The upper allochthon of northwest Iberia represents the most exotic terrane of this part of the European Variscan belt. Recent advances in the metamorphic petrology, structural geology, and geochronology of the upper allochthon in the Órdenes complex are integrated into a synthesis of its tectonic evolution, constraining the main tectonothermal events. Important aspects of this synthesis are (1) the interpretation of Cambro-Ordovician magmatism and earliest metamorphic event, as the result of drifting of a peri-Gondwanan terrane; (2) the subsequent shortening and crustal thickening of the terrane related to its subduction and accretion to Laurussia; (3) a younger cycle of shortening and extension resulting from convergence between Laurussia and Gondwana; and (4) the emplacement of this exotic terrane as the upper allochthon, together with underlying ophiolitic and basal allochthons, during the Laurussia-Gondwana collision. Implications derived from the well-established tectonothermal sequence are discussed in the context of Paleozoic paleogeography and geodynamics. The evolution of this part of the belt is related first to the closure of the Tornquist Ocean, and later to that of the eastern branch of the Rheic Ocean. Furthermore, the relative paleopositions of the upper allochthon and the Iberian autochthon in northern Gondwana are discussed.

Recent advances in geochemical studies of igneous rocks, isotopic age data for magmatism and metamorphism, quantitative pressure-temperature (P-T) estimates of metamorphic evolution, and structural geology in the northwestern Iberian Massif are integrated into a synthesis of the tectonic evolution that places the autochthonous and allochthonous terranes in the framework of Paleozoic plate tectonics. Because northwestern Iberia is free from strike-slip faults of continental scale, it is retrodeformable and preserves valuable information about the orthogonal component of convergence of Gondwana with Laurentia and/or Baltica, and the opening and closure of the Rheic Ocean. The evolution deduced for northwest Iberia is extended to the rest of the Variscan belt in an attempt to develop a three-dimensional interpretation that assigns great importance to the transcurrent components of convergence. Dominant Carboniferous dextral transpression following large Devonian and Early Carboniferous thrusting and recumbent folding is invoked to explain the complexity of the belt without requiring a large number of peri-Gondwanan terranes, and its ophiolites and high-pressure allochthonous units are related to a single oceanic closure. Palinspastic reconstruction of the Variscan massifs and zones cannot be achieved without restoration of terrane transport along the colliding plate margins. A schematic reconstruction is proposed that involves postcollisional strike-slip displacement of ∼3000 km between Laurussia and Gondwana during the Carboniferous.

Allochthonous ophiolitic units in the northwestern Iberian Massif are remnants of peri-Gondwanan Paleozoic oceans sandwiched among other exotic terranes of continental and volcanic-arc derivation. All these terranes define an intricate suture zone that marks the convergence and collision between Laurussia and Gondwana. The suture is defined by three different ophiolitic ensembles: upper ophiolitic units, lower ophiolitic units, and the Somozas mélange. The lower ophiolitic units were derived from an alternation of basalts and sediments intruded by gabbros and scarce granitoids, and they formed during the opening of a marginal basin, the Galician ocean, during Late Cambrian to Early Ordovician time. This ocean was created as a back arc by the severance of a volcanic arc that had developed at the northern margin of Gondwana and formed part of the Rheic oceanic realm. The upper ophiolitic units formed during the Early Devonian from intraoceanic subduction in the early Paleozoic lithosphere of the Rheic Ocean. These suprasubduction ophiolites were formed just before the ocean closed, preceding the collision between Gondwana and Laurussia. The Somozas mélange appears in an anomalous position at the base of the Cabo Ortegal Complex. The ophiolites involved in this tectonic mélange represent an imbricate of highly dismembered oceanic lithosphere, slivers of subducted outer edge of the Gondwanan continental margin, and Paleozoic metasediments of the northern Gondwanan platform. The ophiolites might either record the development of a different peri-Gondwanan oceanic domain, or they might be equivalent to any of the other ophiolitic ensembles, and their anomalous structural position is simply a consequence of complex thrusting.

Two Paleozoic ophiolites of different age mark the Variscan suture in the Cabo Ortegal Complex (northwestern Iberian Massif). The Moeche and Purrido allochthonous ophiolites are structurally located between an exotic terrane of continental affinity (basal units), below, and another terrane with arc affinity (upper units), above. The mafic rocks of the Moeche unit are greenschist with a composition typical of tholeiitic basalt. Their rare earth element (REE) concentrations range from 12 to 31 times the chondritic abundances, with almost flat chondrite-normalized REE patterns ([La N /Yb N ] avg = 0.83), and they have no significant Eu anomalies. According to their contents in some immobile trace elements, the geochemical composition of these metabasalts seems to be transitional between normal mid-ocean-ridge basalt (N-MORB) and island-arc tholeiites. Their abundances in the most immobile trace elements with the highest discriminating power (Th, Nb, Ce, Zr, Ti, and Y), normalized to an average tholeiitic N-MORB, depict quite flat patterns close to the average N-MORB concentrations, although they are slightly enriched. A small negative Nb anomaly points to an origin linked to a subduction zone. The Purrido unit consists of amphibolites with compositions also equivalent to tholeiitic basalt. REE compositions range from 6 to 22 times chondritic abundance, with essentially flat chondrite-normalized REE patterns ([La N /Yb N ] avg = 0.99) and very small positive Eu anomalies ([Eu/Eu*] avg = 1.08). The abundance of some trace elements in these metabasites shows an affinity with island-arc tholeiites, but, unlike the greenschist of the Moeche unit, their N-MORB–normalized trace-element patterns are slightly depleted in Nb, Zr, Ti, and Y, with a somewhat steeper negative Nb anomaly. These features are found in basalts generated in some back-arc basins. Both ophiolites seem to represent oceanic lithosphere that developed in a supra-subduction zone context. The Moeche unit was probably formed during the opening of a back-arc basin at the Cambrian-Ordovician boundary, whereas the Purrido unit was generated later, during the Early Devonian, as a result of the intraoceanic subduction of the older oceanic lithosphere represented by the Moeche unit. The two ophiolites are representative of the Variscan suture in northwest Iberia, and they preserve a significant part of the history of the oceanic realms that closed during the final amalgamation of Pangea.

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Full article available in PDF version.

Full article available in PDF version.