full-lines refer to the three
end-member organizations  (b for basin, g for gorges and s for nested
strath terraces). The squares in dashed-lines refer to examples found
in Albania. f  ${=}$ nested filled terraces  (lower Vjosa River);  f-h
${=}$ nested fill terraces with hidden risers; s-u ${=}$ superposed units 
(Osum River); v ${=}$ valley fill (paleo-Devoll, C\"{e}rrik plain);  f-c
${=}$ strath terraces cut in a previous fill terrace  (upper part of the
Vjosa River);  t-f-c ${=}$ thick sedimentary unit deposited above a
strath cut through a previous fill terrace  (possibly upper part of the
Vjosa River).
(b)~Time evolution of the incision from measurements in the middle
section of Osum river. Note that the tops of the terraces are dated but
not their bases and it is assumed that the strath surface were mainly
linked to a lateral beveling and that deposits occurred fast  (see
discussion). The long-term incision rate is deduced from a linear fit.
(c)~Simplified diagram of the terrace geometry identified along the
Osum River.}
{\vspace*{-2pt}}
\end{figure*}

Difference between aggradation and incision amount is of great
importance when studying nested terraces: The nested terrace geometry
is formed of a succession of thick  (fill) or of thin (strath-related)
sedimentary units depending of the amount of aggradation. The nested
units are separated by 
\mbox{substratum} risers  (Figure~\ref{fig3}s
and~\ref{fig3}f) if the vertical incision between two terraces
deposition phases is greater than the sum of the sediment thickness
deposited during the two phases. On the contrary, there is no
substratum riser if vertical incision is smaller than the sum of the
thickness of the two sedimentary units  (Figure~\ref{fig3}f-h).
Fill-cut terrace is formed if the vertical incision between two
terraces deposition phases is smaller than the sediment thickness
deposited during the first phase of deposition 
(Figure~\ref{fig3}f-c). A superposition of sedimentary units occurs
when the vertical incision is almost null between two phases of
aggradation  (Figure~\ref{fig3}s-u). A combination of superposed
sedimentary units and nested terraces can be inferred in the case of
thick fill-cut terraces  (Figure~\ref{fig3}t-f-c). When the role of
lateral erosion is weak, no strath forms and aggradation occurs above
the steep flank of a valley  (Figure~\ref{fig3}v). In some plain-like
valleys, only aggradation is observed  (Figure~\ref{fig3}b). 

\subsection{Strath versus fill terraces}\label{sec2.2} 
From pioneer  \citep{Gilbert1877} to recent  \citep{Pazzaglia2022}
works, it is classically distinguished fill and strath terraces. A huge
body of published works  [see references in  \citet{Cordieretal2017}
and  \citet{Pazzaglia2022}] based on detailed sedimentological
description of the deposits and application of sedimentological
concepts nourish numerous discussions about their dynamic of 
formation.

In the following, a systematic allostratigraphic unit description,
coupled with a comparison with the geometry of the actual river
morphological flood plain is used and allow a very simple criteria to
distinguish between strath terrace and fill terrace.

Fill terraces are the upper boundary of very thick allostratigraphic
units  (Figure~\ref{fig2}g and~\ref{fig2}h) that could reach
thicknesses of several hundred meters  \citep[e.g.][]{Tofeldetal2017}
and their lower boundary type is various either valley slope,
paleo-soil or strath 
\mbox{surface.}

The lower boundary of the allostratigraphic unit located beneath a
strath terraces is a strath---an erosional surface---nearly
parallel to the top of the terrace and located at the base of a thin
sedimentary sequence  \citep[see][]{Schanzetal2018}. Locally, the
thickness of the unit is null  \citep{Mugnieretal2005,Seongetal2016}
and the top of the strath terrace coincides with the bedrock erosional
surface. 

Straths are observed at the base of allostratigraphic units of very
variable thicknesses  (Supplementary Information,  Figure~5 Appendix),
and the thickness of the alluvial unit is a long-discussed criterion in
the definition of a strath terrace  \citep{Pazzaglia2022}. From an
adaptation of the work of  \citet{Schanzetal2018}, a characteristic
thickness deduced from the present geometry of the morphological flood
plain has been used to separate fill terraces from strath 
\mbox{terraces.}

In the case of a strath terrace, the basal strath was still beveled by
lateral erosion during the high energy hydrological events 
\citep{HancockAnderson2002} that remobilized the coarse deposits 
\citep{LimayeLamb2016}, whereas the morphological flood plain 
\citep{Haueretal2021} was flooded by great inundations 
\citep[e.g.][]{Vandenberghe2015}. The 
{remobilization} and flooding ended
due to the following vertical incision phase. Therefore, the thickness
of the sedimentary unit of a strath terrace scales with the scour depth
of the present channel  \citep{Pazzaglia2022} incised through the
lowermost terrace unit  (T1$_{(\mathrm{e})}$,\break
Figure~\ref{fig2}e).

In the case of a fill terrace, the thickness of the sedimentary unit
had increased due to the vertical aggradation and beveling of the
strath ended as soon as the deposits above the strath became thicker
than the thickness of sediment remobilized during high energy
hydrological events  \citep{SklarDietrich2001}.\looseness=1

In this work, the distinction between a strath terrace and a fill
terrace located above a strath surface is therefore based on the
comparison of the past allostratigraphic geometry  (unit thickness)
with the actual dynamics of the river  (height above the low flow of
the morphological plain still flooded during centennial to millennial
extreme flows). This height varies from one river to another and has
been estimated for each Albanian river.

