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\DOI{10.5802/crgeos.293}
\datereceived{2024-12-19}
\daterevised{2025-04-30}
\dateaccepted{2025-05-07}
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\dateposted{2025-06-16}
\begin{document}

\begin{noXML}

\TopicFR{Tectonique, tectonophysique, g\'eodynamique}
\TopicEN{Tectonics, tectonophysics, geodynamics}

\CDRsetmeta{articletype}{research-article}

\shortrunauthors

\title{Tectonic underplating at intermediate depths in subduction
zones: structural and geochronological evidence preserved in Corsican
ophiolites}

\alttitle{Sous-plaquage tectonique \`{a} des profondeurs
interm\'{e}diaires dans les zones de subduction: \'{e}vidences
structurales et g\'{e}ochronologiques pr\'{e}serv\'{e}es dans les
ophiolites corses}

\author{\firstname{R\'{e}mi} \lastname{Magott}\CDRorcid{0000-0002-3993-2601}}
\address{UMR 7359 CRPG, Universit\'{e} de Lorraine, 15 rue
Notre-Dame-des-Pauvres, 54500 Vandoeuvre-l\`{e}s-Nancy, France} 
%\email[R. Magott]{}

\author{\firstname{Olivier} \lastname{Fabbri}\CDRorcid{0000-0001-8646-9368}\IsCorresp}
\address{UMR CNRS 6249 Chrono-environnement, Universit\'{e} Marie et
Louis Pasteur, 16 route de Gray, 25000 Besan\c{c}on, France} 
\email[O. Fabbri]{olivier.fabbri@univ-fcomte.fr}

\author{\firstname{Patrick} \lastname{Moni\'{e}}\CDRorcid{0000-0003-2448-6335}}
\address{G\'{e}osciences Montpellier, Universit\'{e} de Montpellier,
CNRS, 1 Place Eug\`{e}ne Bataillon, 34095 Montpellier, France} 
%\email[P. Moni\'{e}]{}

\author{\firstname{Marc} \lastname{Fournier}\CDRorcid{0000-0002-9256-0034}}
\address{Institut des Sciences de la Terre de Paris (ISTeP) UMR 7193,
Sorbonne Universit\'{e}, 4 Place Jussieu, 75005 Paris, France} 
%\email[M. Fournier]{}

\keywords{\kwd{Underplating}\kwd{Ophiolite}\kwd{Pseudotachylyte}\kwd{Cataclasite}\kwd{Subduction}\kwd{Micro-seismicity}\kwd{Slow slip}}

\altkeywords{\kwd{Sous-placage}\kwd{Ophiolite}\kwd{Pseudotachylyte}\kwd{Cataclasite}\kwd{Subduction}\kwd{Micro-sismicit\'{e}}\kwd{Glissement lent}}

\thanks{INSU Tellus Al\'{e}as program, OSU THETA SRO
(\textit{Soutien \`{a} la Recherche et \`{a} l'Observation})}

\begin{abstract} 
Peridotites from two ophiolitic units of Alpine Corsica show
outcrop-scale reverse faults whose dip angles follow a bimodal
distribution (weak {vs}. steep angles). These faults are
outlined by pseudotachylyte or cataclasite, and formed concurrently at
35--70 km depths, during the subduction of the Piemonte-Ligurian
oceanic lithosphere beneath Apulia in Cretaceous to Eocene times.
\tsup{40}Ar/\tsup{39}Ar dating of pseudotachylyte indicates a 38.1 Ma
age of formation. The faults are interpreted as reflecting, at the
outcrop-scale, similar structures formed during the propagation, along
the subduction interface, of map-scale flats and ramps delimiting
duplex-like thrust sheets eventually incorporated by underplating to
the upper plate.
\vspace*{3pt}
\end{abstract}

\begin{altabstract} 
Les p\'{e}ridotites de deux unit\'{e}s ophiolitiques de Corse alpine
montrent des failles inverses \`{a} angles de pendage distribu\'{e}s de
mani\`{e}re bimodale (&lt; 10\textdegree\ ou 45--70\textdegree). Ces
failles, jalonn\'{e}es de pseudotachylytes et de cataclasites, se sont
form\'{e}es \`{a} des profondeurs de 35--70 km, pendant la subduction
de la lithosph\`{e}re oc\'{e}anique liguro-pi\'{e}montaise sous la
plaque apulienne entre le Cr\'{e}tac\'{e} sup\'{e}rieur et l'Eoc\`{e}ne
sup\'{e}rieur. La datation \tsup{40}Ar/\tsup{39}Ar d'une
pseudotachylyte indique un \^{a}ge de formation \`{a} 38.1 Ma. Les
failles inverses sont interpr\'{e}t\'{e}es comme des r\'{e}pliques de
chevauchements d\'{e}limitant des \'{e}cailles de type duplex qui sont
ensuite incorpor\'{e}es, par sous-plaquage, \`{a} la plaque
sup\'{e}rieure.
\end{altabstract} 

