# Plate-Scale Strike-Slip Fault System in the Barbados Accretionary Wedge of the Lesser Antilles Subduction Zone

Gaëlle Bénâtre, Nathalie Feuillet, Hélène Carton, Eric Jacques, Thibaud Pichot, Frédérique Leclerc, Christine Deplus

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## ► To cite this version:

Gaëlle Bénâtre, Nathalie Feuillet, Hélène Carton, Eric Jacques, Thibaud Pichot, et al. Plate-Scale Strike-Slip Fault System in the Barbados Accretionary Wedge of the Lesser Antilles Subduction Zone. Journal of Geophysical Research: Solid Earth, 2025, 130 (6), pp.e2024JB030059. 10.1029/2024JB030059. hal-05123213

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Distributed under a Creative Commons Attribution 4.0 International License
# JGR Solid Earth

## RESEARCH ARTICLE

10.1029/2024JB030059

**Key Points:**

* An active left-lateral strike-slip fault, long (~580 km) enough to cause magnitude class 8 earthquakes, is identified in the Barbados wedge

* This fault (Seraphine) accommodates a part of the oblique convergence between the North and South American and the Caribbean plates

* This fault system may be a result of the segmentation of the megathrust and a marker of variable seismic coupling at plate interface

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

G. Bénâtre,
 gaelle.benatre@gmail.com

**Citation:**

Bénâtre, G., Feuillet, N., Carton, H., Jacques, E., Pichot, T., Leclerc, F., & Deplus, C. (2025). Plate-scale strike-slip fault system in the Barbados accretionary wedge of the Lesser Antilles subduction zone. *Journal of Geophysical Research: Solid Earth*, 130, e2024JB030059. https://doi.org/10.1029/2024JB030059

Received 2 AUG 2024

Accepted 14 MAY 2025

**Author Contributions:**

**Conceptualization:** Gaëlle Bénâtre,

Nathalie Feuillet, Hélène Carton,
Eric Jacques, Thibaud Pichot,

Frédérique Leclerc, Christine Deplus

**Formal analysis:** Gaëlle Bénâtre

**Funding acquisition:** Nathalie Feuillet

**Investigation:** Gaëlle Bénâtre,

Nathalie Feuillet, Eric Jacques,

Frédérique Leclerc

**Methodology:** Gaëlle Bénâtre

**Project administration:** Nathalie Feuillet

**Resources:** Nathalie Feuillet,

Hélène Carton

**Supervision:** Nathalie Feuillet,

Hélène Carton, Eric Jacques

**Validation:** Gaëlle Bénâtre

© 2025. The Author(s).

This is an open access article under the
terms of the Creative Commons

Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.

<figure><img src="image_3.png" /></figure>

## Plate-Scale Strike-Slip Fault System in the Barbados Accretionary Wedge of the Lesser Antilles Subduction Zone

Gaëlle Bénâtre¹ ID, Nathalie Feuillet¹, Hélène Carton¹ ID, Eric Jacques¹, Thibaud Pichot² ID,
Frédérique Leclerc³ ID, and Christine Deplus¹ ID

¹Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, France, ²Beicip-Franlab, Rueil-Malmaison, France, ³Université Côte d'Azur, CNRS, IRD, Observatoire de la Côte d'Azur, Géoazur, Nice, France

**Abstract** Fault systems of various geometries develop into accretionary wedges to accommodate slip partitioning of plate convergence in oblique settings. However, how they form, evolve and contribute to the segmentation of the megathrust is still poorly understood. The Barbados accretionary wedge, which results from the subduction of the American plates beneath the Caribbean Plate at 2 cm/yr in a southwesterly direction, is one of the largest on Earth. Here we present a comprehensive morphotectonic study of this wedge based on a new marine geophysical data set acquired during the recent CASEIS marine experiment combined with legacy data sets. From a joint analysis of high-resolution bathymetry and seismic reflection profiles, we characterize a regional-scale left-lateral strike-slip fault system in the wedge, which began to form in Pliocene times and is still active today. It may relay, via a diffuse complex deformation system, to the Bunce left-lateral strike-slip fault in the north and may join the conjugate Kitridge right-lateral fault to the south. Both Bunce and Kitridge faults are themselves linked via relay zones to the main strike-slip fault systems bounding the Caribbean plate. They likely developed along the toe of the backstop and may mark a segmentation of the Lesser Antilles megathrust. The main segment of this system is the newly identified ~580-km-long Seraphine fault, which is long enough to rupture during magnitude class 8 earthquakes. The kinematics of the entire fault system is incompatible with present-day NE-SW compression inferred at the Caribbean/American plate boundary and raises questions about the current geodynamics of the region.

**Plain Language Summary** Subduction zones are regions of plate convergence where a tectonic plate plunges beneath another. In that process different kinds of faults are formed in the overriding plate, responsible for accreting and deforming sediments carried by the incoming oceanic plate, with increased complexity when the convergence is oblique. Here, we examine faults developed within accreted sediments offshore the Lesser Antilles where the North and South American plates subduct beneath the Caribbean plate in a southwesterly direction. Using marine data relying on sound waves to map the ocean floor and probe the sub-seafloor, we identify a major fault zone, the Seraphine fault, along which the seaward block is moving to the NNW and the landward block to the SSE. We show that this fault formed a few million years ago and is still active today. It is long enough (around 580 km) to produce magnitude 8 earthquakes and may contribute to the segmentation of the subduction zone. It is part of a larger fault system on the eastern border of the Caribbean plate that accommodates the oblique component of plate motion. Our study points to an incompatibility between fault kinematics and current convergence direction, raising questions about the region's current geodynamics.

## 1. Introduction

Subduction zones are the locus of intense upper-plate deformation. Where the convergence is oblique, such as in Sumatra, South Ryukyu, and New Zealand, specific patterns of deformation occur. The partitioning into trench-normal and trench-parallel components of motion commonly manifests itself through the development of a strike-slip fault zone in the upper plate. Such strike-slip faults may localize in the volcanic arc, for instance the 1,900-km-long Great Sumatran Fault (e.g., Rafie et al., 2022; Sieh & Natawidjaja, 2000) or the ~2,000-km-long Philippine fault (e.g., Barrier et al., 1991; Lallemand, 1999). The accommodation of the trench-parallel component of slip may also occur closer to the trench, in the fore-arc basin area or in the accretionary wedge (Chemenda et al., 2000; Lallemand, 1999). Meanwhile, the accommodation of the trench-normal component of motion occurs on the megathrust (e.g., Chemenda et al., 2000; Lallemand, 1999).

Analog sand box and numerical models provide insights into the temporal evolution of such systems (e.g., Martinez et al., 2002; McClay et al., 2004). These models show that the formation of a strike-slip fault as a result
**Visualization:** Gaëlle Bénâtre

**Writing – original draft:** Gaëlle Bénâtre,
Nathalie Feuillet, Hélène Carton

**Writing – review & editing:** Eric Jacques,
Thibaud Pichot, Frédérique Leclerc,
Christine Deplus

of partitioning of the deformation is related to the frictional properties on the megathrust and to the degree of obliquity of the convergence (Chemenda et al., 2000; Martinez et al., 2002; McClay et al., 2004; Philippon & Corti, 2016; Pinet & Cobbold, 1992). In most cases, such faults develop near a geological discontinuity along which the deformation localizes (Martinez et al., 2002). When the obliquity is low (<30–45°), the trench-parallel component of slip is accommodated on pre-existing forethrusts or backthrusts (McClay et al., 2004), whereas wrench zones and even flower structures may develop in the case of stronger obliquity (Martinez et al., 2002; McClay et al., 2004). A strong basal friction favors strain partitioning (Chemenda et al., 2000; Philippon & Corti, 2016) as it decreases the obliquity threshold for the partitioning to occur (Martinez et al., 2002). It has been observed that the faults accommodating the trench-parallel component of slip frequently bound the trenchward limit of fore-arc basins, as exemplified by the 100-km long Yaeyama fault (YF) in southern Ryukyu (Dominguez, Lallemand, Malavieille, & Schnürle, 1998; Lallemand et al., 1999), the 350-km-long Mentawai fault (MF) offshore Sumatra (Diament et al., 1992; Kopp et al., 2002; Schlüter et al., 2002), the Bunce fault (BF) in the northern Lesser Antilles (ten Brink et al., 2004; Laurencin et al., 2019) or the 200-km-long Wedge-Boundary Strike-slip Fault (WBSF) in Nankai (Tsuji et al., 2014). The majority of those faults have developed along the toe of the backstop (Chauhan et al., 2009; Lallemand et al., 1999; Laurencin et al., 2019) which is the updip limit of the contact between the crusts of the two converging plates, whereas the WBSF connects on a mega-splay fault branching off the megathrust (Tsuji et al., 2014). Although a few examples of these strike-slip fault systems have been documented in subduction zones, their formation remains poorly understood. This is mainly because we lack examples documented by high-resolution marine data, which would allow us to characterize them at several spatio-temporal scales. Many questions remain about the development of strike-slip faults, including the factors that control their formation, the structural discontinuities and rheological properties that promote their initiation and localization, and their potential impact on seismic and tsunami hazards.

Here we present a new suite of high-resolution marine data that allows us to document a previously unknown active strike-slip fault system in a context of oblique convergence in the Lesser Antilles subduction zone. New data sets, including swath bathymetry, short-streamer multi-channel seismic (MCS) and sub-bottom profiler data, were acquired aboard the Research Vessel *Pourquoi Pas?* (Feuillet, 2016). They were combined with swath bathymetry data and MCS profiles acquired during earlier cruises to identify and characterize tectonic structures and fault systems at the edge of the fore-arc basin and in the accretionary wedge in an area that had been scarcely investigated before. Moreover, these combined data sets allow more complete analysis of the extension of all the mapped structures and the connections between them at a broader scale.

While mapping the faults and deformation, we establish a hierarchy of the structures to better understand their broad-scale geometry and distribution, and thus determine the kinematics of the active deformation. We identify a new fault, the Seraphine fault. Seismic data combined with regional chronostratigraphic results allow us to date the deformation and discuss its evolution. Finally, we discuss our results with the aim to explain the mechanisms that promote slip partitioning in the Lesser Antilles fore-arc region and the implications in terms of seismic hazard.

## 2. Geological Setting and Seismicity

The Lesser Antilles subduction zone (LASZ) (Figure 1) is a convergent margin where the North and South American plates subduct beneath the Caribbean plate at approximately 2 cm/yr (DeMets et al., 2000, 2010). The present-day convergence rate varies significantly between different plate models partly due to a lack of constraints on the movement between the North and South American plates (e.g., Argus et al., 2010; DeMets et al., 2000, 2007, 2010; DeMets & Merkouriev, 2019; Kreemer et al., 2003). Between the volcanic arc and the trench, several fore-arc basins and a wide accretionary wedge have developed since the Late Cretaceous (Gomez et al., 2018). The Barbados accretionary wedge is one of the largest in the world (Noda & Miyakawa, 2017). It is fed by sediments scraped off from the Atlantic seafloor, which were originally eroded and transported from the arc or from the Orinoco delta (Figures 1a and 1b). The wedge becomes wider and thicker toward the Orinoco river outlet (Deville, 2011; Deville et al., 2015; Mascle & Moore, 1986; Seibert et al., 2020). The trench-parallel component of slip increases northward from 2 mm/yr in Martinique (Mq) to 15 mm/yr in Saba (S) in relation with the northward increase of the trench curvature (Feuillet et al., 2010; López et al., 2006).

