The Variscan belt defines an arc in the north-western Iberian Peninsula (Ibero-Armorican Arc) whose core is formed by the Cantabrian Zone (CZ), which represents the foreland fold and thrust belt of the orogen (Fig. 1). This zone consists of Palaeozoic rocks in which two tectonostratigraphic units have been distinguished (Julivert, 1978; Marcos and Pulgar, 1982), whose limit is approximately located by the Devonian–Carboniferous boundary. The pre-orogenic unit is formed of Cambrian to Devonian rocks consisting of alternating carbonate and siliciclastic formations; they form a wedge that thins towards the foreland. The synorogenic unit is formed by several clastic units, also thinning towards the foreland, which filled foredeep basins generated in the front of the main thrust units of the CZ. In this zone, the Variscan deformation occurred during the upper Carboniferous and gave rise to thin-skinned tectonics, with several large thrust units and associated folds (Fig. 1); the units were emplaced in a sequence towards the foreland. The deformation occurred under shallow crustal conditions, so that diagenetic conditions are dominant in the zone and absence of cleavage in the rocks is also dominant. Nevertheless, there are several areas of the CZ where cleavage and very-low- or low-grade metamorphism are present. One of these areas is the south-eastern sector of the CZ. It is a foreland basin that occupies the core of the Ibero-Armorican Arc and has undergone a complex history of sedimentation, deformation, metamorphism and, to a lesser extent, magmatism.

The present study aims to present a model of tectonothermal evolution for the core of the Ibero-Armorican Arc based on conodont colour alteration index (CAI), the Kübler index (KI) of illite, the Árkai index (AI) of chlorite (Chl), the analysis of clay minerals and the rock cleavage development.

The south-eastern sector of the CZ is made up of two units: the Pisuerga–Carrión unit (PCU) and the Valsurbio unit (VU) (Fig. 1). As a consequence of its location in the core of the Ibero-Armorican Arc, the PCU has been thrust over successively by the VU, the Central Coal Basin, the Ponga unit and the Picos de Europa unit (Fig. 1). Thus, the PCU operated as a foreland basin during a great part of the history of the Variscan deformation. This history involved the accumulation of a great thickness of synorogenic Carboniferous sediments, corresponding to clastic wedges associated with the exhumation of the thrust units.

The VU is located to the south of the PCU. Marine facies occur in both units, but from the Emsian the sediments in the latter unit were deposited in deeper waters than those in the former unit. The VU represents an extension of the southern part of the CZ, and the Devonian succession of both is comparable (Koopmans, 1962). The units within the PCU containing Silurian–Devonian rocks have been interpreted as transported from internal areas of the orogenic belt, specifically from the south-eastern extension of the West Asturian–Leonese Zone, currently hidden under the Mesozoic–Cenozoic cover of the Douro basin and located to the south of the VU (Frankenfeld, 1983; Marquínez and Marcos, 1984; Rodríguez Fernández and Heredia, 1987; Rodríguez Fernández, 1994). These units were named “Palentine nappes” by Rodríguez Fernández and Heredia (1987). The PCU and the VU are separated by the León fault (also called “Ruesga fault” in the study area) (Fig. 2) that crosses most of the CZ and whose meaning is controversial (Alonso et al., 2009 and references therein).

The oldest sediments of the study area are Silurian (Wenlock–Pridoli) sandstones and lutites. The Devonian rocks consist of an alternation of carbonate and siliciclastic formations with the facies differences cited above. Mississippian rocks are mainly limestones in the lower part, especially in the VU; upwards a mostly turbiditic sequences appears with common olistoliths in the northern sector and some carbonate levels. The Pennsylvanian succession is dominantly siliciclastic, with a thickness of several thousand metres and synorogenic character. In relation to this synorogenic sedimentation, several syntectonic unconformities have been described, among which the Curavacas unconformity (early Moscovian) can be highlighted by its structural significance (Van Veen, 1965; Lobato, 1977; Alonso and Rodríguez Fernández, 1983; Martín-Merino et al., 2014).

