J.R. Martínez Catalán

Diachronous Variscan tectonothermal activity in the NW Iberian Massif: Evidence from 40Ar/39Ar dating of regional fabrics

Abstract Multigrain concentrates of hornblende and muscovite together with whole-rock slate/phyllite samples have been dated (27 analyses) using 40Ar/39Ar incremental-release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the Variscan Iberian Massif. Hornblende concentrates from amphibolites in the allochthonous Ordenes Complex yield plateau isotope-correlation ages of 425 Ma and 377 Ma. Muscovite concentrates and whole-rock slate/phyllite from this and the Cabo Ortegal Complex yield plateau ages which range from 367 Ma to 295 Ma. Analyses of similar material from the relative autochthon yield plateau ages between 359 Ma and 275 Ma. Muscovite concentrates from three late- to post-kinematic granitic stocks yield plateau ages between 309 Ma and 274 Ma. At least seven of the 40Ar/39Ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks. The remaining analyses may be used to constrain the local age of various Variscan tectonothermal events. The oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones. Middle Devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon, where Variscan deformation initiated in the Upper Devonian and diachronously prograded eastward. The first deformational events recorded in the limit within the internal and external zones occured ca. 20–25 Ma later (lower Namurian). Variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge. However, the entire orogen remained tectonically active with different structural features forming at different times and at different places. An average propagation rate of ca. 5 km/m.y. is suggested by consideration of a 20–25 Ma difference in correlative fabric ages and present separations.

Early Ordovician orogenic event in Galicia (NW Spain): evidence from U–Pb ages in the uppermost unit of the Ordenes Complex

New U–Pb data (zircon, monazite and rutile) obtained from rocks of the uppermost allochthon in the Variscan belt of NW Spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre-Variscan tectonothermal evolution. This evolution is characterised by an Early Ordovician (498–500 Ma) bimodal magmatism followed almost immediately (493–498 Ma) by a Barrovian style metamorphism up to the granulite facies. The metamorphism subsequent to the igneous intrusions requires convergence and crustal thickening in order to generate the Barrovian facies pattern shown by the lithologies of the uppermost allochthon. The almost coeval magmatism and metamorphism, and the chemistry of the metabasites suggest an accretionary complex, probably related to a volcanic arc, as the most probable setting for the origin of the uppermost unit. The implications of the existence of Early Ordovician convergent plate boundaries are discussed in the context of Gondwana–Laurentia–Avalon interactions.

Tectonothermal evolution associated with Variscan crustal extension in the Tormes Gneiss Dome (NW Salamanca, Iberian Massif, Spain)

Abstract The tectonothermal Variscan evolution of the Tormes Gneiss Dome is controlled by a subhorizontal ductile shear zone of crustal scale and extensional characteristics that induced a quick exhumation of the deep parts of the metamorphic complex during crustal thinning. The shearing affected a broad band of metamorphic rocks but, as the temperature decreased, became progressively concentrated in a low-grade ductile detachment that separates two distinct units, characterized by their lithology and different tectonothermal evolution. Kinematic indicators in non-coaxial fabrics show a displacement of the hanging wall to the southeast i.e., parallel to the trend of the foldbelt. The subhorizontal shearing is related to the extensional collapse of the variscan crust, previously thickened during the collision tectonics.

Syn-collisional extensional collapse parallel to the orogenic trend in a domain of steep tectonics: the Salamanca Detachment Zone (Central Iberian Zone, Spain)

The area south of the town of Salamanca is a zone of the Spanish Variscan Belt characterized by a first compressional event which gave rise to steep structures, followed by the development of an extensional ductile shear zone, more than 4 km thick, that is described as the Salamanca Detachment Zone. The strain associated to the detachment is analyzed using quartz pebbles in conglomerates. Comparing the measured strain values with theoretical strain paths, leads to the conclusion that the deformation was approximately of plane strain type. Quartz c-axis fabrics and kinematic criteria indicate that simple shear was a very important component of the deformation, though coaxial components were probably involved. The extensional character of the detachment is indicated by the geometry of the metamorphic zones and the metamorphic evolution. Taken together, the data indicate that the extensional event was related to gravitational collapse induced by the thickening of the continental crust, and that it was syn-collisional. The movement of the hangingwall unit was to the southeast, parallel to the trend of the fold belt, and the translation was of the order of one to a few dozens of km. Though important, this is not exceedingly large, and allowed the preservation of low-grade metamorphic conditions in the hangingwall, preventing it from being affected by extensive brittle tectonic processes.

