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Variscan events: early Palaeozoic continental and late Palaeozoic crustal shortening

K. Weber

SUMMARY: Variscan events are interpreted in terms of a geodynamic process of long duration. It began in the early Palaeozoic, possibly in the late Precambrian in some regions, with widespread rifting of the continental lithosphere. Granulite facies metamorphism, widely in evidence in the Ordovician and Silurian, coincides in time with igneous activity and with continuous accumulation of sediment at the surface. That association is taken to indicate continental rift metamorphism above anomalous regions of the mantle. Folding and metamorphism of what is now regarded as the began early in the . By Upper Devonian at the latest wide areas of crystalline basement had been deeply exposed by erosion. The orogenic crustal shortening which began early in Devonian time induced intensive development of nappe tectonics involving the basement rocks. This resulted in deep-reaching crustal imbrications, especially well shown at the Moldanubian-Saxothuringian zone and Saxothuringian-Rhenohercynian zone boundaries, which evolved to carry crystalline basement rocks towards their foreland regions over distances greater than 100 km. During the course of these nappe developments folding of the adjacent sedimentary troughs proceeded. A geodynamic model of the northern flank of the central European Variscan orogen is presented. (Rb/Sr data are given using ~Rb 87 = 1.42 x 10 -11 y-1. The error limits are taken from the original papers, original data are given in brackets.)

An increasingly wide availability of radiometric initial ratios, these can make up no age-dates in the Variscides (the term should be volumetrically large part of the Variscan crust. understood in the sense of European Gebauer & Gr/inenfelder (1983), on the other Hercynides, cf. Ziegler 1982) has given new hand, have ffsed U-Pb zircon data to suggest impetus to the discussion of the geodynamic that mafic and ultramafic protoliths older than development of the Variscides and of the range 1 Ba are detectable in metasedimentary of age of the events involved. country rocks in the French massif central, in J/iger's (1977) comprehensive account of the Moldanubian region and in the central and radiometric age-dates in central and western western Alps. The metamorphism of these Europe led to the conclusion that oceanic mafic and ultramafic protoliths can, sedimentation in late Precambrian and early nevertheless, be regarded as being everywhere Palaeozoic time was followed by three phases of of early Palaeozoic age, and this introduces the : the Cadomian, the Caledonian and question of the meaning of any 'Caledonian' the Variscan. event in the Variscan basement. A Caledonian Vidal et al. (1981) have shown that from the event in northern Europe is incontrovertibly of evidence of 87Sr/86Sr initial ratios, magmatites orogenic character and is explained there in in the greater part of the central and west terms of the closing of the Iapetus Ocean. But European crust are probably not older than in central and western Europe it has not been 700 Ma. These ratios rise with time, and this clearly established that any major crustal suggests to Vidal et al. (1981) that the shortening was involved in the events which Variscides in central and western Europe can have been called, because of their age, be regarded as a closed system with Variscan Caledonian. magmatic rocks derived as melts in relatively young continental crust. There are, of course, older pre-Cambrian The 'Caledonian' event within the rocks within the Variscan crust. This is especially clear in the northern part of the Variscan orogen Armorican massif (Cogn6 1974; Cogn6 & Certain peculiarities make it difficult to regard Wright 1980; Autran & Cogn6 1980) and in the a 'Caledonian' event within the Variscan realm Bohemian massif (Vejnar 1971; Jakeg et al. as representing an orogenic event in which sig- 1979). However, in the interpretation of Vidal nificant crustal shortening was achieved: et al. (1981) based on the trends of the 87Sr/86Sr (a) During the time span of 500-400 Ma Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

4 K. Weber

FIG. 1. Structural map of the European Variscides (after Engel & Franke 1983). Devonian and Carboniferous flysch at outcrop (close stipple) and presumed extent (spaced stipple). Upper Carboniferous parallic molasse, at outcrop (cross-hatched) and presumed extent (hatched). Arrows: tectonic polarity. Black patches: crystalline nappes in the Saxothuringian zone (from west to east: Mfinchberg massif (MM), Wildenfels, Fankenberg, and Gory Sowie (8)) and in Galicia of NW Spain (from north to south: Hesperian massif, Braganca and Morais). SG: Saxonian Granulitgebirge, 1: Flechtingen Hills, 2: Mountains, 3: Rheinisches Schiefergebirge, 4: Ardennes, 5: , 6: , 7: Thfiringer Wald (Thuringian Forest), 8: Gory Sowie (Eulengebirge), 9: Schwarzwald (), 10: Vosges, 11: Waldviertel of lower Austria, 12: Elbe line, 13: south Portuguese basin, 14: Alto Alentejo, 15: Sierra Morena. i.e. during the Ordovician and Silurian, Den Tex 1978; Kuijper 1979; Den Tex 1981, enormous quantities of granitic melts of calc- 1982; van der Meer Mohr et al. 1981). alkaline to peralkaline composition were pro- The southern part of the Armorican massif duced. These were emplaced in pre- and the Massif Central in France have produced Variscan crust, and were at a later date evidence of abundant pre-orogenic granites in deformed to produce orthogneisses. Such the range 500-400 Ma. Data from these cases orthogneisses produced from pre-orogenic have been summarized by, for example, granitoids are widespread in the Variscan Dornsiepen (1979) and Autran & Cogng basement. Some examples can be cited: (1980). Alkaline to peralkaline granitoids in the Alto In the Schwarzwald, where sedimentation Alentejo in Portugal have, according to Priem began, according to Hofmann & K6hler (1973), et al. (1970) an intrusion age of 470 Ma. These later than 900 Ma, an anatexis to which the alkali granites were later converted into pre-tectonic granites are attributed and a Variscan orthogneisses. In the Cor- diatexis which brought gneissification of gra- doba-Abrantes shear zone, this deformation nites fall in the range of 470-490 Ma. U-Pb age produced mylonites and ultramylonites determinations on zircons from the diatexites (Kosinowski 1982; Sattler-Kosinowski 1982). (Steiger, Bfir & Busch 1972) gave a lower inter- In the Hesperian massif, Kuijper (1979) has cept of 489 -+ 26 Ma (473 -+ 26) published by considered that ages of between 2 and 2.5 Ba Hofmann & K6hler (1973). According to Hof- on detrital zircons from orthogneisses and mann (1979) these data do not provide a suffi- paragneisses indicate the existence of distinctly cient basis for more far-reaching conclusion on old basement rocks. An age of 1.5 Ba is suggested the pre-Devonian orogenic history. On the for sedimentation of the protoliths of the para- other hand, they do serve to show that there is (Kuijper 1979). Rb/Sr whole rock good evidence that the so-called Rotgneisses of ages and likewise U-Pb ages on zircons the Saxothuringian zone (see below) were orig- (lower intercept) from orthogneisses suggest inally emplaced there as granites during the that widespread granitic intrusions took place Ordovician and Silurian. between 500 and 450 Ma (van Calsteren & Intensive early Palaeozoic granitic magmat- Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

