In Western Europe, the deformations related to the opening of the Bay of Biscay and the formation of the Pyrenean belt are well described in the southern part of the Aquitaine Basin, but little is known about the impact of these geodynamic events towards the Northern Aquitaine Platform. In this paper, we combine field observation with Unmanned Aerial Vehicle (UAV) imagery and calcite U-Pb geochronology to determine precisely the tectonic evolution in the Vendée Coastal domain. We evidence two main tectonic events: (1) At the transition between the Late Jurassic to Early Cretaceous, WNW-ESE striking normal faults formed horsts and grabens at the onset of the opening of the Bay of Biscay. The reactivation of Variscan faults during this tectonic event is consistent with oblique extension. This event triggered ascending fluid flows that mix with Basin fluids responsible for barite-pyrite-quartz mineralizations near the unconformity. (2) During the Late Cretaceous, fractures, wide-open folds, veins, and joints are consistent with the N-S shortening direction during the earliest stages of the Pyrenenan compression.

In both cases, the Northern Aquitaine Platform records the early stages of the main regional tectonic events in a far-field position. In the northern Aquitaine Basin, as in many other places in Europe, the tectonic study of sedimentary platforms located far from the plate boundaries provides new constraints on the early diffuse deformation process that predate the main tectonic phases.

Au cours des périodes du Mésozoïque et du Cénozoïque, l’Europe a connu plusieurs évènements tectoniques interprétés comme de la tectonique lointaine. Il peut s’agir d’une tectonique diffuse qui se produit au début d’un évènement tectonique majeur ou de la propagation au loin de la déformation des fronts orogéniques. En Europe de l’Ouest, les déformations liées à l’ouverture du Golfe de Gascogne et à la formation de la chaine des Pyrénées sont bien décrites dans le sud du Bassin aquitain, là où la plate-forme Nord aquitaine reste peu étudiée. Nous avons combiné les observations de terrain, l’imagerie par drone et la datation laser U-Pb sur la calcite pour déterminer l’évolution tectonique du domaine côtier de la Vendée. Nos résultats mettent en évidence l’existence de deux évènements tectoniques majeurs depuis le début du Jurassique : (1) À la transition du Jurassique supérieur au Crétacé inférieur, des failles normales orientées ONO-ESE présentent une organisation en hosts et grabens au début de l’ouverture du Golfe de Gascogne. Leur association avec des failles décrochantes varisques réactivées est cohérente avec une organisation oblique de l’extension. Cet évènement tectonique est accompagné par la circulation d’un fluide profond. (2) Durant le Crétacé supérieur, la mise en place de fractures, de plis ouverts, de veines et de joints est cohérent avec la direction de raccourcissement N-S qui se produit au premiers stades de la compression N-S des Pyrénées.

Dans les deux cas, les premiers stades des principaux épisodes de déformation régionale ont été enregistrés par une tectonique lointaine sur la plateforme Nord aquitaine, avant la localisation de la déformation dans les rifts et dans les Pyrénées. Dans le Bassin aquitain, comme dans de nombreux endroits en Europe, l’étude tectonique des plates-formes sédimentaires situées loin des zones principales de déformation à l’échelle des plaques apporte de nouvelles contraintes sur les processus de déformations précoces et diffus antérieurs aux principales phases tectoniques.

The plate tectonics theory defines lithospheric plates as rigid blocks bounded by deformable boundaries (McKenzie and Parker, 1967). However, numerous observations have evidenced intra-plate deformations, also called “far-field deformations” (e.g., Ziegler et al., 1995). Within the Basin, intra-plate deformations induce burial or uplift, hydrocarbon maturation, fluid flow changes, and mineral depositions (Boiron et al., 2010; Burisch et al., 2022; Gorczyk et al., 2013; Turner and Williams, 2004).

Far-field deformations occurred in divergence and convergence contexts, and their timing relative to rifting or mountain building is variable. In the divergence context, diffuse extensions predating strain localization within rifts are highlighted by uplift, tectonic reactivation, and fracture networks formation (Huerta and Harry, 2007; Seymour et al., 2016; Szymanski et al., 2016). However, this strain focusing within rifts is sometimes counterbalanced by the activation of several décollement levels in the context of multilayered lithosphere leading the deformation to propagate far from the main thinning zone (Clerc et al., 2016; Lagabrielle et al., 2020; Dhifaoui et al., 2021). In a convergence context, intra-plate deformations occur during orogenic processes several hundred kilometers away from the mountain belt front (Jolivet et al., 2021; Lacombe & Mouthereau, 1999). They are accompanied by uplifting following lithospheric folding (Gerbault et al., 1999; Ziegler et al., 1995), tectonic reactivation (Gorczyk et al., 2013; Navabpour et al., 2017; Parizot et al., 2022), and the formation of new fault and joint networks (Duperret et al., 2012; Missenard et al., 2017). However, far-field deformation takes place sometimes at the onset of convergence resulting in the inversion of pre-existing structures such as rift (Frizon et al., 2000) or passive margins (Leffondré et al., 2021, Strzerzynski et al., 2021;) and in the folding of the lithosphere (Cloetingh and Burov, 2011). Whatever the tectonic context, far-field deformation is always linked with vertical motion, tectonic reactivation of basement structure, and fracture network formation.

In Western Europe, two periods are particularly suitable for intra-plate tectonics: 1) the Late Jurassic to early Cretaceous, in a framework of extensional tectonics, which relates to the opening of the North Atlantic Ocean (Ziegler., 1990; Ziegler and Dézès, 2005). In the Aquitaine Basin, deformation is concentrated on a limited area between the Celtaquitaine flexure in the north and the North Pyrenean Fault in the south (Fig. 1; BRGM and Esso-Rep, 1974; Jammes et al., 2009; Issautier et al., 2022; Tugend et al., 2014) suggesting that no diffuse extension is recorded. However, it has also been proposed that the early deformations were not focused and extended north of the Celtaquitaine flexure (Asti et al., 2022). 2) The period from the Late Cretaceous to Eocene, in a setting of coeval compression, related to the Pyrenees orogeny. Late Cretaceous to Eocene intra-plate deformation is particularly well described in Iberia (De Ruig et al., 1990), in the Paris Basin (Blaise et al., 2022; Lacombe and Obert, 2000; Lacombe et al., 1990), southern England (Duperret et al., 2012; Parrish et al., 2018) and the North sea and Central Europe (Dielforder et al., 2019; Evans et al., 2003).