Several studies, mainly based on active-source seismic experiments, have investigated the geometry of the wedge along seismic profiles perpendicular to the LASZ in front of Guadeloupe (e.g., Bangs et al., 2003; Laigle et al., 2013; Westbrook et al., 1988), to the South of Martinique (e.g., Deville, 2011; Deville et al., 2015; Gomez
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Geodynamic setting of the study area. Shaded bathymetry/topography maps (illumination from N270° azimuth) of (a) the Caribbean plate and (b) the Lesser Antilles Subduction Zone (LASZ), forming the boundary between the Caribbean plate (CAR) and the North and South American Plates (NAM and SAM, respectively). Convergence vector striking ~N254° and convergence rate of ~2 cm/yr are indicated using red (NAM/CAR) and black (SAM/CAR) arrows (DeMets et al., 2010). Red transparent lines correspond to simplified fault systems that accommodate strike-slip motion, in the arc (Feuillet, Beauducel, Jacques, et al., 2011; Feuillet et al., 2002, 2010) and fore-arc (Laurencin et al., 2019; ten Brink et al., 2004 for BF and this study for SF and KF). Thick black line is the deformation front (dashed when inferred). In black = major thrust faults. In gray = major blind thrust faults. Purple lines correspond to normal faults (see references in the main text). White line = backstop (Laigle et al., 2013; Paulatto et al., 2017) AP = Anegada Passage, AR = Aves Ridge, BF = Bunce fault, BR = Barracuda ridge, EPF = El Pilar Fault system (Deville, 2011; Garciacaro et al., 2011; Jouanne et al., 2011), MT = Muertos Trough, PRT = Puerto Rico Trench, SLR = Saint Lucia Ridge, TFZ = Tintamarre Fault Zone, TR = Tiburon Ridge. Transparent purple ellipses: possible ruptures of the 1839 and 1843 earthquakes from Feuillet, Beauducel, and Tapponnier (2011). Focal mechanisms of Mw > 6 earthquakes from Global CMT catalog (1–2 and 4 to 8, Dziewonski et al., 1981; Ekström et al., 2012) and Stein et al. (1982) for the 1969 crisis (3): 1 = Mw 6.3 Les Saintes earthquake, 2 = Mw 7.4 earthquake (Feuillet, Beauducel, & Tapponnier, 2011), 3 = Mw 6.3 earthquake from the 1969 Christmas crisis (Stein et al., 1982), 4 = Mw 5.9 earthquake in 2014, 5 = Mw 6.5 earthquake in 2015, 6 = Mw 6.1 earthquake in 1995, 7 = Mw 6.4 earthquake in 2014 and 8 = Mw 6.0 earthquake in 2016. Islands: A and B = Antigua and Barbuda, BI = Barbados Island, Gr = Grenada, Gw = Guadeloupe, Do = Dominica, Mq = Martinique, Mt = Montserrat, PR = Puerto Rico, S. = Saba, St. K and N = Saint Kitts and Nevis, St. L. = Saint Lucia, St. V and Grs = Saint Vincent and the Grenadines. Spurs, Valleys and bassins: AV = Antigua Valley, B-FS = Bertrand-Falmouth Spur, BS = Barbuda Spur; BV = St Barthelemy Valley, DV = Désirade Valley, GB = Grenada basin, KS = Karukera Spur, MB = Martinique Basin TB = Tobago basin and TS = Tintamarre Spur. White arrows indicate the turbidity currents and sediment flux from the Orinoco delta (Deville et al., 2015). Black dashed boxes are the localization of the 3 sectors: Martinique-Antigua sector (Figure 2a), northern sector (Figure 4a) and southern sector (Figure 9a).</figcaption></figure>

et al., 2018; Torrini & Speed, 1989) and to the North of Antigua (e.g., Boucard et al., 2021; Laurencin et al., 2019; ten Brink et al., 2004). South of 14°N, the wedge is characterized by a 350-km-long, 20–60 km wide thin-skinned fold-and-thrust belt (Torrini & Speed, 1989), called the Barbados Ridge, which extends between the Tobago Island (To, on Figure 1b) and the subducted Saint-Lucia ridge (SLR), north of Barbados Island (BI). An accreted terrane (Mesozoic allochthonous Tobago terrane) and the presence of an east-dipping backthrust seem to be at the
origin of the Barbados Ridge, which locally emerges on Barbados Island (Gomez et al., 2018; Torrini & Speed, 1989). In the Martinique-Antigua sector between 14° and 17.5°N, the outer-arc high domain is not very well understood, because the deep seismic reflectors are unclear in the seismic profiles (e.g., Laigle et al., 2013; Westbrook et al., 1988). The deformation affecting the wedge and fore-arc basins is in part controlled by the subduction of two main bathymetric features, the Barracuda (BR) and Tiburon (TR) ridges (Figure 1; Bangs et al., 2003; Laigle et al., 2013; Westbrook et al., 1988). These WNW-ESE striking ridges are up to 400 km long and 50 km wide and culminate at 2,500 m above the abyssal plain. Recent studies interpreted these ridges as the flanks of old fracture zones situated at the boundary between the North and South American plates and having been uplifted by compressional tectonics since the middle or late Miocene (Pichot et al., 2012). The existence of active folding and flower structures in between the two ridges indicates that this deformation is still active (Pichot et al., 2012). In the area where the ridges are subducting, out-of-sequence thrusts located 50–70 km from the trench (Bangs et al., 2003; Westbrook et al., 1988) and deformed fore-arc basins have been reported (e.g., Bangs et al., 2003; Laigle et al., 2013; Westbrook et al., 1988).

In the northern part of the LASZ, north of 17.5°N, the accretionary wedge is sharply crosscut by a 850 km-long, trench-parallel, left-lateral strike-slip fault, the Bunce fault (BF) (ten Brink et al., 2004; Laurencin et al., 2019). The Bunce fault is located 30 km far from the trench and appears to bound the fore-arc basins to the northeast. At depth, MCS data have revealed flower structures typical of strike-slip faulting and suggest that the fault may connect onto the megathrust at the toe of the upper plate backstop (Laurencin et al., 2019). Offshore Barbuda (B on Figure 1b), the eastern tip of the Bunce fault coincides with a kink in the deformation front at a latitude of ~18.5°N (Laurencin et al., 2019). These authors propose that the fault separates the fore-arc basins from a ~30-km-wide accretionary wedge and that it may accommodate the trench-parallel component of motion. The obliquity of the plate convergence increases northward due to the stronger curvature of the eastern boundary of the Caribbean plate. Laurencin et al. (2019) estimated the trench-perpendicular motion by examining the surface trace of a subducted seamount close to the trench and concluded that the angle between this frontal motion and the plate convergence vector is about ~50 ± 10°. From this result, they inferred that ~80 ± 10% of the plate convergence is likely accommodated by a northwestward motion of the accretionary wedge along the Bunce fault. Assuming that this fault is the only one that accommodates by slip partitioning the trench-parallel component of motion, they deduced a slip rate of 1.6 ± 0.2 cm/yr on the Bunce fault. However, other faults located in the volcanic arc and in the fore-arc basins, were inferred to accommodate a fraction of the trench-parallel motion (Feuillet, Beauducel, & Tapponnier, 2011; Feuillet et al., 2002, 2010). Faults belonging to the first family bound arc-perpendicular grabens or half-grabens that cut the fore-arc reef platforms between Guadeloupe Island and the Anegada Passage (Feuillet et al., 2002). Faults in the second family form a large right-stepping *en-échelon* system running along the volcanic arc (Feuillet et al., 2010). Feuillet et al. (2010) inferred that both sets of faults are connected at plate-scale (faults cut through the entire upper-plate crust) and form a left-lateral horsetail. The major faults of the two families may move at rates of a few mm/yr and accommodate a significant part of the trench-parallel motion related to the obliquity of the convergence. The northern fore-arc basins are also crosscut by trench-parallel N300°E-striking normal faults (Tintamarre Fault Zone: TFZ on Figure 1b), the origin of which remains unclear. Recently, Boucard et al. (2021) suggested that these normal faults are the consequence of the basal erosion of the upper plate.

The seismicity rate is low in the LASZ compared to other subduction zones and only a few damaging earthquakes were reported in the historical period (17th century to 1950). Both instrumental seismicity and GNSS data suggest that this subduction zone is uncoupled, meaning that no strain is accumulating along the plate interface to promote large megathrust earthquakes comparable to those reported in Sumatra in 2004 or in Japan in 2011 (Manaker et al., 2008; Symithe et al., 2015; van Rijsingen et al., 2021). This contrasts with longer-term records from coral microatolls, which show a strong coupling over the last decades on the plate interface between at least 30 and 70 km depth (Philibosian et al., 2022; Weil-Accardo et al., 2016). The two largest historical earthquakes occurred in the 19th century. The first one, with maximum intensity of VIII and IX, occurred on 11 January 1839 off the coast of Martinique, destroying the town of Fort-de-France and killing several hundred people. The second one occurred four years later, on 8 February 1843. It was stronger (maximum intensity of X), destroyed the city of Pointe-à-Pitre in Guadeloupe and killed over 1,500 people. The origin and magnitude of those events are still debated. The most recent studies inferred that they were likely magnitude 8-class megathrust events (Feuillet, Beauducel, & Tapponnier, 2011; Hough, 2013).

More recently, other damaging earthquakes have struck the LASZ: the Mw 6.3 Les Saintes earthquake (labeled 1 on Figure 1b) in November 2004 and a more recent event of Mw 7.4 in November 2007 (labeled 2 in Figure 1b)
(Feuillet, Beauducel, & Tapponnier, 2011). The shallow Les Saintes 2004 earthquake ruptured a submarine normal fault of the *en-échelon* system running along the arc (Bazin et al., 2010; Escartín et al., 2016; Feuillet, Beauducel, Jacques, et al., 2011; Hughes et al., 2023). The intermediate-depth 2007 earthquake (152 km deep hypocenter) likely ruptured an intra-slab normal fault (Schlupp et al., 2008). Few Mw 6+ earthquakes have been recorded in the wedge region. The main shock of the December 1969 crisis (on 25 December 1969, labeled 3 in Figure 1b) is one of such events, occurring at 16°N with a magnitude of Mw 7.2 ("ISC Bulletin," 2022; Storchak et al., 2017, 2020). An almost pure normal focal mechanism has been inferred for this earthquake (Stein et al., 1982). The depth of the main event is poorly constrained with values ranging between 2.1 km ("ISC Bulletin," 2022; Storchak et al., 2017, 2020) and 42 km (Stein et al., 1982). The latter authors estimate that the December 1969 earthquakes took place between 20 and 50 km depth, suggesting they occurred within the subducting slab.

More recently, two other seismic crises occurred east of the arc, beneath the accretionary wedge, in 2014 and 2015. The first crisis took place offshore Antigua. It was characterized by a Mw 5.9 main shock (labeled 4 on Figure 1b), followed by shallow ~25-km-deep aftershocks. The locations of the latter indicate that they were intraslab events, occurring at the intersection between the subducting Barracuda ridge and the backstop (González et al., 2017). The second 2015 crisis occurred offshore St Lucia. As the earthquakes occurred far from the seismic network, the uncertainties on hypocenter locations are large and depths down to 140 km were determined (Massin et al., 2021). Nevertheless, this crisis was characterized by mostly shallow (30 km) earthquakes and a Mw 6.5 mainshock (source: GCMT Catalog, Dziewonski et al., 1981; Ekström et al., 2012) with a strike-slip focal mechanism (labeled 5 on Figure 1b). Isolated events have also been recorded (8 March 1995 (labeled 6, Mw 6.1), 18 February 2014 (labeled 7, Mw 6.4), and 19 March 2016 (labeled 8, Mw 6.0) (source: GCMT Catalog, Dziewonski et al., 1981; Ekström et al., 2012).

The Lesser Antilles is thus a low-velocity subduction zone with a highly variable obliquity along strike. Several major ridges are subducting, especially two submarine ridges that are currently deforming the front of the wedge offshore Guadeloupe and Martinique.

# 3. Data and Methods

## 3.1. CASEIS Data

Here, we present bathymetry, short-streamer MCS (Bénâtre et al., 2024a; Feuillet, 2024) and sub-bottom profiler data (Bénâtre et al., 2024b) acquired during the CASEIS, “CAribbean SEISmicity”, cruise conducted from May 28th to 4 July 2016 onboard the French Research Vessel *Pourquoi Pas*? (Feuillet, 2016).

Bathymetry was acquired using a Reson 7,150 multibeam echo-sounder with an operating frequency of 12 kHz for water depths greater than 2,000 m, and of 24 kHz for water depths less than 2,000 m (Text S1 and Figure S1 in Supporting Information S1). The data were processed with the CARAIBES software (now part of GLOBE software by IFREMER, Poncelet et al., 2024).