Variscan events 5 ism is also known from the Alpine basement. has lensoid eclogites and pyriclasites which According to von Raumer (1981) all of the were retrograded during the granulite facies Variscan massifs in the western Alps have metamorphism. The ascent of the granulitic coarse grained granites, produced during a first body to higher crustal levels is associated with anatexis and having intrusion ages between 420 an amphibolite facies overprint and local ana- and 450 Ma (Arnold 1970) which were later texis (anatexis II): features which are especially deformed to become gneisses. U-Pb zircon and evident in the peripheral parts of the Granulit- monazite ages from granitic gneisses in the gebirge area. During the time of the retrograde Swiss Central Alps (K6ppel, G/inthert & metamorphism of the granulitic body its strati- Gr/inenfelder 1981) point to intrusion between graphic envelope therefore experienced a pro- 450 and 400 Ma. Further, pre-Variscan gra- grade metamorphism. Water release during this nites in the eastern Alps show a maximum in prograde metamorphism migrated into the age of intrusion in the range 460-420 Ma marginal parts of the granulitic body and (Sch6nlaub & Scharbert 1978; Schmidt 1976a; brought about anatexis II under amphibolite Heinisch & Schmidt 1976). facies conditions (Behr 1980). Behr's (1961) The timing of magmatic events in the M/inch- analyses of fabrics have shown that quartz fab- berg massif of the southern part of the Saxo- rics in the core granulites are dominantly those thuringian zone is reasonably well established of small lenticular quartzes having small circle by the geochronological investigations carried configurations with girdle axes normal to the out by S611ner, K6hler & Mfiller-Sohnius metamorphic layering. Within the amphibolite (1981) and Gebauer & Gr/inenfelder (1979). facies marginal regime the highly symmetrical According to these studies, sedimentation of small-circle configurations give way to the paragneisses of the 'Liegendserie' is no crossed-girdle fabrics whose opening angle older than 700-1000 Ma. The basaltic pro- decreases with increasing migmatization. toliths of the M/inchberg eclogites are of Cam- Blasto-mylonitic effects at the margins and in brian age. The peak of regional metamorphism the enveloping mica show oblique girdle that led to the formation of eclogites and fabrics. Lister & Dornsiepen (1982) interpret kyanite-staurolite gneisses occurred at about these fabrics to mean that the small-circle pat- 380 Ma. tern in the core granulites is typical of strain (b) The Ordovician-Silurian magmatism is histories intermediate between axially symmet- broadly contemporaneous with a granulite rical shortening and plane strain, whereas the facies metamorphism which was in progress at 90~ crossed girdle patterns at the rims of the depth in many regions where continuous strati- granulitic body are typical of plane strain. The graphic successions of that range of age were oblique girdle fabrics in the surrounding mica- accumulating at the surface. Two regions must schists and gneisses and also the overprinted be regarded as providing classic examples of crossed girdle fabrics with small opening angles granulite facies metamorphism with concurrent reflect a non-coaxial strain path. stratigraphic continuity during the Lower In the region occupied by the Hesperian mas- Palaeozoic. One is the Granulitgebirge in Sax- sif the sedimentary record is practically con- ony and the other is the Hesperian massif. tinuous from late Precambrian to mid- The Granulitgebirge (Figs 1 and 3) lies Devonian (Kuijper 1979; van der Meer Mohr within the Saxothuringian zone, whose weakly etal. 1981). Local gaps can be attributed to metamorphic sedimentary sequence proceeds block faulting. Bimodal volcanism occurs par- in an essentially continuous succession from ticularly in the Cambrian but is also seen in the late Precambrian to Carboniferous. In the Ordovician and Silurian. In the Ordovician, the Granulitgebirge area itself, the stratigraphic episode of heightened igneous activity and succession ranges from Upper Proterozoic to granulite facies metamorphism corresponds Devonian. A pre-granulitic migmatisation with a widening of the area receiving sediment (anatexis 1), whose relics survived in metatectic and a succeeding period of block faulting. structures, was followed at around 450 Ma by The Ordovician granulite facies meta- granulite facies metamorphism (J~iger & Watz- morphism in the Hesperian massif suggest nauer 1969; Watznauer 1974; J~iger 1977) at 10-11 kb and 850~ (Kuijper 1979). Just as approximately 8 kB and 700-800~ (Behr the Granulitgebirge in Saxony lacks preferred 1980; Weber & Behr 1983). These data suggest lattice orientation where granoblastic pyri- middle pressure granulites. clasites show a granulite facies tempering As in the case of the Waldviertel granulites of (Watznauer 1974), so the granoblastic fabric Lower Austria and the granulites of the Hes- which locally survives as a representative of the perian massif, the Granulitgebirge in Saxony granulite facies metamorphism (M1) likewise Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