The first deformation events were pre-Curavacas (prior to or earliest Moscovian) and involved the emplacement of the Palentine nappes and the VU. Two generations of thrusts and associated folds occurred during this episode (Rodríguez Fernández, 1994), which translated sequences northwards. Further, some back thrusts and normal faults occurred.

The post-Curavacas deformation events involved the development of several generations of thrusts and high-angle reverse faults, folds, cleavages and normal faults. Some thrusts have a trend approximately parallel to the basal thrust of the Ponga unit and are probably related to the emplacement of this thrust unit (Rodríguez Fernández and Heredia, 1987), which occurred during the late Moscovian. In the same episode that involved the emplacement of the Picos de Europa thrust unit during the Kasimovian–Gzhelian (Merino-Tomé et al. 2009), N–S shortening occurred in the study area, involving the development of thrusts, high-angle reverse faults and the reactivation of older faults with a movement dominantly southward (Maas, 1974). In addition, upright folds with E–W axial trace developed; among them, the Curavacas–Lechada syncline is remarkable for its notable dimensions (Savage, 1967; Lobato, 1977; Rodríguez Fernández, 1994).

Several cleavages have been recognised in the study area. Among them, a gently dipping cleavage crosscutting folds is the most relevant and has been described by various authors (Van Veen, 1965; Savage, 1967; Lobato, 1977; Van der Pluijm et al., 1986; Rodríguez Fernández, 1994; Marín, 1997; García-López et al., 2007; and reference therein).

A subsequent N–S shortening episode occurred during the Alpine deformation. It involved tightening of folds, local development of crenulation cleavage and reactivation of some faults. It is responsible for the dome geometry of the VU (Marín et al., 1995; Marín, 1997). Another post-Variscan structure of the study area is the Ventaniella fault, which traverses the whole CZ in a NW–SE direction. It is essentially a dextral, strike–slip fault with a net slip of 4–5 km (Julivert et al., 1971) whose activity began in the Permian and continues to the present (López-Fernández et al., 2004).

Outcrops of intrusive igneous rocks are common in the PCU, and their knowledge is important to understand the tectonothermal evolution of this unit. Among these rocks, three granodioritic stocks (Pico Iján, Peña Prieta and Pico Jano; Fig. 2) and many small outcrops can be distinguished. The latter are mainly concentrated in the southern half of the unit.

The stocks of Pico Iján, Peña Prieta and Pico Jano dominantly have granodioritic composition and their intrusion was favoured by the existence of faults (Suárez and García, 1974; Corretgé and Suárez, 1990), having developed a notable aureole of contact metamorphism in the case of the Peña Prieta stock. The porphyroblasts in this aureole are post-tectonic relative to the gently dipping cleavage (Gallastegui et al., 1990; Rodríguez Fernández, 1994). The U/Pb age of the Peña Prieta granitoid is Cisuralian (292+3/-2 Ma after Valverde-Vaquero et al., 1999), and the Pico Iján granitoid is nonconformably covered by Lower Triassic rocks. The large number of small outcrops of igneous rocks that exist in the southern half of the unit are concentrated in two areas (one in the eastern part and another in the western part) joined by a band whose outcrops of igneous rocks have been related to the León fault (Corretgé et al., 1987; Suárez and Corretgé, 1987; Corretgé and Suárez, 1990). All these southern outcrops have been, in general, related to fractures and appear as small stocks, probably apophyses of bigger bodies in depth, and as sills or dikes. Their composition is varied and ranges from granodioritic to gabbroic. In some cases, they developed ore bodies close to their contacts (Martín-Izard et al., 1986).