\subsection{Determination of the geometry of the\newline 
allostratigraphic
units}\label{sec2.3} 
Field survey have been performed along the 7 Albanian rivers to define
the geometries of the allostratigraphic units from the
lithostratigraphy and the geometry of the interface between Quaternary
sediment and bedrock. Our database is formed of approximately one
thousand sites of observations of the sedimentary units where
sedimentary log, photos and precise thickness measurement were
performed. The sub-unit thicknesses and the elevation of the terrace
surface above the present low flow river level were measured 
using a laser rangefinder with an 
\mbox{absolute} accuracy of ${\pm}$ 0.003~m, but the
actual accuracy is  ${\sim}$0.3~m due to the natural roughness of the
surfaces considered. The terraces extends were estimated on, satellite
images, a digital elevation model  [DEM;  \citealp{SRTM2013}] and
topographic maps 
[\citeauthor{InstitutinTopografikteUshtriseTirana1990},
\hyperlinkcite{InstitutinTopografikteUshtriseTirana1990}{1959--1990}].
The river profiles were performed from the DEM and the
paleoriver profiles were deduced from a correlation between terrace
remnants of same age and from their measured elevation above the
present river profile. A total of 5 river profiles and 8 paleoriver
profiles were already presented by  \citet{Carcailletetal2009} and 
\citet{Guzmanetal2013}.

{\vspace*{-2pt}}

\section{Setting}\label{sec3} 

{\vspace*{-1pt}}

\subsection{Paleoclimate of Albania}\label{sec3.1}  

{\vspace*{-1pt}}

The present-day Albanian climate is Mediterranean on the coast with hot
and dry summers and rather mild and rainy winters. In the mountainous
regions, the climate is continental with cold and snow-covered winters.
The vegetation is organized into altitudinal belts: The
meso-Mediterranean zone is restricted to the border of the sea 
\citep{Ozenda1975}, whereas alpine pasture lands are found over the
timberline, currently located around 1900~m a.s.l. 
\citep{Medwecka-Kornasetal1986}.

The paleo climatic records in the Albanian region 
\citep{SanchezGonietal2002,Sadorietal2016} correspond well with the
succession of cold periods followed by rapid warm excursions 
\citep{ClementPeterson2008} found in the eastern Mediterranean Sea 
\citep{Konijnendijketal2015}. Furthermore, declines in precipitation
probably occurred in Western Mediterranean area during cold periods 
\citep{Kalleletal2000} whereas increases occurred during warm intervals
\citep{Toucanneetal2015}. Changes of the vegetation during interstadial
events were evidenced by pronounced reductions in arboreal pollen
frequencies in western Greece  \citep{Tzedakisetal2004,Sadorietal2016}.
\looseness=-1

Glaciers developed in the Mediterranean mountains 
\citep{HughesWoodward2017} during the last glacial stage (MIS 5d to MIS
2). Glacial features are found in the upper part of the
Vjosa-Voidomatis Basin \citep{HughesWoodward2017} where the glacial
equilibrium line altitude (ELA) was close to  ${\sim}$2170~m a.s.l.
The ELA was lower northward, reaching an altitude of  ${\sim}$2000~m
a.s.l. in northeastern Albania  \citep{Kuhlemannetal2009}.

{\vspace*{-2pt}}

\subsection{Rivers and Terraces of Albania}\label{sec3.2} 

{\vspace*{-1pt}}

The main Albanian rivers are, the Vjosa, Osum, Devoll, Shkumbin, Erzen,
Mat and Drin rivers  (Figure~\ref{fig1}). The Mat and Erzen rivers are
smaller and their maximum altitudes are lower  (1728~m a.s.l. in the
Erzen catchment) than the other catchments  (2523~m a.s.l. in the Vjosa
and Osum catchments). 

Fluvial terrace deposition punctuates the vertical incision along the
main Albanian rivers (Supplementary Information,  Figure~1 Appendix).
The terraces of the upper Vjosa River  (Also called Voidomatis), the
middle Vjosa River, the Osum River, the Erzen River and the Drin River
were mapped and dated by  \citet{Woodwardetal2008}, 
\citet{Haueretal2021},  \citet{Carcailletetal2009}, 
\citet{Kocietal2018} and by  \citet{Gemignanietal2022}, respectively.
River and paleo-river profiles of the Osum, Skumbin, Devoll and Vjoja
were already presented by  \citet{Guzmanetal2013} and the terraces of
the Devoll, Shkumbin and Mat rivers were mapped by 
\citet{Guzmanetal2023}. For each river, the successions of terraces
surface are called Tx$_{(\mathrm{river})}$. Units are nonetheless
labelled Ux$_{(\mathrm{river})}$ when the terrace surface, fully
eroded, cannot be defined.

Previous works have already shown that dating of a same terrace level
by different methods generally provide consistent ages 
\citep[e.g.][]{Woodwardetal2008,Vassalloetal2015} and it was therefore
considered  [\citealp{Guzmanetal2023}; this article] a single database
formed of  47~$\mbox{}^{14}$C, 2~$\mbox{}^{36}$Cl,  7~$\mbox{}^{10}$Be,
3 TL, 8 U/Th and 3 ESR dates for the Albanian river terraces. The time
correlations of these terraces are based on this database  (Figure~\ref{fig4}b
and Supplementary Information,  Table~3 Appendix).

