CALEDONIAN AND LATE CALEDONIAN EUROPE : A WORKING HYPOTHESIS INVOLVING TWO CONTRASTED COMPRESSIONAL / EXTENSIONAL SCENARIOS

The tectonomagmatic and metamorphic structuration of the European Caledonian realm suggests that two mutually perpendicular compressional/extensional scenarios developed during the Ordovician-Devonian time-span. As a result of the mid Ordovician Grampian compressional scheme (Caledonian s.s.), a major extensional province developed further east from the Caledonian foldbelt in continental Europe. This scenario ended by early/mid Devonian, with the complete locking of North America, Baltica and Gondwana into a Pangaea supercontinent, thus triggering a contrasted tectonic environment which might be termed <date Caledonian», characterized by compression and plutonism in western continental Europe, and extensional detachment tectonics and volcanism in the northern part of the Caledonian foldbelt (Scotland, southwestern Norway and eastern Greenland). The overall evolution of this realm can be summarized in terms of inversion tectonics (s.l.) processes, with periods of crustal thinning followed by thickenning and vice versa.


Introduction
The western European realm has been traditionally ascribed to either Caledonian (western Scandinavia and most of the British Isles) or Variscan (southern British Isles and western continental Europe) dornains.In this sense, the question of Paleozoic pre-Variscan events, particularly within the continental sector, has been a subject of controversy, as iIlustra-ted by the fact that no specific differentiation between Caledonian and Variscan cycles exists in sorne areas.
Another polernical question is related to the age of the final configuration of Pangaea (either Devonian, or Carboniferous/Permian), an issue which is of vital irnportance for the overall scenario.In this sense, the list of proposals (based on paleomagnetic and/or paleontological data) is very long.The «late 21R Pangaea» hypothesis sustained by Perroud and Bonhommet (1981) and Zonenshain el al. (1985), among others, proposes that this supercontinent was not consolidated till Carboniferous or Permian time, when the northward drifting Gondwana was finally welded to the septentrional continental mass (Laurentia, Baltica, and Armorica).The alternative «early Pangaea» scheme (McKerrow and Ziegler,  1972; Keppie, 1977; Morel and Irving, 1978; Livermore el al., 1986; Miller and Kent, 1988), suggests a supercontinent configuration by early to mid Devonian time.
In this paper, we will argue that the structuration of the European Caledonian realm is characterized by the following points: 1.An early link between the different continental masses shaping Pangaea in this area.2. Two contrasted tectonic scenarios, each of them defined by perpendicular deformational trends: Caledonian s.s., and «late Caledonian».3. A globa-Iized Caledonian cycle ending by late Devonian time, which can be c1early separated from the Variscan one.

