This is the accepted version of the article: Caldera, Norbert; Teixell, Antoni; Griera Artigas, Albert; [et al.]. «Recumbent folding in the Upper Cretaceous Eaux-Chaudes massif : a Helvetic-type nappe in the Pyrenees?». Terra Nova, Published onlince 2021. DOI 10.1111/ter.12517 This version is available at https://ddd.uab.cat/record/235553 under the terms of the license Accepted Article MR NORBERT CALDERA (Orcid ID : 0000-0002-2383-4097) Article type : Review Article Recumbent folding in the Upper Cretaceous Eaux-Chaudes massif: a Helvetic-type nappe in the Pyrenees? Norbert Caldera1, Antonio Teixell1, Albert Griera1, Pierre Labaume2 and Abdeltif Lahfid3 1Departament de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona), Spain 2 Géosciences Montpellier, Université de Montpellier, CNRS, Université des Antilles, 34095 Montpellier, France 3 Bureau de Recherches Géologiques et Minières, Orléans, France Corresponding author: Norbert Caldera (email: norbert.caldera@uab.cat) Significance statement: The Pyrenean orogenic deformation is most often characterized by thrust-fold systems formed under brittle conditions. The Eaux-Chaudes massif is an inlier of Upper Cretaceous sediments within the hinterland Axial Zone of the Pyrenees which allows to characterize the post-Variscan structure in this basement massif. Here we report for the first-time a recumbent fold structure, developed under ductile conditions in the Upper Cretaceous carbonates, with upper Paleozoic metasediments in the core. The fold nappe displays a km-scale, large overturned limb affected by high non-coaxial deformation (mylonitic fabric and strong crystallographic preferred orientation of calcite). Recorded paleotemperatures >300ºC in the Upper Cretaceous rocks are consistent with the microstructure observed. This is a novel deformation style previously unreported for the This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/TER.12517 This article is protected by copyright. All rights reserved alpine Pyrenees, comparable with other analogues as for example the lower Helvetic Accepted Article nappes of the Alps. The results reported in the manuscript are of broaden general interest, especially for the community focused on mountain building processes, and provide a unique example to decipher the thermomechanical evolution in complex hinterland areas, as the Axial Zone of the Pyrenees. Our interpretations are supported with new cartography and structural cross- section, paleotemperature using RSCM technique and microstructure analysis using SEM- EBSD. Abstract We describe a singular structure in Upper Cretaceous rocks of the Eaux-Chaudes massif of the western Pyrenees, consisting of a km-scale fold nappe with a sheared overturned limb. High ductile strain attests a deformation style rarely reported for the alpine Pyrenees, and peak temperature in Upper Cretaceous carbonates is estimated by RSCM paleothermometry in the lower greenschist facies (>300ºC). The normal fold limb retains the original sedimentary textures, while the overturned limb shows calcite crystal-plastic deformation and dynamic recrystallization, with crystallographic preferred orientation. The observed ductility and metamorphic temperature bear similarities with the lower Helvetic nappes of the Alps, suggesting deep burial and/or possibly high geothermal gradient in this part of the Pyrenees. 1 INTRODUCTION In orogens formed by tectonic inversion, the reactivation of extensional structures and the formation of thrust-related basement uplifts commonly accommodate the compressional deformation (e.g. Lacombe & Bellahsen, 2016; Pfiffner, 2017; Herwegh et al., 2020). The interiors of basement uplifts are often poorly understood due to structural complexity and limited outcrops of post-basement rocks. This study focusses on the Axial Zone of the Pyrenees, an Alpine orogen resulting from the inversion of a Mesozoic hyperextended rift (Jammes et al., 2009; Lagabrielle et al., 2010; Masini et al., 2014; Teixell et al., 2016, 2018). The Pyrenees conform to an asymmetric doubly-verging wedge (Choukroune et al., 1989; Muñoz, 1992; Teixell, 1998) which formed from late Cretaceous to early Miocene times This article is protected by copyright. All rights reserved (Alpine orogeny) (Fig. 1). The Axial Zone comprises a system of thrust sheets made of Accepted Article Iberian Paleozoic basement (previously deformed by the Variscan orogeny) which connects to a south-verging thin-skinned thrust system of Mesozoic and Tertiary rocks (South Pyrenean Zone). North of the Axial Zone, the North Pyrenean Zone (NPZ) comprises the inverted Mesozoic basin axis which overrides both the Aquitaine foreland by the North Pyrenean Frontal Thrust and the Axial Zone by the North Pyrenean Fault and the Lakora thrust (Muñoz, 1992; Teixell, 1998; Mouthereau et al., 2014; Teixell et al., 2016). The Eaux-Chaudes massif (ECM) comprises an inlier of Upper Cretaceous rocks surrounded by Paleozoic rocks in the northwestern Axial Zone (Fig. 2). The massif has been classically interpreted as a duplex structure with a roof thrust carrying Paleozoic basement (Ternet, 1965; Déramond et al., 1985; Dumont et al., 2015). The Upper Cretaceous sedimentary cover constitutes a key-marker to constrain the structural style and paleotemperature conditions during the Alpine orogeny. We present a novel view of the Eaux-Chaudes structure (Fig. 3), documented by structural mapping, microstructure and paleotemperature data, which highlights a kilometre- scale recumbent fold nappe. Our observations evidence the importance of alpine ductile folding not recognized hitherto in the hinterland of the Pyrenees. 