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}(a)~Mean thickness of the allostratigraphic units
beneath the paleo-Devoll terraces. The top of T10$_{(\mathrm{pa})}$ is
everywhere fully eroded. The hatched line refers to the maximum height
of T1$_{(\mathrm{pa})}$ above the low flow of the Devoll River and is
the thickness limit used to separate the fill terraces from the strath
terraces in the case of the paleo-Devoll.
(b)~Thickness of unit of  T4$_{(\mathrm{pa})}$ (T4 in the regional
nomenclature) in function of the distance from the sea.
(b)~Thickness of unit of  T4$_{(\mathrm{pa})}$ (T4 in the regional
nomenclature) in function of the distance from the sea.
(c)~Elevation of the top of  T4$_{(\mathrm{pa})}$ (T4 in the regional
nomenclature) above the actual river low flow. Green circles  (3), 
(7),  (8) and  (9) indicate sites where paleoriver profiles are offset
by faults; Red circles  (c--f)~indicate location of the sites where the
long-term incision rate has been estimated   (see Figure~\ref{fig1} for
location Supplementary Information, Table~1 Appendix and
Figure~1 Appendix).
(d)~Present day river profiles  (successively Devoll and lower Skumbin)
and location of the major active faults (LTT for Lushnje--Tepelen\"{e}
Thrust, EG for Elbasan Graben, TT for Tomorrica Thrust, BNF for Bulcar
Normal Fault and KG for Kor\c{c}\"{e} Graben). Locations of the red and
green dots on Figure~\ref{fig1}.}
\end{figure*}

It is found that Holocene terrace level (T1) was related to a rapid
succession of valley incisions and valley fills that occurred during
the warm climatic optimum whereas nine other terrace levels (T2 to T10)
formed during the last glacial stage (MIS 5d to end of MIS 2) and were
nearly synchronously with interstadial transitions toward warmer and
wetter conditions  \citep{Guzmanetal2023}. 

\subsection{Geology and recent tectonics of Albania}\label{sec3.3} 

The Albanian mountains are parts of the fold belt, which was thrusted
westward  \citep{Roureetal2004}. The eastern side of Albanides consist
mainly of Jurassic ophiolites and Mesozoic carbonates, on top of which
sit continental Cenozoic extensional basins. The western side of
Albanides are mainly formed of carbonates topped by Mesozoic or
Cenozoic flysch deposits  \citep{Roureetal2004}. A foreland basin,
filled with Plio-Quaternary molasse deposits, forms a coastal plain 
\citep{Guzmanetal2023}. The synorogenic grabens crosscut the thrust
system  \citep{Aliajetal1996,Guzmanetal2013}. 

The Outer Albanides and the associated foredeep was controlled by a
complex tectonic interplay during the Neogene between a rifting episode
and the usual foredeep flexural evolution above the subducted Adriatic
Sea plate  \citep{Scroccaetal2022}. The roles of gravitational
potential energy and rheology contrasts in mountain belts is still
debated in the development of subparallel thrust and normal faulting in
Albania  \citep{Copleyetal2009}. Furthermore, a clockwise oroclinal
bending  \citep{Handyetal2019} is still in process and induces an
orogen-parallel extension  (NW--SE) in the deformation field 
\citep{Jouanneetal2012}. Therefore, the recent active fault pattern is
complex and delineate numerous tectonic compartments 
(Figure~\ref{fig1}).

Vertical slip rates for active faults in Albania were estimated from
the offset of the paleoriver profiles reconstituted from mapping of
terraces  (${\sim}$for the last 19~ka; green circles on  Figure~\ref{fig1} and
Supplementary Data,  Table~1, Appendix). Thrusts, that extend more than
120 kilometers, have throw rates that varies from 0.2 to 0.8~mm/yr 
\citep{Guzmanetal2013,Kocietal2018} but could reach locally more than
2~mm/yr  \citep{Gachelin1977}, whereas the active faults of the
extensional domain are segmented but are very active, with throw rates
reaching locally 2~mm/yr  \citep{Guzmanetal2013}. Furthermore, an
orogen-perpendicular (SW--NE) uplift gradient is found and an
orogen-parallel (NW--SE) uplift gradient is locally added by the  (far
field) effects of major transverse fault, like the Shkodra-Peja
Transverse Fault Zone (SPFZ)  \citep{Biermannsetal2018}.

The late Pleistocene uplift rate of the compartment between faults,
inferred from the long-term fluvial incision depends from the tectonic
and geographic setting  \citep{Carcailletetal2009}. Each compartment
itself undergoes a variable uplift, linked to folding but above all to
tilting  (Figure~\ref{fig4}b). The incision in the middle of the
compartments are the following  (Supplementary Data,  Figure~1,
Appendix): At the hanging wall of the major Albanides/foreland thrust
boundary, the incision rate is respectively of  1.0~mm/yr along the Mat
River, 1.1~mm/yr along the Skumbin River, 0.8~mm/yr along the lower
part of the Devoll River, and 0.5~mm/yr along the Osum River. It is of
0.46~mm/yr at the hanging-wall of the Tirana Back thrust and 1.1~mm/yr
in the upper part of the Devoll at the hangingwall of the Tomorrica
Thrust. In the Elbasan and the Kor\c{c}\'{e} graben sedimentation is
still occurring  \citep{PriftiMecaj1987} whereas the incision rate is
of 0.13~mm/yr in the Konista\break graben.

\section{Comparison of the allostratigraphic\newline 
organization of the
Albanian terraces}\label{sec4}

\subsection{The Vjosa river terraces}\label{sec4.1} 
The Vjosa River, more than 272~km long, is made up of sections with
very different morphologies  \citep{Haueretal2021}. Ten terrace levels
are identified along the Vjosa River  \citep{Guzmanetal2023}. The
coarse sub-units beneath the terrace surfaces are mainly formed of
rounded fluvial clasts but, angular calcareous clasts, related to
debris flows, are locally intercalated within the fluvial units.
Furthermore, detailed sedimentological studies  \citep{Lewinetal1991}
show a change in the sediment dynamics linked to glacier retreats at
the end of MIS2: Fine-grained slackwater sediments were deposited by
large floods fed by meltwaters from glaciers sometime after 21.3~ka for
T3$_{(\mathrm{vj})}$, and by rainfall-generated
floods between 17.9 and  $16.2~\mathrm{cal}{\cdot}\mathrm{ka}$ for 
T4$_{(\mathrm{vj})}$ \citep{Woodwardetal2008}.

