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\DOI{10.5802/crgeos.225}
\datereceived{2023-03-17}
\daterevised{2023-06-19}
\dateaccepted{2023-06-19}
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\dateposted{2023-07-25}
\begin{document}

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%\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

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\shortrunauthors

\title{Extensional forearc structures at the transition from Alaska to
Aleutian Subduction Zone: slip partitioning, terranes and large
earthquakes}

\alttitle{Structures avant-arc extensives \`a la jonction des subductions
Alaska-Al\'eoutiennes : partitionnement, terranes et grands s\'eismes}

\author{\firstname{Amin} \lastname{Kahrizi}}
\address{Laboratoire de G\'{e}ologie, Ecole Normale Sup\'{e}rieure, 
CNRS UMR 8538, PSL University, Paris, France} 
\email[A. Kahrizi]{kahrizi\_amin@hotmail.com}

\author{\firstname{Matthias} \lastname{Delescluse}\CDRorcid{0000-0002-8374-7358}\IsCorresp}
\addressSameAs{1}{Laboratoire de G\'{e}ologie, 
Ecole Normale Sup\'{e}rieure, CNRS UMR 8538, PSL University, 
Paris, France} 
\email[M. Delescluse]{delescluse@geologie.ens.fr}

\author{\firstname{Nicolas} \lastname{Chamot-Rooke}\CDRorcid{0000-0001-9924-749X}}
\addressSameAs{1}{Laboratoire de G\'{e}ologie, 
Ecole Normale Sup\'{e}rieure, CNRS UMR 8538, PSL University, 
Paris, France} 
\email[N. Chamot-Rooke]{rooke@geologie.ens.fr}

\author{\firstname{Manuel} \lastname{Pubellier}\CDRorcid{0000-0002-3064-9391}}
\addressSameAs{1}{Laboratoire de G\'{e}ologie, 
Ecole Normale Sup\'{e}rieure, CNRS UMR 8538, PSL University, 
Paris, France} 
\email[M. Pubellier]{manupub.pubellier@gmail.com}

\author{\firstname{Anne} \lastname{B\'{e}cel}\CDRorcid{0000-0001-7803-5949}}
\address{Lamont-Doherty Earth Observatory of Columbia University, 
Palisades, NY, USA}
\email[A. B\'{e}cel]{annebcl@ldeo.columbia.edu}

\author{\firstname{Donna} \lastname{Shillington}\CDRorcid{0000-0003-1058-3871}}
\address{School of Earth and Sustainability, 
Northern Arizona University, Flagstaff, AZ, USA}
\email[D. Shillington]{donna.shillington@nau.edu}

\author{\firstname{Mladen} \lastname{Nedimovi\'c}\CDRorcid{0000-0003-0327-5192}}
\addressSameAs{2}{Lamont-Doherty Earth Observatory of Columbia University, 
Palisades, NY, USA}
\address{Dalhousie University, Halifax, NS, Canada}
\email[M. Nedimovi\'c]{mladen@ldeo.columbia.edu}

\author{\firstname{C\'{e}dric} \lastname{Bulois}\CDRorcid{0000-0002-5926-9042}}
\addressSameAs{1}{Laboratoire de G\'{e}ologie, Ecole Normale Sup\'{e}rieure, 
CNRS UMR 8538, PSL University, Paris, France} 
\email[C. Bulois]{bulois@geologie.ens.fr}

\keywords{\kwd{Alaska Subduction Zone}
\kwd{Forearc}
\kwd{Seismic reflection}
\kwd{Terranes}
\kwd{Slip partitioning}
\kwd{Megathrust earthquakes}
\kwd{Tsunamis}}

\altkeywords{\kwd{Subduction Alaska-Aleoutienne}
\kwd{Avant-arc}
\kwd{Sismique}
\kwd{Terranes}
\kwd{Partitionnement}
\kwd{Tsunamis}
\kwd{Grands s\'eismes}}

\begin{abstract} 
The Unimak and Shumagin segments of the Alaska Aleutian Subduction Zone
show extensional deformation of the forearc since the Miocene. Using
legacy seismic profiles and modern multichannel seismic data, we update
the structural map of the area, focusing on the intersection between
trench-parallel, landward-dipping normal faults rooting in the plate
interface and trench-oblique to trench-perpendicular normal faults, all
showing signs of recent activity. We investigate for the first time the
origin of the trench-parallel extension and explain the horsetail
geometry of the Central Sanak Basin as the termination of a
slip-partitioning right-lateral strike slip fault. Re-analysis of
subduction zone thrust earthquakes slip vectors indicates a possible
onset of slip partitioning in the vicinity of the Central Sanak Basin.
In the hypothesis of a continuum of deformation, finite deformation
from normal fault offsets show a slow sliver motion of less than 1
mm/yr, which is below the resolution of GNSS measurements. Both trench
oblique and trench parallel faults have been cited as reactivated
terrane sutures, the presence of which may act as upper-plate
weaknesses needed to allow slip partitioning in a context of low
convergence obliquity. Weak landward-dipping normal faults rooting in
the plate interface have also been linked to the tsunamigenic rupture
of the shallow plate interface. We infer that the trench parallel
Unimak Ridge, associated with the 1946 Mw 8.6 tsunami earthquake, is
the last expression of terrane sutures reactivation before their
westward vanishing in the more recent Aleutian~arc.
\end{abstract}

\begin{altabstract}
Les segments Unimak et Shumagin se situent \`a la jonction des
subductions Alaska et Al\'eoutiennes. L'avant-arc de ces segments subit
une d\'eformation extensive depuis le Mioc\`ene. A l'aide de profils
sismiques existants et de retraitements, nous mettons \`a jour la carte
structurale dans la r\'egion d'interaction entre des failles normales
actives obliques et parall\`eles \`a la fosse. Ces derni\`eres, de
vergence continentale, prennent racine sur l'interface de subduction
\`a  ${\sim}$35~km de profondeur. Les failles accompagnant l'extension
parall\`ele \`a la fosse sont dispos\'ees en queue de cheval autour du
Bassin de Sanak Central, que nous interpr\'etons comme la terminaison
d'une faille de partitionnement d\'ecrochante dextre. Une nouvelle
analyse des vecteurs glissements des s\'eismes sur l'interface de
subduction confirme un d\'ebut de partitionnement d\`es le voisinage du
Bassin de Sanak Central. Dans l'hypoth\`ese d'un continuum de
d\'eformation depuis le Mioc\`ene, la d\'eformation finie issue des
rejets de failles normales indique un mouvement lent du sliver
inf\'erieur au mm/an, en de\c c\`a de la r\'esolution des mesures
g\'eod\'esiques GNSS. Aussi bien les failles parall\`eles qu'obliques
\`a la fosse ont \'et\'e interpr\'et\'ees comme des sutures
r\'eactiv\'ees au sein des terranes, accr\'et\'es depuis le Cr\'etac\'e
le long de la pal\'eo-marge de Bering, ant\'erieure \`a la formation de
l'Arc des Al\'eoutiennes. Ces sutures peuvent correspondre \`a des
zones de faiblesse dans la plaque sup\'erieure, permettant un
partitionnement lent dans un contexte de faible obliquit\'e. Des
failles normales m\'ecaniquement faibles, de vergence continentale, ont
\'egalement \'et\'e propos\'ees comme indice de la rupture
tsunamig\'enique de l'interface de subduction \`a faible profondeur.
Nous en d\'eduisons que la ride Unimak, mur de faille parall\`ele \`a
la fosse longeant la zone de rupture du  \guillemotleft{}~tsunami
earthquake~\guillemotright{} de 1946, est probablement l'expression la
plus occidentale de la r\'eactivation des sutures, du fait de leur
absence dans la subduction Aleoutienne.
\end{altabstract} 

\maketitle

%\vspace*{-1pt}

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\section{Introduction}\label{sec1}
Within the upper plate of subduction zones, forearcs deform in reaction
to a variety of processes occurring during the convergence between two
tectonic plates. A classic example is the indentation of the margin by
the subduction of seamounts from the downgoing plate 
\citep[e.g.][]{Dominguezetal1998}. The varying basal and internal
friction at the base and within the wedge, in addition to accretion or
underplating of incoming sediments, can also destabilize the slope and
lead to extension or compressional deformation of the forearc 
\citep{Dahlenetal1984,Lallemandetal1994,Cubasetal2013}. In
non-accretionary margins, basal erosion 
\citep{vonHueneLallemand1990,vonHueneScholl1991,CliftVannucchi2004} due
to rough oceanic basement and hydrofracturing 
\citep[e.g.][]{LePichonetal1993} often leads to subsidence and
extension of the forearc  (e.g. Figures~\ref{fig1} and~\ref{fig2}),
although uplift and compression are also possible 
\citep[e.g.][]{Noda2016}. Independently of these processes, strike slip
deformation can occur due to slip partitioning of oblique convergence
between two plates  \citep[e.g.][]{McCaffrey1992}, leading to the
trench-parallel motion of a so-called sliver block with respect to 
the arc. 