We acquired 123 short-streamer MCS profiles (Text S1 and Figure S2 in Supporting Information S1), between latitudes 15°N and 19°N, using a 48-channel streamer with a group spacing of 6.25 m (streamer length of 300 m) and a source consisting of two GI airguns (one airgun with two 105 in³ chambers and the other with two 45 in³ chambers). For the first 14 profiles, shooting in time was used (10 s shot interval i.e. approximately 50 m assuming a fixed vessel speed of 9.7 knots), then shooting in distance (50 m shot spacing) was used for the remainder of the profiles. Data processing was performed using ECHOS Software from AspenTech according to the following steps: the data were first re-sampled at 2 ms interval (against 1 ms initially), then a swell-noise attenuation technique and an FK filter (polygon-shaped) were applied, followed by a spherical divergence correction assuming a velocity of 1,500 m/s; data were then sorted into common depth-point (CDP) gathers at 6.25 m spacing, a Normal Move Out (NMO) correction was applied with a uniform velocity of 1,500 m/s, NMO-corrected gathers were stacked, and finally the stacked sections were constant-velocity migrated at 1,500 m/s. We used a velocity of 2,000 m/s (cor-responding to the 4–5 km depth range on the tomographic image of Kopp et al. (2011)) to convert the two-way time thickness of sedimentary units to thickness in meters on the seismic profiles.

The sub-bottom profiles (Text S1 and Figure S3 in Supporting Information S1) were acquired with an 80 ms-long sweep source operating in a frequency range between 1.8 and 5.3 kHz, allowing for a penetration of ~100 m below the seafloor. Processing of sub-bottom profiler data was performed using SEISMIC UNIX (Cohen &
Stockwell, 2010) as follows: generation of chirp source signal, correlation with source signal, spherical divergence correction, calculation of the trace envelope and cancellation of delays. A velocity of 1,800 m/s was used to convert thicknesses from two-way time to actual thicknesses in meters on sub-bottom profiler sections.

## 3.2. Previous Surveys

Swath bathymetry data from previous cruises AGUADOMAR (Deplus, 1998), CARAVAL (Deplus & Boudon, 2002), SISMANTILLES 1 and 2 (Hirn, 2001; Laigle & Lebrun, 2007), GWADASEIS (Feuillet, 2009), ANTIPLAC cruise (Patriat, 2007) and ANTITHESIS I cruise (Marcaillou & Klingelhoefer, 2013) were included in our database to produce a DEM for the entire area with a 100 m grid spacing. In area not covered by the recent cruises, we used data available in the NOAA Multibeam Bathymetry Database and the Global Multi-Resolution Data Synthesis (Carbotte et al., 2004).

We also used processed MCS data from previous surveys: profiles HH' and DD' of SISMANTILLES 2 (Laigle et al., 2013; Laigle & Lebrun, 2007), all MCS profiles of EW9803 (8 strike-lines i.e. arc-parallel lines from S1 to S8 and 5 dip-lines i.e. arc-perpendicular lines from D1 to D5) (Bangs et al., 2003), profile 15 of RC1904 (Aitken et al., 2011; Speed et al., 1984) and several profiles of RC2604 (Bangs et al., 1990; Westbrook et al., 1988). Processed profiles from cruises EW9803, RC1904 and RC2604 were accessed from the Academic Seismic Data Portal at LDEO (previously ASP at UTIG, Christeson et al., 2017) either as stacks (RC1904 and RC2604 cruises) or migrated profiles (EW9803 cruise). The RC1904 cruise (1975) used a 24-channel streamer with a group spacing of 100 m and a shot interval of 20 s (approximately 50 m). The source consisted of a BOLT 1,500c airgun with a 1,864 in³ of volume. For the RC2604 cruise data were recorded on a 48-channel streamer with a group spacing of 50 or 100 m and the shot interval was variable. The source consisted of a BOLT 1,500c airgun with a 2,791 in³ of volume. The SISMANTILLES 2 data were acquired using a 360-channel streamer with a group spacing of 12.5 m and a shot interval of 150 m. The source system was a symmetrical device composed by 8 airguns on both lines plus one airgun on each side of the vessel; the largest volume used was 8,865 in³. Water-velocity migrated profiles used here are similar to those presented in Laigle et al. (2013). For the RC1904 and RC2604 cruise, we corrected the deep-water delays before water-velocity migrating the data.

## 3.3. Mapping Strategy

To map the structures of the LASZ we use a combination of bathymetric data (Seibert & Feuillet, 2024) and seismic data (MCS profiles and chirp sub-bottom profiles). The processed MCS profiles were loaded in a Kingdom™ software project, which allows to precisely locate structures imaged on the profiles with respect to the bathymetry. The bathymetry data and navigation of seismic profiles were loaded in a QGIS® software project.

Using QGIS® software, we first analyzed the seafloor from shaded bathymetric maps with hillshade according to different directions of illumination, as well as slope maps to detect faulting-induced perturbations (i.e., slope breaks related to fault scarps and folding) and other features (i.e., mud volcanoes). Bathymetry maps with gradient contours as well as cross-sections depicting submarine topography were also used to locate the structures and their extent.

In a second step, using seismic profiles, we define the nature of the tectonic features such as normal faults, thrusts faults, folds or strike-slip faults. When structures are not visible in cross-section on seismic profiles (not available or poorly oriented seismic profiles), we use their morphology and extent in the bathymetry data to interpret their nature. Normal fault scarps often appear as well-defined sharp and steep slopes in the bathymetry, whereas folds and traces of thrust faults appear as much smoother reliefs with more gentle slopes. After performing the identification and mapping of faults, we establish a hierarchy among them according to the heights of the scarps (from seismic profiles or local cross-sections) and the length of the faults. The detailed geometry of fault scarps (strike, segmentation, bends, steps) is studied in map view to obtain information on fault kinematics.

We subdivide the study area in three zones: a central (Martinique-Antigua) sector, a Northern (Antigua) sector, and a Southern sector south of Martinique (see boxes on Figure 1b). Our approach is to describe the tectonic features as precisely as possible and at several spatial scales. In this paper we focus on the central part of the accretionary wedge (central high wedge domain), which is the more topographically elevated region of the
wedge, in which we identify new major regional tectonic structures. We describe below the morphology of the seafloor and our tectonic interpretation based on bathymetry and seismic profiles analyses.

Several structures of the Northern sector were investigated by using published MCS and Wide-Angle Seismic profile (WAS) (Boucard et al., 2021; Laurencin et al., 2019) having lower resolution in the subsurface than the CASEIS seismic profiles. Using the new data acquired by the CASEIS cruise in the area, we revisit the main structures and discuss them in the light of the observations made in the central (Martinique-Antigua) sector.

The Barbados sector was not investigated by the CASEIS cruise but published data allow us to determine the extent of structures in this sector (Figure S2 in Supporting Information S1).

To better enhance some morphologies and structures, we provide as Supplementary Figures a number of zooms of bathymetry maps and slope maps, in which all indications and names are reported, as well as non-interpreted seismic profiles.

### 3.4. Chronology From Regional-Scale Seismic Units

Boucard et al. (2021) interpreted a set of MCS and WAS seismic profiles acquired in the northern part of the arc. They described and dated main seismic units, based on correlation with wells and previous interpretations. By comparing our data with the interpretations of Boucard et al. (2021), it was possible to recognize the same main units and date their deformation (Text S2 and Figure S4 in Supporting Information S1). Those units were identified at regional scale in our profiles but also in various other seismic profiles along the arc and in the fore-arc domain north of Antigua (Boucard et al., 2021; Bouysse & Guennoc, 1983; Cornée et al., 2023 and reference therein). In the fore-arc domain, the unit U1 is characterized by a transparent seismic facies, very low amplitude and discontinuous reflectors resting over a thin series of high-amplitude and continuous reflectors of Early to Mid-Miocene age (forming the limit with U0) (Boucard et al., 2021). The unit U2, the base of which is of late Miocene age (discontinuity UB3 in Boucard et al., 2021; Cornée et al., 2023), is made of continuous sub-parallel high-amplitude reflectors characteristic of carbonate environments (Bouysse & Guennoc, 1983). At some places along our profiles, the Unit U2 is composed of several subunits (U2a to U2d). In our profiles, it was also possible to identify a third younger Unit U3 (with a Pleistocene age inferred) restricted to deep fore-arc basins. This unit U3 shows onlap on unit U2.

## 4. Results

### 4.1. Martinique—Antigua Central Sector

The morphology of this central area, shown on Figure 2, Figures S5 and S6 in Supporting Information S1, can be divided in several domains, from west to east. Steep (up to 20°) slopes separate the arc in the west from a ~50 km wide fore-arc domain in the east, where large basins (Karukera basin KaB, Martinique basin MB, Désirade valley DV, in Figure 2a) are encountered with water depths up to 5,800 m (see details in Text S3 in Supporting Information S1). In the central part of the wedge, a shallow, up to 170 km wide domain, which we name the central high wedge domain, can be seen; it is especially shallow in front of the subducting Tiburon ridge (TR) where the minimum water depth reaches 4,600 m. Our study is focused on this domain. Finally, to the east, the fore-arc ends with a 15 km wide frontal domain (Figures 2a and 2b). These four domains are crosscut and shaped by different tectonic structures, which we describe below.

South-east of the Bertrand-Falmouth spur (B-FS), the fore-arc-basins (i.e., Désirade Valley, DV on Figures 1 and 2a) are elongated in a N50°E direction arising from the fact they are in part controlled by arc-perpendicular normal faulting (Feuillet et al., 2002, 2010). East of Karukera Spur (KS), the smallest Karukera basin (KaB) is disrupted by ~N130 ± 10°E-striking linear scarps that are 10–20 km long (white arrows in Figure 2a). Structures displaying the same strike are also observed about 15 km east of the basin and more southward on the Arawak ridge (AR, see details in Text S3 and Figures S7 and S8 in Supporting Information S1). Those scarps are the surface traces of normal faults previously described from seismic data by Laigle et al. (2013). South of the Martinique basin, normal faults bound the graben of the Kalanina basins (KB, described previously by Seibert et al., 2020).

The fore-arc basins domain is limited eastward by ~N-S oriented smooth reliefs (red arrows on Figure 2a, Figures S7 and S8 in Supporting Information S1). In the northern part of the area, these reliefs form 2–3 km wide crests
<figure><img src="image_5.png" /><figcaption>Figure 2.</figcaption></figure>
and lows (Figure 2c). Southward, several of these well-defined parallel reliefs are ~3 km wide, up to 600 m-high, and limit to the east the Martinique basin. They are elongated in N-S direction, and have a convex arcward shape in map view. In between those reliefs whose westward leading flanks dip very steeply (up to 30°), we can observe one-to-two-km-wide flat perched surfaces which are intra-high basins (see bathymetric sections in Figure S7c in Supporting Information S1).

In the central high wedge domain, there are two regions of shallower topography (Figure 2a) that we named the Grande-Terre Hills (GTH, depth between 5,254 m and 4,420 m, Figure 2c and S10a in Supporting Information S1) offshore Guadeloupe, and the Martinique Hills (MH, depth between 4,110 m and 3,240 m, Figures 2f and S10b in Supporting Information S1) offshore Martinique. Those shallow zones are separated by a deeper zone (down to 5,827 m, Figures 2d and 2e; Figures S10c and S10d in Supporting Information S1). The morphology of the deeper zone is itself sharply disrupted by linear, steep and narrow features, several tens of kilometers long and striking N120°E to N180°E (white arrows on Figures 2c, 2d and 2e i.e., SF, WBr, EBr, ES, WS and in sections Figures 2g–2j). Those linear features are either depressions or positive reliefs, with several hundred meters elevation change. They are visible all the way to the northern part of the GTH. Southward of GTH, they connect at the Seraphine basins set (SB, Figure 2a, SBW, SBE, Basin 1 and Basin 2, Figure 2d), which are more or less oblique (N05 to N74°E -striking) elongated deep basins (up to 5,827 m), separated by high ridges (~780 m-high above surrounding seafloor) having the same direction between them (Figure 2d and bathymetric section cc', dd' Figures 2g and 2h). Overall, all these structures form a horsetail-shaped structure in map view, underlined by the red dotted lines NE of Guadeloupe in Figure 1.

Northwards of Grande-Terre Hills (GTH), this horsetail structure is sharply interrupted by 5–15 km long, N100-110°E-striking linear structures forming clear steep steps (slopes of up 25–30°, black arrows in Figure 2c) in the bathymetry. Those steps are in the continuity of the northern flank of the Barracuda ridge, visible eastward of the deformation front.