In its upper section, the river crosses the graben structure of Konista
(Figure~\ref{fig1}) where eight units  (Figure~\ref{fig5}), including
the present-day channel (T1$_{(\mathrm{vj})}$), were identified. All,
except T2$_{(\mathrm{vj})}$ were dated 
\citep{Lewinetal1991,Woodwardetal2008}. The highest terrace
T10$_{(\mathrm{vj})}$ was deposited by a river system with a much
larger catchment that was pirated before 350~ka 
\citep{Macklinetal1997}. The  T10$_{(\mathrm{vj})}$ and
T8$_{(\mathrm{vj})}$ units are related to fill terraces 
\citep{Lewinetal1991,Woodwardetal2008}. The substratum does not outcrop
between T7$_{(\mathrm{vj})}$, T6$_{(\mathrm{vj})}$ and
T4$_{(\mathrm{vj})}$ terraces  \citep{Woodwardetal2008} and they are
either directly deposited above the substratum or are thick fill-cut
terraces formed during a process of down-cutting within the
T8$_{(\mathrm{vj})}$ fill terrace. T3$_{(\mathrm{vj})}$ is superposed
onto unit T4$_{(\mathrm{vj})}$  \citep{Woodwardetal2008} but is locally
a thin ``fill-cut'' terrace cut within T4$_{(\mathrm{vj})}$ 
\citep{Lewinetal1991}. Lastly, T2$_{(\mathrm{vj})}$ is a strath
terrace.

\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig05}
\vspace*{-4pt}
\bcaption{\label{fig5}}{4}{(a)~Simplified diagram of the terrace geometry
identified  along the 7 main Albanian rivers. The horizontal and
vertical axes are not to scale. The terrace colors refer to the
regional nomenclature TX   (see Figure~\ref{fig5}c).
(b)~Typical terrace geometries. Letters s,  f-c, f, f-h, t-f-c,
s-u, v refer to the types of organization of the allostratigraphic
units indicated on Figure~\ref{fig3}. 
(c)~Age distribution of the terraces   
\citep[adapted from][]{Guzmanetal2023}.}
\end{figure*}

In the middle section of the Vjosa River, the long-term incision is
greater than in the upper section  \citep{Guzmanetal2013}. 
\citet{Guzmanetal2023} indicated a level T9$_{(\mathrm{vj})}$ formed of
paleo-meanders cut in T10$_{(\mathrm{vj})}$.  \citet{PriftiMecaj1987}
mapped four fill terrace levels T8$_{(\mathrm{vj})}$ to
T5$_{(\mathrm{vj})}$ and substratum frequently outcrops in the fluvial
risers between the nested fill terraces. Furthermore, a large scale
strath surface is clearly evidenced at the base of
T10$_{(\mathrm{vj})}$ and T5$_{(\mathrm{vj})}$ thick fill terraces 
\citep{Guzmanetal2013}. Finally, T2$_{(\mathrm{vj})}$ is located at the
top of sediments more than 50~meters thick located below the present
river  \citep{Prifti1981}. Geophysics would suggest that these lowest
sediments could be divided into two distinct sedimentary units 
\citep{Prifti1981}. 

\subsection{The paleo-Devoll River terraces}\label{sec4.2}
The Devoll and Shkumbin rivers are presently separated by approximately
5~km at the level of a wind gap where a paleo-Devoll River flowed
northward and was connected to the Shkumbin River till the beginning of
the Holocene  \citep{Guzmanetal2023}. Hence, the terraces located along
the middle reaches of the Devoll and the lower reaches of the Shkumbin
formed a unique terrace system of eleven terrace levels 
\citep{Guzmanetal2023}. 

Two superposed sub-units generally form the units: A thin upper
sedimentary sub-unit  (${\sim}$1~m) is composed of clay, siltstone,
fine sand and organic material. Nevertheless, its thickness reaches
more than 2~m for the oldest terraces  (T12$_{(\mathrm{pa})}$,
T11$_{(\mathrm{pa})}$ and T9$_{(\mathrm{pa})}$) and possibly includes
loess and colluvium deposited after the fluvial story. The basal
sub-units consist of rounded pebbles and cobbles that are supported by
a gravel and sand matrix. Clast size generally decreases towards the
top, while the percentage of matrix increases. Nonetheless, the size of
the coarse material shows complex variations and conglomerate sometimes
alternate with horizontal stratified, fine to coarse sand levels.
Sediments of the basal unit were probably deposited in a braided
alluvial system characterized by cross-stratified rounded pebbles and
cobble levels that alternate with horizontal stratified, fine to coarse
sand levels. Nonetheless, evidence of a meandering environment is found
at the base of the unit U10$_{(\mathrm{pa})}$ whose upper part is fully
eroded  (Figure~\ref{fig6}b) and where sediments dip steeply.

