Tectonics and geothermal gradients from subduction to collision in the NW Variscan Iberian Massif

ABSTRACT The earliest tectonometamorphic record of tectonic slices incorporated to the base of an orogen holds the key to understand how an orogen is built. The tectonic pile of the NW Iberian section of the Variscan Orogen includes tectonic slices separated by crustal-scale thrusts. The earliest tectonometamorphic record in the uppermost parautochthon is calculated at 11–14 kbar and 450–500°C (P-T gradient about 13°C/km), suggesting a subduction-related metamorphic recrystallization at lower pressure than the overlying Lower Allochthon. Early conditions calculated in the autochthon (9–10 kbar and 425–450°C; 16°C/km) point to a relatively ‘cold’ collisional setting. Higher thermal gradients obtained from some sections of the autochthon (11–12 kbar and 700–725°C; 21°C/km) and the Lower Parautochthon (7.5 kbar and 550–700°C; 24–31°C/km), correspond to more advanced and ‘hot’ stages of collision. New U–Pb monazite geochronology indicates a 318–311 Ma age for the final formation of HT domes in the region. We propose the rapid decrease in P-T gradient (from <10 to 16°C/km) documents a fail to sustain further burial along a regular subduction zone. We consider the subsequent increase in the geothermal gradient (from 16 to 31°C/km) as the culmination of previous crustal accretion and the onset of crustal underthrusting and later processes in a collisional stage. We propose these switches in the early tectonometamorphic record of individual tectonic slices as potential markers to track the transition from subduction to collision in collisional orogens.


Introduction
Continental collision is a distinctive process on Earth that occurs after the oceanic crust that separated two pieces of continental crust is consumed at a subduction zone.Subduction and collision are not instantaneous processes, and each of them may take several million years before completion (e.g.Soret et al. 2021).The existence of high-to ultrahigh-pressure continental terranes proves that continental crust, despite buoyant in nature, can be subducted down to upper mantle depths if attached to oceanic lithosphere (Chopin 1984;Andersen et al. 1991;OBrien 2001;Warren et al. 2008;Hacker et al. 2013).Yet, the overall wedge-shaped structure of (thicker) continental crust at its transition to (thinner) oceanic crust on a downgoing plate, together with the denser nature of oceanic relative to continental crust, set up the scenario for an interesting and predictable outcome.Resistance to subduction should increase as thicker and more buoyant crust reaches the trench.This way, is to be expected that new incoming sections of a continental margin undergoing subduction should be subjected to lower P-T gradients compared to others that preceded them.Given that subduction is arguably the first major process in an orogeny to produce a significant and pervasive metamorphic imprint in both oceanic and continental crust, we propose that tracking the early metamorphic evolution of different paleogeographic sections of a continent involved in an orogen is a good test to qualify the type of transition from normal subduction to collision.
The European Variscan Orogen provides a framework with different tectonic slices of continental nature appropriate for this purpose.The parautochthonous domains represent paleogeographic and geodynamic intermediate pieces that separate an autochthon with Gondwanan affinity, resting below, from an upper set of allochthonous continental terranes bearing high-pressure (HP) rocks and accompanied by ophiolites representing a suture zone or zones (Matte and Burg 1981;Arenas et al. 1986;Farias et al. 1987;Martínez Catalán 1990;Ribeiro et al. 1990;Franke 2000;Ballèvre et al. 2009;Díez Fernández et al. 2012a).Here, we present the results of a tectono-metamorphic study of selected rocks from the parautochthonous sections and underlying autochthons of NW Iberia to illustrate how a transition from subduction to collision can be tracked and qualified through the early-Variscan metamorphic record in this part of the European Variscan Belt.Field studies dealing with the tectono-stratigraphic framework and paleogeography of NW Iberia are key to provide consistency to our interpretation (i.e.recognition of individual tectonic slices), so a combination of previous knowledge and new observations regarding regional structural geology are included here alongside new metamorphic and geochronological data.

Geological setting
The Variscan Orogen resulted from the collision of Gondwana, Laurussia and peri-continental terranes in between those major landmasses, after consumption of the oceanic basins that separated all of them (Franke 2000;Matte 2001;Martínez Catalán et al. 2009;Simancas et al. 2009;Díez Fernández et al. 2016).Pieces of this orogen are exposed as crystalline basements along Europe.One of these pieces occurs in western Spain and in Portugal, referred to as the Iberian Massif.The NW Iberian Massif is formed by an allochthonous/parautochthonous tectonic pile, the so-called Galicia -Trás-os-Montes Zone (GTMZ, Farias et al. 1987); Figure 1.GTMZ was emplaced onto the autochthonous Central Iberian Zone (CIZ) during the Variscan Orogeny in early Carboniferous times (Dallmeyer et al. 1997).
The general classification of Variscan tectonic stages recorded in the NW part of the Iberian Massif and used in this work is adapted from Alcock et al. (2009) and Martínez Catalán et al. (2009), Martínez Catalán et al. (2014).We distinguish between contractional (C1, C2  and C3) and extensional (E1, E2) episodes.However, we introduce a modified classification of regional thrusts according to their kinematics.Top-to-the-E/NE thrusts are included as C1 structures together with E-vergent folds and fold-nappes.Top-to-the-S/SE thrusts are considered out-of-sequence C2 thrusts.The metamorphic episodes (M1, M2, and M3) are taken from Arenas et al. (1995) andMartínez Catalán et al. (1996).Correlation between metamorphic episodes and a summary of structural characteristics and reference ages is presented in Table 1.

Overview of the Galicia -Trás-os-Montes zone
GTMZ is divided into the Domain of the Allochthonous Complexes, structurally above, and the parautochthonous Schistose Domain, below.

