The NPZ shows three major geological features.

• (1) The metamorphic Mesozoic sediments are intimately associated with fragments of subcontinental mantle rocks and with very thin tectonic lenses of continental basement rocks.
• (2) The Mesozoic sediments were metamorphosed at low pressure and high- to very high-temperature conditions during the Albo-Cenomanian extension (Albarède and Michard-Vitrac, 1978; Choukroune and Mattauer, 1978; Golberg and Leyreloup, 1990). In the western Pyrenees, the maximal temperature recorded in the metasediments reaches ca. 350 °C and was measured in Albian sediments of the Saraillé massif (Clerc et al., 2015). The Cretaceous metamorphism was thus synchronous with extreme continental thinning and mantle unroofing.
• (3) The Cretaceous rifting led to the opening of basins filled with flysch-type sediments. Opening started during the Albian with disconnected basins filled with the so-called Flysch Noir; these basins were later connected into a wider trough during the Cenomanian (Debroas et al., 2010). A significant feature is that, during extension, the Triassic to Aptian shallow water prerift sediments were decoupled from their basement and were set in tectonic contact on top of the mantle rocks. Such a decoupling of the sedimentary cover was caused either by (1) gravity sliding of sediment rafts into the basins, or by (2) traction-assisted sliding during lateral extraction of the ductile crust (Clerc and Lagabrielle, 2014).

The Chaînons Béarnais correspond to a segment of the west-central ZNP, forming a series of E/W-trending fold-thrust structures in the Mesozoic succession, floored by a south-verging Pyrenean décollement in the Triassic evaporites (Teixell et al., 2016). In their western part, the Chaînons Béarnais comprise from north to south: the south-verging Mail Arrouy thrust and the Sarrance anticline and the north-verging Layens anticline (Castéras, 1970) (Fig. 1b). The base of the Mesozoic sequence is represented by the Triassic Keuper facies with shales, evaporites, breccias and ophites, followed by Jurassic to Aptian platform limestones and dolomites, and Upper Jurassic Aptian limestones, with shale intercalations in the Upper Liassic and Lower Aptian. These sediments represent the original prerift cover of the northern Iberian margin (Canérot and Delavaux, 1986; Canérot et al., 1978), now entirely detached from its original Palaeozoic basement (Lagabrielle et al., 2010; Teixell et al., 2016). The syn-rift Albian–Lower Cenomanian Flysch Noir is preserved within the synclines. This sedimentary succession is intimately associated with peridotite (lherzolite) bodies and scarce Palaeozoic basement exposures, most of them located at the base of the Mesozoic sequence (Fig. 1b). The mantle rocks were incorporated during the Cenozoic in the tectonic wedge as the result of the closure of the unroofed mantle domain. As a whole, the Chaînons Béarnais wedge represents the suture between the Iberian and the European plates (Lagabrielle et al., 2010; Masini et al., 2014, Teixell et al., 2016).

Four main lherzolite bodies are exposed in the western Chaînons Béarnais (Fig. 1b): (1) the Saraillé and (2) the Tos de la Coustette bodies in the core of the Sarrance anticline, (3) the Turon de la Técouère body at the base of the Mail Arrouy thrust unit, and (4) the Urdach body at the western end of the Mail Arrouy. In this paper, we report the results of detailed investigations on the Saraillé massif where the most complete collection of lithologies is exposed.

The Sarrance anticline displays a folded Mesozoic cover and two peridotite bodies associated with tectonic lenses of Palaeozoic rocks, outcropping respectively in the Tos de la Coustette and the Saraillé massif (Castéras, 1970) (Fig. 1b). In the Saraillé massif, the comprehensive association of mantle rocks, Palaeozoic basement rocks and Triassic to Albian metasediments forms a unit, 1 km × 1 km wide, thrust southwards over the Albian Flysch Noir. This unit corresponds to folded Mesozoic metasediments wrapping both mantle and crustal rocks (Fig. 2).

The 1/50 000 scale geological map of Oloron-Sainte-Marie (Castéras, 1970) provides a brief description of the peridotite bodies of the Sarrance anticline with a short interpretation of their geological setting. Following this first description, the peridotite bodies have been the subject of different interpretations:

• (1) they have been first regarded as the result of an early exhumation during the Mid-Jurassic times, followed by the deposition of unconformable Mesozoic sediments over a seafloor comprising ultramafic and Palaeozoic crustal rocks (Duée et al., 1984; Fortané et al., 1986);
• (2) peridotite uplift along Mesozoic faults and their incorporation with a slice of Palaeozoic rocks into the sedimentary cover during the Cenozoic tectonic inversion of the NPZ has been proposed by Canérot and Delavaux (1986);
• (3) the contact between the mantle rocks and the Mesozoic sediments has been attributed to detachment faulting during the Albo-Cenomanian extensional phase, with the Palaeozoic lens corresponding to a remnant of the hyper-extended crust (Lagabrielle et al., 2010).

