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\DOI{10.5802/crgeos.337}
\datereceived{2026-01-20}
\daterevised{2026-04-15}
\dateaccepted{2026-04-16}
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\section*{Declaration of interests}
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\COI{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.}

\dateposted{2026-06-19}
\begin{document}

%\dateposted{2026-02-16}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

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

\title{Was the Pays de Bray fault an active structure during the  Upper
Pleistocene? The contribution of the ``Beauvais La Justice''
palaeolithic site (France)}

\alttitle{La faille du Pays de Bray a-t-elle \'{e}t\'{e} une structure
active  pendant le Pl\'{e}istoc\`{e}ne sup\'{e}rieur ?  Contribution du
site pal\'{e}olithique de Beauvais La Justice}

\author{\firstname{St\'{e}phane} \lastname{Baize}\CDRorcid{0000-0002-7656-1790}\IsCorresp}
\address{ASNR, Direction de la Recherche et de l'Expertise en Environnement 
PSE-ENV/SCAN/BERSSIN, Bureau d'\'{e}valuation des risques sismiques pour la 
s\^{u}ret\'{e} des installations. B\^{a}t. FAHRENHEIT, 92 262, 
Fontenay aux roses Cedex, France}
\email[S. Baize]{stephane.baize@asnr.fr}

\author{\firstname{Pierre} \lastname{Antoine}\CDRorcid{0000-0002-4176-8388}}
\address{UMR 8591 CNRS-Univ. Paris I-UPEC, Laboratoire de G\'{e}ographie Physique, 
Environnements Quaternaires et actuels. 2 rue Henri Dunant, 
F-94 320 Thiais, France}
\email[P. Antoine]{pierre.antoine@lgp.cnrs.fr}

\author{\firstname{Jean-Luc} \lastname{Locht}}
\address{INRAP Institut Recherche en Arch\'{e}ologie Pr\'{e}ventive, 
Centre arch\'{e}ologique de Glisy, 32 Avenue de l'Etoile du Sud, 
80 440 Glisy, France}
\email[J.-L. Locht]{Jean-luc.locht@inrap.fr}

\keywords{\kwd{Upper Pleistocene}
\kwd{Active tectonics}
\kwd{Intraplate seismic hazard}
\kwd{Pays de Bray fault and anticline}
\kwd{Palaeolithic}}

\altkeywords{\kwd{Pl\'{e}istoc\`{e}ne sup\'{e}rieur}
\kwd{Tectonique active}
\kwd{Al\'{e}a sismique intraplaque}
\kwd{Faille du Bray pal\'{e}olithique}}

%\shortrunauthors

\begin{abstract}      
The Paris Basin, long regarded as a region of very low seismicity,
shows new evidence of recent tectonic activity. Re-examination of  the
pedosedimentary sequence uncovered during the 1993 excavation of the
Paleolithic site of Beauvais ``La Justice'' reveals a fault system
cutting through deposits dating from the Upper Pleistocene (${\sim}$60
to 45 ka), as well as their underlying strata (Paleocene and Upper
Cretaceous). These faults, with a total vertical offset up to 25 cm,
were mapped across the excavation and interpreted as tectonic in
origin, excluding periglacial, karstic, or anthropogenic causes. Their
location and geometry suggest a link to deeper crustal structures,
potentially associated with the Pays de Bray anticline and related
fault. This discovery aligns with broader reassessments of intraplate
seismic hazard in France, prompted by the 2019 M${\sim}$5 Le Teil
earthquake which demonstrated that moderate events could produce
surface ruptures. The BLJ reinterpretation challenges the level of our
knowledge of the fault activity and of their contribution to seismic
hazard in the Paris Basin, mirroring how Le Teil reshaped perception of
seismic hazard in the Rh\^{o}ne Valley. The Pays de Bray fold and
fault---or a secondary structure---may have generated the observed
deformations, raising questions about its potential for larger
earthquakes. The study emphasizes the need for further investigations
to refine hazard assessments. It also highlights the value of
interdisciplinary collaboration, to uncover hidden tectonic activity
and improve seismic hazard models in low-seismicity regions.
\looseness=-1
\end{abstract}

\begin{altabstract} 
Le Bassin parisien, une r\'{e}gion de tr\`{e}s faible sismicit\'{e},
r\'{e}v\`{e}le de nouvelles preuves d'une activit\'{e} tectonique
r\'{e}cente. Le r\'{e}examen de la s\'{e}quence
p\'{e}dos\'{e}dimentaire observ\'{e}e en 1993 lors de la fouille du
site pal\'{e}olithique de Beauvais  \og La Justice
\fg montre un syst\`{e}me de failles traversant des
d\'{e}p\^{o}ts du Pl\'{e}istoc\`{e}ne sup\'{e}rieur (${\sim}$60 \`{a}
45 ka) ainsi que leur substrat (Pal\'{e}oc\`{e}ne et Cr\'{e}tac\'{e}
sup\'{e}rieur). Ces failles, pr\'{e}sentant un rejet vertical
cumul\'{e} jusqu'\`{a} 25 cm, ont \'{e}t\'{e} cartographi\'{e}es sur
l'ensemble du site et interpr\'{e}t\'{e}es comme d'origine tectonique,
excluant les causes p\'{e}riglaciaires, karstiques ou anthropiques.
Leur g\'{e}om\'{e}trie et localisation sugg\`{e}rent un lien avec des
structures crustales plus profondes, potentiellement associ\'{e}es
\`{a} l'anticlinal du Pays de Bray et la faille associ\'{e}e. Cette
d\'{e}couverte s'inscrit dans le cadre d'une r\'{e}\'{e}valuation plus
large de l'al\'{e}a sismique intraplaque en France, impuls\'{e}e par le
s\'{e}isme de magnitude ${\sim}$5 du Teil en 2019, qui a
d\'{e}montr\'{e} que des \'{e}v\'{e}nements mod\'{e}r\'{e}s peuvent
produire des ruptures en surface. La r\'{e}interpr\'{e}tation du site
remet en question notre niveau de connaissance de l'activit\'{e} des
failles et de leur contribution \`{a} l'al\'{e}a sismique du Bassin
parisien, comme le s\'{e}isme du Teil a modifi\'{e} notre
appr\'{e}hension de l'al\'{e}a sismique dans la vall\'{e}e du
Rh\^{o}ne. Pli et faille du Pays de Bray --- ou une structure
secondaire --- pourraient \^{e}tre \`{a} l'origine des d\'{e}formations
observ\'{e}es, soulevant des questions sur leur potentiel \`{a}
g\'{e}n\'{e}rer des s\'{e}ismes puissants. L'\'{e}tude souligne la
n\'{e}cessit\'{e} de poursuivre les investigations pour affiner les
\'{e}valuations des al\'{e}as. Elle met \'{e}galement en lumi\`{e}re
l'importance de la collaboration interdisciplinaire, pour
r\'{e}v\'{e}ler des indices tectoniques cach\'{e}s et am\'{e}liorer les
mod\`{e}les d'al\'{e}a sismique dans les r\'{e}gions de faible
sismicit\'{e}. 
\end{altabstract} 

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\defcitealias{Beccalettoetal2011}{ibid.}
\defcitealias{Manchueletal2018}{ibid.}
\defcitealias{Lemoine1911}{ibid.}
\defcitealias{Lochtetal1995}{ibid.}

\section{Introduction}\label{sec1}
The Paris Basin is a region of very low seismicity---even
quasi-aseismic when considering the 
{instrumental} catalog since  1962 
\citep{Caraetal2015}---within the European intraplate platform,
foreland to the Alpine belt that was actively deforming during the
Neogene. Nevertheless, a few historical earthquakes have caused notable
damage  \citep{Manchueletal2018}, and rare potential indications of
\mbox{recent} \mbox{(Plio-Quaternary)} tectonic deformation are documented in
national  databases \citep{Baizeetal2013,Baizeetal2002,Jomardetal2017} 
(Figure~\ref{fig1}). The 2019 Le Teil earthquake (Mw 4.9, November 11,
2019, Ard\`{e}che, Southern France) demonstrated that moderate
earthquakes can produce surface ruptures in a setting analogous to that
of the Paris Basin---an intraplate domain characterized by inherited
Variscan faults cutting through thick Meso-Cenozoic cover  sequences
\citep{Wynsetal2016}. In Ard\`{e}che, for the La Rouvi\`{e}re Fault,
the source of the Le Teil earthquake as well as in the Pays de Caux
and Vexin regions, for the Pays de Bray structure, these faults were
mapped prior to 2019, but their Quaternary activity remained unknown
due to a lack of dedicated investigations and paleoseismological
approaches, despite the presence of significant societal and industrial
stakes (populated areas, industries, ports). Following the Le Teil
earthquake, appropriate studies on the La Rouvi\`{e}re Fault revealed
its prehistoric  activity  \citep{Ritzetal2022}.  In this context of
heightened awareness---marked by limited knowledge, significant stakes,
and seismic hazard---it seemed pertinent to revisit detailed data
collected over 30 years  ago. \looseness=-1