The allochthonous complexes
The Allochthonous Complexes comprise three main sets of tectono-metamorphic units (Arenas et al. 1986(Arenas et al. , 2016)).The top is occupied by an Upper Allochthon made of Neoproterozoic-early Paleozoic metasedimentary rocks and Cambrian igneous rocks.The lower part of this upper set experienced Early Devonian HP-HT metamorphism followed by IP metamorphic recrystallizations (Novo-Fernández et al. 2020).Beneath, the Middle Allochthon comprises ophiolites with both, Upper Cambrian and Early Devonian protolith ages (Arenas et al. 2021).The Lower Allochthon is at the base and also referred to as the Basal Units.The Lower Allochthon consists of Neoproterozoic-early Paleozoic metasedimentary rocks, metabasites, and peralkaline to calcalkaline orthogneisses, affected by a Late Devonian HP-LT metamorphic recrystallization (M1, Arenas et al. 1995;Abati et al. 2010;Díez Fernández et al. 2010) et al. 2014).In Forcarei, a ductile shear zone that occupies the same structural position as LFT shows top-tothe-S and/or top-to-the-N sense of shear (González-Cuadra et al. 2006;Fernández et al. 2011), so LFT has possibly undergone tectonic reactivation/reworking (González-Cuadra et al. 2006).The last movement of LFT is inferred to be younger than 346 Ma (Martínez Catalán et al. 2009).

The parautochthon
The parautochthonous Schistose Domain in Spain (Farias et al. 1987) or Peri-Transmontano Domain in Portugal (Ribeiro 1974) is mainly formed by Paleozoic metasedimentary rocks and some metavolcanic rocks.
In NW Spain it has been traditionally subdivided into three pre-Variscan stratigraphic groups, from bottom to top: Santabaia (Farias et al. 1987), Nogueira and Paraño (Marquínez 1981(Marquínez , 1984)).They were firstly defined in the Schistose Area of Central Galicia and in the A Seara Synform area (SACG and AS, respectively, Figure 1), and then, some or all of them, were also recognized in other areas of the NW Iberian Massif (Verín Synform, Marquínez 1984;Farias et al. 1987;Chantada area, Barrera Morate et al. 1989;Marcos and Llana Fúnez 2002;Monteferro-El Rosal area, Llana-Fúnez 2001).Parautochthonous sequences have also been defined in other areas, such as in the Río Baio Thrust Sheet beneath the Cabo Ortegal Complex (Marcos et al. 2002), where the Queiroga Series, roughly equivalent to the Paraño Group, sits on top of the Loiba Series, roughly equivalent to the Nogueira Group (Marcos and Farias 1999).Silurian graptolites in rocks from the Nogueira Group or in similar graphite-rich schist and lydite formations (see review by Piçarra et al. 2006) have been used as an argument to sustain a supposedly continuous sedimentary sequence of Ordovician to Devonian age in the parautochthonous sequence.However, U-Pb zircon ages obtained from metavolcanic rocks support late Cambrian to Early Ordovician or Ordovician-Silurian ages for the structurally upper group or groups of rocks (Valverde-Vaquero et al. 2005, 2007a;Farias et al. 2014).New data in Portugal led to models considering two imbricated tectonostratigraphic units (Rodrigues et al. 2003(Rodrigues et al. , 2006)).Both units are referred to as the Lower Parautochthon (below), including thick deposits of syn-orogenic rocks piled up in thrust-sheets, and the Upper Parautochthon (above) with pre-orogenic rocks forming fold-nappes (Martínez Catalán et al. 2008, 2016;Meireles 2011;Dias da Silva et al. 2014, 2015;González Clavijo et al. 2016).In an overall approach (e.g.Barrera Morate et al. 1989;Martínez Catalán et al. 1996), the rocks of the parautochthon have undergone Barrovian IP metamorphism (M2) reaching medium-grade conditions in the more internal sections of the GTMZ pile.HT-LP overprinting (M3) is more intense down structure and towards more internal positions within the tectonic pile.The allochthonous/parautochthonous pile was piggyback transported as a whole to its current position above the CIZ autochthons by a new generation of thrusts (Martínez Catalán et al. 2002).One of these structures (Figures 1 and 3) is the Verín Thrust (Farias et al. 1987;Farias 1990), which is equivalent (similar structural position) to the basal thrusts of the Peri-Transmontano Domain (sometimes recognized as the Main Trás-os-Montes Thrust), and to those of the Alcañices Synform.The Verín Thrust has been dated at 340 ± 0.9 Ma and the main thrust in Alcañices at 342.6 ± 0.3 Ma (whole-rock 40 Ar/ 39 Ar analyses, Dallmeyer et al. 1997), corresponding to the C2 stage in Table 1.
After C2 thrusting, the resulting thickened crust was thermally weakened and extended (Alcock et al. 2009(Alcock et al. , 2015;;Martínez Catalán et al. 2009, 2014;Díez Fernández et al. 2012c) in Carboniferous times (extensional stage E1, see Table 1).Some of the main structures affecting the parautochthon during this stage are the top-to-the-NW Bembibre-Pico Sacro Detachment (Martínez Catalán et al. 2002;Gómez Barreiro et al. 2010) 1).Later, C3 upright folding contributed to the regional structural pattern observed in Figure 1, in which the overlying allochthonous pile is preserved as klippen in the cores of C3 synforms and the underlying autochthons crop out in tectonic windows in cores of C3 antiforms.

The autochthonous central Iberian zone
The rocks that conform the CIZ are mostly pre-orogenic and crop out structurally below the parautochthon, either defining tectonic windows or around the GTMZ.In NW Iberia, the autochthonous CIZ includes Neoproterozoic to Early Devonian metasedimentary rocks and late Cambrian to Early Ordovician metavolcanic and metagranitic orthogneisses.The orthogneisses are related to the peraluminous magmatism of the Ollo de Sapo Formation (Lancelot et al. 1985;Bea et al. 2006;Díez Montes 2007;Montero et al. 2009;Díez Montes et al. 2010;Talavera et al. 2013).The CIZ series are affected by E-verging recumbent folds (C1) with an associated axial plane S C1 schistosity (Table 1).C1 folds in the autochthon are cut by C2 thrusts transporting the GTMZ onto the CIZ (Farias 1987(Farias , 1990;;Marcos and Farias 1999;Díez Montes 2007).C2 thrusts can also transport pre-orogenic rocks onto syn-orogenic deposits that cover the CIZ autochthon in the Alcañices Synform show top-to-the-NW shear-sense (Lx E1 ).Similar E1 extensional shear zones are widespread represented along the most internal zones of the CIZ (Doblas et al. 1994;Escuder Viruete et al. 1994, 1998;Díez Balda et al. 1995;Arango et al. 2013;Díez Fernández et al. 2013

Pre-barrovian metamorphism M1
It has been broadly assumed that both, the parautochthon and autochthon, did not undergo metamorphic recrystallization earlier than the syn-collisional, intermediate-P metamorphism, M2.However, Marquínez (1984) reported a foliation earlier than collisional S C1 , preserved as microinclusion trails in albite porphyroblasts from the schists of the Paraño Group in the parautochthon of the SACG, near Forcarei.In the Verín Synform, Farias (1990) also mentions the presence of albite porphyroblasts with an internal schistosity of aligned micro-inclusions of quartz, chlorite and white mica in the uppermost part of the Paraño Group.These are findings we will explore in more detail below.

