Synorogenic marine basins encompass most of the known marine geodynamic settings, from active to passive earthquake-prone margins (Dickinson and Valloni, 1980; Garzanti et al., 2007; DeCelles, 2012). They are found to be associated with Archean to Phanerozoic orogens (e.g., Liang and Li, 2005; Wilmsen et al., 2009; Mulder et al., 2017; Kusky et al., 2020) and hold key evidence to understand the geographic and geodynamic evolution of modern and ancient orogenic belts.

A common sedimentary feature of all synorogenic marine basins is the presence of earthquake-triggered turbiditic flows that promote a variety of sedimentary facies, from cohesive rhythmic flysch sequences to chaotic large-scale mass-wasting bearing heterometric-sized blocks or olistoliths (Franke and Engel, 1986; Coleman and Prior, 1988; Eyles, 1990; Festa et al., 2020), also denominated block-in-matrix formations (hereafter referred as BIMFs) (Festa et al., 2016). Because the most probable source areas of the sediments and olistoliths that fed these basins are in the surrounding orogenically active highs, their stratigraphy can provide important clues on the orogen relief variation along space and time (e.g., Ducassou et al., 2014; Chiocci and Casalbore, 2017).

The synorogenic basins formed during continental convergence are gradually incorporated into the active orogenic edifice as tectonic slices in the accretionary complex, especially at the base, forming a tectonic carpet (Festa et al., 2019; Kusky et al., 2020, and references therein). The progradation of the tectonic front and the basin depocenter favors a systematic intrabasinal sedimentary recycling (i.a. wild flysch) and mixing of synorogenic sediments with other external sources (e.g., Franke and Engel, 1986; Bütler et al., 2011) producing mixed signals of not straightforward paleogeographic interpretation. The key to addressing this problem rests in the regional study of the basin stratigraphy, including the flysch sequences, the mass-wasting deposits and the petrography of the olistoliths (Festa et al., 2019, 2020). Complementary, the detailed geological recognition of the basement and surrounding areas of the synorogenic basins is crucial to identify discriminatory aspects that can help to constrain different variables, such as possible sources, sediment transport distance, regional and local tectonic settings, and paleogeographic limitations (e.g., Alonso et al., 2015; Festa et al., 2016; Krastel et al., 2019).

The sedimentological models can be refined using detrital zircon geochronology. This tool is commonly used to trace source-to-sink relationships, through different statistical approaches that compare the zircon age populations present in detrital rocks (e.g., Meinhold et al., 2011, 2013; Linnemann et al., 2012, 2014). One of the most rigorous procedures for zircon age populations similarity–dissimilarity analysis is multidimensional scaling (MDS) (Vermeech, 2018), which provides a graphic output of the Kolmogorov–Smirnov test when using a large number of samples with individual zircon age populations (Gutiérrez-Alonso et al., 2020; Pereira et al., 2020a).

In the southwestern edge of the European Variscan Belt (Fig. 1), the Iberian Massif preserves some of the best examples of Phanerozoic synorogenic marine basins, which reflect different tectonic settings along the belt during the Upper Devonian–late Carboniferous collision of Laurussia and Gondwana to form Pangea (e.g., Pereira et al., 2012a, b, 2017; Oliveira et al., 2019b). In SW Iberia, the Late Devonian–late Carboniferous flysch basins appear on both sides of the oceanic suture that separates Laurussia from Gondwana (Silva et al., 1990; Braid et al., 2011; Pereira et al., 2012a; Pérez-Cáceres et al., 2017). On the Gondwana-side, the Late Devonian–early Carboniferous marine sedimentation had Gondwana-type sources with massive contribution of intrabasinal volcanism in the Tournaisian–Visean period (Pereira et al., 2012a, 2020a). On the Laurussian side, sediments that filled the synorogenic marine basins resulted from intrabasinal recycling processes and source areas located on both continents. In this case, the sediments were systematically imbricated at the base of the advancing orogenic front, towards inland Laurussia from the Late Devonian (Pulo do Lobo Zone) to the upper Carboniferous (southwestern South Portuguese Zone) (Pereira et al., 2012a, 2020a; Pérez-Cáceres et al., 2017; Braid et al., 2011; Jorge et al., 2013; Rodrigues et al., 2015).

