A tectonic carpet of Variscan flysch at the base of an unrooted accretion prism in NW Iberia: U-Pb zircon age constrains from sediments and volcanic olistoliths

The allochthonous complexes of Galicia – Trás-os-Montes Zone (NW Iberia) are part of the tectonic stack that unrooted the Variscan accretionary prism. They are formed by individual tectonic slices marked by specific tectono15 metamorphic evolution, which was piled up in a piggy-back thrust complex onto its relative autochthon, the Central Iberian Zone (CIZ). Consequently, allochthony decreases towards lower, more external and younger thrust sheets. The lowermost unit of this pile of slivers is known as Schistose Domain or Parautochthon and bears low metamorphic grade, contrasting with the higher temperatures and pressures estimated for the upper allochthonous units, but sharing the stratigraphic sequence with the underlying autochthon. The Parautochthon is divided in two structural and stratigraphic sub-units: i) the Lower (LPa) made of 20 synorogenic flysch-type sediments with varied turbiditic units and olistostrome bodies, showing Upper Devonian-lower Carboniferous age on base of the youngest zircon populations and fossiliferous content; ii) the Upper (UPa), composed of highly deformed pre-orogenic upper Cambrian-Silurian volcano-sedimentary sequence comparable with both the nearby autochthon and the HP-LT Lower Allochthon, laying structurally above. The UPa thrusted onto the LPa by the the Main-Trásos-Montes Thrust; and the LPa detached from the CIZ relative autochthon by a regional structure (Basal Lower Parautochthon 25 Detachment) which follows the favourable Silurian carbonaceous beds. A review on the detrital zircon studies of the synorogenic LPa complemented by 17 new samples geochronology is here presented. The results support the extension of the LPa underneath the NW Iberia allochthonous complexes, from Cabo Ortegal, to Bragança and Morais Massifs. Its current exposure follows the lowermost tectonic boundary between the Galicia – Trás-os-Montes (allochthon) and Central Iberian (autochthon) Zones. Youngest zircon age populations point to a maximum 30 sedimentation age for the LPa formations ranging from Famennian to Serpukhovian and endorse the piggy-back evolution inside this unit, mimicking the general structure of the Galicia – Trás-os-Montes Zone. The zircon populations in the LPa allow constraining the sedimentary provenance areas, showing the intervention of nearby sources (mostly the UPa) and/or multiply recycled/long transport sediments with typically N-Central Gondwana age https://doi.org/10.5194/se-2020-173 Preprint. Discussion started: 15 October 2020 c © Author(s) 2020. CC BY 4.0 License.

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 using MDS (ISOPLOT-R, Vermeech, 2018) performed in this work (described in detail in Supplementary File) offers a new general view of the LPa geotectonic setting at the Variscan times and sets new constrains on the source-areas of the synorogenic sediments and blocks.
This allows a better view on the paleogeography and geodynamic setting of the Late Devonian-lower Carboniferous flysch 105 basins in NW Iberia and their relationship with the surrounding (exposed) tectonic units in the shoulders of the Variscan marine basins.
This led to a later division in two tectonically overriding units, Upper and Lower Parautochthon (UPa and LPa), in the meaning 135 firstly proposed by Rodrigues et al. (2006a;2006b) and updated by Dias da Silva et al. (2014;2015;2016). This division restricts 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 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 (e. g. Martínez Catalán et al., 2016). This proposal required a relocation of the bounding thrust structures of the lower tectonic sheets of the GTMZ: 140 i) the LA-UPa thrust (basal thrust of the Centro-Transmontano thrust complex in the meaning of Ribeiro et al. 1990b) into an upper structural position; ii) the UPa-LPa thrust system named as Main Trás-os-Montes Thrust (MTMT) (Ribeiro, 1974;Ribeiro and Ribeiro, 2004;Meireles et al., 2006;Pereira et al. 2006) to a lower structural position (Dias da Silva et al., 2014). The MTMT is an up to 1000 m thick gently dipping shear-zone responsible by the thrusting of the 145 upper Cambrian-Silurian (pre-orogenic UPa) sequence onto the syn-orogenic LPa, allowing the thickening the upper crust during the Tournaisian-Visean stage (Dias da Silva et al., 2014;2015;2016;2020;Azor et al., 2019);

iii)
At the base of the LPa another major fault structure named the Basal Lower Parautochthon Detachment (BLPD), also gently dipping, separates the syn-orogenic piggy-back imbricated slices from the structurally 150 underlying autochthon (Dias da Silva et al., 2014). The BLPD was developed following a favorable stratigraphic unit, the condensed Silurian autochthonous sequence formed by carbonaceous cherts and graphitic slates (González Clavijo and Martínez Catalán, 2002; Dias da Silva et al., 2014).