\maketitle

{\vspace*{18pt}}

\twocolumngrid

\end{noXML}

\defcitealias{Fabbrietal2018}{ibid.}
\defcitealias{Monieetal2024}{ibid.}
\defcitealias{Hosseinzadehsabetietal2021}{ibid.}
\defcitealias{Onckenetal2021}{ibid.}

\section{Introduction}\label{sec1}

The study of exhumed accretionary complexes, especially of their deep
parts which experienced high pressure/low temperature (HP/LT)
metamorphism and which are exposed in collision belts, provides key
information about the tectonic processes active along subduction zones.
An example of a tectonic process that is suspected to occur at depth in
subduction zones is tectonic underplating, that is, the incorporation
into the upper plate of rocks units (sedimentary rocks and oceanic
crust rocks) originally belonging to the lower (subducting) plate.
Tectonic underplating is assumed to be currently active in \mbox{various}
convergent margins where various pieces of evidence are documented.
Across the North America plate margin in Alaska, seismic reflection
profiles show flat reflectors at 10--25~km depths interpreted as
reverse faults delimiting thrust sheets of material accreted to the
upper plate \citep{Mooreetal1991,Fuisetal2008}. In central Japan,
micro-earthquake distribution at the top of the Philippine Sea plate
subducting beneath Eurasia in the Kanto region is similarly interpreted
as the consequence of active underplating at 15--20~km depths 
\citep{Kimuraetal2010}. In the same area and in the same depth range,
repeatedly occurring micro-earthquakes are associated with slow-slip
events, both possibly \mbox{being} related to underplating
\citep{Kimuraetal2006,Kimuraetal2009}.  Marine seismic reflection
profiling across the Hikurangi margin northeast of New Zealand revealed
a low-seismic velocity zone interpreted as caused by trench-filling
sediments and crustal rocks being currently underplated at 5--35~km
depths  \citep{Bassettetal2010}. Underplating is also inferred from
sedimentological or structural evidence in several exhumed ancient
active margins such as Makran \citep{Burg2018,Plattetal1985}, Kodiak
\citep{Byrne1986, SampleFisher1986}, Shimanto \citep{KimuraMukai1991,
Matsumuraetal2003,Raimbourgetal2014}, or Franciscan complex and Klamath
mountains \citep{Kimuraetal1996,Meneghinietal2009, 
TewksburyChristleetal2021,PlattSchmidt2024}. The long-term record of
underplating is suggested by episodes of coastal area uplift lasting
for several million years \citep{Menantetal2020, Rossettietal2024}.
Lastly, the preservation of imbricated thrust sheets made of
high-pressure/low-temperature (LP/LT) blueschist to eclogite facies
rocks suggests that underplating took place at depths in several
subduction or collision settings \citep{Agardetal2018, 
AngiboustAgard2010,Angiboustetal2014,Angiboustetal2021,
AustrheimBoundy1994,Warrenetal2008a}. 

The aim of this contribution is to interpret previously described
structural data pertaining to pseudotachylyte-bearing faults and
associated cataclastic structures in ultramafic rocks exposed in
ophiolitic units from Alpine Corsica \citep{AndersenAustrheim2006, 
Magottetal2016,Magottetal2017,Fabbrietal2018}, and to discuss a
possible mechanism of incorporation, by tectonic underplating, of the
ultramafic rocks from the subducting lithospheric mantle to the upper
plate. Newly obtained \tsup{40}Ar/\tsup{39}Ar ages of gabbro-hosted
pseudotachylyte veins further provide a time constraint on the proposed
model.

\section{Geological setting}\label{sec2}
\subsection{Alpine Corsica}\label{ssec21}

Alpine Corsica mostly consists of metamorphosed ophiolitic units
interpreted as remnants of the lithosphere of the Piemonte-Ligurian
ocean formed by the sea-floor spreading in the Jurassic between the
European and the Adria plates or an intervening microplate
\citep{ManatschalBernoulli1998,PiccardoGuarnieri2010, 
Ramponeetal2008}. Most palaeogeographic reconstructions propose that,
from the Late Cretaceous to Palaeogene, the Piemonte-Ligurian
lithosphere subducted eastward or southeastward beneath Adria or
beneath an Adria-derived microplate, and was succeeded by a collision
between Europe and Adria in the Eocene to the Oligocene
\citep{Mattaueretal1981,
Malavieilleetal1998,Stampflietal2002,Handyetal2010,
MolliMalavieille2010,Turcoetal2012,VitaleBrovaroneetal2013,
Marronietal2017}. The HP/LP metamorphism recorded in ophiolitic units,
which includes lawsonite-eclogite facies metamorphic rocks
\citep{Ravnaetal2010,VitaleBrovaroneetal2011}, is ascribed to the
subduction of the oceanic lithosphere down to 60--80~km depths. The
initial decompression from lawsonite-eclogite metamorphism took place
at blueschist facies conditions \citep{Ravnaetal2010} and was succeeded
by regional greenschist facies metamorphism during the later stages of
exhumation \citep{Fournieretal1991,Jolivetetal1990}. Radiometric ages
of the HP/LT metamorphism by various methods (\tsup{39}Ar/\tsup{40}Ar
on micas and amphiboles, U--Pb on zircon, U--Pb thermal ionisation mass
spectrometry on rutile, Lu--Hf on garnet and lawsonite) are between 55
and 34~Ma \citep{AmaudricduChaffautSaliot1979,
Beaudoinetal2020,Brunetetal2000,Maggietal2012,Martinetal2011,
VitaleBrovaroneHerwartz2013}. Rb-Sr and \tsup{39}Ar/\tsup{40}Ar ages of
the retrograde greenschist facies metamorphism partly overlap those of
the HP/LT metamorphism, and are comprised between 32~Ma to c.\ 15~Ma
\citep{Beaudoinetal2020, Brunetetal2000, Rossettietal2015}.