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}General context of the site of interest, near
Beauvais. Top: Map of the northwestern part of the Paris Basin.
Seismicity information is reduced to the major events 
\citep{Manchueletal2018}. Major fault traces are from the French
geological survey (BRGM). The Upper Cretaceous Pays de Bray cuesta
marks the lithological boundary between stiff limestones and softer
marls, and underlines the general surface shape of the Pays de Bray
anticline. Neotectonic clues are from the NEOPAL database;
St-Martin-Le-Noeud  \citep{Wyns1980} and Pr\'{e}cy (briefly described
in the text) being the closest sites with neotectonic evidence. sh is
indicative, averaging the data available to the SE in the Paris  basin
\citep{Heidbachetal2025}. Bottom: Simplified geological section redrawn
from  \citet{Gelyetal2014}, based on a depth migrated
seismic-reflection profile. The section illustrates the relationship
between the SW-dipping fault and the surface  folding.}
\end{figure*}

Here, we present evidence suggesting the activity of the Pays de Bray
fault and related anticline (or an associated secondary structure)
during the Upper Pleistocene (Weichselian). The region lacks
high-quality outcrops that would allow for the description and analysis
of tectonic history over the past few million years. As in other parts
of France, we leverage stratigraphic work conducted during
archaeological excavations, such as those at Biache-Saint-Vaast in the
north of  France \citep{Colbeauxetal1981}. Specifically, we draw on
stratigraphic investigations carried out in 1993 along the excavation
of the RN-31 road on the periphery of Beauvais, at the  ``La Justice''
hill (hereafter named BLJ). We describe and analyse a site exhibiting
brittle deformation features affecting a Upper Pleistocene
pedosedimentary sequence including aeolian sands, clayey-sandy slope
deposits and paleosols, and extending through Tertiary layers
(glauconitic sands of the Thanetian) and Upper Cretaceous bedrock
(Campanian chalk). The aeolian sands, making the base of the Quaternary
sequence, yields a rich Middle Palaeolithic archaeological horizon
dated to about 60~ka using thermoluminescence (TL) dating on heated
flint\break  artefacts.

This work represents a significant contribution to the national effort
initiated in recent years under the Epos-France Transverse Thematic
Action on  Seismicity \citep{BaizeRitz2022}. Accumulating such
information will improve the characterization of regional seismic
hazard and ultimately advance our understanding of intraplate
deformation processes, which remain poorly  understood 
\citep{Mazzottietal2020}.

In addition to a precise stratigraphic description, we present detailed
chronological data and outline correlations with the regional Upper
Pleistocene reference  sequence \citep{Antoineetal2016}. When
considered in the context of the regional structural framework, these
results provide new insights into Quaternary tectonic deformation
associated with the Paris Basin's major structure---the Pays de Bray
fault and related anticline---and likely its associated buried crustal 
fault.

\section{Regional tectonic background}\label{sec2} 

\subsection{The Pays de Bray fault and anticline}\label{sec2.1}
The Paris Basin is embedded within a vast Meso-Cenozoic sedimentary
domain characterized by a broad monocline structure dipping toward the
depocenter to the east/south-east. This overlies a Paleozoic and
Proterozoic basement that was structured during the two major Upper
Paleozoic tectonic phases---the Caledonian and Variscan  orogenies
\citep{Gelyetal2014}. At the surface, the Paris Basin is intersected by
several tectonic structures, one of the most significant being the Pays
de Bray fault and related anticline, which extends over 110~km from
Saint-Vaast (20~km south of Dieppe) to Coye-la-For\^{e}t, 10~km
south-west of Senlis  (Figure~\ref{fig1}). The anticline is asymmetric,
with a steep northeastern flank. It has been demonstrated that this
fold is associated at depth with a major fault---the Pays de Bray
Fault---which causes relative uplift of the southwestern block. This is
illustrated by the ECORS seismic profile, which clearly shows a
significant south-dipping discontinuity at  depth
\citep{Cazesetal1985}.

According to seismic tomography \citep{AverbuchPiromallo2012}, the Bray
structure lies vertically above a remnant of a Variscan lithospheric 
slab and corresponds to a paleoslab tear fault. This megastructure
separates two lithospheric domains \mbox{rejuvenated} during the Upper
Paleozoic, within which the roots of the Variscan orogen have been
flattened to similar depths on either side. On a larger scale, Averbuch
and Piromallo (2012) extend this deep-seated Bray structure to the
Wight Fault, though this continuity does not appear to be confirmed in
the sedimentary cover, particularly along the southern margin of the
offshore Dieppe-Hampshire Tertiary  Basin
\citep{Hamblinetal1992,Jollivet-Castellot2018},  contrary to the
proposal by  \citet{Wynsetal2016}. Instead, the Bray structure seems to
branch westward within the sedimentary cover as inverse or fold
structures with a more E--W orientation, potentially connecting to
extensions of the F\'{e}camp Fault or structures bordering the Channel
Basin. These relationships are not confirmed in the Paleozoic basement
\citep{Jollivet-Castellot2018}.\looseness=-1

While geological maps do not indicate any significant structures near
the coastline \citet{Duperretetal2012} propose vertical
reactivation with uplift of the eastern block in the Dieppe area, based
on outcrops observed in coastal cliffs. Southeast of the
surface-expressed fold, deep geophysical data---particularly gravity
data---have led some  authors  \citep{Autranetal1986,LefortAgarwal1996} 
to consider a deep structural continuity between the Bray structure and
the Vittel Fault, which would separate the Saxo-Thuringian (to the
south) and Rheno-Hercynian (to the north) Variscan  zones 
\citep{Ballevreetal2009}. Geological maps reveal a fault bordering the
steep northeastern flank and a few minor oblique E--W faults. This E--W
direction aligns with anomalies derived from 3D gravity modeling, which
places deep E--W anomalies in the basement beneath the Beauvais  area
\citep{Alessandrelloetal1983}.

From the Triassic to the Upper Cretaceous, the Bray Fault was a
syn-sedimentary normal fault, subsiding the southwestern  block
\citep{Guillocheauetal2000}. The so-called Pyrenean inversion, marked
by uplift and significant flexure of the southwestern  block 
\citep{Beccalettoetal2011}, is also reflected in sedimentary facies
changes in Paleogene  layers  \citep{Blondeauetal1964}. Seismic
sections show that the fault reaches the surface in the central part 
(Figure~\ref{fig1}b)  \citep{Gelyetal2014}  but does not propagate
markedly into the shallowest layers toward the tips, such as in the
Oise  Valley  \citep{Beccalettoetal2011}. Instead, it manifests as an
asymmetric fold, particularly obvious on large-scale geological maps.
The seismic section in the Oise Valley shows a 0.15~ms TWT  
\mbox{offset}
\citepalias{Beccalettoetal2011}, which could correspond to an inverse
displacement of approximately 150--200~m at the level of the Lower
Cretaceous. At the surface, the anticline is asymmetric, with a gently
dipping southwestern flank  (5\textdegree--10\textdegree) and a steeper
northeastern flank, reaching up to 45\textdegree\ in the Cretaceous.
The dips diminish rapidly toward the synclinal trough associated with
the anticline along the Th\'{e}rain Valley.

To the south of the fold, in the Pr\'{e}cy quarry, we identified
strike-slip faulting along WNW--ESE planes with dextral slickenlines
developed on reddish weathering clays  (Figure~\ref{fig1}a for
location). These faults are locally bound to decametric karstic
depressions filled with deformed Quaternary sediments 
(Figure~\ref{fig2}). The chaotic character of the deformation features
in the sands is probably related to gravitational and/or periglacial
processes during the karst depression collapse; however, the colocation
of the Tertiary/Quaternary karst and the sharp-shaped fault could
indicate a Quaternary fault rejuvenation. Closer to Beauvais to the 
north, \citep{Wyns1980} shows that the deformation associated with the
fold, such as dextral motions along brittle fold-related faults,
particularly in the Saint-Martin-le-N\oe{}ud quarry 
\mbox{(Figure~\ref{fig1}a),} is compatible with post-Cretaceous N--S
shortening. This contraction has recently been dated between ${\sim}$25
and 50~Ma on the Weald-Boulonnais area, in the northern part of the
Paris  Basin  \citep{Blaiseetal2025}. 

\begin{figure*}
\vspace*{-2pt}
\includegraphics[scale=.98]{fig02}
\vspace*{-2pt}
\caption{\label{fig2}Deformation features of the chalk and karst
deposits in the Pr\'{e}cy-sur-Oise quarry (observations done in 2000).
(A)~The N110\textdegree\ E-striking fault zone delineates a karstic
cavity and its infilling;  (B)~several secondary planes related to this
fault zone show dextral calcite slickenlines;  (C)~internal deformation
(faulting and tilting) of karstic infilling (redeposited loess and
sandy loess) is related to sinkhole development in the karstic void.}
\vspace*{-3pt}
\end{figure*}

The Beauvais  ``La Justice'' (BLJ) study site is located approximately
6~km east of the Pays de Bray anticlinal axis, above geophysical
anomalies that may correspond to buried faults in the substratum.