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig01}
\vspace*{-4pt}
\caption{\label{fig1}The Alaska-Aleutian Subduction Zone (AASZ).
(A)~Overview. Main terrane sutures between Prince William Terranes
(PWT) and Chugach Terrane (CT, from the geology of islands in red)
(dashed pink line) and between Chugach and Peninsular Terrane (PT),
respectively south and north of the dashed purple line. Rupture zones
of large 20th century earthquakes from aftershocks are shown in green
\citep{TapeLomax2022}. Pacific/North-America and Pacific/Bering Plate
convergence vectors as well as Bering/North America motion are from
GSRM v2  \citep{Kreemeretal2014}. YT: Yakutat Terrane. K.I.: Kodiak
Island. (B)~Detailed view of the Unimak, Shumagin and Semidi segments.
Addition of rupture zones (slip $>$ 0.5~m) of the 2020 Mw 7.8 Simeonoff
[thin brown contour from   \citealp{Liuetal2020}] and 2021 Mw 8.2
Chignik (thick brown contour,
\url{https://earthquake.usgs.gov/earthquakes/eventpage/ak0219neiszm/finite-fault}) 
megathrust earthquakes. Average coupling on the segments are simplified
from  \citet{LiFreymueller2018} and  \citet{Briggsetal2023}. Simplified
extensional structures from  \citet{Brunsetal1987} are shown in red.
Sanak I.: Sanak Island. Sh.I: Shumagin Islands.  CSOF: Central Sanak
oblique faults.}
\end{figure*}

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Main seismic lines used in this study, named in
white ellipses. Black normal faults are simplified from 
\citet{Brunsetal1987}.  White thrust lines represent the trench and the
Backstop Splay Fault Zone (BSFZ) delimiting a narrow accretionary wedge
from the margin framework (see text). The yellow question mark locates
the area of shelf edge embayment which had  poor seismic coverage in 
\citet{Brunsetal1987} and  \citet{Horowitzetal1989}. Numbers in colored
squares indicate Figure numbers next to profiles of the same color.
Stars indicate basement highs not aligned with trench parallel ridges
shaded in yellow: Unimak Ridge; Unimak ``Seamount'' (US);  Shumagin
Ridge (SR). WSB: Western Sanak Basin; CSB: Central Sanak Basin; ESB:
Eastern Sanak Basin.}
\vspace*{2.5pt}
\end{figure*}

All these factors relate to time scales much larger than the seismic
cycle, and it is thus difficult to correlate forearc structures with
downdip variations of the seismogenic behaviour of the plate interface 
\citep{Hyndmanetal1997,Oleskevichetal1999} or segmentation of large
earthquake ruptures  \citep[e.g.][]{KelleherMcCann1976}. Nevertheless,
several studies showed some worldwide spatial correlations between
features in residual gravity anomalies, a proxy for tectonic structures
of forearcs and downgoing plates, and large earthquake rupture zones 
\citep{SongSimons2003,Wellsetal2003, BassettWatts2015}. The underlying
processes at work are however still debated and another approach is to
look for some short-term feedback on specific structures after the
occurrence of large earthquakes.  \citet{Tsujietal2011,Tsujietal2013}
imaged an active landward-dipping normal fault bounding a half-graben
just landward of the shallow rupture zone of the tsunamigenic 2011 Mw
9.0 Tohoku megathrust earthquake.  \citet{McKenzieJackson2012} further
showed the systematic occurrence of extensional aftershocks in the
forearc following megathrust earthquakes with a large  shallow plate
interface slip. Both groups associate landward-dipping normal faults to
the ``overshoot'' mechanism  \citep[e.g.][]{Ideetal2011} in which
amplified slip on the shallow plate interface may be due to the release
of gravitational potential energy  \citep{McKenzieJackson2012} along a
weak landward-dipping normal fault. Consequently, detecting such
landward-dipping normal faults in other subduction zones is of
particular interest for the study of tsunamigenic 
hazard \citep[e.g.][]{Beceletal2017}.

The Unimak and Shumagin segments of the Alaska-Aleutian Subduction Zone
(hereafter AASZ)  (Figure~\ref{fig1}) have long been recognized as a
transitional domain between two contrasting subduction regimes, as
these segments encompass the transition from an oceanic--oceanic
subduction along the Aleutian arc to an oceanic--continental subduction
along the Alaska Peninsula 
\citep[e.g.][]{Scholletal1975,Lewisetal1988}, accompanied by a westward
increase in the slab dip angle  \citep{Hayesetal2018,Kuehn2019} and a
contrasted history of terranes docking  (Figure~\ref{fig1}). This makes
the Unimak and Shumagin segments particularly suited to study the
possible influence of forearc structures on the short-term behaviour of
the plate interface, in particular when compared to the neighboring
Semidi segement: From the Shumagin to the Unimak segment of the AASZ,
seismic profiles consistently show landward-dipping, trench-parallel
normal faults  [Figures~\ref{fig1} and~\ref{fig2}; 
\citealp{Brunsetal1987,vonHueneetal2019,Beceletal2017}] controlling the
development of half-grabens  in the forearc. A deep seismic reflection
profile shot over the Shumagin segment shows one major landward-dipping
fault rooting at 35 km depth in the plate interface 
(Figure~\ref{fig3}) possibly indicative of tsunamigenic, shallow
ruptures coeval with megathrusts earthquakes \citep{Beceletal2017}.
Indeed, in the Unimak segment of the AASZ  [Figure~\ref{fig1}; 
\citealp{Milleretal2014}], the landward-dipping normal fault in the
upper plate may influence the seismicity  \citep{vonHueneetal2012} and
follows the rupture zone of the 1946 Mw 8.6 Unimak tsunami--earthquake 
\citep{JohnsonSatake1997,LopezOkal2006,OkalHebert2007,vonHueneetal2012}.
This tsunami earthquake features, by definition \citep{Kanamori1972}, a
shallow rupture zone. Like the Unimak segment, the Shumagin segment 
(Figure~\ref{fig1})---also known as the Shumagin Gap
\citep{Daviesetal1981}---is thought to be poorly locked 
[Figure~\ref{fig1}B; \citealp{FournierFreymueller2007,LiFreymueller2018}]. 
In contrast, the Semidi segment, east of the Shumagin segment, is locked 
\citep{FournierFreymueller2007} and does not show any landward-dipping
normal fault nor half-graben basin  \citep{vonHueneetal2016} as along
the Shumagin segment. In this Semidi segment, recent earthquakes showed
deep ruptures (Figure~\ref{fig1}) in 2020  [Mw 7.8, 
\citealp{Liuetal2020,Jiangetal2022}] and 2021  [Mw 8.2, 
\citealp{Liuetal2022}]. 

