Geol Rundsch (1997) 86 :585 598 Springer-Verlag 1997
ORIGINAL PAPER
J. A. Tait · V. Bachtadse · W. Franke · H. C. Soffel
Geodynamic evolution of the European Variscan fold belt:
palaeomagnetic and geological constraints
Received: 10 June 1996 / Accepted: 18 December 1996
Abstract The Variscan fold belt of Europe resulted Key words Variscan fold belt · Armorica ·
from the collision of Africa, Baltica, Laurentia and the Avalonia · Palaeomagnetism · Palaeogeography
intervening microplates in early Paleozoic times. Over
the past few years, many geological, palaeobiogeo-
graphic and palaeomagnetic studies have led to signifi- Introduction
cant improvements in our understanding of this oro-
genic belt. Whereas it is now fairly well established that
Palaeomagnetic, faunal and geological evidence all es-
Avalonia drifted from the northern margin of Gond- sentially agree that the European sector of the Variscan
wana in Early Ordovician times and collided with
fold belt (VFB) is a collage of Gondwana-derived ter-
Baltica in the late Ordovician/early Silurian, the nature
ranes. These terranes drifted from the northern margin
of the Gondwana derived Armorican microplate is
of Gondwana in early Palaeozoic times, and migrated
more enigmatic. Geological and new palaeomagnetic
northwards to collide with the northern continents. In
data suggest Armorica comprises an assemblage of
recent publications, palaeomagnetists have implied
terranes or microblocks. Palaeobiogeographic data in- that these terranes formed two semi-rigid microplates,
dicate that these terranes had similar drift histories, and
Avalonia and Armorica, whereas geological evidence
the Rheic Ocean separating Avalonia from the Armori- tends towards more mobilistic models in which Ar-
can Terrane Assemblage closed in late Silurian/early
morica and Avalonia themselves are an amalgamation
Devonian times. An early to mid Devonian phase of
of several different terranes. This is mainly due to the
extensional tectonics along this suture zone resulted in
fact that palaeomagnetic data provides information
formation of the relatively narrow Rhenohercynian
on the larger-scale latitudinal and rotational motion of
basin which closed progressively between the late De- the plates, whereas structural and geological evidence
vonian and early Carboniferous. In this contribution,
generally provide smaller-scale details. For example,
we review the constraints provided by palaeomagnetic
the resolution of palaeomagnetic data is such that
data, compare these with geological and palaeobiogeo- separations of the order of up to $500 km cannot be
graphic evidence, and present a sequence of palaeogeo- recognised. Thus, (relatively) small-scale rifting and
graphic reconstructions for these circum-Atlantic
intraplate deformation (where no rotations are
plates and microplates from Ordovician through to
involved) remain unobserved by the palaeomagnetist,
Devonian times.
and the generic term microplate is used in reference to
what the field geologist clearly considers a relatively
loose assemblage of tectonostratigraphic units.
J. A. Tait ( ) · V. Bachtadse · H. C. Soffel
Institut fur Allgemeine und Angewandte Geophysik, Definition of Armorica and Avalonia
¨
Ludwig-Maximilians Universitat, Theresienstrasse 41,
¨
D-80333 Munchen, Germany
¨
The term Armorica was first coined by Van der Voo
Fax: # 89 2394 4205
49
(1979) on the basis of palaeomagnetic studies. He
E-mail: jenny@magbakt.geophysik.uni-muenchen.de
recognised that whereas the early Cambrian segment
W. Franke
of the apparent polar wander (APW) path from the
Institut fur Geowissenschaften und Lithospharenforschung,
¨ ¨
Armorican Massif coincided with that of Gondwana,
Justus-Liebig-Universitat, Senckenbergstrasse 3,
¨
D-35390 Giessen, Germany the late Devonian palaeomagnetic data indicated
586
Palaeozoic times, the implication being that they for-
med a coherent microplate. Whether or not this is valid
has not been fully demonstrated from palaeomagnetic
data, due to the rather sparse data set available for the
European massifs, but faunal similarities indicate that
any separation cannot have been greater than 1500 km
(McKerrow and Cocks 1986). From the geological re-
cord, however, it is clear that Armorica comprised
a number of tectonostratigraphic terranes (Fig. 2), and
thus was a loose assemblage of semi-autonomous ter-
ranes, or even microplates, each of which underwent
similar latitudinal dift histories throughout the early
Palaeozoic. Thus, throughout this paper, the term Ar-
morica is used in a fairly loose sense to refer to the
Armorican Terrane Assemblage (Variscan Europe
south of the Rhenohercynian zone). Discussion of the
validity of this in the light of more recent work will be
given in the final section.
