Bulletin de la Société belge de Géologie T. 93 fasc. 1-2 pp. 33-44 Bruxelles 1984 Bulletin van de Belgische Vereniging voor Geologie T. 93 deel 1-2 blz. 33-44 Brussel 1984

TECTONIC ANTECEDENCE OF VARISCAN GEOLOGY IN BELGIUM

by S. C. MATTHEWS t (*)

In Memoriam On May Sth, 1983, Dr. S. C. MATTHEWS died suddenly. A few days before he sent us the manuscript of the talk he was expected to give at the meeting that the Société belge de Géologie organized to honour their Fast-President and General-Secretaries Ir. A. DELMER et Dr. R. LEGRAND. Unfortunately, because of ill­ ness, he was unable to attend the whole meeting and did not pre­ sent his contribution. S. C. MATTHEWS was born at Muirkirk {Scotland) on 23rd May, 1936. He first attended Glasgow University were he got his B. Sc. Then he was awarded a Shell Studentship and proceeded to the University of Bristol, where he first became Assistant Lecturer in 1960 and was promoted Lecturer in 1963. He got his Ph. D. in 1965. He left the University of Bristol in 1982 for the Department of Paleobiology of the University of Uppsala {Sweden) where he died suddenly and unexpectedly following a heart attack. His scientific carrer was quite eclect:ic andproductive. Indeed, he managed with equal mastery work in three different fields : - Devonian and Carboniferous conodont faunas, mainly in Southwest England; - Studies of early Cambrian skeletal fossils.. This involving exploration of some of the earliest examples of mineralized organic tissues; - Geology of the southwestern part of the British Isles in its Eurooean context. Being a perfect polyglot {Crosbie MATTHEWS conducted his business equally well in English, French and German and had a working knowledge of Swedish and Russian) and a great traveller, he was the right person to make a synthesis of the abundant new information, making it possible to re-assess the character of the Variscan fold­ belt in Central and Western Europe. His main publishing activity for the last few years was in this field and only three years ago he gave for the members of the Société belge de Géologie a conference on "Les principaux traits structuraux de l'Europe. Commentaires de la structure paléozoïque de la Belgique" • The Société belge de Géologie is proud to publish one of the last manuscripts of this great erudite. Eric GROESSENS.

ABSTRACT. - There is a reason to ask whethergeological purposes are well served if we continue to apply terms such as "Caledonian" and "Variscan" to Palaeozoic tectonism in Belgium. Questions can also be raised concerning the state of the late Precambrian in Belgium, particularly whether any resemblance to Precambrian geology in the Armorican Massif is likely to exist. It appears improbable that "Caledonian" metamorphism is developed to any worthwhile degree in the region. "Variscan" tectonism is interpreted as including two effects in the Ardennes and regions to the east. Each is evident first as a heating which induced modifications of basin form then later as more penetrative deformation. One is attribu­ ted to heating in the Ardennes - Ebbe Anticline neighbourhood ("Ardennes effect"). The other is heating advancing northwestward from a source in Central Europe ("Saxothuringian effect"). The two interfere with one another. The Ardennes heating, and a consequent doming, is compared with heating in southwest Ireland. The cause in each of these two cases may be suggested to be mantle delamination precipitated

(*) Enheten for Paleobiologi, Universitetet i Uppsala, Box 564 S-751 22 Uppsala (Sweden) 33 by the closure of the Iapetus Ocean. Pre-existing, essentially late Precambrian features of major structure may have played a part in determining the sites of delamination. The argument suggests that "Variscan" intraplate tectonism may be explained as a consequence of "Caledonian" processes.

INTRODUCTION, The matters involved may be intro­ "Caledonides" are to be thought to be as­ duced by posing three questions sociated with a Caledonian belt which runs from the neighbourhood of Stavanger - What is the justification for applying in Norway southeastward through Europe. the name "Caledonian•;• to tectonic ef­ On this interpretation one regards the fects in the Lower Palaeozoic rocks in Belgian "Caledonides" a as special deve­ Belgium ? lopment on the south flank of an isolated "Zwischenmassif" which could include the - What are our reasons for applying the older geology of Central England, and one name "Variscan" to later Palaeozoic tec­ suspects that where the Zwischenmassif tonism in Belgium ? ends in the east (as the interpretation requires that it do), "Caledonian" tecto­ - What is the nature of pre-Devillien (ap­ nism of the character seen in the Ardennes proximately pre-Cambrian) geology in must also end. KREBS (1976, 1978, 1982) Belgium and adjacent regions ? has treated pre-Devonian geology in the Rheinisches Schiefergebirge, especially The first two questions require ac­ the evidence revealed in the Soest-Erwitte counts of a wide range of information pa­ borehole, in a manner largely in keeping tiently accumulated over many years. We with von GAERTNER'S (1960) second possi­ agree that those who introduced these bility. DUN'iING and WATSON (1977) made a terms to Belgian geology were putting the closer examination of the scattered infor­ best interpretation they could contrive mation on the character of the older geo­ on the information available in their logy of central England (the Zwischenmas­ time. We should ask whether we, by conti­ sif) and offered an interesting distinc­ nuing usage of these terms, are making tion between a London Platform (treated the best employment possible of the fund as a continuation of the Brabant Massif) of information available to us. and an older Midland Craton to the north­ west of it. But they found it difficult The third question is different in to suggest what might have been the con­ kind. We