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Major landward dipping normal faults flanking (A)
Shumagin Ridge and (B) Unimak Ridge (B). Colored arrows
illustrate the possible ``overshoot mechanism'' (see text) linking
shallow subduction interface rupture and landward-dipping normal faults.
(A)~Time-migrated ALEUT Line 5 modified from  \citet{Beceletal2017}.
See inset for location. Orange landward normal fault branches bounding
the buried Shumagin Ridge merge in the deep landward reflector rooting
in the plate interface. The transparent orange wedge in the Eastern
Sanak Basin (E.S.B.) shows the Miocene syn-extension sediments. Horizon
A is the tilted Eocene-eroded paleo-shelf. Yellow dashed lines indicate
a few seaward normal faults, while blue faults are splay faults and
thrusts in the near-trench area. Vertical exaggeration is 2 in the
sediments and around 1 below basement.
(B)~Depth converted poststack time migration of USGS line 203 modified
from  \citet{Milleretal2014}. Purple reflectors are interpreted as
pre-extension. The orange and yellow wedges are syn-extension sediments
rotated by a major landward normal fault tilting the Unimak Ridge
block. Yellow lines represent seaward normal faults and decollements in
weak layers. Vertical\break exaggeration is 2.}
\vspace*{5pt}
\end{figure*}

Trench-parallel faulting is  not the only tectonic deformation recorded
in the forearc of the Shumagin segment. The oblique Central Sanak basin
is located at the western boundary of the Shumagin segment 
(Figure~\ref{fig1}). It is believed to follow a reactivated Beringian
margin structure  \citep{Brunsetal1987,Lewisetal1988} and may
correspond to the eastern end of the 1946 Unimak tsunami earthquake
rupture zone  \citep{Lewisetal1988}. Although there are large
uncertainties in the estimation of the 1946 rupture zone by aftershock
relocations, no aftershocks have been relocated east of the Central
Sanak basin \citep{LopezOkal2006}. In the Eastern Sanak Basin, the
landward-dipping normal fault may also reactivate a pre-existing suture
between accretionary terranes that form the overriding plate 
\citep{Shillingtonetal2022}. Although both oblique and trench-parallel
normal faults have formed basins during the Miocene and have been
described as recently active  [Figure~\ref{fig4}; 
\citealp{Beceletal2017,Brunsetal1987}], the origin of the oblique
extension has never been discussed, nor has been the relationship
between the two extensional systems.

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Structure of the Central Sanak Basin (CSB).
USGS 531 and 533 modified from  \citet{Brunsetal1987} and USGS line 115
crossing the oblique Central Sanak Basin across and along strike. See
inset for location. The surface offset of the main normal fault on
lines 531 and 115 is 80~m while the offset of the early Miocene horizon
A is estimated around 4--5~km. Line 533 shows the basement tilt towards
the SE. The basement high marked by a star is reported in the inset map
and Figure~\ref{fig2}.}
\vspace*{-5pt}
\end{figure*}

Here we use available legacy and modern multichannel seismic datasets
to investigate the extensional features of the forearc in the Unimak
and Shumagin segments of the AASZ. Forearc extensional faults have been
known for a long time to be present in the vicinity of trenches 
\citep[e.g.][]{Aubouinetal1984} in response to various factors
including gravity combined with vertical motion, but also forearc
stretching due to arc-parallel sliver motion in oblique subduction 
\citep[e.g.][]{AveLallemant1996}. Our analysis suggests such a dual
origin for forearc extension. We discuss the possible onset, or
starting point, of Aleutian partitioning around the Sanak Island and
interpret the 280-km-long, landward dipping, trench-parallel normal
fault system from Unimak to Shumagin segments in terms of reactivation
of terrane sutures and seismogenic behaviour of the shallow plate
interface. 

\section{Geological background}\label{sec2}
The Alaska-Aleutian Subduction Zone  is an area of important seismic
and volcanic activity extending from the Alaska landmass and 
Peninsula
%\vadjust{\pagebreak}
to the\unskip\break 
Aleutian Islands  (Figure~\ref{fig1}). It is likely that the
Mesozoic and Tertiary crustal configurations still have an impact on
present-day subduction zone processes. To the east, the modern
subduction encompasses a segment which has a history conforming with
the tectonic evolution of the cordilleras of North America dating back
from the Jurassic  \citep{Dickinson2004}, whereas to the west an island
arc formed to the south of the Aleutian Basin 
\citep{Scholletal1987,Lewisetal1988,Lonsdale1988,Vallieretal1994}. The
US and Canadian western margins are well known to have been constructed
by the collage of a number of accreted exotic terranes originated from
collision around the Paleo-Pacific and the modern Pacific basins. These
terranes were in large part translated along the north 
American margin
by long right-lateral wrench faults 
\citep{Davisetal1978,MongerIrving1980,Coneyetal1980}. Such terrane
docking processes resulted in the Cretaceous accretion of the
Wrangellia--Peninsular Terrane, and more recently the Paleogene
accretion of the Chugach, Prince William, and Yakutat terranes 
[Figure~\ref{fig1}; 
\citealp{PlafkerBerg1994,Plafkeretal1994,GarverDavidson2017}]. 
The docking of the Wrangellia--Peninsular terranes created an orogen that
extended to the north, beyond the South Alaskan margin, along the
Beringian shelf during the Late Cretaceous. The convergence and the
subduction zone were abandoned during the Late Cretaceous
(Maastrichtian) and sealed by Paleogene deposits 
\citep{Worrall1991,Bruhnetal2004}. Relics of the orogen are present as
compressive structures, interpreted from the gravity field under the
shelf as well as from seismic profiles and velocity models 
\citep[e.g.][]{Horowitzetal1989,Shillingtonetal2022}.

After the Late Eocene, subduction resumed along the present-day
Aleutian Arc system, as shown by Oligocene and Early Miocene volcanics
and thick sedimentary deposits in the forearc basin. The trace of the
Beringian suture zone is likely to be marked by the deeply sedimented
St George--North Aleutian basins (St George Basin in 
Figure~\ref{fig1}B), supporting  that the  terranes follow the
abandoned Beringian margin 
[\citealp{Lewisetal1988,Lizarraldeetal2002};  Figure~\ref{fig1}] west
of the Shumagin segment. The Shumagin and Unimak areas are located at
the termination of the Alaskan Peninsula where the Aleutian arc splays
out of the Alaskan Shelf. This area is marked by a drastic change of
the present day configuration of the subduction zone which shows a
sharp change from a thick crust and a large forearc to a narrower
margin  (Figure~\ref{fig1}). Transverse structures such as the Central
Sanak Basin (CSB in  Figures~\ref{fig2} and~\ref{fig4}) mark the
boundary and have an orientation which parallels the former Cretaceous
margin, and therefore the intrinsic structure of the continental margin
may have experienced reactivation. 

Around the present-day shelf edge, trench-parallel basins extend from
west to east of the oblique CSB: the Unimak, Western Sanak and Eastern
Sanak basins respectively  (Figure~\ref{fig2}). All basins are filled
with Miocene to Holocene sediments above a tilted paleoshelf that was
eroded from Oligocene to Miocene  \citep{Brunsetal1987}. This tilted
acoustic basement is ubiquitous in seismic profiles (horizon A, 
see\unskip\break 
Figures~\ref{fig3} and~\ref{fig4}). The paleoshelf has a refraction
velocity of 4.5 km/s and, from nearby island geology, this paleoshelf
basement is inferred to include Paleogene intrusive, arc-affinity rocks
\citep{Brunsetal1987} extending down-slope to the backstop splay faults
zone (BSFZ) which is also the accretionary wedge backstop 
[Figure~\ref{fig2};  \citealp{vonHueneetal2020}]. Landward-dipping
normal faults bound the trench-parallel half-graben basins seaward.
Their footwall marks a linear ridge, which is exposed in the Unimak
basin (Unimak Ridge,  Figures~\ref{fig2} and~\ref{fig3}B) but remains
buried by the sediments of the Eastern Sanak Basin (Shumagin Ridge, 
Figures~\ref{fig2} and~\ref{fig3}A).