Location of the early Palaeozoic Rheic suture zone
between Avalonia and Armorica has been a matter of
some debate but is thought to be located at the south-
Fig. 1 Map of the present-day orientation and location of the
ern margin of the Northern Phyllite zone (NPZ) of
various lower palaeozoic plates and microplates
central Europe (Fig. 2). Greenschist metamorphism
characterises this zone which is a tectonic collage con-
separation of the Amorican Massif from Gondwana, taining remnants of both southern Avalonia and north-
and proximity to northern Europe. Thus, the term ern Armorica, and comprises Ordovician quartzites,
Armorica was adopted to refer to the various terranes Ordovician/Silurian magmatic arc rocks and mid-up-
with Cadomian basement now contained within the per Devonian clastic sequences (Anderle et al. 1995;
Variscan and Alpine fold belts of Europe. However, Franke and Onken 1995). Geological evidence for the
major faunal distinctions between some of these Gond- re-opening of this suture, and formation of the narrow
wana-derived terranes had already been recognised for Rhenohercynian Ocean, in mid to late Devonian times
a number of years. In the early Palaeozoic reconstruc- is found in the association of MORB-type basalts over-
tion of McKerrow and Ziegler (1972), the Avalon and lain by pelagic sediments, and the presence of De-
southern British terranes were placed adjacent to the vonian/Carboniferous subduction-related volcanics in
northern continents with which they had faunal affin- the mid-German Crystalline High (MGCH) located to
ities, whereas southern Europe remained adjacent to the southeast (Fig. 2). The MGCH represents the
Gondwana. Although there was some evidence in the northern margin of Armorica, which, during closure of
palaeomagnetic data set for this configuration (Thomas both the early Palaeozoic Rheic Ocean, and the later
and Briden 1976), it remained a matter of controversy Rhenohercynian Ocean, represented the northwest
(Perroud et al. 1984). Finally, through detailed driving active plate margin of northern Armorica. The
palaeomagnetic studies Trench and Torsvik (1991) MGCH can be traced westwards by boreholes in the
were able to demonstrate that southern Britain drifted Paris basin and is usually correlated with a Norman-
northwards, independent of the Armorican and Iberian nian High concealed under the British Channel
massifs. This microplate was then called Avalonia, and (Holder and Leveridge 1986). Correlation of tectonic
included coastal New England, New Brunswick, Nova units further south is difficult: in the French/English
Scotia and the Avalon Peninsula of Newfoundland all segment of the Variscan fold belt, the Normannian
of which have Cadomian basement and faunal similar- High is juxtaposed against the Amorican massif. In
ities with southern Britain (Fig. 1). Avalonia also ex- central Europe, the MGCH is bounded to the south by
tends eastward into continental Europe to include the the Saxothuringian basin (Fig. 2). This zone represents
Ardennes and the Rhenohercynian zone of the VFB. a Cambro-Ordovician rift basin, infilled by neritic clas-
Thus, the remaining parts of Armorica (sensu Van der tic sediments and bimodal volcanics, which formed
Voo 1979) then comprised the Ibero-Armorican block between the northern driving edge of Armorica and the
and the Bohemian Massif (Fig. 1). It remained adjacent Tepla-Barrandian/Moldanubian region to the south.
to Gondwana, at least during early Ordovician times, Whether or not the Saxothuringian basin reached
and was separated from Avalonia by the Rheic Ocean
(sensu McKerrow et al. 1991). The name Armorica has
&&&&&&&&&&&&&&&&&&&&&&&
subsequently been used by palaeomagnetists to refer to
Fig. 2 Map of the early Palaeozoic tectonostratigraphic terranes in
these three Variscan massifs throughout early central European segment of the Variscan fold belt
587
588
oceanic status is not clear, but there is evidence for
MORB-type gabbros and ophiolites along the steep
southeastward-dipping shear zone which currently sep-
arates this zone from the Moldanubian and Tepla-
Barrandian. The tectonic units comprising the central
European segment of the Armorican Terrane Assem-
blage thus appear to be more complicated than in the
French and Iberian sectors (see Matte 1991). For cen-
tral Europe, Franke et al. (1995) and Franke and On-
ken (1995) have therefore distinguished N-Armorica
(comprising parts of the MGCH and the continental
foundation of the Saxothuringian basin) from South
Armorica (Tepla-Barrandian), with the narrow inter-
vening Saxothuringian basin being interpreted as a fail-
ed rift. However, as is discussed later, in the light of new
preliminary palaeomagnetic data by Kossler et al.
