GR Letter The onset of the assembly of Pangaea in NW Iberia: Constraints on the kinematics of continental subduction Ruben Oiez Fernandez a . * , Jose R. Martinez Catalan a , Ricardo Arenas b, Jacobo Abati b • Departamento de Ge%gia, Universidad de Salamanca, 37008 Salamanca, Spain b Departamento de Petrologia y Geoquimica and Instituto de Geologia Economica, Universidad ComplutenselConsejo Supe rior de Investigaciones Cientificas, 28040 Madrid, Spain ABSTRACT Excellent exposures of high-pressure rocks developed in a Variscan continental subduction system outcrop in NW Iberia. The kinematic criteria provided by the high-pressure metamorphic fabrics can be used to infer tectonic flow within the deep sections of this system. The dominant trend of the ductile flow is oblique to that of the orogenic belt, indicating oblique continental subduction. Its azimuth, a few tens of degrees clock­ wise relative to the orogenic trend, suggests dextral transpression between Gondwana and Laurussia during continental subduction that took place at the Upper Devonian, and provides a consistent kinematic reference for the earliest assembly of Pangaea in NW Iberia. Keywords: Assembly of Pangaea Continental subduction kinematics Tectonics NW Iberia 1. Introduction The oldest evidence on the plate kinematics of a given continent­ continent assembly lies in its continental subduction record preced­ ing collision, and the high- to ultrahigh-pressure (HP jUHP) belts so formed are witness of such processes. Exhumed continental blocks develop a strong overprint of the subduction record as they return to shallower lithospheric levels (e.g. Ring et al., 2007; Zhang et al., 2009; Hacker et al., 2010), where such evidence is mostly preserved in small and disconnected lenses (e.g. eclogite pods; Teyssier et al., 2010), commonly as mineral relicts within a dominant low- to medium-pressure metamorphic matrix. Indeed, recent modeling pre­ dicts large-scale nappe-folding during the process (e.g. Gerya et al., 2002; Warren et al., 2008) that would distort any former geological record (Dlez Fernandez et al., 2011 a). For these reasons, unraveling the earliest kinematic events in ancient collisional orogenies is one of the most critical topics in tectonics, and requires significant struc­ tural and metamorphic insights. The supercontinent Pangaea assembled from the collision between Gondwana and Laurussia, following the closure of the Rheic Ocean dur­ ing the Upper Paleozoic (Matte, 1986; Scotese, 1997; Stampfli and Borer, 2002; Martinez Catalan et al., 2009). The Variscan Belt represents '" Corresponding author. Tel.: +34923 294488; fax: +34923 294514. E-mail addresses: georuben@usai.es (R. Diez Fernandez), jrmC®usales (JR Martinez Cataian), arenas@geo.ucm.es (R. Arenas), abati@geo.ucm.es 0. Abati). that part of this amalgamation preserved in Europe, which started with the subduction of the Gondwana plate in the Upper Devonian (e.g. Schulmann et al., 2005; Ballevre et al., 2009; Abati et al, 2010). Kine­ matic data for this event are lacking. Variations in the PT conditions of the first HP metamorphic event along this margin indicate a west­ dipping (present coordinates) polarity for the continental subduction in NW Iberia (Martinez Catalan et al., 1996). There is a lack of consistent indicators of along-strike components during the subduction, so ideas about the early relative plate movement between the two main land­ masses involved in the assembly of Pangaea are highly speculative (e.g. Arenas et al, 2009). Exposures of well-preserved Variscan HP rocks are rare, but where they occur they provide evidence for the kinematics of continental sub­ duction. We present in situ kinematic criteria from the best outcrops of eclogite rocks developed in the section of the Variscan subduction sys­ tem exposed in NW Iberia, Spain. The data are accompanied by a de­ tailed field analysis and a regional tectonometamorphic synthesis in order to constrain their plate tectonic significance. 