In the Eastern Sanak basin (ESB), reflections from the main
landward-dipping normal fault have been imaged down to the plate
interface at 35~km depth  [\citealp{Beceletal2017}; 
Figure~\ref{fig2}]. In a recent deep seismic refraction study, 
\citet{Shillingtonetal2022} showed that the hanging-wall block has a
higher $P$-wave velocity than that of the seaward footwall block which
includes the ridge. These authors interpret the fault as a former
thrust separating the Chugach Terrane from the Prince William Terrane.
It likely corresponds to a suture previously mapped south of the Kodiak
Island and extrapolated westward towards and beyond the Shumagin
Islands  \citep{Mooreetal1983}. It is thus tempting to consider the
landward flank of the Aleutian-arc-parallel Unimak Ridge to be the
continuation of the Eastern Sanak landward normal fault 
\citep{vonHueneetal2019}. However, the terranes-related structures are
supposed to bend and follow the NW--SE Beringian margin geometry rather
than run along the Aleutian Arc  [\citealp{Lewisetal1988}; 
Figure~\ref{fig1}], in apparent contradiction with the Shumagin and
Unimak trench-parallel ridges being connected and having a common
geological origin  (Figure~\ref{fig4}).

\begin{figure*}
\includegraphics{fig05}
\vspace*{-4pt}
\caption{\label{fig5}Structure of the Eastern Sanak Basin (A)~USGS line
217 Prestack Depth Migration (PSDM), modified from 
\citet{vonHueneetal2019}.  (B)~Reprocessed R/V Ewing line 1235,
modified from  \citet{vonHueneetal2019}. Orange fault is the main Sanak
landward-dipping fault, red fault the  ``secondary'' fault and yellow
faults are seaward-dipping normal faults. Colored sediments are Miocene
sediments above the basement horizon A.  Vertical exaggeration is 2.}
\vspace*{-4pt}
\end{figure*}

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}Upper slope and slope structure at the
intersection between Central Sanak Basin and Eastern Sanak Basin:
Reprocessed ALEUT Line 6 and
interpretation. The black frame locates the Prestack Depth Migrated
image (PSDM) and streamer tomography velocity model in  
Supplementary Figure~S2.
Orange shaded areas show Miocene sediments recording clear activity of
the landward normal faults (red and orange). Yellow faults are all
seaward normal faults. Black lines represent the toe-thrust fault
system downslope while grey shaded areas record the tilting of the
block. Dashed grey lines show weak basement reflectivity tentatively
interpreted as old thrust structures from former accretionary prism or
within terranes. C.S.R.: Central Sanak Ridge; S.R.: Shumagin Ridge.}
\vspace*{-2pt}
\end{figure*}

Indeed, it is the oblique Central Sanak basin that is interpreted as a
reactivation of the Beringian margin structures 
\citep{Brunsetal1987,Lewisetal1988}. Onlaps and unconformities within
all sub-basins point to a Miocene onset of extension on the normal
faults, whether they are trench-parallel or reported to follow the
Beringian margin orientation  (Figures~\ref{fig3} and~\ref{fig4}). All
Sanak basins show  recent---although moderate---activity of both
trench-parallel and oblique normal faults 
\citep{Brunsetal1987,Beceletal2017}. As a result, this raises questions
about the nature of the extension activating the diversely oriented
structures, how the two basin families interact at their intersection 
(Figure~\ref{fig2}) and subsequently whether forearc\unskip\break 
structures can
provide a window into controls on modern deformation and seismogenesis
in this area of the AASZ. 

\section{Data and methods}\label{sec3}
Many seismic profiles used in this study were acquired in the 70's
and 80's and reflection images derived from these data are published
in  \citet{Brunsetal1987} and  \citet{Horowitzetal1989}.  These authors
built isopach maps of the basins fill and interpreted the locations of
the main border normal faults (see a simplified version in 
Figures~\ref{fig1} and~\ref{fig2} and the Horowitz et~al. map in 
Supplementary Figure~S1). Although there has not been a major improvement in density
of profiles since the earlier studies of  \citet{Brunsetal1987} and 
\citet{Horowitzetal1989}, some of the legacy data have been reprocessed
[\citealp{Milleretal2014,vonHueneetal2019}; Figures~\ref{fig3}
and~\ref{fig5}] and some new deep penetration profiles acquired with an
8-km-streamer are available from the ALEUT experiment 
\citep{Shillingtonetal2011,Lietal2018,Beceletal2017}. ALEUT Line 6 
(Figure~\ref{fig6}) is of particular interest since it is located at
the junction of the Central and Eastern Sanak basins, in an area of
poor coverage of the legacy data. The shelf break and slope section of
Line 6 has been reprocessed using the CGG Geovation 
Seismic\unskip\break Imaging
software. The processing workflow includes denoising, SRME and Radon
multiple attenuation and finally Kirchhoff Prestack Time Migration 
[see  \citealp{Liangetal2019}, for more details on the processing 
workflow]. USGS Line 221 was available as a non-migrated stack but is
presented here after application of a post-stack, water velocity
Kirchhoff Migration  (Figure~\ref{fig7}). 

\begin{figure*}
\vspace*{3pt}
\includegraphics{fig07}
\vspace*{4pt}
\caption{\label{fig7}The Central Sanak Basin South Graben. Post-stack
migrated USGS line 221. Black faults are trench-perpendicular normal
faults (see map), misplaced in the simplified structural trend of 
Figure~\ref{fig2}. Purple dashed lines are interpreted to be former
margin low-angle thrusts. Notice the eastward tilt of the basement
horizon A. Orange star shows the same basement high visible in line
USGS 533  (Figure~\ref{fig4}). C.S.R.: Central Sanak Ridge; S.R.:
Shumagin Ridge.}
\vspace*{3pt}
\end{figure*}

Streamer tomography (Supplementary Figure~S2), as published in 
\citet{Beceletal2017}, is also used to detect a basement high beneath
the shelf (small yellow star on  Figure~\ref{fig2}, 100~km from the
trench in  Figure~\ref{fig6}). This high was found difficult to image
by conventional processing of the reflectivity. Long-streamer refracted
first arrivals confirm the 4.5~km/s $P$-wave velocity of the basement
reported by  \citet{Brunsetal1987} (horizon A interpreted as the eroded
paleoshelf). This localized shallow high velocity delimitates a horst
or ridge structure that was neither detected in previous studies, nor
interpreted in  \citet{Beceletal2017}. 

The final structural map of the basins (Figure~\ref{fig8}, with faults
coordinates in Supplementary Material) also uses constraints from the
vertical gradient of the free-air gravity anomaly from DTU13 
\citep{Andersenetal2016} as well as the Analytic Signal (AS) extrema 
[as described in  \citealp{MickusHinojosa2001,Saad2006,DungThanh2016}]
in order to discuss the lateral continuity of the basement structures
seen in 2D seismic profiles  
(Supplementary Figures~S3 and~S4). In this study, we map
faults that have clearly been active since the Miocene with attested 
\citep{Brunsetal1987,Beceletal2017} or suspected continued activity up
to the present. However, we are not able to quantify the variation of
the faults activity through time due to the lack of wells and complex
sedimentary processes around forearc faults 
\citep[e.g.][]{Calvesetal2017,Ratzovetal2010}. In  Figures~\ref{fig3}
to~\ref{fig7}, faults are sometimes not drawn in recent sediments as
their activity is not clearly recorded there, which can be the result
of a regional 4-fold increase in sedimentation rates from Pliocene to
present  [\citealp{vonHueneLaVerne1973},  ODP]. It is thus important to
notice that the apparent decrease in the sedimentary record of faults
activity after the Miocene (e.g.  Figure~\ref{fig5}) may  be an
artifact of the competition between sedimentation and tectonics, as
some clear fault scarps are attested by multibeam bathymetry 
\citep{Beceletal2017}. 

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}Final structural map of the Unimak and
Shumagin segments of the AASZ. Black faults represent
partitioning-related oblique and trench perpendicular faults revealing
a horsetail structure at the onset of slip partitioning. White thrusts
show the trench and the BSFZ (Backstop Splay Fault Zone).
Digitized faults are available in Supplementary Material.}
\end{figure*}

\section{Interpretation results}\label{sec4}

\subsection{Landward-dipping normal faults}\label{sec4.1} 
Figure~\ref{fig3} shows two profiles (USGS 203 and ALEUT~5)
representative of the forearc basin structures of the Unimak and
Eastern Sanak 
basins, respectively. The sub-basement imaging of ALEUT
line 5 indicates that only one of the shallow landward-dipping normal
fault branches connects to the deep landward-dipping reflector
(${\sim}$75~km from the trench) 
\citep{Beceletal2017,Shillingtonetal2022}, while another
landward-dipping normal fault  ${\sim}$20~km NNW (and ${\sim}$95~km
from the trench) appears to root at shallower depth. This 20-km-wide
block between the two landward normal faults is not easy to detect in
the legacy data, except in the reprocessing by  \citet{Milleretal2014}
and  \citet{vonHueneetal2019}, where the ``secondary'' trench-parallel
landward-dipping fault is clearly confirmed all along the Eastern Sanak
basin, and particularly in profile 217 (Red fault in 
Figure~\ref{fig5}). 