¨
(1996) this model can no longer be maintained, and the
Saxothuringian must have formed a separate micro-
plate.
The southernmost part of the Armorican Terrane
Assemblage in central Europe is represented by the
Tepla-Barrandian (Fig. 2), whose stratigraphic and tec-
tonic record are similar to those of the Central Armori-
Fig. 3 Palaeogeographic reconstruction for the early Ordovician
can zone in France, and the Moldanubian zone. The
(Tremadoc) using the data of MacNiocall and Smethurst 1994
latter comprises metamorphic sequences and nappe (Laurentia), Bachtadse and Briden 1991 (Gondwana), Torsvik et al.
1992 (Baltica), Trench et al. 1992 (Avalonia) and Tait et al. 1994a
units with high pressure and mantle rocks implying the
(Armorica)
presence of another major ocean, the Moldanubian
Ocean, to the south. This ocean is thought to be corre-
lated with the French Massif Central Ocean and to
have separated southern Armorica from Gondwana, Palaeomagnetic studies from the Welsh basin dem-
and is termed the Massif Central Moldanubian (MCM) onstrate that in the Tremadoc this part of Avalonia
Ocean. was at palaeolatitudes of approximately 60° south
(Fig. 3; McCabe and Channell 1990; Torsvik and
Trench 1991; Trench and Torsvik 1991). Faunal evid-
ence demonstrates, however, that communication be-
Palaeogeographic reconstructions tween Avalonia and Gondwana was still possible.
Given that palaeomagnetic data constrains only the
Early Ordovician palaeolatitudinal position, this connection can be
achieved if the longitudinal position of Avalonia is
The palaeogeographic scenario for early Ordovician changed such that it is in contact with the northern
times is now fairly well established (Fig. 3). Corre- margin of South America (Fig. 3). This scenario is also
spondence and cluster analyses of the various faunal supported by isotopic analyses of Pre-Cambrian-aged
groups clearly show the three-way nature of the zircon crystals which indicate that Avalonia may have
major continental configurations in which Laurentia, originated from the northern margin of South America
¨
Gondwana and Baltica formed faunally and geographi- (A. Kroner, pers. commun.).
cally discrete elements. From palaeomagnetic evidence Calc-alkaline volcanism, and the development of
it is clear that whereas the northern margin of faunas endemic to Avalonia in Llanvirn times, demon-
Gondwana was situated at high, peri-polar latitudes, strates that Avalonia then started to rift from Gond-
Baltica was inverted with respect to its present- wana (Fig. 4) and Armorica, opening up the Rheic
day orientation and was located at intermediate to Ocean (sensu McKerrow 1991) in its wake. The onset of
high palaeolatitudes (Torsvik et al. 1990; Torsvik Avalonia s northward translation and the change from
et al. 1992). At the same time, Laurentia, which passive to active margin is marked by the occurrence
was largely covered by warm-water carbonates, was of calc-alkaline volcanics of late Tremadocian age
straddling the equator (McKerrow et al. 1991; Van der (Cooper et al. 1993; Kokelaar et al. 1984), and sub-
Voo 1993), and essentially remained in this position duction-related magmatism continued until late Or-
with only minor changes throughout the Ordovician dovician times (Pharaoh et al. 1993). Palaeomagnetic
(Cocks and Fortey 1990; MacNiocall and Smethurst data from southern Britain are in agreement with this
1994). scenario and demonstrate a gradual northward drift in
589
Fig. 4 Palaeogeographic reconstruction for the Arenig/Llanvirn us- Fig. 5 Palaeogeographic reconstruction for the late Ordovician us-
ing the data of MacNiocall and Smethurst 1994 (Laurentia), Bach- ing the data of MacNiocall and Smethurst 1994 (Laurentia), Bach-
tadse and Briden 1991 (Gondwana), Torsvik et al. 1992 (Baltica), tadse and Briden 1990 (Gondwana), Torsvik et al. 1992 (Baltica),
Torsvik et al. 1993 (Avalonia) and Tait et al. 1994a (Armorica) Trench et al. 1992 (Avalonia) and Tait et al. 1995 (Armorica)
the Llanvirn, and by the Llandeilo Avalonia was situ- indicators which show a change from the typical
ated at approximately 45° South (Torsvik et al. 1993;
Hirnantia fauna (Kosov province) to the warmer-water
Trench and Torsvik 1991). Avalonia was separated
Edgewood faunas at the Ordovician/Silurian boundary
from Laurentia to the northwest by the southern
(Owen et al. 1991), and lithological indicators such as
Iapetus Ocean, and from Baltica in the northeast by the
the presence of calcareous oolites in the latest Or-
Tornquist Sea.