\subsection{Recent and current deformation, seismicity}\label{sec2.2} 
In the Paris Basin, there is a general consistency in GNSS velocity
directions, resulting in a significant NE--SW shortening of
approximately  $1\times10^{-9}$~yr$^{-1}$---a value similar to that
observed around the C\'{e}vennes  Fault \citep{Massonetal2019}.
Vertical velocities indicate generalized subsidence 
$({\sim}{-}0.3~\mathrm{mm}{\cdot}\mathrm{yr}^{-1})$, except around Paris.
The amplitudes of seasonal signals are more pronounced in this region
than elsewhere in France, suggesting that hydrological processes may
contribute to the observed \mbox{deformation.} In terms of stress data, there
is a paucity of relevant information due to the absence of recent
earthquakes. However, by extrapolating available 
{breakdown} borehole
data from the southeast part of the Paris Basin, it can be inferred
that the maximum horizontal stress (${\sigma}$H$_{\max}$) is oriented NW--SE,
similar to the Pays de Bray structure  (Figure~\ref{fig1}). 
To explain how significant strain can occur with such low seismicity, 
\citet{Petitetal2019}  propose that the thick Mesozoic sedimentary pile
of the Paris Basin could act as a  ``sticky layer'', binding together
the basement rocks and increasing crustal cohesion, thereby preventing
failure. Nevertheless, the broader region has experienced historical
earthquakes of moderate significance, some of them causing damage. The
strongest recorded earthquake in the region is the Veules-les-Roses
earthquake on December 1, 1769, with its SISFRANCE (the national
reference for historical  seismicity, \url{https://sisfrance.net/})
epicenter situated on the English Channel coast, 20~km west of the
blind Bray structure, with a poor location quality D (uncertainty up to
50~km). With a high epicentral intensity (Io ${=}$ 6.5), its magnitude
and depth were estimated at Mw 5.1 and 14~km,  respectively
\citep{Manchueletal2018}. In the same region, the Bacqueville
earthquake on April 2, 1829, produced moderate effects (epicentral
intensity of 5), with an estimated magnitude and depth of Mw 4.3 and
16~km,  respectively \citepalias{Manchueletal2018}. While these earthquakes
do not appear to be directly linked to the Bray structure, their large
uncertainties in their epicentral locations prevent definitive
conclusions.

\begin{figure*}
\includegraphics{fig03} 
\vspace*{-2pt}
\caption{\label{fig3}Location map of the BLJ site in its local
geological context. The information includes fault and fold axes
traces, stratigraphic labels, all of them from the French geological
survey database (BRGM) (c1: lower Cretaceous; c2: upper Cretaceous; e1:
Paleocene; e2: Eocene; q3: Holocene). Geophysical information is from
BRGM (regional scale gravity anomaly) and from
\citet{Alessandrelloetal1983} ${=}$ (AEA, 1983) (secondary axis of
gravity anomaly). Town names underlined in white locate the 1910
earthquake information discussed in the text.}
\vspace*{-3pt}
\end{figure*}

\citet{Blondeauetal1964}  mention an earthquake that supposedly
occurred on April 27, 1910, in Noailles (see  Figure~\ref{fig3} for
location), thus possibly related to the Pays de Bray fault,  citing
\citet{Lemoine1911}. According to this account, the event was strong
enough to cause the collapse of a building in Ponchon (see location in 
Figure~\ref{fig3}).  However, \citet{Rothe1976} associates this event
with local \mbox{intensities} of 5.5 in Laboissi\`{e}re-en-Thelle and 4.5 in
\mbox{Boncourt,} a portion of Noailles municipality, and questions the
narrative of a building collapse in Ponchon. According to this further
interpretation, the 1910 Noailles earthquake would be a more moderate
event than  proposed by  \citet{Lemoine1911}. The SISFRANCE database,
however, classifies this event as a  ``false earthquake.'' Actually, a
brief search in regional and national press archives via Retronews 
(\url{https://www.retronews.fr/})---a tool of the
\textit{Biblioth\`{e}que Nationale de France}---, reveals a chronicle
that may correspond to a different event in 1910. A paper from 
``\textit{L'Ind\'{e}pendant R\'{e}mois}'' (May 5, 1910) reports a
several-second tremor felt on the night of May 3--4, 1910, in Noailles,
Ponchon, and Laboissi\`{e}re, causing no damage. Thus, there could
effectively be an earthquake in the Noailles area potentially related
to the fault and related anticline activity, but at a different time
and a lower intensity than suggested by  \citetalias{Lemoine1911}. 

Finally, the Breteuil seismic sequence, about 25~km northeast of
Beauvais, culminated on April 30, 1756, with the Wavignies earthquake
(epicentral intensity 6), estimated at Mw 4.4 and 11~km  depth 
\citep{Manchueletal2018}. This earthquake is rather well constrained
(quality B, location uncertainty ${\sim}$10~km), suggesting a possible
association with a basement fault beneath the secondary NW--SE to
NNW--SSE Thieux anticlinal axis (see  Figure~\ref{fig1} for location).

Several geomorphological indicators have led several authors to propose
that the southern flank of the asymmetric Pays de Bray anticline is 
uplifting \citep{Blondeauetal1965,Larue2005,Larue2000,Wyns1977}. The
Oise River, a major tributary of the Seine, crosses the structure
almost perpendicularly, which would presumably induce vertical movement
of the southwestern block. The Oise and its tributary valleys exhibit
upstream congestion near the anticlinal axis, with the river narrowing
and shifting southeastward as it crosses this axis. Concurrently,
valley-floor alluvium appears to thin along the anticlinal axis. Based
on statistical analysis and rigorous extrapolation across the entire
Oise  Valley \citet{Grimaudetal2025}  conclude that this narrowing and
deepening of the valley floor---previously \mbox{highlighted} 
\mbox{upstream} of \mbox{Beaumont-sur-Oise,} a 
\mbox{village} a few km SW of Pr\'{e}cy,  by
\citet{Larue2000}---shows a geometry consistent with the Quaternary
uplift of the Bray anticline. The increased elevation of the chalk
substratum at the Bray anticline area, more pronounced than in other
areas of the same valley, is compatible with active uplift; 
however this fluvial pattern change could
also be explained by local lithological changes, transitioning from
chalk to soft Cenozoic sandy or clayey sediments. Furthermore, similar
variations in substratum altitude at the watershed scale are clearly
linked to meandering patterns or confluences  \citep{Grimaudetal2025}.
According to \citet{Larue2005}, the uplift of the anticlinal axis may
also be responsible for enhanced upstream incision of valleys flowing
south-southwestward along the southern flank of the anticline between
the Oise River and Beauvais, without any lithological change.

Decades of geological and geomorphological research have not produced a
comprehensive neotectonic catalog or demonstrated the tectonic origin
of the few observations  made. \citet{Baizeetal2002} listed a series of
mostly structural or geomorphological indicators, offering low
confidence in terms of active tectonics, and it does not reference any
paleoseismicity clues in the Seine-Maritime, Eure, Somme, or Paris
regions, including the Pays de Bray structure which is actually not
included in the BDFA  database \citep{Jomardetal2017}. The nearest
confirmed evidence of recent tectonics is near another major fault in
the Paris Basin---the Marqueffles Fault---in the Hauts-de-France 
region \citep{Colbeauxetal1981}.

In this context, the discovery and dating of faults cutting through
Quaternary deposits and the underlying Cretaceous substratum in 1993,
during a preventive archaeological excavation at the Middle
Palaeolithic site of Beauvais  ``La Justice'' \citep{Lochtetal1995},
appear particularly significant. This finding is the subject of
original development in the present paper.

