The Mid-Albian unconformity, a key to understand the geodynamics of the North Pyrenean Trough

– The Pyrenees are the result of the structural inversion of a rift of general direction N110E, established at the articulation of the European and Iberian plates. This rift was created by left-lateral transtension with severe crustal thinning during the latest Early Cretaceous and Mid-Cretaceous. Its inversion, which led to the uplifting of the Pyrenean chain, took place gradually, by transpression from east to west, during the Late Cretaceous followed by an Eocene collision. From historical ﬁ eldwork, this note mainly focuses on the rifting period at the scale of the Pyrenean domain. It clearly shows the fundamental role of the stratigraphic approach in structural interpretation. Although synchronous at the scale of the Pyrenees, the Cretaceous extension is more important in the western part of the chain, near the Bay of Biscay. It is therefore at this location, within the Tardets-Mauléon Basin, that the various stages of creation and evolution of the rift could be clearly recognized with, successively: (1) an epirogenic pre-rifting stage, during which the facies lines, submeridian during the Jurassic, gradually change into N110E during the Neocomian before giving way, in the Early Cretaceous (Barremian-Gargasian), to a moderate acceleration of subsidence and, consequently, to the gentle uplift of a southern and western margin, future High Primary Belt (also often called Axial Zone) and Landes Plateau, initiating the next stage; (2) a major, brittle, newly identi ﬁ ed rifting stage in the Latest Aptian (Clansayesian) and Early Albian. It is characterized by the installation of elongated lows on the downthrown side of transverse N20 normal faults, active diapirism of the Triassic evaporites and central detachment of the post-Triassic cover. During the Mid-Albian, a major


Introduction
The North Pyrenean Cretaceous Flysch Trough is a major element in the structure and geological history of the Pyrenees insofar as it is located at the articulation of the Iberian and European margins, the collision of which gave birth to the orogen that we know today (Fig. 1). This area of almost permanent crustal instability has, in recent years, been the subject of much work, particularly in the context of recent multidisciplinary projects such as the Pyramide, Orogen and RGF projects. Despite the results obtained at the end of this work especially on a geophysical point of view including passive imaging (Chevrot et al., 2022), differences remain in the modalities of creation and evolution of this complex trough during the Mesozoic as well as the role it subsequently played in the establishment of the Tertiary Pyrenean chain.
In most recent works, the deformation has been exerted along a vast sub-linear zone, running from the Atlantic to the Mediterranean, bordered by two hyper-stretched continental margins, allowing the exhumation of the mantle in several places. Two distinct evolutionary models are retained: the model of cold, deep passive margins, inspired by the Alpine or Atlantic margins (Jammes, 2009;Jammes et al., 2009, 2010a, 2010b: Lavier and Manatschal, 2006Lescoutre and Manatschal 2020;Masini, 2011;Masini et al., 2014;Tugend et al., 2014;Lescoutre et al., 2021;Saspiturry et al., 2021) and mainly applied to the western Mauléon Basin. The Cretaceous extension is accompanied by the tilting of crustal blocks above one or more detachment surfaces with a very low dip towards the north. These surfaces locally lead to mantle exhumation. The dimension of the crustal blocks, generally large in the proximal part of the margin, becomes smaller in the distal part (Fig. 2); the model of hot passive margins with a crust made ductile by stretching over its entire thickness, carrying "gently sloping basins" (Bodinier et al., 1987(Bodinier et al., , 1988(Bodinier et al., , 2004Clerc 2012;Clerc and Lagabrielle, 2014;Clerc et al., 2015;Corre et al., 2016;Duée et al., 1984;Lagabrielle and Bodinier, 2008;Lagabrielle et al., 2010, 2016, 2019a, 2019b: Manatschal, 2004Manatschal and Bernoulli, 1998;Saspiturry, 2019;Teixell et al., 2016Teixell et al., , 2018. In detail, two major detachments can be distinguished: one, infracrustal, separates the lherzolitic mantle from its Paleozoic cover, the other within the Triassic evaporites causes the generalized detachment of the post-Hercynian series and its centripetal glide. Breccias resulting from the underwater disintegration of the stretched crust are often found mixed with the Cretaceous flysch (Fig. 3).
In previous works (Boirie and Souquet, 1982;Canérot and Delavaux, 1986;Canérot, 1988: Canérot and Lenoble, 1993Canérot et al., 2012;Debroas, 1978, 1987, 1990, 2003Debroas et al., 2010;James and Canérot, 1999;Lenoble and Canérot, 1992;Souquet et al., 1985), the Mid-Cretaceous N-S distension was accompanied by a modest left-lateral strike-slip motion between Europe and Iberia, which could possibly lead to a transpressive component in the most eastern sectors. The North Pyrenean Trough is thus considered to be made up of several "pull-apart" type basins separated from each other by relay zones with thicker crust and shallower facies successions. The Albo-Cenomanian tectonics only led to mantle exhumation in the western Tardets-Mauléon Basin where the distension and the gravimetric signature are the strongest. Subsequently, from the Upper Senonian, the Pyrenean tectonics resulted in an inversion of the Mid-Cretaceous transtensive structures under a diachronous transpression regime initiated in the east and gradually propagating towards the west.
In the present work, we propose to recall the arguments which plead in favour of this last interpretation, which will be consistent with the observed sedimentary history. Our review will first concern the Tardets-Mauléon Basin which, by its western position and its extension, constitutes a key element for our regional interpretation. We will then focus on a selection of the main eastern basins (Lourdes-Bagnères, Baronnies, Camarade, Ballongue, Aulus, Quillan and Saint-Paul de Fenouillet-Boucheville). Despite a stronger deformation and certain structural and geodynamic specificities, they still retain elements well integrating into a logical evolution of the entire North Pyrenean Trough (Fig. 1). Finally, we will see that this evolutionary pattern can find the support of a similar evolution in the less deformed South Aquitaine, Parentis and Basque-Cantabrian Basins.