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig06}
{\vspace*{1pt}}
\caption{\label{fig6}(a)~Nested fill terraces along the Devoll. Details
of T9$_{(\mathrm{pa})}$, U6$_{(\mathrm{pa})}$ and T5$_{(\mathrm{pa})}$
are shown on Figure~\ref{fig6}b and~\ref{fig6}c. (b)~The remnant of the
base of U10$_{(\mathrm{pa})}$ is located beneath the strass surface at
the base of the thick T9$_{(\mathrm{pa})}$.  (c)~Superposition of the
allostratigraphic units  T5$_{(\mathrm{pa})}$ and U6$_{(\mathrm{pa})}$
(upper middle reaches of the Devoll River) above two distinct straths
(A and B). Unit U6$_{(\mathrm{pa})}$  is located between the strath A
and the erosional surface, in the continuity of strath B.}
\end{figure*}

The sedimentary units the paleo-Devoll River were generally deposited
on straths beveled in the flysch or molasse substratum. The substratum
often outcrops in the risers between the nested terraces 
(Figure~\ref{fig6}). Nonetheless, from the mountain front to the
C\"{e}rrik plain, the base of the allostratigraphic unit
T2$_{(\mathrm{pa})}$ is hidden and located several tens meters lower
than the present river level  \citep{Melo1961}, suggesting here a thick
valley fill. 

The morphological flood plain (top surface of T1$_{(\mathrm{pa})}$) was
measured between 2 and 6~m above the low flow river 
(Figure~\ref{fig4}a). T4$_{(\mathrm{pa})}$ and T3$_{(\mathrm{pa})}$,
with thicknesses between 4 and 6~m, are therefore considered as strath
terraces whereas T12$_{(\mathrm{pa})}$, T11$_{(\mathrm{pa})}$,
T9$_{(\mathrm{pa})}$, T8$_{(\mathrm{pa})}$, T7$_{(\mathrm{pa})}$,
pT6$_{(\mathrm{pa})}$, and T5$_{(\mathrm{pa})}$ are fill terraces as
their mean thicknesses are between 10 and 32~m  (Figure~\ref{fig4}a).

In the downstream part of the paleo-Devoll,  T6$_{(\mathrm{pa})}$ and
T5$_{(\mathrm{pa})}$ are stepped terraces where the substratum riser is
hidden. Upstream, T6$_{(\mathrm{pa})}$ is not evidenced, but two
allostratigraphic units are located beneath the surface of
T5$_{(\mathrm{pa})}$  (Figure~\ref{fig5}); the boundary between the two
allostratigraphic units is an erosional surface, also expressed
laterally by a strath surface beveled in the substratum 
(Figure~\ref{fig6}c). The lower allostratigraphic unit extends several
tens of kilometers and is formed of a gravel and pebble sediments
sub-unit overlained by a flood plain deposit sub-unit; The lower
allostratigraphic unit U6$_{(\mathrm{pa})}$ could be the lateral
equivalent of T6$_{(\mathrm{pa})}$, and this upstream evolution from
encased terraces to superposed units could be due to an increase in the
slope of the Paleo-Devoll River between the deposition of
T6$_{(\mathrm{pa})}$ and  T5$_{(\mathrm{pa})}$.

Tectonics locally strongly affects the sedimentation in the
Devoll-Shkumbin area. The Elbasan plain upstream of the junction
between the Paleo-Devoll and the Shkkumbin, is located in a graben. The
faults bounding this graben offset T1, at a rate  ${>}$ 1--2~mm/yr for
the last 6~Ka. In the center of the graben, several sedimentary units
are superposed over more than 100~meters  \citep{Melo1961}. In the
upstream part of the Devoll, a lake developed in the Korca graben till 
19$^{\mathrm{eme}}$ century, but the geometry of sedimentary units is
here unknown.

\subsection{The Osum river terraces}\label{sec4.3} 
In the Osum River area, ten terraces were mapped and 5 dated 
\citep{Carcailletetal2009,Guzmanetal2023} (Figure~\ref{fig5}). In the
Ersek\"{e} graben (upper reach of the Osum River), a thick
superposition od sedimentary units is observed, and
T3$_{(\mathrm{os})}$ is possibly beveled in T7$_{(\mathrm{os})}$
(Supplementary Information,  Figure~2 Appendix). In the middle reaches
of the Osum River T10$_{(\mathrm{os})}$ to T6$_{(\mathrm{os})}$ are
fill terraces  \citep{Carcailletetal2009}; the sedimentary unit linked
to T4$_{(\mathrm{os})}$ is superimposed above another allostatigraphic
unit U5$_{(\mathrm{os})}$ whereas T3$_{(\mathrm{os})}$, 
T2$_{(\mathrm{os})}$ and T1$_{(\mathrm{os})}$ are strath terraces 
\citep{Carcailletetal2009}. A ${\sim}$20~m thick sedimentary unit is
located beneath the present-day river  \citep{PriftiMecaj1987} and is
probably younger than T2$_{(\mathrm{os})}$ and therefore Holocene in
age. This hidden unit would be in this case a T1 unit in the
nomenclature of  Supplementary Information, Table~3 Appendix.

In the middle reaches of the Osum River, gorges more than 50~m deep and
less than  ${\sim}$5~m large were locally incised in a weakly extended
cretaceous limestone outcrop. This zone is located between flysh
outcrops where the long-term incision rate of strath terraces is 
$1.3\pm0.5$~mm/yr  \citep{Guzmanetal2013} (Supplementary  Information
Figure~2 Appendix) and the absence of lateral erosion in the limestone
prevented any beveling during more than 30~Ka. 