2 TECTONIC SETTING Paleotectonic reconstructions place the Upper Cretaceous platform carbonates of the ECM in the upper margin of the Iberian plate during the Mesozoic post-rift stage (e.g. Teixell et al., 2016 and references therein). These carbonates later became sandwiched between Paleozoic rocks during the Pyrenean convergence. The Upper Cretaceous sequence lies unconformably on Silurian to Carboniferous metasedimentary rocks, mainly shales, limestones, and sandstones, and on the late Paleozoic Eaux-Chaudes granite. Occasionally lower Triassic Buntsandstein conglomerate pods are preserved in between the Paleozoic and the Upper Cretaceous (Ternet, 1965). The Cretaceous sequence consists of a lower unit of mudstones and grainstones (“Calcaires des Cañons”, Cenomanian to Santonian) followed by sandstone-shale Flysch of Campanian- Maastrichtian age (Ternet, 1965), a sequence which represents the early stages of This article is protected by copyright. All rights reserved subsidence related to the Pyrenean orogeny (Teixell, 1993). Cenomanian-Turonian Accepted Article formations are discontinuous due to local erosion events, as observed elsewhere in the western Axial Zone (Souquet, 1967; Teixell et al., 1994). The ECM is overlain by the Lakora thrust complex (Fig. 2), here represented by Paleozoic (Montagnon d’Iseye thrust unit) and Upper Triassic rocks (mostly Keuper and ophites, Bedous unit), which is in turn overlain by the detached Jurassic-Lower Cretaceous rocks of the southern Chaînons Béarnais (Ternet et al. 2004, Labaume & Teixell, 2020). The whole system was uplifted in the hanging wall of the Gavarnie thrust that raised the Axial Zone, imparting local backthrusting deformation in the ECM (D2 in Dumont et al., 2015). Thrusting of the Lakora-Eaux-Chaudes units is attributed to the latest Cretaceous to mid-Eocene, while subsequent Gavarnie thrusting in the western Axial Zone is attributed to the late Eocene-early Oligocene (Teixell, 1996; Labaume et al., 2016). 3 METHODOLOGY Field structural analysis and mapping was combined with cross-section construction to interpret the large-scale structure. Samples were analysed by thin section and scanning electron microscopy (SEM) coupled with electron backscattered diffraction (EBSD) and energy dispersive spectrometry (EDS). EBSD data were collected at the University of Montpellier on a CamScan Crystal Probe X500FE. Raman spectroscopy of carbonaceous material (RSCM) was performed at the BRGM (Orléans) to calculate peak temperatures (Tmax) using a diode-pumped solid-state laser source excitation of 514.5 nm. The laser power reaching the sample surface, through the 100x objective of a Leica DM2500 microscope, did not exceed 1 mW. We followed the procedures described by Beyssac et al. (2002, 2003, 2007) and Lahfid et al. (2010). 4 RESULTS We propose a new structural profile of the ECM showing an unexpected large-scale ductile structure defined by the Upper Cretaceous rocks (Figs. 3, 4), consisting of a south- verging ~5km-long recumbent fold nappe cored by folded Silurian-Devonian metasediments with a near-horizontal axial plane and a large overturned limb. The Paleozoic core shows minor folds with subhorizontal axial planes and heterogeneously distributed foliation. The This article is protected by copyright. All rights reserved overturned limb is in thrust contact over an autochthonous panel of weakly deformed Upper Accepted Article Cretaceous carbonates (Figs. 2, 3, 4). Occasional Keuper and ophite slices are found along the contact (Ternet, 1965). The Eaux-Chaudes recumbent structure shows an immersion to the west, consistent with the regional plunge of the western Axial Zone, and hence it disappears laterally below overlying thrust sheets. East of the Ossau valley it is largely eroded, so the valley area provides a unique opportunity to observe the proposed structural relationships. Sample analyses were performed in the Upper Cretaceous of the autochthon, the overturned limb and the normal limb of the fold nappe (Figs. 3, 4). 4.1 Structural analysis In the Upper Cretaceous of the autochthonous and normal-limb domains, sedimentary textures are preserved, despite small-scale brittle faults, and a spaced pressure-solution cleavage. Generally, fossils are unstrained apart from solution seams, and ductile deformation at the grain scale is not observed. EBSD analysis of calcite aggregates show random distribution of crystallographic a-, m- and c-axes (Fig. 5). The average grain size is ~32 µm, ranging between 16-100 µm. Occasionally, deformation twins of type III-IV (Ferrill et al., 2004) are observed in coarser calcite grains. The overturned limb is characterized by strong ductile deformation that obliterates fossil content. It shows a mylonitic foliation subparallel to bedding at the mesoscale, and stretching lineation, both inclined to the north (Fig. 5). Microfabrics are controlled by the rheological behaviour of the mineral phases. In calcite-rich layers, crystal-plastic deformation is dominant with development of a fine-grained matrix (9-28 µm, decreasing grain size with increasing deformation intensity) featuring grain-shape preferred orientation and S-C composite fabrics (Fig. 5). We observed evidence of