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\DOI{10.5802/crgeos.252}
\datereceived{2023-07-05}
\daterevised{2023-11-20}
\dateaccepted{2023-12-21}
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\begin{document}

\begin{noXML}

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

\shortrunauthors

\title{Accretion versus erosion and sediment transfer balance near the
subduction interface}

\alttitle{Accr\'{e}tion versus \'{e}rosion et bilan des transferts
s\'{e}dimentaires au voisinage de l'interface de subduction}

\author{\firstname{Serge} \lastname{Lallemand}\CDRorcid{0000-0003-1924-9423}\IsCorresp}
\address{Geosciences Montpellier, CNRS, Universit\'{e} de Montpellier, 
Universit\'{e} des Antilles, France}
\email[S. Lallemand]{serge.lallemand@umontpellier.fr}

\author{\firstname{Michel} \lastname{Peyret}\CDRorcid{0000-0003-3882-4403}}
\addressSameAs{1}{Geosciences Montpellier, CNRS, Universit\'{e} de Montpellier, 
Universit\'{e} des Antilles, France}

\author{\firstname{Diane} \lastname{Arcay}\CDRorcid{0000-0001-6773-0807}}
\addressSameAs{1}{Geosciences Montpellier, CNRS, Universit\'{e} de Montpellier, 
Universit\'{e} des Antilles, France}

\author{\firstname{Arnauld} \lastname{Heuret}\CDRorcid{0000-0003-0186-5260}}
\address{Geosciences Montpellier, CNRS, Universit\'{e} de Montpellier, 
Universit\'{e} des Antilles, Universit\'{e} de Guyane, France}

\keywords{\kwd{Subduction}
\kwd{Active margins}
\kwd{Oceanic trenches}
\kwd{Subduction channel}
\kwd{Accretion}
\kwd{Erosion}
\kwd{Sediment flux}}

\altkeywords{\kwd{Subduction}
\kwd{Marges actives}
\kwd{Fosses oc\'{e}aniques}
\kwd{Chenal de subduction}
\kwd{Accr\'{e}tion}
\kwd{\'{E}rosion}
\kwd{Flux de s\'{e}diment}}

\begin{abstract} 
The nature and amount of sediment transferred from one plate to the
other near the subduction interface partly determines the tectonic and
seismogenic regime of a margin. Examination of over 500 multichannel
seismic lines has enabled us to build up a global database of
subduction zone front characteristics at unprecedented spatial
resolution. The total thickness of sediment in the trench below the
deformation front, as well as that of the subduction channel at a
distance from the trench, combined with other indices such as the
tectonic regime of the forearc or the migration of the volcanic front,
have enabled us to revisit the accretionary or erosional character of
active margins. Preliminary conclusions, based on ${\sim}$3/4 of
sufficiently documented subduction zones, show a predominance of the
erosive character of subduction over the last million years. The flux
of solid matter through the shallow part of the subduction channel is
of the order of 1.5~km$^{3}$/yr, and that of pore fluids
0.4~km$^{3}$/yr. Some subduction zones, such as the Mediterranean, are
characterized by exceptional solid flux in the channel, while fluid
flux is comparatively moderate. This is due to the fact that channel
sediments are compacted even before subduction. Overall, fluid flux in
the channel is greater under erosive margins, due to the higher rate of
subduction and often higher porosity. Maximum porosity in the channel
is reached when there is neither accretion nor tectonic erosion.
\vspace*{2pt}
\end{abstract}

\begin{altabstract} 
La nature et la quantit\'{e} de s\'{e}diment transf\'{e}r\'{e} d'une
plaque \`{a} l'autre au voisinage de l'interface de subduction
conditionne en partie le r\'{e}gime tectonique et sismog\`{e}ne d'une
marge. L'examen de plus de 500 lignes sismiques multitraces a permis de
constituer une base de donn\'{e}es globale des caract\'{e}ristiques du
front des zones de subduction \`{a} une r\'{e}solution spatiale sans
pr\'{e}c\'{e}dent. L'\'{e}paisseur totale de s\'{e}diment dans la fosse
\`{a} l'aplomb du front de d\'{e}formation ainsi que celle du chenal de
subduction \`{a} distance de la fosse, combin\'{e}es \`{a} d'autres
indices comme le r\'{e}gime tectonique de l'avant-arc ou la migration
du front volcanique nous ont permis de revisiter le caract\`{e}re
accr\'{e}tionnaire ou \'{e}rosif des marges actives. Les premi\`{e}res
conclusions, bas\'{e}es sur ${\sim}$3/4 des zones de subduction
suffisamment document\'{e}es, montrent une pr\'{e}dominance du
caract\`{e}re \'{e}rosif de la subduction sur le dernier million
d'ann\'{e}es. Enfin, le flux de mati\`{e}re solide transitant par le
chenal de subduction superficiel serait de l'ordre de  1.5~km$^{3}$/an,
celui des fluides contenus dans les pores de 0.4~km$^{3}$/an. Certaines
zones de subductions comme en M\'{e}diterran\'{e}e se caract\'{e}risent
par un flux solide dans le chenal exceptionnel tandis que le flux
fluide est comparativement mod\'{e}r\'{e}. Ceci est d\^{u} au fait que
les s\'{e}diments du chenal sont compact\'{e}s avant m\^{e}me de
subduire. Globalement, les flux de fluides transitant dans le chenal
sont plus importants sous les marges \'{e}rosives du fait d'une vitesse
de subduction et d'une porosit\'{e} souvent plus \'{e}lev\'{e}e. La
porosit\'{e} maximale dans le chenal est atteinte lorsqu'il n'y a ni
accr\'{e}tion, ni \'{e}rosion tectonique.
\end{altabstract} 

\maketitle

\vspace*{10pt}

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\section{Introduction}\label{sec1} 
The existence of erosive processes responsible for the destruction of
some active margins was first considered in the early 1970s along the
Chilean subduction based on a mass balance deficit between sediment
inputs from the Andes and those actually observed along the margin and
in the trench  \citep{Scholletal1970}, as well as a distribution of
plutons showing an eastward rejuvenation from Jurassic in the coast to
Cenozoic in the Cordillera  \citep{Rutland1971}. A little later, deep
ocean drilling on active margins in Central America, the Marianas,
Japan, or Peru provided additional support for the tectonic erosion
process that explained both the anomalous subsidence of these margins
(not explained by thermal contraction or sediment loading) and the
landward migration of the volcanic front 
\citep{Mooreetal1986,NatlandTarney1981,Bloomer1983,vonHueneetal1982,Suessetal1988}. 
These findings have challenged the established model of active margin
growth by sediment accretion at the foot of the landward slope 
\citep[e.g.,][]{Seely1979}.  \citet{Aubouinetal1984,Aubouinetal1985},
following an exploration of the Guatemalan margin
\citep{vonHueneAubouin1982}, introduced the
concept of convergent-extensional margin, reflecting that many active
margins are cut by normal faults as in passive margins 
\citep[see also][]{FerriereFaure2024}. 
Proposals to classify active
margins on the basis of their accretionary or erosive characteristics
have emerged since the 1990s as discoveries were made
\citep{VonHueneScholl1991,CliftVannucchi2004}. Nowadays, most authors
still refer to these early classifications that deserve to be updated,
now that we have additional \mbox{observations.} 

Another descriptive feature of active margins concerns the amount of
sediment available at the trench versus that present between the plate
interface and the top of the igneous oceanic crust, that we will
further call the  ``subduction channel''. The thickness of this channel
often reflects the erosive or non-erosive behavior of the subduction.
Similarly, many authors suggested a causal link between an excess
of sediment present in the trench and the generation of
mega-earthquakes  \citep{Ruff1989,Heuretetal2012,Scholletal2015}. 