\section{The Beauvais la Justice site (BLJ)}\label{sec3} 

\subsection{Geological context}\label{sec3.1}
The BLJ site is located on the southwestern flank of a Paleocene hill
(denoted as e1 in  Figure~\ref{fig3}), which itself overlies the Upper
Cretaceous chalk plateau (denoted as c2 in  Figure~\ref{fig3}). It is
situated a few hundred meters from the asymmetric Beauvais
fault-related syncline, which runs beneath the Th\'{e}rain Valley. The
Pays de Bray anticline extends approximately 5~km from the syncline,
exposing Lower Cretaceous (c1) and Upper Jurassic (j3) formations. The
anticlinal axis is accompanied by short NW--SE fault segments at the 
surface  \citep{Blondeauetal1982,Blondeauetal1974,Blondeauetal1965},
primarily between Neufch\^{a}tel and Beauvais, along roughly 60~km of
its length, particularly in the study area. Locally, oblique structures
to the NW--SE axis---sometimes concealed---have been proposed to explain
changes in the axis direction, especially in Beauvais itself 
(Figure~\ref{fig3}). Significant fracturing has been documented in
former quarries and mines, most of which are now filled or 
inaccessible \citep{Blondeauetal1974}, except for rare cases such as
the underground quarry of Saint-Martin-le-N\oe{}ud near  Beauvais
\citep{Wyns1980,Wyns1977}. In an old quarry between Saint-Sulpice and
Abbecourt, approximately 5~km southeast of Beauvais, the fault brings
Gault Clay (Albian) into contact with Turonian chalk. Further north,
about 5~km west of Beauvais, in the former sandpit of
Saint-Germain-la-Poterie, deltaic to continental sandy-clay facies of
the Lower Cretaceous  (``Wealdian'' facies) are affected by a network
of low-displacement faults and significant flexuring of the overlying
Barremian clays.

The former quarry of the La Justice hill is the type locality of the
Bracheux Sands  (``\textit{Sables de Bracheux}''), a stratigraphic
milestone of the Thanetian stage (end of the Paleocene) in the Paris
Basin (${\sim}$55~Ma)  \citep{Wynsetal1981}. Despite being
internationally recognized as a geological heritage site since the
early 1950's, the quarry has been intensely exploited afterwards and
there has not been a proper outcrop in this old sandpit for many years
(see
\url{http://www.donnees.picardie.developpement-durable.gouv.fr/IMG/File/patnat/sites/60-02.pdf}).
Resting unconformably on Campanian chalk, the quarry once exposed a
conglomerate of green-gray glauconitic sands with flint pebbles,
overlain by approximately 5~m of fossiliferous yellowish-grey
sands, capped by a 1-meter-thick layer rich in marine bivalve and
gastropod fossils
(\url{https://www.musee-delapparent.com/fr/tiroirs/8-b-12-thanetien-bracheux-bassin-de-paris-france}).

Regarding Quaternary formations, this sector of the Beauvais plateau is
characterized by a very thin (or locally absent) loess cover overlying an 
\mbox{irregular} layer of clay with flint (``\textit{argile \`{a} silex}'')
derived from the dissolution of the underlying Upper Cretaceous chalk.
Numerous test-pits conducted in 1992 along the A16 highway confirmed
this  observation \citep{AntoineCoudret1991}  revealing a highly
discontinuous loess cover, generally less than 1~m thick, resting on
the clay with flints layer. The highly irregular thickness of this
layer results from multiple phases of chalk dissolution
(sinkholes/dolines) during temperate interglacial and early glacial
periods  \citep{Antoineetal2016}, as well as cryoturbation (chalk
cryo-injection)  during permafrost degradation in full glacial 
conditions \citep{Bertranetal2017}. Significant loess-paleosol
sequences (3 to 4~m on plateaus, locally 5 to 7~m on
east-northeast-facing dry valley slopes) are only found ${\sim}$30~km
to the north-northeast (Fransures region)  \citep{Antoine2019}.

Finally, according to ``\textit{Banque du Sous-Sol}'' borehole database
accessible online (\url{https://infoterre.brgm.fr/}), the
Weichselian-Holocene fill in the Th\'{e}rain Valley reaches ${\sim}$8~m
in thickness within the Beauvais urban area (boreholes BSS000GWLU and
BSS000GWHB). Recent prospection work in the Warluis gravel quarries, a
few kilometers upstream, indicates a relatively thin and condensed (1
to 3~m max.) Weichselian Late Glacial and Holocene sedimentary 
infilling  \citep{Coutardetal2010}. Two additional boreholes downstream
of Beauvais at the wastewater treatment plant (borehole BSS000GWKW)
reveal chalk at depths of 7.5~m (T782626) and 6.80~m (T782628). A
piezometer in the Chouvet quarry provides equivalent values but over
Tertiary sands at the base (S417542 ${=}$ BSS004ASNU).

\vspace*{-2pt}

\subsection{Pleistocene stratigraphic sequence}\label{sec3.2}

\vspace*{-2pt}

\subsubsection{Site overview: the Palaeolithic site of Beauvais La
Justice}\label{sec3.2.1} 
The Palaeolithic site of Beauvais La Justice 
(49\textdegree25$'$53.89$''$N, 2\textdegree7$'$23.28$''$E) is situated
on the edge of a plateau overlooking the right bank of the Th\'{e}rain
Valley, approximately 3~km upstream from the city centre of Beauvais.
Located at an elevation of 89--90~m a.s.l. (NGF reference), 30~m above
the river, and 600~m north of the slope break of the chalk plateau, the
site occupies the northern edge of the La Justice hill, a Tertiary
remnant composed of Bracheux Sands (Thanetian). We analysed the \mbox{LiDAR}
\mbox{Digital} Elevation Model (DEM) available for the study region
(open-access data:
\url{https://cartes.gouv.fr/telechargement/IGNF\_MNT-LIDAR-HD}) 
(Figure~\ref{fig4}). The flat topography of the Chalk plateau is
locally cut by smooth NNE--SSW talwegs flowing southward to the
Th\'{e}rain Valley. Several circular depressions, weakly expressed
(maximum vertical relief of 1~m) and of limited size (diameters less
than 50~m), were identified in the vicinity of the study site, more
than 500~m to the north, east, and south. In the western quadrant, the
topography is heavily altered by anthropogenic modifications,
preventing the identification of natural structures of comparable size.
These circular features may correspond to partially infilled sinkholes
of karstic origin (dolines). Additionally, a morphological trace,
oriented in a direction consistent with the faults observed at the
site, extends eastward. We consulted the past aerial pictures
(\url{https://remonterletemps.ign.fr/}) and this suggests that the
origin of this feature is very probably related to agriculture works,
as it aligns with the boundary of a former parcel limit or track as
shown in the 1950 aerial photo.

\begin{figure*}
\vspace*{1pt}
\includegraphics{fig04}
\vspace*{1pt}
\caption{\label{fig4}(a)~DEM and aerial view of the Beauvais la Justice
area with location of the excavation site along the RN-31 road (DEM
from G\'{e}oportail; aerial view from {\textcopyright} Google Earth
image) and the former ``\textit{Sables de Bracheux}'' sandpit. DEM
shows several topographic features that illustrate the strong
anthropogenic control of landscape. The origin of depressions is
unknown. (b)~Map of the excavation of palaeolithic layers at Beauvais
la Justice with the location of the two main stratigraphic profiles (P1
and P2), a detailed log from P1, the location of the exploration pit
regarding to P2 and the drawing of all the fault strands\break (red lines).}
\vspace*{1pt}
\end{figure*}

The site was discovered in 1993 by J.-F.~Pastre during archaeological
prospection surveys conducted prior to the construction of a bypass
connecting the RN-31 (Beauvais-Clermont) to the A-16 highway 
(Figure~\ref{fig4}). The Beauvais La Justice site (BLJ) underwent an
extensive excavation campaign lasting over five months 
(Figure~\ref{fig5}A), documenting Palaeolithic material in sandy layers
(Figure~\ref{fig5}B), a rich fauna  (Figure~\ref{fig5}D,~E) and
deformation features  (Figure~\ref{fig5}; Plate C) 
\citep{Lochtetal1995,Micheletal1999}.