The North Aquitaine Platform is located along the western coast of France, at 100 and 300km north of the Bay of Biscay and Pyrenees front, respectively (Fig. 1). Here, geophysical investigations point to the absence of large-scale deformation, such as rifts or inverted structures (Bois et al., 1997; Jammes et al., 2009). However, at the outcrop scale, several deformation phases have been described, but no consensus exit: For some authors (Burbaud-Vergneaud, 1987; Bouton and Branger et al., 2007), no tectonic events occurred during the Mesozoic and at least six deformation episodes take place during the Cenozoic. Their timings are only constrained by the analogy of the stress direction with the peri-alpine domain (Bergerat, 1987). For others (Cathelineau et al., 2012, Strzerzynski et al., 2020), normal faults crossing the first Mesozoic sedimentary formation are associated with fluid circulation and mineral deposition. Here, the timing of faulting is indirectly constrained by the datation of the basement alteration and adularization, in a possible relationship with fluid circulation and mineral deposition (Cathelineau et al., 2012). No direct time constraints are available on the Mesozoic to Present tectonic event of the North Aquitaine Platform. No studies encompass the relationship between brittle deformation in the basement units and the sedimentary cover.

The following paper provides new structural and time constraints on North Aquitaine Platform to constrain far-field tectonics in this area and better understand how this can modify the fluid pathways and impact mineral deposition. We especially focused on the criteria characterizing the far-field deformation without a large-scale structure, such as tectonic reactivation of basement structure and fracture network formation. We conducted a geological study across the unconformity that separates the Armorican basement and the Aquitaine Basin. The exceptional outcrop conditions of the Vendée coast allow us to perform a structural survey based on detailed maps assisted by Unmanned Aerial Vehicle (UAV). In addition, we provide new time constrain on the syn-kinematic mineral by direct dating of calcite filling fractures using Laser ablation ICP-MS U-Pb geochronology. Then, we discuss the relationships between outcrop scale deformations and the main tectonic events of the Aquitaine Basin, such as the Bay of Biscay opening and the Pyrennee orogeny.

The Mesozoic North Aquitaine Platform emplaced over the Armorican basement that well recorded the latest stage of the Variscan evolution (Ballèvre et al., 2009; Cagnard et al., 2004; Gapais et al., 2015; Goujou et al., 1994). The latter is characterized by HT-LP metamorphism in a WNW‐ESE extension context (Cagnard et al., 2004; Goujou et al., 1994; Iglésias and Brun, 1976) and that has been dated further north at ca. 320 Ma (Augier et al., 2015; Turrillot et al., 2011). The Armorican basement consists of migmatitic gneiss at the Anse aux Moines, kyanite and sillimanite bearing gneiss that is intruded by aplo-pegmatitic dikes in Cayola Bay and kyanite micaschists in Mine des Sards and Anse de Saint-Nicolas (Fig. 2). Such a distribution of metamorphic rocks is consistent with a decrease of Barrovian-type metamorphism from the N.W. to the S.E., as proposed by Goujou et al., (1994). Ductile structures associated with this metamorphism gradually evolve into brittle structures such as N-S normal faults and a large shear zone at ca. 300 Ma (Turrillot et al., 2011).

Erosion and peneplanation occurring during the Late Paleozoic to Triassic periods are responsible for the continued exhumation of the basement rocks. At the end of the Jurassic, the Armorican basement from Vendée was buried under a thick layer (∼1 km) of platform sediments, causing a maximum temperature rise of 60 °C at the bottom of the Basin (François et al., 2020).

Fluid flows, inducing authigenic minerals precipitation near faults and along sedimentary strata, occurred during the Jurassic − Cretaceous transition as attested by 40Ar/39Ar and K-Ar dating on adularia and clays (illite, smectite, and kaolinite) of Cathelineau et al. (2012), in an N-S extensional setting (Strzerzynski et al., 2020). The numerous normal faults of the Aquitaine Mesozoic units are successively filled by barite, pyrite, quartz, or calcite (Fig. 2c; Strzerzynski et al., 2020). Fluids responsible for mineral deposition have variable temperatures and salinities, suggesting different fluid sources from secondary brines such as the Basin and sea water mixed with rising warm water (Cathelineau et al., 2012). The latter is particularly important along the Vendée coast north of the Aquitaine Basin, as evidenced by the high fluid temperatures (Cathelineau et al., 2012).

These fluid circulations and the associated tectonic event coincide with a major marine regression that induces the emersion of the North Aquitaine Platform in the Early Cretaceous (Fig. 1; Biteau et al., 2006; B.R.G.M., 1974; Curnelle and Dubois, 1986). At that time, the Variscan basement rocks begin to exhume due to the erosion of the Aquitaine Basin sedimentary cover (François et al., 2020). Exhumation is relatively fast as the basement rocks have been near the surface since Cretaceous times (François et al., 2020). Cenozoic structures have been dated by the relationship between continental deposits and regional tectonic (Burbaud-Vergneaud, 1987; Bouton and Branger et al., 2007). It attests to (i) N-S shortening in the Eocene, consistent with Pyrenean orogeny, (ii) an NNE-SSW extension in the Oligocene, compatible with the European Cenozoic rift system (Ziegler and Dèzes, 2005), followed by (iii) an E-W shortening in the Miocene and (iv) an NW-SE shortening from the Pliocene to the present day, which may relate to the Alpine orogeny (Bergerat, 1987).

Our approach is based on three main steps: 1) structural characterization. Thanks to excellent outcrop conditions on the rocky coastal plateau, we propose detailed structural maps constrained by UAV imagery coupled with orientation and length measurements of the structure. 2) relative chronology based on cross-sectional relationships of structural maps, and 3) absolute dating of authigenic minerals.

Structure characterization

The study focuses on fractures, a group of structures encompassing joints and faults (Peacock et al., 2016). Joints are characterized by opening from either side of the fracture (rupture mode I), and faults are associated with movement in the same direction as the fracture (rupture mode II) (Pollard and Aydin, 1988). Fractures can be filled by minerals that form veins and slickensides in the case of mode I openings and mode II slips, respectively (Bons et al., 2012).