Even more recently,  \citet{Lallemandetal2018} and 
\citet{vanRijsingenetal2018,vanRijsingenetal2019} showed that a high
ocean floor roughness at 80--100~km wavelengths tended to reduce
interseismic coupling on the subduction interface, and hence earthquake
magnitude.  \citet{Seno2017} was the first to point out that it was not
so much the thickness of the sediments at the trench but those actually
present in the subduction channel which could condition the
seismogenic behavior of the interface. Here, we propose to complete and
revise the databases describing active margins from the wealth of work
done along subduction zones, in order to verify the assertions made
since the 1980s, which were based on a limited number of reference
seismic profiles.

Based on a comprehensive review of interpreted seismic profiles
published in the literature, we present not only an updated compilation
of trench sediment thickness at an unprecedented density but we also
provide a first order thickness of the subduction channel on the depth
range over which it is imaged. This new database allows us to
significantly revise the characterization (accretionary versus erosive) of
active margins, as well as the solid and fluid fractions of sediment
flowing through the subduction channel.

\section{The NE Japan margin as a case study}\label{sec2} 
Tremendous improvements have been done in the processing of
multichannel seismic lines since the 1980's. It concerns both the
penetration depth and the resolution of the data.  Figure~\ref{fig1} is
an example of the progress made, allowing the NE Japan margin to be
imaged with unprecedented accuracy  \citep{Parketal2021}. The
observation of widespread shallow normal faults cutting through the
Neogene sedimentary cover of the margin had already been reported
\citep{vonHueneLallemand1990} but not their deeper extent. In their
interpretation,  \citet{Parketal2021} identified some of these at
depths greater than 20~km. Their shallow dip suggests the reactivation
of former splay faults into normal faults as similarly observed off
Ecuador \citep{Collotetal2008} or at the transition from Alaska and
Aleutian subduction zones \citep{Kahrizietal2024}. A tiny
accretionary wedge (see thrust faults in red in  Figure~\ref{fig1}),
limited to the foot of the margin, reworks the material slided from the
margin rather than those deposited in the trench to the extent that the
latter is empty of terrigenous deposits. Truncation of margin's basal
reflections along the subduction interface (see green arrow labelled 
``basal erosion'' in  Figure~\ref{fig1}), seaward deepening of a
Miocene subaerial unconformity and sampling of Early Miocene arc
volcanic rocks at less than 100 km (see projection of DSDP site 439 in 
Figure~\ref{fig1}) from the present trench attest for subcrustal
tectonic erosion since at least 20~Ma
\citep{vonHueneetal1980,vonHueneLallemand1990}. Evidence for landward
migration of the volcanic front through time has been reported by 
\citet{Ohguchietal1989}. The ${>}$200~km progressive landward migration
of the volcanic front is illustrated in the  Figure~\ref{fig1} insert.
Downdip thickening of the subduction channel primarily observed by 
\citet{VonHueneetal1994} further north was later observed all
along the NE Japan margin  \citep{Tsuruetal2002}. The thickness of the
channel can locally reach up to 3~km--100~km north of the line
shown in \mbox{Figure~\ref{fig1}.}

\begin{figure*}
\includegraphics{fig01}
\vspace*{3pt}
\caption{\label{fig1}Pre-stack depth migrated seismic profile across
Japan margin off Ishinomaki Cape modified from  \citet{Parketal2021}.
Note the extent of normal faults (in dark blue) beyond the Moho, and
the restriction of the thrust faults (in red) to the frontal part of
the margin. The subduction channel, the top of which is delimited by
the dashed pale blue line, is well imaged down to 19~km.
$T_{\mathrm{channel}}$  refers to the thickness of the subduction
channel, $T_{\mathrm{sed}}$  is the trench fill sediment thickness
measured at deformation front (maximum thickness) and $T_{\mathrm{SP}}$
is the average sediment thickness covering the subducting plate away
from the trench. Updip and downdip limit of the seismogenic zone based
on historical ruptures after  \citet{Nishikawaetal2023}. Considering a
subduction rate of 92~mm/yr, the subducting oceanic crust near the red
star  (2011 Tohoku Eq hypocenter) began to  underthrust ${\sim}$1~Ma
ago. The projection of the DSDP site 439, distant of ${\sim}$200~km
from the present-day active volcanic front, where Early Miocene arc
volcanic rocks were drilled beneath a subaerial unconformity is shown
in purple. Insert: Location of seismic line and volcanic front since
21~Ma after  \citet{Lallemand1995}.}
\vspace*{5pt}
\end{figure*}

Given the progress made and the multitude of seismic lines acquired
along the subduction zones since the 1980's and 1990's, we have
undertaken to update the submap database
(\href{https://submap.gm.umontpellier.fr/}{submap.fr}) concerning not
only the sediment thickness at the trench $T_{\mathrm{sed}}$, but also
that of the sediment cover of the lower plate away from the trench
$T_{\mathrm{SP}}$, and especially that of the subduction channel
$T_{\mathrm{channel}}$ (see  Figure~\ref{fig1}). On the basis of these
elements as well as the morphology of the margin and any additional
elements such as the position of the volcanic arc in the past, we were
led to revisit the classification of active margins into accretion versus
erosion.

\section{Dominantly accretionary versus erosive margins over the last
million years}\label{sec3} 
Sorting active margins into accretionary or erosive types 
(Figure~\ref{fig2}) has long been challenging because the net growth
or decrease of the volume of a margin can only be considered over a
period that allows the integration of spatial (along and across strike)
and temporal variations, typically from 1 to several million years.
Thus, the presence or absence of an active accretionary prism,
especially at the margin front, is not sufficient to characterize the
margin regime. 

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Sketch showing two types of active margins. Left:
Typical accretionary margin undergoing frontal and subcrustal
(underplating) accretion of sediment transferred from the trench or the
subduction channel respectively (see small arrows at the base of
accretionary wedge). The wedge is growing through time, loading and
deflecting the subducting plate (former seafloor in dashed line). The
volcanic arc is generally stable with respect to the overlying plate.
The tectonic regime is generally compressive. Subcrustal removal may
occur (see question mark) so that we prefer to describe this case as
dominantly accretionary. Right: Typical erosive margin undergoing
frontal and subcrustal tectonic erosion feeding the subduction channel
that thickens downdip. The margin is generally narrow because it
oversteepens with time, with a maximum of subsidence trenchward, and
lacks of significant accretionary wedge. The volcanic arc often
migrates landward as the margin shrinks and thins (former seafloor in
dashed line). The tectonic regime is generally extensional.
Underplating at large depths may happen (see question mark) so that we
prefer to describe this case as dominantly erosive.}
\end{figure*}

For a long time, authors have described the structural complexity of
active margins based on the nature of rocks outcropping in the inner
forearcs or on seismic reflection images, shedding light on the first
${\sim}$10~km of crust 
\citep[e.g.,][]{CloosShreve1988a,CloosShreve1988b,VonHueneScholl1991}.
Thus, the notions of wedge-shaped or channel-like subducted sedimentary
units were later imaged, for example, by  \citet{Tsuruetal2002} beneath
the NE Japan margin. Underplating of sedimentary units that were first
underthrust, then incorporated into the margin and ultimately exhumed
from large depths as attested by their blueschist and eclogitic facies,
was primarily suspected in the Franciscan complex in California
\citep{Platt1975,Cloos1986} and then in numerous ancient accretionary
systems  \citep{Angiboustetal2021}.