In the studied area, the three structural units considered in this work (UPa, LPa and CIZ) underwent a regional Barrovian metamorphism (M1) through the early Variscan compressive events (C 1 +C 2 on the Alcock et al., 2015 proposal) which were later followed by a complex extensional (E 1 -M 2 ) and compressive (C 3 -M 3 ) tectonothermal history (Azor et al., 2019; Dias da 160 Silva et al., 2020). Some specialized studies were performed to discriminate if the metamorphic grade at the syn-orogenic units was lower than in the pre-orogenic units by illite crystallinity (Antona and Martínez Catalán, 1990) and Colour Alteration Index in conodonts (Sarmiento and García-López, 1996;Sarmiento et al., 1997) but no conclusive results were attained. This matches with the Matte (1968) conclusions for the autochthonous San Clodio syn-orogenic deposits, supporting the same metamorphism and deformation in the Carboniferous than in the underlying autochthonous Ordovician sequence. content ranges from Furongian to Emsian (González Clavijo et al., 2012;2016). At the upper Rábano Fm. a flyschoid sequence made of phyllite, quartzlitharenite, and local microconglomerate holds exotic, lithified and deformed grains and clasts (Fig. 200 6A, B, and C); including plagioclase and volcanic quartz and explosive quartz shards supporting a vulcanite rich source area for this unit (González Clavijo, 2006). Detrital zircon ages studies performed in this wild-flysch (sample SO-9; situation in Fig. 4) support a syn-orogenic nature and points to an age Tournaisian or younger (González Clavijo et al., 2012;2016;Martínez Catalán et al., 2016). The Rábano Fm. lies on a sheared condensed black Silurian unit which develops a tectonic mélange and, sometimes, a polygenetic mélange where deformation is superimposed to a sedimentary mélange. The 205 discontinuity of the Silurian unit along the tectonic limit suggests that a strong stretching event was concentrated in this band. The tectonically lower multiplex includes the Almendra Fm. (Vacas and Martínez Catalán, 1987) which is a calciturbidite made of phyllite and calcarenite rhythms up to several metres thick (González Clavijo, 2006). Local lenses of conglomerates and microconglomerates holding exotic, lithified and deformed pebbles (Fig. 6F, G and H) of diverse lithologies (phyllite, 215 sandstone, litharenite, quartzite, limestone, ortogneiss, rhyolite, acidic volcanic tuff) were firstly described by Aldaya et al. (1976). Major blocks of black lydite with Silurian graptolites and limestone ( Fig. 5B 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 there in) and the conodonts seized in the calcarenite yielded Lower Devonian age (Sarmiento et al., 1997). Detrital zircon studies in Almendra Fm yielded Mississippian ages (sample SO-14; 220 situation in Fig. 4), thus supporting the Variscan syn-orogenic origin of this unit (Martínez Catalán et al., 2016). As in the other duplexes, the Almendra coherent unit is underlayered by a sheared band made of a block-in-matrix sequence and the condensed Silurian succession, thus forming a polygenetic mélange (Festa et al., 2019;2020) The LPa placed at the eastern rim of the Morais Complex ( Fig. 2 and cross section 6 in Fig. 3 In the Marão range, W of Vila Real (Fig. 2), some tectonic slices appertaining to the GTMZ Parautochthon comprise several flyschoid stratigraphic sequences displaying rhythms of phylite and greywacke with some volcanic tuffs towards the top (Pereira, 1987). They are concordantly above the sheared black Silurian unit mostly made of ampelite and lydite, but also having some quartzite and black limestone discontinuous bodies in the upper levels. As this last unit was dated by graptolites 235 as Silurian (Piçarra et al., 2006), it was proposed, without fossiliferous evidences, a Devonian age for the overlying flyschoid units (Pereira et al., 2006). González Clavijo (2006 supported a correlation between these units and the San Vitero flysch, in the Alcañices synform, on base to the lithologies and the stratigraphic position, thus meaning a possible Tournaisian age (Martínez Catalán et al., 2016). In this work we consider the flyshoid sequences as coherent primary units, and the Silurian condensed sequence as a tectonic mélange placed at the base of every tectonic slice. According our proposal of tectono-240 stratigraphic scheme, all the stacked pile must be considered as belonging to the LPa.