\subsection{The studied units and associated pseudotachylyte veins}
\label{ssec22}

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}(A) Geographical setting of Corsica. (B)
Geological setting of the studied ophiolitic units in Cape Corse.
Insets (A) and (C) refer to maps (A) and (C) of Figure~\ref{fig2}.}
\end{figure}

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig02}
{\vspace*{-4pt}}
\caption{\label{fig2}(A) Geological map of the Monte Maggiore unit. (B)
Cross-section of the Monte Maggiore unit. (C) Geological map of the
Cima di Gratera unit. (D) and (E)  Cross-sections of the Cima di
Gratera unit. Note that, for clarity, late  steeply dipping normal
faults are not depicted on section B--B$'$.}
{\vspace*{-5pt}}
\end{figure*}

This study focuses on two ophiolitic units exposed in the northern part
of Corsica, in Cape Corse, namely the Monte Maggiore and the Cima di
Gratera units (Figure~\ref{fig1}). These units consist of variably
serpentinized lherzolite-type peridotite with subordinate metagabbro
(Figure~\ref{fig2}). The Monte Maggiore unit mostly consists of a
weakly to moderately serpentinized peridotite, inside which gabbro is
only exposed as cm-thick dykes cross-cutting the peridotite
\citep{JacksonOhnenstetter1981}. Pseudotachylyte veins in the Monte
Maggiore unit are rare and are hosted in a peridotite which locally
shows evidence for cataclasis \citep[see below and][]{Fabbrietal2018}.
The Cima di Gratera unit shows, from base to top (Figure~\ref{fig2}d),
(1) a serpentinite substratum including lenses of oceanic gabbro and
thrust sheets of continental crust-derived gneiss
\citep{Lahondere1988}, (2) a lower ultramafic sub-unit consisting of
serpentinite and peridotite, the latter being more abundant upward, and
(3) an upper gabbro sub-unit. The contact between (1) and (2) is a flat
or gently undulating contact ($\upvarphi_{1}$). Immediately above
$\upvarphi_{1}$, and over a thickness of c.\ 100~m, the serpentinite
is strongly foliated. The variety of shear directions and shear senses
indicated by sigmoidal lenses and shear bands precludes a determination
of $\upvarphi_{1}$ kinematics. The contact between (2) and (3) is also
a flat or gently undulating fault ($\upvarphi_{2}$). It is
characterised by heterogeneously distributed brittle and ductile
deformation structures
\citep{Andersenetal2014,Magottetal2016,Magottetal2017}. In some places,
the $\upvarphi_{2}$ contact consists of a thick (${\sim}$0.8~m)
accumulation of pseudotachylyte \citep{Andersenetal2014}. In other
places, the footwall peridotite contains only a few pseudotachylyte
veins, and the hanging-wall gabbro is mylonitic over a thickness of
c.\ 20~m \citep{Magottetal2016}. The foliation is parallel to
$\upvarphi_{2}$ and progressively disappears upwards. In the Cima di
Gratera unit, pseudotachylyte veins are hosted in fresh or weakly
serpentinized ({${&lt;}$}10\% serpentinisation) peridotite and also in the
overlying metagabbro sub-unit
\citep{AndersenAustrheim2006,Andersenetal2008,Andersenetal2014,
AustrheimAndersen2004,Desetaetal2014a,Desetaetal2014b,
Magottetal2016,Magottetal2017}. The veins are widespread across the
entire unit, but their abundance is highest along or near
$\upvarphi_{2}$ (Figure~\ref{fig2}). In the gabbro sub-unit,
immediately above $\upvarphi_{2}$, where pseudotachylyte fault veins
are parallel to the foliation, they are ductilely deformed. Where they
cross-cut the foliation, the veins are free from any ductile
deformation. These relationships show that ductile and brittle
deformation conditions overlapped \citep{Magottetal2016}. Where the
footwall consists of peridotite, abundant pseudotachylyte fault veins
parallel to $\upvarphi_{2}$ are present over a thickness of about
100~m. The vein thickness, lateral extent and abundance decrease
downwards with the increasing distance to $\upvarphi_{2}$. Where the
footwall consists of a serpentinite, pseudotachylyte veins are rare or
absent and the serpentinite acquires a foliation parallel to
$\upvarphi_{2}$ \citep{Magottetal2017}.