The detailed exploration of subduction interfaces, such as in Ecuador,
Chile or Japan for example 
\citep[e.g.,][]{Collotetal2008,Contreras-Reyesetal2010,Parketal2021},
shows that the processes of growth or consumption of the margins evolve
in space and time. It is now possible to detect evidence of subcrustal
erosion down to depths of 20~km on a seismic profile 
(Figure~\ref{fig1}) and to record uplift in the innermost parts
potentially corresponding to the underplating of deeper sedimentary
units like in Chile  \citep{Angiboustetal2021}. 

\begin{figure*}
\vspace*{4pt}
\includegraphics{fig03}
\vspace*{4pt}
\caption{\label{fig3}Distribution of  ``dominantly accretionary'' (blue
trenches) and  ``dominantly erosive'' (red trenches) margins. Grey
trenches correspond either to convergent margins where continental
crust subducts like north of Australia, or where we do not have enough
data to decipher which type is dominant. MED ${=}$ Mediterranean, SEA ${=}$
Southeast Asia, NPA ${=}$ North Pacific, SWP ${=}$ Southwest Pacific, 
SAM~${=}$~South America.}
\vspace*{4pt}
\end{figure*}

\citet{VonHueneScholl1991} and then  \citet{CliftVannucchi2004} 
attempted to estimate the net volume of sedimentary material that is
accreted at convergent margins or subducted beneath the basement rocks.
The former authors thus classified the margins according to whether or not they
are building an accretionary prism while recognizing that the present
state does not necessarily reflect the past regime and that visible
accretion can mask a deep erosive process. From seismic profiles
distributed over 3/4 of the subduction zones, they concluded that 56\%
of the margins presented an accretionary regime against 44\%
non-accretionary. The latter authors calculated mass balances for subduction
margins over longer periods---typically ${>}$10~m.y.---and then
classified the subduction margins into accretionary versus erosive. This
had the effect of switching some segments initially identified as
accretionary, into erosive segments like Japan, Mexico, Panama,
Colombia, Ecuador, N-Peru or Manila. More recently, 
\citet{Festaetal2018} compiled both classifications by retaining
the intent of that of Clift and Vannucchi on the long-term behavior. 
\citet{Noda2016} proposed a classification of forearc basins based on
material transfer between the converging plates like von Huene and
Scholl (accretion versus non-accretion) but also on long-term strain field
in the basin (compressional versus extensional). \looseness=-1

The thickness of sediments observed at a subduction trench
$T_{\mathrm{sed}}$ has been shown to be an important parameter not only
in terms of material fluxes 
\citep{Scholletal1970,VonHueneScholl1991,CliftVannucchi2004} but also
as a parameter that may partly control the seismogenic and tsunamogenic
character of the subduction interface, in particular through the fluid
content of the subducted  section 
\citep{Ruff1989,LePichonetal1993,Heuretetal2012,Scholletal2015,Festaetal2018,Brizzietal2018,Brizzietal2020,Geersen2019}.
At the same time, many authors pointed out the bias of considering
only $T_{\mathrm{sed}}$ insofar as all or part of the sediments present
in the trench could be accreted to the margin and there was ample
evidence of material transfer to the margin or to the subduction
channel  \citep{Heuretetal2012,Scholletal2015,Seno2017}.

The difficulty today, if one wants to test the role of the sedimentary
thickness available at the foot of the margin $T_{\mathrm{sed}}$ or the
one effectively dragged into the subduction $T_{\mathrm{channel}}$, is
to have a reliable database. 

Apart from the pioneering work of 
\citet{Ruff1989} who characterized the 19 margin segments that hosted
an earthquake of $M_w>8$ according to the presence or absence
of an accretionary prism (ETS  ${=}$ Excess Trench Sediment) or horsts
and grabens (HGS) on the subducting plate, there are few studies 
\citep{VonHueneScholl1991,Heuretetal2012,Scholletal2015,Seno2017}, all
based on a limited number of observations, then extrapolated. 

A close
examination of the subduction zones shows that the lateral variations
along strike and downdip are sometimes very important. This is why we
have undertaken to gather as many observations as possible to upgrade
the existing datasets.\looseness=-1

Here, we will consider the mid-term behavior (a few m.y.) of the margin
based on criteria such as (1)~the down-dip variations of the thickness
(thickening/thinning) of the subduction channel, (2)~the tectonic
regime (compressional/extensional) of the margin overriding the
subduction interface, (3)~its vertical motion (subsidence/uplift), and
(4)~the migration of the volcanic arc (landward/seaward) 
(Figure~\ref{fig2}). By doing this along 3/4 of the subduction zones,
mostly the same as  \citet{VonHueneScholl1991} and 
\citet{Festaetal2018}, we have increased the proportion of dominantly
erosive margins (see  Table~\ref{tab1} and  Figure~\ref{fig3}).

\begin{table*}
\caption{\label{tab1}Comparison between global classifications of
margin types with respective segments lengths (this study)}
\begin{tabular}{c}
\tbody
\inlinefig{fx01}
\end{tabular}
\tabnote{Blue (accretion) and red (erosion) trench lengths highlight
major changes with respect to earlier classifications.} 
\end{table*}

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}The calculation of porosity in the subduction
channel at a distance of ${\sim}$15~km from the deformation front is
performed differently depending on whether the channel thickness
$T_{\mathrm{channel}}$ is greater or smaller than the 
$T_{\mathrm{sed}\_\mathrm{co}}$  threshold value corresponding to
$T_{\mathrm{sed}} \times K_{\mathrm{co}}$. If there is frontal
accretion, then only the already compacted basal  section
$(T_{\mathrm{sed}\_\mathrm{base}})$ of the trench sediments will be
entrained, whereas if there is no accretion, the entire
$T_{\mathrm{sed}}$  thickness will be entrained
$(T_{\mathrm{sed}\_\mathrm{base}}= T_{\mathrm{sed}})$. In addition,
sediments eroded from the sole of the margin may be added to the
subducted  section.}
\vspace*{-2pt}
\end{figure*}