South of 16°N, in the MH shallower area, we also observe an alternation of basins and crests in the bathymetric map, but the orientations of the structures are more heterogeneous (Figure 2f and cross-sections 2j). The low bathymetric areas are narrow (up to 4 km wide) intra-wedge (IW) tear-shaped N-S to NE-SW elongated basins. They are bounded by steep (up to 20°) submarine cliffs that are several hundred meters high. Those basins are separated by ~NNW-SSE to NE-SW striking bathymetric highs culminating at 3,300 m depth. East of the basins, trenchward, the seafloor is dissected by several N130° to N150°E-striking long linear structures that are tens of kilometers long. These structures are composing a major fault system that had not been previously mapped and that we name here the Seraphine fault (SF) (Figure 2, see description and map in a later section). Those linear structures form a ~2.4-km-wide depression with a small bathymetric high in its center (section ff', Figure 2j).

On Figure 2a, at a larger scale, we can clearly observe that those linear structures form the southward continuation of those we observed and described in between the MH and GTH (SF, EBr, Wbr on Figures 2c and 2f) over a distance of 180 km and that vanish in the GTH. The SF is 60–100 km far from the trench and is visible as a sharp cut in the bathymetry. It is often associated with either local bathymetric highs or depressions that likely results from off-fault deformations.

Like in the GTH, the structure seems to split into several branches in and south of the MH. Two roughly linear traces can be distinguished: the western structure (WS) is at least 67 km-long and strikes N05°E in its southern part and N165°E in its northern part (Figures 2e and 2f). The 85-km-long eastern structure (ES) strikes N160°E (Figures 2e and 2f). Both western and eastern structures seem to join the Seraphine fault (SF) at 16°N, at a kink

**Figure 2.** (a) Shaded bathymetry map showing the fore-arc basins and accretionary wedge in the Martinique-Antigua central sector (illumination from N270° azimuth). Location is shown in Figure 1. Bathymetry strips along seismic profiles in Figure 3 are marked with white brackets. Dashed orange frame indicates extent of 3D view in Figure 7. (b) Bathymetric cross-sections corresponding to end points A-A' B-B' C-C' on panel (a). Detailed shaded bathymetry close-up views of (c) Grande-Terre Hills (GTH), (d) Seraphine basins (SBW: Seraphine basin west, SBE: Seraphine basin east) (e) Kink basins (KB) and (f) Martinique hills (MH) with locations shown as black frames on panel (a). (g)–(j) Bathymetric cross-sections corresponding to lines on detailed map panels (c), (d), (e) and (f). AR = Arawak Ridge, BAW = Barbados Accretionary Wedge, B-FS = Bertrand-Falmouth Spur, BR = Barracuda Ridge, CHG = Caravelle Half Graben, DB = Désirade Basin, DV = Désirade Valley, EBr = Eastern Branch, EIB = Elongated Basins, ES = Eastern Structure, GTH = Grande-Terre Hills, IWB = Intra-wedge Basins, KaB = Karukera Basin, KB = Kalinina Basins, KS = Karukera Spur, MB = Martinique Basin, MH = Martinique Hills, SB = Seraphine basins, SF = Seraphine Fault, TR = Tiburon Ridge, WBr = West Branch, WS = Western Structure. Red points: location of core CAS16-12PC and CAS16-21PC.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** MCS profiles in the Martinique-Antigua sector (a) CAS016, (b) CAS033 and (c) its close-up view, (d) CAS036 and (e) its close-up view and (g) CAS062 presented with associated bathymetry strips (illumination from N270° azimuth, see Figure 2 for areal extent) and bathymetric cross-sections. (f) Sub-bottom profile CAS130 (location is marked by white line in Figure 2d) acquired in the Seraphine basin along the CAS036 profile. SP = Shot Point. DV = Désirade Valley, KaB = Karukera basin, MB = Martinique Basin. In red are shown the Seraphine fault (SF) and its branches: eastern and western branches (WBr and EBr) in the north on CAS016 and western and eastern structures (WS and ES) on CAS062. SB: Seraphine Basin (West = SBW, East = SBE). IHB: Intra-High basins, VE = vertical exaggeration at the seafloor. High-definition uninterpreted profiles are included in Figure S11 in Supporting Information S1.</figcaption></figure>

bordered to the south by two basins (KB1 and KB2, Figure 2e, section ee' Figure 2i). These linear structures are likely the surface trace of subvertical faults belonging to the same fault system.

Several seismic and sub-bottom profiles cut across the main morphological elements identified in the bathymetry (Figure 3; Figure S11 in Supporting Information S1, profiles locations in Figure 2). The 110 km-long seismic profile CAS016 (Figure 3a) crosses the fore-arc basins of La Désirade and the GTH zone perpendicularly to the linear structures, basins and crests described earlier. The western part of the profile (between CDP 1 and ~8,000) shows high-amplitude, discontinuous, mean frequency reflectors that belong to the sedimentary units U1 and U2, some being clearly folded. The folds are asymmetric, overturned with their axial planes tilted either arcwards or trenchwards (Figure S12a in Supporting Information S1). To the west, the sub-horizontal sediment pile of the fore-arc basin (U3) rests unconformably upon and onlaps the folded units. Between CDP 3,000 and 13,000, a transparent upper unit (U2a), made of sub-horizontal, low amplitude, mean frequency reflectors can be distinguished (highlighted in dark green on Figure 3a). This unit is less deformed than the older unit below, but is also clearly affected by the system of folds and thrusts. East of CDP ~8,000, the seismic units are more transparent and the bathymetry is highly perturbed along the profile with an alternation of basins and ridges. This indicates that the sedimentary units are highly deformed. The seafloor is folded conformably with the sedimentary units, that is, the main bathymetric highs are structural surfaces of anticlines. Across the linear features of the GTH, the Seraphine
branches (WBr and EBr) and associated folds and thrusts arrange to form positive flower structures characteristic of restraining bends along strike-slip faults. The largest conjugate thrusts likely connect at depth on a main vertical strike-slip fault plane. Below the Wbr, the geometry of the thrusts suggests that this connection occurs at a depth equivalent to about 9s TWT (~2,700 m below sea bottom, ~8 km depth). East of the EBr, between CDP 25,000 and 30,000, we can clearly distinguish a highly deformed zone with narrow folds and conjugate thrusts forming imbricated pop-up structures (Figure S12b in Supporting Information S1).

The seismic profile CAS033 (Figure 3b) is 78 km long and crosses the Karukera basin (KaB) and the N150°E striking linear structures in the deeper zone between the GTH and MH. We recognize the three main sedimentary units (U1, U2, U3). Four sub-units of U2 can be distinguished (Ua-d, see details in Figures S4, S13a and S13b in Supporting Information S1). At the western end of the profile, the fan-like geometry of the sediment layers composing the unit U3 in the Karukera basin onlap westward the U2 units that form a larger-scale fold against the arc basement. The reflectors of the U3 unit are growth strata deposited on the eastern limb of this fold. This unit seals the normal fault system. East of the basin, a large unconformity (in purple), extending at least between CDP 2,500 and 6,000 and around 7.0–7.5 s TWT, separates sub-units U2d and U2c from the upper sub-parallel units U2b and U2a. Arcward or trenchward-dipping normal faults offset both U1 and U2 units, up to the seafloor, where they have clear scarps in the bathymetry. East of CDP 9,000, the units U1 to U2c are folded and the geometry of the reflectors composing the units indicates that they are pre-growth strata. The folds are symmetrical with an axial plane gently tilted westward. They affect and thus deform the seafloor. The upper units U2b-U2a pinch out at this place and are less deformed than units U2c-d below the upward propagating of fold and thrust system. The profile cut across the main linear bathymetric high between CDP 14,000 and 16,000. At this place, thrusts arrange to form a positive flower structure characteristic of a strike-slip fault (Seraphine fault, Figures 3b and 3c; Figure S13c in Supporting Information S1). East of CDP ~16,000, the seismic facies is more transparent, which typifies a highly deformed zone. A few arcward-dipping thrusts are visible and are probably in part responsible for this deformation. In this part of the profile, the bathymetry is shallower as a result of thrusting of Domain A.

The seismic profile CAS036 (Figure 3d) crosscuts the Seraphine fault system near the connection with the Seraphine basins. Along this profile, we also acquired a high-resolution sub-bottom profile (CAS-130, Figure 3f) across the Seraphine fault system over a distance of ~13 km. The profile imaged in section acoustically transparent elevated structures that form smooth reliefs at the seafloor. They bound basins composed of strong sub-parallel reflectors down to 0.12 s TWT (~100 m by considering a velocity of 1,800 m/s). Those strong reflectors are the most recent sedimentary units. Some are folded atop of the easternmost transparent acoustic structures. We observed the same suite of units and reflectors in the basins (SBE and SBW) located in between the more elevated zones. They are thicker and form a fan in the SBW basins between shot point (SP) 1,550 and 1,300. At this place the sedimentary units are tilted and thicken eastward, the older the more tilted they appear, attesting for recent and ongoing syn-tectonic deformation. Because of the partial observation of folded strata, we interpret the two elevated, transparent structures as folds; the transparent facies likely reflects intense deformation. The geometry of the most recent sedimentary layers imaged in the sub-bottom profile is typical of fold growth strata (e.g., Ahmadi et al., 2013; Storti & Poblet, 1997; Suppe et al., 1992). Those folds are also clearly visible in the coincident seismic profile CAS036 (Figure 3d). Along the seismic profile CAS036, by using a velocity in the sediment of 1,800 m/s, we estimated that the thickness of the growth strata reaches 180 m in the SBW basin. The growth strata are thinner in the SBE and the western part of SBW basins at the extremities of the profile probably because the sedimentation rates and the accommodation space, created by the deformation, are lower in these areas. The CAS036 profile crosscuts the main branch of the Seraphine fault (Figures 3d, 3e and Figure S14 in Supporting Information S1), which is very linear and sharp in the bathymetry at this place. It is very well imaged as a positive flower structure that is part of a larger fold-and-thrust system, including an impressive anticline, affecting the U1 unit.

South of 15°30N, the 151-km-long seismic profile CAS062 (Figure 3g) crosses the Martinique fore-arc basin and the MH zone perpendicularly to the structures described previously, as well as the frontal domain of the wedge. In the Martinique basin (MB), the upper unit U3 is made of well-stratified, high amplitude, high frequency westward dipping reflectors that downlap the lower unit U2 and display a fan-shape geometry: the older the reflectors, the more they are tilted westward. East of CDP ~4,000, are imaged the N-S elongated up to 600-m reliefs that flank the Martinique basin to the East and that were described earlier in the bathymetry result from folding of the U1 and U2 units. In between folds, perched flat valleys are transported piggy-back basins (IHB1 and IHB2). They are filled with the well-stratified unit (U3), which is characterized by high-amplitude, sub-parallel, slightly folded
<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a) Top: Shaded bathymetry map showing the fore-arc basins and accretionary wedge in the northern sector, north of Antigua (illumination from N225° azimuth). Location is shown in Figure 1. Dashed orange frame indicates extent of 3D view in Figure 5a. Bathymetry strips associated with seismic profiles in Figure 5 are marked as white brackets. Bottom: Bathymetric cross-section corresponding to end points D-D' in panel (a). Detailed shaded bathymetry close-up views and associated cross-sections for (b) Bunce basins and (c) Antigua hills (locations shown with black frames in panel (a)) and (d) northern Antigua hills and (e) southern Antigua (locations shown with black frames in panel (c)). AB = Anguilla basin, AH = Antigua Hills, Antigua V./AV = Antigua valley, BB = Barbuda Basin, BF = Bunce Fault (trace indicated by white arrows), BS = Barbuda Spur, BSc = Barracuda Scarp, BuB = Bunce basins, Gh = Goat Hill, GTH = Grande-Terre Hills, MeB = Meduse basin, Meh = Meduse Hill, Mh = Marigot Hill (sMh: small Marigot Hill), St. Barthelemy V. = Saint Barthelemy valley, Tintamarre S. = Tintamarre Spur. Blue points = local topography low, Red triangle = local topography high, purple triangles = mud volcanoes.</figcaption></figure>

reflectors, with older reflectors exhibiting greater folding. At this place the U2 unit is acoustically transparent, suggesting that it is highly deformed. Trenchward, between CDP numbers ~10,000 and ~24,000, we observe other smaller thrusts and folds in the seismic section. In map view, they are the southward prolongation of the linear features of the MH described in the bathymetry (WS, ES and SF). In the seismic section, they also appear as positive flower structures (Figure S15 in Supporting Information S1). Between CDP number 32,000 and ~43,000, the acoustic facies is dominantly transparent but we can distinguish a few folds and thrust structures that have a clear signature at the seafloor. East of CDP ~43,000, trenchward, we observed the top of the subducting oceanic crust (TOC) between ~7.5 and 8.0 s TWT. The TOC is covered by four well-stratified, sub-parallel units with the upper one being accreted by arcward-dipping thrusts at the trench above the décollement.
## 4.2. Northern Sector: North of Antigua

In this region, the seafloor of the fore-arc domain is deeper (~5,600 m) and smoother than in the central Martinique-Antigua sector. The trench is here up to 7,400 m deep (Figure 4, Figures S16 and S17 in Supporting Information S1). The direction of the trench veers from a ~N-S direction south of the Barracuda ridge to an E-W direction north of the Virgin Islands (VI) and Puerto Rico (PR, Figure 1b), implying a dramatic increase of the obliquity of the convergence in this area. The main topographic features are deep (down to 6,100 m depth) arc-perpendicular valleys (Antigua valley; Saint Barthelemy valley) that are bounded by several steep normal faults scarps (Boucard et al., 2021; Feuillet, Beauducel, & Tapponnier, 2011; Morena et al., 2022) that are up to 1,000 m high.