6 K. Weber shows no PLO. The granulites exhibit a retro- the granulites. It appears difficult, however, to grade course of metamorphism which runs interpret this educt age as an age of sedimenta- through hornblende granulite facies with local tion, because the Gf6hler orthogneiss, which is migmatization to the amphibolite facies to associated with the granulites has given an greenschist facies. The ascent of the granulites educt age of 474 +_ 23 Ma by Arnold (in Schar- to higher crustal levels can be read in relation to bert 1977b). A likely suggestion would be that several phases of deformation. Deformation the Sr homogenization should be related to associated with hornblende granulite facies Ordovician rift metamorphism (see below) with conditions had taken place at a deep crustal the 446 Ma (431) date indicating the granulite level, with temperature between 600 and facies dewatering of the rocks. 750~ and pressure in the range 8-12 kb The leptyno-amphibolitic group of the Massif (Hubregste 1973; Maaskant 1970). Central contains acidic and mafic granulites. According to van Calsteren et al. (1979) the According to Burg (1977) and Burg & Matte migmatization process came to an end at (1978) (and see Burg et al., this volume) the 347 _+ 17 Ma. The main difference between granulite facies metamorphism is older than the granulite facies and hornblende granulite facies main deformation (F 1 and F2) in the Massif metamorphism lies in an increase of P fluid dur- Central. Some granulite bodies contain ghosts ing the M 2 metamorphism (Engels 1972; Kui- of isoclinal folds older than the static recrystal- jper 1979). Amphibolite facies metamorphism lization under granulite facies conditions which, was accompanied by a penetrative deformation according to Dufour, Piboule & Duthou (1983), (F4) responsible for the subvertical, NW-SE took place at 7-8 kb and 800-825~ Granites trending main foliation, preferred lattice orien- of crustal derivation with Rb/Sr whole rock ages tation of hornblende and blastomylonitic tex- between 450 and 550 Ma, which were intruded tures (Kuijper 1979). In the course of the pre-tectonically, were later transformed into further rise of the granulites the deformation orthogneisses under amphibolite facies condi- produced cold worked fabrics. Deformation tions. In the area of the Monts du Lyonnais this history and metamorphic succession are there- amphibolite facies metamorphism took place at fore closely comparable with those in the 5-6 kb and 700-725~ (Dufour et al. 1983). It Granulitgebirge of Saxony. has overprinted the older granulite facies rocks. The Moldanubian granulites in Lower Austria again reveal retrograde metamorphism during the course of tectonic deformation. Granulite facies metamorphism took place, according to Scharbert (1977a), at approxi- mately 11 kb and 760~ These are predomin- antly light-coloured, quartz-rich granulites, with insertions of subsidiary amounts of garnet pyroxenites which bear a granulite facies over- print. These latter rocks, according to Scharbert (1977a), may in their original condition have been upper mantle material which moved into the lower crust where they acquired their granulitic character. The granulite facies rocks of Lower Austria and Czechoslovakia occur in tectonic nappes which rest on rocks of lower metamorphic grades (Fuchs 1971, 1983; Thiele 1976a,b; Tollmann 1982). The retrograded marginal parts of the granu- lite bodies show ribbon quartzes like those in the Granulitgebirge. Their preferred lattice FIG. 2. Diagrammatic sketches of the early orientations indicate non-coaxial deformation. Palaeozoic continental rifting and associ- The primary granulitic fabrics, in contrast, with ated rift metamorphism (a) and the their discoidal quartzes, suggest coaxial defor- development of an injective granulite mation. The age of the granulite facies meta- during later orogenic crustal shortening morphism is 446 + 35 Ma (431 -+ 35) accord- (b). The present SE-vergence of the Sfichs- ische Granulitgebirge (which is not ing to Arnold & Scharbert (1973). An Sr shown in this sketch) results from the homogenization at 485 + 11 Ma (469 -+ 11) is younger back-folding. (Further explana- regarded as indicating the age of the educts of tions in the text.) Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

Variscan events 7

Continental rift metamorphism during an approximately 30-40 km thick continental Ordovician-Silurian time crust. However, the granulite metamorphism in It is in fact not a simple matter to explain the almost all of the Ordovician granulites follows coincidence of granulite facies metamorphism on a pre-granulite migmatization which corres- deep in the crust, intensive pre-tectonic igneous ponds to anatexis I. This is an understandable activity and development of a continuous sedi- relationship, for the granulite facies dewatering mentary sequence at the surface. It is made of deep crustal rocks is a gradual process. The more difficult if one assumes that the process of expulsion of water coincides with an intro- producing granulite facies metamorphism is duction of mantle CO 2. Lead isotope ratios in necessarily bound up with orogenic crustal K-feldspars from several metamorphic and shortening. The fact that many of the early granitic rocks in the southern Schwarzwald, Palaeozoic granitoids predate deformation and which suggest a very early formation of the also the widespread evidence of more or less basement, have been reinterpreted by Kober & uninterrupted Lower Palaeozoic stratigraphic Lippolt (1983) to be the result of crust-mantle successions in many parts of Variscan Europe interaction during anatexis I: mantle lead was would tend to discredit any such assumption. injected upwards out of a degassing mantle reg- The following proposalS on continental rift ion during genesis of 'Caledonian' magma. metamorphism during Ordovician-Silurian Increasing temperature, and expulsion of time are based mainly on the model developed water from the granulites led to the formation by Den Tex, van Calsteren and their co- of calcalkaline , granite magmas which workers for the Hesperian massif. The basic invaded the higher crust and which were later idea is that a continental rift develops on top of deformed, during crustal shortening, to produce an anomalous mantle and the heat transferred orthogneisses such as the Rotgneise in the into the lower crust produces granulite facies Saxothuringian zone. The fact that the Ordovic- metamorphism (Fig. 2a). ian-Silurian granitoids pre-date Variscan From work on recent passive continental deformation, and that the primary quartz fab- margins we learn that rifting processes in conti- rics in the granulites are highly symmetrical, nental crust promote ductile stretching in the leads to an interpretation in terms of meta- lower crust, which contrasts with the brittle morphism under stretching conditions, with a manner of reaction in the upper 10-15 km of dominantly coaxial deformation path. the crust where graben formation proceeds (De Charpal et al. 1978; Montadert et al. 1979; Le Pichon & Sibuet 1981; Le Pichon, Angelier & Sibuet 1982). This concept, introduced by Crustal shortening and the formation McKenzie (1978a,b) has been widely accepted of nappes with rocks in granulite facies (Christie & Slater 1980; Royden, Sclater & yon Herzen 1980) and might provide a means of In order to understand the origin of tectonic interpreting the fact that 'Caledonian' granulite nappes which involve granulitic facies rocks it is facies metamorphism was contemporaneous necessary to consider the rheological charac- with sedimentation and pre-orogenic igneous teristics of such rocks and to be aware of the activity. distinct rheological character of water-rich The continental crust is underlain by litho- amphibolite facies rocks. Quartz-rich crustal spheric mantle. Rifting of the continental crust rocks, in which quartz is the stress-supporting must have some association with a rifting in the mineral, are drastically affected by hydrolytic lithospheric mantle which promotes ascent of weakening and grain boundary migration when hot asthenospheric mantle material. Such a the recrystallization temperature of quartz is convective supply of heat may be regarded as a exceeded. For 500~ and a strain rate of 10 -14 course of heightened temperature at the s -I steady state creep stress of the order of crust-mantle transition. Partial melts of tholeii- 100 b can be assumed. The steady state creep tic composition may be transformed to eclogites stress falls to about 10b for 600~ and to in the higher parts of the upper mantle or at the approximately 1 b for 800~ (Mercier, Ander- crust-mantle boundary (van Calsteren & Den son & Carter 1977). Tex 1978) or else they invade the lower crust A different set of considerations applies in and become metagabbros or amphibolites. rocks free of water. In quartz and feldspar as The PT conditions for granulite facies meta- the stress-supporting minerals, steady state morphism (7-11 kb and 700-850~ suggest creep stresses at 500-600~ can be expected to that granulites could be produced at the base of be of the order of 1-2 kb (Heard 1976). High Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