Barrovian metamorphism M2
The parautochthon in the SACG and AS presents a M2 Barrovian sequence of metamorphic zones (Marquínez and Klein 1982) related to the main episodes of collisional thickening (fold and thrust nappes stage in Martínez Catalán et al. 1996;C1+ C2 in this work).Barrovian zonation is approximately parallel to the trend of major regional structures and is marked by Chl, Bt, Grt, and St in metapelites (Barrera Morate et al. 1989; mineral abbreviations after Whitney and Evans 2010), which appear progressively with increasing structural depth.Ky-bearing metapelites are actually also found in the AS (see new finding below).Medium-P conditions were achieved in the lower structural levels of the parautochthon in other areas of the central GTMZ (Figure 1), such as in the Riás schists (Díez Fernández 2011;Solís-Alulima et al. 2019).
Complete Barrovian sequences (from Chl to Sil metamorphic zones) appear at lower structural levels of the CIZ autochthon, including some Grt, St and Ky-bearing assemblages beneath the Verín Synform (see new finding below).However, in Sanabria region, some pelitic schists close beneath the base of the parautochthon preserve Ky-rich assemblages developed at lower metamorphic grade than the Grt-bearing assemblages in the Barrovian sequences (Díez Montes 2007).This early appearance of Ky was interpreted by the author as evidence of an initial P-T gradient possibly higher than Barrovian during Variscan deformation of the autochthon.

Low-pressure and high-temperature metamorphism M3
In the lower structural sections, IP metamorphic zones are affected by metamorphic HT-LP overprinting and telescoping of isograds related to syn-collisional extension E1 (Martínez Catalán et al. 2014

Materials and methods
This work presents a collection of new data obtained from fieldwork and lab analysis.The results from fieldwork are presented in geological maps, cross-sections and pictures (with accompanying interpretation).Maps are edited at various scales, ranging from a regional synthesis (Figure 1) to local maps 6,7 and 9).Relevant structural and lithological data are displayed in pictures included in Figures 5 and 8, while petrographic observations done in the lab are shown in pictures gathered in Figures 5 and 10.
In order to constrain the Variscan metamorphic conditions and tectonothermal evolution that affected the parautochthon and autochthon of NW Iberia, we have studied several samples of semipelitic and pelitic schists.Calculations were aimed at obtaining an estimation of the effective thickening during continental collision and at qualifying the tectonothermal regime during the early stages of collision throughout the tectonic pile.Microprobe analyses were performed with a JEOL Superprobe JXA-8900 M of the ICTS -Centro Nacional de Microscopía Electrónica, Madrid.Operating conditions were 15 kV accelerating voltage, 20 nA beam current, 2-5 µm beam diameter and 10 ms counting time.The ZAF correction procedure was used.Thermobarometric study has been performed using the multiequilibrium tool Average P-T of THERMOCALC v.3.33 (Powell and Holland 1994;Holland andPowell 1998, 2011), and combining the Massonne and Schreyer (1987) Si-in-phengite geobarometer with the Pl-Ms geothermometer of Green and Usdansky (1986) in low-grade rocks with poor mineral assemblages.Characteristics of the samples, coordinates, thermobarometric methods and P-T results are summarized in Supplementary Table 2.
We also investigated the age of the HT-LP metamorphism related to the construction of gneiss-domes in the region by means of U-Pb monazite geochronology in samples from the Bande-Celanova Dome (location in Figure 9).Information about samples, methods and results is included in section 4.4.and Supplementary Table 6.
One sample from the footwall CIZ autochthons (9223) in the core of the dome, and two samples from the hanging wall Lower Parautochthon (9222 and 9224), in the flank of the dome, were processed and the monazites separated and dated by U-Pb ID-TIMS at the IGME laboratory (Tres Cantos, Spain), following the procedures outlined in Rubio-Ordóñez et al. (2012).

Cabo Ortegal area
The main foliation within the basal shear zone of the Río Baio Thrust Sheet, that is, the basal thrust of the whole parautochthon in this area, shows top-to-the-S kinematic criteria (Lx C2 Figures 5f-7).According to Marcos et al. (2002), top-to-the-SE shearing is superimposed on previous top-to-the-NE shearing (Marcos and Farias 1999), which is consistent with our C1 stage.
The uppermost part of the parautochthonous section below the Cabo Ortegal Complex (e.g.Arenas et al. 2009) has been also considered part of the Lower Allochthon (Ramallal Phyllonites, Marcos and Farias 1999), Figures 1  and 6.Rocks in that section of the parautochthon conform to a high-strain zone.The main foliation shows mineral and stretching lineations trending NNE-SSW and kinematic criteria that indicate top-to-the-NNE shearsense (also reported by Marcos and Farias 1999).We have also found those same features in the upper part of the parautochthon and affecting the alkaline volcanic rocks of the Queiroga Series.In the lower part of the parautochthon, a set of discrete (1-3 m-thick) shear zones related to higher angle (45°) top-to-the-ENE thrusts cut across the main schistosity (Figure 8a), the East-verging (C1) and even upright folds (C3) (Figure 8b).Syn-orogenic sediments appear at the base of the parautochthon in the coastal section of the pile (González Clavijo et al. 2018), but they are also present at higher structural levels within the parautochthon (Figure 7).