\subsection{The Erzen River terraces}\label{sec4.4} 
The terraces of the Erzen River have been mapped by 
\citet{Kocietal2018}. An active back-thrust fault 
[\citealp{Ganasetal2020};  see Figure~\ref{fig1}] offsets the domain
and a time correlation between the terraces on each side allow defining
7 terraces levels  \citep{Guzmanetal2023}. At the transition between
the plain and the first mountain relief, the morphological flood plain
is very large, locally more than 13~m above the low flow river and
dated at ${\sim}1.5~\mathrm{cal}{\cdot}\mathrm{kyr}$ before present 
\citep{Guzmanetal2023}, suggesting a recent  (less than  1.5~kyr)
uplift. To consider the local development of a riser, the morphological
flood plain has been separated in two parts: T1$_{(\mathrm{er})}$ and
T2$_{(\mathrm{er})}$ (Figure~\ref{fig5}). The terrace
T3$_{(\mathrm{er})}$ is highly extended, in peculiar across a wind gap
which corresponds to a paleo river that flowed close to Tirana 
(Figure~\ref{fig1}a). The others terraces T4$_{(\mathrm{er})}$ to
T7$_{(\mathrm{er})}$ are strath terraces as their sedimentary units are
less 4-m thick and lay above strath surfaces  (Supplementary
Information Figure~3 Appendix).

\subsection{The Mat River terraces}\label{sec4.5} 
Nine terraces  (Figure~\ref{fig5}) were mapped along the Mat River 
\citep{Melo1996} and five of these levels dated 
\citep{Guzmanetal2023}. The sedimentary units of the terraces lie above
a strath surface beveled in the bedrock. They are made up of a 3 to 5~m
thick alluvium of gravel and sand that slopes upwards, and an upper
sub-unit of fine sediments of fine sediments 0.5 to 1~m thick. The
thickness of the alluvial unit beneath the highest T9$_{(\mathrm{ma})}$
is nonetheless locally greater and reaches 8~m. The top surface of the
morphological flood plain  T1$_{(\mathrm{ma})}$ was measured between 1
and 5~m above the low flow river, and the terraces of the Mat River are
therefore considered as strath\unskip\break 
terraces.

\subsection{The Drin river terraces}\label{sec4.6} 
The Neogene-Quaternary graben structures control the drainage pattern
the Drin River area  \citep{Aliajetal1996}. At the hangingwall of
Shkoder-Peja (SP) Normal Fault, four terraces are described 
\citep{PashkoAliaj2020,Gemignanietal2022}. T2$_{(\mathrm{dr})}$
and T3$_{(\mathrm{dr})}$ are nested filled terraces, with hidden
risers, that are cut within an older  (Pleistocene or late Pliocene)
lake sediment unit  \citep{Gemignanietal2022}. Upstream, close to
Peshkopia  (Figure~\ref{fig1}), two higher terraces, 
T5$_{(\mathrm{dr})}$ and T6$_{(\mathrm{dr})}$ are described 
\citep{Guzmanetal2023} and the substratum riser crop out between the
thick and nested units  (Figure~\ref{fig5}). Furthermore, strath
surfaces are clearly expressed at the base of T2$_{(\mathrm{dr})}$
and T3$_{(\mathrm{dr})}$.

\section{Discussion}\label{sec5} 

\subsection{The typology of the Albanian terraces}\label{sec5.1} 
Along the seven rivers of Albania, various organizations of the
allostratigraphic units were observed  (Figure~\ref{fig5}b). Despite
this multitude of geometries, one of the most striking result in
Albania is that only the T1 Holocene valley fill units and the
U10$_{(\mathrm{pa})}$ unit are clearly characterized by the absence
of a basal strath. Straths have been found at the base of most of the
allostratigraphic units, regardless of their thickness and the
considered rivers. These terrace units are possibly synchronous with
interstadial climatic events during the last glacial stage 
\citep{Guzmanetal2023}, and their formation can be explained by the
``cold terrace'' model  \citep{Vandenberghe2015}. In this model,
vertical incision is promoted by increased transport capacity during
the transition from cold, dry conditions to warmer, wetter conditions 
\citep[e.g.][]{Fulleretal1998,Vassalloetal2007,BridglandWestaway2008},
while deposition of sedimentary units and beveling of straths have
occurred previously.

\subsection{Why and when thick fill sedimentary units deposited above
strath surfaces?}\label{sec5.2}
We found that most of the terraces are thin strath terraces in the two
small catchments of the Erzen and Mat rivers, but are thick fill units
deposited above the strath surfaces in the larger catchments of the
Vjosa, Osum, paleo-Devoll and Drin rivers. 

These results arise a crucial question: what controls the difference in
development between fill and strath terraces? In other words: why and
when thick fill sedimentary units deposited above strath surfaces? The
uncertainty associated with the proposed temporal correlation between
the terraces of different rivers  \citep{Guzmanetal2023} is too great
to determine whether the synchronism being proposed relates to
sedimentary deposits or to their incision.  Even superposed sample sets
dated in T4$_{(\mathrm{vj})}$ \citep{Lewinetal1991,Woodwardetal2008} do
not allow distinguishing the age of the lowest fill deposits from that
of the highest ones, themselves close to the terrace abandonment age.
Similar situations have already been described elsewhere in the world 
\citep{WegmannPazzaglia2009,Gundersonetal2014,Vignonetal2016}. 
Understanding of what controls the difference in development of fill
and strath terraces therefore allows inference of when the sediments of
the fill terraces were deposited.