The North-Pyrenean Tardets-Mauléon Basin (Western Pyrénées)
Roughly amygdaloid, this North Pyrenean Basin covers an area of over 300 km 2 . It rests to the south on the Hercynian basement of the High Primary Belt, to the west on the Landes Plateau and to the north on the Grand Rieu ridge which provides a transition with the more northerly marginal Arzacq Basin (Fig. 4). The chronology of its extension (lateral onlap or erosional surfaces), the geometry of its filling and the quality of its control (outcrops and various boreholes) make it possible to offer a faithful reconstruction of its geological history.

Installation and geodynamic evolution
Five major stages mark out the post-Hercynian geological history of the Tardets-Mauléon Basin, stages materialized by distinct genetic units, separated by major unconformities of eustatic origin in an active tectonic context: subsidence or uplift with erosion (Souquet et al., 1985).

Pre-rift stage: Triassic to Aptian
Above an eroded Hercynian base, a thick series develops (up to 1000 m), mainly composed of carbonates resting on Upper Triassic evaporites. The presence of this ante-rift salt bearing series plays a fundamental role by decoupling two different structural styles, infra-and supra-saliferous, and playing a role of deformation damper. The mobility of the evaporites, from gentle cushions in the Neocomian evolving into diapiric ridges in the Albian, is an indicator of the intensity of tectonic activity.
The submeridian facies lines in the Jurassic gradually turn to N110E during the Latest Jurassic/Neocomian and the still modest extension is accompanied by an acceleration of subsidence and uplift announcing the future High Primary Belt to the south and the Landes Plateau to the west. The Barremian reaches more than 500 m in thickness on an axis established in the heart of the Orthez (Bellevue borehole), Arzacq then Tarbes Basins (Synthèse Pyrénées, AGSO and BRGM, 2018). On the rising margins, the Jurassic is exposed and eroded (global intra-Valanginian unconformity) then transgressed by Early Cretaceous formations ranging from the Barremian to the Aptian (Canérot et al., 1978).

Major brittle rifting stage: Clansayesian (Latest Aptian-Early Albian)
Although foreseen in works concerning the relative displacement of Iberia related to Europe (Tuckholke et al., 2007;Gong, 2008;Gong et al., 2008), this major, brittle rifting stage has hardly retained the attention from the authors who subsequently worked in the basin considered here. However, it turns out to be fundamental.  Lavier and Manatschal (2006) deals with a 30-km thick crust and a ductile decoupling middle crust. The upper crust includes the pre-rift Fms (after Masini, 2011).
The regional transtension (NW-SE distension accompanied by sinistral strike-slip dominating on the N110 directions) leads to the dislocation of the Jurassic and Early Cretaceous platform along predominantly transverse N20 directions (Canérot and Delavaux, 1986;Canérot, 2017). These N20 directions (St-Jean Pied-de-Port, Saison, Barlanès, Ossau) act as normal faults, favouring the development of elongated N20 half-grabens, the diapirism of Triassic evaporites ( Fig. 5C and D) and the centripetal detachment of the Jurassic cover (Fig. 6A). On the low side of the fault blocks, the variations in thickness are important, highlighted by the sudden lateral change between platform Melobesia Limestones (Vimport facies with Agardhiellopsis cretacea LEM. and Kymalithon belgicum LEM. and EMB.) and basinal black marls with sponge spicules. Though slumped northward, probably at the end of the considered rifting stage, the Lichançumendy massif is a well preserved example of block tilting related to wrench faulting (Fig. 7). Here, on the southern border of the Tardets-Sorholus Basin, the Clansayesian inner carbonate platform interfingers northwestward with black spicule marls. Prograding reefs and mudmounds develop at the transition between inner and outershelf.
It is important to recall that the amount of left-lateral motion is modest as shown by the lack of a continuous North Pyrenean Fault zone in the western Pyrenees and the absence of important offset of the meridian facies in the structural units lying south of the Eaux Chaudes.
In the Central Pyrenees, where the stratigraphic evolution is very comparable (Souquet and Peybernès, 1991), the latter yielded ammonites from the Mid-to Late Clansayesian (Hypacanthoplites zone) and from the Early Albian (Tardefurcata zone). This evolutionary pattern is particularly well illustrated by the Arbailles fault block, tilted towards the SE, in the direction of the Saison fault ( Fig. 5A and B). Extentional crustal faulting results in the creation of severe sedimentary slopes on which "mudmound-type" constructions can develop. They are well preserved in the eastern end of the Mail Arrouy fault block (Arudy) tilted towards the ENE. This fundamental step therefore gradually leads to the settlement of a black marl basin in a deep and anoxic environment with a complex bathymetry pattern made up of blocks tilted in transverse submeridian directions. The depocenters at this stage are still small compared to the next deepening stage of the Black Flysch. Bordered to the south by the High Primary Belt and to the west by the Landes Plateau/Basque Massifs, they do not yet communicate with the Basque-Cantabrian Basin except by shallow sea environments (Fig. 6B).