This study presents new data and analysis of synorogenic rocks from marine basins located in the hinterland (internal zones) of the Variscan belt in NW Iberia. While synorogenic basins in the external zones of the orogen (the foreland fold and thrust belt) have been classically classified as foreland basins laying on top of north Gondwana Cambrian to Upper Devonian passive margin sequences (Marcos and Pulgar, 1982; Pastor-Galán et al., 2013; Gutiérrez-Alonso et al., 2015), the basins studied in this work are located in the hinterland and were deposited over rocks showing pervasive strain and that were metamorphosed to different degrees (Martínez Catalán et al., 2004, 2008, 2016; Dias da Silva et al., 2015). The sedimentary sources of the synorogenic flysch and BIMF deposits are related to the development and unrooting of a Variscan accretionary prism (the Galicia–Trás-os-Montes Zone) onto Gondwana, and to the development of a peripheral bulge affecting the extensive passive margin of Gondwana (e.g., González Clavijo and Martínez Catalán, 2002; Keller et al., 2008; Dias da Silva et al., 2015). Some of the Variscan hinterland synorogenic basins have been incorporated into the base of the allochthonous wedge as a parautochthonous unit and then emplaced onto the autochthonous terrain of NW Iberia (Dias da Silva et al., 2015, González Clavijo et al., 2016). The basin depocenter migrated towards inland Gondwana from the Late Devonian to the late Carboniferous following the progression of the orogenic front, from the Galicia–Trás-os-Montes Zone (GTMZ) to the Central Iberian Zone (CIZ), where synorogenic deposits are preserved in the San Clodio Series (Martínez Catalán et al., 2016), possibly following to the West Asturian–Leonese zone (WALZ), although no synorogenic deposits linked to this basin are preserved on it, and finally to the foreland Cantabrian Zone (CZ) (Merino-Tomé et al., 2017; Gutiérrez-Alonso et al., 2020).

In both NW and SW Iberian synorogenic basins, zircon geochronology has been used to constrain sedimentary provenance based on the fingerprint of sources and basin stratigraphic units (Braid et al., 2011; Pereira et al., 2012a; 2014; Jorge et al., 2013; Pastor-Galán et al., 2013; Rodrigues et al., 2015; Martínez Catalán et al., 2016; Pérez-Cáceres et al., 2017). While in the southwest and in the CZ recent works have demonstrated the importance of the MDS statistical approach in identifying the relationships between source and sink (Gutiérrez-Alonso et al., 2020; Pereira et al., 2020b), in NW Iberian Variscan hinterland this approach has not been applied to date.

In this work we present new field observations that revise previous interpretations, supported by new structural data and U–Pb geochronology of igneous and detrital zircon grains, which enables a new vision of the parautochthonous units of the GTMZ here referred to as Upper Parautochthon (UPa; preorogenic) and Lower Parautochthon (LPa; synorogenic) (Dias da Silva et al., 2015). Our findings support the extension of these units to different sectors of the GTMZ, extending beyond the area covered by the UPa and LPa to virtually below all the allochthonous complexes of NW Iberia. Field work has revealed the diverse types of flysch complexes and mélanges present in the LPa, commonly obscured by Variscan polyphasic and pervasive deformation. Tectonic and sedimentary mélanges have been recognized; they combine to produce polygenetic mélanges using the terminology by Festa et al. (2019, 2020). Moreover, the detrital zircon age fingerprinting offers a new general view of the LPa geotectonic setting at Variscan times and sets new constrains on the possible source areas of the synorogenic sediments, olistoliths and blocks. We consider that this review led to a better understanding on the paleogeography and geodynamic setting of the Late Devonian–lower Carboniferous flysch basins in NW Iberia and its incorporation into a general model for the synorogenic basins of the Iberian Variscan Massif.

Within the rootless Variscan accretionary wedge which forms the Variscan Massif of NW Iberia – the so-called GTMZ (Figs. 2 and 3) (Ribeiro, 1974; Schermmeron and Kotsch, 1984; Martínez Catalán et al., 2009; Ballèvre et al., 2014; Martínez Catalán et al., 2014; Azor et al., 2019) – a significantly distinct structural unit has been identified and interpreted as a remnant of former oceanic realms (Lower Ordovician Rheic Ocean and an Early Devonian suprasubduction ophiolite). This tectonic unit marks the putative suture zone of the Laurussia–Gondwana continental collision that partially led to the formation of Pangea; this unit is known as a Middle Allochthon (MA) or Ophiolitic unit (Gómez Barreiro et al., 2007; Martínez Catalán et al., 2009; Stampfli et al., 2013; Ballèvre et al., 2014; Arenas and Sánchez-Martínez, 2015; Azor et al., 2019) and currently separates the so-called Upper Allochthon (upper units) and the Lower Allochthon (basal units). The Upper Allochthon (UA) is considered a far-traveled ribbon-shaped terrane that drifted away in the Lower Ordovician from the Gondwanan margin during the opening of the Rheic Ocean and accreted to Laurussia in the Silurian (Gómez-Barreiro et al., 2007). It includes two tectonically stacked units, presenting early-Variscan (ca. 390–380 Ma) HP/HT and Cambro-Ordovician IP/M to HT metamorphism respectively (Martínez Catalán et al., 2019). The Lower Allochthon (LA) is made of a set of nappe folds and tectonic slices (Farias et al., 1987; Díez Fernández et al., 2010; Dias da Silva et al., 2014, 2015) considered to represent the most seaward rim of continental Gondwana (Murphy et al., 2008) which underwent continental subduction (HP/L to MT metamorphism) and obduction (retrogression to amphibolite and greenschist facies) recording the inception of Variscan continental collision at ca. 370–360 Ma (Munhá et al., 1984; Gil Ibarguchi and Dallmeyer, 1991; Arenas et al., 1995, 1997; Gil Ibarguchi, 1995; Rubio Pascual et al., 2002; Rodríguez et al., 2003; López-Carmona et al., 2010, 2014). This latter unit, (LA) despite being interpreted as part of the Gondwanan passive margin ensemble, and being part of the lower plate in the collisional edifice, is classically considered as part of the allochthonous realm in the region but not belonging to the exotic terrains (Ribeiro, 2013; Ribeiro and Sanderson, 1996; Ribeiro et al., 2007). Structurally below the LA, a tectonic unit displaying low metamorphic grade separates the above-mentioned allochthons from their relative autochthon, the Central Iberian Zone (CIZ). This unit, named Schistose Domain (Farias et al., 1987) or Parautochthon (Pa) (Ribeiro et al., 1990; Martínez Catalán et al., 1997), was considered as a thick Silurian sequence, lacking correlation with the condensed Silurian graphite-rich sequences of the underlying autochthon (CIZ). Nevertheless, the paleogeographic affinity of both domains was highlighted by identical north Gondwana Silurian graptolite and conodont faunas (Sarmiento et al., 1998; Piçarra et al., 2003, 2006a, b). However, the stratigraphy and structural features of the lowermost tectonic sheets of the Pa, directly above the CIZ, were interpreted as a synorogenic basin with possible (Middle–Late) Devonian age (Antona and Martínez Catalán, 1990; González Clavijo and Martínez Catalán, 2002; Martínez Catalán et al., 2004; Pereira et al., 2009; Rodrigues et al., 2013).