ALEUT line 6 (Figure~\ref{fig6}) runs at the junction between the
trench-parallel Eastern Sanak Basin and the oblique Central Sanak
Basin, in an area of local shelf break embayment  (Figure~\ref{fig2}).
In comparison with the eastern trench perpendicular lines (e.g. ALEUT
5, USGS 217, see  Figure~\ref{fig3} and~\ref{fig5}), we observe a more
complex basement morphology  (Figure~\ref{fig6}). 
\citet{vonHueneetal2019} interpret the horst immediately seaward of the
shelf break (yellow star 80 km from the trench in  Figure~\ref{fig6})
as the Shumagin Ridge, but none of the basement horsts recognized in
ALEUT line 6 are actually aligned with the Unimak and Shumagin ridges 
(Figure~\ref{fig2}). Therefore, this observation  challenges the
continuity of both ridges inferred in previously published
interpretations. On the other hand, the main upper slope
landward-dipping fault in line~6 (red in  Figure~\ref{fig6}) may be the
continuation of the aforementioned ``secondary'' landward-dipping fault
in ALEUT line 5, USGS 217 and LDEO 1235 (red fault in 
Figures~\ref{fig3} and~\ref{fig5}). There is however no clear evidence
for a continued ``main'' landward-dipping normal fault (orange fault in
Figure~\ref{fig5}), and many line 6 faults, such as the numerous
seaward-dipping normal faults, are difficult to identify in the
eastern lines.

\subsection{Seaward-dipping normal faults}\label{sec4.2}
The basement deformation is much more distributed along the ALEUT line
6 compared to the other lines. The  presence of several basement
horsts, including one detected by traveltime tomography 
\mbox{(Supplementary} \mbox{Figure~S2),}
implies significant activity of normal faults seaward of the Sanak
basin(s) that are not so developed in profiles to the east of ALEUT
line 6  (Figures~\ref{fig3}A and~\ref{fig5}). Most eastern profiles
indeed show half-grabens bounded by major landward-dipping faults and
only a few minor normal faults farther seaward (e. g. ALEUT Line 5, 
Figure~\ref{fig3}A). \looseness=-1

Another difference is the relatively thick sedimentary pile observed
from the shelf to the lower slope  (Figure~\ref{fig5}), with the
seaward-dipping faults offsetting the entire sedimentary column, from
seafloor to basement, without clear syn-extension wedges. In contrast,
two landward-dipping normal faults are associated with the Miocene
sediment wedges (just above basement in both cases). As a result, we
favour a more recent activity of the seaward-dipping faults when
compared to the landward-dipping
faults.

Down the slope, ALEUT line 6 features a large tilted block bounded by
seaward-dipping growth normal faults and landward-dipping toe-thrust
faults  (Figure~\ref{fig6}). These observations all point to a slope
collapse  posterior to the formation of the Sanak Basin. In the entire
legacy and modern dataset, such a slope-collapse is observed in a
limited area in the vicinity of the shelf break embayment 
(Figure~\ref{fig6}), corresponding to the junction between the oblique
Central Sanak Basin and the Eastern Sanak Basin. 

\subsection{Trench-oblique and trench-perpendicular structures in the
Central Sanak and Unimak basins}\label{sec4.3}
The main NW--SE oriented fault forming the oblique half graben (Lines
531 and 115,  Figure~\ref{fig4}, simplified in map  Figures~\ref{fig1}
and~\ref{fig2}) is clearly expressed at the center of the oblique
Central Sanak Basin, halfway between the Sanak Island and the shelf
break. The fault surface throw indicates clear recent activity 
(Figure~\ref{fig4}). Closer to the shelf edge and slope, USGS Line 221 
shows that the similarly verging normal faults  (Figure~\ref{fig7},
18--20~km), SW-bounding the Central Sanak Basin, are not in continuity
of the main NW--SE oriented fault in lines 531 and 115 
(Figure~\ref{fig4}). These faults are also not responsible for the tilt
of the basement, as Horizon A is observed dipping ENE in Line 221 
(Figure~\ref{fig7}) instead of WSW in lines 531  (Figure~\ref{fig4}).
Instead, the faults bounding the basement high in lines 533 and 221
(orange star in  Figures~\ref{fig4} and Figure~\ref{fig7},
respectively), are likely responsible for tilting horizon A. Therefore,
these faults are interpreted to be trench-perpendicular and not
trench-parallel, in agreement with a steeper basement flank in line 221
(shot perpendicularly to the fault,  Figure~\ref{fig7}) when compared
to the same basement high flank in line 533 (shot obliquely to the
fault,  Figure~\ref{fig4}). This orientation of the near shelf edge
full graben  (Figure~\ref{fig7}) matches the vertical gradient of the
Satellite Free Air gravity Anomalies 
(Supplementary Figures~S3 and~S4). These
trench-perpendicular structures are also reported away from  the main
basins using seismic profiles and gravity data  (Figure~\ref{fig8}). An
intermediate NW--SE orientation of normal fault is also mapped south of
the main Sanak island \citep{Moore1974}. 
These observations highlight
that the so-called ``oblique'' Central Sanak basin does not only
feature trench-oblique structures but also diffuse trench perpendicular
structures. 

Another oblique structure, parallel to the ones in the Central Sanak
basin, is inferred separating the Unimak basin from the Western Sanak
basin in  \citet{Brunsetal1987}  (Figure~\ref{fig4}). Multibeam
bathymetry data that were not available in the 80's and now included
in the GMRT v4.0 dataset  \citep{Ryanetal2009}, show a clear offset of
the Unimak ridge  (Figure~\ref{fig4}) separating it from the ``Unimak
Seamount'', in the continuation of the fault inferred by 
\citet{Brunsetal1987}. This offset could indicate transtensional
faulting at the southwestern boundary of the ``Sanak Island 
High''\break (Figure~\ref{fig8}).

\subsection{A revised structural map of the Unimak and Shumagin
segments}\label{sec4.4}
Figure~\ref{fig8} summarizes all the observations. Compared to previous
maps (e.g.  
Supplementary Figure~S1), the reprocessed profiles and the modern ALEUT
data constrain a continuous system of two landward-dipping,
trench-parallel normal faults in the entire Eastern Sanak Basin (main
orange and secondary red faults in  Figure~\ref{fig8}), with a more
seaward location of the main (orange) landward-dipping normal fault in
the  ``slope collapse'' area of ALEUT line 6, where seaward-dipping
normal faults (yellow) are numerous. The main landward-dipping fault is
also offset with respect to the Unimak Seamount/Ridge, limiting the
slope collapse to the intersection of the Eastern and Central Sanak
basins. Many dashed lines indicate inferred faults either from other
maps or interpreted from gravity  
(Supplementary Figure~S4) and/or unclear seismic
data. The fault pattern specifically related to the Central Sanak Basin
(black) is discussed in the next sections. 

\section{Discussion}\label{sec5}

\subsection{Nature of the trench-parallel extension in the oblique
Central Sanak Basin: onset of the Aleutian strain
partitioning?}\label{sec5.1}
\citet{Lewisetal1988} and  \citet{Horowitzetal1989} suggested that the
Central Sanak Basin is part of a larger NW--SE wrench corridor
re-activating the Cretaceous Beringian margin. This would require
differential motion between the Alaskan Peninsula and the Aleutian arc
and transtension along the paleo-suture. No such motion was detected in
GNSS studies of the AASZ  \citep[e.g.][]{LiFreymueller2018} and,
although past re-activation cannot be ruled out, it cannot explain the
present-day activity of the Central Sanak Basin. Alternatively, 
\citet{Brunsetal1987} argued that the Central Sanak Basin, and possibly
the Unimak Basin, may have evolved as pull-apart basins in a
trench-parallel, right-lateral strike-slip system. The northward
bending of the Shumagin Formation at Sanak Island would follow a relay
zone offsetting two strike-slip fault branches, the main eastern branch
being either the Border Ranges Fault \citep{Fisher1981} or any other
terrane boundary. 