Earlier metamorphic studies in the CZ using CAI and/or KI methods have shown the existence of areas with very-low- or low-grade metamorphism in the PCU and the VU (Raven and van der Pluijm, 1986; Keller and Krumm, 1993; Marín et al., 1996; Marín, 1997; Köberle et al., 1998; García-López et al., 1999, 2007, 2013; Bastida et al., 2002; Clauer and Weh, 2014, and references therein). Similar results have been obtained for the southern part of the study area using coal rank and vitrinite reflectance (Colmenero and Prado, 1993; Llorens et al., 2006; Colmenero et al., 2008; Clauer and Weh, 2014). This metamorphism has been described as associated with a late-orogenic extensional event and with the corresponding cleavage (García-López et al., 1999, 2007, 2013; Bastida et al., 2002). Analysis of ore deposits and of the tectonothermal evolution in the neighbouring unit of the Picos de Europa has defined a subsequent hydrothermal episode during the Permian (Gómez-Fernández et al., 1993, 2000; Bastida et al., 2004). Brime and Valín (2006) have suggested a hydrothermal origin for mineral associations with chloritoid (Cld) and pyrophyllite (Prl) found in samples of pelitic rocks collected in the study area. K–Ar dating of illite in samples collected in the southern part of the study area identified four thermal episodes (Clauer and Weh, 2014), namely at (1) 293 ± 3 Ma (Cisuralian), (2) 268 ± 6 Ma (Guadalupian), (3) 243 ± 5 Ma (middle Triassic) and (4) 175 ± 6 Ma (early–middle Jurassic). Low-temperature thermochronology studies by Fillon et al. (2016) in three samples of Westphalian sandstones collected in the northern and central parts of the PCU, provided cooling ages below 180 ∘C by 37–39 Ma. They inferred Cenozoic erosion of a rock thickness between 6.4 and 8 km (assuming a steady-state geothermal gradient of 25 ∘C km-1).

In order to facilitate description, the samples have been mainly grouped in the following areas, namely Liébana, Valdeón, Yuso–Carrión, Pisuerga, Riaño–Cervera and Valsurbio (Fig. 4) (cf. Martín-Merino et al., 2014).

The distribution of the different grade indicators used in the study area shows, in general, an acceptable correlation between them, although some discrepancies have been observed. All methods coincide in pointing out the location of the areas with higher metamorphic grade, being the CAI and the AI the indicators that tend to give the highest and the lowest grade respectively. All indices point to an irregular distribution of the areas with very-low-grade metamorphism. Diacaizonal areas are limited to the south-eastern sector. A discrepancy is observed in the eastern Yuso–Carrión area between clay mineral and CAI data; a notable number of CAI values systematically indicates ancaizonal conditions, whereas clay assemblages with abundant I/S and Kln, KI and AI indicate diagenetic conditions.

The correlation among the different indicators that can be used to establish the metamorhic grade is challenging due to several factors, such as the different kinetics of the processes that modify the colour of the conodonts and the transformation of the clay minerals and the different influence of fluids in limestones and pelitic rocks. These processes could explain the discrepancies observed.

In the context of the CZ, the low-grade extensional metamorphism of the PCU extends westwards and allows an elongated area to be defined that can be followed up to the Central Coal Basin (Fig. 1) (Aller, 1981, 1986; Brime, 1985; Aller et al., 1987, 2005; García-López et al., 2007). The biggest width of this zone is in the study area, coinciding with the core of the Ibero-Armorican Arc, where a special evolution occurred in the context of the CZ. The last stages of arc tightening gave rise to strong shortening with formation of folds and cleavage (S1) in the core of the arc, and the corresponding crustal thickening and heating (late Gzhelian). Subsequently, a gravitational instability in this core produced an extensional episode during which low-grade metamorphism and subhorizontal cleavage (S2) crosscutting previous folds developed in some areas. This evolution continued during the Cisuralian with the intrusion of igneous bodies and associated contact metamorphism whose minerals post-date the S2 cleavage. The existence of a crustal thickening followed by a gravitational instability in the core of the Ibero-Armorican Arc has also been proposed by other authors (Gutiérrez-Alonso et al., 2004, 2011). In the Central Coal Basin, the metamorphism associated with subhorizontal cleavage and crosscutting folds has been related to the possible existence of igneous bodies in depth (Aller, 1986) or to the rise of fluids along faults, especially the León fault (Aller et al., 2005). In the metamorphic southern part of the study area (VU), the metamorphism disappears westward, so that the adjacent western unit of the Esla nappe region is not metamorphic (García-López et al., 2013; Valín and Brime, unpublished data).