In the two studied areas, late vertical to steeply dipping faults
cross-cut the ophiolitic units
\citep{Lahondereetal1992,Magottetal2016}.  In the Monte Maggiore unit,
due to poor exposure, the sense of displacement along these late faults
remains undetermined. In the Cima di Gratera area, these faults offset
the contact $\upvarphi_{2}$ with a normal sense. This displacement is
likely related to the Oligocene to Miocene extension that led to the
formation of the Liguro-Provencal basin and the Tyrrhenian sea on each
side of Corsica \citep{Danieletal1996,Brunetetal2000}.
{\vspace*{-6pt}}

\subsection{Estimates of metamorphic conditions in the studied
ophiolitic units}\label{ssec23}

In the Monte Maggiore unit, the peridotite-hosted gabbro contains
mineral assemblages and textures (partial replacement of plagioclase by
jadeite ${+}$ zoisite ${+}$ paragonite ${+}$ chlorite, glaucophane
overgrowths around primary clinopyroxene) typical of blueschist facies
metamorphic conditions (450~\textdegree C, 0.85~GPa)
\citep{JacksonOhnenstetter1981, Magottetal2020,Nicolletetal2001} which
show that the gabbro and the host peridotite were subducted to minimum
depths of 30--35~km. The peak metamorphic conditions recorded by mafic
rocks in and around the Cima di Gratera unit plot in the field of
lawsonite-bearing blueschist facies conditions, with temperature
estimates at 400--430~\textdegree C and pressure estimates at
1--1.8~GPa
\citep{VitaleBrovaroneetal2011,VitaleBrovaroneetal2013,VitaleBrovaroneetal2014,VitaleBrovaroneetal2021}. 
The pressure \mbox{estimates} suggest that the Cima di Gratera unit was
subducted to depths between 30 and 60~km.{\vspace*{-3pt}}

\subsection{Metamorphic conditions recorded by the pseudotachylyte
veins}\label{ssec24}

In the Cima di Gratera ultramafic sub-unit, the chemical composition of
clino- and ortho-pyroxene microlites in the pseudotachylyte veins
suggests crystallisation under 1 to 1.5~GPa ambient pressures 
\citep{AndersenAustrheim2006,Andersenetal2008}. These pressure
estimates were lowered to 0.5~GPa (${\sim}$17~km) 
\citep{Ferreetal2017,Hosseinzadehsabetietal2021},  based on the
presence of dendritic magnetite microlites and vesicles in the
ultramafic pseudotachylyte. In the mafic sub-unit veins, the presence
of omphacite microlites \citep{Desetaetal2014a,Magottetal2016},
indicates pressure conditions of at least 1~GPa, in agreement with
estimates by \citet{AustrheimAndersen2004}.  Despite the limitations
mentioned above, most inferred pressures are around or above 1~GPa in
the two sub-units, suggesting burial depths of at least 30--35~km. It
is worth being noted that, based on the presence of neocrystallised
spinel in an occurrence of peridotite-hosted pseudotachylyte located in
the central part of Alpine Corsica, \citet{VitaleBrovaroneetal2021}
provide a minimum pressure estimate of 0.8~GPa, which is consistent
with the pressure estimates in the areas studied here.{\vspace*{-3pt}}

\section{Structural and geochronological results}\label{sec3}
\subsection{Pseudotachylyte veins, cataclastic layers, and cataclastic
foliation and their spatial association}\label{ssec31}

In the ultramafic sub-units, pseudotachylyte fault veins are often
flanked by cataclastic peridotite. In the Cima di Gratera sub-unit, the
degree of cataclasis progressively increases with proximity to the
veins, with a transition from proto-cataclastic peridotite at a
distance from the veins to ultra-cataclastic peridotite close to the
veins \citep{Magottetal2016}. The timing of cataclasis overlaps that of
frictional melting, as attested by the reworking of pseudodachylyte
fragments in cataclastic rocks or by pseudotachylyte veins
cross-cutting cataclasite. In the Monte Maggiore unit, pseudotachylyte
fault veins are associated with cataclasite layers whose thickness is
between a few millimetres and 2~cm 
\citep[Figure~\ref{fig3},][]{Fabbrietal2018}.
The cataclastic layers have sharp planar
boundaries with the host rock and consist of minute fragments of
peridotite-forming minerals or mineral assemblages
\citepalias{Fabbrietal2018}. In most cases, pseudotachylyte veins and
cataclasite layers are parallel. Cataclasis also affects the
pseudotachylyte-hosting peridotite or serpentinized peridotite in a
more diffuse manner (Figure~\ref{fig3}), under the form of several
metres thick foliated zones which, due to the scarcity of exposure,
could not be mapped in detail. Inside these zones, the peridotite is
foliated and serpentinized. Thin section observation shows that the
foliation is cataclastic. Infra-millimetric layers of mineral fragments
separate millimetres-thick layers inside which minerals are still
recognisable, although variously serpentinized \citepalias{Fabbrietal2018}.
Serpentinisation increases from the infra-millimetric layers outwards.
In most occurrences, the foliated zones are flat-lying
(Figures~\ref{fig4}B and~\ref{fig5}A). In some exposures, the
cataclastic foliation dips moderately to steeply (Figures~\ref{fig4}B 
and~\ref{fig5}B).