The peridotites of the Saraillé massif form a 500 m-long and 100 m-thick lens-shaped body overlying 10–20 m-thick tectonic lenses of various continental rocks. Both the mantle body and the continental lenses are in tectonic contact with the Mesozoic sediments. At the top of the lherzolite body, the contact with Jurassic dolostones is outlined by a lenticular, strongly sheared, talc-rich layer up to 15 m thick (Lagabrielle et al., 2010). As discussed below, this layer is thought to be the record of intense fluid circulations coeval with mantle exhumation. The Saraillé mantle rocks are made of two types of peridotites: spinel-lherzolites and pyroxenites, mostly websterite. Pyroxenites are represented by numerous centimetre-thick parallel layers within the lherzolite body. These layers are part of an early mantle history of the lherzolite (Gaudichet, 1974) and are associated with refertilisation processes identified in other mantle bodies of the NPZ (Le Roux et al., 2007). Refertilisation by secondary melts, first described in the Lherz mantle body (central NPZ), involves addition of Cpx and dissolution of Ol at the expense of a primary harzburgite. Such a phenomenon is thought to have occurred during the ascent of the mantle in extensional context in lithospheric conditions (Kaczmarek and Müntener, 2010; Le Roux et al., 2007; Müntener and Piccardo, 2003). The Saraillé lherzolites are highly serpentinized with chrysotile and lizardite as dominant (95%) phases (Ferreira, 2013). The temperature of the fluids responsible for the mantle hydration is estimated at ca. 175 °C (Ferreira, 2013). In addition, the serpentinites are crosscut by late antigorite veins, resulting from the circulation of metamorphic fluids (Ferreira, 2013).

Gaudichet (1974) revealed the presence of a metamorphic mineral association of newly-formed muscovite, chlorite and albite, observed in the whole Mesozoic cover of the Sarrance anticline, including the Albian Flysch Noir. In addition, this author showed that the Albian flysch is characterized by the development of at least two cleavages. The earliest one (S1), parallel to bedding (S0), is underlined by the alignment of syn-kinematic muscovite and chlorite flakes. The metamorphic imprint is homogeneous across the Sarrance anticline and there is no increase of the metamorphic conditions towards the Saraillé lherzolite body. In their first detailed mapping of the Saraillé massif, Fortané et al. (1986) described the foliated talc-rich layer located between the lherzolites and the Mesozoic sediments. They evidenced the presence of talc, clinochlore and pyrite, an association representative of greenschist facies conditions (250–350 °C). Similar temperatures have been obtained recently by Raman spectrometry analysis of organic matter in the Aptian limestones and Albian flysch, the highest temperature being found close to the Saraillé lherzolite (Clerc et al., 2015).

As shown on the panorama in Fig. 2 and on the map and cross-sections in Fig. 3, our field investigations reveal that the structure of the Saraillé massif corresponds to a recumbent fold of Mesozoic sedimentary cover wrapping the main lens-shaped unit of mantle rocks and associated slices of Palaeozoic basement rocks. These Palaeozoic lenses also follow the fold geometry, but are absent at the upper contact of the peridotite body, which is there directly against the sedimentary cover. The geometry of the folded Mesozoic cover exhibits a very strong asymmetry between the normal and the reverse limbs, with stretching of the reverse limb shown by the considerable thickness reduction of the carbonate sequence. Other small lenses of crustal and mantle rocks occur in the northern part of the massif (Laünde Pass; Fig. 3). The mantle rocks, the continental basement rocks and the sedimentary cover are all separated from each other by tectonic contacts.

On its northeastern side, the Saraillé massif rests above brecciated Triassic sediments forming the core of the Sarrance anticline. Both the Saraillé massif and the Triassic sediments are thrust southwards above the Albian Flysch Noir of the Lourdios syncline by a subhorizontal contact (Castéras, 1970; Lagabrielle et al., 2010).