are not richly well informed on tinuation of these two elements south­ pre-Devillien geology in Belgium. We may ward into regions where the stamp of nevertheless approach a solution to the later, Variscan tectonism is assumed to do­ question, which is of considerable inte­ minate the character of the pre-Mesozoic rest, by less direct means. geology. DUNNING (1980) later touchedon AUTRAN and GUILLOT's (1977) suggestion It should be said that this explo­ of a Caledonide belt extending from the ration of ideas is offered in amity by an Massif Central to Newfoundland and the observer who has for a number of years Appalachians and ventured the thought and for a variety of reasons had cause to that any eastward continuation of such be involved with Belgium sources of geo­ a belt toward the Bohemian Massif (i. e. logical information, but who has always a belt such as von GAERTNER's 1960 possi­ found it difficult to digest published bility 1 would prescribe) "must constitute interpretations of tectonic relationships some kind of 'Pan-African~-type inter­ between Belgium and other regions. nides to all the external Caledonian fold-zones to the north, that is, Brabant, Ardennes and Rügen-Pomerania". THE CALEDONIAN QUESTION, DUNNING's (1980) inclination was in fa­ veur of an interpretation that would be FOURMARIER (e. g. FOURMARIER, 1931, consistent with von GAERTNER's (1960) 1954), whose opinions on tectonism in possibility 2. He, like FOURMARIER be­ Belgium have carried great authority, fore him, found it difficult to identify sought a relationship between Caledonian concrete connexions between the geology tectonism in Belgium and Caledonian tecto­ of the Brabant Massif and the geology of nism in Wales. A larger view of the pro­ the Caledonides in the western and blems involved was presented by northern parts of the British Isles. von GAERTNER (1960), who offered two al­ ternative interpretations of the setting It is evident that the problems of the several massifs which produce of choice presented to the 1960 Interna­ Lower Palaeozoic outcrop in Belgium and tional Geological Congress by von GAERTNER northern France : either (von GAERTNER's were still extant when DUNNING wrote on first possibility, which he regarded as the occasion of the 1980 International providing the better accomodation of the Geological Congress, and this in spite Belgian evidence) the Brabant and of the fact that the intervening twenty Ardennes massifs are included in a Cale­ years had brought the influence of plate donian belt which swings from Wales tectonic thinking to discussion of ques­ through Belgium and in its course farther tions of these kinds. Plate tectonics eastward subsumes Caledonian geology (as has, if anything, done more to expose von GAERTNER took it to be) in the West the isolated state of "Caledonian" tecto­ Sudeten and the Lysa Gora, or nism in Belgium and northern France than (von GAERTNER's second possibility, ,·hich to explain it. If one accepts an inter­ is more in keeping with the preferences pretation of the Caledonides of the kind of a number of more recent commentators advanced first by DEWEY (1969) and re­ on these matters - see, for example, cently redeveloped (DEWEY, 1982), what ZIEGLER 1978, 1982) the Central European bearing does it have on "Caledonian"

34 tectonism in the Ardennes and the Brabant in the matrix both in morphology and in Massif ? Which sources of energy were tap­ details of their internal fabric and ped in order to drive tectonic processes likewise His observation that cleavage in there in pre-Devonian time ? pre-Devonian slates within Gedinnian con­ glomerate does not exert any control on Caledonian relationships within the shape of the clasts but is disposed Belgium and northern France (GEUKENS 1950; in the same attitude in adjacent clasts LEGRAND 1968; MICHOT 1976, 1978, 1980; whatever their shape. The onus is now on BEUGNIES, DUMONT, GEUKENS, MORTELMANS and anyone who insists on a Caledonian meta­ VANGUESTAINE, 1976; WALTHER 1978, 1980) in­ morphism in the Ardennes to demonstrate volve the problem of understanding why an a metamorphic effect peculiar to the pre­ Ardennes basin failed (so that that area Devonian rocks. has no representation of a sequence be­ tween mid Ordovician and low Devonian) in The polyphase structures in the what is regarded as an early Caledonian Ardennes are therefore suspected to post­ effect and thus presents a contrast with "date the development of the basal the Brabant Massif, where the Lower Palaeo­ Gedinnian unconformity. More obscure is zoic sequence continues later and the tec­ the meaning one should then attach to any tonic effect is regarded as late Caledo­ suggestion that earlier and later phases nian. MICHOT (1978) makes the point that of Variscan deformation should be distin­ farther to the west, in the Liévin area guished. for example, no such break is developed and he infers that there was a certain A set of rocks which, in spite of "bombement" (doming), fading westward, in the apparently small total area occupied the structural relief. VANGUESTAINE at outcrop, is of some significance in (1973, 1974) has brought new palaeontolo­ any attempt to unravel the geological his­ gical information to discussion of these tory of the Ardennes, is the Venn intru­ problems and it is now established that sions (SCHERP 1959; KRAMM 1982). These mid Ordovician ages are represented in porphyritic tonalites and associated do­ the higher part of the Salmien succession. lerites outcrop among the pre-Devonian It therefore becomes clear that the Lower rocks in the core of the regional struc­ Palaeozoic succession in the Ardennes in­ ture. They are themselves affected by cludes an earlier sequence (comprising a metamorphism, which appears to have its major part of the known thickness) which best development in the south flank of is dominantly of shallow water character, the regional structure. KRAMM (1982), and a later part, ranging into the Ordovi­ who has contributed an impressive amount cian, in which there are minor amounts of of new information and intelligent in­ what may be flyschoid sediments. Any sug­ sight, disposes of earlier suggestions gestion that a Caledonian geosyncline is (e.g. STAINIER 1930; KREBS and WACJŒND'.)RF represented does little to penetrate the 1974) thatmetamorphism in the Ardennes character of the geology. Indeed, given is contact metamorphism in the neigh­ the balance of sedimentary character in bourhood of such intrusions. He comments the succession and the lack of any signi­ that the layout of grade-boundaries is ficant volume of basic igneous rocks, more in keeping with a pattern of regio­ such a suggestion might be entirely inap­ nal metamorphism, a view which is consis­ propriate. A possibly more fertile tent with evidence of geothermal gra­ thought is that the Devillien, Revinien, dients commonly of the order of 60°/km Salmien accumulation in the Ardennes may in the Hohe Venn, the Givonne Massif show something of the character of an in­ (BEUGNIES 1976) and in the Bastogne area version structure. It is, however, readi­ (de BETHUNE 1980). If they are affected ly recognized that these are a geological­ by regional metamorphism and if the meta­ ly unobliging set of rocks, and the pre­ morphism post-dates the Gedinnian conglo­ sent lack of information on the exact merates, the Venn intrusions cannot be form of the Lower Palaeozoic basin is regarded as Caledonian. The rocks avail­ understandable. able at exposure are deeply affected by weathering (SCHERP 1959) and relatively li ttle is known about their chemistry - THE VARISCAN QUESTION. for example, there has been no special study of the carbonate component of the The literature records a history tonalites reported by VAN WAMBEKE (1955) of controversy as to what is Caledonian and SCHERP (1959). and what Variscan in the structure and metamorphism in the Ardennes. It is not The intrusions are metamorphosed. in doubt that more than one generation They are also cleaved. All that can be of structure is represented (see, for said of their age is that they are de­ example, PLESSMANN 1959; KRAMM 1982) and monstrably post-Salmien, arguably (by it is not in doubt that an unconformity reference of the metamorphism) post­ separates the basal Devonian from the pre­ Gedinnian, and that they predate some Devonian rocks. But the tenor of recent part of the history of development of findings (SCHREYER 1975; BEUGNIES, 1976; what is regarded as Variscan cleavage. BEUGNIES, CHAVEPEYER and MERCIER, 1981; A further comment on their possible signi­ DANDOIS 1981; KRAMM 1982) appears to fa­ ficance appears below. veur the view that the metamorphism took place within the range of age represented These interesting and insuffi­ by the succession above the unconformity. ciently well understood intrusive rocks SCHREYER'S (1975) study of garnet growth could be labelled "intra-Variscan"; but in pebbles contained within Gedinnian con­ any reference to Variscan (or Hercynian glomerate is critically important. So, see MATTHEWS and FORD (in press) in Be1gium too, is KRAMM'S (1982) observation that and northern France immediately requires chloritoids within Salmien clasts in qualification. The reason is that seve­ Gedinnian conglomerate resemble chloritoids ral of the characteristic features of 35 Variscan geology in the Reinisches Schie­ from the Ligérien tect~nism (PRUVOST fergebirge to the east do not occur here 1949, AUTRAN and COGNE, 1980) of there is no comparable development o.f ba­ Silurian-Devonian time recorded in the sina l ("herzynische") facies in the geology of south Brittany. Further, Devonian and there are none of the strata­ our information on Ordovician in bound ore deposi ts (e. g. Meggen, Rammelsberg) Belgium and northern France suggests that are present in Germany. Belgium and that the succession includes no repre­ northern France have no worthwhile repre-' senta tion of the Grès Armoricain which sentation of the basic volcanism that is is such a conspicuous feature of prominent in the Middle Devonian and Lower Ordovician successions in northwest Carboniferous in the Rheinisches Schiefer­ France and regions wider afield. We gebi rge. Further, there is no equivalent have no direct information on the na­ of the Carboniferous turbidite influx ture of the base of the Devillien and ("Kulmgrauwacke"). These are absent in no means of making comparison with re­ Belgium, and yet they are all represented lationships near the base of the Brio­ again farther to the west in southwest verian in western France; but the evi­ England (MATTHEWS 1977). This interruption dence already available should be suf­ of basin development does nothing to encou­ ficient to cause one to hesitate befo­ rage acceptance of ZIEGLER's (1978, 1982) re assuming that Belgium is underlain proposal of a Cornwall-Rhenish Basin. by pre-Brioverian basement of the kind exposed in northwest France and the One may note the large-scale thrusting Channel Islands. MICHOT (1980) has affecting Carboniferous rocks in Belgium offered an indirect argument in favour and may take this to justify reference to of a suggestion that Dalslandian may Variscan tectonism; but this, again, does be an appropriate label for some com­ not promote the idea that Belgium is tec­ ponent of the crust in this region. tonically at one with Germany, for the northern part of the Rheinisches Schiefer­ 2. Any such proposal would be at varian­ gebi rge shows no evidence of an eastward ce with a suggestion VOLL (1978 and continuation of the Belgian style of thrust­ fide MURAWSKI 1981) makes on the basis ing. MATTHEWS (1983a) has remarked that of rock-samples brought from depth in the Palaeozoic geology of Belgium provides the crust by the Tertiary volcanism what some would accept as a clear expres­ in the Eifel region. He is prepared, sion of a