graptolites (Pereira et al., 2009), are here interpreted as the age of olistoliths. Two samples (CR-ZR-01, and MEI-ZR-01, situation in Fig. 4, coordinates in Table 1 Ribeiro (1974) and Silurian with upper small patches of Lower Devonian by Rodrigues et al. (2010). These late authors mapped in this area the MTMT, which places tectonically the UPa onto the LPa. The here considered LPa syn-orogenic sequence has flyschoid features and it is made of centimetre to metre rhythms of pelite and greywacke ( Fig. 5F) overlying a black ampelite and lydite Silurian sequence with a superimposed strong shearing (Rodrigues, 2008). The proposed Silurian-Devonian age was 275 based on graptolites found in the black lydites (Piçarra et al, 2006) Table 1 and geochronology in Supplementary File) suggest that the black chert bodies are Silurian olistoliths, and/or the graptolite samples were picked up exclusively in the underlying Silurian black unit. The attained zircon ages (SI-4, SI-5) and the flyschoid features enable us to consider this area 280 as a coherent primary unit with olistoliths overlying a tectonic mélange developed in the Silurian carbonaceous rocks.
To the W, in the Vila Pouca de Aguiar region ( Fig. 2), the LPa comprised several units limited by thrust planes and bearing stratigraphic successions with changing names during the last years (Rodrigues, 2008) but sharing flyschoid characteristics (Ribeiro, 1974;Ribeiro et al., 1993;Noronha et al., 1998;Ribeiro, 1998;Rodrigues, 2008). The generalized description for all the successions is a rhythmic sequence (millimetre to metre thick) of pelite and greywacke (quartzwacke towards the base) 285 very rich in plagioclase, thus suggesting a great volcanic input in the basin (Rodrigues, 2008). This group of units include lensshaped bodies of different sizes made of: grey quartzite, black limestone, acidic metavolcanic rocks, and black lydite and ampelite, being the last two intensely sheared in most of the occurrences (Rodrigues, 2008). After the field exploration we envisage this group of formations as coherent primary units with block-in-matrix parts, which slid along the underlying sheared Silurian black sequence (tectonic mélange) to form duplexes. The age has been considered Silurian-Devonian based on 290 lithological correlation with nearby formations (Ribeiro, 1974;Noronha et al., 1998;Ribeiro, 1998;Pereira, 2000) and Silurian graptolites found by Piçarra et al. (2006). One sample (AD-PO-48B, situation in Fig. 4, coordinates in Table 1  section, at the upper part of the LPa, the presence of exotic and lithified pebbles and greywacke grains was recorded by Ribeiro and Ribeiro (1974). These authors describe (i) epizonal fragments: phyllite, quartz-phyllite, quartzite, acidic volcanic tuffs, black ampelite and lydite; and (ii) meso-catazonal fragments: paragneiss (albite, chlorite and K feldspar), blastomylonites, and biotite-garnet gneisses. In this work metre blocks of those materials have been found in a wider area (Fig. 5G, H and 3A), 300 where some rhyolite, acidic volcanic tuff, black lydite and limestone hectometre-long olistoliths were also identified. The base of the LPa in this section displays the same black Silurian condensed sequence previously described in other areas, and it is also deformed for a complex shear band which in some places also involves block-in-matrix sedimentary bodies, thus creating a polygenetic mélange. The existence of a tectonic duplex consisting in slices repeating the general architecture above The Picon Beach exposure at Cabo Ortegal Allochthonous Complex is placed E of the Ortigueira locality, in the Galicia 310 northern coastline (Fig. 2, and cross section 1 in Fig. 3). There, above the sheared black Silurian sequence placed on the top of the BLPD, a tectonic slice formed by low metamorphic grade fine grey sandstones, flyschoid sequences, and discontinuous block-in-matrix bodies led us to consider it a Variscan syn-orogenic deposit. Our detrital zircon study (PICON-2; situation in Fig. 3, coordinates in Table 1 and geochronology in Supplementary File) yielded a clear Tournaisian or younger age (YZ: 350 Ma; MDA: 357 Ma) (SI-6) thus supporting that the LPa extends as far as the northern Spain coast. 315

Lower Parautochthon magmatic olistoliths, their ages and possible source-areas
In several lithostratigraphic units of the LPa exotic and lithified grains, clasts, minor blocks and olistoliths have been identified, some of them previously deformed and metamorphosed (Ribeiro and Ribeiro, 1974;Aldaya et al., 1976;González Clavijo and Martínez Catalán, 2002;Martínez Catalán et., 2016). These fragments presence was considered a proof of its syn-orogenic character, and also evidence that the basin was fed from areas of the Variscan belt already deformed and metamorphosed 320 (Antona and Martínez Catalán, 1990;González Clavijo and Martínez Catalán, 2002;Martínez Catalán et al, 2004;. Complementarily, as above mentioned for the different areas, the fauna and flora findings in those materials display ages from Lower Ordovician to Middle Devonian; this dispersion suggesting the samples were taken in olistoliths as the encompassing material is clearly syn-orogenic on base to the stratigraphic features and the detrital zircon ages (González Clavijo et al., 2016).