The attempt to better understand the tectonostratigraphy of the Pa led to a later division in two tectonically stacked units, Upper and Lower Parautochthon (UPa and LPa), in the sense firstly proposed by Rodrigues et al. (2006, 2013) and updated by Dias da Silva et al. (2014, 2015, 2016). This division limits the UPa to a pre-Variscan upper Cambrian–Silurian sequence comparable with the CIZ and LA that was affected by Variscan recumbent folds and thrusts, and it defines the LPa as an imbricated thrust sequence bearing slices of a foreland synorogenic basin, with the younger slices in the transition to the CIZ (Martínez Catalán et al., 2016). The thrust fault structures bounding the lower tectonic sheets of the GTMZ are as follows (Figs. 2 and 3): i.

The LA Basal Thrust (LABT, Figs. 3 and 4) or basal thrust of the Centro-Transmontano thrust complex (in the meaning of Ribeiro et al., 1990) represents the roofing thrust of the Parautochthon.

In the area studied, the UPa and CIZ underwent regional Barrovian metamorphism (M1) through the early Variscan compressive events (C1 + C2 on the Martínez-Catalán et al., 2014 proposal; ca. 360–330 Ma) which were followed by a complex extensional (E1 - M2; ca. 340–320 Ma) and compressional (C3 - M3; ca. 318–300 Ma) tectonothermal history (Dallmeyer et al., 1997; Azor et al., 2019; Dias da Silva et al., 2021, and references therein).

The LPa synorogenic ensemble was also affected by anchizone to low-grade (chlorite zone) metamorphism. Attempts to discriminate if the metamorphic grade was lower in the synorogenic units than in the preorogenic units using illite crystallinity (Antona and Martínez Catalán, 1990) and the Colour Alteration Index in conodonts (Sarmiento and García-López, 1996; Sarmiento et al., 1997) were inconclusive. Petrographic observations made by Matte (1968) in the synorogenic deposits of the San Clodio in the CIZ to the southeast of Monforte (Fig. 2) and in the underlying Ordovician sequence, and by Dias da Silva et al. (2020) in the LPa and CIZ in the eastern rim of Morais Complex, show similar low-grade epizone metamorphism in both pre- and synorogenic sequences. However, the San Clodio flysch rests unconformably above the reverse limb of a large C1 recumbent syncline whose axial planar cleavage is more evolved than that of the flysch above (Martínez Catalán et al., 2016). And it is also older: ca. 360 Ma (Dallmeyer et al., 1997), while detrital zircons are as young as 324 Ma in the San Clodio Series and 340 Ma in the synorogenic deposits of Trás-os-Montes (Martínez Catalán et al., 2004, 2008, 2016), the age of emplacement of the allochthonous complexes during C2. The first foliation in the preorogenic metasediments of the UPa and CIZ is axially planar to recumbent folds of the C1 event and predates the main foliation in the synorogenic deposits. But a second, low-grade penetrative foliation was developed in the UPa, LPa and CIZ during the emplacement of the Allochthon (C2). This second regional foliation is the one showing similar aspect and metamorphic conditions in both UPa and LPa ensembles, but in the latter it represents the first tectonic fabric.