\section{Methodology}\label{sec4}

\subsection{Sediment thicknesses estimation}\label{sec4.1} 
We have (re)analyzed more than 500 multichannel seismic profiles
published in 170${+}$ articles covering 88\% of the oceanic subduction
zones (see  Table~S1 in Supplementary Material). We still lack precise
information for some subduction zones: Solomons, Halmahera,
Philippines, Sulu, Cotobato, Sangihe, N-Sulawesi. Only the data we were
able to verify are reported in this study. Instead of providing average
values based on one or two profiles, we list measured values of $Z_t$,
$T_{\mathrm{SP}}$, $T_{\mathrm{sed}}$ and $T_{\mathrm{channel}}$ line
by line even when distant by only few kilometers (see 
Figures~\ref{fig1} and~\ref{fig4} for definition and  Table~S1
(Supplementary Material) for access to the complete \mbox{database).} 
Each~line, generally normal to the trench, is located by its intersection
(latitude, longitude) with the trench. $Z_t$ is the trench depth in
kilometers measured on the seismic line. $T_{\mathrm{SP}}$ is the
average sediment thickness covering the subducting plate away from the
trench measured on the seismic line. We do not specify the distance from
the deformation front at which we measure $T_{\mathrm{SP}}$, as this
can vary enormously depending on terrigenous inputs. Our aim is to
provide a value for the thickness of the oceanic cover away from the
terrigenous inputs that concentrate in the trench. This can be very
close to the deformation front if there are no terrigenous inputs, as
along most intra-oceanic subduction zones, or several hundred km away
if the trench coincides with a detrital fan, as in Cascadia or in the
Bay of Bengal. $T_{\mathrm{sed}}$ is the trench fill sediment thickness
measured at deformation front down to the top of the oceanic crust and
$T_{\mathrm{channel}}$ refers to the thickness of the subduction
channel between the plate boundary and the top of oceanic crust, close
or near the updip limit of the seismogenic zone, as defined for example
in  \citet{Heuretetal2011}. We will further consider a mean distance of
${\sim}$15~km from the deformation front, given that it may vary from
10 to 30--40~km. As often observed, the thickness of the channel may
vary significantly as pore fluids are expelled downdip but also as a
function material transfer from or to the margin. This, in conjunction
with the fact that the channel imagery is highly variable depending on
the quality of the seismic data, especially at great depths (a depth of
20~km, as shown in  Figure~\ref{fig1}, remains exceptional), means that
the value provided should not be considered as an absolute value but
rather as an averaged representative value of the profile. The
thicknesses in kilometers are either issued from depth  sections or
converted from two-way-travel time sections using simplistic empirical
laws assuming that sound velocities range from 2 to 3 km/s depending on
the thickness for the trench fill sediment. It has sometimes happened
that very different values are proposed for  sections-depths of the
same seismic line in different papers. In this case, we decided after a
short investigation which solution made the most sense to us. Again, we
do not claim here to provide an exact value that would require
knowledge of the seismic velocities on the  section-time concerned but
an order of magnitude that is close to reality. By doing this, 
we can reasonably assume an average error between 10 and
20\% depending on the quality of the seismic data. The density of the
dataset varies greatly from region to region. We 
extrapolated the margin type to along-strike distances never greater
than  200~km when the lateral variations seemed minor. Regarding
$T_{\mathrm{sed}}$ and $T_{\mathrm{channel}}$, we did not extrapolate
at distances larger than 50~km away from a line. In order to feed the
submap database with a step distance of around  2\textdegree, we
sometimes had to extrapolate the values of $T_{\mathrm{sed}}$,
$T_{\mathrm{channel}}$ and $T_{\mathrm{SP}}$ along the trenches, based
on the nearest measurement, but also on morphological criteria such as
the bathymetric profile of the trench. In such case, the symbols of the
parameters are then accompanied by an asterisk, ${T_{\mathrm{sed}}}^*$,
${T_{\mathrm{SP}}}^*$, ${T_{\mathrm{channel}}}^*$. We provide the source
for each seismic\break line. \looseness=1

\subsection{Margin type characterization}\label{sec4.2} 
The margin type MT  ``dominantly accretionary'' or ``dominantly
erosive'' over the last few million years has been updated 
(Figures~\ref{fig2} and~\ref{fig3}) mainly based on data from the
literature but also according to the criteria mentioned in previous 
section ($T_{\mathrm{sed}}$--$T_{\mathrm{channel}}$, extensional versus
compressional regime, margin subsidence/uplift, volcanic front
migration). 

Raw data set is accessible as supplementary information. Mean values
every ${\sim}$200~km of trench are accessible on line at the following
url: \href{https://submap.gm.umontpellier.fr/}{submap.fr}.

\subsection{Sediment flux calculation}\label{sec4.3} 
To estimate the balance of solid or fluid fraction transiting through the
subduction channel, we need to estimate the porosity of the sequence in
the channel, as well as its thickness. To do this, we convert the depth
versus porosity curves of trench floor sedimentary deposits obtained
from DSDP and ODP cores and seismic velocities  [Figure~S1 in
Supplementary Material extrapolated in depth from 
\citealp{VonHueneScholl1991}] into a thickness versus mean porosity
curve  (Figure~S2 in Supplementary Material). Then, we considered 2
distinct cases, depending on whether part of the sedimentary sequence
was frontally accreted or not  (Figure~\ref{fig4}). In the first case
(frontal accretion), the channel thickness $T_{\mathrm{channel}}$ will
be less than that of the sedimentary sequence at the trench
$T_{\mathrm{sed}}$ compacted by a factor $K_{\mathrm{co}}$, i.e.
$T_{\mathrm{sed}\_\mathrm{co}}$. It means that the incoming  section
subducting at trench consists only in the lower part of the trench
sediment  section further called $T_{\mathrm{sed}\_\mathrm{base}}$. This
implies that the mean porosity of the subducting sediment layer
$\Phi_{\mathrm{sed}\_\mathrm{base}}$ is less than that of the
whole trench sequence, i.e., $\Phi_{\mathrm{sed}}$. The average
porosity in the channel $\Phi_{\mathrm{channel}}$ will be reduced
by a factor $K_r$ compared with that of the subducted  section
$\Phi_{\mathrm{sed}\_\mathrm{base}}$. The second case, without
frontal accretion, generally corresponds to subcrustal erosion if
$T_{\mathrm{channel}}$ is greater than $T_{\mathrm{sed}\_\mathrm{co}}$.
The channel sequence will therefore be composed of a basal  section
issued from the trench $T_{\mathrm{sed}\_\mathrm{co}}$ with a porosity
$\theta_{\mathrm{sed}\_\mathrm{co}}$, and an upper  section
issued from margin removal $T_{\mathrm{ero}}$ with an average porosity
$\Phi_{\mathrm{ero}}<10\%$ \citep{SafferTobin2011} that we set to 5\%.
Details of the calculations used to determine an average porosity
reduction coefficient $K_r$ between the incoming  section at the trench
and the channel at an average distance of ${\sim}$15~km from the
deformation front are given in the Supplementary Material, as well as
for the compaction coefficient $K_{\mathrm{co}}$ between the thickness
of incoming sediment and that in the channel. The compaction
coefficient $K_{\mathrm{co}}$ naturally depends on the initial porosity
measured at the trench in the incoming sequence,\break i.e.,
$\Phi_{\mathrm{sed}}$ or $\Phi_{\mathrm{sed}\_\mathrm{base}}$.

Calculating the solid and fluid fluxes entrained in each subduction
zone requires processing the data obtained along the 535 unevenly
distributed profiles in such a way as to provide the relevant
information per representative transect along each subduction zone. Of
the 260 submap database transects, only 116 were able to be assigned
with a mean $T_{\mathrm{channel}}$ value, due to their proximity to a
seismic line allowing the channel to be imaged at a distance of at
least 10~km from the deformation front. We therefore first estimated
the solid and fluid fractions along each of the 116 constrained
transects (in green in Table~S2 in Supplementary Material) using the
estimated porosity $\Phi_{\mathrm{channel}}$, and then calculated the
fluxes per km of trench by multiplying the solid or liquid fraction by
the normal component of the subduction velocity recently updated in the
submap database  (Table~S2). We then extrapolated these values along
the subduction zones, weighting them by the width sampled on each
transect, which enabled us to cover around 70\% of the total length of
the subduction zones.