Trying to confirm this hypothesis, a U-Pb zircon geochronology study on vulcanite bodies inside the LPa was performed in 325 four olistoliths ( Fig. 4) and completed with previous data from references. (Farias et al., 2014;González Clavijo et al., 2016).
The complete description of the samples and the geochronology study is presented in the Supplementary File. Situation of each sample is in Fig. 4 and coordinates are in Table 1.

Magmatic olistoliths ages results 330
As a complementary study of the detrital zircon in flyschoid sequences, four magmatic rock olistoliths were studied in our research, all of them located in the Alcañices synform and northern Bragança complex areas.

Olistoliths magmatic ages from references
In the Alcañices synform a quantity of vulcanite olistoliths has been identified; all of them of rhyolite to dacite composition, and often forming big clusters with a NW-SE attitude.
Previous research (González Clavijo et al., 2016) obtained an age of the Nuez olistolith (NUEZ-01; situation in Fig. 4), one of the major blocks forming a several kilometer long cluster included in an olistostrome inside the syn-orogenic Rábano Fm, towards the S of the synform. This block contains two volcanic facies: dacite lava and dacitic pyroclastic tuff (Ancochea et al. 1988), belonging the attained age to the latter. A LA-ICP-MS U-Pb in zircon concordant age of 497 ± 2 Ma (lowermost Furongian) was achieved, which was considered magmatic.
In the N of the same synform other volcanic body, Figueruela (COS-8; situation in Fig. 4), was dated by SHRIMP-II U-Pb analysis (Farias et al., 2014) and interpreted as a flow of dacitic lava interlayered in the Paraño Group of the Galicia Schistose 365 Domain or Parautochthon sensu lato. A thorough field review of the area shows a major cluster of olistoliths mainly composed of dacite and rhyolite lavas and tuffs, but also containing quartzite and black lydite big blocks. In our reinterpretation the Figueruela dacite is an olistolith contained in a basal block-in-matrix unit placed below the San Vitero coherent primary unit and above the black Silurian condensed unit. In this section, parts of the block-in-matrix unit and the Silurian black sequence are sheared, forming a polygenetic mélange in the meaning proposed by Festa et al. (2019Festa et al. ( , 2020. Thus, in our general model, 370 the Figueruela dacite belongs to the syn-orogenic LPa as proposed by González Clavijo et al. (2016). The SHRIMP U/Pb in zircon age is 488.7 ± 3.7 Ma (around the limit Furongian/Tremadocian) and is considered magmatic.
Between the Alcañices and Verín synforms, at the N of the Bragança Allochthonous Complex , other big volcanic body was dated by Farias et al. (2014) and named as the Soutelo rhyolite (COS-7; situation in Fig. 4). This sample was dated by the same analytical method than the Figueruela dacite yielding a 499.8 ± 3.7 Ma (upper Miaolingian). This massive rhyolitic lava was 375 also included in the Paraño group and considered a volcanic event among the sedimentary sequence. Our field study disclosed the existence of a huge cluster of olistoliths of diverse lithologies as black lydite, grey quartzite, greywacke, limestone, rhyolitic lavas, and acidic pyroclastic tuffs, being the last two the most abundant types. Complementarily, this major block-in-matrix unit is placed on top of an intensely deformed condensed black Silurian unit (a tectonic mélange). For all these reasons we consider that the rhyolite volcanic body analysed in Soutelo is also an olistolith inside the syn-orogenic LPa. 380

The possible sources of the magmatic olistoliths are in the UPa
The UPa unit, as defined for Dias da Silva et al. (2014) below the Morais Complex, contains an Upper Cambrian to Silurian detrital sequence with minor limestones and interbedded voluminous volcanism (Pereira et al., 2000;2006). The main volcanic events are, from bottom to top, Mora acid and basic volcanites (Dias da Silva, 2014; Dias da Silva et al., 2014;Díez-Montes 385 et al., 2015); the traditionally named as Saldanha gneiss (Ribeiro, 1974;Ribeiro and Ribeiro, 2004;Pereira et la., 2006;Pereira et al., 2008) which actually is a rhyolitic dome composed by lavas and volcanic tuffs (Dias da Silva et al., 2014;Díez-Montes et al., 2015); and finally the big acid and basic volcanic half of the Volcano Siliceous Complex in the higher part of the UPa, also known as Peso Volcano-Sedimentary Complex (Ribeiro, 1974;Pereira et al., 2006 Sample P381 (situation in Fig. 4, coordinates in Table 1