Bande-Celanova Dome and A Seara Synform
The mechanical contact between the Lower Allochthon and the underlying parautochthon or autochthon in the most internal zones of the GTMZ is affected by extensional detachments (E1) crosscutting the nappe stack (Martínez Catalán et al. 2002).The same detachment system affects the contact between the parautochthon and the underlying autochthon in the Padrón Dome (Díez Fernández et al. 2012c, 2017, 2019).
In the Bande-Celanova Dome (Figures 1 and 2), HT paragneisses and Early-Middle Ordovician orthogneisses of the CIZ (Talavera et al. 2013) occupy the core of the dome (Barrera Morate et al. 1989;Martínez Catalán et al. 2014).The parautochthonous section (Figure 9) is formed by a C1 structure of imbricates and NE-verging tight isoclinal folds, made of rocks of the Santabaia Gp (including Early Ordovician orthogneisses, U-Pb (TIMS) on zircons, Valverde-Vaquero pers.comm.) and Nogueira Gp (Silurian), which are overthrust by the Paraño Group (also including Early Ordovician orthogneisses, U-Pb (TIMS) on zircons, Valverde-Vaquero pers.comm.).Díez Montes et al. (2014) considered the base of the parautochthonous section as an extensional detachment.This E1 structure is well exposed in the Arnoia River.The Arnoia Detachment is a low-angle, >3 km-thick extensional shear zone (Figure 8c) featured by a L-S (Lx E1 -S E1 ) mylonitic foliation bearing top-to-the-N/NW kinematic criteria at the base, followed by a pervasive S E1 crenulation cleavage and S-C structures to the top, cropping out around the A Seara Synform.
The fabric in these metaigneous rock ranges from preserved trachytic textures to L-S mylonites.The preferred orientation of quartz veinlets, K-feldspar porphyroclast tails and riebeckite acicular prisms defines an E-W to ENE-WSW stretching and mineral lineation (Figure 5b).Shear sense criteria associated with the main foliation within the shear zone indicates top-tothe-E/NE shearing.This upper nappe (Figure 4), previously considered a part of the parautochthonous Paraño Gp (e.g.Farias 1990), and its basal thrust (FT), have cartographic continuity to the SE with the Lower Allochthon nappe in the Bragança Complex and the CTT, respectively (Ribeiro 1974;Ribeiro et al. 1990).
The Verín Thrust also presents a thick low-angle shear zone affecting the base of the parautochthon Figures 3  and 4; (Verín Thrust, Farias 1990), but with consistent top-to-the-SE kinematic criteria (constrained by stretching and mineral lineations), such as asymmetric boudins and drag folds in quartz veins.
The internal structure of the parautochthon is also defined by other faults that parallel the main boundaries of the parautochthon (Figure 4).One of them is responsible for the stacking of Paraño Gp phyllites onto the Nogueira Gp or onto its syn-orogenic cover of greywackes, slates, conglomerates and block-in-matrix deposits (Figure 5c).Five samples of fine-grained synorogenic sediments have been unsuccessfully studied in search of palynomorphs (Z.Pereira, pers. comm.).However, the Nogueira Gp includes here graphite-rich rocks with Silurian graptolites (Verín site, Piçarra et al. 2006), and the syn-orogenic sediments contain blocks of quartzite, felsic volcanic rocks and also of similar graphite-rich rocks with Silurian graptolites in the Campobecerros and Servoi sites (Piçarra et al. 2006 and references therein), as well as blocks of marble with Silurian conodonts (G.Sarmiento, pers.comm.).The synorogenic rocks are tectonically imbricated and thicken towards the SE.In the Servoi outcrop, the rocks are imbricated between two different sections of Paraño Gp (Figure 4) and their main foliation presents top-tothe-SE kinematic criteria (Figure 5d).The rocks from similar outcrops to the E show small trachyte clasts (Figure 5e), probably sourced from the Ordovician-Silurian volcanic rocks that are exposed in the Lower Allochthon nappe.

Pre-Barrovian metamorphism M1
In the Upper Parautochthon of the central part of the GTMZ, the Ab porphyroblasts grown as part of the main foliation in schists of the Paraño Gp contain an internal schistosity that includes mineral relics of an initial episode of HP-LT metamorphic recrystallization M1 (see calculations below).This internal fabric is defined by aligned microinclusions of Qz, white mica, Chl, Rt, Ilm, Tur and organic matter (Figure 10(a,b,c)).This fabric reminds of the internal foliation that can be observed in a similar textural position in Ab-bearing schists of the overlying Lower Allochthon (e.g.Santiago schists; Arenas et al. 1995;Martínez Catalán et al. 1996) and Middle Allochthon (e.g.Ceán schists; López-Carmona et al. 2014).This metamorphism is related to the process of continental subduction prior to continental collision (Arenas et al. 1995;Martínez Catalán et al. 1996;Díez Fernández et al. 2011).In the Lower Allochthon nappe on top of the Verín Synform, the alkaline metavolcanic rocks present metamorphic growth of acicular riebeckite around relic (igneous?)aegirine crystals.We relate this growth, alongside the presence of Ab-bearing schists (Farias 1990), to the pre-Barrovian metamorphism.

Barrovian metamorphism M2
We have found Ky-bearing metapelites (not previously observed) structurally below rocks of the Barrovian metamorphic zone of St. Ky-bearing assemblages have been observed in the lower sections of the parautochthon in the central area of the GTMZ (AS, Figure 9).They have also been identified in the underlying CIZ rocks, towards more external eastern zones (Verín, Figure 4), where orogenic thickening was presumably lesser.In the westernmost section of the GTMZ, we have observed Grt + St assemblages in metapelites (possibly preserved M2 assemblage, not totally recrystallized under low-P conditions) along the structurally deepest section of the CIZ autochthon in Monteferro-El Rosal area.