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig05} 
\vspace*{-5pt}
\caption{\label{fig5}General and detail information about the Beauvais
La Justice site. (A)~General view of the archaeological excavation at
Beauvais La Justice (March 1993) with the location of the southern part
of profile P1(Photo J.-L. Locht).  (B)~Close-up of unit 6 (central part
of the excavation) (Photo P. Antoine): U6a---laminated aeolian sands
with aeolian ripples;  U6b---Homogeneous aeolian sands with ripples and
abundant reworked fragments of Tertiary marine mollusc shells and an in
situ Middle Palaeolithic archaeological level dated at  $55.6 \pm 4$~ka
at the base;  (C)~photo detail of a fault with an apparent normal throw
of 14~cm affecting the Quaternary sands of Units U5 and U6 and the
Tertiary green sands (Bracheux Sands: Sbx) (Photo P. Antoine); 
(D)~close-up of remnants of mammoth teeth (photo J.-L. Locht); 
(E)~reindeer antler and flint artefact from the lower archaeological
level (base of U6b) (Photo J.-L. Locht).}
\end{figure*}

The BLJ site represents one of the few documented examples of
Neanderthal occupation dating to the Weichselian Lower Pleniglacial
(${\sim}$60\,000 years ago) in northern  France \citep{Lochtetal1995}. 
The lithic assemblage is characterized by discoidal core reduction,
with  tools including simple scrapers and pseudo-Levallois points 
\citepalias{Lochtetal1995}. During the 1993 excavation campaign, over
13\,000 flint artifacts and many large mammal bone remains were
recovered from a total excavation area of 763~m$^{2}$. Evidence of
fireplaces, including heated flint flakes and blocks, as well as
charred bone fragments, was also identified. The heated flints provided
a reliable and precise thermoluminescence (TL) date for the
archaeological layer embedded within a wind-blown sand deposit rich in
reworked Tertiary shell fragments (Unit~U6b,  Figure~\ref{fig5}B and 
Figure~\ref{fig6}). The high concentration of carbonate debris
facilitated the exceptional preservation of large mammal bone remains
in an otherwise acidic sandy environment, where such remains are
typically degraded (Figure~\ref{fig5}D and E).

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig06}
\vspace*{-3pt}
\caption{\label{fig6}Stratigraphic and chronological framework of the
Beauvais la Justice site. (A)~{Profile P1} with the location of the
reference log (B), and of the area affected by faults (ZF) to the SE
end of the section (description of the units: see text). 
(B)~{Reference log of BLJ site (see text for explanation).}
(C)~{Stratigraphic correlations} between Beauvais ``La Justice'', the
regional reference section of Villiers Adam ``Le Chamesson'' 
\citep[][modified]{Antoineetal2003}
and the global reference sequence for Last Glacial loess-palaeosols
records in northern France \citep[based on][modified: last 60\,000
years only]{Antoineetal2016}. Tertiary Bedrock: SBx: Bracheux sands
(Thanetien); SBp: Beauchamp sands (Stampian). Quaternary sequence
(lower part not represented): 1:~surface soil;  2:~homogeneous
calcareous loess;  3:~Cryoturbated humic tundra gley (Nagelbeek tongue
horizon);  4:~carbonated laminated loess with cryo-desiccation
micro-cracks;  5:~cryoturbated tundra gley doublet with intermediary
loess;  6:~homogeneous calcareous loess;  7:~cryoturbated tundra gley
horizon and large ice-wedges network;  8:~arctic brown soil; 9:~sandy
silts or calcareous loess;  10:~greyish tundra gley horizon;
11:~Greyish arctic meadow soil horizon;  12:~Boreal brown soil horizon;
13:~heterogeneous bedded slope deposits and sandy silts (thermokarst
gully infilling);  14:~homogeneous calcareous loess;  15:~thin tundra
gley doublet including a calcareous loess unit;  16:~homogeneous
brownish non-calcareous colluvial silts/incipient soil (arctic meadow
horizon). Graphic symbols.  (A)~Ap horizon of topsoil;  (B)~Bt horizon
of topsoil;  (C)~Calcareous loess;  (D)~Bw horizons of Boreal to Arctic
brown soils;  (E)~Tundra gley horizons (GL Hz, Gelic gleysols); 
(F)~Weak greyish arctic-meadow soil horizon;  (G)~Stratified sandy
loamy colluvial deposits (infilling previously incised topography or
gullies).  (H)~Large ice wedge casts features and associated erosion
features (thermokarst gullies). (MD)
Major erosional unconformity.}
\vspace*{-3pt}
\end{figure*}

During the archaeological excavation, faults were exposed in the
sections  (Figure~\ref{fig5}C), and a detailed planimetric survey of
these structures was conducted  (Figure~\ref{fig4}). Additionally, two
stratigraphic profiles, measuring 45 and 25~m in length (P1 and P2,
respectively), corresponding to the two embankments of the future road,
were documented at a 1:10 scale 
\mbox{(Figures~\ref{fig6}} and~\ref{fig7}).
These profiles were supplemented in the southeastern part of the
northern profile (P2) by a deep test pit that extended through the
Bracheux Sands down to the Campanian chalk  (Figure~\ref{fig7}).
Finally, a series of hand-auger boreholes (STx) systematically
identified the base of the Thanetian sands (Cx)  (Figure~\ref{fig6}A).

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig07}
\vspace*{-3pt}
\caption{\label{fig7}Focus on the Beauvais la Justice profile P2 where
the fault zone (10~m wide) is well represented. (A)~Stratigraphic
interpretation of Profile P2 (see Figure~\ref{fig6} caption for
description of stratigraphic units) with detail of the most external
fault F1 (see B and Figure~\ref{fig4} for location) and close-up of the
pit crossing the fault F3 (C) down to the chalk bedrock between
distance 17.5 and 19.00~m along the section (description of the units:
see text). The fault zone F2 is complex, with apparent normal and
reverse faults of opposite dip directions, together with thickness
changes of affected horizons (U6). Individual vertical separation
around faults range between a few cm to 14~cm. SBx: Bracheux sands
(Thanetian).}
\vspace*{-3pt}
\end{figure*}

\subsubsection{Stratigraphy, palaeoenvironments and
dating}\label{sec3.2.2} 

\paragraph{Description and palaeoenvironmental interpretation of
stratigraphic units  (Figure~\myRef{fig6}) Quaternary and topsoil units}

\begin{itemize}
\item 
U0 (0.3--0.75~m): Greyish-brown silty-clay loam with scattered flint
fragments. Ploughing horizon (Ap) of surface soil, primarily composed
of colluvial silty-sand material, largely stripped from the site.
\item 
U1 (50--100~cm): Reddish-brown silty-clay loam with high root porosity
and diffuse prismatic structure. Bt horizon of brown leached soil
(Luvisol), locally degraded by hydromorphy. 
\item 
U0 \& U1 Represent the post-glacial topsoil.
\item 
U2 (preserved only in P1, between ${\sim}$22--35~m): Homogeneous,
non-calcareous, 
\mbox{light-yellow} sand infilling degraded ice-wedge casts
(permafrost features).
\item 
U3 (50--80~cm): Grey-brown clayey-humic silt with polyhedral structure.
Humic Arctic medow soil horizon is heavily degraded by surface soil
processes.
\item 
U4 (100--120~cm): Non-calcareous, brown silty-clayey loam with diffuse
lamellar structure, significant root porosity, and numerous
ferromanganese concretions. Boreal brown type horizon (Bw) developed on
silty sand during interstadial period(s). From regional stratigraphic
correlation, this unit is associated with the early Middle Pleniglacial
(MIS~3).\looseness=-1
\item 
U5 (120--220~cm): Laminated, grey-yellow sandy-clay deposits,
predominantly sandy, with grey clayey layers, micro-channels
\mbox{(2--20~cm),} small cryoturbations (2--5~cm) and rare syngenetic small
frost cracks. Periglacial slope deposits reworking surrounding Tertiary
sediments by hillwash and solifluction processes. Compared to
present-day processes in cold regions under bare soil environments the
whole unit could have deposited in a very short time (${\approx}$1000
years?).
\item 
U6a (20--30~cm): Homogeneous, non-calcareous, yellow to light
yellow-brown aeolian sand with irregular millimeter-scale darker layers
(organic matter) (Plate B). This units contains the upper Palaeolithic
levels in its middle section. Aeolian sands are deposited in a
periglacial fully open landscape.
\item 
U6b (5--10~cm): Light yellow-brown sand with abundant reworked marine
shell fragments (Thanetian) and oblique stratification (aeolian
ripples) (Plate B). The lower boundary of this unit is marked by
scattered pebbles and numerous wind-polished flints. The main
Palaeolithic level (Lower) is preserved at the base of U6b and locally
rests directly on the Tertiary sand surface (SBx). This level yielded
calcined bone fragments associated with traces of burnt soil,
fireplaces fuelled by the combustion of large mammal bones. Heated
flint artefacts were also found, and the thermoluminescence dating of
these provided a robust chronological constrain for this layer (end of
the Weichselian Pleniglacial, end of MIS~4).
\end{itemize}

\paragraph{Bedrock Units}
\begin{itemize}

\item 
{
SBx (100--180~cm): Light green-grey well-sorted sands locally
containing marine shell layers (Bracheux Sands, Thanetian).
\looseness=1

}

\item 
Cx (10--30~cm): Heterometric flint gravel with greenish flint and
pebbles (glauconite coating) in a sandy matrix, forming the interface
between the Bracheux Sands and the underlying chalk bedrock.