To the west, the fore-arc high domain is sharply cut by the strike-slip Bunce fault (BF, Laurencin et al., 2019), which has a clear linear trace in the bathymetry and is oriented N115°E, at this place between 63°W and 61°W (white arrows on Figures 4a and 4b). Southwest of the fault, the seafloor is much deeper and forms the round-shaped (9 km long, 6 km large) Anguilla basin (AB), the deepest basin of this sector (maximum depth 6,766 m; Morena et al., 2022). At the eastern tip of the Bunce Fault, a set of alternating topographic ~6 km long crests and basins with gentle bounding slopes (10°, Bunce basins, Bub1 to BuB7) extend over a distance of 50 km. They bound to the west the Goat Hill (Gh), which culminates at 5,382 m water depth (section b*b** on Figure 4b). Their strikes veer counterclockwise to N150°E when approaching the trace of the BF faults they branch. Several similar ~N170-180°E-striking 7–10 km long structures spaced from 1 to 20 km apart are seen in the area (black arrows on Figure 4b, section b*b** on Figure 4b).

South of the N160 ± 10°E trending ridges and crests bounding the Bunce basins, we observe a series of arc-parallel reliefs that extend over a distance of ~150 km to the north of Antigua hills (AH, Figure 4c). One example is the ~6-km-wide high, N120°E Marigot Hill (Mh, Figure 4b), which culminates at -5,000 m. These reliefs are bounded by sharp N120 ± 05°E striking scarps (red arrows on Figures 4b, 4c, and 4d).

South of those reliefs, the seafloor of the fore-arc region is strongly disrupted by a dense series of ~N130 ± 20E° short (10 km), small (~100-m high) and steep (20°) scarps parallel to the trench (black arrows and purple lines with ticks on Figures 4c–4e). These scarps were inferred to be those of normal faults, downstream of the Saint Barthelemy valley (Boucard et al., 2021). This normal fault system is 10–50 km wide and faults are observed over a distance of 200 km all along the zone downslope of the arc basement across the Saint Barthelemy and Antigua Valleys, and northeast of Meduse Hill (Meh) (Figure 4c). It joins eastward another ~100 km-long normal fault system over 100 km, for a total length of 300 km toward the trench, and limit the northern side of the GTH in the Central domain (Barracuda scarp, BSc on Figure 4e).

The seismic profile CAS104 (Figure 5a and Figure S18a in Supporting Information S1, see location on Figure 4a) crosses perpendicularly the Bunce fault, as well as the crests and basins of the Bunce basins, to the southwest. In the NNE part of the profile (between CDP ~0 and 4,000), the acoustic facies is semi-transparent with chaotic reflectors of variable amplitude. Some arcward-dipping thrusts clearly offset and control the bathymetry there. Across the linear Bunce fault, folds and thrusts arrange to form a positive flower structure. In the southern part of the profile, we recognize the seismic units U2 and U3 that are made of higher amplitude and more continuous reflectors than U1. Like in the central Martinique-Antigua zone, the unit U2 is folded conformably with the seafloor forming a series of basins (synclines) and ridges (anticlines) as described earlier from the bathymetry. Arcward or trenchward-dipping thrusts cut the folds, with some of the thrusts reaching the surface. The upper unit U3 (in yellow), which rests unconformably on U2, is composed of well-stratified, continuous, low-frequency, medium-amplitude reflectors that are growth strata having a fan-shaped geometry. Unit 3 fills the BuB syncline basins as they form.

The fan-shaped geometry of the upper reflectors of U3 is also seen in sub-bottom profile CAS-451 (Figure 5b and Figure S18b) acquired along the CAS104 profile. Strong reflectors in particular for the Bub1 basin (between SP 1,000 and 400) are seen down to 0.07 s TWT (~60 m by considering a velocity of 1,800 m/s) and are tilted southward; the older reflectors appear more tilted, implying that the tectonic deformation is active.

The seismic profile CAS098 (Figure 5c and Figure S18c in Supporting Information S1, see location Figure 4a) crosses and images the normal faults set observed downslope of the arc basement, close to the Barbuda Spur. In this profile we can clearly identify the reflector of the units U1 and U2. Unit U2 is 480–520 m thick and is composed of three sub-units. The set of high amplitude reflectors, dated by Boucard et al. (2021) to be of mid-
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Seismic profiles in the northern sector with associated bathymetry information (a) Three-dimensional view looking toward west showing MCS line CAS104 and shaded bathymetry (shading according to slope values). BuB = Bunce basin, BV = St. Barthelemy Valley and TS = Tintamarre Spur (b) close-up of MCS CAS104 profile (top) at the location of the CAS-451 sub-bottom profile (bottom) (c) MCS profile CAS098 presented with associated bathymetry strip (illumination from N225° azimuth). BF = Bunce fault, BuB = Bunce basins. VE = vertical exaggeration. High-definition uninterpreted profiles are included in Figure S18 in Supporting Information S1.</figcaption></figure>

Miocene age, are also imaged below the transparent unit U1 (in blue Figure 5c). All these units are offset significantly (up to 190 m) by normal faults having scarps visible in the bathymetry. The seafloor appears to be offset by the same amount as the older seismic units implying that the faults postdate the deposition of U1 and U2 units, and are likely recent and active. The fault system is dense and the faults are very close to each other implying that some probably also connect at shallow depth. The penetration of the seismic profile is too low to image the extent of the fault system at depth. From MCS data, Boucard et al. (2021) suggested that most of them
connect at depth near the plate interface. Such geometry is typical of a negative flower structure and suggests a large component of strike-slip motion.

# 5. Interpretation

## 5.1. The Seraphine Strike-Slip Fault System

In the Martinique-Antigua sector of the LASZ, we identified: (a) the main strike-slip Seraphine fault (SF) and its branches (i.e., Wbr, EBr, WS and ES) forming narrow, long and very linear flower structures in seismic sections; (b) several fold-and-thrust belts on both sides of the strike-slip faults; and (c) a dense N130 ± 10°E striking network of shallow normal faults that deform the fore-arc domain and that are sealed in the fore-arc basins by the most recent sedimentary unit U3 (Figures 6 and 7, and Text S3 and Figures S7, S8 and S11–S15 in Supporting Information S1). The main strike-slip fault limits two domains, named A and B (Figure 7) that have different bathymetric and acoustic facies: transparent units highly deformed to the east (Domain A), and more or less folded stratified units and fan-shaped fore-arc basins to the west (Domain B). The elongated arc-perpendicular 20–30 km long reliefs that delimit the Désirade and Martinique fore-arc basins to the east are the traces on the seafloor of a 280 km-long fold-and-thrust system located west of the Seraphine fault (Th1 to Th10 indicated in gray Figure 6). Its vergence is opposite to the main frontal thrust. In map view, it connects to the Seraphine main fault, east of the Karukera spur, implying that both systems also connect at depth and are coevally active. West of the Seraphine fault, north of 16°N, the folds and thrusts are organized in an *en échelons* pattern with a mean trend of N170°E, implying that they accommodate a sinistral strike-slip component of motion along a direction nearly parallel to the main trace of the Seraphine fault.

Other fold-and-thrust systems connect onto the Seraphine fault and its branches to form a 50–90 km wide zone of wrenching in the highest, central part of the accretionary wedge. The thrusts and folds bounding the Seraphine sub-basins arrange to form a compressional horsetail at the northern tip of the main strike-slip fault. Those located east of Martinique, along with the secondary branches of the Seraphine system (WS-ES in the south; WBr-EBr in the north), form a second contractional horsetail at the southern tip of the main Seraphine fault. The geometry of the whole system is in keeping with the sinistral motion along the main Seraphine fault (Figure 6b).

In the GTH and MH areas, the Seraphine fault is complex, exhibiting several branches and horsetail structures and the deformation is overall more diffuse and distributed over a wider area. These zones of higher topography are located in the prolongation of the Barracuda and Tiburon ridges, respectively. The subduction of those reliefs may disturb locally the regional stresses and the expression of strike-slip deformation (Dominguez, Lallemand, Malavieille, & von Huene, 1998; Dominguez et al., 2000).

The presence of horsetails at both tips of the fault might also suggest that the Seraphine fault is propagating toward both the North and the South (Perrin et al., 2016).

Seismic profiles help to determine the chronology of the deformation. The fan-shaped geometry of sedimentary strata of the most recent unit U3 in the fore-arc, in intra-wedge piggy-back basins and in the Seraphine basins indicate that they are syntectonic growth strata related to the development of the folds in the wrenching zone.

Two of these basins were sampled during the CASEIS cruise (red points Figures 2a and 2d). Sedimentation rates of 10–12 cm/kyr are estimated over the Holocene period from ¹⁴C dating in the Seraphine and IHB2 piggy-back basins (Seibert, 2019). Making the hypothesis that those rates are constant over the Plio-Quaternary period, we estimated a first-order age of 1.5 Ma for the base of the growth strata and thus for the onset of folding in the Seraphine basins. This indicates very recent, late Pleistocene deformation, and suggests that the whole strike-slip system is recent and active. The thickness of the sedimentary pile of the deeper and most-developed intra-arc piggy-back basins east of the Martinique basin reaches at least 650 m (by considering a velocity of 1,800 m/s). Again, under the first-order assumption that the sedimentation rate remained constant over a longer period, the basal units of the basin would be of Pliocene age. The age of the unconformity at the base of unit U2 is estimated to be late Messinian to early Zanclean (Cornée et al., 2023), indicating that unit U2 was deposited shortly before the Pliocene. This unit thus predates the folding that occurred in the Pliocene or Pleistocene and is still active today.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Shaded bathymetry map (illumination from N270° azimuth) of the Martinique-Antigua sector (similar to Figure 2a) showing morphotectonic structures of the fore-arc basins and accretionary wedge. Seraphine Fault (SF) (along with its branches ES, WS, EBr and WBr) is shown in red. In black = thrust faults. In gray = blind thrust faults. Purple lines = normal faults, blue double-arrows lines = folds. AR = Arawak Ridge, BAW = Barbados Accretionary Wedge, B-FS = Bertrand-Falmouth Spur, BR = Barracuda Ridge, DV = Désirade Valley, GTH = Grande-Terre Hills, KaB = Karukera Basin, KB = Kalinina Basins, KS = Karukera Spur, MB = Martinique Basin, MH = Martinique Hills, SB = Seraphine basins, TR = Tiburon Ridge. Blue points = local topography low, Red triangle = local topography high. Dashed white line = backstop (Laigle et al., 2013; Paulatto et al., 2017). Light gray dashed lines = extension of the Barracuda and Tiburon Ridges underneath the wedge (b) Sketch depicting the kinematics of the Seraphine fault.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Three-dimensional view looking toward south showing MCS line CAS033 and shaded bathymetry (shading according to slope values). In red = Seraphine fault and the eastern and western segments of the fault (ES and WS). In black = thrust faults. In gray = blind thrust faults. In purple = normal faults. In dark blue = folds. In light blue = blind folds. VE = vertical exaggeration.</figcaption></figure>

## 5.2. The Bunce Fault and Link With the Seraphine System

The Seraphine strike-slip system ends sharply northward, against a dense, 10–50 km wide and 300 km long system of normal faults in the prolongation of the Barracuda ridge. In map view (Figure 8), this Meduse-Barbuda fault zone (MBFS) joins, at its western end, another normal fault system, the Tintamarre Fault Zone (TFZ, Boucard et al., 2021). The latter forms, from the mouth of the Saint-Barthelemy valley to the Antigua hills, a narrow and linear array of faults over hundreds of kilometers, while in depth, it forms negative flower structures (Boucard et al., 2021). This suggests that it likely accommodates distributed extensional deformation above a main strike-slip fault system. This system develops above a bend in the subduction interface, as shown by the MCS profiles presented in Boucard et al. (2021) and the WAS velocity model of Laurencin et al. (2018), and thus likely accommodates slip partitioning originating from this bend at depth. We infer that this fault system is a relay between the Seraphine and Bunce fault systems (purple area in Figure 8).