The existence of hydrothermal alteration has been suggested by Brime and Valín (2006) and Clauer and Weh (2014). The common occurrence of Cld and Prl and the irregular distribution of CAI values, the wide range in the high CAI values in some samples and textural alterations of conodonts agree with the occurrence of hydrothermal fluids and possible subsurface igneous bodies. As for the thermal events dated by Clauer and Weh (illite K–Ar; 2014), the first has an age (293 ± 3 Ma), comparable to that of the Peña Prieta granitoid, and the others (Guadalupian, middle Triassic and early–middle Jurassic) are probably related to the crustal extension associated with the Basque–Cantabrian basin. Specific structural evidences of these three later events have not been found and their temperatures were probably lower than those of the late-Variscan extensional episode, which, being related to igneous intrusions, probably gave rise to the palaeothermal peak.

The zircon (U–Th) / He ages obtained by Fillon et al. (2016) in three samples from our study area indicate that they had probably a temperature above 180 ∘C before exhumation started by 37–39 Ma ago (Eocene). This is consistent with the local occurrence of crenulation cleavage associated with Alpine ductile deformation.

The proposed tectonothermal evolution of the studied region from Variscan up to Alpine times is summarised in Table 1.

Study of the low-grade metamorphic rocks and associated structures in the south-eastern sector of the CZ has allowed a model for the tectonothermal evolution of the core of an arcuate orogenic belt to be developed. This core, although a part of a foreland fold and thrust belt where diagenetic conditions are dominant, portrays a complex evolution due to its special location inside the belt. It was thrust during the Carboniferous by large units from south, west and north, which resulted in a great accumulation of syntectonic sediments and development of unconformities and structures. The latter arose as a result of compression in different directions that also generated a crustal thickening. The emplacement of the last thrust unit (Picos de Europa unit) produced a N–S shortening that generated thrusts and associated folds. Shortly afterwards, upright folds with E–W trend and associated cleavage (S1) developed, mainly in the northern part of the sector. The ductile deformation occurred under thermal conditions which reached the anchizone and the ancaizone in some areas. This represents the first tectonothermal event registered in the south-eastern sector of the CZ.

At the end of the Variscan deformation, gravitational instability gave rise to an extensional episode and the corresponding crustal thinning. During this event an increase in the thermal gradient enabled ductile deformation and low-grade metamorphism to take place in some areas (second tectonothermal event), mainly in the central part (Curavacas–Lechada syncline) and the southern part (Valsurbio unit) of the sector. The ductile deformation produced gently dipping cleavage (S2) crosscutting earlier upright F1 folds. Some small, open cascade folds were also produced. The metamorphism reached epizonal and epicaizonal conditions during this event. The process culminated in the Cisuralian with the intrusion of igneous rocks and the development of contact metamorphism around the larger igneous bodies. In the case of Peña Prieta granodiorite, post-S1 andalusite porphyroblasts developed. Hydrothermal fluids were common during this extensional episode, resulting in the development of Prl and Cld, textural alteration and high CAI dispersions in conodont samples.

As a whole, the metamorphic grade is independent of the stratigraphic location of the samples and the trend of the main structures, indicating the late-orogenic character of the tectonothermal events. The thermal level decreases progressively northwards inside the adjacent Picos de Europa unit. However, the metamorphism associated with the extensional episode in the PCU is extended westwards as an elongated area whose development was probably favoured by the rise of fluids along faults, especially along the León fault.