We systematically measured the structure’ strike, dip, and dip direction in the field with a compass and reported mineral fills (G.P.S. coordinates and sampling).

UAV imagery

UAV imagery was deployed in all areas on the tidal rocky shelf. The UAV is a DJI Phantom 4 equipped with a 12 Mpixels camera. Picture acquisition was made at an altitude of 70 m. Then, photographs were merged into four high-resolution orthomosaics. The spatial resolution is 30 × 30 mm/pixel.

We provide a high-resolution geological map of the tidal rocky shelf by manually digitizing fractures and remarkable levels under the G.I.S. environment (QGIS 3.16 software). We selected fractures at least 10 m long for better clarity on high-resolution geological maps. Indeed, shorter fractures could be the response to rearrangement with little deviation due to a small-scale stress field. Strike and length were calculated from fractures traces in Q.G.I.S., and length-weighted rose diagrams were produced by multiplying the number of each fracture measurement by its length (for example, an N90° striking fracture of 3m long is considered as “N90; N90; N90” in the rose diagram). Length-weighted rose diagrams were displayed with GeoRose 0.5.1 software.

Relative chronology

The relative chronology between fractures is based on three topological relationships (Fig. 3; Sanderson and Nixon, 2015): isolated, intersecting, and abutting fracture terminations. Isolated fracture terminations, with an “I” geometry and “X” shape intersection without mineral fillings, don’t allow a precise relative chronology to be obtained. If the fractures have an “X” shape intersection that could form under the same stress field and for faults opposite shear senses, in that case, they are considered synchronous as conjugated fractures (Anderson, 1951; Daubrée, 1878; cited by Dennis, 1967). Abutment geometry refers to a “T” shape geometry with one fracture stopping against another, meaning it terminates against a pre-existing fracture (Sanderson and Nixon, 2015).

Mineral precipitation could be synchronous, anterior, or posterior to the fractures (Fig. 3). Synchronous mineral fills joints, veins, and slickenfibres on fault planes (Bons et al., 2012). Except for slickenfibres, other observations, such as the mineral texture and microstructure, help to decipher the synchronicity of the mineral filling. If the duration between fault activity and mineral precipitation is smaller than the dating uncertainties, they are interpreted as synchronous (Roberts and Holdsworth, 2022). Anterior mineral fillings in a new fracture are interpretable with an abutment or crosscutting geometry on the anterior mineral filling. The late mineral filling requires additional chronological evidence, such as a crosscutting relationship with other geological features, to be distinguished from synchronous mineral fillings. In all cases, inherited fractures that reopen by forming veins do not open in type I mode and have an oblique component (Fig. 3). They act as a relay structure between newly formed veins (Virgo et al., 2014) unless there is a separate tectonic event with the same stress field, which is unlikely.

Calcite U-Pb geochronology

We selected calcite samples from veins with a clear orientation and chronology. The selected calcite samples comprise rhombohedral blocky crystals 0.5 to 3 cm wide.

Individual calcite crystals from the three samples were hand-picked and polished in an epoxy mount. They were imaged using cold cathodoluminescence, and no zoning was observed. U-Pb analyses on calcite were performed at the Plateforme GeOHeLiS, Géosciences Rennes (Université de Rennes) using an ESI NWR193UC Excimer laser coupled to an Agilent 7700x, Q-ICP-MS equipped with a dual pumping system to enhance sensitivity. Ablation spots of 100 μm diameter with a repetition rate of 10 Hz and a fluence of 6 J/cm2 were used. The general protocol follows the one in Coltat et al. (2019) and is fully described in the Supplementary data with the instrumental condition table. WC1 (Roberts et al., 2017) is the primary reference material. Data reduction was performed with Iolite v4 software (Paton et al., 2011) using U-Pb VizualAge UComPbine D.R.S. (Chew et al., 2014). Sample age calculations and Tera-Wasserburg diagrams were made using IsoplotR (Vermeesch, 2018). All uncertainties are quoted at the 2σ level. Propagation of excess scatter on primary R.M. is propagated to individual ratio measurements. Systematic uncertainty on the ratios of primary R.M. and decay constants uncertainties are propagated to the final age calculation. Propagation is made by quadratic addition following Horstwood et al. (2016).

The field observations assisted by UAV imaging of the different lithologies and the brittle structure are obtained on six outcrops along the Vendée Coast from northwest to southeast (Fig. 2) as follows: the Anse aux Moines (Fig. 4), the Cayola Bay (Fig. 5), the Mine des Sards (Fig. 6), the Payré Estuary (Fig. 7), the Anse de Saint-Nicolas (Fig. 8) and Four à Chaux (Fig. 9).

Lithologies

Basement unit

The basement unit (B.U.) has been studied in detail at four sites from west to east: Anse aux Moines, Cayola Bay, Mine des Sards, and Anse de Saint-Nicolas (Fig. 2a). Each site has a similar pattern corresponding to a tidal rocky shelf of several tens of meters wide separated from a 10 to 15 meters high coastal cliff by a more or less-developed beach, such as the cliff of the Anse aux Moines (Fig. 4). In each site, the basement unit occupies the tidal rocky shelf, and the unconformity separating the basement unit and the sedimentary cover can be observed along the coastal cliff in many places. Horizontal and panoramic orthoimages obtained by UAV (Figs. 5a, 6c, and 8b and in Supplementary material) provide practical support for G.I.S. tectonic structure mapping and field observation localization (Figs. 5, 6, and 8).

In the B.U., the gneissic foliation is synchronous with metamorphism (Goujou et al., 1994) and magmatic intrusion (Cagnard et al., 2004). The foliation strikes between N90° and N110° and plunges to the N with an angle between 40° and 60° at the Anse aux Moines (Figs. 2a and 4), Cayola Bay (Fig. 5), the Mine des Sards (Fig. 6) and turns to N160−50°E at the Anse de Saint-Nicolas (Fig. 8). In Cayola Bay, dikes are more or less transposed into the foliation (Fig. 5a-b).