North of this composite relay system, the ~10-to-20-km-long folds limiting the Bunce basins connect to the Bunce fault northward (Figure 8a) forming in map view a contractional horsetail structure at the eastern tip of the Bunce fault that is compatible with sinistral motion along the Bunce fault. This horsetail structure marks a compressive relay between the Bunce fault and a diffuse zone of strike-slip motion on the Tintamarre Fault Zone (TFZ). The limit is underlined by nearly trench-parallel striking faults, the Marigot Hills Fault Zone, forming the Marigot Hills (Figure 8).

As observed in the central Martinique-Antigua sector, the folds of the Bunce basins postdate the deposition of U2. The deformation in this sector is thus coeval to that of the Seraphine system, likely recent and active.

## 5.3. The Seraphine System Southwards, Link With the Kitridge Fault

South of the central Martinique-Antigua sector, the bathymetric data are sparser and we have no high-resolution seismic profiles that would allow to image the wedge at depth. In this sector, the seafloor is shallower (~2,700 m) and the trench is ~N-S striking (Figure 9 and Figure S19 in Supporting Information S1). The main topographic
<figure><img src="image_11.png" /><figcaption>**Figure 8.** (a) Shaded bathymetry map of the northern sector (illumination from N225° azimuth, similar to Figure 4a) showing morphotectonic structures of the fore-arc domain. The Bunce fault (BF, Laurencin et al., 2019) and Seraphine fault in the south are shown in red. In black = thrust faults. In gray = blind thrust faults. Purple lines = normal faults, blue double-arrows lines = folds. (b) Tectonic map depicting the link between the Bunce and Seraphine fault systems. Red: Strike-slip faults, Blue: Normal faults of the arc, Purple: TFZ and MBFS normal fault systems and other fore-arc normal faults, Black = thrust faults, Gray = blind thrust faults, Blue double arrows: en-echelon folds. AB = Anguilla basin, AH = Antigua Hills, Ag = Antigua, Antigua V. = Antigua valley, Ba = Barbuda, BB = Barbuda basin, BF = Bunce Fault, BR = Barracuda ridge, BS = Barbuda Spur, BuB = Bunce basins, Do = Dominica, DV = Désirade valley, Gh = Goat Hill, GTH = Grande-Terre Hills, Gw = Guadeloupe, KS = Karukera Spur, MB = Martinique Basin, MBFS = Meduse-Barbuda Fault System, MeB = Meduse basin, Meh = Meduse Hill, MH = Martinique Hills, MHFZ = Marigot Hills Fault Zone, Mq = Martinique, Mt = Montserrat, PRT = Puerto Rico Trench, SB = Seraphine basins, SF = Seraphine fault, St. Barthelemy V. = Saint Barthelemy valley, TFZ = Tintamarre Fault Zone, TR = Tiburon ridge, TS = Tintamarre Spur.</figcaption></figure>

features are the Barbados ridge, which is up to 340 m above sea level at Mount Hillaby on the Barbados Island, and is bounded eastward by folds and thrust faults (Deville, 2011; Gomez et al., 2018; Torrini & Speed, 1989) that separate the ridge from the 2,700 m deep Barbados basin. North of Barbados Island, the Hellene mount culminates at ~2,600 m below sea level. East of it, a very linear and sharp ~110 km long and very narrow relief lies in the prolongation of the Seraphine fault (SF) described in the Martinique-Antigua sector, in a topographically lower part of the wedge (~3,500 m) down to the latitude 14°N. This strongly suggests that this relief is the surface trace of the Seraphine fault. East of the Barbados Island, the wedge is sharply cut by a ~N160°E-striking linear trace extending over ~110 km. This cut is particularly visible between the Cobbler and Crab highs (red arrow on Figure 9b) where it forms a ~2-km wide and ~350 m deep depression. Thirty-five kilometers separate this structure from the trace of the Seraphine fault north of 14°N. This linear trace is likely another segment of the Seraphine fault system, and we label it “SF” on our interpreted maps (Figures 9a and 9b). South of the Cobbler and Crab highs, within a highly deformed area, some structures striking N25–45°E and N110–130°E (black arrows on Figure 9b) clearly connect on this possible segment of the Seraphine fault and may have been offset by the fault by a maximum of 2 km (white arrows Figure 9b and Figure S19d in Supporting Information S1).

Farther south, near the Kitridge High (KH), the trace of the Seraphine fault is lost because we lack high-resolution bathymetric data. Southwest of Kitridge High, Kitridge Basin (KB) is bordered on its seaward side by a curved structure, which connects on a ~N50°E linear structure (red arrows on Figure 9c). This linear structure could be a strike-slip fault and we named it the Kitridge fault (KF) (Figure 9c). This inferred fault splits into several branches at its northeastern tip. South of the Kitridge Basin, it is divided in two parts with its western branch extending southward to the Crane basin (black arrows on Figure 9d). The total length of this fault would be ~100 km. South
<figure><img src="image_12.png" /><figcaption>**Figure 9.** (a) Shaded bathymetry map (illumination from N270° azimuth) showing morphotectonic structures in the southern sector, near Barbados Island (location in Figure 1). Close-up views of (b) the southern part of the Seraphine fault and (c) the junction between the Seraphine and the Kitridge faults, with location shown as black frames in panel (a). (d) Associated cross-sections. SF = Seraphine fault, KB = Kitridge basin, KF = Kitridge fault, KH = Kitridge high. Blue points = local topography low, Red triangle = local topography high.</figcaption></figure>

of the Kitridge fault, the Barbados basin and the Crane basin delimit a shallower area that is bounded by thrust faults (Figures 9a and 9c, Gomez et al., 2018).

East of the Kitridge fault, two ~60°E-striking southern branches are located at about 10 km from each other (Figure 9a, thin red lines). The southeastern one is located at the limit with frontal trench-parallel thrust faults (e.g., Deville, 2011). A similar structure (oriented ~N50°E) close to the front is visible northward at the latitude of Cobbler and Crab highs (Figure 9a, thin red line).

The Seraphine fault thus appears to prolong southwards to connect to a structure that has a clear trace in the bathymetry, and that we named the Kitridge fault. However more data are needed to confirm this.
<figure><img src="image_13.png" /><figcaption>**Figure 10.** (a) Tectonic structures of the Lesser Antilles Subduction Zone plotted onto bathymetry from 10°N to 20°N (b) Close-up view centered on the Seraphine fault (location marked by dashed box in panel (a)) showing earthquake epicenters ("ISC Bulletin," 2022; CDSA catalog: Massin et al., 2021) and focal mechanisms (GCMT catalog: Dziewonski et al., 1981; Ekström et al., 2012; Stein et al., 1982 for the December 1969 event) (c) Schematic interpretation of the Bunce-Seraphine-Kitridge fault system showing main fault traces and compressive relay zones. Stress direction σ1 and σ3 based on the geometry of the junction between Seraphine and Kitridge faults. Gray arrows are the obliquity (Figure S21 in Supporting Information S1).</figcaption></figure>

# 6. Discussion

## 6.1. Arc-Scale Kinematics of the Seraphine System

At the scale of the entire subduction zone, the sinistral strike-slip Seraphine and Bunce faults are part of a very large strike-slip system developed within the entire Barbados accretionary wedge and subduction fore-arc domain (Figure 10a). The whole system is made of very linear and localized fault zones (in red) terminating as contractional horsetail structures and separated from one another by regions of more diffuse deformation consisting of gentle restraining (green) and releasing (purple) bands of deformation (Figure 10c). We infer that the Seraphine fault extends in the southern sector, east of Barbados Island, as a distinct segment. South of Barbados, this fault segment connects to another strike-slip fault, the Kitridge fault, which strikes perpendicular to the Seraphine system. Assuming both structures are conjugate and active coevally, the Kitridge fault would accommodate right-lateral strike-slip motion. South of the Kitridge fault, the Crane Hills can be interpreted as a compressive relay zone between the Kitridge fault and the eastern end of the El Pilar and other strike-slip faults bounding the Caribbean plate to the south. To the north, contractional or extensional releasing bends (Tintamarre Fault Zone) link the Seraphine and Bunce faults in a complex manner. Several hypotheses can be proposed to explain the presence of these normal faults. Boucard et al. (2021) suggested that the TFZ system of normal faults is due to basal erosion of the overriding arc basement. However, at this location, the slab becomes steeper (Laurencin et al., 2018) and as observed elsewhere (Meyer et al., 1998; Philip & Meghraoui, 1983) normal faults may develop in the upper plate to accommodate the change in geometry of the plate interface. An alternative interpretation could be that those faults mark an along-dip transition in the frictional properties of the megathrust
(Cubas, Avouac, Leroy, & Pons, 2013). It was proposed that such faults could be a marker of rupture propagation into the shallow part of the megathrust (Cubas, Avouac, Leroy, & Pons, 2013).

Farther east, north of the GTH zone, the TFZ normal fault system joins another normal fault system located above the northern flank of the subducting Barracuda ridge, and therefore it could in part have developed due to mechanical heterogeneities of the plate interface above the subducting relief. We identify similar normal faults oblique to the trench in the fore-arc domain offshore Guadeloupe. Strikingly, they are also located in the continuity of the Tiburon ridge. They cut a relief (Arawak ridge) that certainly results from the subduction of the ridge. Those normal faults form a dense and restricted network and may be possibly associated with mud volcanism (purple triangle in Figure 6, see details in Text S3 and Figure S9 in Supporting Information S1). At other subduction zones worldwide, normal faults have been identified and inferred to be due to the subduction of ridges, for instance above the Gagua Ridge in the Ryukyu subduction zone (Dominguez, Lallemand, Malavieille, & Schnürle, 1998) or above the Juan Fernandez Ridge in the Chile subduction zone (Laursen et al., 2002). Analog models of seamount subduction show the development of normal fault systems in the area of subsidence developing in the wake of a subducted seamount (Dominguez, Lallemand, Malavieille, & von Huene, 1998).

In the northern sector, the deformation is thus complex with strike-slip and normal fault systems and horsetail structures. The whole suite of mapped structures is however kinematically compatible, and our seismic data indicate that they are coeval. They have developed to accommodate the increasing obliquity of the convergence, the bending of the slab, and also the subduction of the Barracuda and Tiburon ridges. The TFZ normal fault system may play the role of a diffuse relay zone, transferring the deformation between the Seraphine and Bunce faults, in an area where no localized structure is identified. A complex arrangement of strike-slip, thrust and normal faulting has previously been documented as a result of slip partitioning at an oblique subduction in the Nankai area (Tsuji et al., 2014).