Variscan Front, but lacks most on that evidence, to contemplate the of what are taken to be characteristic possibility that the crust sampled features of Variscan geology as a whole in may be largerly of Variscan establish­ the Rheinisches Schiefergebirge or in ment. southwest England. Words like "stability" and "consolidation" regularly arise in res­ We return later to the question ponse to any reference to the apparently of the nature of the pre-Devonian in idiosyncratic tectonic behaviour of Belgium Belgium and northern France. and northern France. They acknowledge the existence of some special problem, but do little to make the problem clearer and do EASTWARD FROM THE ARDENNES, nothing to explain it. One wonders what "consolidation" might mean ? There is brief mention above of suggestions (von GAERTNER 1960; KREBS 1976, 1978, 1982) that Caledonian tec­ THE PRE-DEVILLIAN QUESTION, tonism may have been active eastward of the Ardennes. Given the difficulty Questions concerning relative'~ta­ of distinguishing anything that is bility" or "consolidation " of the Arden­ Caledonian from what is probably poly­ nes region serve to introduce the problem phase Variscan tectonism in the of estimating the nature of the deeper, Ardennes, the significance of an east­ entirely unexposed geology in Belgium and ward expression of Ardennes-associated northern France. Only two matters need to tectonism is not immediately obvious - be discussed at this stage of the argu­ certainly, bland application of the la­ ment : bel "Caledonian" does not seem to be jus­ tified. Stratigraphie evidence opens up 1. KLEIN (1978) introduced the idea of a an opportunity of a clearer view of what "socle mou". This was his conception was involved. of the meaning of polyphase tectonism and since there is polyphase tectonism KEGEL (1950) produced a survey in both Brittany and the Ardennes he of stratigraphie relationships in the took these two to be examples of tee tonie Rheinisches Schiefergebirge. It has reactivation as he understooù it. But been complemented by SCHMIDT's (1952) the two regions show more of diversity work on the Devonian around the Hohe than of unity : it would be wrong to Venn and has been reconsidered that dur­ conclude that they have tectonic ing three decades in which we have themes,orphases, in common, and it is acquired more and more detailed sedimen­ to be doubted whether the example of tary information ~see, for example, the Massif Armoricain offers us worth­ FRANKE, EDER, ENGEL and LANGENSTRASSEN while instruction on the nature of the 1978; GOLDRING and LANGENSTRASSEN 1979) pre-Devillien geology in Belgium and the meaning of KEGEL's (1950) maps and northern France. Sorne differences be­ their information on variations of thick­ tween the two are already obvious in ness have never impressed themselves the exposed geology. It is, for exam­ sufficiently firmly on our thinking. ple, clear that whatever interpreta­ tion we put on "Caledonian" breaks that PAPROTH (1976) follows KEGEL interrupt the pre-Devonian succession (1950) in showing successive troughs in the Ardennes and the Brabant Massif, (her TROGE I, II, III), meaning successive we are seeing something quite different 36 loci of maximum stratigraphie thickness, Devillien, Revinien, Salmien in the migrating southeastward away from the Ardennes (with the obvious exception eastern end (as now seen) of th~ Ardennes of the fact that the Ardennes sequence during early Devonian time. TROG IV (a represents a much longer span of time). Siegerland s!nk) no longer conforms to rhat Comments on details of structure there pattern. TROG V, in the eastern part of supplied by Borrodaille (1977) could be the Rheinisches Schiefergebirge, shows that said to lack the ~eological resonance in mid Devonian time the locus of maximum one senses in DVORÀK (1973), who inter­ thickness had shifted northward, north of preted the Siegerland Anticline as an the Siegerland, and TROG VI represents a inversion structure. The compression further shift in that direction, which which his proposal requires first beco­ brought maximum thickness of the late mes plausible if one thinks of a weak Devonian sequence to a site in the north­ thermal bulge, an "Ardennes effect" on western part of the Rheinisches Schieferge­ the north of the Siegerland and a birge. Regional basin evolution continued stronger "Saxothuringian effect" impin­ later on a broader pattern, with develop­ ging on the sedimentary prism from the ment of the Subvariscan Foredeep still far­ southeast. ther to the north. 2. The interfering effects would combine PAPROTH (1976) noted that the to produce a depression in the eastern shifts of the loci of maximum thickness part of the Rheinisches Schiefergebir­ trace out an anticlockwise spiral in the ge (TRÔG V) in mid Devonian time and Rheinisches Schiefergebirge and asked what would thus provide a set of circumstan­ might be the meaning of such a pattern of ces in which it is understandable that behaviour. It is possible that two ini­ Eifel accumulations of carbonate are of tially separate influences were at work. platform type, in contrast to build­ One is a doming, active in the Ardennes ups in the Rheinisches Schiefergebirge region and fading eastward of there, which which had more opportunity for upward induced the progressive shift of maximum growth. thickness souteastward and away from the Ardennes during early Devonian time. The 3. Heating proceeding from two separate other is a new influence on relief, essen­ sources is evident first in modifica­ tially a heating whose "epicentre" was in tions of basin form then later, when the evolving Saxothuringian Zone (BEHR rocks were thermally weakened to a 1978; ENGEL and FRANKE 1982) in Devonian sufficient degree, in deformation and time, whose effect in terms of thermal bul­ production of cleavage. AHRENDT, ging caused the new (to the Rheinisches HUNZIKER and WEBER (1978) have