Our mapping reveals that various lithologies from the continental crust form thin, elongated tectonic lenses exposed beneath and south of the main mantle body on the western flank of the massif and, to the north, in a poorly exposed area at the Laünde pass (Fig. 3a). The western lenses display a well-developed tectonic foliation (Plate 1A) paralleling the contact with the peridotites (Fig. 3b). We collected twenty-six samples from these crustal lenses for microscope study. Seventeen samples are quartz-rich mylonites or chlorite- and mica-rich mylonites, deriving from Palaeozoic metasediments (Plate 1, B and C). Locally, the mylonitic foliation is crosscut and offset by decametric brittle faults and by cataclastic shear bands. Weakly deformed metasediments are rare: only one sample of quartzite and one sample of folded marble were collected. The remaining nine samples are undeformed quartz-albitites and one sample of granite.

Numerous millimetre-to-centimetre-thick albitite veinlets and veins cross-cut the foliation in mylonites close to the contact with mantle rocks (Plate 1C). In the eastern Pyrenees, albitites are known as a product of fluid/rock interactions during a syn-extension metasomatic event lasting ca.15 Ma between 100 and 98 Ma (Boulvais et al., 2007; Fallourd et al., 2014; Poujol et al., 2010).

The upper part of the Saraillé massif consists of a Mesozoic sequence including Triassic to Albian metasediments (Fig. 3). The Triassic is poorly exposed north of the massif and seems to be tectonically dismembered. It includes motley calcschists, cellular dolomite, brecciated limestones as well as dolostones and ophites. Ophite is a tholeiitic mafic igneous rock of dolerite type, characteristic of Triassic magmatic events (Castéras, 1970). Above the main peridotite body, the metasedimentary cover is base-missing and begins by a layer of Jurassic dolomite up to one hundred meters thick. This dolomite layer can correspond to the Callovian–Oxfordian, but with a lighter grey colour and a finer grain size than its regional facies. The Kimmeridgian–Neocomian corresponds to an up to 40 m-thick alternation of decimetre-thick beds of dolomite, limestones and phyllite-rich limestone. The thickness of the latter is much lower than the regional thickness (250 m in the northern limb of the Sarrance anticline). The Upper Aptian is represented by Urgonian facies platform limestones largely exposed on the top and southern flank of the Saraillé massif (Fig. 2). The Albian Flysch Noir follows the Urgonian limestones on the south-eastern flank of the massif and lies below the sole thrust of the massif. It comprises an alternation of black marls, silts and limestones.

The Jurassic to Aptian carbonates are strongly recrystallized, as well as the Albian limestones on a local scale. In the entire Mesozoic sequence, a S1 foliation always parallels the stratigraphic bedding (S0). The foliation, of variable intensity, is defined by the flattening of paleontological and sedimentological objects such as macrofossils, microfossils and biogenic clasts, perpendicular to the S0 plane. At the thin-section scale, flattening is not only observed in discrete deformed bands paralleling S0, but it can also affect the entire rock as observed in the carbonates sampled close to the basal contact with the mantle rocks (e.g., SAR 5 Plate 1D). The longest calcite veins observed in the carbonates parallel S0 and therefore also underline the foliation. A striking feature is that these veins are boudinaged, indicating extension in the S0/S1 plane consistent with the overall deformation pattern. Younger small veins cut obliquely to the S0/S1 planes. The Albian limestone beds in the footwall of the thrust also feature prefolding strong boudinage with thick interboudin quartz and calcite veins. As a whole, this deformation pattern suggests an early flattening of the sedimentary pile that occurred during the Albo-Cenomanian extensional phase.

3.4.1 Contact between mantle rocks and continental basement

3.4.2 Contact between mantle rocks and prerift metasediments

During the Albo-Cenomanian, the NPZ domain underwent extension that led to the formation of passive margins as the result of extreme crustal thinning and unroofing of the subcontinental mantle (Jammes et al., 2009; Lagabrielle et al., 2010; Tugend et al., 2014; Masini et al., 2014, Teixell et al., 2016). The Saraillé massif is unique as it records most of the successive stages of the tectonic processes leading to crustal hyperthinning. Based on field observations and microscopic analyses, our study allows us to reconstruct an evolutionary model for these processes.

Firstly, as revealed by the nature of the contact between mantle rocks and the sialic basement, crustal thinning has been accompanied by the activation of a major ductile detachment fault (Fig. 4). Motion along the detachment led to the individualization of tectonic lenses at all scales, limited by mylonitic shear bands and defining a phacoidal fabric. The important lithological variety of the crustal rocks of the Saraillé massif can be explained by the fact that tectonic lenticulation and extreme thinning of the crust predated mantle denudation. In other words, the crustal rocks have been sampled and left along the detachment during the extensional phase to form a succession of discontinuous lenses above the denudated mantle (Fig. 4). Ultimately, the detachment evolved toward cataclastic deformation due to exhumation processes in the latest stages of rifting (Fig. 1b).