dovician of the Oslo graben. Development of north-
With regards to Armorica, it is now clear that the
west-directed subduction along the eastern margin of
three constituent massifs (Iberian, Armorican and Bo- Laurentia, and the accretion of the Southern Uplands
hemian) were all situated at high peripolar latitudes
trench deposits in the early Caradoc, indicates gradual
(Fig. 4) in the early Ordovician (Cogne´ 1988; Duff 1979;
closing of the Iapetus Sea between Laurentia and Bal-
Perroud and Bonhommet 1981; Perroud et al. 1986;
tica (McKerrow et al. 1991). Avalonia continued to
Tait et al. 1994a). This is contrary to the model of Krs
drift northwards, above a southerly directed subduc-
et al. (1986, 1987) which, based on palaeomagnetic
tion zone, thus narrowing both the Tornquist and the
evidence, placed the Bohemian Massif at intermediate
southern sector of the Iapetus. By the late Caradoc, the
palaeolatitudes. It has since been shown, however, that
shallow-shelf faunas of Avalonia were similar to those
these data are the result of remagnetisation and incom- of Baltica, and by the Silurian the ostracods were
plete separation of palaeomagnetic vectors, as sugges- identical (Berdan 1990). Subduction-related mag-
ted by Piper (1987) and Van der Voo (1993).
matism along the leading edge of Avalonia stopped in
the Ashgill, and the presence of Ashgill unconformities
and low-grade metamorphism in Silurian sequences of
Late Ordovician/Early Silurian Wales, England and Norway are thought to mark the
time of collision (McKerrow et al. 1991; Pharaoh et al.
Laurentia, Baltica and Avalonia 1993; Roberts 1980; Woodcock 1990). Magmatic activ-
ity is variable along the northern margin of Avalonia.
The palaeoposition of Laurentia changed only margin- In the Welsh basin, volcanism was bimodal with minor
ally and in the late Ordovician was still straddling the calc-alkaline expression, whereas to the east magmatic
equator (Fig. 5). Baltica, however, had drifted north- activity was primarily calc-alkaline, peaking in early
wards while rotating anti-clockwise and was situated at (mid) Caradoc times in west (east) England, and in
more temperate palaeolatitudes (Torsvik et al. 1992). Ashgill times in Belgium (Pharaoh et al. 1993). The
This is also demonstrated by palaeobiogeographic reason for the diachroneity along the Avalonian
590
margin is unclear, but might reflect oblique conver- model also involves unlikely drift velocities for Gond-
gence and/or rotation of the microplate (Oliver et al. wana of the order of 25 cm/years. Detailed discussion
1993; Pharaoh et al. 1993). Thus, from geological and of these two alternative models is beyond the scope of
faunal evidence it is fairly clear that the Tornquist Sea this review, but we adopt the simpler model as the basis
separating Avalonia and Baltica had closed before the for our palaeogeographic reconstructions (Fig. 5). This
start of the Silurian, prior to closure of the Iapetus model is in better agreement with the palaeoclimatic
Ocean between Avalonia and Laurentia. evidence given above, and we refer to Bachtadse and
Palaeomagnetic data provide contrasting models for Briden (1991) and Bachtadse et al. (1995) for detailed
the palaeoposition of Avalonia in Siluro-Ordovician discussion.
times. Two early studies of the Borrowdale volcanics, The Ordovician glaciation was followed by wide-
North England, which are stratigraphically con- spread transgression which caused breakdown of
strained as latest Llanvirn to early Ashgill in age sug- faunal communities and endemism (Brenchley 1984).