Structural and stratigraphic meaning of the Lower Parautochthon synorogenic basins 410
The studied samples containing Variscan zircons make possible to extend the LPa to areas bearing sequences pointing to a sedimentary environment (flyschoid with or without broken beds, block-in-matrix facies, olitostromes, slump folds) previously not openly stated as Variscan syn-orogenic deposits. From the eastern Bragança and Morais complexes rim area, where the LPa was defined (Dias da Silva et al., 2014;2015;2020), this unit may be spread by the S following the GTMZ edge through 1 datum from references in other olistolith (COS-7, Farias et al., 2014), support the existence of the LPa rocks in several stacked slices as the Variscan syn-orogenic sequence (Tournaisian age or younger) holds upper Cambrian and Ordovician volcanic glided blocks (Fig. 9A), besides previously findings of graptolite-rich Silurian lydites (e.g. Meireles et al. 1999aMeireles et al. , 1999bPiçarra et al., 2006aPiçarra et al., , 2006b. A similar arrangement was unveiled in the Alcañices Synform from 2 new samples picked in olistoliths (PET-01 and RAB-01) plus 2 from references (NUEZ, González Clavijo et al., 2016 andCOS-8, Farias et al., 2014) yielding upper Cambrian to Ordovician ages in blocks which are surrounded by the Upper Devonian to Mississippian syn-orogenic sequence, among other Lower-Ordovician-Lower Devonian fossil-bearing sedimentary olistoliths These two last zones, Alcañices and N Bragança, have continuity with the Nogueria Group at the Verín synform (Farias, 1990), which is the upper part of the originally defined group (Marquínez, 1984). In our field work, a coherent primary unit made of rhythms up 430 to decimetre composed of phyllite and greywacke has been identified in several zones (Fig. 9B), but greenish fine litharenite thick beds are also present close to the town of Verín. Block-in-matrix lensoidal bodies enclosing native and exotic blocks were also observed ( Fig. 9C and D), some of them big enough to be considered olistoliths (mainly made of quartzite, black lydite, black limestone, and acidic vulcanite, both lava and tuff). Towards the base, black lydite and ampelite beds are frequent and they are strongly sheared, thus forming a tectonic mélange involving syn-orogenic and black Silurian rocks (Fig. 9E). It 435 can be also envisaged as a very complex arrangement of minor tectonic slices forming an intricate multiplex system, where the horses repeat both stratigraphic units (Silurian and syn-orogenic). In the most oriental part of the SW limb ( Fig. 2 and cross section 3 in Fig. 3) this unit share the same characteristics but some observed olistholits are clearly made of UPa rocks, as they show the materials and polyphasic pervasive deformation characteristic of the tectonically overlaying unit (Fig. 9F, G and H).
In that Portuguese section, this unit has been named Lower Schist Formation and according Pereira et al. (2000) is composed 440 of phyllite and greywacke in fine rhythms, being the last very rich in plagioclase, thus supporting a source area rich in volcanic rocks. According the here adopted terminology (Festa et al., 2019(Festa et al., , 2020 its basal stretch is made of several tectonic slices mixing the block-in-matrix sequence and the black Silurian sheared rocks, thus being considered a polygenetic mélange. This work field revision on the Verín synform geology allows proposing in addition that the core of this late Variscan synform is the enlargement of the UPa for several reasons: 445 i) the map continuity with the Bragança Complex UPa (Fig. 2); ii) the existence of a major thrust underlying the unit (Farias, 1990), which is here reinterpreted as the MTMT following Dias da Silva (2014) 7proposal; iii) it is made of low metamorphic grade pervasively deformed detrital rocks like the sequence forming the UPa in Morais and Bragança (Nuño Ortea et al., 1981;Alonso, J.L, et al., 1981;Farias, 1990); 450 iv) it contains volcanic interbedded bodies (Nuño Ortea et al., 1981;Alonso, J.L, et al., 1981;Farias, 1990;Valverde Vaquero et al., 2007); v) a continuous white quartzite bed displaying the synform (Fig. 2) (Farias et 460 al., 1987;Ribeiro et al.,1990;Martínez Catalán et al., 1997). The internal structure of Rio Baio slice is complex, holding a greenschist facies detrital sequence which includes folded quartzites and volcanic rocks (Arce Duarte and Fernández Tomás, 1976;Arce Duarte et al., 1977;Fernández Pompa and Piera Rodríguez, 1975;Fernández Pompa et al., 1976;Marcos and Farias, 1997), namely the Loiba dacites, Costa Xuncos rhyolites and Queiroga