2. Geologic setting The allochthonous complexes of NW Iberia comprise a nappe stack of allochthonous units, in which the Neoproterozoic and Paleozoic geodynamic evolution of the northern peri-Gondwanan realm is pre­ served as a collage of exotic terranes that delineate a piece of the su­ ture zone of the Rheic Ocean (Fig. 1; Martinez Catalan et al., 2009). The peripheral and outermost domains are placed on top. The upper­ most thrusts consist of an imbricated Cambro-Ordovician continental VARISCAN BELT • Allochthonous complexes D Autochthon § External thrust belt 53 and foredeep basin , ••• •. Local attitude of . •• ••. the orogenic belt ............. Alpine front ............... Variscan thrusts .....L...oL.. Main Variscan thrusts separating zones Main Variscan normal -- and strike-slip faults � D 1 tectonic flow (L 1 ) Fig. 1. Location of the study area in the Variscan belt. The orientation of D1 tectonic flow in relation to the trend of the orogenic belt is shown. Data from NW Iberia (pub­ lished in this article) and from lIe de Groix, France (Philippon et ai., 2009). arc detached from Gondwana (Abati et al., 1999; Gomez Barreiro et al., 2007). The middle units are ophiolitic, and represent vestiges of Rheic Ocean lithosphere (Sanchez Martlnez et al., 2009). The basal units out­ line the most external part of the Gondwana margin (Dlez Fernandez et al., 201 0; Dlez Fernandez et al., 2011 b). The allochthonous pile rests on top of a parautochthonous imbricate thrust sheet that occupies an intermediate position in the regional structural pile, separating the allochthonous complexes from the autochthonous series of the Iberian Massif (Martlnez Catalan et al., 2007). Variscan deformation started in the basal units, and developed under HP/UHP conditions (01; Dlez Fernandez et al., 2011a). This event records the subduction of Gondwana beneath an accretionary complex and Laurussia (Fig. 2), and is the focus of this article. 3. The relicts of continental subduction The basal units include large, lens-shaped, orthogneiss massifs sur­ rolll1ded by albite-bearing schists and paragneisses, and alternating with mafic rocks (Fig. 3a). The metamorphic conditions reached within the subduction wedge range between blueschist and eclogite facies, whereas the early exhumation (02) developed lll1der amphibolite to UPPER DEVONIAN (ca. 380·370 Ma) s Fig. 2. Continental subduction model for the northern Gondwana margin exposed in NW Iberia. greenschist facies conditions (Rodrtguez et al., 2003). The HP mineral as­ semblage (51) in the metasedimentary rocks consists of quartz + phen­ gite + garnet± rutile ± epidote in the lower structural sections (Dlez Fernandez et al., 2011a), and chloritoid + garnet± glaucophane + phengite + paragonite + chlorite + epidote + rutile + illmenite + quartz in the upper sections (L6pez-Carmona et al., 2010). These assemblages usually occur as mineral trails within syn-exhumation albite porphyro­ blasts (Arenas et al., 1995), and have not been considered for kinematic analysis. A foliation developed in most of the orthogneisses during the ear­ liest stages of exhumation (Dlez Fernandez et al., 2011 a), but relict coronitic garnets in igneous biotite provide evidence for the subduc­ tion event (Gil Ibarguchi, 1995). More importantly, non­ retrogressed HP fabrics can be found in tonalitic orthogneisses, which also enclose subconcordant lenses of eclogite. Both the tonali­ tic orthogneisses and the eclogites are exposed together in the Malpica-Tui Complex north of the Fervenza reservoir (Fig. 3a and b), where they represent large bodies, several hundreds of meters thick that escaped retrogression during exhumation. The 01 assemblage (51) of the tonalitic orthogneisses includes ompha­ cite + garnet + quartz + zoisite + phengite + rutile + kyanite ± apatite ± zircon, and defines a tectonic banding in which quartz-rich layers alter­ nate with melanoaatic layers and lenses with nemato-lepidoblastic tex­ ture (Fig. 4a). The eclogites are fine- to medium-grained rocks with grano-nematoblastic texture. 