Sedimentary units

We divided the Aquitaine Basin series into five sedimentary units called SU1, SU2, SU3, SU4, and SU5 (Fig. 2b) based on the sediment’s age and lithologies and, thus, different mechanical behavior. The shallow dip (10°) of the bedding to the N or the S defines several large open folds offering good outcrop conditions on the plateau in each unit (Fig. 2a).

SU1 and SU2 have been dated from the Hettangian age on paleontological contents (Bouton, 2004; Goujou et al., 1994; Granier et al., 2015). They can be observed along the coastal cliffs of each site and on the part of the tidal rocky shelf in the Anse aux Moines, Cayola Bay, the Mine des Sards, and the Anse de Saint Nicolas (Figs. 4, 5, 6, and 8). SU1 lies directly on the B.U. (Figs. 4a, 5d-e, 6a-b, and 8a). It consists of a 1 to 4 m thick grey clay-rich unit intercalated with layers of yellow dolomitic limestones (Figs. 4a, 5d-e, and 6b) except at the Anse de Saint-Nicolas area where it is replaced by a 20 cm thick conglomerate bed (Fig. 8a; Goujou et al., 1994).

The SU2 consists of a layer of dolomitic limestones 5 to 10 m thick with bed thickness ranging from 5 to 30 cm (Figs. 4a, 5d-e, and 6b). In this unit, karsts are abundant in some places, especially at the Mine des Sards and the Anse de Saint-Nicolas, filled with barite and quartz (Fig. 6g-h). Karst shapes are mostly lenticular (Fig. 6g), parallel to the bedding with elongated WNW-ESE-tubular shapes. Silicification of SU2 limestones is very common in the Anse aux Moines, the Mine des Sards, and the Anse de Saint-Nicolas (Figs. 4a, 5d-e, 6a, and 8a). In Cayola Bay (Fig. 5d-e), silicification only develops around faults crosscutting the BU/SU1 unconformity.

The SU3 is dated to the Plienbachian (Fauré and Bohain, 2017). The hardground at the top of SU2, the pebbles layer at the base of SU3, and the absence of Early Sinemurian fauna have been interpreted as evidence of emersion of the area between the Late Hettangian and Early Sinemurian (Fauré and Bohain, 2017). SU3 is visible on the coastal cliffs of the Mine des Sards and the Payré estuary (Figs. 7a and 10c) and has been mapped on the tidal rocky shelf of the Anse de Saint-Nicolas (Fig. 8b). It consists of limestones with abundant Pecten and Belemnite fossils alternating with a shale-rich layer. Similarly to SU2, SU3 is silicified in places such as the Mine des Sards.

The SU4 has been dated from the Toarcian to the Aalenian using paleontological contents (See reference in Goujou et al., 1994). It is observed at the Anse de Saint-Nicolas coastal cliff (Fig. 8b) and, depending on the amount of sand and the tide, at the foot of the beach between the Anse de Saint-Nicolas and the Four à Chaux (Fig. 9a). It consists of a 20 m thick succession of blue shale and rare limestones that forms an impermeable layer on the scale of the northern Aquitaine Basin (Cathelineau et al., 2012).

Based on paleontological contents, the SU5 has been dated from the Bajocian to the Callovian (Goujou et al., 1994, and references therein). It outcrops at Four à Chaux and extends several kilometers to the east. It is a 100 m thick unit. It comprises white limestone beds 10 to 50 cm thick alternating with centimetric marl layers (Fig. 9d).

Brittle structure of the Vendée coastal domain

In the field, we observed fractures with sometimes mineral filling (veins) and faults with offset up to 10m from the B.U. to SU5. Slickensides are preserved in SU2 and SU5, giving the offset’s orientation, but alteration (argillization and silicification) has erased them in B.U., making this measurement impossible. We estimate movement mainly by considering remarkable markers offsets such as dike, unconformity, lithological contacts, quartz lenses, and sedimentary beds.

Anse aux Moines outcrop

The westernmost outcrop is located at the Anse aux Moines (Figs. 2a and 4). It consists of a 150 m-long coastal cliff-oriented NNW-SSE. Here, the tidal rocky shelf is rarely visible at low tide, and a thick pebble beach limits observation of the cliff (Fig. 4). The outcrop consists of SU1 and SU2 rocks bounded north and south by the migmatitic gneisses forming the B.U. To the north, the B.U. is covered by SU1 and SU2 (Fig. 4a). The contact between the B.U. and the SU1 and SU2 bedding are parallel and inclined together to the south, suggesting that the unconformity outcrops here (Fig. 4a-d). The contact between B.U. and SU1 is steeper to the south and underlined by an N116−69°NNE normal fault plane (Fig. 4a). The fault zone is underlined by a 4 m thick fault gouge zone and strongly deformed SU1 beds (Fig. 4a-c). Silicification is well-developed, especially within the SU2 and the fault gouge breccias. Quartz also fills a 50 cm thick vein striking parallel to the fault (Fig. 4b). The structural pattern is consistent with a half-graben structure bounded by a north-dipping fault and tilted toward the south.

Cayola Bay outcrop

Cayola Bay is located at the estuary of the Combe River (Fig. 2a). It is a triangular-shaped bay bounded by two 500-m-long coastal cliffs. The one situated west is striking NNE-SSW, and the other, NW-SE. B.U.’s tidal rocky shelf is present primarily on either side of the bay, and rocks from the SU1 and SU2 are visible on the coastal cliff. We focused here on the eastern part of the bay, where the unconformity is more visible.

On Cayola Bay (Fig. 5), we measured three populations of oriented brittle structure: N160° to N-S, N30° to N50° and N110° to N130° respectively. The N110−130° fault population is the only one to displace the unconformity suggesting an activity post-dating the Hettangian (Fig. 5d-e). In contrast, faults oriented N160° to N-S and N30° to N50° are sealed by the unconformity, suggesting a pre-Hettangian activity (Fig. 5d-e).

Aplo-pegmatitic dikes injected into the B.U. allow us to estimate the apparent offset of the faults. N160° to N-S striking faults cut aplo-pegmatitic dikes with an apparent dextral offset (Fig. 5a-b). At several locations, their orientation continuously evolves from an NW-SE to an N-S strike with the same sense of shearing (Fig. 5a-b). This suggests that N160° to N-S oriented faults may have been active during the dike emplacement. N30° to N50° striking faults cut aplo-pegmatitic dikes with apparent sinistral offsets (Fig. 5c). Together, the N160° to N-S striking faults and the N30° to N50° striking faults may have been active simultaneously as conjugated faults.