Low-pressure and high-temperature metamorphism M3
A pervasive low-dipping foliation (S E1 ) that overprints former foliations dominates the internal structure in the lowest structural levels of the parautochthon around the high-grade gneiss domes.The S E1 fabric defines some hundreds of meters-thick extensional shear bands that juxtapose migmatized rocks of the underlying autochthon against lower-grade rocks of the parautochthon and allochthon, such as in the Bande-Celanova and Padrón domes, or in the base of the SACG.The main foliation in the underlying migmatites parallels S E1 in the overlying domains, and in both sections kinematic criteria indicate similar shear sense for their respective main foliation.In the Arnoia Detachment (Figures 2 and 9), the M2 Barrovian zonation in the lower structural levels of the parautochthon is telescoped.Grt, St and Ky isograds are anomalously close to one another by the effect of the extensional deformation E1, and the rocks within the telescoped section are affected by HT-LP metamorphic overprinting featured by S E1 assemblages with Sil, and new Bt in the lower structural levels of the Arnoia Detachment.Upward and yet within the Arnoia Detachment, S E1 changes to a S-C fabric that is characterized by newly-formed Chl, thus defining a normal (sharp) thermal gradient related to this extensional shear zone.

Thermobarometric calculations
We have studied two samples of semipelitic Ab porphyroblast-bearing schist from the uppermost section of the parautochthon and two samples of pelitic schist from its lowermost structural levels.We also studied two samples from the nearby autochthons, including one sample of Ky-bearing, low-grade pelitic schist from the upper structural levels, and one Grt-Sil schist from the mesozonal structural levels located below.Sample locations are indicated in Figure 1 and in Supplementary Table 2.

Parautochthonous units
The uppermost parautochthonous nappe in the SACG is mostly formed by pelitic and semipelitic schists, which contain Ab porphyroblasts (Figure 10(a,b,c); samples 1 and 2 in Supplementary Table 3) with inclusion trails made of Qz, Chl, Tur, Rt, Ilm and rare white mica (WM).These minerals define an internal schistosity (S i ) that is previous to the main external schistosity (S e = S C1 ).Ab porphyroblasts are typically <0.5 mm in size.Their chemical composition is 98.44-99.63Ab% in the inclusionrich cores, while the outer rims are frequently inclusionfree and their composition is 89.94-95.94Ab%.WM from the S i show high silica contents (maximum values of 3.43-3.55Si) p.f.u.(Supplementary Table 3).Combined Massonne and Schreyer (1987) Si-in-phengite geobarometer and Pl-Ms geothermometer (Green and Usdansky 1986) suggest minimum pressure conditions around 11-14 kbar and temperatures of 450-500°C for the early metamorphic recrystallization of these rocks, just below the Ab = Jd + Qz reaction (Figure 11).
Ab-bearing schists are lacking in the lower structural levels of the parautochthon.In turn, the Barrovian episode of metamorphic recrystallization M2, and the HT-LP M3, affected those rocks with variable intensity.The higher metamorphic conditions were achieved in central areas of the GTMZ.Two samples of pelitic schists from the Lower Parautochthonous nappe were studied using Average P-T THERMOCALC v.3.33.Mineral assemblages used and the estimations obtained are shown in Supplementary Table 2 (samples 3 and 4).The mineral chemistry data are shown in Supplementary Table 4. Sample 3 (Figure 10d) was collected in the lower section affected by the Arnoia Detachment and yielded an isobaric heating from 595 ± 22°C at 7.4 ± 0.9 kbar to 696 ± 25°C at 7.6 ± 0.6 kbar (fields 3c-Grt center, 3 r-Grt rim, respectively, in Figure 11).Sample 4 was taken 100 m structurally above sample 3, and its P-T evolution define a small retrograde path from 663 ± 26°C at 6.6 ± 1.1 kbar to 637 ± 27°C at 5.4 ± 1.1 kbar (fields 4c-Grt centre, 4 r-Grt rim, respectively, in Figure 11), which is consistent with the latest cooling (M3) in the extensional detachment.

Autochthons
In the Sanabria region, some low-grade metapelites from the structural upper sections of the autochthon (close beneath the GTMZ tectonic pile) are formed by Kybearing (Late Ordovician) schists and veins that show low-temperature assemblages with high-silica WM, Chl, Rt, Ilm and Ap (Figure 10e, sample 5 in Supplementary Table 3).The rocks are Bt and Pl-free.The Si-in-Ph geobarometer (Massonne and Schreyer 1987) yields minimum pressures of 8.5 kbar for temperatures that cannot be much higher than the pyrophillite = Ky + Qz + H 2 O reaction (Winkler 1976), proceeding at 425-450°C (Figure 11).On the other hand, the Grt-Sil schists (Early Ordovician) from the Villadepera Antiform (sample 6, Figure 10f, mineral chemistry data in Supplementary Table 5) are representative of mid-crustal sections of the autochthon affected by Barrovian metamorphism.Average P-T analyses on the mineral assemblage of this rock are shown in Supplementary Table 2, and yield conditions near 11-12 kbar and 705-720°C (6c-Grt centre, 6 r-Grt rim, Figure 11).
Samples 9222 and 9224: These two samples are Silzone metasedimentary rocks of the Lower Parautochthon (Santabaia Gp) from the hanging wall of the Arnoia Detachment.In sample 9222, two monazite fractions (M1, M2) have been dated (Supplementary Table 6), and fraction M1 9222 also provided concordant monazite at 311-312 Ma (Figure 12), as in the case of the footwall 9223 sample.Contrarily to sample 9222, sample 9224 preserves earlier Grt-St mineral assemblage and it occurs at a slightly higher structural position across the detachment zone.Three monazite fractions (9224 M1, M2 and M3) have been analysed (Supplementary Table 6).They are concordant and provide a Concordia age of 317.7 ± 0.7 Ma (MSWD 0.24; Figure 12).Given the spread of the error ellipses, to avoid underestimating the error we prefer to double the error and quote an age of 317.7 ± 1.4 Ma as our best estimate for the age of monazite crystallization.
Samples 9222 and 9223, despite their different structural positions (hanging wall and footwall, respectively), show identical ages in their M1 fractions, which allows considering a combined Concordia age of 311 ± 1 Ma (MSWD 0.09) as the best estimate for the age of the thermal homogenization of the gneiss-dome, this is, after the thermal perturbation related to the extensional detachment (post-tectonic stage E1, metamorphic stage M3, Table 1).Thence, 317.7 ± 1.4 Ma age of sample 9224 corresponds to a previous tectonic stage, which regionally fits to the end of E1 or beginning of C3 (ending of metamorphic stage M3 to post-M3, Table 1).
In addition, two anomalous ages have been obtained from the M2 fractions (Figure 13).Fraction M2 9223 overlaps Concordia at 340 Ma with a large analytical error, while M2 9222 is on a reverse Discordia suggesting a Cambro-Ordovician age.A Cambro-Ordovician age was also found in a sample from the NW flank of the Bande-Celanova Dome (M1 9221, unpublished data), Figure 13.The significance of these two fractions have to be considered with caution, since fraction M2 9223 could be a mixture of Cambro-Ordovician and ca.311 Ma Variscan monazite (Figure 13), and further analyses are needed to confirm the extent and nature of the pre-Variscan monazite.