measu­ Schiefergebirge in mid Devonian time) red the northward progress of defor­ northward shift of maximum thickness and mation (330 Ma in the Taunus to 300 Ma whose ef fect in terms of thermal weakening in the northeastern Rheinisches (WEBER 1981; KUZNIR and PARK 1982) caused Schiefergebirge) proceeding in conse­ the northward progression of deformation quence of the "Saxothuringian effect". recorded in slates from the eastern In the Ardennes, deformation may have Rheinisches Schiefergebirge by AHRENDT, begun earlier in consequence of the HUNZIKER and WEBER (1978). local, independent accession of heating which was first expressed as a doming Low-mid Devonian stratigraphie re­ in the Ardennes and the region east­ lations in the Reinisches Schiefergebirge ward from there. KRAMM (1982) may not and the area to the wes t of it may, on this be justified in assuming that AHRENDT, view, involve more than a southward failu­ HUNZIKER and WEBER's (1978) datings of re of a Caledonian Molasse (FRANKE, EDER, deformation in the eastern Rheinisches ENGEL and LANGENSTRASSEN 1978; BEHR, Schiefergebirge apply in the Ardennes WALLISER and WEBER 1980) and more than the also. MICHOT, FRANSSEN and LEDENT problem of identifying a boundary between (1973) have reported preliminary in­ a northwest European and a southeast vestigation of dates in Belgium and European affiliation in Devonian strati­ have offered a figure of 305-310 Ma. graphy (KEGEL 1950). The Devonian in the Any fuller investigation would involve Rheinisches Schiefergebirge would here be recalculation of that result. It treated instead in terms of an interplay should also include some discussion between delivery of clastics, abundant in of the meaning of a single date, given early Devonian time, less so later, from that deformation may have been in pro­ a direction around northwest (essentially gress during a period of time and a "Caledonian" source) and receipt of clas­ was apparently polyphase. Preferably, tics in basins whose form and whose capaci­ if there is to be comparison with ty to acconunodate sediment were subject to AHRENDT, HUNZIKER and WEBER's (1978) modification by a regional doming (an results, the datings should be done on "Ardennes effect") already evident early rocks which represent primary ash-fall in Devonian time and by a second tendency rather than on run of the mill samples to produce positive relief (a "Saxothurin­ of slate which may include detrital gian effect") which began to .impose itself argon. on the geological development of the Rheinisches Schiefergebirge in mid Devo­ An open possibility, in the pre­ nian time. The following considerations sent state of the evidence, is that in are involved. the Ardennes a first generation of struc­ tures emerged for local reasons of heat­ 1. The Siegerland (GROESSENS, CONIL and ing, and that a later generation was STREEL 1982),where the tendency in mi­ produced when the region was eventually gration of maximum thickness changes overtaken by a stronger flux of heating, its course, is of special interest. the "Saxothuringian effect", with special Its succession is of a character which features developing as stress was brought would, broadly, compare with the to bear on the already to some extent 37 determined form of an Ardennes dame and on a special heating ("Ardennes effect") the special qualities developed in the li­ within the Rhenohercynian Zone, which thostratigraphic succession (especially may be suggested to be first evident on the availability of anhydrite : PIRLET a southwestward traverse at Soest­ 1972; BOUCKAERT, DELMER and GRAULICH, 1g77) Erwitte, is progressively more clearly there. expressed southwestward from there, cames to a culmination in the Ardennes re­ gion itself and may be regarded as WESTWARD TO BELGIUM AND BEYOND. fadingwestward from ·there in MICHOT's (1978) proposal of a "bombement" (but One should ask whether any addi­ not a Caledonian bombement) which is no tional evidence in the geology of the longer effective at Liévin. Questions Rheinisches Schiefergebirge would be con­ arise : what becomes of the Rhenohercy- sistent with the interpretation sketched . nian Zone wi thin Belgium, and why does above. Evidence which may have a bearing it not resume its normal character in on the argument cames from the Soest- the west when the doming has faded ? Erwi tte borehole. Dark slates with grap­ And does that problem become clearer if. tolite remains (TEICHMÜLLER 1978) can, in we appeal to the idea of a Variscan geo­ KREBS' (1978, 1982) view, be incorporated syncl ine ? in a proposal of Caledonian tectonism. The steady northward younging of AHRENDT, HUNZIKER and WEBER's (1978) K/Ar dates fal­ COMPARISON WITH SOUTHWEST IRELAND. ters in this neighbourhood and an anoma­ lous older (340 Ma) result appears. KRAMM The proposal is that an elongate, (1982) suggests that the 340 Ma figure may dome-like structure was developing du­ represent a mixture of Caledonian and Va­ ring Devonian and possibly into Carboni­ riscan ages; but it is now doubtful whe­ ferous time, and expressed itself on a ther the term "Caledonian" is in any sense long axis (Ebbe Anticline to Liévin) applicable in this area. The anomalous re­ which measures over 300 kilometres. Œose sult may better deserve to be regarded as to its culmination, in the Ardennes, representing interference of a remote east­ there are the Venn intrusions (the ward Ardennes effect and the Saxothuringian depth from which they were derived has effect which the main run of results not been estimated) which have been meta­ describes. morphosed to some degree and which car­ ry a cleavage. One thinks to suggest a FRANKE, EDER, ENGEL and LANGEN­ comparison with southwest Ireland, where STP.ASSEN (1978) remark that features an elongate dame-structure has been the of the geology which can be traced from site of Carboniferous heating, where in­ the Harz into the eastern part of