\subsubsection{The influence of eustatism, catchment size and lithology
in terrace formation}\label{sec5.2.1}
The influences of eustatism,
catchment size or lithology in terrace formation have been proposed for
solving the fill versus strath terrace dilemma:
\begin{itemize}
\item
An upstream evolution along coastal rivers has been demonstrated 
\citep{Merrittsetal1994}, 
with strath terraces found upstream of fill
terraces linked to sea-level rise. In Albania, the effect sea-level
highstand is only expressed for the Holocene, with shoreline
modifications  \citep{Fouacheetal2010}
and diversions of the Drin 
\citep{Ganasetal2020}, Skumbin, Seman  \citep{Guzmanetal2023} and Vjoja
\citep{Fouacheetal2010}
rivers above their sedimentary fans in the
foreland. In the lower part of the mountain belt, the T1 terraces are
linked to strong aggradations along the Vjoje, Osum, Devoll and Drin
rivers  (Figure~\ref{fig5}), as well as the deviations of the Devoll
and Erzen rivers; they are probably linked to the upstream influence of
the mid-Holocene sea-level highstand  \citep{Somozaetal1991}. A late
Holocene vertical incision follows the formation of fill-cut terraces
beveled by lateral erosion in the thick Holocene valley fills. The T1
terrace is therefore consistent with the ``warm'' unit model 
\citep{Vandenberghe2015}, 
but it is the only one in Albania.
\item
In Albania the correlation found between the catchment size and the
thickness of the sedimentary units suggests that catchment size is a
first order parameter for the genesis of a particular type of terraces.
Nonetheless, the catchment areas located upstream of the fill terraces
of the upper Vjosa and upper Osum  (110~km$^{2}$ and  70~km$^{2}$,
respectively) are of the same order as the catchments upstream of
strath terraces of the Mat and Erzen  (130 and 70~km$^{2}$,
respectively). Therefore, the regional distribution of the fill or
strath terraces in Albania is not simply a consequence of the size of
the catchments.
\item
\citet{WegmannPazzaglia2009} showed in the Northern Apennines that
strath terraces develop in catchments with siliciclastic lithologies
whereas fill terraces develop in catchments with carbonate bedrocks. In
Albania, all the rivers intersect a wide variety of geological zones 
\citep{Roureetal2004}. No clear lithology dominances are evidenced for
the bedload of the different Albanian river catchments 
\citep{Woodwardetal2008,Xhaferrietal2020}. No relation between
thickness of the sedimentary unit and clast lithology is found in
Albania. Thus, the bedrock lithology does not influence the type of
terraces in Albania, although strath surfaces are generally beveled in
flysch formation.
\end{itemize}

\subsubsection{The influence of tectonics in terrace
formation}\label{sec5.2.2} 
The tectonic settings control the long-term incision rates in the
Albania-Greece area 
\citep{Carcailletetal2009,Guzmanetal2013,Biermannsetal2018} and the
active faults locally offset the paleoriver profiles deduced from the
top of the terrace fly  [\citealp{Guzmanetal2013}; Supplementary
Information, Table~1 Appendix; Figure~\ref{fig4}c].

\citet{Vignonetal2016} have shown that displacement along faults,
mainly related to earthquakes close to the surface 
\citep{Mugnieretal2005}, could affect the thickness of a sedimentary
unit if it occurs during its deposition, with a thinning of the unit
above the uplifted compartment. Due to the moderate seismic intensity
in Albania   [fewer M 6.7 earthquakes and less than a few meters of
surface rupture movement, from  \citealp{Pondrellietal2006}], this
influence is nevertheless weak, although observed locally at the
intersection of the Devoll and Bulcar Normal Fault 
(Figure~\ref{fig4}b). Therefore, active tectonics does not transform a
strath terrace into a thick fill terrace.

Furthermore, the regional distribution between fill or strath terraces
is not linked to differences in the long-term incision rates: Terraces
in the Erzen and Mat catchments are strath terraces, although the
incision rates of the latter are double than the incision rates of the
former  (Red dots a and b on  Figure~\ref{fig1}); the thickest fill
terraces both developed in the zone with the fastest incision rate
(Upper Devoll) and the zone with the smallest incision rate 
(Konista graben of the Upper Vjosa) (Red dots f and h on 
Figure~\ref{fig1}).

Therefore, tectonics cannot explain alone the regional distribution of
the fill and strath terraces in Albania.

\subsubsection{The sediment production of the glaciers and the
paraglacial domain}\label{sec5.2.3}  
The sediment production of the glaciers and the paraglacial domain is
of great importance and is particularly significant during the
reduction of glacier size  \citep{Ballantyne2002}. There are several
evidences of ongoing paraglacial adjustment linked to the present-day
glacial retreat  \citep{Guillonetal2022}: Meltwater flow is greatly
increased and downstream sediment discharge is influenced by sediment
reworking and changes in fluvio-glacial connectivity 
\citep{Tunnicliffeetal2012,Ravazzietal2012}.

Glaciers developed during the glacial stages in northwestern Greece and
Albania \citep{HughesWoodward2017}.
Nonetheless, no paleo-glacier has been
evidenced to date for the Osum and Devoll river catchments, whose
surfaces above the 2174 m-paleo-ELA are 12 and  14~km$^{2}$,
respectively. The surface above the paleo-ELA is significant only in
the case of the Vjosa River where it reaches  120~km$^{2}$ (see 
Figure~\ref{fig1}b and Supplementary Information, Figure~1 Appendix).

The bedrock beneath the Vjosa River glaciers is limestone, and the
increased proportion of limestone clasts in T3 to T8 suggests a
significant contribution from glacial erosion 
\citep{Woodwardetal2008}. Nonetheless, ice only covered less than 10\%
of the upper catchment during the last glacial stage  (MIS 5d to 2)
whereas the paraglacial domain inherited from the previous glacial
stages was extended  \citep{Woodwardetal2008}. Therefore, the great
aggradation phases in the upper reaches of the Vjosa River during the
last glacial stage was partly linked to the direct glacial sediment
input and probably enhanced by the remobilization of paraglacial
sediment. 