Reappraisal of major Variscan thrusts in the parautochthon of NW Iberia
The study of Ab porphyroblasts and their mineral microinclusions from rocks that are restricted to some uppermost structural sections of the parautochthon shows that chemical compositions of plagioclases and white micas are similar to those of other Ab-bearing schists in HP-LT units of the GTMZ (Santiago schists, Arenas et al. 1995;Ceán schists, López-Carmona et al. 2010).Our thermobarometric calculations on these rocks yield peak-P conditions around 11-14 kbar, indicating some lesser burial than usually obtained in the Lower Allochthon of the GTMZ for its early metamorphic recrystallization (>16 kbar, e.g.Gil Ibarguchi 1991; Arenas et al. 1995;Rubio Pascual et al. 2002;Rodríguez et al. 2003;López-Carmona et al. 2010, 2013, 2014), but close to those obtained by López-Carmona et al. (2013) in some rocks of the Middle Allochthon in the Ceán Unit (12-14 kbar and 350-380°C), and to the peak-P conditions (9-14 kbar) estimated for the overlying Lower Allochthon in the Lalín-Forcarei unit (Martínez Catalán et al. 1996).This also implies the existence of major contractive structures within the upper part of the parautochthon in the SACG (Figure 2), and at the base of the formerly alleged upper section of the parautochthon in the Verín Synform.Such last structure would be the Fumaces Thrust (Figures 3  and 4) and its continuation to the basal thrust of the Centro-Transmontano Domain in the Bragança Allochthonous Complex (Ribeiro 1974).Shear sense criteria observed in the tectonic fabrics of the Fumaces Thrust are orthogonal to the Variscan trend and consistent with both, the tectonic stacking of the Lower Allochthon (Lalín -Forcarei Thrust, Figure 2), and the first contractive deformations in the autochthons (C1).For this reason, we consider the Fumaces Thrust to be a C1 structure, whether or not its foliation is the first schistosity developed within the shear zone.However, the basal levels of the uppermost parautochthon in the SACG do not show C1, but complex C2-E1 structures, so it is difficult to correlate both thrusts.
Under the Cabo Ortegal Complex, the uppermost structural section of the parautochthon includes Early Ordovician alkaline metavolcanic rocks (Figure 6).The M1 HP-LT metamorphic recrystallization is apparently lacking in this part of the parautochthon.However, thrusting in the upper section of the parautochthon in this part of the orogen is inferred by other means.Middle Ordovician phyllites of the Queiroga Series (Paraño Group) occur structurally below the Early Ordovician alkaline metavolcanic rocks cited above (Valverde-Vaquero et al. 2005).According to our field-work, also those Middle Ordovician phyllites and other rocks corresponding to the Paraño Gp, including late Cambrian-Early Ordovician metavolcanic rocks (Farias et al. 2014;P. Valverde-Vaquero, pers. comm.) are on top of the younger Nogueira Group rocks in Cabo Ortegal (upper part of the Loiba Series with Silurian graphite-rich rocks, Figures 6 and 7), in Verín (Figures 3 and 4) and in A Seara (Figures 2 and 9), or onto a Variscan syn-orogenic cover made of post-Silurian sediments.Collectively, all these observations suggest an imbricated structure of the preorogenic units of the parautochthon.At the same time, given the HP-LT early metamorphic evolution recorded in some of its sections, the resulting uppermost parautochthonous nappe in the SACG could be alternatively considered the lowest, perhaps a minor slice or slices, of the Lower Allochthon in the region.This could be extended to the uppermost part of the parautochthon in Cabo Ortegal, as the rifting-related alkaline/peralkaline magmatism is a characteristic feature of the Lower Allochthon in NW Iberia (Díez Fernández et al. 2012d), and of other HP-LT units of the Variscan Massif (Díez Fernández et al. 2015).
The top-to-the-SE kinematic criteria observed in the main foliation associated with the Verín Thrust (Figures 3 and 4) and in the foliation that is superimposed to the Río Baio Thrust Sheet (Figures 5f-7) are near parallel to the regional Variscan trend.Such kinematics implies these thrusts, or the reworking of these faults, are C2 structures.Top-to-the-S/SE contractive structures are kinematically equivalent to the 'out-of-sequence thrusts' observed within the Allochthonous Complexes (Martínez Catalán et al. 2002;D4 in Gómez Barreiro et al. 2006).Accordingly, C2 tectonic transport was not accommodated through a single fault but probably through an imbricate set of faults.Therefore, the change from C1 to C2 contraction is not a simple progression of tangential deformation outward from the orogenic hinterland but a stage when the previous nappe stack was reworked/reactivated under a different stress field.Some works have proposed that the syn-orogenic sediments in the NW Iberian Variscan front were unconformably deposited at the front of nappes (Dias da Silva et al. 2015;Martínez Catalán et al. 2016;González Clavijo et al. 2021), so syn-orogenic sediments were progressively involved in the thrust-and-nappe tectonics.Such a process would explain the imbrication of pre-orogenic and syn-orogenic rocks that we observe in the Verín Synform (Figure 4) and Cabo Ortegal (Figure 7).Kinematic criteria observed in the main foliation around these imbricates indicate top-to-the-S/SE shear sense, so this imbrication can be considered as a C2 process related to the emplacement of the GTMZ onto the CIZ autochthons.The series of syn-orogenic sediments in the Verín Synform thicken towards the E and SE (more external zones; Figure 4), becoming a main part of the materials column in the Lower Parautochthon of Trás-os-Montes (Rodrigues et al. 2003(Rodrigues et al. , 2006;;Dias da Silva et al. 2014, 2015) and Alcañices (González Clavijo et al. 2016).