Three major N110° to N130° striking faults were mapped on the tidal flat and the coastal cliff (Fig. 5a-e). No dike offsets were observed because these faults strike in the same direction as metamorphic foliation and aplo-pegmatitic dikes (Fig. 5a-e). However, in the coastal cliff, the unconformity is crosscut by these three faults with normal motion (Fig. 5d-e). Around N110° to N130° striking normal faults, SU2 limestones are locally silicified over a width of 1 to 3 m (Fig. 5d-e). In the B.U., the northernmost fault is filled with pyrite mixed with host rocks (Fig. 5a). The opposite dipping of N110° to N130° striking normal faults toward Cayola Bay delineates a graben where the unconformity is lowered by 5 to 10 m (Fig. 5a).

Mine des Sards outcrop

The Mine des Sards is located between Cayola Bay and Bourgenay (Fig. 2a). It is a large area where B.U. is observed on the tidal rocky shelf, and SU1 and SU2 are primarily visible on the coastal cliff (Fig. 2a). Unfortunately, the unconformity is rarely visible as it is covered by beach pebble deposits.

The area’s western boundary consists of an N160° and vertical fault that cuts SU1 to SU3 and uplifts B.U. (Fig. 2a). Here, we estimate a vertical offset of 5 to 10 m high because the uppermost SU3 is in direct contact with B.U. The latter changes from kyanite micaschists to the east of the fault to kyanite-sillimanite gneisses to the west. Such a change suggests that either the vertical offset was underestimated or the fault developed on a pre-existing contact (Fig. 2a). To the east, the B.U. is progressively covered by sedimentary units and disappears on the east side of Bourgenay (Fig. 2a).

The orthoimage in Figure 6 is located around 500 m east of the Mine des Sards (Fig. 2a) in an area where the unconformity is well exposed. Here on the tidal rock platform, we measured four populations of brittle structure according to their orientations: N-S, N40° to N60°, N110° to N130°, and N140° to N160°. On the coastal cliff, the main N110° to N130° striking fault (oriented N118°−73°ENE) cuts the unconformity, SU1 and SU2 (Fig. 6a-c) and therefore is active after the Hettangian. The activity of the N-S, N40 to N60 striking fault population predates the Hettangian as they are always sealed by the unconformity (Fig. 6b). The relationship between the N140° to N160° striking fault population and the sedimentary units (SU1 and SU2) is more controversial as most of the structures are sealed by the unconformity (Fig. 6b) except one fault (oriented N151°−57°ENE) that produces a 1 m offset (Fig. 6a-c).

The N-S faults are vertical or dip mainly to the east, and their movement is not constrained because of bad outcropping conditions (Fig. 6b-c). The N40° to N60° and N140° to N160° striking faults are left and right lateral strike-slip faults, as evidenced by the offset of the foliation and the quartz lenses (Fig. 6d-e). They form an “X” shaped intersection compatible with conjugated fault abutting on N-S faults (Fig. 6f). All these faults are sealed by the unconformity (Fig. 6b) except one fault oriented N140° to N160° that cuts the unconformity (Fig. 6a-c).

A main N110° to N130° striking fault can be followed at the foot of the pebble beach and on both sides of a little cape (Fig. 6a-c). Here, the 3-4 m high unconformity offset is consistent with a normal motion. In SU2, pervasive silicification follows the normal fault planes and, in some places, propagates horizontally into the inter-stratum spaces. To the east, this normal fault abuts the N140° to N160° normal fault that cuts the unconformity (Fig. 6a-c). Despite their different orientations, both of these normal faults are filled by pyrite (Fig. 6c). This N140° to N160° normal fault could have been reactivated at the same time as the N110° to N130° faults.

Payré Estuary outcrop

The Payré Estuary is a 250 m-long cliff area where SU2 and SU3 outcrop (Figs. 2a and 7a).

SU2 and SU3 are cut by a network of minor normal faults showing an offset of up to 10 cm (Fig. 7a). Besides, vertical E-W fractures (Fig. 7c) are filled with barite and calcite abutting on the previous normal fault network (Fig. 7a). At the fracture scale, the calcite veins crosscut the barite vein (Fig. 7b). A Last calcite mineralization stage is locally observed in the coating (concretionary habit) both in the fractures and the interbeds.

Anse de Saint-Nicolas outcrop

The Anse de Saint-Nicolas is a large area where unconformity can be followed in the middle of the coastal cliff over a distance of 1.5 km (Fig. 2a). The mapped zone is located at the eastern boundary of the cliff where B.U. and SU2 to SU3 are observed on the tidal rocky shelf (Fig. 8b). In the following section, we described the structure within the B.U. and those in SU2 to SU3.

The B.U. outcrops on the western half of the map (Fig. 8a-b). The foliation of the B.U. is striking N135° to N0° and dips to the east. In the field, we measured two populations of faults: 1) a west-dipping, N160°-trending normal faults spaced 20 to 50 cm apart (Fig. 8a) and 2) N40° to N60°-trending vertical left-lateral strike-slip faults (Fig. 8h). Both populations are sealed by the unconformity (Fig. 8a) and predate the Hettangian. The N40° to N60° striking faults are less numerous and locally form deformation corridors where pyrite is abundant (Fig. 8b). The orthoimage reveals N90° to N110° striking faults parallel to the coastal cliff (Fig. 8a-b). They abut on N40° to N60° striking faults (Fig. 8c) and are locally filled with barite, pyrite, and quartz (Fig. 8f).

SU2 and SU3 outcrop on the eastern half of the map (Fig. 8a-b) in this area, the structural pattern is dominated mainly by N90° to N110° striking fractures consisting of faults, veins, and joints (Fig. 8b). The fault offsets are consistent with a normal motion and coherent with a vertical slickenside on a fault plane (Fig. 8e). The N90° to N110° striking veins are filled mainly with barite, pyrite (Fig. 8g), and occasionally with quartz (Fig. 2a). Geodic barites are abundant in the E-W corridors (Fig. 8b-g). The unconformity is visible on the coastal cliff and in most distal parts of the rocky tidal shelf, where a 500 m-long outcrop of silicified SU2 is preserved (Fig. 8b). As at the Anse aux Moines, Cayola Bay, and the Mine des Sards (Figs. 4, 5, and 5), silicification of SU2 develops over a few meters around N90° to N110° striking normal faults, potentially increasing SU2’s resistance to erosion.