8 K. Weber steady state creep must be assumed to apply in Upper Devonian rests on crystalline basement. basic granulites and in eclogites also. Early The folding and metamorphism must be older granulites, later involved in crustal shortening, than 375 Ma (360 Ma), because all of the late- will therefore, even at high temperatures, to post-tectonic granites are younger than that behave mechanically in a much 'stiffer' fashion (Brewer & Lippolt 1974). Deep-reaching ero- than would 'wet' rocks. sion of the basement during this time is sug- Consequently, granulites, in the course of gested also by the fact that the post-tectonic crustal shortening, will promote the formation granites, according to Emmermann (1976), of large-scale fold structures. Since the granu- were emplaced at high crustal level, within lites are overlain by water-rich, amphibolite rocks which must have been at greater depth facies, migmatitic rocks, they may penetrate the during the preceding anatexis. Indications of superjacent parts of the crust in the form of relatively early folding and metamorphism are 'injective' folds. The ascent of the granulites available in an Sr-homogenization earlier than would here be ascribed to amplification of 370 Ma (358 Ma) in the phyllites of the north- large-scale folds (Fig. 2b) rather than to a ern Vosges (Steige and Vill~ schists) which diapiric mechanism of the kind proposed by Clauer & Bonhomme (1970) interpreted as an Lehmann (1984), Watznauer (1974) and Behr age of metamorphism. (1961, 1980). The granulites take on a retro- Rb/Sr isochron ages on diatectic and anatec- grade effect where they are in relatively close tic granites in the Massif Central give 375 Ma in association with their envelopes, whereas these the Limousin (Duthou 1978), in the Vend6e surrounding rocks, as in the case of the 385 Ma, and 375Ma at Morbihan (Vidal Granulitgebirge in Saxony, show a prograde 1976), suggest a minimum age of the meta- metamorphism due to the ascending hot granu- morphism (Autran & Cogn6 1980). K/Ar ages lites, which can lead to the development of a on metamorphic hornblendes and Rb/Sr ages zone of contact metamorphism (Behr 1961). on muscovites, all in the range 360-350 Ma Anatexis II relates to this orogenic crustal (Autran & Cogn~ 1980), date the cooling of the shortening. During the amplification process basement to 500-400~ Intrusion of the post- the peripheral parts of the granulitic mass and tectonic granites begins, as in the Schwarzwald, the country rocks around take on a blasto- in the higher part of the Upper Devonian. mylonitic deformative effect (again, the Stratigraphic evidence, too, suggests a pre- Granulitgebirge provides examples). The obli- Upper Devonian age for the basement. North que girdle fabrics and the overprinted cross gir- of Lyons the most highly metamorphic rocks in dle fabrics reflect the non-coaxial strain path the Massif Central are overlain by epizonal taken during the course of amplification of the volcano-clastic Upper Devonian sediments, large-scale granulitic fold. which are in turn overlain by non-metamorphic The Granulitgebirge in Saxony shows a south- Vis~an (Burg & Matte 1978). eastwards tectonic vergence which is due to a This series of examples could be extended to later, SE directed tectonic overprint recogniz- include other Variscan basement complexes. In able in other parts of the Saxothuringian zone what follows, the single further case of the (Weber & Behr 1983; Franke, this volume). mid-German crystalline rise will be examined. That local redeformation apart, the Granulit- Here, too, it is possible to recognize a post- gebirge did not develop tectonically beyond the Silurian, pre-Upper Devonian folding and stage of the 'injective' folding. Elsewhere, e.g. in metamorphism. the Moldanubian of the Bohemian massif, in the The mid-German crystalline rise (Scholtz Hesperian massif, in the catazonal complexes of 1930; Brinkmann 1948) forms the northern Braganca-Moreis and the French Massif Cen- part of the Saxothuringian zone (Fig. 3). It can tral, a more effective crustal shortening has be followed from the Saar district in the west to produced extensive nappe complexes involving at least the Elbe line in the east. Metamorphic granulite facies rocks. These, in all of the cases and magmatic rocks of the mid-German crystal- mentioned, now rest on much more weakly line rise are exposed in the Odenwald, Spes- metamorphic rocks. sart, Kyffhfiuser and crystalline com- As in the basement complexes of the Saxo- plexes. Radiometric and stratigraphic evidence thuringian zone, it is possible to recognize point to the existence of an orogenic event dur- pre-Middle Devonian folding and meta- ing Lower Devonian time (Fig. 3). morphism in the basement of the central zone Stratigraphic evidence of pre-Middle Devo- of the Variscides. In the southern part of the nian deformation and metamorphism is Vosges (Maass & Stoppel 1982) and in the available in the Saar region, where non- Schwarzwald (Maass 1981) non-metamorphic metamorphic sediments of Middle Devonian Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

Variscan events 9

F1G. 3. Structural map of the Rhenohercynian and Saxothuringian zones.

age, encountered in the Saar 1 borehole, rest on bros to younger diorites and granodiorites. The chloritized albite granite of Lower Devonian age intrusion of this igneous sequence post-dates (394 -+ 24 Ma; Lenz & Mfiller 1976). the regional metamorphism (Maggetti 1975). The metasediments of the Spessart possibly Table 1 summarizes the available radiometric represent a time span from late Precambrian in data from the Odenwald and Spessart. The the south to at least Ordovician in the north metamorphism must be younger than the 'red- (Matthes 1954; Bederke 1957; Okrusch, Streit ' intrusions and older than the horn- & Weinelt 1967; Matthes & Okrusch 1977). In blende cooling ages. Therefore, the discordia the Odenwald (B611steiner Odenwald) and in intercept age of 380 Ma determined by Todt the Spessart pretectonic granites were emp- (1979) on zircons of the grain-size fraction laced 398-419 Ma ago (Kreuzer etal. 1973; <2 m from the B611steiner Odenwald grano- Lippolt, Barany & Raczek 1976). The granites dioritic gneiss and its metasedimentary gneiss were transformed during later metamorphism cover could correspond approximately to the into muscovite-biotite gneisses. These gneisses time of regional metamorphism. In that case, are regarded as equivalents of the widespread the regional metamorphism would be of the Saxothuringian Rotgneis (Scheumann 1932, same age as in the Mtinchberg Massif. 1939; Bederke 1957; Matthes & Okrusch After the time of deformation and regional 1965, 1977). metamorphism, or more probably starting at In the central part of the crystalline Spessart some point during this time, fast uplift of the this metamorphism took place at 5-6 kB and mid-German crystalline rise must have taken 600-650~ (Matthes & Okrusch 1977). In the place. In the area of the Saar 1 borehole a western part of the Odenwald (Bergstr~Ber Lower Devonian crystalline basement is Odenwald) the conditions of regional meta- covered by non-metamorphic Middle Devonian morphism were 4-6 kb and 650-670~ sediments (Zimmerle 1976). In the Odenwald (Okrusch et al. 1975). 10-12 km of rocks must have been eroded The exact age of this regional metamorphism prior to the emplacement of the igneous intru- is not known. But it can be roughly bracketed sions since the depth of intrusions was about by the following considerations. The 3.5-5 km and this magmatic event post-dates Bergstrfiger Odenwald is dominated by a sequ- the regional metamorphism (Maggetti 1974, ence of plutonic rocks ranging from older gab- 1975) that took place at a depth of about Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