rhyolites, (Arenas, 1984(Arenas, , 1988Ancochea et al., 1988). The Rio Baio sequence was considered Silurian by the fossiliferous content of some levels (Matte, 1968;Romariz, 465 1969;Iglesias and Robardet, 1980;Piçarra et al., 2006); nevertheless, a field reappraisal considered those Silurian levels placed below the Rio Baio unit (Valverde Vaquero et al., 2005). The base of the Rio Baio thrust sheet is detached from the autochthonous CIZ by a thrust fault developed in black Silurian rocks (here inferred as the LPBD). Immediately above the LPBD, a low metamorphic grade detrital sequence is exposed at the coast line, where the Picón-2 sample was collected (Figs. Based on the results of the 17 samples here presented plus previous research data we propose the LPa Variscan syn-orogenic structural unit is general in the NW Iberia, forming a tectonic carpet which separates the GTMZ and the CIZ. Nevertheless, as can be seen in Figs. 2 and 3, the LPa is not observed in some reaches of the zones limit for different reasons. In some parts Variscan granitoids have intruded, erasing the previous geological information. Between the Cabo Ortegal and the Bragança 480 complexes and in the northern Porto sector the available data from references not conclusively support the existence of synorogenic sequences which could be endorsed to the LPa; and no detrital zircon study oriented to this target have been performed yet, being a future aim of the research team. Finally, it is worth to highlight the existence of a detached remnant of LPa sequences (San Clodio series) preserved at the core of a late Variscan syncline in the autochthonous side of the limit (CIZ) not far away from the LPa/CIZ boundary. 485

2, 3 and 4) and the detrital zircon study supports the
The strongly deformed black Silurian condensed sequence present at the base of the every LPa tectonic slice, and frequently separated from the syn-orogenic sequence for a thrust fault, must be considered a tectonic mélange in the meaning proposed by Festa et al. (2019;2020) as it also incorporates tectonic blocks and olistoliths from the base of the syn-orogenic sequences; so, when the shearing band incorporates glided blocks (Figs. 7 and 9) it could be better classified as a polygenetic mélange (Festa et al., 2019;2020  This offscraping mechanism is supported in the study area by the Silurian rocks found in the autochthonous unit (CIZ) at the eastern part of the Alcañices Synform (González Clavijo, 2006).

Source-areas of the siliciclastic rocks and olistoliths in the Lower Parautochthon: MDS results
We used Multidimentional Scaling (MDS) (ISOPLOT-R by Vermeech, 2018) to compare the new and the already published data on the zircon age populations of the NW Iberia synorogenic basins, with potential sources within the Iberian Massif terranes (see Supplementary File for more details, and complete U-Pb age datasets in Supplementary Tables 18 and 19). This approach has proven successful in the improvement of paleogeographic reconstruction models in the southwestern branch of 500 the Iberian Variscan belt (Pereira et al., 2020a;2020b) where the zircon age fingerprinting of the possible sedimentary sources and the Devono-Carboniferous synorogenic marine basins have shown these basins were fed by sediments from both continental margins, Laurussia and Gondwana, and from a "missing" Variscan volcanic arc (Pereira et al., 2012).
In this study, the zircon age data used to represent the possible source areas of the synorogenic marine basins of NW Iberia is a data selection of published U-Pb ages of detrital zircons of pre-Upper Devonian siliciclastic rocks of the NW Iberian 505 Autochthon and Allochthonous Complexes. The geochronological data gathered by Puetz et al. (2018) and Stephan et al. (2018) (reference samples in Supplementary Table 18) were used in combination to have the best quantity and quality of data.
We have performed a quality test to the U-Pb isotopic data in each sample, recalculating all the zircon ages following the procedure used in our samples. This dataset is used to fingerprint the source areas using MDS and works as a tool to plot our team age data collection of the synorogenic siliciclastic rock samples, which was expanded in this work from 13 to a total of 510 24 samples. We have also included new zircon age data on volcanic rocks from the UPa (2 samples) and from large olistoliths in the LPa (4 samples) to compare their age spectra with the detrital zircon samples, thus tracing the source areas for some of the large olistoliths and the flysch sequence.