51 consists of garnet + omphacite + zoisite + rutile + phengite + quartz + kyanite (Fig. 4b). 51 is the main foliation within the HP bodies, and its minerals define a miner­ al and a stretching lineation (Lt). These rock types provide clues to the metamorphic and deformational conditions at P>2 GPa (2.4- 2.6 GPa and 615 cC; Rodrtguez et al., 2003), and the age of the oldest dated Variscan deformation (372 ± 3 Ma; Abati et al., 2010). In the tonalitic orthogneisses, chlorite, white mica and epidote may occur within Dl-garnet, whereas in the eclogites, glaucophane may be trapped within Dl-garnet (Fig. 4c). Such inclusions witness previous colder conditions in a HP prograde P-T -t path, suggesting that 51 repre­ sents a fossil relict of the continental subduction. 4. Kinematic criteria and structural framework The sense of rotation of the vorticity vector of the 01 tectonic flow was determined by using offset mesoscopic and microscopic features in 51. Kinematic criteria include G-type porphyroclasts and lenses, asymmetrical boudinage, 5C composite foliation or er fabrics, and intra­ folial asymmetrical recumbent folds (Fig. 4d and e). 51 is a flat-lying fo­ liation bent into open upright synforms (see local orientation in Fig. 3a) and shows consistent top-to-the-NE shear-sense (Fig. 3c). 01 folds axes have NW-SE trends (1202/162) and are perpendicular to LJ, the mean trend of which in the tonalitic orthogneisses and eclogites is 302/212 (Fig. 3c), thus constraining the 01 tectonic flow to a NE-SW vector. The early exhumation of the basal units was driven by large fold­ nappe structures (02). Progressive exhumation started with the Fervenza thrust, which was followed by the propagation towards the foreland of a train of recumbent folds, and replaced by the Lalln-Forcarei thrust (Fig. 3d; Dlez Fernandez et al., 2011a). These events preceded the em­ placement of the suture zone onto the Gondwana mainland by out­ of-sequence thrusting (03; Martlnez Catalan et al., 2002). Crustal thickening was followed by the gravitational collapse of the colli­ sional wedge (04; Gomez Barreiro et al., 2010; Dlez Fernandez et al., accepted), wrench tectonics and upright folding (05)' and oroclinal bending (Weil et al., 2000). In the light of such a complex scenario, the interpretation of the 01 flow relies on whether or not the relative orien­ tations of the indicators were modified during the exhumation. The tonalitic orthogneiss bodies occur in the hanging wall of the Fervenza thrust, in the normal limb of a O2 fold (Fig. 3b), in a domain free from strike-slip shear zones, and at the hinge zone of a late, open upright synform. Although the tonalitic orthogneisses are surrounded � Extensiona.1 detachment --Contact _Fault _Axial trace: 02 anticline --.-Axial trace: 02 syrdine +Axial trace: antilorm ........ Axial trace: syrdine ......... S, --- S2 ff�rum15',!;�?fokls -..S, �Ss "'""'-.. L, (trerK! arK! pllJl"'l93) � L2(trerK! arK! pllJl"'l93) R3ral�lim orthogmisses � Osstretchirg lineation AI�line ortmgmisses Gran�ic orthogmisses: k3ss! more deformed Granodioritic ortmg __ less! more delormed Metagreyw_1 Albite-bearirg schists am parag Tona.litic orthogmisses Ecbg�es, retroeclog_ arK! amphibolit RELATIVE AUTOCHTHON !;W���������������C�Olniall� iti ic�orthogneisses DDD Schists_! Paramigmat! Orthog (this study) VARISCAN GRANITOIDS ����(di)�i§���DJ� �� SYi""�O Oim �ffi