A commonly invoked mechanism to explain arc-parallel extension of
forearc terranes is the variation of obliquity of the subduction
inherent to the non-linear shape of the arc. Increasing obliquity along
arcuate forearcs contributes not only to large-scale displacement of
slivers  \citep[e.g.][]{Jarrard1986}, but also to arc-parallel
stretching and the formation of oblique basins 
\citep{EkstromEngdahl1989,AveLallemantGuth1990,McCaffrey1992,AveLallemant1996,
McCaffrey1996}.
The onset of strain partitioning is commonly marked in the upper plate
by one single deep graben more or less perpendicular to the arc,
forming a wide intra-arc strait at the trailing end of the sliver.
Typical well-documented examples gathered in 
\citet{AveLallemantGuth1990} are the Bussol Strait in the Kuril arc 
\citep{Kimura1986,DeMets1992}, the Semangko Graben (Sunda Strait) in
the Sunda arc  \citep{HuchonLePichon1984,Schluteretal2002}, and
possibly the Miyako Depression in the southern Ryukyu arc 
\citep{KuramotoKonishi1989}. We argue here that the Central Sanak Basin
may belong to the same type of basins that find their origin in the
accommodation of subduction obliquity in the upper plate at the very
starting point of partitioning.

In the Aleutian arc, several canyons or passes have been identified as
potential areas of localized transtension between rotating blocks
translated with the sliver  \citep{Geistetal1988,RyanScholl1989}. These
transtensional areas mainly locate in the Central Aleutian arc and it
is generally accepted that no such partitioning is active in the
eastern Aleutians, the obliquity remaining below the threshold
necessary to trigger sliver motion based on a balance of forces acting
on the subduction thrust and the vertical shear plane 
\citep{McCaffrey1992}. However, the analysis of the Aleutian slip
vectors used to estimate the amount of obliquity led to somewhat
conflicting results for the location of onset of partitioning. 
\citet{EkstromEngdahl1989} showed that slip vectors were oriented more
normal to the arc everywhere west of 165\textdegree\ W (more or less the
longitude of Unalaska Island, see  Figure~\ref{fig1}B). These authors
made the striking observation that ``slip partitioning appears to occur
even where plate motion is nearly normal to the arc'', and concluded
that the strike-slip zone had to be particularly weak in the Aleutians.
Using an updated centroid-moment tensor (CMT) catalog, 
\citet{McCaffrey1992} proposed that similarly to the Sunda arc, a
threshold value of obliquity had to be reached for partitioning to be
effective, and he further estimated that this value was in the range
25--40\textdegree\ for the Aleutians, above the 25\textdegree\ value found
for Sunda. At odds with Ekstr\"{o}m and Engdahl interpretation,
McCaffrey concluded to a strong upper plate.

In the last decades, GPS measurements made available for some of the
islands of the Aleutian arc motivated re-examinations of the slip
vector analysis  \citep{AveLallemantOldow2000,CrossFreymueller2008}. We
next discuss the implications of these and our own update of slip
vectors for the arc-parallel extension around Sanak Island.

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}Azimuth plot of  $\mathrm{Mw}>5.8$ thrust
earthquake slip vectors extracted from the GCMT catalog 
\citep{Dziewonskietal1981,Ekstrometal2012} for the 1976--2022 period
(hypocenter depth $<$ 50~km and plunge tension axis
${>}45\text{\textdegree}$). Black points are  $\mathrm{Mw}>7$ with
uncertainties from moment tensor errors. The 2021 Mw8.2 Chignik and the
2020 Mw7.8 Simeonof megathrusts are quoted C and S respectively. Blue
line is the arc-normal azimuth while the red line is the Pacific
(PA)/Alaska Peninsula block (AP) convergence direction from 
\citet{ElliottFreymueller2020}, corresponding to full partitioning and
zero partitioning, respectively. The thick orange dashed line is the
predicted slip vector azimuth in the half-partitioned case
[transcurrent rate equal to half the full-partitioning rate, or 
$k=0.50$; see text and  \citealp{CrossFreymueller2008}]. The grey
shaded area indicates the longitude of the Central Sanak horsetail
structure and the Sanak Island High (see  Figure~\ref{fig8}). See
Supplementary Figure~S5 
for a similar figure with the Pacific/Bering convergence
direction from GSRMv2  \citep{Kreemeretal2014}.}
\vspace*{1pt}
\end{figure*}

\subsection{Onset of partitioning: GPS kinematic constraints and finite
deformation}\label{sec5.2}
Solving simultaneously for the elastic coupling on the plate interface
and the arc-velocity,  \citet{CrossFreymueller2008} include a Bering
Sea plate motion in their analysis  [see \citealp{Lietal2016} for an
update of the Bering block motion as well as 
\citealp{Finzeletal2011,Finzeletal2015}].  Their model predicts active
dextral strike-slip in the back-arc for the central and western
Aleutians, based on significant motion of the arc with respect to the
Bering Sea plate detected by GPS. East of  175\textdegree\ W, 
\citet{CrossFreymueller2008} show, from GPS velocities, that the arc
islands virtually belong to the Bering plate and behave as a rigid
block, a result confirmed by more recent studies 
\citep{Lietal2016,ElliottFreymueller2020,XueFreymueller2020}. At
these longitudes, slip vectors from thrust earthquakes in the forearc
are however systematically oriented between the Pacific--Bering
convergence vector and the arc-normal, as in the early study of 
\citet{EkstromEngdahl1989}. The slip vectors also predict sinistral
strike-slip in the forearc of Kodiak Island, in agreement with GPS and
faults mapped offshore  \citep{Carveretal2008}. The neutral point where
partitioning switches from right-lateral to left-lateral lies 
158\textdegree\ W---east of the Shumagin  Islands---suggesting that the
Sanak Island may well be in the area of onset of dextral 
shear partitioning. \looseness=-1

To illustrate these observations,  Figure~\ref{fig9} is a reappraisal
of the slip vector analysis using an updated catalog for the CMTs 
\citep{Ekstrometal2012} and the Pacific-Alaska Peninsula block
convergence from  \citet{ElliottFreymueller2020}. An alternative using
the GSRM-v2 solution for the Pacific--Bering convergence 
\citep{Kreemeretal2014} is presented in  
Supplementary Figure~S5. The main difference
with previous analysis is that the convergence azimuth gets closer to
the arc-normal, thus predicting smaller arc-parallel motion. The pivot
point remains located in the
Shumagin\unskip\break  
area, close to the 2020  Simeonof
and 2021 Chignik megathrusts, so that the slip partitioning hypothesis
still holds for the Central Sanak Basin. However, around the neutral
point, errors in the azimuth of the slip vectors become as large as the
predicted differential angle between convergence and arc-normal
azimuths, so that partitioning cannot be firmly established in the
Unimak segment. 

The maximum rate of extension across the Central Sanak Basin can be
estimated from the GPS. GPS stations on Sanak Island  and those located
north of the faults mapped in  Figure~\ref{fig8} seem to belong to the
same Peninsula block  \citep{Lietal2016,ElliottFreymueller2020}.
Given the quite large error ellipses, a 1~mm/yr motion cannot be
excluded between Sanak Island and the Alaska Peninsula, but this is far
less than the model predictions. Previous studies find a close-to-half
partitioning ratio, whether a Pacific/North America convergence  [0.6
in  \citealp{EkstromEngdahl1989}; 0.35 to 0.6 in 
\citealp{AveLallemantOldow2000}] or a Pacific/Bering convergence is
considered  [0.55 in  \citealp{CrossFreymueller2008}; 0.5 in 
Figure~\ref{fig9} of this study]. In this latter case, arc-parallel
velocity at Sanak would be about 4~mm/yr. However, the shear rate is
directly proportional to the partitioning ratio which cannot be
estimated around the neutral point.  \citet{AveLallemantOldow2000}
showed that this ratio is highly variable along the Aleutians, some
areas being even totally non-partitioned despite highly oblique
convergence. Beyond the simple analytical solutions for the critical
obliquity that would trigger partitioning 
\citep{Fitch1972,Beck1983,McCaffrey1992}, numerical models show that
weak zones in the upper plate control the amount of arc-parallel motion
almost as much as the obliquity itself. In their Andes model, 
\citet{SchuttWhipp2020} showed that for an obliquity of 
10\textdegree\ and a convergence of 70~mm/yr, the fore-arc moves
coherently at a velocity of 1 to 2~mm/yr in the case of low strength on
the shear plane accommodating the arc-parallel motion. 
Figure~\ref{fig9} suggests an obliquity of  
5\textdegree--7\textdegree\ at Sanak Island for a 68~mm/yr convergence,
in the right range for millimetric motion. 