grain boundary migration, such as lobate and interfingering grain boundaries and subgrains. Well-developed CPO is recorded, with c-axes oriented normal to the foliation, and a- and m- axes defining a girdle parallel to the foliation (Fig. 5). Aligned oxide minerals (<1 vol%) parallel to the main foliation enhance the anisotropic fabric. Thin dolomite-rich layers are observed in some samples. These layers show moderate plastic deformation, with occasional fold streaks and asymmetric This article is protected by copyright. All rights reserved porphyroclasts (Fig. 6). Quartz- and phyllosilicate-rich layers (e.g. Buntsandstein and Accepted Article Cretaceous clastic limestones) show brittle-to-ductile behaviour, with weak intragranular deformation (undulose extinction) and slight evidence of quartz recrystallization by bulging. S-C fabrics and asymmetric porphyroclasts attest a non-coaxial deformation and evidence the south-directed tectonic transport previously reported by Ternet (1965), Majesté- Menjoulas (1979), Dumont et al. (2015) and Cochelin (2016). Microstructure and EBSD of monomineralic calcite aggregates suggest plastic deformation by dislocation creep accommodated by slip on basal planes parallel to the a- and m-axes. 4.2 RSCM paleothermometry Raman Spectroscopy of Carbonaceous Material is based on the analysis of the degree of structural organization of carbonaceous material (CM) as a proxy for the Tmax. CM structure is not sensitive to retrograde transformations, remaining on the maximum temperature reached during metamorphism (Beyssac et al., 2002). Raman spectra are composed in the first region (Tuinstra & Koenig, 1970; Nemanich & Solin, 1979) by the graphite band (G) and defect bands (D1-4). The spectra from the autochthonous and overturned limb were fitted by the Beyssac et al. (2002) method, since it is well adapted to the temperature range 320-650ºC. Samples from the normal limb best fittings were obtained by the method of Lahfid and others (2010), which works better in the temperature range 150-320ºC. In either case, Tmax values obtained for all samples range between 315-350ºC (Fig. 7 and Table 1). Representative Raman spectra (Fig. 7) from the undeformed autochthonous unit and overturned limb show differences between the intensities of D1 and G bands with D1 band more intense. This indicates a temperature higher than 330ºC. In the normal limb, both D1 and G bands show a very similar intensity, implying a lower temperature of approximately 315ºC. 5 DISCUSSION The ECM represents the inverted Iberian rifted margin where Upper Cretaceous rocks have been ductilely folded. Large-scale recumbent folding is accompanied by high-strain shearing in the overturned limb, with extensive crystal-plastic deformation and This article is protected by copyright. All rights reserved recrystallization, not reported hitherto for the compressional deformation of the post- Accepted Article Paleozoic rocks of the Pyrenees. The Pyrenean Axial Zone is interpreted as a stack of basement units buried and deformed during the Cenozoic at temperatures in general <300ºC (e.g. Fitzgerald et al., 1999; Jolivet et al., 2007; Metcalf et al., 2009; Abd Elmola et al., 2017; Bellahsen et al., 2019). This work shows for the first-time evidence for ductile basement-involved thin-skinned deformation (i.e. Pfiffner, 2017) in the Pyrenees, affecting the upper Paleozoic metasediments and the overlying Mesozoic sedimentary rocks. RSCM geothermometric results for the Jurassic-Lower Cretaceous rocks of the NPZ in the vicinity are in a range of 300-400ºC (Clerc et al., 2015; Corre, 2017). However, this metamorphic peak is associated with the mid-Cretaceous episode of crustal hyperextension in the Pyrenean rift (Golberg & Leyreloup, 1990; Clerc & Lagabrielle, 2014; Saspiturry et al., 2020), and not with the Alpine orogeny as in the Upper Cretaceous at Eaux-Chaudes. The microstructural features observed at ECM are consistent with the RSCM estimates. Type IV deformation twins are indicative of temperatures >250ºC (Ferrill et al., 2004), complete recrystallization of calcite is commonly reported at temperatures >300ºC (Weber et al., 2001), and incipient bulging recrystallization in quartz is documented for the range 300-400ºC (Stipp et al., 2002). Our proposed cross-sectional geometry differs notably from previously published structural interpretations (Ternet, 1965; Déramond et al., 1985; Dumont et al., 2015; Cochelin, 2016). The recumbent fold poses the question of the mechanical behaviour of the Paleozoic metasediments located in the deformed core of the structure. The bulk of these rocks is rich in phyllosilicates, i.e. slates with limestone and sandstone intercalations of Devonian age, and graphitic Silurian slates. The main foliation observed in these rocks is consistent with the large structure, although could partly derive from an earlier Variscan fabric (Dumont et al., 2015; Cochelin, 2016) that was reworked during Alpine deformation. Mapping suggests that the Eaux-Chaudes fold nappe was detached in the weak Silurian rocks. The contact that is usually mapped as the Eaux-Chaudes thrust in the Ossau valley (e.g. Ternet et al., 2004) is here reinterpreted as an overturned stratigraphic boundary between Paleozoic and Upper Cretaceous carbonates, supported by (1) the occurrence of This article is protected by copyright. All rights reserved overturned Buntsandstein pods along the contact, (2) a