the trusions in small volume derived from Rheinisches Schiefergebirge tend to lose considerable depth have been emplaced, their identities in the neighbourhood of and where one finds that the intrusions the Siegerland Anticline, where a simila­ are housed in a doming produced by in­ ri ty to geology west of the Rhine and into version. The doming was already in pro­ the Ardennes is already detectible. One gress when Variscan folding took place, accepts these points without necessarily and continued its development later to subscribing to their further comment that produce distortion of some of the Varis­ the new influence emerging is a pronounced can folds (MATTHEWS, NAYLOR and stability from the upper Emsian onward. SEVASTOPULO 1983; MATTHEWS in press). The following questions of possible r:esem­ KAISER, PAPROTH and STADTLER's blance to the Ardennes suggest themsel­ (1978, fig. 4) map of the western part of ves : the Ebbe Anticline (and including the site of the Schwartzbachtal borehole) shows a 1. In bath cases there is an earlier and palaeogeography which would be in accord a later "Variscan" tectonic effect. with proposals offered here. The inversion in County Cork has been interpreted as the consequence of a Farther to the west, across the mantle delamination (BIRD 1979) trig­ Rhine, the Eifeler Nord-Sud Zone is a pro­ gered by the collision which closed minent element in the regional structure. the Iapetus Ocean - i.e. much of what It, like the Krefelder Gewëlbe farther is regarded as "Variscan" tectonism north, has had an influence on tectonic in southwest Ireland is to be attri­ development from Palaeozoic through into buted to a "Caledonian" cause. Mesozoic time. When the Eifeler Nord-Sud Zone was first instituted is not under­ 2. The Ardennes and County Cork differ stood. It is suspected that it was alrea­ in certain respects. In County Cork dy active when the synclines that contain development of a large half-graben, the Middle Devonian limestones in the the Munster Basin, preceded the do­ Eifel came into being (SCHENK 1937). But ming. Nothing of comparable charac­ it is not clear whether that pattern of ter is preserved in the Ardennes. If folds is better explained by invocation such a depression had at one time of a north-south basement structure than developed there, before heating su­ by a suggestion of accommodation to the pervened, it would have been of form of a growing (Ardennes) dame. Possi­ Silurian age. A slightly improbable bly bath influences were at work. alternative view of the Ardennes is that a precursory depression is re­ BEHR, WALLISER and WEBER (1980), presented in the thick early Palaeo­ like FRANKE, EDER, ENGEL and LANGENSTRASSEN zoic sequence. The improbability is (1978), make the prosaic assumption that in the lenght of time involved (cf. the Ardennes is to be treated as part of calculations in BIRD 1979). If the the Rhenohercynian Zone. Such an inter­ early Palaeozoic depression and the pretation would mask the significance of later Palaeozoic doming are nevertheless

38 to be suspected to be in sorne way rela­ effect", one is required to explain ted, the relationship rnay be that of a why this should have produced a rnuch double inversion (cf. KENT's 1980 refe­ more powerful development of thrusting rence to a late Carboniferous inversion in Belgiurn (possibly comparable in sca­ on the sarne site as a late Cretaceous le with "Saxothuringian" thrusting in effect of rnuch the sarne kind in north­ the southeast Rheinisches Schieferge­ eas t England). birge : WEBER 1981) than it did in the adjacent, northwestern part of the 3. The timing of events is different in Rheinisches Schiefergebirge. The cri­ County Cork and the Ardennes. In County tical influence here seerns to have Cork the inversion, identified as the been the availability of evaporites in beginnings of dorning (i. e. in the li­ the Upper Devonian and Lower Carboni­ tera! sense of words an eversion) took ferous (DELMER, GRAULICH and LEGRAND, place, according to stratigraphie evi­ 1978) and the avhilability and prirna­ dence, in Viséan tirne. In the Ardennes, ry distribution of these in turn rnay stratigraphie evidence suggests that do­ have been influenced by the Ardennes rning was already in effect during early dorning. One would wish to know whether Devonian tirne. If delarnination theory there is any evidence of major thrust­ applies in both cases, it does not ne­ ing south of the original limit of cessarily follow that there should be distribution of anhydrite. This might an exact match in timing. Heat lost set contraints on proposals of the from the mantle in two separate instan­ arnount of movement on the Dinant Nappe ces should not necessarily induce ef­ (BLESS, BOUCKAERT and PAPROTH, 1980 : fects such as dorning sirnultaneously in see further comment below on that sarne the geology at high level in both re­ need) - and one would wish to ask, gions. Variation in crustal thickness when the structural succession north is one obvious reason why the two heat­ of the Ardennes is better understood ings should not keep pace wi th one in depth than it is now, whether ear­ another. Equally, one should not ex­ ly Devonian stratigraphy there shows pect that rocks in both regions arri­ a pattern of progressive migration of ved simultaneously at critical thresh­ maximum thickness away frorn the olds of thermal weakening. A separa­ Ardennes, à rnirror image of the pat­ te consideration in the rnatter of ti­ tern to the southwest. This would ming is that the tirne of inception of greatly reinforce the idea of an delarnination cannot be exactly identi­ Ardennes dorning already in progress fied because the collision is not dat­ in early Devonian tirne. ed · with any great accuracy. These are arguments based on a 4. Brief mention should be made of an appa­ fairly free-ranging treatrnent of strati­ rent sirnilarity which rnay or rnay not graphie evidence