Our seismic data set alone does not allow us to image the Seraphine fault deeper than ~9 s TWT (~8 km). By combining our observations with information from various available data sets in earlier published work, we show that the Seraphine fault coincides with a main interface above a slight bend in the megathrust (Figure 11). The fault separates two domains: (a) a zone of accreted sediments (the accretionary wedge) to the east; (b) a deformed sediment sequence atop of a high-velocity zone. This high-velocity zone has been interpreted to correspond to an accreted ridge, playing the role of backstop to the west (Bangs et al., 2003; Kopp et al., 2011). These two domains coincide with the domains A and B identified with our data. At their boundary, Kopp et al. (2011) inferred the existence of a high-angle fault dipping toward the trench and branching at the decollement at the toe of the backstop. It is likely that their seismic profile imaged locally a branch of the Seraphine fault system. Given its geometry at the surface, the Seraphine fault is probably almost sub-vertical where it is well localized, with linear sharp traces, tens of kilometers long, visible in the bathymetry. Its geometry may however vary where the deformation is more distributed along wider flower structures. The Seraphine and Bunce faults thus share common characteristics (they are both parallel to the trench, separating the fore-arc domain and the accretionary wedge, branching down to the subducting slab) and are thus certainly part of a main (~1,600 km long) plate-scale strike-slip fault system that partitions the oblique convergence between the American and Caribbean plates (Figure 12). In the southern sector, the offshore-Barbados segment of the Seraphine fault and the Kitridge fault are both situated far from the fore-arc basin (~150–200 km from the Tobago basin), maybe implying that the backstop is located farther east in that area, and may be made of material composing the metamorphic older prism as described by Gomez et al. (2018).

It is noteworthy that the Seraphine fault in the accretionary wedge is parallel and is compatible in term of kinematics (sinistral strike-slip motion) with the active *en échelon* normal fault system identified between Saba and Martinique along the volcanic arc (Feuillet, Beauducel, & Tapponnier, 2011; Feuillet et al., 2002, 2010). Both faults may move together to accommodate the slip partitioning originating from the oblique convergence. Furthermore, the trench-perpendicular family of normal faults in the arc is well expressed in the area between the Bunce fault and the Seraphine fault. This “arc” system may accommodate the entire trench-parallel component of motion, in a sector where it is difficult to form a localized strike-slip fault in the wedge. A similar pattern of faults was documented in other regions such as the Sunda subduction zone, where both the Sumatra fault in the arc and the Mentawai Fault in the wedge accommodate the trench-parallel component of the convergence (Chemenda et al., 2000; Diament et al., 1992; Sieh & Natawidjaja, 2000).
<figure><img src="image_14.png" /></figure>

**Figure 11.** Three-dimensional block diagram of the Lesser Antilles Subduction Zone looking from the latitude of Dominica northward. Location of frontal cross-section is shown in Figure 8b. Seraphine fault and eastern portion of the Bunce Fault are marked in red. Geometry of the deeper part of the subducting plate from Bie et al. (2020), Paulatto et al. (2017) and Ruiz et al. (2013). Geometry of the shallower part of the slab and the backstop from Bangs et al. (1990, 2003); Kopp et al. (2011); Laigle et al. (2013); Li (2019); Westbrook (1975); Westbrook et al. (1988) and this study. Geometry of the Caribbean crust west of Dominica is from Garrocq et al. (2021). Morphology of the fore-arc from Bangs et al. (1990), Kopp et al. (2011), Laigle et al. (2013), Li (2019), Westbrook (1975) and Westbrook et al. (1988) and this study. AB = Anguilla Basin, Ag = Antigua, AM crust = American crust, AV = Antigua Valley, Ba = Barbuda, BF = Bunce Fault, BR = Barracuda Ridge, Do = Dominica, DV = Désirade Valley, Gw = Guadeloupe, KaB = Karukera Basin, KS = Karukera Spur, MeB = Meduse Basin, Mq = Martinique, PRT = Puerto Rico Trench, SB = Seraphine Basin, SF = Seraphine Fault, St. K. and N. = Saint Kitts and Nevis, St. L. = Saint Lucia, TR = Tiburon Ridge and TS = Tintamarre Spur. VE = vertical exaggeration. See main text for references to along-dip segments (labeled A, B, C) and along-strike segments (labeled 1 and 2). Isotherms (gray lines) from Ezenwaka et al. (2022) and Gutscher et al. (2013).

## 6.2. Obliquity of the Convergence

Few strike-slip faults have been observed within accretionary wedges worldwide. The best examples are the Yaeyama fault (YF) in southern Ryukyu (Lallemand et al., 1999), the Mentawai fault (MF) in Sumatra (Chauhan et al., 2009; Diament et al., 1992; Kopp et al., 2011; Schlüter et al., 2002; Singh et al., 2010) and the Wedge Boundary Strike-slip fault (WBSF) in Nankai (Tsuji et al., 2014). All show linear traces parallel to the accretionary wedge front, similarly to the Bunce and Seraphine faults. They are also all located at the boundary between the accretionary wedge and the fore-arc basin area and branch to the decollement to accommodate the slip partitioning of these oblique convergent margins.

It is inferred that such faults develop along subduction zones displaying a significant obliquity. The Yaeyama and Mentawai faults developed in a context of high obliquity (40–60°), whereas the WBSF in Nankai developed in a context of lower obliquity (15–45°, Tsuji et al., 2014). Analog models tend to show that the obliquity and the friction on the decollement are the main parameters controlling the expression of strain partitioning. An obliquity of at least 30° is required for a zone of wrenching to develop, and the higher the friction on the decollement, the lower the obliquity threshold (Chemenda et al., 2000; Martinez et al., 2002; McClay et al., 2004). When the obliquity is low, the deformation is more distributed; in addition, the strike-slip deformation tends to localize on pre-existing structures such as thrust faults (McClay et al., 2004) or at a mechanical discontinuity such as the edge of the backstop that is a mechanical discontinuity (Martinez et al., 2002).
<figure><img src="image_15.png" /><figcaption>**Figure 12.** Combined bathymetry/topography map of the eastern part of the Caribbean plate showing the Lesser Antilles Subduction Zone in the east, the Septentrional and El Pilar fault systems in the north and south, respectively, and connections with the Bunce-Seraphine-Kitridge fault system. El Pilar Fault system from Jouanne et al. (2011), Garciacaro et al. (2011) and Deville (2011). Septentrional Fault from DeMets et al. (2000) and Possee et al. (2020). Western Bunce Fault from ten Brink et al. (2004). Mq = Martinique, S. = Saba.</figcaption></figure>

By using the NAM/CAR and SAM/CAR Euler poles and the angular velocities from the plate model MORVEL2010 (DeMets et al., 2010), we calculated the obliquity at the front of the accretionary wedge at several points along the trench and also along the arc. The latter values were indeed calculated to avoid uncertainties related to the small-scale variations in the orientation of the front due for example, to front retreat in the zone where the Barracuda and Tiburon ridges enter the subduction (Figure 10c; Figure S21 and Table S1 in Supporting Information S1). At each location, we estimated the angle between the convergence vector and the direction orthogonal to the trench or to the arc (McCaffrey, 1992; Philippon & Corti, 2016). Due to the strong curvature of the arc and subduction front, the obliquity varies over short distances. In the Martinique-Antigua sector, we find low obliquity values, between 10° and 22°, either directed toward the south or toward the north. North of 17°N, the obliquity increases up to 27° toward the north and up to 50° west of ~18°30′N at the latitude of Tintamarre Spur, where the front strongly arches counterclockwise (Laurencin et al., 2019). In the southern sector the obliquity increases from 12° to 62° toward the south. The obliquity values obtained when the calculation is done with respect to the volcanic arc are overall similar (Text S4, Table S1 and Figure S21 in Supporting Information S1). At the latitude of Dominica Island, where the direction of the arc veers counterclockwise to a northwesterly direction, we calculated that the trench-parallel motion is null before changing to an opposite direction more southwards.

By using the Nuvel1 plate model (DeMets et al., 1994), the azimuth of the convergence vector becomes ~N261°E, implying an increase of the obliquity by ~7° with respect to the ~N254°E-oriented convergence vector of the MORVEL model (DeMets et al., 2010). The calculated obliquity nonetheless remains low in the Martinique-Antigua sector (i.e., central sector). The Seraphine fault is thus developing in an unusual context of low obliquity in the central sector. Moreover, the Seraphine fault system extends southward (south of latitude 13°N), where plate motion models predict a dextral motion parallel to the trench. We also showed that the Seraphine system terminates suddenly south of the Saint-Lucia ridge and likely connects to a conjugate fault (Kitridge fault). The orientation and kinematics of both structures would be compatible with a WNW-ESE-striking direction of compression, instead of a WSW-ENE direction expected from the orientation of the
convergence estimated by the most recent plate models. There are still few GNSS data to improve the ITRF models on the motion of the Caribbean plate and large uncertainties remain on the location of its Euler pole (Altamimi et al., 2023). The MORVEL2010 (DeMets et al., 2010) model is mainly based on GNSS data from stations located far from plate boundaries (almost 100 km). These stations are undergoing diffuse deformation over a wide area (Feuillet, Beauducel, & Tapponnier, 2011; Kreemer et al., 2014; Styron et al., 2020) not taken into account in the model (DeMets et al., 2010). A clockwise rotation by 10–15° of the azimuth of the CAR/NAM and CAR/SAM convergence vectors compared to Nuvel1 and Morvel2010 plate models would be sufficient to better explain the geometry and kinematics of the Seraphine fault. These values are however larger than the uncertainties on the azimuth of the NAM/CAR determined by DeMets et al. (2007). Plate motion reconstructions using GPlates (Müller et al., 2018; Seton et al., 2012) predict an azimuth of the convergence between the Caribbean and American plates of 290 ± 10°E between 8 and 2 Ma in Guadeloupe (Figure S22 in Supporting Information S1). Those values are more compatible with the kinematics of the Seraphine/Kitridge fault systems, which may have developed in a context of higher obliquity during the Pliocene.

It is also possible that the Seraphine fault developed in an area where the regional stress field was perturbed by
several processes affecting the accretionary wedge (transient strain changes, mechanical instabilities along the
plate interface, changes in the geometry of the subducting slab, subduction of topographic highs (e.g., Bonnardot
et al., 2007; Greenfield et al., 2021; Hampel et al., 2004; Martinod et al., 2013; Rosenbaum & Mo, 2011). At the
Nankai subduction zone, large local variations of the regional stress field have been observed in the wedge across
the WBSF but no clear interpretation was proposed to explain these variations (Tsuji et al., 2014). It is interesting
to note that the Seraphine fault system shows its best-expressed section between the subducting Tiburon and
Barracuda ridges. The subduction of these ridges may have locally modified the stress field and facilitated the
development of the Seraphine fault. However, models are needed to confirm this hypothesis. In any case, the
subduction of the ridges locally affects the Seraphine fault system, which exhibits larger positive flowers
structures in the prolongation of the two subducting ridges.

6.3. Implications for the Behavior of the Subduction Megathrust

The seismic behavior of the subduction megathrust and the deformation of the accretionary wedge depend on frictional forces at the plate interface (megathrust) and on the mechanical properties of the medium (Cubas, Avouac, Leroy, & Pons, 2013; Cubas, Avouac, Souloumiac, & Leroy, 2013; Fuller et al., 2006; Moreno et al., 2018; Scholz, 1998). Sandbox analog models of oblique convergence show that strike-slip fault systems develop in between a stable domain (backstop, proto-wedge) and a deforming wedge of imbricated thrusts (Martinez et al., 2002). The Seraphine fault is thus likely a good indicator of the position of the backstop toe (Figure 11). This position was inferred by Laigle et al. (2013) from the interpretations of several MCS profiles (white dashed line on Figures 8b, 10a and 12) and roughly coincides with the location of the western secondary branches of the Seraphine fault.