large-scale folded Paleozoic Accepted Article stratigraphy cored by Silurian slates, and (3) the recognition by Ternet (1965) of an overturned panel involving Paleozoic and Cretaceous rocks at Bouerzy and Cézy peaks (Fig. 2) in the hinge of the structure. Along the Ossau valley, the highly-deformed Upper Cretaceous carbonates of the overturned limb are in sharp contact with the low deformed and sub-horizontal autochthonous Upper Cretaceous carbonates (Figs. 3, 4). The Keuper and ophite bodies along the contact may represent relicts of the Triassic sole of the overlying Lakora thrust sheet, squeezed out from the faulted (welded) synform during recumbent folding (Fig. 8). This Pyrenean structure has similarities to the lower Helvetic Morcles-Doldenhorn nappes of the Alps, which are recumbent folds with long overturned limbs involving low- grade Mesozoic metasedimentary rocks (e.g. Ramsay, 1981; Burkhard, 1988; Pfiffner, 1993; Casey & Dietrich, 1997; Herwegh & Pfiffner, 2005). Paleotemperature data (315-350ºC), grain size (9-28 µm) and microstructure (dislocation creep-induced textures, dynamic recrystallization and associated grain size reduction) are also comparable. For example, Austin et al. (2008) reported average grain size between 7-22 µm for samples deformed in a temperature range 337-358ºC in the Morcles Nappe. Similar grain sizes and paleotemperatures were also reported for the Doldenhorn and Glarus nappes by Herwegh & Pfiffner (2005) and Ebert et al. (2007, 2008). A challenge resides in explaining the high temperatures recorded by the Upper Cretaceous rocks in the ECM. In the Helvetic case, high temperature can be explained by tectonic burial by the overlying Penninic and Austroalpine nappes, with a structural pile >15 km (Dietrich & Casey, 1989; Pfiffner et al., 1997; Herwegh & Pfiffner, 2005; Nibourel et al., 2018). Such deep burial is more difficult to envisage for the Pyrenees where the thrust stack is considered much thinner (e.g. Teixell, 1998; Bellahsen et al., 2019). The measured ~30ºC difference between the normal and overturned limbs at Eaux-Chaudes, for samples ~1 km apart vertically, could indicate a normal geothermal gradient in the fold domain, although this difference lies within the RSCM error margins and, therefore, cannot be conclusive. A 30ºC/km gradient for the Iberian margin is near to the 34ºC/km gradient estimated for the This article is protected by copyright. All rights reserved European conjugate margin by Saspiturry et al. (2020). However, a 30ºC/km gradient would Accepted Article imply ~11 km of burial over the fold nappe, which is hard to reconcile with the tectonic pile that can be reconstructed above (Labaume & Teixell, 2020). Labaume et al. (2016), based on thermochronology-assisted restoration, estimated ~8 km of total burial over the Eaux- Chaudes thrust units (including synorogenic Flysch), representing a 40ºC/km gradient using our paleotemperatures. The available observations cannot unequivocally discriminate between deep burial or high geothermal gradient, the latter a relict of the Mesozoic thinning phase as proposed by Bellahsen et al. (2019) for other parts of the Iberian paleomargin in the Axial Zone. 6 CONCLUSIONS We propose a novel structural interpretation for the Upper Cretaceous of the Eaux-Chaudes massif with a style reported for the first time in post-Paleozoic rocks of the Pyrenees. It consists of a large recumbent fold nappe, cored by Paleozoic phyllosilicate-rich metasediments and comprising a km-long overturned and sheared limb, showing similarities with the fold nappes of the lower Helvetic Alps. According to RSCM paleothermometry, the Upper Cretaceous carbonates of the Eaux-Chaudes nappe reached the lower greenschist facies with peak temperature of ~350ºC. Mylonitic foliation and stretching lineation are well-developed in the overturned fold limb. Crystallographic and shape-preferred orientations in calcite are consistent with plastic deformation and extensive dynamic recrystallization indicating top-to-the-south shear sense. These observations evidence a high temperature of the Axial Zone during Alpine deformation and rends challenging its interpretation between a deep burial and/or a high geothermal gradient relict from the Mesozoic thinning phase. ACKNOWLEDGMENTS This work was supported by the Spanish MINECO/MCIU projects CGL2014-54180 and PGC2018-093903-B-C21. We thank the OROGEN project for enabling the acquisition of RSCM data, cofounded by Total S.A., BRGM and Institut national de sciences de l’Univers (INSU). Fabrice Barou is thanked for EBSD acquisition assistance and data treatment at This article is protected by copyright. All rights reserved Géosciences Montpellier. We thank T. Dumont, D. Westerman, B. Cochelin and anonymous Accepted Article reviewers for comments that helped to improve the original manuscript. DATA AVAILABILITY STATEMENT The data that support the findings of this study are available from the corresponding author upon a reasonable request. CONFLICT OF INTEREST: None. REFERENCES CITED Abd Elmola, A., Charpentier, D., Buatier, M., Lanari, P. & Monié, P. (2017). Textural- chemical changes and deformation conditions registered by phyllosilicates in a fault zone (Pic de Port Vieux thrust, Pyrenees). Applied Clay Science, 144, 88-103. https://doi.org/10.1016/j.clay.2017.05.008 Austin, N., Evans, B., Herwegh, M. & Ebert, A. (2008). Strain localization in the Morcles nappe (Helvetic Alps, Switzerland). Swiss Journal of Geosciences, 101, 341-360. https://doi.org/10.1007/s00015-008-1264-2 Bellahsen, N., Bayet, L., Denele, Y., Waldner, M., Airaghi, L., Rosenberg, C., Dubacq, B., Mouthereau, F., Bernet, M., Pik, R., Lahfid, A. & Vacherat, A. (2019). Shortening of the axial zone, Pyrenees: Shortening sequence, upper crustal mylonites and crustal strength. Tectonophysics, 766, 433-452. https://doi.org/10.1016/j.tecto.2019.06.002 Beyssac, O., Goffé, B., Chopin, C. & Rouzaud, J.