Late rifting stage (Mid-Late Albian and Early Cenomanian)
It is characterized by a major tectonic and stratigraphic reorganization during the Mid-Albian, and we will see that this major event presents a very broad regional extension. The mantle rise of Urdach is dated from this episode (see below). The uplift of the margins is accelerating with erosion and the arrival of siliciclastic flysch-type sedimentation in a basin whose only depoaxis is newly oriented NW-SE. Extension now becomes moderate and submeridian, concentrated on a few major basin border faults. Mantle cooling causes the rapid deepening/collapse of the entire basin which combined with the eustatic rise, makes the Black Flysch Group to transgress widely, particularly towards the west where the connection is made with the Basque-Cantabrian Basin (outcrops, boreholes of Hasparren and Ustaritz). This new Mauléon Basin is centred on the town of Tardets-Sorholus where its thickness reaches 4 km (Fig. 8A). It will be noted that the depoaxis has moved further south than the Barremian one of the pre-rift stage. The sediments unconformably cover their deeply faulted Albo-Clansayesian or older substrate. The salt tectonics continues with the submeridian, centripetal slide of the ante-rift blanket on a sole of Triassic clay and salt. The diapirism seems less active, limited to the central part of the basin along the Saison fault (Roquiague diapir). The end of this episode is marked by the eustatic Mid-Cenomanian unconformity in the context of ending tectonic activity, the deformation only concentrating on a very few major border faults with limited throw (Fig. 8B).
The dominant turbiditic sedimentation is accompanied by slope conglomerates present on the active asymmetric southern border, the Igounce and Mendibelza fan deltas, facing the heart of the Tardets Basin. They progressively seal the steep fault steps of the Iberian margin and are dated Latest Early Albian to Middle and Late Albian (Boirie and Souquet, 1982). During the Vraconnian (Latest Albian) fanglomerates triggered by an eustatic drop event reached the central part of the basin (Erretzu pebbles and cobbles) testifying to the gradual filling of the basin.
It is also at the end of the Vraconnian and at the very beginning of the Cenomanian (Souquet et al., 1985), that the famous Urdach breccias are deposited. They rework lherzolite elements testifying to the existence of a syn-sedimentary mantle exposure at the seafloor, as recognized by the entire scientific community (Roux, 1983;Duée et al., 1984 Fault line scarp et Masini, 2011;Canérot et al., 2012;Corre et al., 2016;Canérot, 2017;Lagabrielle et al., 2019a). Therefore, the Urdach-Les Pernes site deserves a specific description (see below) because by itself it summarizes the history of the Tardets-Mauléon Basin.
2.1.4 Post-rift stage (Late Cenomanian to Senonian p.p.) Crustal faulting associated with rifting is now practically inactive and the basin, centred on the northern sector of Mauléon ( Fig. 9A) continues to subside and spread laterally in all directions. The submarine topography is smoother, and oxygenation improves (Grey Flysch). The sedimentation, represented by a carbonate then clay-sandstone flysch, admits lateral equivalents of limestones and breccias. These breccias are essentially triggered by collapses of the carbonate margin at the edge of the basin during eustatic falls, they will only cease with the flooding of the "Canyon Limestone" platform during the Campanian maximum transgression (Navarelles calcschists). The presence of the Col d'Osquich megaturbidite, of Turono-Coniacian age and of southern origin, is known from the Barlanès valley (Les Pernes) to the Atlantic coast, testifying to the size of the sedimentary receptacle and the absence of notable internal fracturing within the new basin ( Fig. 9B).

Pyrenean structural inversion stage (Late Cretaceous [?] and Eocene)
The structures emplaced by transtension during the Mid-Cretaceous period are taken up in transpression (N-S compression with a NNW-SSE dextral component) from the Late Cretaceous and/or the Paleocene. These structures appear better expressed in outcrop around the lherzolite massif of Urdach-Les Pernes (see below), or on the peripheral area (Ursuya, Arbailles, Sarrance). It is also at this time that the inversion of the old border faults of the Black Flysch Trough leads to thrusts of regional extension directed in opposite direction, to the north (North-Pyrenean Thrust) and to the south (Lakhora Thrust) (Canérot, 2008: Jammes, 2009Masini, 2011;Corre, 2017;Saspiturry, 2019;Lagabrielle et al., 2019b). The N20 transverse faults are also reactivated into dextral transfer faults (St-Jean-Pied-de-Port, Saison, Barlanès) or even thrusts (Ossau).

The Urdach-Les Pernes lherzolite Massif
Running for about 20 km between the Ossau and Aspe valleys, the Mailh Arrouy has retained its block structure tilted towards the east with a thick (2000 m) syn-rift wedge which disappears to the west under the post-rift Cenomanian flysch cover. The Urdach-Les Pernes lherzolite Massif thus appears at the head of a large, tilted block associated to the submeridian Barlanès Fault. It corresponds as well to the eastern edge of the Albian Tardets-Mauléon Flysch Trough. This sector is of particular interest because it summarizes by itself the geological history of the Tardets-Mauléon Basin (Fig. 10), namely: the ante-rift stage (I), widely exposed in the reliefs of Bisarce (Jurassic) and Pic Bellevue (Lower Cretaceous). Protected by a basal Triassic detachment in the evaporites, the Mesozoic hardly underwent ductile deformation during the Mid-cretaceous extension phase. Contrary to the interpretation of Corre (2017) the major brittle syn-rift stage of Clansayesian-Early Albian age, characterized by the tilting of the Mail Arrouy block to the east with passage from Urgonian limestones to black marls (IIa) at the Soum of Ségu and diapirism of the Triassic evaporites at the level of the transverse structure of Asasp (IIb). It is at the end of this episode, around À105 to À108 Ma (Mid-Albian) that the Urdach's lherzolite is emplaced at shallow depth (dating by gabbros in Masini et al., 2014). After a rapid ascent (barely 1 Ma according to Azambre et al., 1991) through a thinned crust, the mantle was exhumed along the Barlanès Fault at the seafloor under low temperature conditions as indicated by the presence of ophicalcites. Recent studies (Ducoux et al., 2021) show that in the North Pyrenean hyper extended rift system HT/LP metamorphic event occurred during this major brittle syn-rift stage, inducing high extension and a resulting low sedimentation rate; relations with the Paleozoic basement and the lherzolitic mantle undoubtedly exhumed by hyper-extension at the end of the Lower Cretaceous. This is a unique setting in the entire Pyrénées.

Discussion. The Mid-Albian unconformity in the Tardets-Mauleon Basin
In the recent works concerning the geodynamic evolution of the considered basin and specially for its southern part (Arbailles area), the authors provide different models and ages for the Cretaceous rifting stage, ranging from ante Albian (conceptual model of "raft tectonics" in Lagabrielle et al., 2010, Fig . 13A), Albian Masini et al., 2014, Fig 13B) or Albian to Cenomanian (Clerc and Lagabrielle 2014;Corre et al., 2016;Lagabrielle et al., 2019a;Teixell et al., 2016;Saspiturry, 2019;Saspiturry et al., 2021). The present proposal of a Mid-Albian unconformity led us to separate two rifting stages: the Clansayesian-Lower Albian brittle main rifting stage is herein mentioned for the first time in the considered area. suddenly overlain everywhere by the turbiditic Flysch Noir.
On the northern border of the Igountze Massif (Fig. 4) the same unconformity separates (Canérot, 2008) the Palaeozoic basement from the overlying Late Albian fossiliferous (ammonites) Flysch including breccias and Clansayesian carbonate olistoliths (Fig. 13D). In our interpretation this transitional step is also underlined by the northward gravity sliding of the Lichançumendy, Arguibelle, Layens and Ourdinse units (location in Fig. 4). The resulting Mid-Albian unconformity is likely a composite unconformity aggregating several onlapping sequences of eustatic origin in an active tectonic setting. Below biostratigraphic resolution its precise age is here difficult to define further.