Finite deformation can be crudely estimated from the amount of slip on
the normal faults bounding the Central Sanak Basin. The largest fault
in seismic line 531 features a ${\sim}$5~km offset over ${\sim}$23~Myrs
(the early Miocene age of horizon A,  Figure~\ref{fig4}), which leads
to a minimum averaged extension rate of 0.2~mm/yr\unskip\break 
assuming a 
45\textdegree\ dipping normal fault and a continuum of deformation
until today. Considering that other faults around may contribute to
extension in a more diffuse way, and a younger mid-Miocene onset of
extension (15 Ma), we estimate that the maximum extension rate would
hardly exceed 1~mm/yr. These motions are likely too small to be
detected by GPS or quantified from slip vectors, but in the range of
the numerical model predictions. 

\subsection{Other structural evidences for an onset of
partitioning}\label{sec5.3}
The amount of arc-parallel extension around Sanak Island is hard to
quantify, but putting together all the various constraints, we infer
that a plausible mechanism for the formation of the Central Sanak Basin
is the onset of dextral shear partitioning in a context of slowly
increasing obliquity. This hypothesis requires the presence of a
right-lateral strike slip fault west of Sanak Island, parallel to the
islands of the arc. Some evidence for such a fault is actually found in
the northernmost termination of USGS seismic line 203 
(Figure~\ref{fig10}), a seismic line crossing this potential
partitioning strike slip fault area. The line shows a faulted area
(from km-5 to km-25 on the profile) that may be interpreted as a flower
structure, as typically found along large-scale strike-slip faults. As
hypothesized by  \citet{CrossFreymueller2008}, the partitioning
structures unveiled in  Figure~\ref{fig8} are all within the forearc.
This location explains the discrepancy between the GPS measurements in
the Peninsula, that do not show any partitioning, and the forearc
earthquakes slip vectors that do.

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig10}
{\vspace*{1pt}}
\caption{\label{fig10}Non-migrated stack of the northern section of
USGS profile 203 (see  Figure~\ref{fig8} for location) with our
interpretation of a flower structure where partitioning strike-slip
motion is expected to localize.}
{\vspace*{1pt}}
\end{figure*}

Active trench-perpendicular normal faults in the Central Sanak Basin 
(Figures~\ref{fig7} and~\ref{fig8}) also call for trench parallel
extension rather than the wrench re-activation of oblique Beringian
structures. The finite amount of extensional deformation is kilometric
up to ten kilometers at most, so that the faults remain essentially an
immature horsetail-like structure at the trailing end of a nascent
sliver  (Figures~\ref{fig8} and~\ref{fig11}). The Central Sanak Basin
vanishes towards the trench slope where it interferes with extensional
structures of the active margin itself, the trench acting as a
free-slip boundary. The fault pattern is well reproduced in analog
models of pull-apart basins  \citep{McClayDooley1995} or nascent
horsetail  \citep{BasileBrun1999}, including the trench-perpendicular
faults. Multi-directional extensional faults are also documented around
the Sunda Strait at the trailing edge of the Sumatra sliver, the main
oblique Semangko graben interfering with slope parallel faults 
\citep{Schluteretal2002}, or at the Hikurangi subduction zone in New
Zealand  \citep{Chanieretal1999}.

In the shelf edge area of the Central Sanak Basin, trench perpendicular
normal faults are not far from landward and seaward-dipping trench
parallel normal faults  (Figures~\ref{fig7} and~\ref{fig8}), which
indicates almost equal horizontal principal stresses, leading to the
predominance of the vertical principal stress, favoring gravity
collapse. In this context, the discontinuities between the
trench-parallel Shumagin Ridge, collapsed Shumagin Ridge, Unimak
Seamount and Unimak Ridge  (Figure~\ref{fig11}) can all be interpreted
as the intersection between the partitioning horsetail transtensional
faults and the landward dipping normal fault system extending from the
Unimak to Shumagin segments.

\begin{figure*}
\includegraphics{fig11}
\caption{\label{fig11}Synthesis map using the residual gravity
anomalies of the AASZ from  \citet{BassettWatts2015} showing near
trench and near arc polarity reversal through the Central Sanak Basin
because of the slab dip angle increase towards the South-West. Large
earthquake aftershocks zones in green are those from 
Figures~\ref{fig1}A and~B. Terrane boundaries (Chugach/PWT:
pink; Chugach/Peninsular: purple) can sometimes be followed in the
gravity anomalies. We propose a westward extension of the terrane
boundary along the trench-parallel ridge system (in red) and a junction
with the outer Beringian margin similar to  \citet{BassettWatts2015}.
Simplified partitioning strike-slip fault and horsetail are shown,
intersecting the trench-parallel ridges. The dashed continuation of the
strike slip-fault toward Unalaska Island is only a guess. Red islands
are reported to be part of the Chugach Terrane  \citep{Mooreetal1983},
while the red hexagon is a Beringian margin canyon where turbidites
originating from rocks similar to the Shumagin Formation were dredged 
\citep{Moore1973}. White dashed lines represent the shelf edge. Blue
lines represent the orientation of magnetic anomalies (EMAG) and show
the clear variation of the orientation of the subducting oceanic fabric
(see text).  Sh. I.: Shumagin Islands.}
\end{figure*}

As discussed in the previous section, partitioning at a low angle of
obliquity requires weak contacts in the forearc, which are likely
present within the Chugach terranes. Transpressional terrane docking 
\citep{PubellierCobbold1996,Pubellieretal1996} occurred at the pivot
point between a more or less convergence-normal oriented trench and a
very oblique Beringian margin  \citep{MarlowCooper1980} so that around
Sanak, both the Beringian sub-parallel shear-related sutures (Central
Sanak horsetail, formerly Sanak ``wrench zone'') and frontal thrust
related sutures  \citep{Shillingtonetal2022} can be reactivated
nowadays  (Figure~\ref{fig11}). 

\subsection{The landward-dipping normal fault system and the
reactivation of terrane sutures}\label{sec5.4}
Extensional deformation related to unstable slopes or general
subsidence is commonly found in forearcs. Basal tectonic erosion 
\citep[e.g.][]{vonHueneScholl1991} and/or underplating of sediments 
\citep[e.g.][]{Chanieretal1999} likely drive the long term extension in
the studied area  \citep{Brunsetal1987} and may also explain the paired
residual gravity anomaly in the Unimak segment  [Figure~\ref{fig11}, 
\citealp{BassettWatts2015}]. However, such extensional deformation
rarely localises on a single  structure. Although slightly offset,
ridges associated with landward dipping-normal faults form a more or
less 280-km-long structure running from the Unimak segment to the
Shumagin segment some 60 to 80 km away from the trench (red faults in 
Figure~\ref{fig11}), featuring several kilometres of vertical throw,
which is quite unprecedented among the worldwide extensional forearcs.
If, as discussed in the previous section, reactivation of
trench-parallel thrust sutures can explain this peculiar observation,
it is interesting to understand why there is no reactivation west of
the Unimak segment and East of the Shumagin segment. 