10 K. Weber

TABLE 1. Radiometric data from the Odenwald and Spessart (Rb/Sr ages are recalculated using ;t = 1.42 • 10-Nyr -1)

1 'Red-gneiss' intrusion 398-419 Rb/Sr whole rocks (Kreuzer et al. 1973; Lippolt et al. 1976) Spessart and B611stein Odenwald 2 Granodioritic gneiss and paragneiss 380 U/Pb zircon, grain-size fraction < 2 #m cover of the B611stein Odenwald (Todt 1979) age of intrusion or metamorphism? 3 Metamorphism at 4-6 kB and Spessart 5-6 kB/600-650~ (Matthes & 600-650~ (regional metamorphism) Okrusch 1977), Odenwald 4-6 kB/650- 670~ (Okrusch et al. 1975) 4 Cooling of the metamorphic rocks to Approx. 370 K/Ar hornblende (Kreuzer & Harre about 400-500~ 1975) Odenwald 5 Intrusion of gabbros, diorites and Post-regional Maggetti (1974, 1975) Odenwald granodiorites. Depth of intrusion metamorphism but 3.5-5 km (1-1.5 kB) syntectonic 6 Cooling of the magmatites to about 340 (northern K/Ar Hornblende (Kreuzer & Harre 400-500~ part) 335 (southern 1975; Hellmann et al. 1982) part) Odenwald 7 Cooling ofthe Odenwald to about 330-325 K/Ar biotite (Kreuzer & Harre 1975; 300~ Hellmann eta'. 1982) Odenwald

15 km. The K/Ar cooling ages of the magmatic The Carboniferous event rocks cannot be far from the intrusion age, because the closing temperature of hornblende The major phase of folding and metamorphism is about 400-500~ (Kreuzer & Harre 1975), occurred early in Devonian time in the Variscan and the temperature of the country rocks has basement, but the weakly metamorphic external been estimated by Maggetti (1974, 1975) at zones of the Variscides were first folded during about 200~ The cooling of the igneous rocks the course of the Carboniferous. This latter as indicated by the K/Ar cooling ages of horn- episode, however, did bring significant further blende and biotite (Kreuzer & Harre 1975; shortening in the central zones of the Varis- Hellmann, Lippolt & Todt 1982) proceeded cides, documented in folding and imbrication of from 400-500~ at about 342-335 Ma, to weakly metamorphic Devonian and Carbon- 200-300~ at about 330-325 Ma. It should be iferous sequences which are discordant on crys- mentioned that the cooling ages of hornblende talline basement. The style of deformation was and biotite in the northern Odenwald seem to now different. The uplift due to orogenic short- be somewhat older than those of the southern ening and the consequent cooling had set the Odenwald. stage for inhomogeneous deformation of the During the time of uplift, deformation con- crust with, at macroscopic scale, dominantly tinued in the Odenwald and Spessart. This is brittle behaviour. At mesoscopic and micro- indicated by the flaser-gneiss-fabric of some of scopic scales this deformation shows itself as the biotite-diorites and granodiorites. This blastomylonites and at still lower temperatures fabric is interpreted by Nickel & Maggetti and/or higher strain rates by development of (1974) and Maggetti & Nickel (1976) as the ultramylonites. Such mylonite zones are nor- result of syntectonic intrusion. It is furthermore mally interpreted as shear zones. They arise as indicated by the widespread syntectonic retro- the reactions of a cooling and therefore more grade metamorphism and phyllonitization rigid crust to orogenic foreshortening. Such (Murawski 1958) under greenschist facies con- deep-reaching shear zones, associated with ditions (Matthes 1954; Matthes & Okrusch uprise of crystalline basement, are found at, for 1977; Gabert 1957). In the quartzite-micaschist example, the northern margin of the mid- series of the northern Spessart this diaphtho- German crystalline rise, and at the boundary resis is related to the south-facing late tectonic between the Saxothuringian and Moldanubian overthrust (Michelbach overthrust) of the zones along the Erbendorf line, and in the north-western amphibolite paragneiss (Bederke northern Schwarzwald and the northern part of 1957; Gabert 1957; Plessmann 1957; the Vosges. A further example is the Baden- Murawski 1958). weiler-Lenzkirch zone in the southern Schwarzwald. Deep-reaching crustal imbrica- tion is especially well displayed at the southern Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

Variscan events 11

however, a very broad statement. The wide- spread volcanism, already evident in the Upper Devonian and continuing into the Permian, would suggest the intervention of phases of crustal stretching. Ascent of the early Variscan basement can also be documented in the greywacke influxes into the adjacent troughs. The example of the Rheinisches Schiefergebirge shows that erosion of the mid-German crystalline rise and receipt of turbidites in a basin on its north began at least as early as low Upper Devonian. These Upper Devonian greywackes are encountered today as allochthonous units in the southern Harz, the southern part of the Hesse depression ('Hessische Senke') and the southern Rheinische Schiefergebirge (Figs 3 & 4). They were at one time parts of a nappe complex con- tinuously developed from the southern Schiefergebirge to the southern Harz (Engel et al. 1983; Weber 1978, 1981; Weber & Behr 1983). The original site of deposition of these greywackes is not now seen--it was over- ridden by the mid-German crystalline rise. From this one can estimate that the mid- German crystalline rise must have travelled at least 100 km toward its foreland and one notes FIG. 4. Diagrammatic sketch of the that the greywacke nappes, for their part, have development of the Giessen greywacke suffered deformation which has gone as far as nappe. producing recumbent folds. The boundary between the mid-German limit of the Ossa Morena zone, where tectonic crystalline rise and the Rhenohercynian zone is transport is toward the south Portuguese one of the most significant boundaries in the basin--the evidence is well seen in the Variscides. Its geodynamic character as a zone Arancena area in southern Spain. of deep-reaching crustal imbrication (a subflu- In eastern Bavaria a NW-SE belt of blasto- ence zone) was recognized by Weber (1978, mylonitic gneisses (Perlgneise) is exposed. The 1981). The transition from the crystalline rocks mylonitic overprint of pre-existing para- and of the mid-German crystalline rise into the very orthogneisses which began under PT condi- low grade metamorphic state of the rocks in the tions of 550~ and 2.8 kB (Blfimel, 1983, per- Rhenohercynian zone is represented in a zone sonal communication), ended in the Perlgneise of phyUitic rocks which can be traced from with the paragenesis quartz + white mica + Dtippenweiler in the northern part of the Saar chlorite + albite in low temperature ultra- north-eastward to the Wippra zone in the mylonites (Bltimel 1977). Rb/Sr thin slab southern Harz Mountains (Figs 3 & 5). Deep measurements on these blastomylonites carried boreholes show that this northern phyllite zone out by K6hler, Christinas & M/iller-Sohnius continues farther to the NE. In the Wippra zone (1983) suggest an intrusion age of 474 _+ 18 Ma Ordovician to Upper Devonian sedimentary for the protoliths of the orthogneisses and a rocks are known. In the southern part of the 346 _+ 29 Ma age for the blastomylonitic over- Hessische Senke and in the southern print with the production of the Perlgneise. bimodal metavolcanites are widespread. Further uprise along this shear zone in the The more acid variants are in some cases course of the Carboniferous can be estimated to ignimbrites. The presence of Mg-riebeckite have continued until at least 310 Ma on the indicates alkali volcanism. The age of the educts basis of biotite cooling ages. of these metavolcanites is not known. It may, The progressive development of these large however, be suggested that these bimodal vol- shear zones provides a basis for suggesting canites possibly belong among the widely more or less continuous crustal shortening dur- developed Ordovician rift-related volcanics. ing the Devonian and Carboniferous. This is, It is the case that in the remainder of the Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