The basins of the Central and Eastern Pyrenees
The Pyrenean deformation, increasing from west to east, does not allow an analysis comparable to the one proposed for the Tardets-Mauléon Basin. However, in the light of the observations made in this western sector, we can propose a stratigraphic, structural, and geodynamic interpretation for five of the main basins located east, those of Lourdes-Bagnères, Baronnies, Ballongue, Aulus and Saint-Paul de Fenouillet-Boucheville, which belong of the Metamorphic Internal Zone ("slate" flysch). It will continue with the summary review of the more northerly basins of Camarade and Quillan, characterized by an "external", non-metamorphic black flysch and established within the North and Sub-Pyrenean Zones (Fig. 1).

The Lourdes-Bagnères Basin
Located east of the Tardets-Mauléon Basin, it has been little studied in recent years due to the absence of outcrops of lherzolite (Fig. 1). Yet it was the subject of in-depth stratigraphic and structural studies (Souquet and Mediavilla, 1976;Debroas, 1978;Debroas et al., 1978) during the preparation of the geological maps at 1:50,000 of Lourdes (BRGM, N°XVI 46, 1970) and Bagnères-de-Bigorre (BRGM, N°XVII 46, 1989). We will simply recall the characteristics that allow its integration into the regional geodynamic diagram of the North Pyrenean Trough proposed here.
The major Clansayesian-Early Albian syn-rift stage is well illustrated in particular in the western part of the domain, at the level of the Gave de Pau valley where the Urgonian platform is affected by a N20 transverse set of faults, known as the Lugagnan Fault Zone, accompanied by diapirism (Aspin and Cité de l'Ophite) and probably causing the outcropping during the Albian of the Paleozoic basement (tectonic lense of the Pic du Ger above the Cité de l'Ophite and breccias with large Paleozoic elements from the Pont Neuf of Lugagnan). The following late rifting stage is mainly represented by the accumulation of a thick Albo-Cenomanian series (over 3000 m) of Black Flysch accompanied by border breccias (Jarret, Neuilh, Bagnères), onlapping towards the east and the south the Early Cretaceous and Jurassic bedrock (basal unconformity of the Black Flysch). Approaching the transverse, eastern fault of Bagnères, these weakly metamorphosed sediments directly cover the basement largely exposed. Small syenite massifs of mantle origin, contemporaneous with this flysch have been mentioned, including a new one at the Montagne de Castets, inserted in the Lugagnan fault system. Note that the Neuilh Breccia contains a matrix of eruptive material (Capdecomme et al., 1965). The existence of Mid-Cretaceous folds associated to the opening of the basin by transtension, however, has not been clearly demonstrated.

Saint-Palais Fault
The presence of numerous directional accidents characterizing the N80 "Bigorre Fault" masks the relationship between the Black Flysch and the Senonian flysch on the southern edge of the trough. As in the previous example, these Late Cretaceous sediments reflect the post-rift widening of the trough towards the south and the High Primary Belt.
The Pyrenean tectonics is clearly reflected in the reactivation in transpression (dextral W-E strike-slip fault) of the old transtensive faults, the Lourdes-Bagnères Fault in the north and the Bigorre Fault in the south. The echelon folds and the SW-NE thrusts, perfectly preserved in the western part  of the basin, on the outskirts of Lourdes, are a perfect illustration of this. All of these deformations give the basin a characteristic "pull-apart" geometry.

The Baronnies Basin
Established in the central part of the North Pyrenean Trough (Fig. 1), the Baronnies Basin has been the subject of detailed mapping, supported by stratigraphic and sedimentological data (Souquet et al., 1985;Azambre et al., 1991;Debroas, 1990Debroas, , 2003Canérot et al., 2012). Its study is therefore very valuable for the overall interpretation of the North Pyrenean Trough.
The basin fill (Fig. 14) involves syn-rift sediments of the major brittle phase (Urgonian and Clansayesian complex 1000 to 2000 m thick), followed by a thick late syn-rift discordant series (4000 m of weakly metamorphic Black Flysch), then by a post-rift succession (Cenomano-Turonian grey flysch) and finally Sub-Pyrenean sedimentary formations (Turonian to Eocene). The Black Flysch is here dated Mid and Late Albian to Early Cenomanian by ammonites and calcareous nannoplankton (Debroas, 2003). It should be noted that the breccias including Paleozoic elements recently discovered on the working face of the Beyrède-Jumet marble quarry (Ducoux, 2017), within the Cenomano-Turonian carbonate flysch, reflect, the same as in the Tardets The study of the Baronnies Black Flysch reveals the presence of a narrow primitive south-eastern depocenter, of Clansayesian and Early Albian age, SW-NE oriented, which gradually widens towards the NW by integrating younger crustal panels. The presence of discordant and diachronous border breccias, associated with each of the three sequences that make up the Black Flysch Group (Batsère, Esconnets and Mauvezin breccias) testifies to the individualization of steep normal faults dipping to the SE. They reflect the rapid collapse of the basin and its enlargement. These breccias rework the Hercynian metamorphic basement (Cambro-Silurian) from the Bagnères-de-Bigorre massif. The latter separates the Baronnies Basin from the Lourdes one to the west. The metamorphism associated with crustal thinning thus generated is greater in the SE (mesozone) where the thickest flysch cover is observed, rather than in the NW (anchizone), with less flysch cover (Azambre et al., 1991see also Ducoux, 2017, p. 420, with Raman temperature ranging between 310 and 430°C). Regional Pyrenean metamorphism therefore clearly results from an interaction between thermal flux associated with rifting, and syn-and/or post-rift sedimentary thickness, which explains its variable age from basin to basin (from Late Albian to Early Senonian). This static metamorphism is thus associated with a S1 foliation parallel or subparallel to stratification (Ducoux, 2017). We note that the Avezac peridotite (avezacite) is located in the heart of the depocenter, at the articulation of the tilted blocks of Prat and Bourg. The total absence of breccias in the neighbouring Black Flysch supports the interpretation that this mantle rock remained in depth during rifting and was only later thrusted with the Mesozoic section, during Pyrenean compression. Thus, created by left-lateral transtension in the Cretaceous (Clansayesian-Early Albian) along EW border faults (Neste to the north and Bigorre to the south), the Baronnies Basin was subsequently inverted during the Tertiary along these border faults, this time under dextral transpression (Fig. 14). The better control than in the case of the Tardets-Mauléon Basin, undoubtedly gives this basin the character of a "pull-apart" type basin.