gest palaeolatitudes of 15° South (Faller et al. 1977; Although analyses of pandemic faunas are of little use
Piper 1979), placing Avalonia adjacent to Baltica. in determining palaeogeographic boundaries, they can
However, the analytical techniques and selection cri- be used as indicators of latitudinal and climatic vari-
teria employed in these studies do not stand up to ations. Statistical analyses of early Palaeozoic
modern reliability requirements. In a more recent study chitinozoans (Achab et al. 1991) demonstrate that in
of this volcanic complex, Channell and McCabe (1992) early Ordovician times essentially three groups
identified a secondary magnetisation direction which Laurentia, Baltica and Gondwana (including Armorica
indicates palaeolatitudes of 43° South. This remag- and Avalonia) can be identified. Although by late
netisation is thought to be related to late-stage or Ordovician the faunas become more heterogeneous,
syn-volcanic hydrothermal activity and as such is close- the basic subdivision between the three groups can still
ly bracketed with the rock age, i.e. latest Llanvirn to be made. By the Lower Silurian, however, only two
early Ashgill. These results are in good agreement with groups can be clearly identified: the first corresponds to
Siluro-Ordovician data from western Avalonia which, northern Africa, with the second comprising Laurentia,
similarly, indicate palaeolatitudes of approximately 40° Baltica, Avalonia and Armorica.
South (Johnson and Van der Voo 1990). However, With regard to the more endemic faunas, although
assuming relatively constant drift rates, it has been diversities are much lower in the Ashgill, brachiopod
argued that shallower palaeolatitudes are more likely faunas do show a degree of provincialisation with dis-
(Trench et al. 1992), given that Avalonia drifted from tinct latitudinal zonation and climatic variation. Gond-
45° south to 13° South between the Llandeilo and the wana was characterised by the colder water atypical
Wenlock (Torsvik et al. 1993). Taking into account the Hirnantia fauna (Bani Province). Armorica shows
palaeobiogeographic and more recent geological evid- a variation in faunal characteristics, but the variation is
ence, the reconstruction of Trench et al. (1992) is used in of a spatial rather than temporal nature. The sedimen-
Fig. 5, placing Avalonia between 30 and 40° South in tary record on the Spanish Peninsula is characterised
the late Ordovician, close to the southern margin of by the predominance of the cold water Bani province
Baltica. indicating the increased influence of the African ice
sheet as compared with faunas of the Bohemian massif
which are typical Hirnantia faunas of the Kosov Prov-
Armorica and Gondwana ince. This faunal distinction between the Iberian and
Bohemian massifs is not considered to indicate major
The latest Ordovician is characterised by a period of separation of these elements for which there is little
global cooling, and the presence of a large ice sheet geological evidence, but rather a climatic gradient be-
centred over North Africa suggests that the northern tween these two extremities of the Armorican terrane
margin of Gondwana remained at high palaeolatitudes assemblage. From facies studies, Young (1990a) sug-
throughout the Ordovician (Beuf et al. 1971; Crowley gests that the massifs of Armorica were elongated
et al. 1991; Brenchley et al. 1994). north/south. This would allow for a latitudinal range of
Palaeomagnetic data from Gondwana are ambigu- approximately 20° between Iberia and the more north-
ous, and essentially two opposing models for the APW erly Bohemia.
path have been put forward in order to connect the The presence of Ashgillian glacio-marine sediments
well-established cluster of Ordovician palaeomagnetic in the Spanish Peninsula, northern France, and the
poles in northern Africa with the equally well-defined Bohemian Massif could be taken as evidence for some-
position of the Carboniferous south pole to the east of what higher palaeolatitudes. However, these diamic-
southern Africa. Whereas the first model connects these tites were deposited from floating or seasonal ice
two clusters by a direct track, the second model is more (Brenchley et al. 1991) and not from the main ice sheet
complex involving repeated collision between Gond- itself. Recent palaeomagnetic data from the Tepla-
wana and the northern continents, first in early Silurian Barrandian of the Bohemian massif (Tait et al. 1995)
and then again in Carboniferous times. This latter show that this part of Armorica was at intermediate
591
palaeolatitudes in the late Ordovician (Fig. 5). This
confirms the idea of Owen et al. (1991) that the effects of
Ashgillian cooling could be felt at intermediate
palaeolatitudes, thus accommodating the deposition
and development of colder-water facies and faunas in
previously warmer water regions.