12 K. Weber

FIG. 5. Diagrammatic cross-section through the Rhenohercynian crust in the area of the Rheinisches Schiefergebirge. The lower cross-section shows the position of horizons of high magentotelluric conductivity (conductivity data after Giese et al. 1983).

Rhenohercynian zone (with the exception of have come to the conclusion that the meta- parts of the Stavelot-Venn massif--Kramm morphic conditions were 10-12 kb at 1982; Schreyer & Abraham 1978) meta- 400-450~ These findings (they correspond to morphic temperatures did not exceed 350~ a geothermal gradient of approximately and in most places were in the range 10~ km -~) would resemble what is proposed 200-300~ But 400-450~ was reached in for B-subduction zones. But since the ophiolites the northern phyllite zone (Meisl 1970). and typical of B-subduction are lack- Taking account of the structural evolution of ing, as is also any indication of a paired meta- the northern phyllite zone and of the likely orig- morphic belt, it is reasonable to await further inal configuration of sedimentary thicknesses in information before proceeding to any relatively the southern part of the Rheinische ambitious interpretation of these estimates of Schiefergebirge, Weber (1978, 1981) has pro- pressure. posed that the rocks of the northern phyllite zone Typical of the whole orogenic character of were at one time at considerably greater depth. the Rhenohercynian zone is a NW directed This suggestion has been confirmed by work polarity (Figs 5 & 6). This is evident not only done by Massonne & Schreyer (1983). Using a in the strong NW vergence of the folding and new phengite barometer Massonne & Schreyer thrust structures (Weber 1978, 1981) it is also Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

Variscan events 13

oo

~,..~

.=0

0 Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

14 K. Weber developed in the flysch sediments which were et al. 1983; Franke, this volume) are no less accommodated in a succession of basins which impressive in the Variscides than those nappes migrated from south to north in late Devonian directed southward (Burg & Matte 1978; Burg and Lower Carboniferous time, but also in the et al. 1984; Tollmann 1982; Engel, Feist & migration of folding and metamorphism from Franke 1978, 1981). south to north. K/Ar datings on white micas Compared to the Alps or the Himalayas, for from rocks which were originally vitric tufts in example, the European Variscides show a the Rheinische Schiefergebirge, and which were clearly expressed bilateral symmetry. This bi- syntectonically recrystallized at temperatures lateral symmetry invites the suggestion that below 350~ indicate that the prograde meta- similar geodynamic processes were producing morphism is of an age around 330 Ma in the crustal shortening on both flanks of the orogen. southern part of the Rheinische Schiefergebirge It is, however, evident that a proposal of classi- and 300 Ma in the northern part (Ahrendt, cal B-subduction applies altogether less readily Hunziker & Weber 1978; Ahrendt et al. on the north side than on the south. 1983). (Fig. 3). The beginning of prograde The geodynamic model presented in what fol- metamo/'phism in the southern part of the lows is based on the evidence of the north flank Rheinische Schiefergebirge is therefore con- of the European Variscides, especially the temporaneous with the cooling of the igneous Rhenohercynian zone and is based on the view rocks in the Odenwald to temperatures around that this was an essentially ensialic tectonic pro- 300~ (Table 1). cess. It takes account of the well determined polarity of structural and metamorphic developments. Geodynamic interpretation Geometric and kinematic analyses of the The preceding discussion makes it clear that exposed suprastructures show (Weber 1978, the geodynamic development of the European 1981) that the frequent NW verging overthrusts Variscides cannot be treated as an exclusively in the Rheinische Schiefergebirge are listric Carboniferous set of events. The whole scheme overthrusts. These overthrusts arise at different of evolution began with continental rifting in depths out of horizontal thrust planes. Major the early Palaeozoic which had an association structures can thus be related to the horizontal with metamorphism and anatexis in the lower layering in the infrastructure, which has been crust. One cannot exclude the possibility that interpreted by Weber (1978, 1981) as resulting the process of continental rifting may locally from a horizontal flow regime. The horizontal have led to limited developments of oceanic thrust planes which pass upward into listric crust. overthrusts tend to produce an uncoupling of Orogenic crustal shortening began with the the infrastructure from the folded and imbricate Acadian event in the early Devonian. suprastructure (Fig. 5). B-subduction during this episode seems likely Based on homologous temperatures for wet only in the neighbourhood of the Ligerian granite solidus, Vetter & Meissner (1979) have suture. How and indeed whether such a suture divided the continental crust into two layers: an continued towards what later became the upper layer with T/Tm< 0.6-0.65 showing Alpine region is not known. This being the case, brittle response to stresses and where steady considerable uncertainty surrounds any sugges- state creep does not seem to be possible in tion that the Acadian event is the result of a times < 10~Ma, and a lower layer with widely developed B-subduction operating on TIT m > 0.6-0.65 showing transient creep in its the southern side of Variscides. Interpretation upper part and steady state creep in its lower of the Rhenohercynian depositional basin as a part. The transition from brittle to creep- Devonian and Lower Carboniferous back-arc dominated behaviour lies, in areas of higher basin (Ziegler 1982; Floyd 1982) is tempting. heat flow, at a depth of 10-20 km, and in older But one would encounter serious difficulty in continental shields at a depth of 40-60 kin. proceeding from there to geodynamic explana- In view of the high heat flow in the Variscan tion of the Saxothuringian depositional site, orogen this proposed division is in good agree- with its quasi-continuous late Precambrian to ment with the structural interpretations and the Lower Carboniferous sequence. Further, there seismic and rheological data mentioned above. is the problem that the northward directed Behr (1978), Weber (1978, 1981) and Behr nappe and thrust tectonics (Weber 1978, 1981; & Weber (1980) have applied the term 'subflu- Weber & Behr 1983; Enge[ et al. 1983; Behr ence' (in the sense in which it was used by Amp- 1978; Behr, Engel & Franke 1980, 1982; ferer 1906; see also Schmidt 1976b) to the Meissner, Bartelsen & Murawski 1981; Giese progress of underflowing mass transfer of mat- Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