The Ballongue Basin
It is a narrow North-Pyrenean Basin, squeezed along the North-Pyrenean Fault of the Central Pyrenees (Fig. 1) and belonging to the Metamorphic Internal Zone. Relatively well preserved in its western part, on the two slopes of the Bouigane valley, it stretches further to the east in various tectonic slices pinched between the North Pyrenean Ariège, Castillon and Arize massifs (Fig. 15).
As in the examples of the Baronnies, Lourdes-Bagnères and Tardets-Mauléon Basins, the ante-rift substrate is, this time again, made up of carbonates Jurassic and Lower Cretaceous in age. The major Clansayesian-Early Albian rifting stage is represented by the fracturing of Melobesia Limestones interfingering with black marls (Debroas and Souquet, 1976;Debroas, 1985). This series (0 to 100 m thick) only exists locally on the northern flank of the basin where it is truncated by the cartographic unconformity of the Mid-Albian which underlines the importance of the erosion preceding the deposit of the Black Flysch (Debroas, 2009).
The late syn-rift Black Flysch Group is dated from Mid-Albian to Late Albian and possibly to basal Cenomanian (Debroas, 1987(Debroas, , 2009. The Black Flysch is here also called "slate flysch" because of the strong metamorphism that affects its entire section, whose thickness can reach 3000 m. These schists cover the previous series in angular discordance, sealing the previous faults. On the margins of the basin, the Flysch Noir is replaced by reduced series on eroded and karstified bedrocks: Balacet limestone to the south, on the High Primary Belt, marls and olistolites from Lachein to the north, karst and "red series" on the Arbas block, emphasizing once again the transgressive character of the Late Albian. This transgressive slate flysch involves breccias at its base that are divided into two essential groups (Fig. 15A): the eastward onlapping Castel-Nérou breccias (up to a few hundred meters thick) which mainly contain Aptian and Clansayesian limestone elements and mark out SW-NE trending paleo-faults; the Alos breccias with Mesozoic or Paleozoic and often granitic elements on the southern edge of the basin, also deeply fractured by the North Pyrenean Fault. It should be noted that despite the presence of eight small massifs of lherzolite, no element of it has been found resedimented in the Black Flysch (Debroas, 1987) which leads to consider that the mantle was not brought to seafloor during the rifting period although the continental crust had probably been severely thinned considering the considerable  Fig. 15. Simplified geological map of the Ballongue Basin (after Debroas, 1987). sediment thickness and the local level of rather high metamorphism and the presence of the second strongest gravity anomaly after the Mauléon Basin.
The location of these breccias, their composition and their relationship with the Black Flysch allow us to characterize the Ballongue Basin as a triangular basin, limited, like those of Lourdes and Baronnies, by faults active during its establishment, mainly in the Early Albian and rapidly subsiding during the Late Albian. As in the western examples, the current directions are centripetal indicating that the Albian basins are still isolated, anoxic and poorly communicate with each other. The E-W North-Pyrenean Fault acted as a strike-slip fault with a 20 km left-lateral offset as shown by the current position of the Rogalle granite (Fig. 16A), the only possible source of the Alos breccias (Debroas, 2009). The north-western transverse border fault played as a normal fault downthrown to the SE. Several other accidents of the same orientation, made up of ante-flysch terrains and tilted towards the NW are known in the central part of the basin. Finally, the Cap de Broc fault, with a SE-NW orientation, acted as another left-lateral strike-slip fault, allowing the basin to widen towards the NW. The sedimentary polarities (Debroas, 1987) are consistent with such a basin geometry (Fig. 16B). Thus interpreted, the Ballongue basin fits well with a triangular pull-apart basin associated to a W-E strike-slip motion whose 20 km offset is estimated.
The post-rift cover of Late Cretaceous age is absent by erosion in the Ballongue Basin itself but has been preserved in the marginal basins of Arbas to the north and Uchentein to the south, where a weakly metamorphosed fucoid flysch of Lower Senonian age has been recognized. The Tertiary Pyrenean deformation subsequently deeply affected the entire region considered here. The structural organization of the tectonic slices that surround the Castillon massif seems compatible with its SE-NW, dextral motion along the North-Pyrenean Fault (Fig. 16B).