In SU2 and SU3, two further populations of smaller fractures striking from N30° to N50° and N150° to N170° (Fig. 8b-h). As no offsets of the sedimentary beddings are observed here, this suggests a type I rupture mode (i.e., joints). These smaller fractures are locally filled with pyrite, while barite is always absent (Fig. 8g). No structure striking between N150° and N170° are visible at the map scale, but they do appear at high resolution (Fig. 8d-g). In many locations, abutment relationships indicate that N150° and N170° striking joints post-dates N90° to N110° striking fractures (Fig. 8d-g). The N30° to N50° striking fracture population is well expressed at the map scale, where it forms a linear corridor with abundant pyrite embedding angular clasts but with no visible motion (Fig. 8b). However, the geometric relationship between this corridor and the N90° to N110° striking faults remains unclear. At high resolution, the “X” shape geometry of the N150° to N170° striking joints and the N30° to N50° striking joints (Fig. 8d-g) suggests that they act as conjugated joints.

Four à Chaux outcrop

Four à Chaux is located east of Anse de Saint-Nicolas (Fig. 2a), where SU4 and especially SU5 are observed on the tidal rocky shelf and the coastal cliff (Fig. 9a-d). A major fault described by Ters and Gabilly (1986) draws SU5 down to the level of SU4 in the north-western part of the mapped area.

Field measurements (Fig. 9c) and the ortho-image in Figure 9a-b highlight three fracture populations oriented respectively: N100° to N120°, N40° to N60° and N160° to N-S°. Fractures oriented N100° to N120° are the longest structure with local barite and calcite fillings (Fig. 9a-e). On the coastal cliff, the slickensides on a fault plane oriented N104°−75°SW are compatible with a normal motion (Fig. 9e).

The N40° to N60° and N160° to N-S° fractures are the shortest; they are visible only at a high resolution between the beach and the coastal cliff (Fig. 9b). These two fracture populations are abutting and thus post-dated on the N100° to N120° striking fractures (Fig. 9b). The geometrical relationship between the N40° to N60° and the N160° to N-S° striking fractures are an “X” shape intersection suggesting that these two populations are coeval (Fig. 9b). No offsets of the sedimentary bedding are observed here, suggesting a type I opening mode (i.e., joints). No mineralization fills these joints (Fig. 9d).

Calcite U-Pb geochronology

The three samples for U-Pb dating are from calcite veins sampled at Four à Chaux (ASN201 sample; Fig. 9a) and at the Payré Estuary (PN201 and PP5 samples; Fig. 7a).

ASN201 has been sampled on the tidal rocky shelf of Four à Chaux composed of SU5 strata (Fig. 10a), where calcite forms 10 cm thick veins with dilational vein textures (Fig. 10b; Roberts and Holdsworth, 2022). Its orientation is N100°−52°S (Fig. 10b), and at the map scale, it is the continuation of the major normal fault N104°−75°SW (Figs. 9a and 10a). On the cliffs, slickensides from the fault plane indicate a normal sense of shearing and the occurrence of rosette-shaped barite with calcite on the fault plane surface (Fig. 9d-e). The mineral deposition is synchronous with the fault activity (Roberts and Holdsworth, 2022). ASN201 sample consists of yellowish subtransparent rhombohedral calcite crystals 0.5 to 1 cm wide. At a small scale, the calcite vein hosts crackle-breccia of SU5 embedded by the same blocky calcite crystals (Fig. 10c). This observation demonstrates the synchronous precipitation of calcite with the faulting that affects SU5. The lower intercept on the Tera-Wasserburg Concordia diagram yields a date of 144.1 ± 26.2 / 26.4 Ma (n = 20, MSWD = 0.68; Fig. 10g).

Samples PN201 and PP5 come from the Payré Estuary coastal cliff (Figs. 2a and 7). PN201 and PP5 are oriented N86°−84°S and N95°−88°S respectively (Fig. 5d). These veins are hosted in SU2, and by place, the veins crosscut barite veins, like the sample PN201 (Figs. 7b and 10d). This shows that two fracturing with N-S opening occurred at the Payré Estuary, the first with fractures filled by barite and the second with calcite. In detail, the calcite consists of cloudy yellowish-white rhombohedral crystals 1 to 3 cm wide. At a small scale, in a smaller sample than PN201 from the Payré Estuary cliff, a barite vein is crosscut by a calcite vein constituted of blocky crystals (Fig. 10f). Another calcite veinlet filled a fracture crosscutting the same barite vein (Fig. 10f). The morphology of the calcite veins could suggest a single phase opening during crack-seal, and so, possible synchronicity of calcite filling with fracturing (Roberts and Holdsworth, 2022). The lower intercept on the Tera-Wasserburg Concordia diagram yield dates of 72.5 ± 11.5 / 11.6 Ma (n = 20, MSWD = 0.45) and 79.7 ± 22.3 / 22.4 Ma (n = 20, MSWD = 0.85), for PP5 and PN201 calcites respectively (Fig. 10 h-i).

Chronology of brittle structure

From site to site, we sorted the joints and faults in the Vendée coastal domain according to their 1) orientations, 2) relationships to the Aquitaine Basin unconformity, and 3) mineral fill (Tab. 1). We obtained five populations of faults and two populations of joints (referred to after that as FP-1 to FP-5 and JP-1 and JP-2).

The FP-1 fracture family consists of N-S striking normal faults crossing only B.U. (Figs. 5, 6, and 8). They are locally associated with the emplacement of aplo-pegmatitic dikes (Fig. 5), suggesting that they were active at the end of the Variscan ductile deformation dated from Late Carboniferous by Turrillot et al. (2011) and Augier et al. (2015).