\section{Results}\label{sec5}

\subsection{Margin type}\label{sec5.1}
Our updated dataset allowed us to significantly modify the distribution
of margin types and, at the end, increase the proportion of 
``dominantly erosive'' margins compared with former studies. We count
in this study (see  Table~\ref{tab1} and Table~S1 in Supplementary
Material for details and references)  25,420~km of  ``dominantly
erosive'' margins, 18,500~km of ``dominantly accretionary'' margins and
13,470~km of either undetermined type or where oceanic subduction
laterally switches into continental subduction like north of Hispaniola
or Australia. In such cases, it is impossible to measure the sediment
thickness because the continental basement is either poorly defined or
not imaged at all. Based on the 3/4 of subduction zones for which it
has been \mbox{possible} to \mbox{determine} the dominant margin regime, at least in
the submerged part, we observe 42\% accreting margins (25,420~km) and
58\% eroding margins (18,500~km). However, we do not rule out the
possibility that some erosive margins may simultaneously record
subcrustal accretion far from the deformation front or that some
margins assumed to be accreting are subject to deep subcrustal erosion.

\begin{table*}
\caption{\label{tab2}Digest of Table~S1 in Supplementary 
Material\vspace*{4pt}}
\begin{tabular}{c}
\tbody
\inlinefig{fx02}
\end{tabular}
\tabnote{Trench segments by region (MED  ${=}$ Mediterranean, MAK  ${=}$
Makran, N-PAC  ${=}$ North Pacific, CARIBBEANS, S-AMERICA  ${=}$ Andes and
S-Sandwich, SE-ASIA  ${=}$ Philippine Sea and Sunda, SW-PAC  ${=}$
Southwest Pacific) were firstly defined according to margin type MT,
then sediment thickness at trench $T_{\mathrm{sed}}$  and finally
sediment thickness below the plate boundary $T_{\mathrm{channel}}$,
each of them showing little variations within a given trench segment.
Mean thickness values are reported in km for each segment.} 
\vspace*{1pc}
\end{table*}

We confirm the accretionary character of the Mediterranean, Makran,
South Lesser Antilles, Sumatra-Andaman, Alaska, Cascadia, Nankai and
South Hikurangi margins  (Figure~\ref{fig3}). We also confirm the
erosional character of the Puerto Rico, South Sandwich, New Hebrides,
Izu-Bonin-Mariana or Tonga-Kermadec margins. Some inconsistencies with
previous classifications  (Table~\ref{tab1}) are linked to the time
period concerned (current  versus 1--2~m.y. versus 10 m.y.). Discrepancy is
greatest when  \citet{VonHueneScholl1991} list non-accretionary
margins, while we list erosive margins. When re-examining the type of
margin, we based our analysis---as indicated above---on a set of criteria
and not just on the excess or deficit of sediment present in the
subduction channel (see  Table~\ref{tab2}). The differences with
earlier studies include about 600 km of the Lesser Antilles margin
north of its intercept with the Tiburon Ridge. The forearc underwent
there a dramatic subsidence and extension since middle Miocene
(${\sim}$16~Ma) which is attributed to severe basal erosion following
the subduction of the southern extent of the Bahamas Bank 
\citep{Boucardetal2021}. Further west, the Puerto-Rico-Virgin-Islands
margin recorded an even stronger episode of tectonic erosion 
\citep{Grindlayetal2005}. Most of the Central America forearc turned
into  ``dominantly erosive'' margins  \citep{Pradaetal2023} except the
northernmost and southernmost terminations 
\citep{Bartolomeetal2011,MacKayMoore1990}. The Central Aleutians margin
is currently considered as accretionary based on the well-developed
accretionary wedge but  \citet{JichaKay2018} demonstrated that the
landward arc migration increased to the west and is greatest over the
last 5 m.y. Furthermore, they interpret adakite-like lavas as
additional evidence for ongoing subduction-erosion. Northeast Japan and
Northern Andes (South Colombia--Ecuador--Peru) turned to exhibit all
characters of subduction-erosion even if some local sediment accretion
may occur at the margin front 
\citep[e.g.,][]{Collotetal2004,Marcaillouetal2006}. The Chilean margin
shows lateral variations already highlighted in 
previous classifications, 
with a predominantly erosive behavior, albeit
accretionary evidence in the center. On the other hand, the location of
accretionary sectors between 33.4 and  38.2\textdegree S and south of
54.1\textdegree S is more precise in our study. We agree with 
\citet{Festaetal2018} that the margin south of Java is dominantly
erosive and that the Manila subduction zone is accretionary over its
entire length. The Ryukyu margin is still dominantly erosive but we
have changed the status of its southernmost termination, between Taiwan
and the intersection with the Gagua ridge where sedimentary accretion
dominates  \citep{Lallemandetal2013,Nishizawaetal2017}. Finally, we
have characterized new margins such as in the Caribbean Sea
(North-Panama, South-Caribbeans, Muertos) exhibiting sedimentary
accretion or Puysegur south of New Zealand being mainly\break erosive.

\subsection{Total sediment thickness in the trench and the subduction
channel}\label{sec5.2} 
Based on a re-examination of 500 multichannel seismic lines across the
subduction zones, we have\unskip\break
updated the database firstly published by 
\citet{Heuretetal2012} not only regarding the thickness of incoming
sediment $T_{\mathrm{sed}}$ but also those of the subduction channel
$T_{\mathrm{channel}}$ when available as well as those of the
sedimentary cover away from the trench area ($T_{\mathrm{SP}}$, see 
Table~S1 for data and references line by line). An extract of the
database each  2\textdegree\ is provided on the submap web-tool
(\href{https://submap.gm.umontpellier.fr/}{submap.fr}). We have chosen
to represent the $T_{\mathrm{sed}}$ and $T_{\mathrm{channel}}$ data in
visual form with a color code against the margin types (see
explanations in  Figure~\ref{fig5}). The maximum thickness of sediment
available in the trench $T_{\mathrm{sed}}$ is plotted on the ocean
side. The thickness of the channel is represented indirectly by
subtracting it from the thickness of the sediments in the trench
($T_{\mathrm{sed}}$--$T_{\mathrm{channel}}$), 
so that negative values (in
red) indicate an excess of subducted sediments over incoming sediments,
and positive values (in blue) indicate a deficit of sediments when the
upper (or entire)  section of entering sediment accrete at the margin
front, for example. In absolute terms, knowing that subducted sediments
are compacting, it would be necessary to compare $T_{\mathrm{channel}}$
and $T_{\mathrm{sed}\_\mathrm{co}}$, but as the difference 
is\unskip\break  
minor, we
have contented ourselves with plotting
$T_{\mathrm{sed}}$--$T_{\mathrm{channel}}$. As the spacing between seismic
profiles is extremely variable from one region to another, from less
than 1~km to over 100~km, we plotted the average of $T_{\mathrm{sed}}$
and ${T_{\mathrm{sed}}}^*$ or $T_{\mathrm{channel}}$ and
${T_{\mathrm{channel}}}^*$ values as a colored bar every km,
perpendicular to the trench, within a sliding window of 100~km along
the trench. In other words, empty sectors mean that no values could be
measured or estimated at a distance of less than 50~km.