The Aulus Basin
This new flysch basin follows a N110E direction between the Trois Seigneurs Massif and the High Primary Belt to the north and south respectively. Deeply deformed by Pyrenean tectonics and intensely metamorphosed, its infill reveals a series comparable to that of the units which have just been described further to the west (Fig. 17): a set of ante-rift series including Triassic, Jurassic and Lower Cretaceous (Clansayesian-Early Albian); a fragment of Albo-Cenomanian Black Flysch preserved in the only tectonic lense of the Oust forest; finally, various pinched lenses of Senonian fucoid flysch on the northern front of the High Primary Belt.
The Aulus Basin was made famous by the presence of several small massifs of peridotites of mantle origin called lherzolites (Lacroix, 1894(Lacroix, , 1900  and even would have been reworked in Albo-Cenomanian sedimentary marine breccias, known as "Lherz B." (Bodinier et al., 1990;Lagabrielle and Bodinier, 2008;Clerc, 2012;Clerc et al., 2012Clerc et al., , 2015Lagabrielle et al., 2016;Uzel et al., 2020); for others the same lherzolites would have remained below the infra-saliferous Triassic décollement surface (Colchen et al., 1997;Ternet et al., 1998;Canérot et al., 2012;Debroas, 2003;Debroas and Azambre, 2012;Debroas et al., 2013). According to us, the Lherz breccias, actually involve three types of formations: the Agnes Pass breccias of diapiric origin (collapse breccias), probably of Early Cretaceous age and completely independent from the outcrops of lherzolites. The local section shows clearly rough megabreccias reworking Jurassic limestones and dolostones ( Fig. 18B and G). Close to the pass, metamorphic limestones of Muschelkalk age, occupying the core of the diapir (under the Triassic salt level) are well bedded and devoid of breccias (Fig. 18H); the cataclasic breccias (the true Lherz breccias) located close to the different lherzolite bodies. They developed in contact with the peridotites when they were still intracrustal and suffered metamorphism towards the Mid-Cretaceous period (Ternet et al., 1998;Canérot et al., 2012). They rework metamorphosed angular carbonate (mainly Jurassic) and lherzolitic elements and are devoid of stratification ( Fig. 18A and B). For us, their interpretation as sedimentary breccias located at the base of the Albian flysch (Lagabrielle and Bodinier, 2008;Clerc et al., 2012;Lagabrielle et al., 2016;Uzel et al., 2020) cannot be accepted; and finally the Coumettes breccias, more recently defined (Debroas et al., 2013), outcropping along the Coumettes gully ( Fig. 18C-F). They correspond to local pocketlike sub-horizontally bedded infillings at the surface of the Jurassic carbonates and of the cataclasic breccias, involving coarse brown-coloured breccias, conglomerates, sandstones and siltstones. They are considered as posttectonic, continental sediments of karstic origin and Tertiary age.
So, in our opinion, within the Aulus Basin, there is no evidence of an underwater mantle exhumation at seafloor during the Albian. It is to be noted that the only Black Flysch that is preserved in the Lherz region (about 10 km west in the sector of the Ustou forest) is completely devoid of breccias. It has been established (Clerc et al., 2015;Lagabrielle et al., 2016) that the Jurassic carbonates outcropping around the peridotites underwent a high thermal metamorphism (600°C) during the Late Cenomanian and the Turonian, implying the existence of a thick post-Jurassic sedimentary cover now disappeared by erosion. The Senonian flysch preserved in the tectonic fault steps against the High Primary Belt is, on the other hand, little metamorphosed.
The tectonic complications and the absence of complete Early Cretaceous cover make it difficult to assess the Albo-Aptian left-lateral transtension, previously well illustrated by the Baronnies   by the "en echelon" arrangement of large fold axis with a dominant W-E orientation (Fig. 18A).

The St-Paul de Fenouillet-Boucheville Basin
In the eastern part of the North Pyrenean Trough, the Mid-Cretaceous extension between Europe and Iberia has given rise to a complex basin stretching from west to east between the sub-Pyrenean Mouthoumet Massif and the High Primary Belt (Fig. 1). Two sub-basins can be distinguished, that of Saint-Paul de Fenouillet to the north and that of Boucheville to the south, separated by the intermediate Agly massif (Chelalou et al., 2016) (Fig. 19).

Geodynamic evolution
The intense deformation and the high degree of metamorphism, especially in the southern sub-basin (Boulvais, 2016), make it difficult to assess the different stages of rifting. The major syn-rift stage seems however marked, here again, by the Clansayesian dislocation of the Urgo-Aptian platform leading to tilted blocks with migration of Triassic evaporites along with transverse N20 (?) or N110 active faults, the creation of steep slopes as well as the sudden lateral passage from limestones to spicule-bearing black marls. The late syn-rift stage follows, illustrated by the deposition of a Black Flysch now very deeply eroded and absent in the southern Boucheville sub-basin. On the northern edge of the St-Paul de Fenouillet sub-basin, approaching the Mouthoumet Massif, one can nevertheless observe (Peybernès, 1976) the transgressive and sometimes discordant nature of this flysch on its Albo-Aptian substratum, thus highlighting, once again, a Black Flysch basal angular unconformity of Mid-Albian age. Mid-Cretaceous folding has been reported, probably associated with this left-lateral transtension (Clerc and Lagabrielle, 2014;Chelalou et al., 2016). The Late Cretaceous cover is not preserved within the Saint-Paul de Fenouillet-Boucheville Basin but we know (Bilotte, 1985) that it onlaps further north to gradually overlap unconformably the Triassic, Jurassic and Lower Cretaceous cover of the Mouthoumet Massif.
Pyrenean transpression affects the entire remaining infill of the Saint-Paul de Fenouillet and Boucheville Sub-Basins. The authors agree that the compression began here as early as the Senonian (Late Santonian to Early Maastrichtian) as in the whole Eastern Pyrenees (Bilotte, 1985;Charrière and Durand-Delga, 2004;Durand-Delga and Charrière, 2012).