Variscan events 15 erial suggested to have been active in the lower those on the north a northward vergence could crustduring the evolution of an orogen. Weber then be seen to be consistent with the assump- (1981) has proposed that the cause of the tion of convergent movement in the litho- subfluence is a relative movement of continental spheric mantle. For mechanical and geometrical crust with respect to lithospheric mantle. reasons, such a convergence is possible only if This assumption is based on the fact that in subduction of lithospheric mantle is involved. the neighbourhood of the Moho there is a criti- The causes of such a subduction are unknown. cal lithological change. Predominantly peridoti- Since the lithospheric mantle is, by reason of its tic composition is suggested for the lithospheric higher density as compared to the astheno- mantle (Ringwood 1975). Olivine and pyrox- sphere below, in an unstable equilibrium, it ene can be regarded as the stress supporting could be suggested that a potential instability minerals in such rocks. Available theological established during an earlier rifting stage might data on olivine as well as on rocks of peridotitic during later convergent plate movement lead to composition indicate very high steady state downward detachment of lithospheric mantle creep stresses, extending up to several ('A-subduction', Weber 1981). Descent of kilobars, in the Moho range of temperature of large volumes of lithospheric mantle into the 500-600~ (Meissner & Strehlau 1982; Heard asthenosphere would require, in exchange, 1976; Stocker & Ashby 1973; Ashby & Verrall upward transfer of large volumes of astheno- 1977; Mercier 1980; Goetze 1978; Post 1977; spheric material (Fig. 7). Kirby 1980; Carter 1976; Nicolas & Poirier Since the Iapetus Ocean, to the north of the 1976). It is then to be anticipated that there is a Variscides, had closed, since the opening of the well-developed, geodynamically effective Atlantic had not yet begun, and since the pres- rheological boundary zone between the 'dry' ence of the Gondwana continent in the south (a peridotitic upper mantle and the 'wet' possible ocean was closed, at least by Upper quartz-feldspar-rich continental crust. In some Devonian time) excluded availability there of areas of former continental rift zones where an any mid-ocean ridge at which mass- intervening layer of higher stiffness can be equilibration could be effected, any subducting developed at the base of the continental crust segment of lithospheric mantle has to retreat in some perturbations can occur at the crust-man- order to allow mass-equilibration. A possible tle boundary which give rise to the formation of means of return is available in Andrews & granulite nappe complexes as described above. Sleep's (1974) model of forced convection, A relative movement of the lithospheric man- below a crustal segment, induced by the sub- tle in a sense opposite to the one indicated at ducting slab of lithospheric mantle (Fig. 7). This high levels by the tectonic structures would Andrews-Sleep cell would shift, during the provide a plausible explanation of the tectonic retreat of the descending lithospheric mantle, movement picture. The fact that the tectonic outwards toward the margin of the developing structures on the southern side of the European orogen. In the case of the bilateral Variscides Variscides have a southward vergence and we have to assume two such Andrews-Sleep

,j NW o~ ~ co~~ SE

F 1 - folding F 2 - backfotding subsequent rifting

0 1O0km

FIG. 7. Diagrammatic sketch of the geodynamic development of the northern branch of the mid-European Variscides. (Further explanation in the text.) Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

16 K. Weber cells operating on either side of the orogen and Franke 1984) and the southernmost part of the in opposite senses. (In the sketch in Fig. 7, only Rhenohercynian zone. Also the SE-facing the northern A-subduction is shown.) structures in the non-metamorphic sediments of An assumption basic to this model is that the the Saarbriicken anticline and the post- orogenic shortening took place in a wide region metamorphic SE-directed, suprastructural of crustal convergence. Considerations arising overthrusts of Dfippenweiler and the northern from the history of development of the orogen Spessart (Michelbach overthrust) can be attri- require that any explanation should also take buted to this secondary stress field. into account the fact that earlier processes of The presence of an antivergent homoaxial continental rifting have introduced mechanical F 2 folding is a general phenomenon in the more inhomogeneities which led to local and regional internal parts of the Variscan orogen which is perturbations in the subsequent orogenic evolu- not only seen in Europe. The mechanism pro- tion. posed above could provide a possibility of under- A further assumption is that the regional standing this phenomenon. stresses were first resolved in what later became The Ordovician granulites seem to have been the central region of the orogen. The siting of formed under a geothermal gradient of about that particular region prescribed the zone in 20-30~ -1 (Zwart & Dornsiepen 1978; which lithospheric mantle became detached Behr et al. 1980). During Acadian time (Lower from the crust and so initiated the subduction to Middle Devonian) the gradient increased to processes. In the central European Variscides 30-40~ km -1, and in the Carboniferous and the zone thus prescribed is the Moldanubian Permian perhaps locally reached even zone. The central zone is also the region in 80-100~ km -1 (Zwart 1976; Zwart & Dorn- which uprise of hot asthenospheric material siepen 1978; Behr etal. 1980; Weber 1978; began and initiated thermal weakening of the Buntebarth 1982; Buntebarth, Koppe & crust. Teichmfiller 1982). The steepest increase is to Away from the central zone, in places where be observed during the Upper Carboniferous continental crust and lithospheric mantle and Lower Permian. Thus, in the Variscan remained in contact with one another, there crust, there was a pronounced increase of the must nevertheless have been relative movement geothermal gradient from the early Palaeozoic of these two. The frictional stresses induced (largely Ordovician) rifting stage to the final could have brought about a deformation of the stage of the Variscan orogeny. lower crust. The frictional stresses need not, A cause of the steepening of geothermal gra- however, have been so great that the entire dients in the course of the Variscan progression crust was overtaken by this deformation. One of events may lie in the buffering of endo- could in this way explain the Lower Devonian thermic prograde metamorphic reactions. age (392 + 10Ma, Schoell, Leuz & Harre The rate of metamorphism is determined by 1973) of deformation and metamorphism of the the net input of heat into the metamorphic pile, Ecker Gneiss (Fig. 2), which later, by means the enthalpy of metamorphic reactions and the that are not yet understood, became involved in heat capacity of the rock-forming minerals. The the late Carboniferous folding and meta- net input of heat can be understood as the sum morphism that are imposed on the sedimentary of heat which enters the system and which is rocks of the Harz Mountains. generated inside the metamorphic pile minus As the two subduction fronts withdraw from the heat which leaves by advection and conduc- one another a new episode begins in the tion. development of the two flanks of the orogen. The suggestions would be that this buffer Interaction between the descending slabs of effect was especially effective in, for example, lithospheric mantle and the opposed sense of the Saxothuringian zone with its thick pile of rotation in the Andrews-Sleep cells produces a sediments reaching back in time to late Pre- compressive stress field between the slab and its cambrian. Steeper geothermal gradients associ- associated Andrews-Sleep cell, the effect of ated with the rift metamorphism would in such which spread into the crust (Fig. 7). circumstances be expected to exist in the deeper This second deformation must, however, crust only. The whole thickness of crust, on an because of the sense of rotation of the overall average, would suggest only a low geo- Andrews-Sleep cell, be antivergent to the first thermal gradient. Acadian metamorphism deformation. This is a possible explanation of shows widespread occurrences of kyanite- the south-eastward vergence of the isoclinal, bearing middle pressure metamorphic assemb- synmetamorphic F 2 folds encountered in the lages. They reflect the temperature increase at Saxothuringian zone (Weber & Behr 1983; mid-levels of the crust. Later, when nappe Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