SEISMIC STRUCTURE OF THE NORTHERN CONTINENTAL MARGIN OF SPAIN FROM ESCIN DEEP SEISMIC PROFILES

Abstract By the end of the Carboniferous, the crust of the continental shelf in northwestern Spain was made up of deeply rooted structures related to the Variscan collision. From Permian to Triassic times the tectonic setting had changed to mainly extensional and the northern Iberian continental margin underwent rifting during Late Jurassic-Early Cretaceous times, along with sea-floor spreading and the opening of the Bay of Biscay until the Late Cretaceous. Subsequently, the northern Iberian margin was active during the north-south convergence of Eurasia and Iberia in the Tertiary. A multichannel seismic experiment, consisting of two profiles, one north-south (ESCIN-4) crossing the platform margin offshore Asturias, and another (ESCIN-3) crossing the platform margin to the northwest of Galicia, was designed to study the structure of the northern Iberian margin. The ESCIN-4 stacked section reveals inverted structures in the upper crust within the Le Danois Basin. North of the steep continental slope, ESCIN-4 shows a thick sedimentary package from 6 to 9.5 s, two-way travel time (TWT). Within this latter package, a 40-km-long, north-tapering wedge of inclined, mainly south-dipping reflections is thought to represent a buried, Alpine-age accretionary prism. In the north western part of the ESCIN-3 (ESCIN-3-1) stacked section, horizontal reflections from 6.5 to 8.5 s correspond to an undisturbed package of sediments lying above oceanic-type basement. In this part of the line, a few kilometres long, strong horizontal reflection at 11.2 s within the basement may represent an oceanic Moho reflection. Also, a band of reflections dips gently towards the southeast, from the base of the gently dipping continental slope. The part of ESCIN-3 line that runs parallel to the NW-Galicia coast (ESCIN-3-2), is characterized by bright, continuous lower crustal reflections from 8 to 10 s. Beneath the lower crustal reflectivity, a band of strong reflections dips gently toward the southwest from 10 to 13.5 s. The part of ESCIN-3 that parallels the northern margin (ESCIN 3-3), shows good reflectivity in all levels. Upper crustal reflections image the sedimentary fill of probable Mesozoic to recent basins. Mid-crustal reflectivity is characterized by dipping reflections until 8 s that are probably related to compressional Variscan features. The lower crustal level shows ‘layered’ reflections between 8 and 12 s. Dipping reflections are found below the continental Moho.

Lower Paleozoic extensional tectonics in the limit between the West Asturian-Leonese and Central Iberian Zones of the Variscan Fold-Belt in NW Spain