-N. (2002). Raman spectra of carbonaceous material in metasediments: A new geothermometer. Journal of Metamorphic Geology, 20, 859-871. https://doi.org/10.1046/j.1525-1314.2002.00408.x Beyssac, O., Goffé, B., Petitet, J.-P., Froigneux, E., Moreau, M. & Rouzaud, J.-N. (2003). On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 59, 2267-2276. https://doi.org/10.1016/S1386-1425(03)00070-2 This article is protected by copyright. All rights reserved Beyssac, O., Simoes, M., Avouac, J. P., Farley, K. A., Chen, Y.-G., Chan, Y.-C. & Goffé, B. Accepted Article (2007). Late Cenozoic metamorphic evolution and exhumation of Taiwan. Tectonics, 26. https://doi.org/10.1029/2006TC002064 Burkhard, M. (1988). L’Helvetique de la bordure occidentale du massif de l’Aar (evolution tectonique et métamorphique). Eclogae geologicae Helvetiae, 81, 63-114. Casey, M. & Dietrich, D. (1997). Overthrust shear in mountain building. In: Sengupta S. (eds) Evolution of Geological Structures in Micro- to Macro-scales, 119-142. Springer. https://doi.org/10.1007/978-94-011-5870-1_8 Choukroune, P., and ECORS Team. (1989). The ECORS deep seismic profile reflection data and the overall structure of an orogenic belt. Tectonics, 8, 23-29. Clerc, C. & Lagabrielle, Y. (2014). Thermal control on the modes of crustal thinning leading to mantle exhumation: Insights from the Cretaceous Pyrenean hot paleomargins. Tectonics, 33, 1340-1359. https://doi.org/10.1002/2013TC003471 Clerc, C., Lahfid, A., Monié, P., Lagabrielle, Y., Chopin, C., Poujol, M., Boulvais, P., Ringenbach, J. C., Masini, E. & de St Blanquat, M. (2015). High-temperature metamorphism during extreme thinning of the continental crust: A reappraisal of the North Pyrenean passive paleomargin. Solid Earth, 6, 643-668. https://doi.org/10.5194/se-6-643-2015 Cochelin, B. (2016). Champ de deformation du socle paléozoïque des Pyrénées. Sciences de la Terre. Université Toulouse 3 Paul Sabatier (UT3 Paul Sabatier). Français. Corre, B. (2017). La bordure nord de la plaque Ibérique à l'Albo-Cénomanien. Architecture d'une marge passive de type ductile (Chaînons Béarnais, Pyrénées Occidentales). Sciences de la Terre. Université Rennes 1, PhD thesis. France. Déramond, J., Graham, R., Hossack, H., Baby, J. R., Crouzet, P. & Crouzet, G. (1985). Nouveau modéle de la chaîne des Pyrénées. Comptes Rendus de l’Académie des Sciences, Paris, 301, 1213-1216. Dietrich, D. & Casey, M. (1989). A new tectonic model for the Helvetic nappes. Geological Society, London, Special Publications, 45, 47-63. https://doi.org/10.1144/GSL.SP.1989.045.01.03 This article is protected by copyright. All rights reserved Dumont, T., Replumaz, A., Rouméjon, S., Briais, A., Rigo, A. & Bouillin, J.-P. (2015). Accepted Article Microseismicity of the Béarn range: Reactivation of inversion and collision structures at the northern edge of the Iberian plate. Tectonics, 34, 934-950. https://doi.org/10.1002/2014TC003816 Ebert, A., Herwegh, M., Evans, B., Pfiffner, A., Austin, N. & Vennemann, T. (2007). Microfabrics in carbonate mylonites along a large-scale shear zone (Helvetic Alps). Tectonophysics, 444, 1-26. https://doi.org/10.1016/j.tecto.2007.07.004 Ebert, A., Herwegh, M., Berger, A. & Pfiffner, O. A. (2008). Grain coarsening maps for polymineralic carbonate mylonites: a calibration based on data from different Helvetic nappes (Switzerland). Tectonophysics, 457, 128-142. https://doi.org/10.1016/j.tecto.2008.05.007 Ferrill, D. A., Morris, A. P., Evans, M. A., Burkhard, M., Groshong Jr., R. H. & Onasch, C. M. (2004). Calcite twin morphology: a low-temperature deformation geothermometer. Journal of Structural Geolgoy, 26 (8), 1521-1529. https://doi.org/10.1016/j.jsg.2003.11.028 Fitzgerald, P. G., Muñoz, J. A., Coney, P. J. & Baldwin, S.L. (1999). Asymmetric exhumation across the Pyrenean orogen: implications for the tectonic evolution of a collisional orogen. Earth and Planetary Science Letters, 173 (3), 157-170. https://doi.org/10.1016/S0012-821X(99)00225-3 Golberg, J. M. & Leyreloup, A. F. (1990). High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France). Contributions to Mineralogy and Petrology, 104, 194-207. https://doi.org/10.1007/BF00306443 Herwegh, M. & Pfiffner, O. A. (2005). Tectono-metamorphic evolution of a nappe stack: A case study of the Swiss Alps. Tectonophysics, 404, 55-76. https://doi.org/10.1016/j.tecto.2005.05.002 Herwegh, M., Berger, A., Glotzbach, C., Wangenheim, S., Mock, S., Wehrens, P., Baumberger, R., Egli, D. & Kissling, E. (2020). Late stages of continent-continent collision: Timing, kinematic, evolution and exhumation of the Northern rim (Aar Massif) This article is protected by copyright. All rights reserved of the Alps. Earth-Science Reviews, 200, 102959. Accepted Article https://doi.org/10.1016/j.earscirev.2019.102959 Jammes, S., Manatschal, G., Lavier, L. & Masini, E. (2009). Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees. Tectonics, 28, 1-24. https://doi.org/10.1029/2008TC002406 Jolivet, M., Labaume, P., Monié, P., Brunel, M., Arnaud, N. & Campani, M. (2007). Thermochronology constraints for the propagation sequence of the south Pyrenean basement thrust system (France-Spain). Tectonics, 26 (5), 1-17. https://doi.org/10.1029/2006TC002080 Labaume, P., Meresse, F., Jolivet, M., Teixell, A. & Lahfid, A. (2016). Tectonothermal history of an exhumed thrust-sheet-top basin: An example from the south Pyrenean thrust belt. Tectonics, 35, 1280-1313. https://doi.org/10.1002/2016TC004192 Labaume, P. & Teixell, A. (2020). Evolution of salt structures of the Pyrenean rift (Chaînons Béarnais, France): From hyper-extension to tectonic inversion. Tectonophysics, 785, 228451. https://doi.org/10.1016/j.tecto.2020.228451 Lacombe, O. & Bellahsen, N. (2016). Thick-skinned tectonics and basement-involved fold- thrust belts: Insights from selected Cenozoic orogens. Geological Magazine, 153, 763- 810. https://doi.org/10.1017/S0016756816000078 Lagabrielle, Y., Labaume, P. & de Saint Blanquat, M. (2010). Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies. Tectonics, 29, 1-26. https://doi.org/10.1029/2009TC002588 Lahfid, A., Beyssac, O., Deville, E., Negro, F., Chopin, C. & Goffé, B. (2010). Evolution of the Raman spectrum of carbonaceous material in low‐grade metasediments of the Glarus Alps (Switzerland). Terra Nova, 22. 354-360. https://doi.org/10.1111/j.1365- 3121.2010.00956.x Majesté-Menjoulas, C. (1979). Evolution alpine d’un segment de la chaîne varisque. Nappe de Gavarnie, chevauchements Cinq-Monts-Gentiane (Pyrénées centrales et occidentales). Thesis Université of Toulouse, 343 p. This article is protected by copyright. All rights reserved Masini, E., Manatschal, G., Tugend, J., Mohn, G. & Flament, J.-M. (2014). The tectono- Accepted Article sedimentary evolution of a hyper-extended rift basin: The example of the Arzacq- Mauléon rift system (Western Pyrenees, SW France). International Journal of Earth Sciences, 103, 1569-1596. https://doi.org/10.1007/s00531-014-1023-8 Metcalf, J. R., Fitzgerald, P. G., Baldwin, S.L. & Muñoz, J. A. (2009). Thermochronology of a convergent orogen: Constraints on the timing of thrust faulting and subsequent exhumation of the Maladeta Pluton in the Central Pyrenean Axial Zone. Earth and Planetary Science Letters, 287 (3-4), 488-503. https://doi.org/10.1016/j.epsl.2009.08.036 Mouthereau, F., Filleaudeau, P.-Y., Vacherat, A., Pik, R., Lacombe, O., Fellin, M. G., Castelltort, S., Christophoul, F. & Masini, E. (2014). Placing limits to shortening evolution in the Pyrenees: Role of margin architecture and implications for the Iberia/Europe convergence. Tectonics, 33, 2283-2314. https://doi.org/10.1002/2014TC003663 Muñoz, J. A. (1992). Evolution of a continental collision belt: ECORS-Pyrenees crustal balanced cross-section. Thrust Tectonics, 235-246. https://doi.org/10.1007/978-94-011- 3066-0_21 Nemanich, R. J. & Solin, S. A. (1979). First- and second-order Raman scattering from finite- size crystals of graphite. Physical Review B, 20, 392-401. https://doi.org/10.1103/PhysRevB.20.392 Nibourel, L., Berger, A., Egli, D., Luensdorf, N. K. & Herwegh, M. (2018). Large vertical displacements of a crystalline massif recorded by Raman thermometry. Geology, 46, 879-882. https://doi.org/10.1130/G45121.1 Pfiffner, O. A. (1993). The structure of the Helvetic nappes and its relation to the mechanical stratigraphy. Journal of Structural Geology, 15, 511-521. https://doi.org/10.1016/0191- 8141(93)90145-Z Pfiffner, O. A., Lehner, P., Heitzmann, P., Mueller, St. & Steck, A. (1997). Deep Structure of the Swiss Alps: Results from NFP 20. Birkhäuser Verlag, Basel, Switzerland, 460pp. ISBN 3-7643 5254 X, Hardback, DM 228. Pfiffner, O. A. (2017). Thick-skinned and thin-skinned tectonics: A global perspective. Geosciences, 7 (3), 71. https://doi.org/10.3390/geosciences7030071 This article is protected by copyright. All rights reserved Ramsay, J. G. (1981). Tectonics of the Helvetic nappes. Geological Society, London, Accepted Article Special Publications, 9, 293-309. https://doi.org/10.1144/GSL.SP.1981.009.01.26 Saspiturry, N., Lahfid, A., Baudin, T., Guillou-Frottier, L., Razin, P., Issautier, B., Le Bayon, B., Serrano, O., Lagabrielle, Y & Corre, B. (2020). Paleogeothermal Gradients Across an Inverted Hyperextended Rift System: Example of the Mauléon Fossil Rift (Western Pyrenees). Tectonics, 39. https://doi.org/10.1029/2020TC006206 Souquet, P. (1967). Le Crétacé supérieur sudpyrénéen en Catalogne, Aragon et Navarre. Thèse d’Etat, Université de Toulouse, 529 p. Stipp, M., Stünitz, H., Heilbronner, R. & Schmid, S. M. (2002). Dynamic recrystallization of quartz: Correlation between natural and experimental conditions. Geological Society of London, Special Publications, 200 (1), 171-190. https://doi.org/10.1144/GSL.SP.2001.200.01.11 Teixell, A. (1993). Coupe géologique du massif d'Igountze: implications sur l'évolution structurale de la bordure sud de la Zone nord-pyrénéenne occidentale. Comptes Rendus de l'Académie des Sciences, Paris, 316, 1789-1796. Teixell, A., García Sansegundo, J., & Zamorano, M. (1994). Memoria de la Hoja nº 144 (Anso). Mapa Geológico de España E. 1:50.000 (MAGNA), Segunda Serie, Primera edición. IGME, 62 p. Teixell, A. (1996). The Ansó transect of the southern Pyrenees: Basement and cover thrust geometries. Journal of the Geological Society of London, 153, 301-310. https://doi.org/10.1144/gsjgs.153.2.0301 Teixell, A. (1998). Crustal structure and orogenic material budget in the west central Pyrenees. Tectonics, 17 (3), 395-406. https://doi.org/10.1029/98TC00561 Teixell, A., Labaume, P. & Lagabrielle, Y. (2016). The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario. Comptes Rendus – Geoscience, 348, 257-267. https://doi.org/10.1016/j.crte.2015.10.010 Teixell, A., Labaume, P., Ayarza, P., Espurt, N., de Saint Blanquart, M. & Lagabrielle, Y. (2018). Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and This article is protected by copyright. All rights reserved new interpretations from recent concepts and data. Tectonophysics, 724-725, 149-170. Accepted Article https://doi.org/10.1016/j.tecto.2018.01.009 Ternet, Y. (1965). Étude du synclinal complexe des Eaux-Chaudes (Basses-Pyrénées), Thèse Doctorat 3º Cycle: Faculté des sciences de l’Université de Toulouse, 332 p. Ternet, Y., Majeste-Menjoulas, C., Canerot, J., Baudin, T., Cocherie, A., Guerrot, C. & Rossi, P. (2004). Carte géologique de la France: Laruns-Somport. Bureau de Recherches Géologiques et Minières, scale 1:50.000, feuille nº1069. Tuinstra, F. & Koenig, J. L. (1970). Raman Spectrum of Graphite. The Journal of Chemical Physics, 53, 1126-1130. https://doi.org/10.1063/1.1674108 Weber, J. C., Ferrill, D. A. & Roden-Tice M. K. (2001). Calcite and quartz microestructural geothermometry of low-grade metasedimentary rocks, Northern Range, Trinidad. Journal of Structural Geology, 23 (1), 93-112. https://doi.org/10.1016/S0191- 8141(00)00066-3 Figure captions FIGURE 1. (A) Geologic sketch map of the Pyrenees showing the location of the Eaux-Chaudes massif. White frame indicates the mapped area (Fig. 2) and white line indicates cross-section in Figure 1B. (B) Crustal cross-section of the Pyrenees west of the study zone (simplified from Teixell et al., 2016) showing the tectonic setting of the: Eaux-Chaudes structure (red frame); CBB: Chaînons Béarnais Belt; IM: Igountze Mendibelza; GT: Gavarnie thrust; LT: Lakora thrust; NPZ: North- Pyrenean Zone; NPF: North-Pyrenean fault; NPFT: North-Pyrenean Frontal thrust; SPZ: South- Pyrenean Zone; SPFT: South-Pyrenean Frontal Thrust. FIGURE 2. (A) Geological map of the Chaînons Béarnais belt (North-Pyrenean Zone) and Eaux- Chaudes massif, location in Figure 1a; NPF: North-Pyrenean fault. Modified from Labaume & Teixell (2020). (B) Detailed geological map of the study area in the Ossau Valley, western Eaux-Chaudes massif (location in Fig. 2b). Data compiled from published maps (Ternet, 1965; Ternet et al., 2004), completed with additional own field studies. White line indicates cross-section in Figure 3. This article is protected by copyright. All rights reserved FIGURE 3. Cross-section of the recumbent Eaux-Chaudes fold nappe in the western side of the Accepted Article Ossau valley (see Fig. 2b for location). FIGURE 4. Outcrop photo and interpreted profile of the Eaux-Chaudes nappe structure at the western side of the Ossau valley (see Fig. 2b for location). Gray arrows indicate bedding polarity. Colour codes correspond to those given in Figure 3. FIGURE 5. (A) Equal area upper hemisphere pole figures of calcite a-, m- and c-axes measured by EBSD for the undeformed Upper Cretaceous limestone from the normal limb of the Eaux-Chaudes fold nappe (sample 17NC51, cut perpendicular to bedding), showing random crystallographic orientation. (B) Band contrast image and Inverse Pole Figure (IPF) map for sample 17NC51. (C) Stereonet reference frame with xyz spatial axes for deformed samples of the overturned limb. Pole figures and images correspond to the xz plane. (D) Equal area upper hemisphere pole figures of calcite a-, m- and c-axes fabrics deformed of sample 17NC26, showing a moderate crystallographic preferred orientation. (E) Band contrast image and IPF map of sample 17NC26 showing S-C composite structure. (See Fig. 2b and table 1 for sample location). FIGURE 6. Deformed samples from the overturned limb at the locality of Cambeilh (see Fig. 2 for location). Arrows indicate the shear direction. (A) Asymmetric dolomite porphyroclasts in a dark recrystallized calcite matrix. (B) Intramylonitic folds (dashed white lines) in sheared limestone. (C) Stretching lineation (dashed white lines) on the mylonitic foliation planes. All of these structures indicate top to the south sense of tectonic transport. (D), (E); Stereographic projection of (D) main foliation and (E) stretching lineation measured in the Upper Cretaceous rocks. (lower hemisphere, equal area). FIGURE 7. Representative Raman spectra of carbonaceous material and photomicrographs of three samples from the Upper Cretaceous rocks, in a vertical profile across the Eaux-Chaudes fold nappe. (A) Undeformed sample from the normal limb of the recumbent fold. (B) Deformed sample from the overturned limb of the recumbent fold. (C) Undeformed sample from the autochthon (see Fig. 2b and table 1 for sample location). FIGURE 8. Cartoon (not to scale) illustrating the development and evolution of the Eaux-Chaudes recumbent fold during the D1 stage of the Pyrenean compressional deformation. Thickness of Upper Triassic and subsequent units are poorly constrained. Relicts of Upper Triassic rocks (Keuper and This article is protected by copyright. All rights reserved ophites) between the overturned limb and autochthon remain from its extrusion while the fold nappe Accepted Article progressively evolved. TABLE 1. Summary of samples used