Discussion: question about the age and origin of the breccias outcropping in the Agly Massif
The Agly Massif shows different outcrops of breccias, especially developed in its central part (Fig. 19A), near Ansignan (Serre Vergès) and Lansac (Le Roc de Lansac) and more to the south, close to the Agly valley (Roquo Roujo). In the geological map of Rivesaltes, these breccias have been considered as post Albian and possibly Eocène in age (Mattauer and Proust, 1962;Fonteilles et al., 1993). These authors argued that the reworked elements affected by the Mid-Cretaceous metamorphism were involved in an unmetamorphosed cement. In a recent work (Motus et al., 2022), the considered Ansignan, Roc de Lansac and Roquo Roujo breccias are interpreted as marine sediments closely related to the Mid-Cretaceous rift system leading by extension and coverbasement decoupling to the creation of the St-Paul de Fenouillet and Boucheville basins (Fig. 19B). Five main facies have been described. They overlie the Mesozoic cover (Triassic to Aptian pre-rift system) and interfinger with the syn-rift (Albian) infilling. This way, North-South extension leads to the creation of a southward necking zone in the European crust and to mantle exhumation along the Boucheville Sub-Basin.
Our geodynamic interpretation is quite different. For us, there is no flysch sediments preserved in the St-Paul de Fenouillet and Boucheville Sub-Basins and no evidence of mantle exhumation (breccias reworking the peridotites are unknown). As suggested by the authors of the Rivesaltes map, the Ansignan, Roc de Lanzac and Roquo Roujo breccias correspond to Tertiary continental sediments ( Fig. 1C and field photographs). The elements, originated from the underlying Mesozoic carbonates (to-day converted into narrow tectonic slices) are generally highly metamorphosed and diversely coloured, whereas the non epigenized cement is red-coloured by ferric oxydes.
These outcrops can be included within the tectono karstic brecciated zone which has been described in different outcrops running along the Pyrenees, from the eastern Corbières to the western Basque country (Canérot, 2008). They are closely related to the early rising of the Eocene belt along the North Iberian Fault Zone (European and Iberian facing crusts area). Karst evolution continued all along the Tertiary times, leading in some places to the present open karst systems.

The North Pyrenean Basins
In a more northerly position than the ones reviewed above there are several other Albian basins of variable extension, very weakly metamorphosed and separated from the latter, by a "median high" (Peybernès, 1976;Debroas, 1990Debroas, , 1995Souquet and Peybernès, 1991). They are established on a weakly thinned crust and devoid of lherzolites. Only two basins will be considered here (Fig. 1 (Fig. 20). The structural organization of the basin and the evolution of its infill (migration of the discordant flysch series towards the NW) is interpreted (Souquet and Peybernès, 1991)   This uplifted block is then finally transgressed, without brittle deformation, by a 100 m of Late Albian and finally by the Cenomanian series (Aragon, 1988). This tectonic setting, including the erosive discordance of the Mid-Albian and a late post-rift transgression recalls those just described, notably in the Ballongue Basin (Arbas "red series").

Comparison with the South Aquitaine, Parentis and Basque-Cantabrian Basins
This new interpretation of the Albo-Aptian rifting from the Tardets-Mauléon Basin to the Mediterranean is consistent with that which has been recognized in the southern Aquitaine subsurface and in the Parentis Basin or further west, in the Cantabrian Basin.

The South Aquitaine Basin
The South Aquitaine southern border corresponds to the northern faulted margin of the external Black Flysch (Souquet et al., 1977) that has triggered the emergence of the North Pyrenean Thrust Front during the Pyrenean convergence particularly because it was the site of diapirs and salt ridges (Triassic outcrops on the surface and hundreds of meters thick evaporites found in the subsurface in the boreholes of Belair, Ossun, Lannemezan for example). On the southern Aquitaine platform itself Early Albian deposits are very thick in the Arzacq-Lacq Basin, faulting is dense along its southern part suggesting gliding into the neighbouring Black Flych Trough. The regime changes from Mid-Albian when tectonic activity becomes limited to a few major basement faults and the platform is tilted down to the north. The southern margin along the future North Pyrenean Thrust Front is uplifted and eroded while Late Albian deposition becomes essentially limited to the compensation synclines of the Audignon and Maubourguet salt ridges which host more than 2000 m of Late Albian marls in the boreholes of Nassiet 2 and Audignon 4 (Serrano et al., 2006).

The Parentis Basin
The Parentis Basin benefits of a modest Pyrenean inversion making it easier to decipher its evolution during the rifting Cretaceous period in addition to abundant oil exploration data although for the most part remaining confidential (Biteau et al., 2006).
In general, this basin appears to be asymmetrical, plunging southward to the foot of the Landes Plateau. The supra-salt prerift series are affected by peripheral tearing/extension ("break away zones") and gliding which allows it to be superimposed  Souquet and Peybernès, 1991).

Mid Albian unconformity
at the bottom of the basin, almost without any brittle deformation (mild coeval compressional anticlines), on its highly stretched ante-salt base. The seismic data clearly shows the Mid-Albian unconformity sealing the previous faults and overlain by the draping Late Albian-Early Cenomanian Marl Series. These two distinct structural units, long mapped by petroleum geologists, were recognized by Jammes (2009) and applied to the Pyrenean domain to juxtapose supra-salt series and mantle. The tectono-sedimentary sequences described above are here remarkably illustrated seismically. We will find the same division in Mathieu (1986) (Elf-Aquitaine BCREDP Vol. 10, No. 1). Initiated at the end of the Jurassic and accelerating during the Neocomian, the extension culminates in the Early Albian with a general NW-SE direction. Coeval with the Landes Plateau uplift it results in a thick main depocenter oriented N60, the N130 direction (Antarès ridge) playing rather the role of a transform zone. The Late Albian Marl Series (which includes turbidite facies) is equivalent to the Pyrenean Black Flysch. Well documented from seismic and boreholes, its base (Mid-Albian unconformity) is clearly discordant and seals the previous faults, being erosive on the basin margins. As already described in the Pyrenean region this unit is largely transgressive (Fig. 21).
Two notable differences with the Pyrenean domain should be mentioned: the basin closing towards the east onshore does not allow any significant E-W strike-slip offset; the Marl Series devoid of faulting is clearly post-rift here. Rifting might thus have ceased slightly earlier in the Parentis than in the Pyrenean domain while its other connection around the south of the Landes Plateau was more successful and extended into the Late Albian.