The internal zones of the orogenic belts are considered areas with scarcely preserved related synorogenic sequences because of the subsequent denudation caused by the orogenic relief (Martínez Catalán et al., 2008), but they might be preserved in the core of synclines or below post-depositional thrust. In other parts of the Variscan belt, Franke and Engel (1986) described tectonic slices carrying synorogenic sedimentary units from more internal areas. In NW Iberia, while there is a complete record of the synorogenic deposits in the foreland fold and thrust belt (CZ; e.g., Marcos and Pulgar, 1982; Merino-Tomé et al., 2017), in the deeply eroded internal part of the chain, the first identified synorogenic sequence was the San Clodio Series (Matte, 1968), which is preserved at the core of the late-Variscan Sil Syncline in northern Iberia (Figs. 2, 4 and 5, and cross section 2 in Fig. 3). It consists of a rhythmic turbiditic sequence of pelite and greywacke (Riemer, 1966; Pérez-Estaún, 1974) including Upper Devonian or even older fossil plant fragments (Pérez-Estaún, 1974). Previous detrital zircon studies (samples SO-1 and SO-2; location of all samples from previous studies are plotted in the map of Fig. SF1.1 of the Supplement) have reinforced the synorogenic character and supported an Upper Mississippian maximum depositional age (Martínez Catalán et al., 2004) according to the youngest ages found in the detrital zircon population of ca. 324 Ma. Towards the base it displays exotic lithic blocks and pebbles, that is, extrabasinal rocks derived from the basement exposed in the basin surrounding highs. These include carbonaceous chert, quartzite, slate, gneiss and granite. Intrabasinal (“native”) lithified intraclasts and soft pebbles also occur (Riemer, 1966). The San Clodio Series is often separated from the underlying Ordovician formations by a few meters of Silurian black shales (SCSSs) which were mylonitized forming a basal detachment (Barrera Morate et al., 1989). However, the unconformity was preserved from reactivation at a few places (Martínez Catalán et al., 2016). Following Festa et al. (2019, 2020) terminology, the San Clodio Series represents a coherent primary succession above a sedimentary block-in-matrix unit (olistostrome).

The presence of a variably deformed SCSS unit below the synorogenic lithostratigraphic units in the LPa, as described for the Sil Syncline, is a constant feature in all the areas incorporated into this study. The BLPD is a first-order structure that forms a complex arrangement of stacked tectonic slices depicting diverse patterns. Locally, this shear band only deforms the lower part of the SCSSs, preserving the sedimentary unconformity at the base of the synorogenic unit (Alcañices syncline; González Clavijo, 2006). In other places the BLPD involves the entire SCSSs with upper and lower shear bands that limit lower-order shear band structures merging to create a first-order S/C structure (eastern Morais Complex; Dias da Silva, 2014). In many sections, deformation along the BLPD involves the BIMF deposits placed at the base of the LPa, thus resulting in a polygenetic mélange in the Festa et al. (2019) meaning. Also common is the presence of lower-order thrust duplexes within the LPa that are rooted in the BLPD. In these cases (San Vitero Formation in the Alcañices Syncline, described in this section) the LPa is internally repeated by a stack of imbricated tectonic slices each with the SCSSs at the base. The lensoid shape of the SCSSs along the BLPD and associated structures also suggests that it was submitted to a strong tectonic pinching (thinning) and swelling (thickening) during thrusting. However, deformation is not always pervasive, as several size lenses of comparatively undeformed SCSS preserved fossiliferous content at many localities (Romariz, 1962, 1969; Quiroga de la Vega, 1981; González Clavijo et al., 1997; Piçarra et al., 2006b), defining all the Silurian graptolite biozones but lower Rhuddanian and upper Ludfordian stages as well as the Pridoli series (González Clavijo, 2006; Piçarra et al., 2006b).

The most extensive outcrop of the synorogenic rocks studied occurs in the periphery (structurally below) of the Bragança Complex (Figs. 2 and 4). To the east of this allochthonous complex, the core of the Late Variscan Alcañices Synform (Figs. 2, 4 and 5; cross sections 4 and 5 in Fig. 3) is formed by several LPa synorogenic units tectonically piled up in a number of imbricated thrust units (C2) folded by a train of NW–SE-trending upright folds (C3 - M3) (González Clavijo and Martínez Catalán, 2002; González-Clavijo et al., 2012). From the top (more internal) structural position to the bottom (more external) the synorogenic units are named: Gimonde, Rábano, San Vitero and Almendra formations. According to the maximum depositional age (MDA) obtained through detrital zircon U–Pb geochronology (Upper Devonian to uppermost Mississippian), these formations include progressively younger rocks from the more internal to the more external ones, thus suggesting migration of the depocenter toward the relative autochthon coeval to the Parautochthon stacking (González-Clavijo et al., 2012; Martínez Catalán et al., 2016).

The structurally highest Gimonde Formation (Pereira et al., 1999; Meireles et al., 1999a, b) is formed by finely bedded phyllites and metagreywackes, and scarce polymictic microconglomerate lenses containing exotic clasts and native intraclasts. Its age has been considered Upper Devonian on base of fossil plant debris (Teixeira and Pais, 1973) and palynomorphs (Pereira et al., 1999). However, detrital zircon ages (samples SO-7, SO-8, SO-9, SO-12; Martínez Catalán et al., 2016) imply an early Carboniferous (Tournaisian–Visean) MDA.