Early Variscan P-T gradients across the Variscan nappe pile
The early metamorphic evolution of the CIZ autochthon in NW Iberia defines a path that passes through Ky stability conditions at relatively low grade, such as in the Sanabria region (Figure 11; 9-10 kbar and temperatures of 425-450°C).This evolution is related to C1-C2 thickening and confirms early Variscan burial under a P-T gradient higher than classical Barrovian (early M2 geothermal gradient of 16°C/km) (Díez Montes 2007) and yet not typical for continental subduction.Subsequent metamorphism developed under Grt, St and Ky stability conditions typical for mid-P Barrovian metamorphism, as supported by our data from the Villadepera Antiform (Figure 11; 11-12 kbar and temperatures of 700°C; M2 geothermal gradient about 21°C/ km), similar to other autochthonous sections across the Iberian Massif (Escuder Viruete et al. 2000;Díez Montes 2007;Rubio Pascual et al. 2013, 2016).The internal structure of the CIZ autochthon lacks of crustal-scale thrusts (no tectonostratigraphic evidence or large-scale metamorphic inversion is observed).Accordingly, the early and subsequent metamorphic record of the CIZ autochthon is compatible with that of a piece of crust that remained internally coherent during burial and early exhumation.Our P-T calculations indicate that the hanging wall during the burial of the CIZ autochthon in Sanabria and Villadepera (GTMZ wedge) was ~27 km thick.
Maximum pressures calculated on the samples from the Lower Parautochthon in the Arnoia Detachment are lower than those from the CIZ autochthon (Figure 11; 7.5 kbar and temperatures of 600-700°C; geothermal gradient about 29°C/km).This section of the parautochthon shared with the CIZ autochthon a path through regional Grt, St and Ky stability conditions of Barrovian metamorphism (M2), but is later noticeably affected by E1 and the related HT-LP metamorphism, just like the autochthon in the Bande-Celanova and other gneiss domes.Our P-T calculations indicate that post-accretion exhumation in the Lower Parautochthon (change from M2 to M3) proceeded under a tectonic pile (GTMZ) >22.5 km thick.
The P-T conditions obtained for the earliest mineral assemblage observed in the rocks of the uppermost parautochthon point to an early burial under a HP-LT gradient (a minimum of 11-14 kbar and temperatures of 450-500°C; geothermal gradient about 13°C/km), typical for continental subduction.Subsequent evolution would include their exhumation during C1 in a syn-collisional setting, with a slight increase in T during decompression.The early evolution differs from that of the underlying units (with lower P-T gradients) and approaches that of the overlying Lower Allochthon featured by blueschists and eclogites, with higher P-T gradients (5-10°C/km; Arenas et al. 1995;Gil Ibarguchi 1995;Martínez Catalán et al. 1996;Rodríguez et al. 2003;López-Carmona et al. 2010, 2013, 2014).
Overall, every major thrust that has been revisited, reconsidered or identified in this work by structural and/ or tectonostratigraphic criteria bound a section of the Variscan tectonic pile of NW Iberia that experienced a slightly different early metamorphic evolution.Metamorphic P-T gradients decrease down structure, from 5°C to 10°C/km in the overlying Lower Allochthon, through to 13°C/km in the uppermost parautochthon and 16-21°C/km in the autochthon (Figure 14).Primary Variscan faults separating these domains formed in-sequence, are younger down structure and crustal-scale, so the decrease in P-T gradient reflects increasing resistance to subduction from a progressively thicker wedge-shaped continental margin reaching a subduction trench.An apparent exception to this trend is the Lower Parautochthon, (metamorphic gradient of 29°C/km).The lower parautochthonous samples 3 and 4 in (Figure 11) were selected from the extensional Arnoia Detachment, so this particular record is probably not so early, and it is marking the transition from Barrovian collisional to HT-LP syn-collisional extension gradients.According to our U-Pb data on monazites, this transition was completed between 318 and 311 Ma, which is consistent with the ages obtained by (Dallmeyer et al. 1997;Díez Montes 2007;Valverde-Vaquero et al. 2007b) (Table 1) for the end of E1 and the late deformations in the region (C3 Steep folds and transcurrent strike-slip shear zones, E2 late extensional detachments).Our data are also consistent with recent U-Pb SHRIMP and CA-ID-TIMS ages in early granodiorites dating the first Variscan magmatic pulses in the region in 315-326 Ma (González Menéndez et al. 2021).From a structural point of view, these changes in gradient coincide with changes in how the orogenic crust is built.Accretion of different tectonic slices (under high P-T gradients) dominates the tectonic style during the stage dominated by subduction,  whereas the subsequent building and thickening of the orogen is assisted by progressive underthrusting of a single (the underlying autochthon remained basically coherent during its burial), yet thicker tectonic slice under a moderate P-T gradient.We propose all these switches in the early tectonometamorphic record as potential markers to track the transition from subduction to collision in a collisional orogen.
In NW Iberia, the transition from regular subduction in the Late Devonian (record in the Lower Allochthon s.s.) to collision in the Carboniferous is smooth and probably conditioned by paleogeography.The progressive (instead of abrupt) lessening that is observed in the early P-T gradients down structure in the NW Iberian Variscan tectonic pile favors a wedged (instead of necked) pre-orogenic local structure in the lower plate  to Variscan subduction/collision, which would include the Lower Allochthon, the parautochthon, and the autochthon.