\subsubsection{Role of the altitudinal distribution of the\newline
vegetation}\label{sec5.2.4}  
The altitudinal distribution of the vegetation could have controlled
terrace formation. gkIn a general manner, vegetation changes enhance
the effect of precipitation and temperature on the sediment supply and
water runoff  \citep[e.g.][]{Bull1991,Kasseetal2005}. Our data indicate
that the altitude distribution of the catchments  (Figure~\ref{fig1}b)
modulates the geomorphic response of the fluvial system: fill terraces
develop in catchments with large high-altitude zones while strath
terraces in catchments with smaller high-altitude zones 
(Figure~\ref{fig7}). We suggest that this difference in terrace
formation is mainly related to the vegetation contrast between the
lower and higher part of the range 
\citep{Tzedakisetal2004,Wohlfarthetal2008} that is enhanced during cold
and dry periods  \citep{Kalleletal2000,Toucanneetal2015}:

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Diagram illustrating the relationships between
hillslope-channel coupling and the succession of aggradation, vertical
incision and lateral erosion during hydro-climatic cycles in the
Albanian rivers. The left and right sides refer to large catchments
with a large proportion above 1700~m asl and to small catchments
located beneath 1700~m asl, respectively. (a)~During periods of cold
and dry conditions, a balance between transport capacity and sediment
supply favors a large morphological flood plain in large and small
catchments due to lateral erosion and sediment transfer. (b)~At the
very beginning of a period of warm and humid conditions, pulse of
aggradation occurs in the large catchments. (c)~During the period of
warm and humid conditions, vertical incision occurs everywhere.} 
\end{figure}

\begin{itemize}
\item
In the high-altitude part of the large catchments, steppe vegetation
prevailed during cold and dry periods \citep{Sadorietal2016} and the
vegetal cover was probably not continuous. Hillslope processes heavily
affected these zones and produced a lot of sediments. A pulse of
sediment supply at the beginning of the warming period would correspond
to a remobilization, during the latency time needed for the vegetation
to recolonize the slopes, and under the effect of the increase in
runoff, of the sediments stored during the cold period on the slopes.
Then, fill terraces formed rapidly during these climatic 
\mbox{transitions.}
\item
In the small catchments, foothills remained covered over their whole
area by vegetation that protected their hillslopes from erosion as they
were less dry and less cold than the high parts of the large
catchments; the vegetation had a buffer effect and attenuated the
evolutions of sediment supply and transport capacity. This would have
maintained a near equilibrium balance in the rivers favoring the
lateral beveling that led to the formation of strath terraces.
\end{itemize}

In summary, the pulses of sediment supply at the transition between
cold and warmer periods in Albania may be a combination of
(1)~sediments provided by areas that have become ice-free as a result
of glacier retreat, (2)~sediments inherited from previous glaciations
and remobilized by increased precipitation and erosion, and
(3)~hillslope sediments supplied during the latency period between
climate change and vegetation change in high altitude 
\mbox{catchments.}

\section{Conclusions}\label{sec6} 
The geomorphologic study of the seven major Albanian rivers is
presented in this work. The allostratigraphy organization of the
flights of terraces is compared between these rivers. The abandonment
of the Albanian terrace surfaces is possibly in phase with climatic
variations but our results show that the geomorphic responses of the
river systems to climatic variations are influenced by a combination of
processes. The geomorphic responses of the river systems are also
influenced by the lithology, the tectonic settings, the eustatism and
the catchments altitudinal distribution.
\begin{itemize}
\item 
The medium strength of the bedrock lithology (mainly flysch) in
numerous parts of 
Albania favors lateral erosion and strath
\mbox{development.} The tectonic setting is characterized by a moderate uplift
rate (0.1 to 1~mm/yr) and the rather slight difference between the
amount of the successive incision and deposition events both favors the
formation of nested terraces and their preservation. The exceptional
succession of Albanian terraces is probably due to this combination of
a medium strength of the bedrock and of a moderate uplift rate (0.1 to
1~mm/yr). 
\item
The influence of the altitudinal distribution of the catchments on the
geomorphic responses of the river system is complex and includes the
vegetation spreading and glaciers size.
\end{itemize}

In the small catchments, during cold and dry periods, most of the
hillslopes continued to be protected by the vegetation; an equilibrium
was approached between sediment supply and transport capacity. In the
upper part of the large catchments, the vegetation cover had
significantly decreased, leading to increased erosion of the
hillslopes; more sediment was available, while dry conditions
maintained a low transport capacity of the rivers.

It is suggested that, during the rapid transition to warmer conditions,
the increase in precipitation favored the supply of sediments from
previously un-vegetated or glaciated areas. This would induce a
sedimentary impulse that would create thick sedimentary units affected
by vertical incision as soon as the sedimentary stock provided by the
slopes was exhausted or stabilized by the vegetation. These processes
would favor the development of fill terraces in large catchments but
not in small ones.

\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}
The authors would like to thank the NATO SFP 977993 and the Science for
Peace team for supporting this work. OG thanks the Simon Bolivar
University for granting him permission to carry out his doctoral
studies. This publication was also made possible through support
provided by the IRD-DPF. We warmly thank the staff of the AMS facility
ASTER at the Centre Europ\'{e}en de Recherche et d'Enseignement des
G\'{e}osciences de l'Environnement (CNRS, France) for technical
assistance during the $\mbox{}^{10}$Be measurements.

\back{}

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

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-278-suppl.pdf}}

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