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig03}
{\vspace*{-7pt}}
\caption{\label{fig3}\fontsize{9.7}{11}\selectfont 
Cataclastic foliation and pseudotachylyte and
cataclasite veins in the Monte Maggiore ultramafics. (A) Flat-lying
cataclastic foliation. (B) Gently  dipping cataclastic foliation (2)
deflected by a flat-lying cataclastic  foliation (1). (C) Serpentinized
peridotite hand-sample- hosting flat-lying pseudotachylyte veins. (D)
Foliated serpentinite hand-sample showing a  flat-lying pseudotachylyte
vein and a cataclastic layer.}
{\vspace*{-3pt}}
\end{figure*}

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}\tsup{40}Ar/\tsup{39}Ar age spectrum of the
gabbro-hosted pseudotachylyte injection vein COR H-8A. The double arrow
indicates the range of released cumulative \tsup{39}Ar (\%) regarded as
a plateau from which the 38.1~Ma age of formation is derived.}
\end{figure}

\begin{figure*}
\includegraphics{fig05}
{\vspace*{2pt}}
\caption{\label{fig5}Attitude of pseudotachylyte veins, cataclastic
layers and cataclastic foliation in ultramafic rocks. (A) Lower
hemisphere equal-area projection of poles to pseudotachylyte fault
veins in the Cima di Gratera ultramafic sub-unit (56 measurements). (B)
Lower hemisphere equal-area projection of poles to pseudotachylyte
fault veins (solid circles), cataclastic layers (triangles) and
cataclastic foliation (open circles) surfaces in the Monte Maggiore
ultramafic rocks (61 measurements). (C) Histogram showing the
distribution of dip angles for pseudotachylyte fault veins at Cima di
Gratera. (D) Same for Monte Maggiore.}
\end{figure*}

\subsection{Age of formation of the pseudotachylyte veins}
\label{ssec32}

In order to determine whether the pseudotachylyte veins were formed
during oceanic accretion along normal faults at the mid-oceanic ridge,
before subduction by outer-rise normal faulting near the trench, or
during subduction along the plate interface, \tsup{40}Ar/\tsup{39}Ar
laser probe dating of pseudotachylyte samples was carried out
\citep{Monieetal2024}. Due to their low K content, peridotite-hosted
veins did not provide meaningful results. Conversely, three
gabbro-hosted veins with sufficient K content were dated. Each age
spectrum was obtained on a single millimitre-sized, thin and
translucent fragment of pseudotachylyte glassy matrix optically free of
any survivor clasts. The samples were step-wise heated for 30~s with a
CO\tsub{2} laser using a beam diameter of about 300~$\upmu$m.  Slightly
moving the sample under the laser beam ensures homogeneous heating.
After purification, the gases were introduced into an Argus VI mass
spectrometer and analysed by peak switching. Ages were calculated using
the ArArCALC software. More details can be found in
\citet{Monieetal2024} and references therein. The obtained ages date
the cooling of the melt, a few tens of seconds or a few minutes after
seismic slip-related frictional melting. One sample (COR H-8A) provides
a reliable age at 38.1 ${\pm}$ 1.1~Ma and a total gas age of 37.0
${\pm}$ 0.4~Ma (Figure~\ref{fig4} and Supplementary Material). A second
sample (COR G-17A) yields 42 ${\pm}$ 8~Ma. The large error margin
regarding this age is due to excess argon. The third dated sample gives
a poorly constrained age at \mbox{55 ${\pm}$ 11~Ma.} The large error margin is
here also due to excess argon. The reliability and significance of
these three ages are discussed below (Section~\ref{ssec41}), but the
obtained values confirm that the studied pseudotachylyte veins were
formed under HP/LT metamorphic conditions during subduction.
{\vspace*{-6pt}}

\subsection{Bimodal distribution of dip angles of pseudotachylyte
veins, cataclastic layers and cataclastic foliation, and relative
chronology}\label{ssec33}

As reported by \citet{AndersenAustrheim2006} and by
\citet{Magottetal2016}, in the Cima di Gratera ultramafic sub-unit, the
attitude of pseudotachylyte fault veins is clustered around N0 ${\pm}$
45\textdegree, but their dip angles show a bimodal distribution. They
either dip gently (0--10\textdegree) or steeply (45--70\textdegree)
eastward (Figures~\ref{fig5} and~\ref{fig6}). A similar bimodal
distribution is observed for pseudotachylyte fault veins, cataclastic
layers and cataclastic foliation in the Monte Maggiore area
(Figure~\ref{fig5}).