Conclusions
Some uppermost parts of the parautochthon of NW Iberian Massif experienced early Variscan HP-LT metamorphism (M1: 450-500°C; 11-14 kbar; geothermal gradient about 13°C/km), compatible with a continental subduction process during the Variscan Orogeny.The uppermost parautochthon was emplaced onto the rest of the Upper Parautochthon by means of east-directed thrusts (C1), such as the Fumaces Thrust in the Verín Synform (Figure 14).C1 deformation produced tectonic imbrications, folds, and large fold-nappes in the preorogenic sections of the Parautochthonous Domain, both in the hanging wall and footwall to C1 thrusts.Subsequent south-directed thrusting (C2) transported parts of the initial orogenic crust onto the autochthon of the Central Iberian Zone, affecting both previous and coeval syn-orogenic sediments during the process.The resulting thickened crust was then attenuated (E1) by conjugate top-to-the-N/NW and top-to-the-S/SE extensional detachments, which culminated the faultbounded tectonic pile formed in NW Iberia during the early stages of the Variscan Orogeny.
If the early tectonometamorphic evolution is observed, each fault-bounded tectonic slice in the Variscan tectonic pile of NW Iberia was deformed and accreted to the existing orogenic crust under a slightly different P-T gradient.We interpret the progressive decrease in P-T gradient and peak-P of tectonic slices accreted to the base of an orogenic crust (such as the Variscan) as a resistance to subduction of the lower plate.These changes may reflect the transition from a subductiondominated stage to a collision-dominated one.
(González Clavijo 2006; González Clavijo and Martínez Catalán 2002; González Clavijo et al. 2016; Farias and Marcos 2019) Figure 3.In the core of gneiss domes and C3 antiforms, S C1 in the CIZ autochthon is usually transposed by a flat-lying S C2 foliation.Stretching lineations and asymmetric microstructures indicate top-to-the-SE shear-sense during C2 (Lx C2 ) and probably E1, while upper crustal sections (here represented by the GTMZ) ; Rubio Pascual et al. 2013, 2016; Díez Fernández and Pereira 2016).From a metamorphic point of view, the CIZ rocks cropping out from immediately around the GTMZ to the central areas of the Iberian Massif are characterized by complete Barrovian sequences (M2 in our work) dated at 337-347 Ma, which are variably overprinted by HT-LP (M3) metamorphism (corresponding to M1 and M2, respectively, in Martínez Catalán et al. 2014 and geochronological references therein).

Figure 5 .
Figure 5. A) Outcrop image of mylonites in the Fumaces Thrust, Verín Synform.Criteria indicate top-to-the-E sense of shear.b) Microscope aspect of a peralkaline gneiss, Verín Synform.K-feldspar porphyroclasts in a matrix of feldspar and oriented crystals of blue riebeckite.Other samples may show radial growth of riebeckite around dark green small crystals of aegirine.c) Field image of volcanic rock blocks (white) in dark pelitic-rich matrix.Syn-orogenic rocks covering the Nogueira Gp in the northern part of the Verín Synform.d) Field aspect of quartzite blocks (white) in pelite-psammite-rich matrix.Sigma structure of individual blocks indicates top-to thesouth (right side of the picture) sense of shearing.Syn-orogenic sediments covering some lower part of the Paraño Gp in the western limb of the Verín Synform.The outcrop includes blocks of lydite with Silurian graptolite fauna similar to those from Nogueira Gp (see map in Figure 4 and references therein).e) Photomicrograph of syn-orogenic conglomeratic sandstone, east of the Verín Synform.Fragments are formed by quartz, quartzite (Qtzt), slate, schist (Sch) and trachyte (Trch).f) N-S stretching lineation in dark reddish pebbles.Conglomeratic quartzite, lower part of the Queiroga Series, Cabo Ortegal.

Figure 6 .
Figure 6.Tectonic scheme of the parautochthon around the Cabo Ortegal Complex.Based on Marcos and Farias (1999), Marcos et al. (2002), Arenas et al. (2009) and own data.Arrows indicate relative sense of top displacement.

Figure 7 .
Figure 7. Geological map and cross-section of the NE area of the parauchthouton beneath the Cabo Ortegal Complex.Based on Marcos et al. (2002), Arenas et al. (2009), Díez Montes et al. (2014) and new data.Double arrow in Lx indicates the relative sense of movement of the top.Explanation in the text.

Figure 8 .
Figure 8. A) Discrete shear band indicating top-to-the-E late thrusting (LT) in Silurian rocks, crosscutting the C2 schistosity of Ordovician phyllites.Autochthonous section immediately beneath the parautochthon in Cabo Ortegal.b) Isoclinal C1 folds affected by an upright C3 antiform and both crosscut by discrete shear bands related to top-to-the-E late thrusting (LT).Phyllites and metagreywackes, lower part of the Queiroga Series, Cabo Ortegal.c) Outcrop image of metavolcanic rocks in the Arnoia Detachment zone.Kinematic criteria indicate E1 top-to-the-NW sense of shear.

Figure 9 .
Figure 9. Geological map and cross-section of the Arnoia Detachment zone.Based on Barrera Morate et al. (1989), Díez Montes et al. (2014) and new data.Double arrow in Lx indicates the relative sense of movement of the top.Location of thermobarometry and monazite geochronology samples in the Bande-Celanova Dome.Explanation in the text.

Figure 10 .
Figure 10.A) Albite porphyroblast-bearing schist, SACG.Microinclusions defining the internal schistosity can be formed by Qz, WM, Chl, Rt, Ilm, Tur and organic matter.The outer rims of the porphyroblasts are frequently free of inclusions.b, c) Petrographical aspects of an Ab porphyroblast showing oriented inclusions of WM and Rt crystals.Parallel and crossed polars, respectively.d) Petrographical aspect of a Grt-Sil schist, lower section of the parautochthon in the Arnoia Detachment.e) Photomicrograph of a Ky-bearing schist, Sanabria, CIZ.Most of this rock is formed by kyanite, white mica and rutile.f) Microscope aspect of Grt-Sil schist, Villadepera Antiform, CIZ.

Figure 11 .
Figure 11.Thermobarometric results and inferred P-T paths for the studied samples.Methods and sample estimates are explained in the text and Table 2. Area name abbreviations as in Figures 1 and 2. CI-1 and CI-2 paths of autochthonous sections from Central Iberia (after Rubio Pascual et al. 2013) are shown for comparison purposes.

Figure 14 .
Figure 14.Synthetic cross-section along the study zone resuming the main data obtained in the work.Stages A to E show sequential evolution of geothermal gradients.Area and structure name abbreviations as in Figures 1-3.

Table 1 .
Summary of tectonic events used in this work.