\begin{figure*}
\includegraphics{fig05}
\vspace*{-2pt}
\caption{\label{fig5}Close-up view of margin type and related mean
sediment thickness available at trench and within the subduction
channel in the North Pacific area. Total sediment thickness in the
trench $T_{\mathrm{sed}}$  is plotted in yellow to green colors seaward
of the trench axis at the resolution of available MCS data. Landward of
the trench axis, we plotted the difference in thickness between the
incoming sediments and those of the subduction channel
$T_{\mathrm{sed}}$--$T_{\mathrm{channel}}$. Blue colors mean that the
upper part of trench sediment is offscrapped to build the accretionary
wedge like off Kamtchatka, whereas red colors mean that the thickness
of the subduction channel exceeds those of the entering  section like
off Tohoku (NE Japan). EUR: Eurasian plate. NAM: North American plate.
PAC: Pacific plate.}
\vspace*{-5pt}
\end{figure*}

Figure~\ref{fig5} and Table~\ref{tab2} show that we now have an almost
complete coverage of $T_{\mathrm{sed}}$ and less coverage of
$T_{\mathrm{channel}}$. Generally speaking, and with rare exceptions,
an excess of sediment in the channel in relation to the thickness of
the incoming  section highlights the erosive nature of the margin.
Positive $T_{\mathrm{sed}}$--$T_{\mathrm{channel}}$ values, reflecting
frontal accretion, are sometimes associated with margins with a
dominant erosive character (e.g., Central Aleutians 
179--187.4\textdegree E, Colombia 1.7--3.5\textdegree N, North
Hikurangi  37.8--39\textdegree S). Most of the time, this is due to a
bias in $T_{\mathrm{channel}}$ estimation, when its measurement could
only be carried out at a short distance from the trench and is not
corrected for compaction. For this reason, this value should be used
with caution. 

At the world scale (Figure~\ref{fig6}), we observe that thin trench
fill, typically less than 0.8 km, often faces erosive margins (South
Kuril Islands, NE Japan, Izu-Bonin-Mariana, Ryukyu, Java, Northern Lesser
Antilles, Northern Andes, Tonga-Kermadec, New-Hebrides) and thick
trench fill, typically more than 0.8 km, often faces accreting margins
(Nankai, Manila, Sumatra-Andaman, Alaska, Cascadia, Southern Lesser
Antilles, Panama-Venezuela, Hikurangi, Makran or Mediterranean).
However, there are some exceptions to this association, such as
Northern Kuril-Kamtchatka where accretionary processes may prevail
while $T_{\mathrm{sed}}$ does not exceed 0.7~km 
\citep{Gnibidenkoetal1983,Klaeschenetal1994,Baranovetal2022}. We 
classified the Colombia margin as subject to subduction erosion while
mean $T_{\mathrm{sed}}$ between  1.7\textdegree N and 3.5\textdegree S
is 3.3 km and mean $T_{\mathrm{channel}}$ is 1.4 km. According to 
\citet{Collotetal2008}, coeval basal erosion of the outer wedge
proceeds pervasively together with deeper underplating with a balance
in favor of erosion over the mid-term. Splay faults are activated as
new plate boundaries as margin slices are progressively dragged down
the subduction zone. As described above, despite the 2 km sediment
thickness in the trench in the central Aleutians between 179 and 
187.4\textdegree E, the recent landward migration of the arc as well as
the composition of the lavas argues for deep tectonic erosion 
\citep{JichaKay2018}. South Chile, between 39 and 53\textdegree S,
North New-Hebrides, between 12 and  17.2\textdegree S, South Puysegur,
between 46 and  49\textdegree S, Hispaniola and Puerto-Rico combine
trench sediment thicknesses more than 1.4~km, thick subduction channels
(${\geq}$1.2~km) and typical characteristics of erosive margins
(narrow, steep, sometimes extensional).\looseness=1

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}Margin type and related mean sediment thickness
available at trench and within the subduction channel around the world
except the North Pacific area. See also explanations in legend of
Figure~\ref{fig5}. AFR  ${=}$ Africa plate, ANT  ${=}$ Antarctica plate,
ARA  ${=}$ Arabia plate, COC  ${=}$ Cocos plate, IND  ${=}$ India plate, NAZ 
${=}$ Nazca plate, PHS  ${=}$ Philippine Sea plate, SAM  ${=}$ South America
plate, SCO  ${=}$ Scotia plate.}
\vspace*{-2pt}
\end{figure*}

We observe large variations of $T_{\mathrm{sed}}$ from one trench to
another: 0.54 ${\pm}$ 0.4~km from NE Japan to Kamtchatka, 1.98 ${\pm}$
1.0~km along the Aleutians-Alaska or 3.27 ${\pm}$ 0.5~km off Cascadia,
but also from one profile to the adjacent one, even at short spacing
interval like in the central Aleutians for example. Significant local
variations in sediment thickness along the trenches are often due to
major offsets in the subducting basement, linked to the presence of
seamounts or fracture zones, for example.

\subsection{Volumes of solid and fluid transiting through the
subduction channel}\label{sec5.3} 
The large number of observations taken along most of the subduction
zones enables us to update estimates of solid and fluid fluxes
through the subduction channel. In order to compare our results with
those proposed by  \citet{VonHueneScholl1991}, we attempted to use
a similar approach. First, we determined the solid and liquid fraction
of the subducted sedimentary  section on the basis of average porosity
(see Section~\ref{sec4}). The major differences, apart from the
quantity of data analyzed, consist in measuring the thickness of the
channel at an average distance of ${\sim}$15~km from the deformation
front, taking into account compaction in the channel, updating
subduction velocities and sometimes differences in trench length (see
Supplementary Material for details on the procedure). We then estimated
material fluxes per km of trench in areas where we had
$T_{\mathrm{channel}}$ values, and extrapolated these data along the
subduction zone. In the absence of constraints on a given subduction
zone, such as the Sunda region north of Australia, New Britain and the
Solomons, Izu-Bonin, Yap-Palau, Puerto-Rico, Panama, Hjort or South
Sandwich, we did not provide estimates. The details of the calculations
are provided in the Supplementary material  (Table~S2) and average
porosities and fluxes values per km of constrained trench and per
trench are given in  Table~\ref{tab3}.

\begin{table*}
\caption{\label{tab3}Mean porosities $\Phi_{\mathrm{sed}}$  and 
$\Phi_{\mathrm{channel}}$ and solid and fluid fluxes dragged into the
subduction channel per km of trench and extrapolated per trench}
\begin{tabular}{c}
\tbody
\inlinefig{fx03}
\end{tabular}
\tabnote{The last 3 columns are values of $\Phi_{\mathrm{sed}}$ and
material fluxes reported from  \citet{VonHueneScholl1991} for
comparison, given that the mode of calculations slightly differs (see
main text).}
\vspace*{-2pt}
\end{table*}

Our results show very strong variations from one subduction zone to
another, between sectors where fluxes are gigantic, as in the Aegean,
Central America, Chile, New Hebrides and Aleutians, and sectors where
fluxes are negligible, as in the Andaman-Sumatra, Ryukyus or
Tonga-Kermadec zones. We also note that pore water fluxes are certainly
also significant in the same regions where solid fluxes are important,
but not in the same proportions. For example, most of the water flowing
through the\unskip\break 
subduction channel is concentrated under Central America,
Peru--Chile, and under the New Hebrides, Kuril Islands and Aleutians. In
particular, the Aegean subduction zone is characterized by the largest
solid flow for a very small fluid flow. On the basis of 70\% cumulative
trench length, we estimate that every million of years for the past few
million years,  1463~km$^{3}$ and 396~km$^{3}$ of solid sediment and
pore water respectively have been transiting through the subduction
channel, having subducted some ${\sim}$15~km from the deformation front.
In  Table~\ref{tab3}, we report the values obtained by 
\citet{VonHueneScholl1991}, 
bearing in mind that they only provided solid fluxes
based on the porosity of the sediment column estimated at the trench.
Surprisingly, in the end, we arrive at the same order of magnitude for
solid flux, i.e.  1.5~km$^{3}$/yr along 70\% of the subduction zones.
In detail, we can see large discrepancies with our estimates (e.g.,
Aegean, Japan, Central America, Ryukyus, Nankai, New Hebrides,
Cascadia, Andaman, Sumatra, Tonga, Kermadec), which can be explained by
the smaller number of constrained transects more than 30 years ago, but
also by the fact that we have taken into account the first stages of
compaction.