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig06}
{\vspace*{-4pt}}
\caption{\label{fig6}Pseudotachylyte and cataclasite in the Cima di
Gratera ultramafic sub-unit. (A)--(B) Flat-lying pseudotachylyte veins
(light brown) cross-cutting steeply dipping veins (dark brown) near
$\upvarphi_{2}$ contact. (C)--(D) Steeply dipping pseudotachylyte veins
associated to cataclasite bands (dark brown in (C)). (E)~Thin section
showing association of cataclastic bands with flat-lying
pseudotachylyte. (F)~Polyphase pseudotachylyte/cataclasite flat-lying
shear band displaying S--C microstructure.}
{\vspace*{-4pt}}
\end{figure*}

Whatever the unit, no relative chronology between flat-lying and
steeply dipping veins can be ascertained. In some instances, flat-lying
veins cross-cut steeply dipping ones, but the reverse relationship is
also observed. This lack of unambiguous chronology suggests a
contemporaneity between the two types of fault veins. Similarly, no
relative \mbox{chronology} can be established for the cataclasite layers in
the Monte Maggiore unit. Regarding the diffuse cataclastic foliation
observed at Monte Maggiore, the flat-lying foliation offsets the
steeply dipping one, with a marked deflection, suggesting that the
former is younger than the latter (Figure~\ref{fig3}B).

\subsection{Sense of displacement along the pseudotachylyte veins and
cataclastic layers}\label{ssec34}

The sense of displacement along pseudotachylyte veins and cataclastic
layers is determined at the hand-sample or thin-section scale using
various indicators such as offset markers or secondary fracture
networks \citep{Fabbrietal2018,Magottetal2016}. Given the N0 ${\pm}$
45\textdegree\ strike of pseudotachylyte veins and cataclastic layers,
the sense of displacement is top-to-the-west, southwest or northwest.
Such a kinematics is in agreement with an eastward subducting
Piemonte-Ligurian oceanic lithosphere and top-to-the west (antithetic)
reverse faulting in the subduction zone
\citep{FaureMalavieille1981,Mattaueretal1981,MolliMalavieille2010}. The
top-to-the-west sense of shear is \mbox{questioned} by
\citet{Hosseinzadehsabetietal2021}. According to this study, magnetic
fabrics frozen in the Cima di Gratera pseudotachylyte veins would
indicate top-to-the-east shear senses during the formation of the melt.
Among the various kinematics of intraslab earthquakes at 35--70~km
depths, normal (i.e., synthetic) reactivations are reported from
several subduction zones (see Section~\ref{ssec43} below). The
top-to-the-east shear sense obtained by
\citetalias{Hosseinzadehsabetietal2021} could be relevant to these normal
(synthetic) reactivations.

\section{Interpretation and discussion}\label{sec4}
\subsection{Significance of the Eocene ages of pseudotachylyte veins}
\label{ssec41}

As mentioned above, \tsup{40}Ar/\tsup{39}Ar laser probe dating of the
glassy matrix of pseudotachylyte veins hosted in a gabbro in the Cima
di Gratera area yields a reliable age at 38.1 ${\pm}$ 1.1~Ma
(Figure~\ref{fig3}), that is, Bartonian (middle-late Eocene) and two
less constrained ages at 42 ${\pm}$ 8~Ma (Lutetian) and 55 ${\pm}$
11~Ma (Ypresian). For these last two samples, the large error margins
are due to excess argon contamination. Despite the uncertainties, the
three ages fall in the range of 50--33~Ma ages so far obtained for the
Alpine HP/LT metamorphic peaks in the internal Alps
\citep{Bonnetetal2022}. Restricting the results to the Piemonte-Liguria
units (Zermatt-Saas, Rocciavr\`{e}, Monviso, Voltri), the obtained
pseudotachylyte vein ages also fall in the 50--38~Ma range of HP/LT
metamorphic peak ages. Lastly, the pseudotachylyte ages compare well
with the 37.5--34.1~Ma range of HP/LT metamorphism obtained in Corsica
by zircon U/Pb, garnet Lu/Hf or lawsonite Lu/Hf methods
\citep{Martinetal2011,VitaleBrovaroneHerwartz2013}.  This confirms that
the studied pseudotachylyte veins were formed during subduction, and
not during normal faulting at the mid-oceanic ridge or at the trench
outer rise where the subducting plate is{\unskip\break} being bent.

\subsection{Tentative interpretation of basal contacts and 
pseudotachylyte veins, cataclastic layers and cataclastic
foliation}\label{ssec42}