The structurally underlying Rábano Formation (González Clavijo and Martínez Catalán, 2002), also called External Gimonde in Martínez-Catalán et al. (2016), comprises diverse lithologies being the most abundant a BIMF sequence displaying large exotic olistoliths of deformed rhyolites and dacites, felsic metatuffs, epiclastic rocks, white and grey quartzite, Silurian lydite (black radiolarite) and ampelite (carbonaceous shale), greywacke, phyllite, and limestone (Fig. 6a). The ages of these blocks (sometimes hundreds of meters in length) based on fossils and U–Pb zircon ages range from Furongian to Emsian (González Clavijo et al., 2016). At the uppermost Rábano Formation a flyschoid sequence made of phyllite, quartzlitharenite, and local polygenic microconglomerate holds deformed exotic clasts and lithified intraclasts (Fig. 7a, b, and c), including plagioclase and volcanic quartz mineraloclasts and quartz shards indicating a nearby volcanic source (González Clavijo, 2006). Detrital zircon ages performed in this wild flysch (sample SO-6) support a synorogenic nature and point to a Visean MDA (González Clavijo et al., 2016; Martínez Catalán et al., 2016).

The San Vitero Formation (Martínez García, 1972) is a flysch made of up to meter-thick phyllite and quartzlitharenite rhythms and local lenses of polygenic microconglomerates holding exotic clasts and lithified intraclasts (Figs. 7d, e and 8a, b) (González Clavijo and Martínez Catalán, 2002). This unit was considered Upper Devonian or younger based on fossil plant debris (Teixeira and Pais, 1973) but detrital zircon studies (samples SO-4, SO-5 and SO-13) support a Tournaisian MDA (Martínez Catalán et al., 2016).

The structurally lower imbricated thrust system hosts the Almendra Formation (Vacas and Martínez Catalán, 1987) which is a calciturbidite made of phyllite and calcarenite rhythms up to several meters thick (González Clavijo, 2006). Local lenses of polygenic conglomerates and microconglomerates holding exotic clasts and pebbles, and lithified intraclasts were firstly described by Aldaya et al. (1976). Their lithologies include phyllite, sandstone, litharenite, quartzite, limestone, orthogneiss, rhyolite and felsic volcanic tuff (Fig. 7f, g and h). Major blocks of lydite with Silurian graptolites and limestones (Fig. 6b and c) containing abundant fossils (bioclasts of corals, sciphocrinoides, bivalves, gastropods and tentaculites) have been identified (González Clavijo and Martínez Catalán, 2002; González Clavijo et al., 2016 and references therein). Conodonts found in calcarenite yielded a Lower Devonian age (Sarmiento et al., 1997). Detrital zircon studies in the Almendra Formation indicated a Visean MDA (sample SO-14), thus supporting the Variscan synorogenic origin of this unit (Martínez Catalán et al., 2016).

The LPa emplaced at the eastern rim of the Morais Complex (Figs. 2, 4 and 5; cross section 6 in Fig. 3) was described by Dias da Silva (2014) as a turbiditic synorogenic sequence comprising two stratigraphic units (Travanca and Vila Chã Formations). They consist of coherent primary units, broken bed units and lenses of block-in-matrix units displaying hectometer-sized olistoliths. Clasts include native intraclasts and soft clasts as well as exotic deformed lydite, ampelite and quartzite. These units lack fossil-based ages. A palynomorph study performed by Gil Machado (in Dias da Silva, 2014) was unfruitful because of poor pollen preservation owing to a Variscan thermal imprint. Detrital zircon studies (samples VC-21ZIR, VC-45ZIR and VC-57ZIR) suggest a Devonian MDA (ca. 390 Ma; Dias da Silva et al., 2015).

In the Marão Range, west of Vila Real (Fig. 2), the westernmost extent of the rocks studied, tectonic slices appertaining to the Pa comprise several turbiditic sequences (Mouquim, Canadelo and Santos Formations) displaying rhythms of phyllite and greywacke with some intercalations of volcanic tuffs towards the top (Pereira, 1987). As they lie concordantly above the SCSSs dated by graptolites (Piçarra et al., 2006b), a Devonian age for the flyschoid units was proposed, without fossiliferous evidence (Pereira et al., 2006). González Clavijo (2006) supported a correlation between these units and the San Vitero flysch, in the Alcañices synform, on the basis of the lithologies and the stratigraphic position, and for that reason here a possible Tournaisian MDA is proposed. According to our proposal of a tectonostratigraphic scheme, all the stacked pile must be considered as belonging to the LPa.

The known existence of synorogenic LPa units, partially encircling the GTMZ to the east, fostered the present research and aimed to recognize their possible extension to other areas of the GTMZ (Fig. 2). In this work 17 samples from areas surrounding the Bragança, Morais and Cabo Ortegal complexes were used for zircon U–Pb geochronology (location of all samples in the map of Fig. SF1.1; coordinates in Table SF1).

The detailed description of the new zircon geochronology study is presented in the Supplement and in Figs. SI-1 to SI-9. The complete dataset with the new U–Pb isotopic analyses is given in Tables S1–S17 in the Supplement. The reference and complementary U–Pb zircon age datasets used in the multidimensional scaling (MDS) process and other statistical procedures are in Tables S18 and S19.