Crustal thinning was synchronous with the sliding of the sedimentary cover towards the distal parts of the margins, the Triassic evaporites playing the role of an efficient detachment layer (Jammes et al., 2010) (Fig. 4). In contrast to the nearby Urdach massif, the Saraillé peridotites never formed the seafloor of the extensional Cretaceous basin; they were only unroofed beneath extremely thinned basement rocks during the Albo-Cenomanian (Lagabrielle et al., 2010). Decoupling of the sedimentary cover from its crustal basement occurred while the continental crust underwent lateral extraction between two convergent detachment faults located respectively at the mantle–basement interface and at the basement–cover interface (Clerc and Lagabrielle, 2014). The prerift sediments are in tectonic contact with the mantle rocks only between the tectonic crustal lenses. Extensional ductile deformation of the sedimentary cover along the detachment fault resulted in the development of subtractive contacts associated with a strong bedding-parallel foliation and a drastic thickness reduction of the stratigraphic pile. The detachment between the mantle rocks and the sedimentary cover has been the locus of intense metasomatic circulations responsible for the mineralization of the talc/chlorite-rich layer (Fig. 4). The foliation of this layer indicates synkinematic development during the Albian-Cenomanian rifting episode. The relatively high temperature allowing the ductile deformation of the sedimentary cover and the talc-chlorite development was possible thanks to two geological processes: (1) the ascent of the subcontinental mantle which caused a steep geothermal gradient; and (2) the synchronous sedimentation of the Albian–Cenomanian flysch that probably acted as a blanket on the basin, facilitating the temperature increase (Clerc et al., 2015).

It still remains an important issue to decipher the timing of folding of the Saraillé massif structure. Folding may have occurred during the extensional phase (Albian–Cenomanian) in relation with salt tectonics (James and Canérot, 1999), or during the compressional phase of the Pyrenean orogeny starting in the Late Santonian.

The Saraillé talc/chlorite-rich layer shares some characteristics with metasomatic layers known in the eastern Pyrenees such as one of the largest deposits of talc ore worldwide, in the Saint-Barthélémy North-Pyrenean massif (Trimouns: Fortuné, 1971; Moine et al., 1989). At Trimouns, the hydrothermal activity responsible for the talc formation has been dated between 112 Ma and 97 Ma (Schärer et al., 1999). It was thus contemporaneous with the opening of the Albian–Cenomanian basins. However, this major ore deposit is located in a pure continental environment in contrast with the Saraillé massif. To our knowledge, the Saraillé massif is the only known example worldwide of a preserved detachment system involving subcontinental mantle, thinned continental basement and prerift metasediments.

Nowadays, numerous models of passive margin formation do exist (see review in Huismans and Beaumont, 2011). Recently, Clerc and Lagabrielle (2014) distinguished a cold rifted margin model from a hot rifted margin model. In the first one, the proximal part of the margin is characterized by a system of tilted blocks and the distal part exhibits an exhumed mantle domain partly covered by continental allochthons. The prerift sedimentary cover remains welded on the tilted blocks and on the continental allochthons. A typical example of such a system is represented by the Galicia/Iberia margin (Boillot et al., 1995; Péron-Pinvidic and Manatschal, 2009; Reston, 2007). The hot margin model differs from the cold margin one by its distal part where mantle is exhumed between lenses of crust deformed in a ductile manner. In this case, the prerift sedimentary cover is also affected by a syn-metamorphic ductile deformation.

In the Saraillé massif, we observed the following major features consistent with the model of distal, hot passive margins with exhumed mantle rocks proposed by Clerc and Lagabrielle (2014) for the preorogenic Pyrenees: (i) As a consequence of the Albian-Cenomanian rifting episode, the subcontinental mantle is now in direct tectonic contact with the prerift sedimentary sequence. (ii) This prerift sedimentary sequence has experienced syn-extension ductile deformation during the rifting along a major detachment. Features (i) and (ii) are both the result of the lateral extraction of the continental crust, the upper crust showing a ductile behaviour. (iii) The detachment faults separating the mantle rocks from the prerift sediments and from the hyperthinned continental crust were the loci of intense hot fluid circulations producing talc and chlorite mineralizations.