The shaping oC the European Caledonides
The controversial issue of a Caledonian continental Europe is not new, and in fact, pre-Variscan unconformities, tectono-metamorphic events, and magmatism have long been recognized in this realm (Alvarado, 1980;Autran and Peterlongo, 1980;Behr el al., 1980;Weber, 1984;Serrano Pinto, 1987, among others).However, these phenomena have been mostly regarded as local peculiarities, and no c1ear relationships to the main Caledonian foldbelt have been established.Additionally, the existence of tectonic events taking place in continental Europe, while the main Caledonian foldbelt was being shaped westward, favours the hypothesis of an early Paleozoic unified and unstable domain in the continental European realm, thus posing serious doubts on the models based on a series of microplates adrift at different paleolatitudes.Prior to the Caledonian cycle, this realm was affected by the Cadomian orogenesis (Precambrian/Cambrian), a scenario which changed in continental Europe with the onset of extensional conditions (Autran and Cogne, 1980) heralding the Caledonian.Sawkins and Burke (1980) suggest that extension continued in continental Europe during more advanced stages of the Caledonian cycle.
sional basins in which the Old Red Sandstone facies were deposited and volcanics extruded (extensional event E2).However, the stresses responsible for this collapse were mainly due to intraplate unstable conditions triggered by the previously overthickened orogenic welt.If we assume that an early or mid Devonian Pangaea existed, as suggested by sorne authors (McKerrow and Ziegler, 1972;Keppie, 1977;Morel and Irving, 1978;Gambles, 1984;Livermore el al., 1986;Miller and Kent, 1988), then the finallocking of North America, Baltica and Gondwana (370 Ma, Gambles, 1984) can adequately account for the compressional event C2.Since by Devonian time, the British segment of Iapetus had closed (Dewey, 1982;Dewey and Shackleton, 1984), it is conceivable that the main stress component may have been provided by the northward drifting and collision of Gondwana against Europe by early/mid Devonian time.
Finally, soon after the cessation of this «late Caledonian» scheme, new extensional conditions prevailed during Dinantian time in most of Europe, a si-tuation which already belongs to the Variscan cycle.During this event the following features developed: the Basin/Block province, volcanism, and the so-ca-lIed lrish-type ore deposits in Britain and Ireland (Leeder, 1982;Evans, 1987); the «Basin and Swell» zones in Germany (Luettig, 1980); basic and acid volcanism in France (Chauvel and Robardet, 1980); and rifting and bimodal volcanism in southern Portugal and Spain (Swakins and Burke, 1980).To the latter is associated a major volcanosedimentary complex hosting sorne of the most outstanding massive sulphide deposits of the world (e.g.Aznalcollar, Neves Corvo, Río Tinto).This extensional episode, the first of two during the Variscan cycle (a late Variscan extensional event is also recorded in France, Germany and Spain; Menard and Molnar, 1988; Doblas el al.,  1988), ended by late Visean time, when a major Variscan compressional episode induced folding, metamorphism and plutonism throughout continental western Europe (Sudetic phase).

The extensional events: Contrasted genetic models
The case of the «late Caledonian» extensional event (E2; post-early Devonian-pre Dinantian) is analyzed first, as a model has already been proposed to explain it.As suggested by McClay el al. (1986), extensional detachment tectonics (as a consequence of the gravitational collapse of a previously overthickened orogenic welt) seems a plausible mechanism to account for the northern Devonian basins in which the Old Red Sandstone facies were deposited during this event.In this sense, the «late Caledonian» Basin and Range-type extensional province thus defined, bears many similarities with southern Spain and northern Africa during late Alpine time (Neogene), when the overthickened Betic-Rif orogenic edifice collapsed through detachment systems (Doblas and Oyarzun, 1989).Even if intraplate stresses arising from a gravitationally unstable crustal welt seem to be the major cause of this extension, the final locking of North America, Baltica and Gondwana (generating a northward-directed compressional stress pattern), may have been the triggering event contributing to release the stresses generated within the Caledonian foldbelt, thus allowing the extensional collapse of its northern sector.By contrast, the Ordovician-earIy Devonian extensional event (El) developed as related to different circumstances.Since no previous overthickening can be advocated for continental western Europe, then another cause must account for this extension.In this sense, stresses arising frorn the building of collisional-type mountain belts are known to trigger extensional provinces in their continental forelands (e.g.Alps, IlIies and Greiner, 1978; Hima-M DOBLAS.R.OYARZUN layas, Tapponnier el al., 1982).Therefore, it can be argued that El was a direct consequence of the compressional stresses generated throughout the Caledonian front during the Cl episode.This scheme also provides an adequate explanation for the strike-slip zones of deformation (Dewey, 1982;Dewey and Shackelton, 1984) observed in the northern Caledonian foldbelt (fig.lA), since is typical of a Himalayan-type collision/indentation model (Tapponier el  al., 1982) as the one suggested here.Finally, we suggest that these two Caledonian s.s.compressional (Cl) and extensional (El) dornains rnight have represented unstable intercratonic corridors, differentially deformed in between three rnajor Precambrian shields (Gondwana, Baltica, and Laurentian-Greenland; fig.2).Moreover, the central bending of the Caledonian compressional belt, and the eastward divergence of the extensional trends in continental Europe, would be a final result of the continued convergence between Laurentian-Greenland, Baltica, and Gondwana.
Finally, the overall evolution of the European Caledonides can be ascribed to an «inversion-type» (s.l.) tectonic model, in that, regions subject to crustal thickenning were later thinned (e.g.northern Caledonides; Cl followed by E2) and vice versa (e.g.western continental Europe; El followed by C2).