and on interpretations be accidenta!. The Glandore High of the tectonic consequences (regarded (NAYLOR, JONES and MATTHEWS 1974; as Variscan) of heating. It remains to NAYLOR, SEVASTOPULO, SLEEMAN and REILLY be seen whether the suggestion of two 1981, esp. fig. 5) is an approximately tectonic effects ("Ardennes" and north-south structure sited close to "Saxothuringian") rnay eventually be the transverse axis of the dorning in brought into coherence with classical County Cork. The offshore evidence German work on structural geology in re­ southward of Ireland shows that its ac­ gions close to the Ardennes (e. g. CLOOS tivity continued in Mesozoic time. The 1950; HOEPPENER 1957 especially; ENGELS resernblance to the Eifeler Nord-Sud 1959) or with WEBER's (1978, 1981) in­ Zone rnay or may not be a rnatter of coin­ creasingly more widely effective analy­ cidence. ses of the different generations of structure in the Rheinisches Schieferge­ 5. Brief mention rnay be made of an obvious birge. difference. There is a conspicuous cop­ per anornaly associated with the geology of southwest Ireland. One presurnes THE INFLUENCE OF PRE-VARISCAN TECTONISM, that a copper anornaly in the Ardennes would not have escaped the attention of The discussion has touched on the Belgian geologists, given thei r aware­ question of the continuance of the Rheno­ ness of copper in Africa. hercynian Zone in the southwest where the "bombement" fades. The likelihood 6. One possible sirnilarity deserves fuller is that developrnent of "Variscan" tecto­ examination. What has been regarded as nisrn ends at a major pre-Variscan struc­ the Variscan Front in southwest Ireland ture running northwestward,frorn Paris is reverse faulting on the northern toward southern England (GERARD and flank of the growing darne. The faulting WEBER, 1971; WEBER 1973, 1980). This fades eastward as the dorning fades. structure continued in activity and has There rnay be a resernblance to Belgium, shown sorne rnovement in sub-Recent tirne where the Faille du Midi is well-devel­ (FOURNIGUET, VOGT and WEBER 1981). It oped along the front of the Ardennes èan be treated as part of a pattern of dorning, but fades eastward (MATTHEWS pre-Variscan structure in Central and 1g74) into the Rheinisches Schieferge­ Western Europe discussed by MATTHEWS birge as the dorning fades. Whatever (1984, in press). These pre-Variscan the irivolvernent of the culmination of structures are essentially riftings a first structure, it is evident that which can be argued to have had their the major thrusting in Belgiurn is to a inception in late Precarnbrian-early large extent an expression of a second Palaeozoic tirne. The severely deforrned generation of deforrnation, with strong state of such a rift in Central Europe northwestward drive. If it is perrnis­ is what is identified as the Saxothurin­ sible to think of this second genera­ gian Zone there (BEHR 1978; BEHR, ENGEL tion of structure as the "Sa.'

CONCLUSIONS, These are proposals which pay no respect to the idea that Palaeozoic geolo­ Fig. 1 - Heavy dashed lines : a pattern of pre­ gy in Belgium came early to a state of Variscan structure in Central and Western stability or "consolidation" (see a re­ Europe. Large crosses : major Variscan cent example of that view in ZIEGLER 1982). heating ("Saxothuringian effect"). Small The idea of consolidation seems perverse crosses : two other sites of heating when one considers, for example, the evi­ ("Ardennes effect" and the in some respects dence of the Saint-Ghislain borehole comparable case in southwest Ireland). (DEJONGHE, DELMER and GROESSENS, 1976; DELMER, GRAULICH and LEGRAND, 1978; GROESSENS, CONIL and HENNEBERT, 1979) which shows availability of anhydrite in and FRANKE 1982). The belts of rifting thickness of the order of a thousand me­ are quasicontinuous, but imposed Variscan tres and more and which implies produc­ heating and deformation is not everywhere tion of a considerable degree of structu­ as advanced as in Central Europe. The pat­ ral relief in order to accommodate such a tern of rifting runs northwestward through a pile. Or one might cite the matter of France to southern England and continues the long-term activity of the Eifeler clown the Bristol Channel to the Celtic Sea Nord-Sud Zone or the question of the an­ (MATTHEWS, in press). The geology of Brittany tiqui ty of the Rhine Graben structure as is not involved : nothing there deserves examples of continuing tectonic activity to be interpreted as a continuation of the in the general region of Belgium. The Saxothuringian Zone. The Rhenohercynian Bray structure running northwestward Zone, as known in the Harz and the through France is still active and the Rheinisches Schiefergebirge, ends in anhydrite which did much to lubricate Belgium and northern France. It is not late Palaeozoic structural transpositions continuously developed toward southwest in Belgium is still available. England, where there was a quite separate occasion of Variscan tectonism. It is difficult to account for Caledonian tectonism in Belgium and nor­ A point of particular relevance to thern France in terms of a plate tecton.ic present arguments is that the two sites of model. It is difficult to summon up any presumed delamination, Ardennes and County plate tectonic model specific to the Va­ Cork, appear to occupy comparable posi­ riscides which would promote a better un­ tions in relation to the earlier established derstanding of Variscan tectonism in pattern of rifting. Each is set on Belgium. A curious (as it may appear at the northwest sicle of a run of the rifting first sight) outcome of the argument here which trends southwest. The implication is that "Variscan" tectonism in the is ~hat the existence of rift-structures, Ardennes (as in southwest Ireland) may be so disposed, may have tripped the delami­ more readily explicable if one treats it nation process at a time when the