The FP-2 fracture family consists of NE-SW striking sinistral faults that displace beddings relics in B.U. micaschists and magmatic dikes with an apparent horizontal offset of up to 5 m (Figs. 6 and 8). The FP-2 is sealed by the Aquitaine Basin unconformity everywhere in the B.U., indicating that they formed before the Hettangian. Because no magmatic dike follows the fault plane of FP-2, we propose that FP-2 post-dates FP-1. In the Anse de Saint-Nicolas area, NE-SW joints of the same orientation crosscut SU2 and SU3. No strike-slip motions were observed there. As these joints are not observed in SU4 and SU5, FP-2 is potentially reactivated between Pliensbachian and Toarcian times. However, as FP-2 from B.U., SU2, and SU3 are filled with pyrite, the reactivation can also occur during the mineral deposition (referred to as FP-2 reactivated in Tab. 1). In this case, the absence of the NE-SW joint in SU4 and SU5 may be explained by the rheologic barrier of the SU4 marls, as in the case of mineral deposits.

The FP-3 fracture family consists of NW-SE striking dextral faults that cut and shift bedding relics in B.U. micaschists and aplo-pegmatitic dikes (Fig. 6). The relationship between FP-3 and the Aquitaine Basin unconformity is ambiguous: on the one hand, FP-3 is sealed by the unconformity in most cases, indicating that it formed before the Hettangian. At B.U., it abuts FP-1 and thus is posterior to FP-1 (Fig. 6), and the intersecting relationship between FP-3 and FP-2 suggests that they formed simultaneously. On the other hand, two major faults from FP-3 crosscut the unconformity with a normal motion, and SU2 and SU3 at the contact of these two faults are silicified (Figs. 2 and 6). These mean the reactivation of FP-3 per place (referred to as FP-3 reactivated in Tab. 1).

The FP-4 fracture family consists of WNW-ESE striking normal faults that affect both B.U. and SU1 to SU5 (Figs. 49) and with the Aquitaine Basin unconformity (Figs 46). The FP-4 structure family forms horsts and grabens visible at map scale (Figs. 2, 4, and 5). FP-4 abuts FP-2 and FP-3, suggesting that these latter predate FP-4 although they crosscut the unconformity (Figs. 2 and 6). We observed barite, pyrite, quartz, and calcite-filled veins and fault planes in the SU1 to SU5 as previously described by Cathelineau et al. (2012) and Strzerzynski et al. (2020) but also in the B.U. (Fig. 8). We also provide field evidence of focused silicification of SU2 and SU3 immediately around FP-4 faults that crosscut the unconformity (Fig. 4). Given this evidence, we propose that the fluid flow related to mineral deposition and silicification is contemporaneous with normal faulting. Calcite from sample ASN201 which fills an FP-4 affecting SU5 yields a U-Pb date of 144.1 ± 26.2 Ma (Fig. 10). In the same fault plane, calcite is associated with barite (Fig. 9a-e). This date overlaps the 155 to 145 Ma interval proposed in the northern Aquitaine Basin (40Ar/39Ar and K-Ar dating on adularia and clays by Cathelineau et al., 2012). It reflects the timing of mineral deposition and WNW-ESE striking normal faulting (Fig. 9).

The FP-4 fracture family consists of WNW-ESE striking calcite veins that affect SU2 in the Payré Estuary (Fig. 7). The FP-5 veins crosscut the FP-4 barite veins (Fig. 7) and abut on FP-4 normal fault. Calcite from samples PP5 and PN201 that fills FP-5 veins yield a U-Pb date of 72.5 ± 11.5 Ma and 79.7 ± 22.3 Ma, respectively (Fig. 10). These ages are significantly younger than the 170 to 118 Ma range proposed for FP-4 and agree with the relative chronology (Fig. 7).

The JP-1 and JP-2 joint families, oriented NE-SW and NNW-SSE, respectively (Fig. 9), only develop in SU5 (Fig. 9). Both are abutting on, and thus they post-date FP4. The absence of barite, pyrite, quartz, and calcite filling the joints confirms that JP-1 and JP-2 post-date mineral deposition (Fig. 9). In the absence of observed relationships with FP-5, the relative timing remains unconstrained.

Late-Variscan tectonic evolution

Three populations of faults (FP-1, FP-2, and FP-3) sealed by unconformity take place before the Hettangian (Fig. 11). The orientation and movements of FP-1 are consistent with E-W extension post-dating HT-LP Variscan metamorphism (Cagnard et al., 2004; Goujou et al., 1994; Iglésias and Brun, 1976). A population of similar faults observed in the west Armorican Massif has been dated between 302 and 298 Ma (Turrillot et al., 2011). The geometric relationship between FP-3 and FP-2 suggests synchronous and conjugate slip faults compatible with σ1 and σ3, oriented N-S and E-W, respectively (Fig. 6) and consistent with the dextral movement of the South Armorican shear zone (Fig. 2) that ended circa 300 Ma (Augier et al., 2015). Considering this, we propose that brittle faults of the Vendée coastal domain sealed by the Aquitaine Basin unconformity occur during the Late Carboniferous (Fig. 11). No further deformation is recognized until the deposition of SU1.

Late-Jurassic Early Cretaceous extension and fluid event

The FP-4 structure are compatible with a subhorizontal σ3 oriented N-S, as Strzerzynski et al. (2020) proposed.Throughout the study area, tectonic activity is synchronous with fluid transfer and mineral deposition (Cathelineau et al., 2012; Strzerzynski et al., 2020) (Figs. 2 and 11). The close relationship between high-temperature mineral deposition ( > 250 °C), silicification, and the rooting of FP-4 normal faults in the B.U. suggests that some of the fluids involved are crustal in origin and ascend from several kilometers depth (Cathelineau et al., 2012). Our new age on FP-4 calcite constrains the chronology of N-S extension in the Vendée coastal domain at the Jurassic-Cretaceous transition (Fig. 10 and 11). This age is consistent with previous dating of adularia and clay minerals (Cathelineau et al., 2012), meaning the dated minerals precipitated simultaneously. This event predates the main extensional phase of Aquitaine Basin rifting and oceanic spreading in the Bay of Biscay (Fig. 1; Tugend et al., 2014, 2015) and is related to the earliest extensive deformation (Asti et al., 2022).