ZusammenfassungDie Grenze zwischen der West-Asturischen-Leonesischen und Zentral-Iberischen Zone im NW der Iberischen Halbinsel wird aus dem Gebiet von Caurel-Truchas (Provinz Orense und León) beschrieben. Aufgrund der Verteilung der sedimentären Formationen, sowie der variszischen Strukturen wird abgeleitet, daß der Übergang zwischen den beiden Zonen durch ein Störungssystem bedingt wurde, das zumindest zwischen Ordovizium und Untersilur aktiv war. Dabei handelte es sich vermutlich um Abschiebungen mit einer Horizontalverschiebungskomponente. Diese Ausbildung führte zu einem Halbgraben mit darin abgelagerter sedimentärer Syn-Riftsequenz. Letztere wurde während der Anfangsphase der variszischen Tektonik invertiert, so daß es zu einem Rampen-Antiklinal-Synklinal Paar kam, welches die wichtigsten Falten der Gegend bildet. Die variable Geometrie der Falten wird zum einen der Existenz von frühen En-échelon Falten zugeschrieben, die durch die Horizontalverschiebungskomponente der Störungen hervorgerufen wurden; zum anderen sind sie das Resultat der veränderlichen Intensität der Scherspannung während der ersten variszischen Phase.AbstractThe limit between the West Asturian-Leonese and Central-Iberian Zones in the NW of the Iberian Peninsula is described in the area of Caurel-Truchas (Provinces of Orense and León). From the distribution of the sedimentary formations and the Variscan structures it is inferred that the transition between the two zones was a fault system, which was active during, at least, the Ordovician and Lower Silurian. The faults were supposedly normal, though they had probably some strike-slip component, and gave rise to a half-graben in which a syn-rift sequence was deposited. The latter underwent inversion at the onset of the Variscan tectonism, producing a ramp anticline-syncline pair that forms the more important folds in the area. The varying geometry of these folds is interpreted partly as being due to the existence of previous en échelon folds related to the strike-slip component of the faults and, partly, as a result of the variable intensity of the shear strain undergone during the first variscan phase.ResumenSe describe el límite entre las Zonas Asturoccidental-Leonesa y Centroibérica en el NO de la Península Ibérica en el área del Caurel-Truchas (Provincias de Orense y León). Partiendo de la distribución de las formaciones sedimentarias y de la geometría de las estructuras varíscicas, se deduce que la transición entre las dos zonas fue un sistema de fallas, que fue activo al menos durante el Ordovício y el Silúrico Inferior. Se supone que la principal componente de las fallas fue normal, aunque debió de existir también una componente de desgarre, y su juego originó un semi-graben en el que se acumuló una secuencia sin-rift. Esta última experimentó una inversión al principio de la tectogénesis Varíscica, formándose un par de pliegues de rampa que son los pliegues más importantes del área. La geometría variable de esos pliegues se interpreta como debida, en parte, a la existencia previa de pliegues en escalón relacionados con la componente de desgarre de las fallas y, en parte, como consecuencia de la variable intensidad de la deformación por cizallamiento sufrida durante la primera fase de deformación varíscica.Краткое содержаниеОписана граница межд у западной астурийск о-леонской и центрально-иберийс кой зонами на северо-западе Ибер ийского полуострова в регионе Caurel-Truchas, провинции Orense и León. На основании распр еделения осадочных формаций и варисских структур приходят к выводу, что переход ме жду обеими зонами обусловлен системой нарушений, которая проявляла активност ь в период между ордов иком и нижним силуром. Здес ь речь идет, вероятно, о сбросе с ко мпонентами горизонт ального сдвига. Это привело к о бразованию полуграбена с отложи вшимися там осадочны ми свитами одновременн о с появлением рифта. Полуграбен оказался опрокинутым во время начальной фазы варис ской тектоники, что привело к появлению а нтиклинально-синкли нальной рампы и ведущих склад ок в данной области. Различия в ко нфигурации складок приписывают, во-первы х, наличию предшевств овавших кулисообразных скла док, которые вызвали нарушения в р езультате горизонта льного надвита, а, во-вторых, о ни являются следствием различно й интенсивности напр яжения скола во время первой фазы варисского горообразовательно го процесса.

P-T evolution of eclogites from the Agualada Unit (Ordenes Complex, northwest Iberian Massif, Spain): Implications for crustal subduction

Abstract Eclogite lenses in the Agualada Unit (western Ordenes Complex, Spain) contain the peak mineral assemblage garnet (prograde rim: Alm = 48 mol%, Prp = 30 mol%), omphacite (Jd max = 36 mol%), quartz, rutile and rare zoisite, which equilibrated at T = 700 °C and P > 12–14 kbar. Garnet shows discontinuous growth zoning, with a pyrope-poor intermediate zone (Alm = 51 mol%, Prp = 10 mol%) between a core zone where pyrope is slightly higher (Alm = 46 mol%, Prp = 16 mol%) and areas just inward from the rims where the maximum pyrope contents (Alm = 48 mol%, Prp = 30 mol%) are recorded. In atoll interiors, garnet contains inclusions of a first generation of omphacite (Jd max = 40 mol%). This omphacite is replaced in the matrix by a second generation (Jd max = 36 mol%) with higher Fe/Fe + Mg ratio. The compositions of garnet and omphacite suggest a complex syneclogitic tectonothermal evolution for the Agualada Unit, characterized by: (1) eclogite-facies metamorphism ( T = 585 °C, P > 12–13 kbar), followed by (2) cooling during a slight decompression ( T = 500 °C, P > 11–12 kbar), and (3) a final increase in P and T to form the garnet rim-matrix omphacite mineral assemblage. The Agualada Unit is part of a subduction complex which affected the Gondwana margin at the beginning of the Variscan cycle. The P-T evolution of the Agualada eclogites is closely related to the structural evolution of the accretionary complex and the whole orogenic wedge. The cooling event recorded by the Agualada eclogites may have resulted from the accretion of a new colder crustal slice under the unit, whereas the final progradation reflects the emplacement of the Agualada Unit directly under the mantle wedge. This evolution fits well with previously presented the retical models, both for the tectonothermal evolution of accretionary complexes and for the dynamic evolution of orogenic wedges. P-T paths such as the one for the Agualada Unit eclogites, probably reflect a prolonged structural evolution. Although evidently rarely preserved, such paths are probably the rule rather than the exception during plate convergence.