Variscan events 17 development became more intense they pro- subduction implies strong convergent move- ceeded into higher levels of the crust. There, ments. Such a movement pattern does not allow such rocks took on a high temperature/low the kind of crustal spreading encountered in pressure overprint. marginal seas. The widespread surface near very weakly Only the late orogenic magmatic activity metamorphosed rocks, which were never can be understood as an expression of 'subse- deeply buried and which first encountered quent' rifting processes induced into the crust deformation and metamorphism at a later stage by the ascending limbs of Andrews-Sleep cells of orogeny, also became exposed to Abukuma- at a time when during the late stages of type metamorphic conditions. Here it is neces- A-subduction and retreat of the subducting sary to take the view that in addition to possibly slabs the convergent movements became less enhanced radioactive heat production and and less effective. Finally during the Lower synorogenic granite intrusion the underlying Permian the subcrustal subduction and crustal metamorphic crust exerted a 'socle effect', convergence came to an end. which accentuated the upward transfer of man- tle heat which is brought at the base of the crust by Andrews-Sleep convection. This 'late orogenic' heat is also regarded as being respons- Conclusions ible for the Abukuma-type overprint of the older higher grade metamorphic rocks which The so-called 'Caledonian' thermal event rep- were brought into a near-surface position by resents one of the main problems of the nappe and thrust tectonics. Variscan crustal development of central, west- The regions subjected to pre-Middle Devo- ern and southern Europe. nian folding and prograde regional meta- The geological data discussed in this paper morphism had already been deeply eroded by allow an interpretation of this event as resulting the Middle Devonian. Therefore, younger sedi- from continental rift processes. These took ments suffered a weak prograde meta- place on top of an anomalous mantle which morphism, whereas the older metamorphic induced igneous and bimodal volcanic activity rocks were uplifted and subjected to retrograde during almost continuous sedimentation, and overprinting under Abukuma-type meta- high grade metamorphism at the base of the morphic conditions. The development is, how- crust. ever, not uniform, but mirrors heterogeneities These rifting processes could have taken of the crustal structure and the heat flow. There place inside a stationary crustal field between are areas with strong late to post-tectonic igne- Laurasia and Gondwana as supposed by Zwart ous activity, e.g. the Odenwald, which contrasts & Dornsiepen (1978) and Weber and Behr with the neighbouring Saar-Province where the (1983) or, in the sense of Ziegler (1982), at the post-Lower Devonian sediments overlying the northern border of Gondwana. From here, crystalline basement have remained non- Cadomian (Panafrican) consolidated crustal metamorphic even at a depth of 5000 m. fragments in the form of allochthonous terranes In the case of the subsequent Permian mag- were transported to the north where they were matism the generation of the rhyolitic magmas incorporated by collision into the Variscan belt. requires a high temperature at the crust-mantle More palaeomagnetic data are necessary to boundary. The model in Fig. 7 attributes the prove these models. Nevertheless, the existence high temperature to the ascending limb of an of Lower Palaeozoic rift processes seems to be Andrews-Sleep cell that migrates towards the well established, no matter which of the two orogenic foreland. The fact that rhyolitic vol- models one prefers. canism had already occurred in the Black For- The main phase of crustal shortening and est in the Lower Carboniferous allows the accompanying regional metamorphism in the interpretation that the ascending limb of the central parts of the Variscan orogen took place convection cell has spread out, from the Lower during the Lower Devonian. However, crustal Carboniferous to the Permian, from the Black shortening and thrust and nappe tectonics were Forest to the Saar-Nahe trough. active in the central zones up to the end of the It could be assumed that the convective Carboniferous, and the external zones of the uprise of asthenospheric material below the Variscides were first deformed at this time. crust produces spreading movements in the The former rift zones were the sites from overlying crust similar to back-arc situations. which deep reaching crustal imbrications (sub- However, the bipolar structure of the Variscan fluence zones in the sense of Weber 1978,1981) orogen and the assumed bilateral subcrustal and nappe tectonics developed and which trace Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

18 K. Weber out the main structural boundaries, e.g. the mass-equilibration could be effected. There- boundaries between the Rhenohercynian and fore, any subducting segment of lithospheric Saxothuringian zones and the Saxothuringian mantle has to retreat in order to allow mass- and Moldanubian zones. equilibration. This mass-equilibration which A-subduction, i.e. subduction of lithospheric takes place in front of the subducting slabs leads mantle underneath continental crust to an uprise of hot asthenospheric material at (Ampferer-subduction in the sense of Weber the base of the overlying continental crust. High 1981 or delamination in the sense of Bird heat flow (low pressure/high temperature 1978), is regarded as the driving mechanism of metamorphism) and the formation of a second- crustal shortening. A-subduction follows ary stress field (back-folding) might be attri- B-subduction when the collisional stage is buted to forced convection possibly in the form reached. That applies especially to the Ligerian of an Andrews-Sleep cell. suture. Whether small oceanic basins have been Finally, the formation of vast masses of late developed in other parts of the Variscides, par- to post-orogenic granites, of bimodal volcanics ticularly in the northern part along the bound- and ignimbrites might be interpreted as the aries between the Rhenohercynian and Saxo- result of a restricted subsequent rifting event, thuringian zones and Saxothuringian and which evolved on top of the convecting Moldanubian zones cannot yet definitely be asthenosphere when A-subduction and crustal answered. However, the main effects which can convergence gradually ceased. be observed there are the result of A-sub- duction. ACKNOWLEDGMENTS: I owe my thanks to H. Ahrendt, A peculiarity in the development of the H. J. Behr, W. Engel and W. Franke for many helpful European Variscides in comparison to Cordille- discussions and to S. C. Matthews for providing the ran and island arc type orogens is the missing translation. availability of any mid-ocean ridges at which

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KLAUS WEBER, Geologisch-Pal/iontologisches Institut und Museum Goldschmidt- str, 3, D-3400, G6ttingen, Germany.