in Raman and EBSD study. Zone indicates the structural position of samples (Au: Autochthonous; OL: Overturned Limb; NL: Normal Limb). RA1 and RA2 values were retrieved from peak-fitting of Raman spectra (mean and standard deviation). RA1, (D1+D4)/(D1+D2+D3+D4+G); RA2, (D1+D4)/(D2+D3+G); Maximum temperatures were estimated after the methods by Beyssac et al. (2002) and Lahfid et al. (2010). Asterisks indicate EBSD analysed samples. This article is protected by copyright. All rights reserved TABLE 1. RESULTS OF RSCM Sample Latitude N Longitude W Zone R2 parameter RA1 parameter Number T (ºC) SD Mean SD Mean SD of spectra Mean 17NC21 42º 56’44.69’’ 0 º 26’ 39.45’’ Au 0.67 0.03 - - 17 340 8 17NC26* 42º 58’3.27’’ 0 º 26’ 20.03’’ OL 0.65 0.02 - - 16 348 8 17NC27* 42º 58’14.39’’ 0 º 26’ 17.28’’ OL 0.64 0.02 - - 22 355 7 17NC34* 42º 55’52.68’’ 0 º 25’ 52.96’’ OL 0.66 0.02 - - 21 344 6 17NC37* 42º 56’7.22’’ 0 º 25’ 22.92’’ OL 0.68 0.02 - - 18 337 8 18NC04 42º 56’2.59’’ 0 º 27’ 7.40’’ OL 0.68 0.03 - - 15 334 7 18NC46 42º 57’19.84’’ 0 º 29’ 21.50’’ NL 0.70 0.01 - - 15 328 5 18NC47 42º 57’17.08’’ 0 º 29’ 21.61’’ NL 0.72 0.01 - - 12 321 4 18NC16 42º 58’5.81’’ 0 º 29’ 36.55’’ NL - - 0.62 0.01 14 311 10 18NC17 42º 58’6.62’’ 0 º 29’ 37.71’’ NL - - 0.63 0.01 10 316 15 18NC20 42º 58’0.87’’ 0 º 29’ 34.21’’ NL - - 0.63 0.01 14 317 11 18NC37 42º 57’46.53’’ 0 º 29’ 37.65’’ NL - - 0.63 0.01 10 313 15 18NC48 42º 57’5.41’’ 0 º 29’ 17.86’’ NL - - 0.63 0.01 12 315 13 17NC51* 42º 59’13.80’’ 0 º 30’ 27.30’’ NL - - - - - - - This article is protected by copyright. All rights reserved A) 0º 2º Pyrenees Fig. 1b AQUITAINE BASIN N NPFT IM CBB Fig. 2 NPZ LT 43ºN Eaux-Chaudes GT NPF JACA AXIAL ZONE BASIN EBRO BASIN SPZ SPFT 42ºN 0 100 km Tertiary in forelands Mesozoic Pyrenean Tertiary Paleozoic B) SSW SOUTH PYRENEAN ZONE NORTH PYRENEAN ZONE NNE Lakora EBRO BASIN JACA BASIN AXIAL ZONE thrust AQUITAINE BASIN SPFT CBB NPFT 0 Gavarnie thrust Broto thrust Guarga thrust 10 km V=H U. Oligocene-Miocene molasse Upper Cretaceous Upper Triassic in Keuper facies Continental basement + Lower - Middle Triassic U. Eocene - L. Oligocene molasse Lower Cretaceous (upper crust) Eocene flysch and limestone Jurassic Lower crust A) W0°40' Mai l Ar W0°30' W0°20' rou y S ar ran C ce H A I S I R N A N E A O N B LI S C Q OUZ FA OM UL Igun N P F T tze Laye ns N43°00' Bedous N43°00' Arbéost B E Laruns D O S T H R U S T LAKOR U S L A R U N U N I T S A Th - . gon Mon tagnon Ber Iseye -C UX HA EA UD ES Ga Eau E A ve x- Ch U X- Th. d’ aud B O AS PE es m N N E S assi f T h . A X Fig. 2b I A L Eaux- Chaud es Z O granit e N E N Gav ed ’O S S AU 5 km W0°40' W0°30' W0°20' Campanian-Maastrichtian Detachment contact Jurassic to lower Albian carbonates Lherzolite (flysch) Thrust fault Cenomanian to Santonian Middle-Upper Triassic (Keuper facies, Upper Cretaceous alkaline Thrust on reactivated (south: carbonates ; north: flysch) Muschelkalk carbonates and ophite) magmatic rocks diapir/weld (intrusive sheets Upper Aptian to lower Cenomanian a) Paleozoic metasediments and Welded diapir and pillow lavas) (Spicule marls and Black Flysch Group) Lower Triassic sandstone a b Normal fault (Iguntze: Mendibelza conglomerates; b) Late Variscan granitoids Arbéost, Bazès and Gallagos: Undifferentiated fault mainly carbonates) or diapir contact 0º27’W B) 52 41 Laruns 17NC51 21 45 49 49 49 13 52 48 90 Montagnon 34 17NC27 45 d’Iseye 47 18NC16 17NC26 18NC17 20 34 23 18NC20 23 18NC37 41 42 23 46 22 15 18 09 37 80 18NC46 23 69 68 18NC47 38 Les 25 66 57 50 Goust Eaux-Chaudes 18NC48 20 26 18 Pic du 70 17NC21 Gourzy Pic de 15 Bouerzy 25 45 70 48 22 29 55 29 17NC37 Cabane 36 52 50 du Cambeilh 55 18NC04 09 23 Ossau Valley 41 27 17NC34 42º55’N 36 49 31 57 Figs. 3-4 Pic de Cézy N 1 km Detachment contact Quaternary Keuper and ophites Upper Devonian Thrust fault Campanian - Maastrichtian Buntsandstein Lower Devonian Coniacian - Santonian Eaux-Chaudes granite Silurian Welded contact Cenomanian - Turonian Carboniferous Indiferentiated fault Sample location SW NE Lakora thrust Montagnon d’Iseye Normal limb Eaux-Chaudes nappe Overtu rned li mb Autoc hthon ous Axial Zone 1 km H=V Campanian - Maastrichtian Buntsandstein Lower Devonian Coniacian - Santonian Eaux-Chaudes granite Silurian Cenomanian - Turonian Carboniferous Welded contact Keuper and ophites Upper Devonian Montagnon d’Iseye Goust S Lakora thrust N Axial Zone Eaux-Chaudes nappe 1 Km A) 17NC51 B) (1100) (1010) (0001) (1120) m 1.15 c 1.4 a 1.2 (0001) 1.3 1.15 1.1 (0110) 1.2 1.1 1.05 1.1 1.05 x x x 1 1 1 0.9 0.95 0.95 0.8 0.9 z 0.9 z z 500 µm 500 µm n = 36518 Max: 1.2 Max: 1.4 Max: 1.2 C) Stereonet reference frame E) (1100) z shear c-plane and z stretching lineation (0001) (0110) x y x foliation Equal area projection y s-foliation S upper hemisphere D) 17NC26 C (1010) (0001) (1120) m c a 2.5 1.5 1.6 1.4 1.3 2 1.4 1.2 1.1 1.2 x x 1.5 x 1 0.9 1 0.8 1 200 µm 200 µm 0.8 0.7 0.6 0.6 z z 0.5 z n = 8647 Max: 1.6 Max: 2.6 Max: 1.8 A) N B) N C) N 0 10cm 0 10cm 0 10cm D) Foliation E) Stretching lineation 20 19 18 n = 82 n = 47 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Percent per 1% area D1 G A) 18NC16 Normal Limb 311ºC (SD=10) 500 µm B) 17NC27 Reverse Limb 355ºC (SD=7) 100 µm C) 17NC21 Autochthonous 340ºC (SD=8) 1000 µm 1000 1500 2000 Raman shift cm-1 A) Lakora thrust B) D1 C) Montagnon d’Iseye ECM D1 Upper Cretaceous Eaux-Chaudes granite Welded contact Upper Triassic Paleozoic and lower Triassic Thrust contact