The western end of the Unimak Ridge, a tilted sedimentary block 
[\citealp{Milleretal2014}, see also  Figure~\ref{fig3}B], vanishes
south-westward within the Unimak basin, in what is usually already
considered to be the Aleutian-arc domain. This is also the western
boundary of the Unimak tsunami--earthquake rupture zone  [e.g. 
\citealp{LopezOkal2006};  Figure~\ref{fig11}]. Further west lies the
Unalaska gap and the 1957 megathrust\unskip\break earthquake rupture zone 
(Figure~\ref{fig11}). The southwest of the Unimak basin shows no
obvious change in trench sedimentation, elastic coupling 
\citep[e.g.][]{CrossFreymueller2008} or downgoing plate structure 
(Figure~\ref{fig11}). As a result, it is tempting to explain the
western vanishing of the Unimak ridge by the absence of pre-existing
suture west of the 1946 Unimak tsunami earthquake rupture area. 
Figure~\ref{fig11} shows that it is still possible to link the western
edge of the Unimak ridge with the Beringian margin as suggested in 
\citet{BassettWatts2015}. Other previous studies do not show such a
``widening'' of the terrane docking area and consider a bend of the
terranes around  Sanak Island. However, considering the Sanak Island
High  (Figure~\ref{fig8}) as the western limit of the terranes does not
explain the presence of the Unimak Ridge and its virtual alignment with
Unimak Seamount and Shumagin Ridge  (Figure~\ref{fig11}).  Besides, 
\citet{Horowitzetal1989} report thrust structures similar to those seen
beneath the Central Sanak Basin  (Figure~\ref{fig7}), but in the Unimak
segment  (Supplementary Figure~S1), which would indicate that the
Unimak basin and ridge were once part of the former margin too.

At the eastern end of the landward-dipping normal fault system, 
\citet{Shillingtonetal2022} show the reactivation of a major thrust
suture zone within the terranes. This suture zone is expected to extend
off Kodiak Island  (Figure~\ref{fig11}) but normal faulting
reactivation actually vanishes just west of the Shumagin Islands 
(Figure~\ref{fig11}). A possible explanation is that the distance
between the major terrane suture and the trench becomes too large 
(Figure~\ref{fig11}) to allow the ``overshoot mechanism'' 
\citep{Ideetal2011,Tsujietal2011,McKenzieJackson2012} activating the
landward-dipping fault-planes and allowing rupture of the shallow
interface as during the archetypal 1946 Unimak tsunami earthquake or the 2011
Tohoku megathrust earthquake. 

To further illustrate the possible link between the shallow interface
behaviour and the normal reactivation of the terrane thrust sutures,
mid-slope terraces are also observed trenchward of the Unimak and Sanak
Basins, along the 280-km-long section of trench-parallel ridges 
(Figure~\ref{fig8}). These terraces are located up-slope of near-trench
reverse splay faults  [BSFZ in  \citealp{vonHueneetal2020}, see also 
Figure~\ref{fig8}] separating the margin framework from small frontal
wedges. The epicenter of the 1946 Unimak tsunami earthquake is believed
to be located in the vicinity of such a splay fault 
\citep{vonHueneetal2020}. The terraces are less developed in the
Eastern Sanak Basin and did not develop further east than the Sanak
Basin  (Figure~\ref{fig1}B). They only appear between the
landward-dipping normal faults, where they exist, and the BSFZ, and may
be another morphological hint at preferential slip on the shallow
interface and/or the near trench splay faults, again vanishing when the
sutures are too far from the trench.

\subsection{The role of fluids?}\label{sec5.5}
The Chugach-Prince William suture is also present in the Semidi segment
but there are no trench-parallel half graben basins there, which, as
discussed, could indicate that the suture zone is too far from the
trench. Another solution would involve variation of additional
properties besides the need of a pre-existing fault. Indeed, many other
parameters are varying from the Shumagin to the Semidi segment of the
AASZ, like for instance elastic coupling and fluid release, in relation
with properties of the downgoing plate.

The subducting oceanic plate fabric shows contrasting orientations 
(Figure~\ref{fig11}): (1)~${\sim}$E--W oriented ridge fabric
formed at the Kula--Pacific extinct spreading center 
\citep{Byrne1979,Lonsdale1988}, sub-parallel to the trench, dominate
west of the Shumagin segment, while (2)~${\sim}$N--S oriented
ridge fabric formed at the Vancouver-Pacific or Farallon-Pacific
extinct spreading center  \citep{AtwaterMenard1970,StockMolnar1988}
prevail to the east. This abrupt variation in the plate fabric relates
to the presence of a fossil triple junction located at the eastern
boundary of the Shumagin segment  \citep{StockMolnar1988}. 
\citet{Shillingtonetal2015} show an increase in present-day
bending-related extension within the incoming plate toward the west,
where the trench sub-parallel Kula--Pacific oceanic fabric 
(Figure~\ref{fig1}A) normal faults can be easily reactivated 
\citep{Aubouinetal1981,Masson1991,Billenetal2007}. These western,
highly faulted areas correlate with a reduction in $P$-wave velocity
interpreted to be due to hydration of the crust and upper mantle. The
background plate-interface seismicity may in turn be affected when
fluids are released and migrates to the plate interface 
\citep{Shillingtonetal2015,Acquistoetal2022}. \citet{Lietal2018} also
image an increase in the subducting sedimentary layer thickness from
west to east towards the Semidi segment, where thicker Zodiac Fan
sediments  \citep{Stevensonetal1983} are subducting. More sediments and
less bend faulting towards the eastern Semidi segment may contribute to
a net decrease in plate roughness and favour increased plate interface
locking and reduced background seismicity 
\citep{Ruff1989,WangBilek2014,Lallemandetal2018}.

In Japan, fluids have been collected around the Tohoku landward-dipping
normal fault  \citep{Sanoetal2014} and geochemical analyses show that
these come from the plate interface through the fault 
\citep{Sanoetal2014,Parketal2021}, potentially reducing the friction 
\citep{McKenzieJackson2012}. Since more fluids from the downgoing plate
may be released from the Shumagin to the Unimak segment, it is a factor
that may be essential in the activity of the landward-dipping normal
faults and that may also participate in the weakness of the
partitioning\unskip\break  
faults. 

\section{Conclusion}\label{sec6}
In this reappraisal of the forearc structures in the transition zone
between Aleutian and Alaskan subduction zones using legacy and modern
seismic data, we show that the Central Sanak basin likely localizes the
onset of Aleutian strain partitioning, in agreement with plate
convergence obliquity extracted from thrust earthquake rake vectors.
The trench-parallel extension rate is estimated from the finite
deformation to be small (less than 1~mm/yr) lower than or close to the
resolution of the available, non conclusive GPS measurements. 

This onset of Aleutian strain partitioning is another transitional
feature between Alaskan and Aleutian subduction zones, in addition to
slab dip, downgoing plate fabric orientation, trench sedimentation. We
review the other forearc structures and their potential link with the
subduction zone interface behaviour and segmentation. Trench
perpendicular extension localising on landward-dipping normal faults
may be linked to tsunamigenic shallow coseismic slip only if such
normal faults feature very low friction, which we interpret
geologically as reactivation of pre-existing and potentially fluid-rich
structures within the terranes. This study aims to emphasise the
perennial importance of mapping geological structures to better assess
the seismogenic behaviour of subduction zones, in addition to modern
instrumentation.

\section*{Conflicts of interest}
Authors have no conflict of interest to declare.

\section*{Acknowledgements}
We thank Jeff Freymueller and an anonymous reviewer for their very
useful comments. Reflection images resulting from basic processing of
most legacy profiles are available in the GeoMapApp database
(\url{https://www.geomapapp.org/})  and the US Geological
Survey (USGS) data portal
(\url{https://walrus.wr.usgs.gov/namss/search/}). Data from the 1994
R/V Maurice Ewing survey EW9409 
\citep{McGearyetal2018} and from MGL1110
(ALEUT cruise) are available from the Marine Geoscience Data System
(MGDS) data portal
(\href{https://www.marine-geo.org/tools/new\_search/search\_map.php}{https://www.marine-geo.org/{\break}tools/new\_search/search\_map.php}).

\CDRGrant[MGDS]{EW9409}
\CDRGrant[MGDS]{MGL1110}

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\section*{Supplementary data}

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Supporting information for this article is available on the journal's
website under \printDOI\ or from the author. 

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\CDRsupplementaryTwotypes[application/zip]{supplementary-material}{\cdrattach{crgeos-225-suppl.zip}}

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