Iapetus as being largely a consequence of a "Ca­ Ocean had closed and the inertia of the ledonian" scheme of events. The lesson southeast plate was obliging the litho­ is, perhaps, that we should not strive sphere to deform. too hard to separate Caledonian from Variscan (MATTHEWS 1978). We should not In a more 1ocal tectonic context, regard the unconformity at the base of one would direct particular attention to the Gedinnian around the Ardennes as the the Givonne Massif (BEUGNIES 1976). most important single consideration in Should the large underlying basic body to any attempt to understand Palaeozoic tec­ which the positive gravity anomaly is due tonism in the region. Nor should we in­ be conceived to relate to delamination, sist that local accessions of heating or to earlier tectonism? If there is a drove tectonic processes everywhere at relationship between the insertion of such the same rate. Certainly, there seems a body and potassium metasomatism in the to be little justification nowadays for Upper Palaeozoic rocks above (BEUGNIES the opinion that all in Europe came to 1976), does this age-relationship settle some common climax which deserves the the question of the body's tectonic affi­ name "Asturian" (cf . MA TTHEWS 1 9 8 3 c) . liation ? And does the same evidence suggest that no major structural detach­ Structures established earlier, ment underlies the Givonne Massif ? In within what was the plate on the south­ this again, one finds cause to resist eas t sicle of the Iapetus Ocean in early 40 Palaeozoic time, nave exertea an inriuence BEUGNIES, A. (1976) - Structure et métamorphisme on later Palaeozoic tectonism. Nothing in du Paléozoïque de la région de Muno, un the layout of the earlier Palaeozoic struc­ secteur clef du domaine hercynien de ture in the neighbourhood of Belgium sug­ 1 'Ardenne. Ann. Mines de Belgique, gests a relationship with Brittany (even 6, 481-509. although the Armorican Massif, like the Ardennes and adjacent massifs, carries the BEUGNIES, A., CHAVEPEYER, G. & MERCIER, M. imprint of more than one generatio~ of (1981) - Sur le métamorphisme de la par­ tectonism). The influence of earlier geo­ tie méridionale du Massif cambrien de logy suggests that VOLL (1978) may be mis­ Rocroi. Les zones à chloritoïde et à taken in regarding the crust in this re­ ilménite. Ann. Soa. géol. Nord, C 131- gion as being largely a Variscën produc­ 138. tion. And one notes, in TEICHMULLER and TEICHMÜLLER (1979, fig. 2), a suggestion BEUGNIES, A., DUMONT, P., GEUKENS, F. MORTELMANS, that in the southern part of the Ardennes, G. & VANGUESTAINE, M. (1976) - Essai de a deep reflector may be the top of the synthèse du Cambrien de l'Ardenne. Ann. Precambrian rather than any continuation Soa. géol. Nord, 96, 263-273. of a Variscan structural surface. BIRD, P. (1979) - Continental delamination and the Colorado Plateau. J. Geophys. Res., ACKNOWLEDGEMENTS, 84, 7561-7571.

I have in the past issued all too BLESS, M., BOUCKAERT, J. & PAPROTH, E. (1980) - brief comments on geology in Belgium (e.g. Pre-Permian around the Brabant Massif in "Belgium is structurally complex but tec­ Belgium, the Netherlands and Germany. tonically relatively simple" : MATTHEWS Meded. Rijks geol. Dienst, 32, 3-13. 1977) and I have regretted the lack of an opportunity to offer a fuller statement BORRODAILE, G. J. (1977) - On cleavage and of my views. I am grateful that such an strain : results of a study in West opportunity has corne now, and am particu­ Germany using tectonically deformed larly glad that it cornes on the pleasant sand dykes. Jl. geol. Soa. Lond., occasion of our doing homage to MM. DELMER 133' 146-164 . and LEGRAND. I thank Dr. Eric GROESSENS, a good colleague, for extending to me the BOUCKAERT, J,, DELMER, A. & GRAULICH, J. M. Society's invitation to take part in the (1977) - La structure varisque de Symposium. Monica SIEWERTZ (Uppsala) l'Ardenne : Essai d'interprétation. produced the typescript. She, too, deser­ Meded. Rijks Geol. Dienst, 28, 133-134. ves my thanks. CLOOS, H. (1950) - Gang und Gehwerk einer Falte. z. deutsah. geol. Ges., 100 (for 1948), REFERENCES, 290-303.

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42 MATTHEWS, S. C. (Ms) - Caledonian and Variscan SCHMIDT, Wo. (1952) - Die palaogeographische Ent­ treated as component parts of one chapter wicklung des linkstheinischen Schiefer- · of tectonism. gebirges vom Kambrium bis zum Oberkarbon. Z. deutsch. geol. Ges., 103 (for 1951), MATTHEWS, s. C. & FORD, I. H. (in press) - Armorican 151-177. Hercynian, Variscan and certain other foreign terms. Pr>oc. Ussher Soc. SCHREYER, W. (1975) - New petrologie evidence for Hercynian metamorphism in the Venn­ MATTHEWS, S. C., NAYLOR, D. & STEVASTOPULO, G.D. Stave lot Massif, Belgium. Geol. Rdsch., (1983) - Palaeozoic sedimentary sequence 64, 819-830. as a reflection of deep structure in sou­ thern Ireland. Sedimentary Geology (de STAINIER, X. (1930) - Le métamorphisme des régions Raaf volume), 34. de Bastogne et de Vielsalm. Bull. Soc. belge Géol. Paléont. Hydrol.,39, 112-156. MICHOT, P. (1976) - Le segment varisque et son an­ técédent calédonien dans le cadre de la TEICHMÜLLER, M. (1978) - Nachweis von Graptoli­ Belgique et les régions limitrophes. then-Periderm in geschieferten Gesteinen Nova Acta Leopoldina, N. F. 45 : 224, mit Hilfe kohlenpetrographischen Metho­ 201-228. den. N. Jb. Geol. Palaont. Mh., 1978, 430-447. MICHOT, P. (1980) - Le segment tectogénique calé­ donien belge. Mém. Cl. Sei. Acad. R. TEICHMÜLLER, M. & TEICHMÜLLER, R. (1979) - Ein Belg., 43, 9-61 (l980a). Inkohlungsprofile entlang der linksrhei­ nischen Geotraverse von Schleiden nach MICHOT, P. (1980) - Belgique. Introduction à la Aachen und die Inkohlung in der Nord­ géologie générale (Excursion : 211 A), Sud Zone der Eifel. Fortschr. Geol. 487-576. Publ. 26ème C. G. I. Paris. Rheinld Westf., 27, 323-355.

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