In addition, high friction is required on the shallow part of the megathrust to develop a strike-slip fault system in a context of low obliquity (Martinez et al., 2002). The development of the Seraphine fault may also originate from a downdip zonation of the friction. It was inferred that an increase in friction on the megathrust toward the trench contributes to the development of backthrusts in the wedge (Cubas, Avouac, Souloumiac, & Leroy, 2013). This is consistent with our observations, as the fore-arc basins of the central sector are bounded by fold-and-thrust systems verging toward the arc. Moreno et al. (2018) propose that the downdip zonation of the friction controls the mechanism and timing of large megathrust earthquakes. Earthquakes rupturing the deeper part of the megathrust where the friction is lower can be more frequent and precede larger earthquakes on the shallower part of the megathrust (Moreno et al., 2018). The Seraphine fault may be a marker of along-dip segmentation of the megathrust and constitute a persistent updip limit of a deep megathrust segment (Figure 11). The megathrust may thus be composed of three main segments: a ~115 km wide segment A under the wedge (the wedge segment) extending between the Seraphine fault system and the subduction front, a 75-km wide segment B below the Caribbean crust and the backstop material (the interplate segment) and a deeper 75 km-wide segment C (the mantle segment). Segment C was inferred from coral microatoll studies (Philibosian et al., 2022; Weil-Accardo et al., 2016) and the location of instrumental and historical seismicity that indicates an unusually wide coupling zone reaching 60 km depth in the Lesser Antilles. In particular, it was inferred that the 1839, 1843 and 1946 earthquakes ruptured the plate interface below the Caribbean Moho (Feuillet, Beauducel, & Tapponnier, 2011; Weil-Accardo et al., 2016).
Each segment may have its own seismic history but may, once in a while, rupture all together during a megathrust event breaking its entire downdip width (265 km, Figure 11). From paleoseismological investigations, Seibert et al. (2024) showed that the LASZ is also segmented along strike with a 150 km-long segment located in between the Tiburon and Barracuda ridges. They proposed that the ridges may act as long-term barriers preventing rupture propagation along strike. This is also where the Seraphine fault system is most continuous and best expressed in the morphology. The different segments of the Bunce/Seraphine/Kitridge system and their kinematics may also underline an along-strike segmentation of the subduction zone. In between the well-defined sharp and linear traces of the Seraphine and Bunce faults, there is a complex relay zone composed of trench-parallel normal or reverse faults and restraining bands. This area lies in the prolongation of the subducting Barracuda ridge. From modeling, it was inferred that the subduction of such reliefs modifies the megathrust fault strength, the recurrence times of earthquakes and the interseismic coupling, with the reliefs thus acting as seismic barriers (Menichelli et al., 2023). When subducting, the ridges fracture the surrounding upper plate (Dominguez, Lallemand, Malavieille, & Schnürle, 1998; Dominguez, Lallemand, Malavieille, & von Huene, 1998) and drag fluid-rich sediments, overall contributing to increasing the fluid pressure and decreasing the friction (Menichelli et al., 2023 and reference therein). It is therefore possible that the friction decreases where the ridges are subducted, preventing the formation of a well-localized strike-slip fault, even in a context of increasing obliquity like in the area between the Bunce and Seraphine fault.

## 6.4. Related Seismicity and Seismic Hazard

The Seraphine fault is likely recent (of Plio-Quaternary age) and active. Limited seismicity has been detected thus far along accretionary wedge strike-slip faults worldwide, with the exception of the Mentawai fault (Chauhan et al., 2009; Singh et al., 2010). To examine if the Seraphine fault has given rise to instrumentally recorded earthquakes, we plot earthquake data from the GCMT catalog (Dziewonski et al., 1981; Ekström et al., 2012) events of Magnitude 6+ with epicenters within 50 km of the fault trace (Figure 10b). Several seismic clusters including Mw 6+ earthquakes have occurred near this fault (Figure 10b), for example, in December 1969 (labeled “1”, Stein et al., 1983, 1982) in the prolongation of the Tiburon ridge, then on 16 May 2014 (Mw 5.9, labeled “2”) in the prolongation of the Barracuda ridge, and on 16 July 2015 (Mw 6.5, labeled “3”) east of Saint Lucia. Two additional significant earthquakes of Mw 6+ have occurred near the Seraphine fault: on 8 March 1995 (Mw 6.1, labeled “4”) and on 19 March 2016 (Mw 6.0, labeled “5”) (source: GCMT catalog, Dziewonski et al., 1981; Ekström et al., 2012). As the earthquakes occurred 100 km to the east of the seismic network, there is considerable uncertainty associated to their epicentral positions (Massin et al., 2021), making their location uncertain. Based on the focal mechanisms of the main event of the 1969 cluster, it was proposed that the earthquakes occurred in the subducting slab (Stein et al., 1982). The origin of the other events is unknown, but all exhibit focal mechanisms incompatible with left-lateral movement along the Seraphine fault. All the Mw 6+ and other aforementioned significant earthquakes likely did not rupture the Seraphine fault, but it is noteworthy that they occurred in its vicinity. Both the presence of the fault and this seismicity may reflect mechanical instabilities and partitioning of the strain at depth within the wedge or along the plate interface near the connection with the branches of the Seraphine fault. This is also where a bend in the slab is observed (Figure 11, e.g. Kopp et al., 2011; Ruiz et al., 2013).

Although there is no evidence of significant instrumental earthquakes rupturing the Seraphine fault, we cannot rule out that it could be a potential source of large earthquakes. From latitude 17°30'N to latitude 12°30'N, the fault extends over ~580 km (Figure 12), with continuous, almost linear segments up to 180 km in length. It likely branches to the decollement, extending in depth for as much as 10 km (which is the thickness of the accretionary wedge beneath the surface trace of the fault, at the toe of the backstop, from Kopp et al., 2011; Laigle et al., 2013; Westbrook et al., 1988). By using the empirical relationships of Wells and Coppersmith (1994), which relate the magnitude of an earthquake to the fault surface, we estimate that a rupture of a 180 km-long segment of the fault could cause a Mw 7+ earthquake. A rupture of the entire Seraphine fault system would generate an earthquake of larger magnitude, reaching magnitude 8. In the Martinique-Antigua sector, we calculated an obliquity of the convergence that implies a trench-parallel velocity of 25%–30% of the total convergence (Text S4, Table S1 and Figure S21 in Supporting Information S1). By considering that the Seraphine fault system fully partitions the convergence, it would slip at a mean rate of 0.6 cm/yr (three times lower than the slip rates estimated for the Bunce fault by Laurencin et al. (2019), 1.6 cm/yr). Coseismic ruptures associated to magnitude 8 earthquakes have an average displacement between 5 and 8 m (Wells & Coppersmith, 1994). This would imply recurrence
times of about a thousand of years for these earthquakes. For a magnitude 7 earthquake, the average displacement is 1 m, which corresponds to a recurrence time of ~170 years.

Another source of hazards is the fact that slip on accretionary wedge structures can be triggered during a megathrust earthquake, as has been suggested for the West Andaman fault (and backthrusts at depth) offshore North Sumatra: this fault appears to have acted as a secondary source of the 26 December 2004 tsunami (Chauhan et al., 2009). Rupture on the Seraphine fault and its associated thrust faults in the GTH and MH regions could similarly be triggered during a megathrust earthquake resulting in a secondary but significant source of moment release and of tsunami.

## 7. Conclusions

The joint analysis of swath bathymetry and seismic data from the CASEIS cruise and legacy cruises, conducted for the first time along the entire Lesser Antilles subduction zone, sheds new light on the geometry of tectonic deformations affecting the fore-arc domain in the Martinique-Antigua sector. The data have allowed the discovery of the Seraphine fault, a 580-km-long trench-parallel strike-slip fault system, composed of a main linear strand and a set of secondary branches and associated thrusts, lying at the boundary with the fore-arc basins. This sinistral strike-slip fault seems active and long enough to be capable of causing large earthquakes of up to magnitude 8. The geometry of the Seraphine fault is unusual compared with other known fore-arc strike-slip faults. In Sumatra or Nankai, strike-slip faults dip toward the arc at depth with a thrust movement, whereas the Seraphine fault is sub-vertical.

Strikingly, the Seraphine fault is located in the Martinique-Antigua sector, where the obliquity of the convergence is however weak, also corresponding to the region between the subducted traces of the Barracuda ridge and the Tiburon ridges. Therefore, we suggest that high friction on the decollement is needed in the shallower part of the megathrust, updip of the Seraphine fault, to develop this structure. Further investigations are needed to understand the role of the subducting ridges on the formation and evolution of the Seraphine fault.

The Seraphine fault is part of a plate-scale strike-slip fault system that runs parallel to the accretionary wedge front over ~1,600 km. Well-expressed fault segments, such as the Bunce, Seraphine and Kitridge faults, are separated by relay zones, in particular in the zone in the continuity of the Barracuda ridge, which was previously identified as a megathrust segment boundary. Thus, the geometry of the plate-scale strike-slip fault system might be an indicator of along-strike segmentation of the megathrust. This system is exceptional by its extension and by its connection to major strike-slip faults of the Caribbean plate: the Septentrional and El Pilar fault systems. The kinematics of the entire fault system is incompatible with present-day NE-SW compression inferred at the Caribbean/American plate boundary and raise questions about the current geodynamics of the region. Further investigations via modeling are needed to better understand the link between this fault and the mechanical behavior of the megathrust underneath.

## Data Availability Statement

All geophysical data from CASEIS cruise are stored on the SISMER Database, available through the drop down menu "Data managed by SISMER" on the cruise page (Feuillet, 2016; https://campagnes.flotteoceanographique.fr/campagnes/16001800/fr/). MCS profiles presented in this study are available on SEANOE as raw data (Feuillet, 2024) and as processed data (Bénâtre et al., 2024a). Sub-bottom profiles (Bénâtre et al., 2024b) and bathymetry compilation (Seibert & Feuillet, 2024) are available on SEANOE. Other data used in this study are available on NOAA (NOAA National Centers For Environmental Information, 2023) and GMRT (Carbotte et al., 2004). Other geophysical data used are available through the referenced publications and on Academic Seismic Portal at LDEO (Christeson et al., 2017) and SISMER for French cruises (Deplus, 1998; Deplus & Boudon, 2002; Feuillet, 2009; Hirn, 2001; Laigle & Lebrun, 2007; Marcaillou & Klingelhoefer, 2013; Patriat, 2007). MCS and sub-bottom profiler data processing were performed using AspenTech ECHOS (https://www.aspentech.com/en/products/sse/aspen-echos) and Seismic Unix (Cohen & Stockwell, 2010) softwares, respectively. Seismic interpretation was carried out using Kingdom software from S&P Global (https://kingdom.ihs.com/). Bathymetry data processing were performed using CARAIBES software, part of the GLOBE software (Poncelet et al., 2024). The sedimentary cores are on the repository of the Museum National d'Histoire Naturelle. The data are accessible using the following link and with the inventory name GS-MNHN followed by
Acknowledgments

We thank the GENAVIR captain, officers and crew of R/V Pourquoi Pas? (Flotte océanographique française) and the CASEIS scientific cruise party (DOI: 10.17600/16001800). In particular we thank Jean-Marie Saurel and Muriel Laurencin for their contribution regarding MCS and sub-bottom profiler data processing. We also thank the scientific parties of cruises referenced in this publication that collected the legacy data. This study was supported by IPGP and funded by French ANR CARQUAKES (ANR-17-CE3-0006) and INTERREG Caraïbes PREST (Plateforme REgionale de Surveillance Tellurique) project, FEDER European Community (program number CCI, 2014TC16RFTN008). We thank Jean-Bernard de Chabaler as P.I. of this project. We also thank Chloé Seibert for her advice and help on core data interpretation. We thank the reviewers, Stéphane Dominguez and an anonymous reviewer for their contributions that helped to improve the manuscript.

the core number: https://science.mnhn.fr/institution/mnhn/collection/gs/item/search/form. Most maps were produced with QGis (https://www.qgis.org) and 3D block models were produced with ArcGIS (https://www.esri.com/en-us/arcgis). Focal mechanisms were plotted on Qgis using a Python script by Jérémy Billant (https://geolgis.wordpress.com/2016/11/14/plot-focal-mechanisms-using-qgis/). The plate motions are based on the Plate Motion Calculator service provided by the NSF GAGE Facility, operated by EarthScope Consortium (https://www.unavco.org/software/geodetic-utilities/plate-motion-calculator/plate-motion-calculator.html), with support from the National Science Foundation, the National Aeronautics and Space Administration, and the U.S. Geological Survey under NSF Cooperative Agreement EAR-1724794. Seismicity data used are from the CMT catalog (https://www.globalcmt.org/, Dziewonski et al., 1981; Ekström et al., 2012), the “ISC Bulletin” (2022) and Storchak et al. (2017, 2020) and the CDSA data set (Massin et al., 2021, http://www.seismes-antilles.fr/).

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