\item 
Cr: In situ chalk bedrock of the Upper Cretaceous (Campanian).
\end{itemize}

\subsubsection{Chronoclimatic and chronostratigraphic\newline
interpretation}\label{sec3.2.3} 
The chronoclimatic interpretation of the Beauvais sequence is based on
the comparison of its pedosedimentary record with the regional
reference sequence for the last Interglacial--Glacial cycle
\citep{Antoineetal2016} (Figure~\ref{fig6}C). This interpretation
relies mainly on stratigraphic correlation with the Villiers-Adam
reference sequence, located 40~km to the south near the Oise Valley,
which provides the most complete stratigraphic record for this region
at the southern margin of the loess  zone \citep{Antoineetal2003}. It
is further supported by available dating, particularly a
thermoluminescence (TL) date on heated flints (55.6 ${\pm}$ 4~ka) in
Unit 6, considered the most reliable.

Across the excavated area, boreholes and observations made after the
excavation during road construction revealed that the Bracheux Sands
(SBx) rest unconformably on the Campanian chalk, with an erosional
surface that appears relatively flat and 
\mbox{devoid} of dissolution features
such as sinkholes or dolines. This unconformity is marked by the Cx
gravel layer, composed of green-patina flints embedded in a glauconitic
sand matrix, typical of the base of the Thanetian  transgression 
\citep{Wynsetal1981}.

The oldest Quaternary unit (U6b) is separated from the underlying
Thanetian sands by an erosional unconformity, resulting from an episode
of intense aeolian deflation. This Pleistocene deposit consists of
homogeneous aeolian sand rich in reworked Tertiary marine shell
fragments. Unit U6b contains the lower Palaeolithic level,
characterized by knapped flints and large mammal remains. In an acidic
sandy environment, these faunal remains were exceptionally preserved
due to the high carbonate content from abundant reworked Tertiary
fossil fragments. Faunal analysis indicates the dominance of reindeer 
(Figure~\ref{fig5}E), along with woolly rhinoceros, mammoth 
(Figure~\ref{fig5}D), and horse, suggesting a steppe environment with a
cold, dry, and continental climate, corroborated by rodent fauna
studies. The evolutionary stage of species such as woolly rhinoceros
and mammoth aligns with an attribution to the early Middle Weichselian.
In its middle and upper sections, Unit U6a exhibits fine, dark, wavy
silty laminae  (Figure~\ref{fig5}B), indicative of nivo-eolian
processes like those described in modern Arctic environments, where
particles are trapped on the snow surface and deposited during spring
thaw. Given this sedimentary facies, faunal content typical of a cold
steppe, TL dating of heated flints (55.6 ${\pm}$ 4~ka), and
stratigraphic correlations with the Villiers-Adam reference sequence,
units U6b and U6a are attributed to the end of the Weichselian Lower
Pleniglacial, corresponding to the end of Marine Isotope Stage MIS~4.
This arid period, characterized by intense deflation and aeolian
sedimentation, is generally poorly represented regionally. Only a few
profiles, such as those at Havrincourt in northern  France
\citep{Antoineetal2014} and Harmignies in Belgium
\citep{Haesaertsetal2016}, record the deposition of the first
allochthonous calcareous loess, marking the Lower Pleniglacial. Dated
to ${\sim}$60\,000 years, these deposits coincide with the first major
sea-level drop (${{\sim}{-}}$90 to ${-}$100~m) \citep{Siddalletal2003},
which exposed deflation zones associated with paleo-fluvial networks in
the Eastern English  Channel\break \citep{Antoine2019}.

Unit U5, which systematically overlies these aeolian sands, consists of
a thick, laminated 
\mbox{sandy-clay} formation with scattered gravel and
micro-cryodesiccation cracks. Its structure and composition are typical
of a humid periglacial context, with extensive evidence of gelifluction
and intense slope runoff  (Figures~\ref{fig5} and~\ref{fig6}). The base
of this slope unit is highly discordant with the underlying U6 sands,
from which it likely eroded a significant portion. Its facies,
characterized by intense runoff structures with oblique,
heterogeneously filled bedding, implies a large water supply which, in
these arid periglacial environments, is primarily available during
melting events of ice trapped in ice-rich permafrost, particularly in
large ice-wedge networks. By comparison with the Villiers-Adam
reference section, the U5 deposits are interpreted as those trapped in
a large thermokarst incision gully \mbox{cutting} into the  slope
\citep{Antoineetal2003}. Given its stratigraphic position beneath the
interstadial brown soil complex (U4--U3) and the dating of the
underlying aeolian sands, the U5 deposit at BLJ, resulting from an
intense and widespread slope erosion episode, serves as a marker of the
Lower to Middle Pleniglacial transition. This major slope erosion event
has also been documented in the Somme Valley at  Morcourt
\citep{Sambourgetal2025}, and in the Nussloch reference section in 
Germany \citep{Antoineetal2001,Rousseauetal2002}. The end of this
erosive episode marks a return to a drier environment, with slope
stabilization in a context where aeolian sedimentation reactivates,
favouring the deposition of homogeneous silty aeolian sands, as seen at
Villiers-Adam  (Figure~\ref{fig6}C, Unit 13). Given the reduced loess
dynamics in Western Europe during this period ---and particularly in
northern France and the Paris Basin---, the Tertiary sands constitute
easily re-mobilizable material through aeolian deflation and
redeposition. It is suggested that this erosive phase, which
rejuvenated the topography and remobilized a large quantity of Tertiary
sands, may have renewed the sedimentary stock available for aeolian
deflation in the form of foot-slope spreads and/or sand bars in fluvial
valleys such as the Oise or Th\'{e}rain rivers. At BLJ, the end of this
episode is marked by the deposition of increasingly homogeneous
silty-sandy aeolian material toward the top of Unit 5, like that
observed at Villiers-Adam\break  (Figure~\ref{fig6}C).

The BLJ sequence is subsequently characterized by a compact, brown,
silty-clay loam corresponding to a Bw horizon of a particularly thick
boreal brown soil (U4). Its stratigraphic position and facies suggest a
correlation with the main soil constituting the base of the
Saint-Acheul-Villiers-Adam Soil Complex  (Figure~\ref{fig6}, Unit 12),
dated to ${\sim}$55--45 ka and attributed to the early Middle
Pleniglacial (early MIS 3). Above this soil, in the eastern section
(P1), a greyish-brown silty-clay loam horizon (U3) is locally observed
over ${\sim}$15~m. This horizon, slightly less compact than the U4
soil, exhibits a poorly developed isohumic soil facies of the arctic
meadow type, later enriched in clay due to its proximity to the topsoil
Bt horizon. It is comparable to the small horizon described in the
Middle Pleniglacial soil complex at Villiers-Adam  (Figure~\ref{fig6},
Unit 11), and possibly to that described at Nussloch at the top of the
upper Gr\"{a}selberg soil, dated to 45\,000 years and attributed to the
end of interstadial GI-12 in the NGRIP  record
\citep{Rasmussenetal2014}.

The end of the sequence, represented by non-calcareous yellow aeolian
sands (U2), is preserved only in the fillings of large ice-wedge
pseudomorphs that open at the top of the U3 soil  (Figure~\ref{fig5}).
Given its stratigraphic position, the size of the associated ice-wedge
pseudomorphs, and the limited sedimentary record attributable to this
period in the Beauvais area, this facies is assigned to an
indeterminate Upper Pleniglacial (${\sim}$30--20~ka). Its purely sandy
nature likely represents an initial stage of the Upper Pleniglacial,
before the deposition of typical calcareous loess became widespread.
Even at Villiers-Adam, where Upper Pleniglacial loess is present, this
period is only preserved in localized sedimentary traps linked to
thermokarst incision  valleys \citep[][Figure~\ref{fig6}C]{Antoineetal2003}.

Finally, beneath a ploughing horizon (Ap/U1), locally developed on
recent agricultural colluvium up to 75~cm thick toward the southeast,
the BLJ sequence concludes with a thick, compact, reddish-brown Bt
horizon of leached brown soil (Luvisol) on silty-clay loam. This unit
represents the weathering record under a temperate oceanic climate and
deciduous forest of the silty-sandy material previously deposited
during the Upper Pleniglacial, following the aeolian sands of U2. Its
formation corresponds to the climatic improvements of the Late Glacial
and then to the Holocene Interglacial.

As shown by its truncated surface and the thickening of colluvial
deposits covering it toward the southeast (U1), this soil is strongly
truncated by 
\mbox{anthropogenic} erosion processes, which, according to
recent studies, intensified regionally during the Bronze  Age
\citep{Pastreetal2015}.