The age data of the possible source areas was selected according a conceptual paleogeographic model for the Upper Devonianlower Carboniferous (as provided in Dias da Silva et al., 2015 andMartínez Catalán et al., 2016). Following the reasoning 515 explained in Supplementary File, we define Source A samples as representative of the peripheral bulge developed in the Autochton (CIZ, WALZ-CZ and OMZ) and the UPa slice as the most continental section of North-central Gondwana margin; Source B reflects the NW Iberian Allochthon (GTMZ), defined as an accretionary complex emplaced onto the autochthon in the Devonian-Carboniferous using the Parautochthon as the lower tectonic sheet. We highlight that although the UPa is in Source A, it could have been at either side of the synorogenic basin margins, as belonging to the peripheral bulge in early 520 Variscan times (Late Devonian), or to the GTMZ basal tectonic sheet in the early Carboniferous, as the tectonic front moved towards inland Gondwana. In this way, Source A samples were grouped by stratigraphic age, considering that the general stratigraphy of the Autochthon and UPa was not substantially shuffled by major Variscan thrust zones as in the case of the GTMZ allochthons. In Source B, the GTMZ Allochthonous complexes were separated according to their tectono-metamorphic https://doi.org/10.5194/se-2020-173 Preprint. Discussion  and/or B), but they also represent different grades of sediment mixing and recycling (Fig. 10).
The analysis of the MDS diagram ( Fig. 10) and age distribution plots (Figs. 11 and 12) demonstrated that there is no specific pattern in the provenance of sediments in time and space. It is interesting to see dramatic provenance changes along and across the same stratigraphic units, sometimes with samples collected in different beds that are a few centimeters apart (e.g. samples MIR-41 and AD-PO-49). In this way, we checked that zircon provenance varies substantially or circumstantially, with 540 sediments coming for both sources at the same time and/or in different sectors of the marine synorogenic basins preserved in the LPa. Another interesting aspect is the representation mixing between age groups, with sediment recycling leading to dilution of sources towards more typical Gondwana (e.g. Group 7 and Cluster 1 showing a trend to Group 5), or reversely with younger samples falling closer to the allochthonous complexes (e.g. trend in Group 7, from samples SO-14, SO-1 and SO-2 towards the Upper Allochthon reference population). 545 These fluctuations reveal drastic variations of the topographic highs surrounding the synorogenic basin at both margins (accretionary complex and peripheral bulge) in the Upper Devonian-lower Carboniferous (Fig. 13). The tectonic activity that controls the basin shape and sedimentation was also capable to trigger highly erosive, large-scale mass-wasting that forms the large olistolith-bearing BIMF deposits. The synorogenic marine sediments (cohesive flysch and BIMF) were gradually incorporated at the base of the accretionary wedge as a tectonic carpet, forming polygenic mélanges. The rapid frontal accretion 550 of trench turbidites (Kusky et al., 2020) allowed the fast exhumation of the marine sediments, leading to their recycling within the basin (wild-flysch).

Origin of Varican zircons
One contrasting aspect of NW Iberia synorogenic basins with other Variscan belt sectors, such as SW Iberia (e.g. Pereira et al 2020a, 2020bPérez-Cáceres et al., 2017), is the lack of Variscan ages in the populations in our study case. Only 14 samples 555 out of 24 have a minor population of Variscan zircons, comparing with the predominant Variscan zircon populations in most of the synorogenic formations in SW Iberia. The synorogenic Variscan zircons are represented in all studied formations of the LPa, independently of the age cluster they belong (the clusters mostly define the "old" zircon age population patterns) (Figs. 11 and 12). Because there are no evidences of syn-sedimentary vulcanism associated to the synorogenic marine basins of NW Iberia (it is amagmatic, whereas in SW 560 Iberia the vulcanism is persistent; Oliveira et al., 2019aOliveira et al., , 2019b, one must explain the origin of the (scarce) Variscan zircon populations in the LPa.
To verify the sources for the Variscan zircons in the studied basins, we had to check the main zircon forming events represented in the Allochthonous Complexes (GTMZ) and in the underlying autochthon (CIZ, WALZ-CZ and OMZ), and in other Variscan sectors. The oldest Variscan zircon populations in the LPa range from c.a. 400 to 380 Ma, which can be related to the HT(HP) 565 metamorphism in the Upper Allochthon (Gómez- Barreiro et al., 2006Barreiro et al., , 2007. A second younger age group, between 380-370 Ma can have their origin in the metamorphic zircons of the Middle Allochthon (Pin et al. 2006;Arenas et al., 2007;Arenas and Sánchez-Martínez, 2015;Santos Zalduegui et al., 1996; and references within). In the same way, the age group including zircon ages in the range 370-360 Ma can have their source in the HT(HP) metamorphic rocks of the Lower Allochthon (Abati et al., 2010;Díez Fernández et al., 2011;Santos Zalduegui et al., 1995). While these "oldest" Variscan ages are found in a 570 relatively small number in the Allochthonous Complexes (Source B), younger age groups can be also identified in the underlying autochthon. In this manner, ages in the range of 340-320 Ma are commonly associated to a HT-LP regional tectonometamorphic event with magmatic flare-ups at 340, 335 and 320 Ma, that affects both autochthonous and allochthonous domains in Iberia (ZCI, WALZ, OMZ and ZGTM) (Dias da Silva et al., 2018;Díez Férnandez andPereira, 2016, Díez Fernández et al., 2017;Gutierrez-Alonso et al., 2018;López-Moro et al., 2017;Martínez Catalán et al., 2003). So, the most 575 probable source for this zircon ages lye in the autochthon (Source A) and GTMZ (Source B).