In this section, the youngest zircon grain and zircon population ages of our new results are discussed for the different parts of the synorogenic carpet and compared with published data.

The Meirinhos area, to south of the Morais Complex (Figs. 2, 4 and 5; cross section 4 in Fig. 3), was divided in two stratigraphic units: Meirinhos and Casal do Rato following Pereira et al. (2009) and Rodrigues et al. (2003). They were considered synorogenic flyschoid deposits bearing olistoliths of quartzite, phyllite, greywacke, felsic and mafic volcanic tuffs, limestone, ampelite and lydite by Pereira et al. (2009). However, different age, stratigraphic features and structure were proposed (Sá et al., 2014) based on the reappraisal of the Lower Ordovician trilobite Cruziana ichnofossils in Armorican-type quartzites originally described by Ribeiro (1974) in this sector. Field data allow us to confirm the earlier proposal and the recognition of new synorogenic features as slump folds, disrupted beds, and olistostromes including hectometer-sized lydite and quartzite olistoliths (Fig. 6d and e). In both stratigraphic units, blocks, cobbles and pebbles of native (lithified intraclasts, soft pebbles) and exotic (quartzites, ampelite and lydite, felsic and mafic volcanic rocks, limestone) sources have been identified. Biostratigraphic ages of the olistoliths range from early Ordovician (trilobite tracks, Ribeiro, 1974; Sá et al., 2014) to Silurian (graptolites, Pereira et al., 2009). Two samples of medium-grained greenish greywacke belonging to the turbiditic sequence (CR-ZR-01 and MEI-ZR-01; see location, coordinates and geochronology of all geochronology samples in the Supplement) were collected in this area with a youngest single zircon (YZ) with Upper Ordovician age (439±8 Ma for CR-ZR-01 and 443±6 Ma for MEI-ZR-01) and MDA (i.e., concordant age given by the youngest zircon population in the 90 % or 95 % concordance interval; it may not include the YZ) of 467 ± 4 Ma (CR-ZR-01) and 486 ± 3 Ma (MEI-ZR-01) (Supplement; Fig. SI-1A and B). These ages do not support a synorogenic character nor a Lower Ordovician age for these siliciclastic rocks. However, the sedimentary features and the cartographic and structural continuity and correlation with the previously described LPa unit east of the Morais Complex (i.e., Travanca Formation in Dias da Silva et al., 2015) make it possible to propose a Mississippian MDA for these synorogenic siliciclastic rocks, with contribution of early Silurian, Middle Ordovician and Tremadocian (possibly magmatic) zircon sources.

West of Mirandela, around the village of Suçães, there is a tectonic window (Rodrigues et al., 2010) partially controlled by late- or post-Variscan NNE–SSW subvertical faults (Figs. 2, 4 and 5; cross sections 3 and 6 in Fig. 3) which displays a turbiditic sequence attributed to the Devonian (Ribeiro, 1974) or to the Silurian with small patches of Lower Devonian siliciclastic rocks in the upper stratigraphic positions (Rodrigues et al., 2010). The MTMT was firstly mapped in this area by Rodrigues et al. (2010), tectonically separating the UPa overturned fold sequence from the LPa imbricated thrust complex. The proposed Silurian–Devonian age was based on graptolite assemblages preserved in the Silurian lydites (Piçarra et al, 2006b). Field and geochronological data allow us to infer that the LPa in this sector is also a synorogenic sequence, presenting the classical flyschoid features (Fig. 6f) (made of centimeter to meter beds of pelite and greywacke; Rodrigues, 2008). Scarce Variscan detrital zircon grains (Famennian) were found in the greywacke layers of the flysch sequence, in the Upper Schists Formation (MIR-41: YZ = 369 ± 7 Ma, MDA = 497 ± 5 Ma, Fig. SI-4A; AD-PO-49: YZ = 468 ± 39 Ma, MDA = 494 ± 27 Ma, Fig. SI-4B; AD-PO-55: YZ = 444 ± 26 Ma, MDA = 488 ± 16 Ma, Fig. SI-5A) and in the culminating slate and greywacke formation (sample AD-PO-57: YZ = 372 ± 6 Ma, MDA=488 ± 16 Ma, Fig. SI-5B). The combination of field and geochronology data suggest that the graptolite-rich lydites belong to exotic olistoliths or to the SCSSs at the base of the synorogenic tectonic slices. Detrital zircon ages and the lithostratigraphy of these stratigraphic units enable us to consider them as coherent primary units with olistoliths, overlying a tectonic mélange developed in the SCSSs.