\subsection{The fault network}\label{sec3.3} 
During the excavation and logging, numerous faults affecting part of
the Quaternary deposits were observed in the southeastern sector 
(Figure~\ref{fig4}), between ${+}$35 and ${+}$40~m on profile P1 
(Figure~\ref{fig6}) and between ${+}$15 and ${+}$25~m on profile P2 
(Figure~\ref{fig7}). The fractures are commonly coated by white
calcite, which correspond to secondary precipitation associated with
roots activity. These structures clearly cut through Units U5 and U6
and the Bracheux Sands (SBx). They were precisely mapped on the surface
of the archaeological excavation  (Figure~\ref{fig4}). The surveys
indicate that the fault segments span a width of 5 to 10~m, exhibiting
a consistent E--W directional trend across the site and a kinematic
coherence, with downthrown blocks to the north. Vertical displacements
of a few centimeters to about 10~cm are documented on individual
segments, which are short ranging from 1 to 5~m in length 
(Figure~\ref{fig4}). They are recognized across the entire width of the
excavation (25~m) at the level of the archaeological layer.

The vertical continuity of the northernmost structure (named F3) was
demonstrated thanks to the excavation of a 2-meter-deep test pit along
the southern end of P2  (Figure~\ref{fig7}). In this pit dug down to
the chalk, the fault cuts through Units U5 and U6, the Bracheux Sands
(SBx), and the flint gravel layer forming the interface with the chalk
(Cx). An apparent normal offset of 12 to 14~cm was measured on this
particularly clear structure at the interface between the Bracheux
Sands (green) and the grey-yellow Quaternary aeolian sands (U6a and
U6b). At the base of the pit, the offset of the chalk surface is
approximately 10~cm. The other main faults, labelled F2 and F1 (from
north to south), were not explored beyond the Pleistocene/Thanetian
interface.

The stratigraphic observations presented above demonstrate that the 
N-90\textdegree\ faults clearly affect part of the Quaternary deposits
(U6, U5 and the base of U4), with maximum vertical offsets of about
10~cm on individual fault segments. The cyclostratigraphic analysis of
the BLJ sequence and the dating of the U6 sands suggest that the
youngest unit affected by these faults could be the base of U4, which
is capped by the apparently undisturbed major core of U4. This would
lead to an  {event} attributable to the first half of the Middle
Weichselian Pleniglacial,  {after} ${\sim}${56} ${\pm}$ {5~ky} 
(Figure~\ref{fig6}). Due to the proximity to the surface
and the pedological alterations associated with the post-glacial soil
(U1), as well as the clayey (plastic) nature of the U4 material, it is
challenging to really constrain the upper limit of the deformation
which seems to stop within U4 (${\sim}$50 ky, according to regional
stratigraphic correlations) and we cannot exclude that the deformation
reaches the upper part of U4. Since these faults disappear within Unit
4, we infer that their activity ceased before the formation of the
humic soil of Unit 3, and thus before 45\,000 years ago.

\section{Discussion}\label{sec4} 

\subsection{Origin of the deformations observed at\newline
Beauvais}\label{sec4.1} 
In the broader regional context of northern France, the brittle
structures cutting through dated Quaternary deposits, their Tertiary
and Cretaceous substratum, as observed at Beauvais  ``La Justice'',
constitute an exceptional example,  alongside Biache-Saint-Vaast
\citep{Colbeauxetal1981}. While numerous faults have been documented by
various authors in the Quaternary deposits of northern France, these
typically result from periglacial processes (e.g., ice-wedge casts,
slope decompression) or, more commonly, from localized dissolution of
the underlying chalk substratum like karst sinkholes and  dolines
\citep{Antoineetal2016,AntoineLimondin-Lozouet2024,Baizeetal2007,
Bertranetal2017,VanVliet-Lanoeetal2017}.

However, at Beauvais La Justice, no doline or karst sinkhole
deformations were observed at the scale of the site, despite a
systematic search for the Quaternary-Cretaceous contact during the
excavation and subsequent observations during the RN-31 roadworks,
which revealed a rather regular chalk surface and an absence of karst
cavities or depressions in this area. While anthropogenic cavities
(e.g., marl or underground quarries) can also cause surface
deformations, no such cavities are recorded in the national database
within the site or slightly to the north to explain the observed
deformations, despite the risk classification of the municipality of 
Beauvais \mbox{\citep{PannetSchroetter2011}}. The hypothesis of a landslide
linked to the proximity of a gently sloping hillside toward the
Th\'{e}rain river in the southwest of the section is unlikely, despite
the colluvial nature of the youngest affected deposits (U5), as it
would be incompatible with the northward dip of the normal faults. A
periglacial origin also seems unlikely: the brittle deformations at BLJ
are rooted in the substratum, are mechanically consistent from top to
bottom, and bear no morphological resemblance to any periglacial
structures in northwestern France, such as ice-wedge pseudomorphs,
cryoturbation involutions, or sediment-filled pots. Additionally,
glacitectonic deformation can be ruled out, as the region was never
occupied by ice.

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig08}
\vspace*{2pt}
\caption{\label{fig8}Tectonic and kinematic interpretation of the P2
profile, accounting for the units and faults geometry. We highlight the
major role of F2 in separating the two blocks vertically offset
(0.25--0.3~m) and the secondary impact of other faults, including F1
and F3. F2 fault zone shows a complex pattern of apparent reverse and
normal offset, with abrupt thickness changes and mismatching layers
apart strands. This geometry could be explained by a combination of dip
and strike-slip motion on curved and merging planes (F1/F2 fault zone,
at least). Interestingly, the stratigraphic context constrains a
probable single event deforming the Cs to U5/base of U4 pack, capped by
undisturbed sediment of the U4 unit.}
\end{figure*}

Thus, a tectonic origin appears to be the most plausible explanation
for these deformations. In such a hypothesis, we propose an attempt to
characterize and quantify the tectonic deformation. When considered as
a whole, the fault zone (F1 to F3) separates two vertically offset
blocks by approximately 25~cm  (Figure~\ref{fig7}). The largest
vertical displacement is concentrated around fault zone F2, which
exhibits a complex pattern with both apparently reverse and normal
offsets and opposite dip directions. If we assume that all segments are
connected at depth to a single fault, this pattern can be interpreted
coherently in terms of kinematics, with a curved/listric geometry for
the F2--F3 ensemble  (Figure~\ref{fig8}). When additionally considering
the change of thickness and mismatching of U6 across a fault segment
between F1 and F2, we could propose a lateral component of the
movement.  Figure~\ref{fig8} shows a right-lateral component without
any constraint (it could be left-lateral).

The arrangement of these structures reveals strong segmentation, with
parallel meter-scale strands spaced a few meters apart, grouping into
clusters. While the excavation area is extensive, it remains too
limited to fully understand the organization of a potential coseismic
surface rupture. For comparison, similarly complex geometries are
common at similar scales, as illustrated by the 2016 Norcia earthquake
(Mw 6.5) in  Italy \citep[][]{Villanietal2018} where ground ruptures
exhibited intricate and segmented patterns. 

Therefore, the BLJ fault system may represent a localized expression of
a larger, more complex rupture system, possibly linked to deep-seated
tectonic structures. Further investigations, \mbox{including}
\mbox{high-resolution} geophysical imaging and paleoseismic trenching,
would be necessary to clarify the three-dimensional geometry and
kinematic behaviour of this fault network. This study and related
archeological framework would be useful to analyse those further
investigations.

\subsection{Implications for seismic hazard assessment}\label{sec4.2} 
The recent age of tectonic deformations at BLJ raises questions about
our understanding of seismic hazard. The region is classified as an
area of very low seismicity, with no mandatory earthquake-resistant
building regulation, even for critical infrastructure (buildings and
bridges). Critical plants potentially  belong to the same
seismotectonic zone  as the BLJ site according to  the
\citep{Baizeetal2013} seismotectonic scheme, and to date their hazard
could be controlled by the reference earthquake of 
Veules-Les-Roses (1769). This event is characterized by an
epicentral intensity of 6.5 and an equivalent magnitude/depth pair of
Mw 5.1 and $H = 14$~km \citep{Manchueletal2018}. If the BLJ
\mbox{observations} are associated with a prehistoric 
\mbox{earthquake} \mbox{involving}
surface rupture, questions arise in terms of seismic hazard and its
potential reassessment in the region. What magnitude could this
paleoearthquake have had? Which structure generated this event? What is
the potential of this structure in terms of maximum earthquakes and
their recurrence intervals? Could a similar feature exist elsewhere in
the same seismotectonic zone and  where?