There are two sets of apparently contradictory re-
sults from coeval sequences in northern France, which
indicate either intermediate (45° south, Crozon Penin-
sula; Perroud et al. 1983) or high (76° south, Thouars
massif; Perroud and Van der Voo 1985) palaeolati-
tudes. Although the rocks studied by Perroud et al.
(1983) are known to be late Ordovician in age, K/Ar
studies of the dolerite sills yield ages ranging from 190
to 300 m.y. This dispersal is thought to be related to
a complex thermal history which, as mentioned by the
original authors, will certainly have had an effect on the
palaeomagnetic record. This, and the lack of any field
tests with which to constrain the age of magnetisation,
renders the results unreliable. Whereas the intrusive
rocks of the Thouars massif yield an Rb/Sr age of
444$9 Ma (Bernard-Griffiths and Le Me´ tour 1979),
there is again no structural control for the palaeomag-
Fig. 6 Palaeogeographic reconstruction for the Siluro/Devonian
netic results, although the original authors argue that
using the data of MacNiocall and Smethurst 1994 (Laurentia), Van
any major tilt (i.e.'20°) of the complex is unlikely. der Voo 1993 (Gondwana), Douglass 1988 (Baltica), Torsvik et al.
1993 (Avalonia) and Tait et al. 1994b (Armorica)
Thus, from the palaeomagnetic data alone the
palaeoposition of the Amorican Massif is unclear and
the question as to whether it had a similar
(northward directed?) subduction at the southern mar-
palaeolatitudinal drift history to Bohemia remains un-
gin of Avalonia and gradual closure of the Rheic Ocean
answered. Biostratigraphic and lithological indicators,
between Avalonia and Armorica in the Ordovician-
however, show no evidence for any major separation
Silurian (see reviews by Anderle et al. 1995; Franke and
between the Armorican and Bohemian Massifs. The
Onken 1995; Franke et al. 1995).
similarity of Silurian ostracods from these two regions
(Kr\ íz\ and Paris 1982) would make a major oceanic
separation in early Silurian times unlikely unless ex-
tremely high drift rates were involved. Until further Late Silurian/Early Devonian
palaeomagnetic data are forthcoming, the palaeolati-
tudes obtained for the Bohemian Massif are considered By latest Silurian times, Baltica had reached an equato-
representative for the Armorican Terrane Assemblage rial position and was in its present-day orientation
and are used for the palaeogeographic reconstruction (Fig. 6). Final closure of the northern Iapetus Ocean
given in Fig. 5. between Laurentia and Baltica (Scandian orogeny) was
The reconstruction given in Fig. 5 clearly illustrates essentially of a longitudinal nature, and timing of defor-
that for the late Ordovician Armorica (or at least the mation during the Siluro-Devonian relies principally
Bohemian Massif) was separated from the northern on biostratigraphic control and isotopic dating. Defor-
margin of Gondwana by an ocean of approximately mation related to the Scandian orogeny was polyphase
3000 km width (the Massif Central Moldanubian in Scandinavia and concentrated in the time range late
Ocean). At some time between Llanvirn and Caradoc Llandovery to early Devonian (Roberts 1988). The first
times, it rifted from Gondwana and started to drift appearance of Laurentian-derived sediments occurred
northwards towards the northern continents. This is in in southern Norway and Sweden in the late Llandovery
marked contrast to previously published palaeogeo- (Bassett 1985), and the main period of nappe emplace-
graphic models which have generally assumed that ment and deformation along the eastern margin of the
Armorica remained adjacent to the northern margin of orogen has been dated as mid Silurian. In Scotland,
Gondwana. Evidence for gradual closure of the Rheic calc-alkaline volcanism related to subduction on the
Ocean between Avalonia and Armorica is found in the northwest margin of the Iapetus Ocean continued into
presence of upper Ordovician-Silurian calc-alkaline the Lower Devonian (Smith 1995; Thirlwall 1988), sub-
rocks in the southern Taunus segment of the NPZ duction ceased in early Devonian times and collision is
(Sommermann et al. 1990, 1992). These have been inter- marked by late Silurian deformation. In the Ap-
preted as remnants of an island arc, and thus testify to palachians, subduction below Laurentia continued into
592
the Devonian as Avalonia swung in towards the Franke et al. 1995). Collision is also constrained by
Laurentian margin (McKerrow et al. 1991). early Famennian flysch greywackes encountered in the
In central Europe, the Rheic Ocean separating Ava- southernmost part of the Saxothuringian basin, which
lonia from Armorica was also closing, only to be were deposited on Saxothuringian continental base-
opened again in early-mid Devonian times with devel- ment. This indicates that any oceanic separation be-
opment of the Rhenohercynian basin (Franke and On- tween the Tepla-Barrandian source area and the
ken 1995). Palaeomagnetic data indicates that by the Saxothuringian foreland had disappeared by that time
late Silurian Bohemia had reached fairly shallow (Franke and Engel 1986). The Famennian flysch con-
¨
palaeolatitudes (ca. 30° south; Fig. 6), and was thus tains detrital zircon dated at 380 Ma (Schafer and Dörr
¨
adjacent to the southern margin of Baltica/Avalonia 1994; Dorr et al. 1989). This leaves no doubt that the
(Tait et al. 1994b; Channell et al. 1992). Two other continental crust of the Tepla-Barrandian and the
Siluro-Devonian palaeopoles are available for Ar- Saxothuringian had made contact before the Famen-
morica, from the San Pedro redbeds in Asturia, Spain nian, probably in the late middle Devonian (ca. 380 Ma).