To the west, in the Vila Pouca de Aguiar area (Fig. 2), the LPa comprise several tectonically stacked flysch units limited by thrust planes (Ribeiro, 1974; Ribeiro et al., 1993; Noronha et al., 1998; Ribeiro, 1998; Rodrigues, 2008). All units are formed by turbidite sequences, with millimeter- to meter-thick interbedded pelites and greywackes (quartzwacke towards the base). The coarse-grained layers are very rich in plagioclase, thus suggesting a near-source (felsic) volcanic input into the basin (Rodrigues, 2008). Differently sized lens-shaped bodies of grey quartzite, black limestone, felsic metavolcanic rocks and highly sheared SCSSs are described within these units (Rodrigues, 2008). Our field research permitted us to define these LPa formations as coherent primary units with block-in-matrix deposits exposing exotic fragments in a synorogenic convolute sediment tectonically repeated by imbricated thrust faults. These units are currently considered as Silurian–Devonian based on Silurian graptolites preserved in the lydites (Piçarra et al., 2006b) and by lithological comparison with similar formations to the east of the Bragança and Morais Complexes (Ribeiro, 1974; Noronha et al., 1998; Ribeiro, 1998; Pereira et al., 2000). The detrital zircon geochronology of one sample of lithoclastic coarse-grained arkose (AD-PO-48B) returned a Famennian–Tournaisian youngest single zircon and maximum depositional age (YZ: 355 ± 34 Ma; MDA: 364 ± 22 Ma; Fig. SI-3B).

At the northern edge of the Bragança Complex (Figs. 2, 4, 5 and 9; cross section 3 in Fig. 3), the Upper Allochthon high P/high T rocks tectonically overlay the LPa. Field work confirmed the absence of the UPa stratigraphic units and the discontinuous cartographic outline of the Middle and Lower Allochthon (Meireles et al., 1999a, b). These units were tectonically thinned by an extensional shear zone or truncated by an out-of-sequence thrust system. In the upper structural part of the LPa, Ribeiro and Ribeiro (1974) noticed the presence of differently sized rock fragments in the stratigraphic sequence. They describe (i) epizonal fragments such as phyllite, quartz-phyllite, quartzite, felsic tuffs and rhyolites, ampelite, and lydite, and (ii) meso-catazonal fragments of paragneiss (albite, chlorite and K feldspar), blastomylonite, and biotite–garnet gneiss. We have verified that these rocks appear as centimeter- to hectometer-sized olistoliths, being dispersed in a wider area than previously estimated (Figs. 6g, h and 10a). The olistoliths usually cluster within large mass-wasting deposits (BIMFs) or occur as isolated bodies in the flysch sequence. The native rock blocks consist of fragments of consolidated fine- to coarse-grained greywacke beds among other siltstone–sandstone intraclasts and pelitic soft pebbles. The exotic blocks include highly deformed lydites and ampelites, rhyolites, felsic tuffs, quartzites, and limestones. Regional work in this area (Meireles, 2013) divides this sector LPa in four formations: Coroto, Rio de Onor, Soutelo and Gimonde, all of them bearing flyschoid characteristics and olistoliths (Figs. 4 and 5). In some of these units (Coroto and Soutelo) the limestone blocks yield upper Silurian to Lower Devonian crinoids (Meireles, 2013). Inherited Cambrian–Middle Ordovician acritarchs and lower–middle Silurian palynomorphs were also found in the Rio de Onor Formation (Pereira et al., 1999). In the Soutelo Formation, we also report the presence of a sandy–quartzitic olistolith with poorly preserved brachiopods of the “Lingula” genus (Sofia Pereira and Jorge Colmenar, personal communication, 2017), which are particularly common in the Lower Ordovician Armorican quartzite of the CIZ (Marão Range; Coke et al., 2001). Three samples collected in the Gimonde Formation (Meireles et al., 1999a, b) were used for geochronology of detrital zircon grains: GIM-ZR-01 (microconglomerate), EC-PO-293 and AD-PO-66 (quartz-lithic sandstones). Although two of these samples have yielded Silurian (EC-PO-293: YZ = 426 ± 44 Ma and MDA = 435 ± 29 Ma; Fig. SI-2B) and Furongian (AD-PO-66: YZ = 431 ± 31 Ma and MDA = 483 ± 18 Ma; Fig. SI-3A) youngest zircon ages, one sample has Variscan detrital zircon ages (GIM-ZR-01: YZ=327 ± 9 Ma and MDA = 354 ± 3 Ma; Fig. SI-2A) confirming the results obtained in previous studies (samples SO-9 and SO-12 in Martínez Catalán et al., 2016), which allowed the characterization of the Gimonde Formation as a Tournaisian–Visean synorogenic stratigraphic unit.

The Picón Beach at Cabo Ortegal Complex is placed east of the Ortigueira locality, in the Galicia northern coastline (Figs. 2, 4 and 5; cross section 1 in Fig. 3). There, the Loiba unit of the Rio Baio thrust sheet (Marcos et al., 2002) is structurally on top of the tectonically sheared (autochthonous) SCSSs that define the BLPD. This tectonic unit is formed by low metamorphic grade flysch sequences and discontinuous lens-shaped BIMFs. Its field aspects and structural relationships with the overlying and underlying tectonostratigraphic units led us to consider this unit as part of the Variscan synorogenic marine basin. The detrital geochronology of a fine-grained quartzite sample (PICON-2) has yielded a Tournaisian maximum depositional age (YZ: 350 ± 7 Ma; MDA: 357 ± 4 Ma; Fig. SI-6) thus supporting the extent of the LPa from northeast Portugal to the northern Spanish coast.

