Hercynjan pa/aeomagnetism of Europe: arguments for large sca/e comp/ex thin-skinned and thick-skinned rotations y. BACHTADSE Department of Earth Selences, Oxford University, Parks Road, Oxford, OXI 3PR, UK ABSTRACT Palaeomagnetic data for ihe Devonian and Carboniferous of the Eu- ropean Hercynides and from their stable foreland (northern Europe and the BriLish Isles) display a very coherenL pattern in inclinaLion, pointing towards consolidation of Hercynian Europe during Lhe mid Lo late Devo- man. Declination data however, show a significant dispersion and can be used in order to unravel the deformation history of the mountain belt. Differences between the observed palaeomagnetic declinations and those expeeted for the specific areas and the age of magnetizaLion can be corre- lated to the changes in regional sLrike. Linear regression analysis of the de- clination deviations, shows a very strong correlaLion (close to unity) for [hose areas, where thin-skinned nappe emplacement has been most pre- valent. This suggests the predominantly primary characLer of the strucLu- ral curvature. Data from presumably autochthonous parts of Lhe orogen are more scattered but nevertheless yield a significant, but shallower re- gression line. A L-test of the slope against zero slope is signifieant aL the 95 0/o confidence level, indicating secondary (oroclinal) bending of an ori- ginally deformed fold system for the autochLohonous parts of the rnoun- tain beIL. It can be inferred, that the present shape of Lhe Hercynian moun- tarn belL is the resulL of combined recurrent deep-reaching deformation of the lithosphere and the superimposed effects of thin-skinned thrust rota- tions, compatible with geodynamic modeis involving the indentation of Hercynian Europe by a microplate or an African promontory during the Hercynian orogeny. 148 J/~ Baehtadse INTRODUCTION ‘¡he geodynamic evolution of the European Hercynides has been the subject of intense research during the last two decades and raiher anta- gonistic scenarios for the Palaeozoic crustal consolidation of Europe have been proposed by nurnerous authors (e.g. Lorenz and Nicholís, 1984, Behr el al.., 1984, Maite, 1986, Ziegler, 1986 and references Iherein). I-lowe- ver, a unifying model for I-Iercynian geodynamics, which integrates geophysical data and geological observations has not been brought for- ward yet. Ihe Hercynian foid belt of Europe is generally characterized by a cen- tral polymetamorphic crystalline beiL, bordered on both sides by ouLward facing foid and thrust belts, consisting of unmetamorphosed or low grade sedirnentary and volcanic assernblages. Although there is evidence for a localized tectonic event during the Devonian (Acadian), the main oroge- níc pulse has been shown to he Carboniferous (i.e. Late Visean to Namu- rian) in age (Behr et al., 1984 and references therein>. The most promi- nent feature of the European Hercynides is the well pronounced curvatu- re of the rnountain belt as defined by the change in the structural trend (Fig. 1). Ihis is rnost spectacular around the Bay of Biscay, where the to- tal bending of the Ibero-Armorican arc in a pre-Mesozoic reconstruction 1 .‘ ( ¡ Fig. 1. Geological sketch map of the European Hercynides in pre-Mesozoic configuration (Van der ‘Voo, 1969). Sho’wn are ihe directiens of Ihe siructural trend (1), ihe norihein ovcrthrust (2), and ihe directions of Devonian and Carbonifereus paleomagnetic declina- iions from autochthonous (3) and allochthonous (4) areas. A: Carpathians; B: Central Eu- ropc (Han Mountains and Franconian Forest); C: Ardenne-Fifel; D: Massif Central; E: Wa- les; E: Armorica; G: Cantabria; H: Galicia-Castilla. Ilerc~’nian palaeomagnetism of Europe: arguments... 149 (Van der Voo, 1969) approximaLes 165’ (Ries and Shackleton, 1976). In Central Europe, Lhe change from the Hercynian (NW-SE) to the Variscan (SW-NE) sLriking structural trend is less dramaLie but still amounts to about 40’. Finally a change in the structural trend of about 80’ can be ob- served in Lhe Carpathians. Based on palaeomagnetic (Van der Voo, 1979) and general geological evidence (e.g. Ziegler, 1986 and references therein), it is now widely ac- cepted, that the Palaeozoic consolidation of Western Europe was contro- lled by the convergence of three major plates (Laurentia, Baltica and Gondwana) as well as by the amalgamation and deformation of srnaller, Gondwana-derived, crusLal elements such as the Armorica microplaLe (Van der Voo, 1979) and the Iberian and Austro-alpine allochthonous terranes (Ziegler, 1983). In this paper a brief review of the Devonian to Carboniferous palaeomagnetic data from Central Europe will be given and an attempt will be made Lo elucidate a sLrucLural and geodynamic in- terpretaLion. ‘¡HE PALAEOMAGNETIC REFERENCE FRAMEWORK ‘¡he strucLural significance of palaeomagnetic daLa from deformed a- reas hinges crucially on the quality of the reference daLa base from a sta- ble foreland or craton. It is now widely accepted, that Laurentia, Baltica, the British Isles south of the lapetus and the Central