The Bray Fault is absent from the French Database of Potentially Active
Faults (BDFA)  \citep{Jomardetal2017} but described as a neotectonic
structure with very low  activity in the  \citep{Baizeetal2013}
database. It is the largest geological structure near the BLJ site,
consisting of a deeply rooted fault dipping southwestward. While it
outcrops only partially along its extension at depth, it likely extends
to the contact between the Lower and Upper Cretaceous, approximately
5--6~km southwest of the site. This major fault is associated with an
asymmetric  anticline and syncline  \citep{Gelyetal2014}, with the
syncline axis passing about 500~m south of the site. Gravimetric
data suggest that the bedrock of the study area may include a secondary
E--W transverse structure, roughly at the same latitude as another NE--SW
trending brittle structure  (Figure~\ref{fig3}). 

The tectonic deformations at BLJ are thus not located directly above a
mapped fault but seem collocated with a fold structure and potentially
above a geophysical anomaly with a same direction. One hypothesis is
that those features in BLJ represent distributed deformations
associated with a major coseismic rupture on a distant main fault, such
as the Bray Fault. However, this fault is too distant (over 5~km),
making such distributed ruptures with high amplitudes (25~cm) very
unlikely. These would typically be associated with major events of
magnitudes greater than 6.5 or even  7
\citep{Nurminenetal2022,Visinietal2025}, which seem unrealistic in the
context of the Paris Basin.

An alternative possibility is that the observed deformations correspond
to the rupture of an unmapped ${\sim}$E--W fault, a secondary crustal
fault structurally related to the Bray Fault, during a significant
earthquake. Using empirical scaling relationships, the magnitude of
such an earthquake can be estimated between Mw 6.2 and  6.5 for normal
or strike-slip mechanisms if the deformation is due to a unique event,
according to the documented offset (${\sim}$25~cm) across the entire
fault width (${\sim}$5--7~m)  based on the \citet{WellsCoppersmith1994}
relationship applicable to active tectonic contexts, and Mw 5.3 to 5.65
for dip-slip and strike-slip earthquakes (respectively)  using
\citet{Leonard2014}  intraplate scaling law. To date, we have no clear
evidence of multiple events in the observed stratigraphy: we cannot
confidently interpret any of the upward attenuating deformation
features  (Figure~\ref{fig7}) for proper fault terminations which would
attest to recurring events. Moreover, the spatial distribution of
faults visible on map is not an argument of multiple events, as this
pattern is commonly observed during modern Mw ${\sim}$ 5.5--6.5
earthquakes' surveys  \citep{Nurminenetal2022}. The geometric
relationships and the ages obtained for the affected and unaffected
formations, ${\sim}$55 ${\pm}$ 5~ka and 45~ka respectively, therefore
suggest that the site records only one event since the deposition of
the aeolian sands containing the Palaeolithic layer. Therefore, no
usable information is available on the recurrence interval of
equivalent events or the possible slip rate of the underlying fault.
The available data remains too fragmentary to extend beyond the
assertion of active tectonics within the last 60~ka in 
the~\mbox{region.}

Regardless of the magnitude of the interpreted paleoearthquake,
questions remain about the potential of the Bray Fault, even if the
reported observations are not linked to it but to a peripheral
structure. Large earthquakes on transform interplate and mature faults
are commonly triggered by branch and immature faults (e.g.\ 2002 M7.9
Denali (Alaska), 2016 M7.8 Kaikoura (New Zealand) or 2023 M7.7 Pazarcik
(Turk\"{i}ye)) \citep{SteinBird2024}. The region of interest is under a
drastically different seismotectonic environment and regime; however
one could question the possibility that the potentially BLJ structure
could trigger a larger earthquake on the crustalscale, (probably)
mature and old buried Bray fault. Structural complexities are also
thought to be a location of stress buildup at the initiation of large
intraplate  earthquakes \citep{Talwani2017}. The seismogenic thickness
(10--15~km), along with the lateral and depth extent of the nearby Bray
structure, thus raises questions about its capacity to generate large
earthquakes capable of producing damaging ground motions and surface
ruptures.

The timing and location of this event questions the triggering
phenomenon. Actually, the age bracket of the event, between 55 and 45
ky, more or less \mbox{coincides} with the retreat of the MIS 4
Fennoscandia-British Islands icesheet that were partly covering
Northern  Europe \citep{Batcheloretal2019}. The BLJ site
being located on the southern margin of this massive icesheet, it could
have been influenced by glacio-isostatic adjustments (GIA) that likely
modified the strain-rate and stress conditions as during and after the
Late Glacial  Maximum  \citep{Craigetal2023}. MIS 4 ice sheet southern
boundary is much less constrained than that of MIS 2
\citep{Batcheloretal2019}, its exact location in relation to the BLJ
site is uncertain; however, it was at least 400~km to the
north-northeast.

Even if the far-field tectonic loading may remain the long-term control
of loading and mechanism of faulting, transient changes are prone to
trigger seismicity on critically stressed faults, especially during the
early period of post-glacial adjustment. One important consequence of
such a triggering process would be that, at first glance, the hazard
would have been larger at that time than during the present days and
the near  future \citep{Linsalataetal2026}. Actually, despite a large
number of observations in the framework of archaeological rescue and
soundings, there has never been an observation of LGM-related
post-glacial event. However, as  stated by \citet{Wangetal2021}, the
current ``no (measurable) strain'' does not mean ``no hazard'' in the
context of a visco-elastic earth responding to a large-scale isostatic
post-glacial rebound, especially dealing with a fault deeply seated in
the crust like the Pays De Bray structure. In other words, the early
loading of a fault, caused by GIA, may still be stored, and it may
still be close to failure and available for further events.

In any case, these results encourage further detailed exploration of
the potential underlying structure, using high-precision near-surface
geophysical methods (e.g.\ ground-penetrating radar, S-wave reflection
seismics, electrical resistivity tomography) and additional
paleoseismic excavations, as well as a reassessment of the whole Bray
Fault, which has been overlooked until now.

\section{Conclusion}\label{sec5}
More than 30 years after the initial study of the Beauvais La Justice
site, we report here observations concerning a fault system (N
90\textdegree) cutting across a Weichselian pedosedimentary sequence
including at its base a Middle Paleolithic site preserved in an aeolian
sand layer well-dated to nearly 60 ka using TL on burned flint
artefacts. A re-examination of the stratigraphic sections of this site,
originally analysed for paleoanthropological and paleoenvironmental
purposes, from a tectonic perspective reveals that the geometry,
kinematics, and spatial arrangement of the deformations---within the
local morphologic context---are consistent with a tectonic origin.

In the context of heightened seismic risk awareness following the Le
Teil earthquake, we emphasize the urgent need to re-examine all
potential indicators of neotectonic activity documented in existing
databases. It is then essential to collect evidence of potentially
tectonic deformation from local geoscience stakeholders, whether they
are involved in the Epos-France FACT consortium. We demonstrate here
that French archaeologists and geomorphologists, who---through the
national preventive archaeology law in France---have acquired
unparalleled territorial knowledge of anthropogenic remains and
associated stratigraphic horizons, can significantly contribute to this
effort.

Moreover, to inform seismic hazard models, we argue that these data
must be supplemented by broader acquisitions at the scale of associated
geological structures capable of generating paleoearthquakes. This
includes quantitative geomorphology and paleoseismology, as well as the
exploitation of available geophysical data, whether from subsurface
exploration (e.g., reflection seismics) or regional deformation
quantification (geodesy or 
\mbox{seismology).}

This is particularly relevant for large nearby regional structures,
such as the Pays de Bray anticline and its associated crustal fault,
whose Quaternary activity remains poorly characterized. A
comprehensive, multidisciplinary approach is necessary to refine
seismic hazard assessments and better constrain the potential for
future earthquakes in intraplate regions like France.

\section*{Acknowledgments}
We sincerely thank Klaus Reicherter, Brigitte Van Vliet-Lano\"e and an
anonymous reviewer, as well as Eric Calais, Editor-in-Chief at the
journal ``Comptes Rendus G\'eoscience Earth Sciences'', for their
insightful comments and constructive feedback. They greatly contributed
to improving the quality of this manuscript. This work is part of the
collective effort of the national community gathered within the Active
Faults Study Group, Epos-France/ATTS/FACT.
Archeological survey was led by J.-L.\ Locht with continuous
stratigraphic monitoring by P. Antoine (Figures B, C, D; Plate A). Large
mammal remains were analysed by P. Antoine (CNRS-USTL-Lille), while
thermoluminescence dating of archaeologically heated flints was
performed by N. Debenham (TL Survey Nottingham). Concurrently, 
V. Michel and J.-J. Bahain (IPH-MNHN, Paris) conducted U/Th and combined
U/Th-ESR dating on dental enamel of large mammals, yielding results
consistent with those obtained from thermoluminescence.

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