The Basque-Cantabrian Basin
The Early Cretaceous geodynamic evolution of this wide western basin presents very strong similarities (Garcia-Mondejar et al., 1996;Martin-Chivelet et al., 2002;Poprawski, 2012;Poprawski et al., 2014) with those which have been described here before. The same development of a trend of small basins associated with active normal N20 faults during the Clansayesian and the Early Albian is described in the Basque-Cantabrian Trough along the Gernika coast. The role of these transverse accidents is reflected, as in the Tardets-Mauléon Basin, by lateral facies change between Urgonian limestones established on the crest of tilted blocks and black spicule marls in the lows. Here again, normal faulting is accompanied by diapirism of the Triassic evaporites (Bakio, Guernika). The Mid-Albian has been recognized as a period of regional tectono-sedimentary reorganization with uplift, erosion, and sedimentary hiatus (loricatus and lautus zones) as reported by Rosales (1999), Garcia-Mondejar et al. (2005), or Agirrezabala (2008). Then follow the Late Albian-Early Cenomanian Black Flysch deposits, several kilometers thick, characterized by a rapid deepening and the end of tectonic activity on most of the previous faults. It now occupies a large single N110 oriented basin and transgresses its substratum. Only the southern margin of the Landes Plateau and Cinco-Villas remain active at this time (Martin-Chivelet et al., 2002). Note, during the Vraconnian-Early Cenomanian, the return of  coarse sandstones (Cabo Villano Formation in Poprawski et al., 2014), a pulsation probably of eustatic origin, which evokes the Erretzu conglomerates in the Tardets-Mauléon Basin. The Pyrenean tectonic deformation, moderate, hardly alters the Cretaceous stratigraphic and tectonic organization.

Conclusion
The Mid-Cretaceous North Pyrenean Flysch Trough is composed of several basins established on a continental crust thinned by stretching between the European and Iberian plates. These basins are arranged in relays separated from each other by intermediate areas which have retained a thicker crust.
By its western position, its wide extension and the thickness of its infill, moderately deformed by Pyrenean convergence, the Tardets-Mauléon Basin has preserved the best evidence of this geodynamic evolution. It comprises a major brittle rifting stage of Clansayesian (Latest Aptian) to Early Albian age, newly highlighted and characterized by the dislocation of the ante-rift, Jurassic-Cretaceous platform, and its break-up into tilted fault blocks. The latter, generated by N110 directional accidents with a left-lateral component, are separated from each other by transverse normal faults directed N20 (transtension regime in NW-SE direction). These accidents lead to the formation of small basins interpreted as pull-apart basins. They are partially filled with proximal Melobesia Limestones passing laterally to distal black spicule marls. The presence of thick salt-bearing beds in the ante-rift series introduces a complex structural style where the extension triggers the mobility of the evaporites and can result in a sliding of the post-Triassic pre-rift cover towards the basin centre. During the Mid-Albian, a major unconformity, underlined by a surface of erosion announces a new structural organization, most probably related to a very rapid mantle ascent (Azambre et al., 1991). The following Late Albian to Early Cenomanian late rifting stage is only affected by a moderate and simple N-S extension. With the end of the mantle rise follows its rapid cooling and a generalized collapse of the expanding Black Flysch Trough while the uplifted Iberian margin (asymmetric rift) feeds a thick turbiditic sedimentation. This unit is largely transgressive. The previous extensive and widespread brittle phase is now replaced by extension limited to a very few active major border faults.
During the Upper Cretaceous, the post-rift stage is marked by the continued enlarging of a passively subsiding basin, devoid of any significant faulting. In the Tertiary, the transpressive Pyrenean tectonics reactivates the former N110 faults into thrusts with a dextral strike-slip component, in particular on the northern and southern margins of the Albian flych basin.
Established on the northern edge of the Tardets-Mauléon Basin, the Col d'Urdach-Les Pernes sector is of particular importance in terms of history of the North Pyrenean Flysch Trough. It is in fact the only place in the Pyrenees where the Mid-Cretaceous transtension undoubtedly led to the seafloor exposure of the earth's mantle. It may well be that the decollement of the supra-salt series toward the bottom of the basin depocenters have made it very difficult for the mantle to get across this structurally disconnected and mobile cover of Jurassic/Lower Cretaceous series explaining thereby the so frequent association between deep highly stretched crustal/ mantle rocks and Triassic/Jurassic strata.
Well illustrated in the Tardets-Mauléon Basin, a similar evolution, although obscured by the compressional deformation, can be recognized in the more eastern basins of Lourdes-Bagnères, Baronnies, Ballongue, Aulus or St-Paul de Fenouillet-Boucheville, as well as in the non-metamorphic North Pyrenean basins of Camarade and Quillan. The best evidence is actually preserved further away in the South Aquitaine, Parentis and Basque-Cantabrian Basins, where Pyrenean tectonics hardly disturbed the Mid-Cretaceous paleogeographic organization and the draping of most the faults by the Late Albian transgressive series. At this regional scale, we will note the alternating asymmetry of the rift basins around the Landes Plateau: Parentis basin down to the south, Basque-Cantabrian Basin down to the north and Tardets-Mauléon Basin down to the south. The corridor of the described Early Albian "pull-apart" basins is only found in the North Pyrenean Zone and in the Basque Basin (Lekeitio and Oiz) while absent on the adjacent platforms. It will be recalled that the extent of the strike-slip component is limited and does not significantly offset the Jurassic facies, transverse to the North Pyrenean Zone.
Thus interpreted, the North-Pyrenean Cretaceous Flysch Trough corresponds to a corridor of distinct pull-apart basins whose history and evolution are harmoniously integrated into the regional geodynamics between the Europe and Iberian plates with successively, an aborted attempt to open an oceanic basin extending the Bay of Biscay to the east, then a gradual closure, this time from east to west, of this basin giving way to the Pyrenean chain.
Enlarging the field for thoughts, we can consider that the Mid-Albian reorganization is probably an Atlantic event as a coeval rifting evolution could be followed further to the Goban Spur on the northwestern margin of the Bay of Biscaye offshore Ireland (Sibuet et al., 1985;Masson and Montadert, 1985) and even offshore Newfoundland in the Jeanne d'Arc Basin (Sinclair, 1995). Jumping to another Atlantic transform zone it might be linked to descriptions given offshore Guiana with a Mid-Albian major angular unconformity and erosion followed by the start of oceanic accretion of the Equatorial Atlantic (Sapin et al., 2016).