European pre-Va- riscan basement (sLable Europe), have been assembled during the Caledo- nian orogeny (e.g. Soper and Hutton, 1984). Devonian and Carboniferous palaeornagnetic data from the Baltic shield and the British Isles can the- refore be used as reference for Lhe geodynamic and structural interpreta- tion of palaeomagnetic data from 11w European Hereynides. ‘¡he various proposed apparent polar wander (APW) paths for stable Europe (Briden et al., 1973, McElhinny, 1973, French, 1976, Duff, 1980) agree raLher well for the Silurian Lo early Carboniferous period. Differen- ces ofup Lo 10’ in the longitudinal position of the Carboniferous poles bet- ween various proposed models (Duff, 1980; Briden, 1973, McElhinny, 1973) are however conspicuous, and point towards unresolved problems in age calibration (Perroud, 1986). Following a different une of argument, Edel (1 987a) postulated thaL notably the Devonian and Carboniferous parts of Lhe APW path of stable Europe have been severely affected by Perrno-Carboniferous remagnetizations and Lherefore cannot be used as a reference for the interpretation of pre-Variscan directions. Conse- quently, he proposed to use the APW path for the Russian platform instead. ‘¡he rather tight cluster of Devonian to Permian poles from European Russia (Khramov et al., 1981) however, which are noL signi- ficantly different from western European Permian palaeopoles, itidicates 150 U. Baehtadse unresolved pervasive Kiaman (Permo-Carboniferous) remagnetization of the Russian Palaeozoic rocks. Lacks fo accessibility of the original publi- cations withdraws those dala from any further serious evaluation. ‘¡he more vigorous use of modern statistical methods and reliability criteria resulted in several published mean palaeopole positions for stable Europe (Table 1) considered to be more credible and used as reference for this pa- per. PALAEOMAGNETIC FRAMEWORK OF THE EUROPEAN HERCY- NIDES Widespread Kiaman remagnetization is a predominan[ feature of the European Palaeozic palaeomagnetic record (Creer, 1 968). Although we are still far from understanding the dynamics of this conLinení, it be- comes more and more obvious, that the remagnetization process is controlled by chemical rather than thermal condiíions (Courtillot et al., 1986). Fluid migration related to Hercynian thrusting can poten- tially provide the chemical conditions for such a large scale event (McCa- be et al., 1983). It comes therefore notas a surprise, that only a minority of the reliable palaeomagnetic pole positions tabulated (Table 2) are thought to refleet primary (e.g. prefolding) directions. Only five primary palaeomagnetic palaeopoles have been reponed for the Devonian so far, namely from the Iberian Meseta (Perroud and Bon- hommet, 1984), Lhe Harz mountains and Franconian Forest (Bachtadse, 1984), Wales (McClelland Brown, 1983) and more recently from the Holy Cross Mountains, Poland (Lewandowski et al., 1987). Except for the Po- TABLE 1 Siluro-Devonian to Permian reference peles for stable Europe Pote Age Lat. Long. A95 N Reference ‘E Siluro-Devonian —2 321 Duff, 1980 Mid-Devenian 18 332 3 II Pesonen et al., 1987 Late Devonian Early Carbonifereus 30 333 3 8 Duff, 1980 Mid to late Carbonifcrous 37 341 3 6 Frey and Cex, 1987 Late Carboniferous 40 344 3 21 Frey and Cox, 1987 Lal. and Long, are latisude and longilude of Ihe palacopole posilion u degrees SouIh ancí Ezíl, respecíively. N it dic number of dala entries aud .495 it dic radius of Ihe cone of confidence al dic 95% probability le-va. Ilercvnian palaeomagnetisni of Europe: argurnenis... 151 ES .5555 50 55 oSEo, 5555 55555555’55~55 .5.5.5 55-55 555~55555555~ 5 565 55555 5 5555 ~5555 0a2 55= - O 55 5. 55 0.55 555555 5 55 55.55555. 55550 5 55 55 -p 05. — E—:— tOs -“ O5550 55555555 ~~00 5o~- -~“000 5—— .55.55.55.55 E A 550 5555550 0055 555555 0.5.> 55.55550. 53555555 75555 55555555 ‘-<“E EE0~ “EQ o 00 (.3000>000000 0.”0 00-j oDE tECE E 0 0002 2 — - $~55 ¡ -¡rl E sg 5 0 7 5 5 O 5 55 5. -5 0 YO E 5555 550 - 70 Eso. - . ‘5 5 .5 55 55-55 .55555 0. ~--.55555551 ~0t-~ 6655’ 55 ~o~oE .tE~5ss # ~ UZa~57EoEEE ~50 ~5535555< 05955<0 55555 ~sEóo 5.~. 55 55 ~ GZ25555 ‘055<55 0s.o5555 055 05 152 V. Baehtadse lish result, [he primary character of those magnetizations has been sup- ported by positive fold tests. Ihe Devonian palaeomagnetic inclinations (Table 2) show a raLher good agreement wiLh Ihe inclinations derived from the mid to late Devo- man reference pole for stable Europe (Table 1), putting Armorica next to Laurentia Lo the East and Baltica Lo the North. Any wide Palaeozoic ocean (cg. mid Lo late Devonian - early Carboniferous, see Johnson, 1974) must therefore he indentified South of the Central Crystalline belt (Moldanu- bian) of Lhe Hercynian orogen. Althought a detailed review of the Palaeozoic palaeomagnetism of Gondwana is beyond Lhe seope of this account, several recent studies of Palaeozoic rocks from Gondwana will be discussed briefiy since their in- terpretation has a direct bearing on the understanding of the tectonic evo- lution of Hercynian Europe.