Tectonic setting of the Monte Castelo gabbro (Ordenes Complex, northwestern Iberian Massif): Evidence for an arc-related terrane in the hanging wall to the Variscan suture

The Ordenes Complex is the largest of the allochthonous assemblages containing the Variscan suture in the northwestern Iberian Massif. Its uppermost tectonostrati-graphic unit overlies the ophiolitic units, and consists of a thick metasedimentary sequence, the Ordenes Series, intruded by orthogneisses and gabbros. In the lower part of the Ordenes Series, the large Monte Castelo gabbro (∼150 km 2 ) is surrounded by high-grade migmatitic paragneisses. Several shear zones cutting across the gabbro massif depict intermediate-pressure granulite facies, indicating a metamorphic evolution common with the surrounding paragneisses. Recent U-Pb geochronological data prove that the main tectonothermal evolution of the Ordenes Series took place in Late Cambrian-Early Ordovician time. These data suggest that the intrusion of the Monte Castelo gabbro (499 ± 2 Ma; U-Pb in zircons) was immediately followed by a Barrov-ian metamorphic episode that reached the granulite facies (493-498 Ma; U-Pb in monazites). A later Variscan overprint is indicated by U-Pb rutile ages of 380-390 Ma. Considering the geochronological evidence for almost coeval magmatism and metamorphism during the Early Ordovician, together with the geochemical characteristics of the Monte Castelo gabbro, an accretionary complex related to a Late Cambrian-Early Ordovician island arc appears as the more probable setting for the uppermost allochthonous terrane in the northwest of the Iberian Massif. This implies the presence of a convergent plate boundary in the oceanic realm between Laurentia and Gondwana, or close to it, during the early Paleozoic. The island arc later became involved in the Variscan convergence and accretion.

Allochthonous Units in the Variscan Belt of NW Iberia: Terranes and Accretionary History

The allochthonous complexes of NW Iberia can be correlated with the paleogeographic realms involved in the Variscan collision. Assuming that the existing ophiolites represent the suture formed by the closure of a Paleozoic ocean, the units in the footwall to the suture correspond to the outer edge of the Gondwana continental margin, formed after the opening of the ocean. The units in the hangingwall to the suture are interpreted as pieces of a continental fragment detached from Gondwana, which registered an orogenic event in the Lower Ordovician, shortly after its separation. The correlation establishes a link between circum-Atlantic terranes and the Rheic ocean, and makes it possible to draw a relatively simple scenario of the successive recorded tectonothermal events. Following the amalgamation of Avalon to Laurentia, the remaining outboard terranes underwent a progressive accretion to the Laurentian continental margin that ended with the collision between this continent and Gondwana. The accretionary history, as deduced from the tectonothermal events recognised mainly in the Galician allochthonous complexes (NW Spain), includes partial subduction of the continental margin of Gondwana, during the Middle Devonian.

Fabric Development in a Middle Devonian Intraoceanic Subduction Regime: The Careon Ophiolite (Northwest Spain)

A Middle Devonian suprasubduction zone ophiolite, the Careón Unit (northwest Spain), displays amphibolite‐facies ductile deformation fabrics related to the onset of the Rheic Ocean closure. Two different fabrics, an early high‐T foliation and a subsequent lower‐T foliation, each of which characterized by distinct deformation mechanisms, have been identified in two distinct crustal‐scale shear zones of the same ophiolitic thrust sheet. Combined quantitative texture analysis by electron backscattered diffraction and time‐of‐flight neutron diffraction, were carried out on the shear zones and correlated with micro‐ and macrostructural data. The results indicate that the regional lineation and shear zone kinematics (east‐west, top‐to‐the‐east) represent fabrics developed essentially during the intraoceanic subduction of the Rheic Ocean, and their orientation may be considered a reference vector for convergence models in this part of the Variscan belt.