In all lithostratigraphic units of the LPa exotic (extrabasinal) and native (intrabasinal) grains, clasts, pebbles and large olistoliths have been identified. Exotic rock fragments show presedimentary mild to high deformation and metamorphic aspects, contrasting with the usually poorly deformed, low-metamorphic flyschoid sequence. The native fragments include soft pebbles and intraclasts, often showing synsedimentary deformation features (slump folds, boudinage, bedding disruption and convolute bedding of turbiditic aspect: Fig. 6d and e). The most consolidated sedimentary fragments suggest that they were recycled within the basin, as a wild flysch (Ribeiro and Ribeiro, 1974; Aldaya et al., 1976; González Clavijo and Martínez Catalán, 2002; Martínez Catalán et., 2016). These fragments were considered a proof of its synorogenic character, and also evidence that the basin was fed from areas of the Variscan belt already deformed and metamorphosed (Antona and Martínez Catalán, 1990; González Clavijo and Martínez Catalán, 2002; Martínez Catalán et al., 2004, 2008). Complementarily, the fossil flora, fauna, and ichnofossil findings in the exotic olistoliths display ages from Lower Ordovician to Middle Devonian (Fig. 5). This wide range of biostratigraphic ages seems to indicate that fossil findings belong to rock blocks within a synorogenic turbiditic unit, which is confirmed by stratigraphic features and detrital zircon ages (González Clavijo et al., 2016). Trying to confirm this hypothesis, a U–Pb zircon geochronology study on volcanic rocks considered to represent olistoliths from the LPa was performed (Fig. SF1.1). We have complemented our study with the integration of already published U–Pb zircon age data from other volcanic olistoliths in the LPa (Farias et al., 2014; González Clavijo et al., 2016). The complete description of the samples, their location and their geochronology is presented in the Supplement.

New results from field and geochronology studies on the Variscan (ca. 400–320 Ma) hinterland synorogenic marine deposits of the Parautochthon in NW Iberia are presented. They surround large parts of the Galicia–Trás-os-Montes Zone (GTMZ), separating the Allochthon and Parautochthon from the structurally underlying Autochthon of the Central Iberian Zone (CIZ). The new data are useful for better understanding the relationship of the NW Iberian Variscan hinterland marine basin with the structurally underlying and overlying units. We show that the two units defined in the Parautochthon at the eastern rim of the Morais and Bragança complexes cover an area larger than previously estimated, being exposed from Cabo Ortegal (NW Spain) to Trás-os-Montes (NE Portugal). The existence of a preorogenic highly folded Upper Parautochthon (UPa) and an imbricated Lower Parautochthon (LPa) composed of slices of Devono-Carboniferous turbidites and tectonically scrapped autochthonous Silurian carbonaceous–siliceous slates (SCSSs) is a general attribute of the whole NW Iberian Parautochthon. Regional tectonic and stratigraphic features show that the LPa represents a synorogenic basin that was gradually incorporated into the base of the allochthonous accretionary wedge while it was being emplaced onto the northern Gondwana margin, forming a continuous tectonic carpet at the base of the GTMZ.

Analysis of the stratigraphic and tectonic features of the synorogenic flysch in the LPa highlights the relevance of large-scale mass wasting deposits in the sequence. Block-in-matrix formations (BIMFs) represent sedimentary mélanges including large olistoliths derived from the accretionary wedge and probably the Autochthon exposed in a forebulge in front of the wedge. They appear surrounded by a chaotic matrix with slump folds and broken beds of the flysch sequence. The BIMFs formed due to gravitational instability triggered by tectonic activity within and at both margins of the basin. These deposits are frequently tectonized, forming imbricated thrust complexes with polygenic mélanges and tectonically scrapped autochthonous SCSSs at the base.

Zircon geochronology of the LPa siliciclastic rocks and of magmatic olistoliths derived from the UPa and the Autochthon constrains the provenance of the sediments and blocks in the LPa. Our study confirms the synorogenic nature of the LPa stratigraphic units, which include Emsian to Serpukhovian detrital zircon grains. These Variscan grains probably derive from nearby sources located in the allochthonous complexes (metamorphic ages of 400–360 Ma), and from Variscan granitoids and migmatites (ca. 340–320 Ma). A comparison of the detrital zircon populations of the NW Iberian synorogenic marine deposits, including the magmatic and inherited ages now obtained in the Middle Ordovician–Silurian volcanic rocks of the UPa (source) and LPa (olistoliths), with a compilation of reference samples from possible source areas, showed evidence of direct source-to-sink relationships of the Variscan hinterland basin with the accretionary complex (GTMZ) and the peripheral bulge in the Autochthon. Multidimensional scaling analysis showed evidence intrabasinal sediment recycling and mixing of sources in time and space. These are explained by (i) the tectonic instabilities within the basin and in its margins, (ii) the migration of the depocenter towards inland Gondwana, and (iii) the gradual incorporation of the synorogenic basin into the accretionary wedge leading to its denudation and recycling.