The barite, pyrite, and quartz deposits that fill several faults FP-2 and FP-3 (Fig. 8) show that during the Jurassic-Cretaceous transition, at the Jurassic-Cretaceous transition, fluids flow along both newly formed (FP-4) and reactivated (FP-2 and FP-3) structure. In addition, we document the tectonic reactivation of the inherited strike-slip fault FP-3 during the Jurassic-Cretaceous transition fluid event (Fig. 6). The involvement of the Late Variscan inherited structure during the rifting phase of the Aquitaine Basin has been proposed before based on seismic profiles (Ducoux et al., 2021; Issautier et al., 2020; Manatschal et al., 2015) but never observed in the field. The Vendée coastal domain provides a unique opportunity to observe the association of newly formed and inherited faults during this rifting phase. This fault association agrees with the oblique rifting model for the Bay of Biscay proposed by Asti et al. (2022).

Pyrenean compression

A calcite-filled FP-5 fracture dated between 84 and 61Ma (Fig. 10) and joints JP-1 and JP-2 post-date the mineralization and extension of the Jurassic-Cretaceous transition. If calcite infilling is synchronous with FP-5, the latter forms with the onset of shortening along the Eurasian-Iberian plate boundary (Fig. 1). At this time, shortening is assumed to follow an NNE-SSW direction (Rocher et al., 2000) parallel to the σ3 opening direction of the FP-5 stage veins (Figs. 7 and 11). In convergent domains, subhorizontal σ3 strike in the same direction as large-scale convergence has already been described before and during thrusting (Lacombe, 2010, 2012). When it precedes horizontal shortening, subhorizontal σ3 stress is related to extension into the peripheral foredeep bulge (Bradley and Kidd, 1991; Doglioni, 1995; Langhi et al., 2011; Tavani et al., 2015; Turcotte and Schubert, 2002). When subhorizontal σ3 stress coincides with shortening, it corresponds to limited extension at fold hinges (Cooper, 1992; Hancock, 1985).

During the Late Cretaceous (pre-Santonian), the extension observed in the Aquitaine Basin foreland is associated with mantle exhumation leading to high geothermal gradients (e.g., Angrand et al., 2018; Clerc et al., 2012; Issautier et al., 2022; Lagabrielle and Bodinier, 2008; Lagabrielle et al., 2010). This thermal influence does not affect the northern Aquitaine Basin. In the Vendée coastal domain, two joints (JP-1 and 2) and folds (Fig. 2) could be due to NNE-SSW compressive deformations (Fig. 11) after the extension of the Late Jurassic and Early Cretaceous. Thus, compressive tectonics related to the Campanian Iberia-Eurasia convergence (Issautier et al., 2022; Mouthereau et al., 2014; Roure et al., 1989; Vergès et al., 1995, 2002) is observed during the Late Cretaceous up to 400 km north of the Pyrenean belt.

Diffuse versus focused strains in the Aquitaine-Pyrenean domain

Our new constraints on the timing of the deformations affecting the North Aquitaine Platform provide insight into how diffused tectonics focused on limited areas during kinematic changes at plate boundaries.

The exhumation of the Armorican Massif and the French Massif Central began in the Late Jurassic and/or Early Cretaceous from the initiation of the opening of the Bay of Biscay (Figs. 1 and 12; Barbar and et al., 2020; François et al., 2020). Such uplift coincides with forming of new fault networks (FP-4) on the North Aquitaine Platform, compatible with subhorizontal σ3 oriented N-S and the tectonic reactivation of inherited faults (FP-3). In the Paris Basin, this episode results in large-scale folding, forced regression, and unconformity deposition of Purbeck facies (Brigaud et al., 2018). These observations provide good arguments for intra-plate deformation (e.g., Parizot et al., 2022; Ziegler et al., 1995).

Because it occurred 30 Ma before the main Aquitaine rifting event and is located 350 km north of the main rifts in the Parentis and the Arzack-Mauléon Basins, we proposed that extension in the Vendée coastal domain at the Jurassic to Cretaceous transition is an example of diffuse extension terrain preceding the focus of strain during rifting (Fig. 12; Asti et al., 2022; Huerta and Harry, 2007; Seymour et al., 2016; Szymanski et al., 2016).

A network of calcite-filled conjugate joints and faults (FP-5) (Fig. 7), crosscutting the previous fractures (FP-4), are Late Cretaceous, i.e., from the early stages of the Iberian-Eurasian convergence. At this time, no temperature drop occurred in the Armorican Massif and the French Massif Central (Barbarand et al., 2020; François et al., 2020), suggesting that shortening is not associated with significant uplift of the North Aquitaine Platform. As no relief is documented (Huyghe et al., 2012), such compressive deformation cannot be related to wedge propagation. It is more likely related to far-field intra-plate tectonics (Fig. 12; Gorczyk et al., 2013) predating the formation of the Pyrenean relief and the Eocene propagation of the deformation in the North Aquitain Platform (Brown et al., 2021), the Paris basin (Blaise et al., 2022) and the English Channel (Parrish et al., 2018).

In this study, we applied field and UAV imagery methods in the Vendée remarkable outcrops to decipher the evolution of brittle tectonics of the north of the Aquitaine Basin during the opening of the Bay of Biscay and the Iberia-Eurasia convergence.

We showed that: (1) During the opening of the Bay of Biscay, newly formed normal faults associated with reactivated Late Variscan faults provide evidence of oblique rifting. (2) Compressive tectonics related to the Iberia-Eurasia convergence occurred during the Late Cretaceous 400 km north of the Pyrenean belt. (3) Both the extensional and compressive tectonics are related to diffuse intra-plate tectonics before the focus of deformation on continental rifts and the Bay of Biscay during the Early Cretaceous and on the Pyrenenan- Cantabrian margin during the Cenozoic.

The TELLUS-CESSUR and AO-OSUNA programs have funded the research presented in this article. The authors thank Thomas Blaise and Michael Jentzer for helpful and constructive manuscript reviews.

Cite this article as: Bouat L, Strzerzynski P, Mourgues R, Branquet Y, Cogné N, Barré G, Gardien V. 2023. Early, far-field and diffuse tectonics records in the North Aquitaine Basin (France), BSGF - Earth Sciences Bulletin 194: 17.

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