Figure~\ref{fig8} depicts a possible scenario in which a basal
decoupling surface propagates through the subducting lithosphere,
allowing formation and imbrication of ultramafic thrust sheets. A
present-day analogue for this scenario can be found in seismic
reflection profiles across the Sumatra trench where the Indo-Australian
oceanic lithosphere subducts beneath the Sunda block
\citep{Singhetal2012}, or across the Nankai Trough where the Philippine
Sea plate subducts beneath Eurasia \citep{Tsujietal2009,Tsujietal2013}.
In the Corsican case, the basal surface could be $\upvarphi$ at Monte
Maggiore or $\upvarphi_{1}$ at Cima di Gratera. First-order steeply
dipping ramps delimiting the thrust sheets have not been identified in
the studied areas. However, the vertical to steeply dipping faults
mapped in the studied areas
\citep{Lahondereetal1992,Magottetal2016,Magottetal2017}, which likely
accommodated late, post-metamorphic crustal extension in Oligocene to
Miocene times \citep{Danieletal1996}, could be possible candidates.
However, due to poor exposure conditions, the early, pre-to
syn-metamorphic history, if any, of these faults is unknown, and the
existence of first-order ramps remains hypothetical. Above the basal
contacts $\upvarphi$ or $\upvarphi_{1}$ (Figures~\ref{fig7} 
and~\ref{fig8}), steeply dipping pseudotachylyte veins, cataclastic
layers and cataclastic foliation could correspond to second-order
displacement surfaces parallel to first-order ramps while flat-lying or
gently dipping pseudotachylyte veins, cataclastic layers and
cataclastic foliation could correspond to second-order displacement
surfaces parallel to first-order contacts below ($\upvarphi$ or
$\upvarphi_{1}$) or\break above~($\upvarphi_{2}$).

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig07}
{\vspace*{-2pt}}
\caption{\label{fig7}Summary of the geometry of pseudotachylyte veins,
cataclastic layers and cataclastic foliation in the Monte Maggiore and
Cima di Gratera units. Keys to numbers: (1) Foliated paragneiss or
orthogneiss of the continental crust-derived basal units. (2) Foliated
serpentinite along the basal thrust surfaces. (3)~Steeply dipping
cataclastic foliation. (4) Flat-lying cataclastic foliation. (5)
Flat-lying pseudotachylyte fault veins and cataclastic zones. (6)
Steeply dipping pseudotachylyte fault veins and cataclastic zones.}
{\vspace*{-2pt}}
\end{figure*}

\begin{figure}
\includegraphics{fig08}
\caption{\label{fig8}Interpretative model for underplating of the
uppermost part of the subducting Piemonte-Ligurian oceanic lithosphere.
(A) General framework showing outer rise bending-related normal
faulting near the trench, and propagation and underplating at depth.
(B) Enlarged view of the forward propagation of the thrust sheets
through imbrication. Keys to numbers: (1) Propagating basal thrust
surface. (2) Soon-to-be incorporated thrust sheet. (3) Outer-rise
normal fault prone to reactivation as a thrust sheet-bounding fault.
(4) Underplated thrust sheet. (5) Metamorphosed basalts and sedimentary
cover. (6) Flat-lying structures (pseudotachylyte fault veins,
cataclastic veins, cataclastic foliation). (7) Steeply dipping
structures (pseudotachylyte fault veins, cataclastic layers,
cataclastic foliation).}
\end{figure}

The hypothetical steeply dipping first-order ramps delimiting the
thrust sheets could be reactivated normal faults formed at the trench
outer rise, where the lithosphere is bent before being subducted
\citep{Kobayashietal1998,Masson1991}. If this hypothesis is valid, then
the thrust sheet thickness should be of the same order of magnitude as
the spacing of outer-rise faults, that is, 2--10~km
\citep{Kobayashietal1998, Raneroetal2005}.

\subsection{Comparison with intermediate-depth intraslab earthquakes}
\label{ssec43}

The tentative interpretation proposed above may be similar to
mechanisms of tectonic incorporation and transfer of material from the
subducting to the overriding plate documented in active subduction
zones. A significant part of the earthquakes occurring in subducting
slabs at 30--90~km or larger depths is \mbox{interpreted} as the reactivation
of so-called outer-rise normal faults formed during the bending of the
subducting oceanic plate near the trench \citep{Barnhartetal2014, 
Hasegawaetal2007,Jiaoetal2000,Layetal2013,Malatestaetal2024,
Marotetal2012,Ohtaetal2011,OkadaHasegawa2003,Raneroetal2005, 
Warren2014,Warrenetal2007,Warrenetal2008b}. Such outer-rise faults dip
about 60\textdegree\ (either landward or seaward) and can therefore
correspond to steeply dipping fault surfaces assumed here. Among these
faults, the candidates that would satisfy the comparison with the
Corsican case should dip landward and should be reactivated with a
reverse sense of slip. Flat-lying analogues should be parallel to the
slab upper surface and should also be reactivated with a reverse sense
of slip. A possible modern analogue is a sequence of earthquakes (two
main shocks and many aftershocks) that occurred in 2002--2003 in the
subducting Pacific plate beneath NE Japan \citep{Hasegawaetal2007,
OkadaHasegawa2003}. Two seismic fault surfaces were identified at a
30--70~km depth range. A reverse fault parallel to the slab upper
surface, and a landward-dipping reverse fault making an angle of about
60\textdegree\ with the slab upper surface. This pair of faults appears
as an analogue to the flat/ramp system mentioned above. Another
analogue, although geometrically less constrained than the NE Japan
case, is provided by micro-seismic activity at a 15--20~km depth range
in central Japan. Repeating micro-earthquakes and slow-slip events
occur near the boundary between the subducting Philippine Sea oceanic
plate and the overriding Eurasian plate
\citep{Katoetal2010,Kimuraetal2010}.  Seismic reflection profiles
suggest slicing (off peeling) of the top of the subducting plate and
incorporation of material into the upper plate \citep{Kimuraetal2010}.