\section{Discussion}\label{sec6}

\subsection{$T_{\mathrm{sed}}$ and $T_{\mathrm{channel}}$  may be
decorrelated}\label{sec6.1} 
We have reported the values of  Table~\ref{tab2}, mean
$T_{\mathrm{sed}}$, mean  $T_{\mathrm{channel}}$ and margin type MT
for each trench segment, on a diagram  (Figure~\ref{fig8}) after
sorting the trench segments, first based on their associated MT and
second based on decreasing $T_{\mathrm{sed}}$.

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Map of solid (sediment) and fluid (pore water)
fluxes estimated from the solid and fluid fractions of the subduction
channel, having travelled approximately 15~km from the deformation
front, by subduction zone in thousands of km$^3$ per million years.}
\end{figure*}

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}Diagram sorting the trench segments first as a
function of margin type (grey domains) and second as a function of
decreasing $T_{\mathrm{sed}}$  mean values (blue dots and curve).
Superimposed are the $T_{\mathrm{channel}}$  mean values (orange) when
available. Note that $T_{\mathrm{channel}}$  is often equal or larger
than $T_{\mathrm{sed}}$  for dominantly erosive trench segments.}
\end{figure*}

Although not all trench segments have information on the thickness of
sediment in the subduction channel, we note that, apart from the major
accretionary prisms: Mediterranean, Makran or Southern Lesser Antilles,
there is no obvious correspondence or proportionality between the
thickness of incoming sediment and that of the channel. However, we
note that $T_{\mathrm{channel}}$ is systematically lower than
$T_{\mathrm{sed}}$ in accreting margins, whereas it is more or less
close to $T_{\mathrm{sed}}$ in eroding margins. This diagram provides
information primarily on the thickness of frontally accreted sediment
or, on the contrary, the material removed from the prism (subcrustal
erosion) that feeds the subduction channel at the front of the prism
where it can still be imaged seismically. Even if, overall, the erosive
margins are characterized by a small gap between $T_{\mathrm{sed}}$ and
$T_{\mathrm{channel}}$, we can see that frontal accretion is carried
over along some trench segments, such as in southern Colombia, central
Aleutians, Kermadec or the Ryukyus. This means that deep subcrustal
erosion of the margin exceeds frontal accretion in these areas. 

\vspace*{-3pt}
\subsection{Porosity variations in the subduction
channel}\label{sec6.2} 
\vspace*{-2pt}
The sediment porosity in the subduction channel
$\Phi_{\mathrm{channel}}$ depends on those of the incoming
sequence (beneath the accreted sequence in case of frontal accretion,
$\Phi_{\mathrm{sed}\_\mathrm{base}}$), a porosity reduction
factor $K_r$, and the porosity $\Phi_{\mathrm{ero}}$ of the
material subcrustally eroded from the margin (in case of erosion).
Assuming  $K_r = 0.85$ and  $\Phi_{\mathrm{ero}} = 5\%$, it is possible
to express $\Phi_{\mathrm{channel}}$ as a function of
$T_{\mathrm{sed}}$ and $T_{\mathrm{channel}}$ (see Section~\ref{sec4}
and Supplementary Material).  Figure~\ref{fig9} shows that maximum
porosities are obtained close to the threshold value where 
$T_{\mathrm{channel}}=  T_{\mathrm{sed}\_\mathrm{co}}$ is represented
by the straight line. The values obtained on the 116 constrained
transects are plotted on the diagram, with triangles representing cases
with frontal accretion (below the threshold line) and dots representing
cases with erosion (above the threshold line). A wide dispersion can be
observed, reflecting the variety of thicknesses in the trench
$T_{\mathrm{sed}}$ as well as those in the channel
$T_{\mathrm{channel}}$, but on average, porosities are slightly higher
for erosive margins, despite the fact that some of the subducted
material only has a porosity of 5\%. This is because the entire
sequence at the trench is subducted under erosive conditions, and the
upper part of the sequence at the trench also has the highest porosity.

\begin{figure}
\includegraphics{fig09}
\caption{\label{fig9}Predicted mean porosity in the subduction channel
at a distance of ${\sim}$15~km from the deformation front as a
function of $T_{\mathrm{sed}}$  and $T_{\mathrm{channel}}$. The dots
(erosive margins) and triangles (accretionary margins) represent the
values corresponding to the 116 transects for which we have measured
both $T_{\mathrm{channel}}$  and $T_{\mathrm{sed}}$  (Table~S2). We
have also drawn the threshold line between frontal accretion and
subcrustal erosion when  $T_{\mathrm{channel}}=
T_{\mathrm{sed}\_\mathrm{co}}$.}
\end{figure}

\subsection{Solid and fluid volumes dragged at depth within the
subduction channel}\label{sec6.3} 
Given that we only estimate the volumes of solid or fluid
transiting through the shallow subduction channel, and therefore do not
predict transfers between the margin and the subducting plate that
may occur deeper down, we have nevertheless updated previous estimates.
Our first observation is that $T_{\mathrm{channel}}$ determines the
volume of subducted material, whereas $T_{\mathrm{sed}}$ will have
essentially an effect on $\Phi_{\mathrm{sed}}$. Indeed, some
subduction zones characterized by thick trench fill like Andaman,
Sumatra or Cascadia-Vancouver are associated with very little sediment
flux. The reason is that most of the sedimentary pile at trench is
accreted frontally. The flux of solid material dragged in depth is
decorrelated from the subduction rate as illustrated by the 
Aegean--Cyprean subduction zone which allows the most solid sediment to pass
through  (Figure~\ref{fig7}), 
while the average subduction rate (normal
component) is only 27~mm/yr  (Table~S2). This is due to the fact that
the thickness of the incoming sediments is great (as is the length of
the trench), but as a result their porosity is low (17.2\%), as they
were already compacted before subducting into the channel. As a result,
the volume of fluid entrained in this subduction zone is comparatively
very low. Another observation is that the greatest volumes of fluid in
the channel globally subduct along erosive margins. This may result
from the combination of a larger channel porosity and a higher
subduction rate. As indicated above  (Section~\ref{sec5}), our overall
values for subducted solid matter flux are comparable to those
estimated by  \citet{VonHueneScholl1991}. In contrast, the volume of
subducted fluid is significantly lower in our study, mainly due to the
fact that we took into account both the  ``pre-compaction'' of
incoming sediments in accretion cases and then applied a porosity
reduction factor in the channel. In the end, we obtain 
${\sim}$0.4~km$^3$/yr of fluids entrained in the channel along 
40,450~km of trench, whereas  \citet{VonHueneScholl1991} proposed
${\sim}$0.8~km$^{3}$/yr along 43,450~km of trench.