On the other hand, the 370-340 Ma age spectrum is not very easy to explain because it is not easily found in the nearby sources.
In other sectors of the Variscan belt there are several evidences of explosive vulcanism synchronous with the synorogenic sedimentation, such as in the South Portuguese Zone or in Ossa Morena Zone (e.g. Tournaisian-Visean magmatism, e.g. Pereira et al. 2020a;Oliveira et al., 2019a). This kind of magmatism can provide a shower of some airborne zircons into this relatively 580 far basin as the ash cloud falls (Fig. 13). This explanation can also be used to other erratic zircon ages, with ("missing") magmatic arcs that were active during the Upper Devonian (Pereira et al., 2012). Also, HT-LP metamorphic events that are described in the French Massif Central (Gèret Dome, c.a. 365 Ma, Faure et al. 2009) which cut the root zone of the GTMZ accretionary complex, can be considered as a source. Although this zone is currently far away, they could have been closer to the synorogenic basins in the Upper Devonian-early Carboniferous. 585

Conclusions
We present in this paper new results from field and geochronology studies on the Variscan orogeny (390-300 Ma) foreland marine basins of NW Iberia, that rim the unrooted accretionary complexes of the Galicia-Trás-os-Montes Zone (GTMZ), and outline its boundary with the structurally underlying autochthon, the Central Iberian Zone (CIZ).
The relationship of this foreland marine basin with the structurally underlying and overlying units has been successfully 590 established in the revised sectors of NW Iberia. We reveal that both parautochthonous units of the GTMZ, as defined in the eastern rim of the Morais and Bragança complexes, cover a larger area 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 thrust-complex Lower Parautochthon (LPa) composed of slices of Devono-Carboniferous turbidites and tectonically scrapped autochthonous Silurian strata becomes a general architecture for the NW Iberia. Regional tectonic 595 and stratigraphic aspects show that the LPa is a syn-orogenic basin that was gradually incorporated into the base of the accretionary wedge while it was expanding towards Gondwana, defining a continuous tectonic carpet at the base of the GTMZ.
A detailed analysis of the stratigraphic and tectonic aspects of the synorogenic flysch in the LPa highlight the abundance and the importance of large-scale mass wasting deposits in the sequence. These deposits originated Block-in-Matrix formations (BIMF), sedimentary mélanges with large olistoliths with exotic natures (with origin in the accretionary wedge and in the 600 autochthon exposed in the forebulge) surrounded by a chaotic matrix with slump folds and broken beds of the flysch sequence.
These deposits were triggered by an intense tectonic activity within the foreland basin and at both margins. The BIMF are frequently tectonized, forming thrust multiplexes with polygenic mélanges and tectonically scrapped autochthonous Silurian black shales at the base.
The zircon geochronology of the LPa siliciclastic rocks, and magmatic rocks from the UPa and the LPa (olistoliths), has 605 constrained the provenance of the sediments and blocks in this sector of the Variscan foreland basin. Our study confirmed the synorogenic nature of the LPa stratigraphic units, all presenting Variscan zircons with Famennian to Serpukhovian ages, with possible sources in the allochthonous complexes (390-365 Ma), in the HT-LP metamorphic domes exposed in the root zone of the GMTZ (365 Ma) and in the autochthon (340-320 Ma), or they can be airborne zircons carried in ash clouds coming from the Variscan magmatic arc(s) (365-340 Ma). 610 The older zircon age populations were compared with reference samples from possible source-areas using fingerprinting with Multidimentional Scaling (MDS). The associations of the synorogenic sediments with the reference populations, including the magmatic and inherited ages now obtained in the Middle Ordovician-Silurian volcanic rocks of the UPa (source) and LPa (olistoliths), allowed direct source-to-sink relationships of the foreland basin with the accretionary complex (GTMZ) and the peripheral bulge (autochthon). The MDS analysis demonstrates intrabasin sediment recycling and mixing of sources in time 615 and space, highlighting the tectonic instabilities within the basin and in its margins, the migration of the depocenter towards inland Gondwana, and gradual incorporation of the foreland basin in the accretionary wedge that led its exhumation and recycling.    Legend is in Fig. 11. See text and Supplementary File for a detailed description. https://doi.org/10.5194/se-2020-173 Preprint. Discussion started: 15 October 2020 c Author(s) 2020. CC BY 4.0 License.