\subsection{Magnitude of the seismic ruptures and possible relationship
between cataclasis and slow slip} \label{ssec44}

Among the pseudotachylyte veins, cataclastic layers and cataclastic
foliation studied here, pseudotachylyte fault veins result from
co-seismic displacement. An estimation of the magnitude of the
earthquakes leading to the formation of these veins could extend or
strengthen the comparison with present-day intra-slab seismicity. This
estimation is however approximate because of several unknowns. (1) A
given vein may correspond to a unique earthquake or to several ones.
(2) Conversely, a given earthquake can produce one or several veins,
and in the case of several veins, the exact number cannot be readily
determined. (3) The volume of formed pseudotachylyte is difficult to
estimate, because of the two-dimensional exposure limitation or the
uncertainty regarding the volume of melt injected in non-exposed or
eroded injection veins.

The pseudotachylyte veins described above and which are interpreted to
correspond to the tectonic slicing of packages of subducting slab
mantle rocks have lateral extensions of less than a few metres 
({${&lt;}$}8~m) and thicknesses of a few millimetres (2.5--5~mm on
average). For any such fault vein, the amount of melt exposed to a
single vein remains small, in the order of a few tens of cm\tsup{3} to
no more than 1~m\tsup{3}. If a fault vein results from frictional
melting during one earthquake, the small volume of melt suggests that
the magnitude of the corresponding event was small. Using the same
reasoning to estimate the magnitude of an earthquake from the amount of
melt produced by this earthquake as in \citet{Magottetal2020}, the
above-mentioned volumes (50~cm\tsup{3}--1~m\tsup{3}) could have
been produced during events with magnitudes between ${-}$1 and ${+}$1.8
(Supplementary Material 4). Such magnitudes fall in the range of
magnitudes of micro-earthquakes, that is, earthquakes with magnitudes
not exceeding ${\sim}$3. This analysis does not apply to several metres
long and and several tens of centimetres thick pseudotachylyte veins
described along $\upvarphi_{2}$ by \citet{AndersenAustrheim2006} and 
\citet{Andersenetal2014}. Indeed, as shown by
\citet{AndersenAustrheim2006} and \citet{Andersenetal2014}, these major
veins were formed during large-magnitude (M ${\sim}$ 7 or 8) events with
large rupture areas (megathrust earthquakes).

The cataclastic fabric and the cataclastic layers associated with
pseudotachylyte veins in the Monte Maggiore unit could result from slow
slip, as proposed by \citet{BehrBurgmann2021}, \citet{Burgmann2018} and
\citet{Onckenetal2021}. Slow slip collectively refers to fault
displacement velocities between $10^{-8}$ and $10^{-3}$~m/s, that is,
intermediate between \mbox{steady-state} aseismic sliding 
($10^{-11}$--$10^{-10}$~m/s) and rapid ($10^{-1}$--1~m/s) seismic slip
\citepalias{Onckenetal2021}.

\section{Conclusion}\label{sec5}

The two ophiolitic units of Cima di Gratera and Monte Maggiore in Cape
Corse, Alpine Corsica, display minor (metre-scale) structures
(pseudotachylyte veins, cataclastic layers and cataclastic foliation)
that can be interpreted as the result from reverse faulting and slicing
of the uppermost mantle of the Piemonte-Ligurian slab under
lawsonite-bearing blueschist or lawsonite-bearing eclogite facies
conditions, that is at depths about or larger than c.\ 30~km.
Geometry, nature (pseudotachylyte or cataclasite) and kinematics of
these structures suggest that they can represent second- or
higher-order thrust surfaces that formed parallel to first-order flat
or ramp faults delimiting map-scale thrust sheets. If first-order flats
can correspond to flat-lying tectonic boundaries with kilometric
extension, first-order ramps are not unambiguously recognised in the
field. Late (Oligocene to Miocene) steep to vertical faults could
correspond to former ramps, but their early (Eocene or older) history
is still to be documented. Despite this limitation, the scenario
proposed here provides a picture to what may be presently taking place
at intermediate depths (${\sim}$30~km or deeper) in subducting slabs.
Incipient slicing was seismogenic (frictional melting) or aseismic
(cataclasis), the latter being possibly the signature of slow slip.

\section*{Declaration of interests}

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

\section*{Acknowledgements}

This study was supported by the INSU Tellus Al\'{e}as program and by
the OSU THETA SRO (\textit{Soutien \`{a} la Recherche et \`{a}
l'Observation}).

\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-293-suppl.pdf}}

\nocite{ChoyBoatwright1995}

\printbibliography
\refinput{crgeos20241070-reference.tex}

\end{document}
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