\subsection{$T_{\mathrm{sed}}$ and seismogenic potential along the
subduction interface}\label{sec6.4} 
As we have already pointed out, several studies suggest that subduction
mega-earthquakes occur preferentially along opposite sediment-filled
trenches  \citep{Ruff1989,Heuretetal2012,Scholletal2015}. 
\citet{Geersen2019} noted that sediment-starved trenches and rough
subducting plates are conducive to tsunami earthquakes. As Geersen,
some of the formerly cited authors consider that $T_{\mathrm{sed}}$
alone is not a good proxy, as the thickness of sediment overlying the
subducting plate in the seismogenic zone can sometimes differ quite
widely from that available in the trench.  \citet{Scholletal2015}, for
example, noted the correspondence between most $M_{w}\geq8.5$
earthquakes and high $T_{\mathrm{sed}}$ values, but pointed out that
some $M_{w}\geq8.0$ earthquakes could sometimes occur along weakly
sediment-filled trenches facing erosive margins, i.e., the products of
subcrustal erosion thicken the subduction channel.  \citet{Seno2017}
comes to the same conclusion and then provides a rough estimate of the
correction to be made to $T_{\mathrm{sed}}$. As can be seen, the
general idea is that interseismic coupling seems to be stronger with a
smooth subduction interface  \citep{Lallemandetal2018} and a large
radius of curvature  \citep{Bleteryetal2016}, hence the concern to
describe the roughness of this interface as closely as possible. While
it remains impossible to image the interface over the entire depth
range of the seismogenic zone, we provide here an estimate of
$T_{\mathrm{channel}}$ close or at the updip limit of the seismogenic
zone [${\sim}$11 ${\pm}$ 4~km in average according to 
\citealp{Heuretetal2011}] in a number of regions. The analysis of this
new dataset with the objective to evaluate and eventually revisit our
former conclusions based on a less complete dataset is under process.

\subsection{Nature of the incoming sediment}\label{sec6.5} 
It is not just the thickness of the sediments in the trench or channel
that is important, but recent studies show that their nature and spatial
distribution are likely to play a role in seismogenic potential. It
appears that the mud-rich mass transport deposits (MTDs) modify the
architecture and properties of the subduction channel 
\citep{Festaetal2018,Geersenetal2020}. Heterogeneous fabric and fluid
content possibly favor slow ruptures, particularly in shallow parts.
Magmatic intrusions (sills) within the sedimentary cover appear as
patches in few areas around the rupture area of the 2011 Tohoku
earthquake.  \citet{Fujieetal2020} thus suggest that disturbance and
thermal metamorphism associated with this recent volcanic activity
modify and shape the size and distribution of interplate earthquakes
off NE Japan. In the Nankai subduction zone,  \citet{ParkJamaliHondori2023}
suggest that the underthrust turbidites cause low pore-fluid
overpressure and high effective vertical stress across the decollement
inhibiting the slow earthquakes occurrence. Similar heterogeneities
were also found in the shallow part of the subduction interface along
the Hikurangi margin  \citep{Gaseetal2022}. There, they consist of
pelagic carbonates versus heterogeneous volcaniclastics. The presence of
pelagic carbonates above thickened non-volcanic siliciclastic sediments
plays a key role in smoothing the rough topography of the subducting
plate and higher coupling that could be prone to large ruptures. In
this study, we do not directly address the question of the nature of
the incoming sediments, as this would require lengthy developments, but
we provide whenever possible a proxy for the contribution of
terrigenous inputs via the submarine drainage network thanks to the
difference between $T_{\mathrm{sed}}$ and $T_{\mathrm{SP}}$ 
(Table~S1). Unsurprisingly, we find the great detrital fans such as the
Nile in the Mediterranean, the Indus or Bengal in the Makran and
Andaman, the Astoria and Nitinat in the Cascades or the Orinoco in the
Lesser Antilles. 

\subsection{Uncertainties on plate interface transient
migration}\label{sec6.6}
Mass transfer occurs in the vicinity of the plate boundary landward of
the deformation front from the subducting to the overriding plate
(underplating) or the opposite (basal erosion). By nature, field
evidences of erosive structures in the vicinity of exhumed subduction
channels are tiny  \citep{Bachmannetal2009,Vannucchietal2010}. Such a
mass transfer process assumes that the subduction interface migrates
upwards pervasively via hydrofracturing of the overriding plate sole 
\citep[e.g.,][]{VonHueneetal2004} or transiently in the wake of a
seamount, for example  \citep[e.g.,][]{Lallemandetal1994}. Similarly,
certain discontinuities along the interface, such as variations in
fluid pressure or rheology, can cause downward migration of the
d\'{e}collement, leading to the incorporation of a subduction channel
slice at the margin  [underplating, 
\citealp{SampleMoore1987,VonHueneScholl1991,Raimbourgetal2019,Angiboustetal2021}]. 
Today, it is relatively easy to trace the d\'{e}collement at shallow
levels, based on seismic reflection amplitudes and polarity reversals.
Sometimes, we can observe the ghosts of abandoned
paleo-d\'{e}collements such as in the Nankai accretionary wedge  [deep
strong reflector of  \citealp{Parketal2002}]. Based on the estimates of
basal versus internal friction along the Chilean margin, 
\citet{Cubasetal2022} were able to characterize the sectors of the
subduction interface where deformation was distributed, i.e. where
forethrusts paralleled the interface. The authors interpret this type
of condition (small differences in friction) as favorable to
underplating, but it could just as easily 
favor basal erosion.\unskip\break The
distribution of the deformation around the plate interface over the
last Ma makes analysis (margin type, $T_{\mathrm{channel}}$) complex
on the basis of the current image of the margin alone.

\section{Conclusion}\label{sec7}
We provide in this study an exhaustive database describing the lateral
variations of the sediment thickness covering the oceanic plate when
approaching a subduction zone, at the deformation front in the trench
and in the shallow portion (${<}$20~km) of the subduction channel. The
unprecedented density of the data acquired opens the door to future
\mbox{multivariate} analyses, notably concerning the dependence of the
seismogenic behavior of the subduction interface on the thickness of
the subduction channel versus that of the sediments present in the trench.
A mass balance of material transfer from the trench to depths through
the subduction channel, as proposed by \citet{VonHueneScholl1991}, is
updated by analyzing fluxes, knowing the convergence rate and
making assumptions on the sediment porosity, over the last m.y.
Maximum porosity in the channel is reached when there is no accretion
or tectonic erosion.

With the caveat that the observations used to carry out this study can
be traced back to the last m.y., while the seismic lines or morphology
reflect the current to recent state, and the vertical motions of the
margin or the migration of the volcanic arc incorporates a longer
period, we propose an adjustment in the tectonic style of certain
margins, with a final erosive dominance.

\section*{Dedication}
The first author would like to dedicate this study to Roland von Huene,
Jean Aubouin's partner at a time when the process of subcrustal
tectonic erosion of active margins had only just been revealed and was
the subject of much criticism.

\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} 
We are grateful to Jin-Oh Park who kindly provided us the seismic line
of Figure~\ref{fig1}. We would like to thank Hugues Raimbourg for his
detailed review, and in particular his encouragement to revisit
material balance on the basis of our new data. We would also like to
thank the anonymous reviewer. Finally, the first author is particularly
grateful to one of the co-editors of this special issue, Laurent
Jolivet, for prompting him to update his knowledge of a subject that
first attracted his attention some thirty years ago.

\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-252-suppl.pdf}}

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