(Perroud and Bonhommet 1984), and from Almaden in As discussed above, the position of Gondwana in
southern Spain (Perroud et al. 1991). Whereas it has Silurian times remains controversial. Adopting the
subsequently been demonstrated that the Almaden more complex APW path implies closure of a wide
rocks have been remagnetised, the San Pedro results ocean between Baltica/Laurentia and Gondwana by
indicate palaeolatitudes of approximately 20° south for the early Silurian, with subsequent re-opening of this
Iberia. Within the error limits, this is in fairly good ocean during the late Silurian/early Devonian. From
agreement with the Bohemian data. However, as the late Devonian onward, this ocean then closed again
pointed out by the original authors the age of magnet- and the classic Pangea assemblage was established in
isation in the San Pedro sediments is fairly loosely the Carboniferous. However, the more conservative,
constrained and could have been acquired anywhere and preferred, APW path places the northern margin of
between the Siluro-Devonian and early Carboniferous. Gondwana at more intermediate palaeolatitudes, with
Nevertheless, the results indicate that the ocean sep- the presence of a late Silurian ocean of approximately
arating Armorica from the northern continents had 2500 km width between Gondwana and the northern
closed, or had at least been reduced below the continents (Massif Central Moldanubian Ocean).
palaeomagnetic detection limit, by this time (Fig. 6). There is evidence for Silurian oceanic crust in the
This scenario is in good agreement with palaeobiogeo- southeastern part of the Moldanubian (Finger and
graphic indicators which show a complete interchange Quadt 1995), and in the Sudetes (circum Sowie-Gory
of Armorican and Baltic faunas by the late Silurian, ophiolite; Oliver et al. 1993) which is compatible with
and the first warm-water carbonates appeared in the the more conservative APW path. Furthermore, evid-
Tepla-Barrandian in late Silurian/early Devonian ence for a Silurian age collisional event has never been
times, and in the Iberian and Amorican Massifs in the detected anywhere to the south of the Tepla-Barran-
early Devonian. An important feature to be recognised dian. All petrological and isotopic evidence available
is that all recently published palaeomagnetic data from suggest that the allochthonous Moldanubian
the early Palaeozoic of Bohemia place the Tepla-Bar- granulites and associated high-grade rocks were for-
randian in an inverted position with respect to its med around 340 Ma (Medaris et al. 1995).
present-day orientation, indicating a post-Silurian anti-
clockwise rotation of up to 140°. Although, as discussed
previously, reliable palaeomagnetic data from the Ar-
morican and Iberian Massifs are lacking, it is con- Late Devonian/Carboniferous
sidered unlikey that the rotation relates to the Armori-
can assemblage as a whole. Recent palaeomagnetic Whereas the major oceans separating the various
results from Silurian carbonate rocks of the plates now comprising Europe had closed by the De-
Saxothuringian basin (Kossler et al. 1996) confirm this vonian, final consolidation into the present-day config-
¨
and demonstrate that this region did not take part in uration of Variscan Europe did not occur until some
the post-Silurian rotation, thus restricting it to a more time later. From geological evidence it is clear that this
local level involving the Tepla-Barrandian. period was a time of large-scale shearing, transpres-
Palaeomagnetically this rotation is constrained to sional movement with the opening of small oceanic
have occurred sometime between the early Devonian basins (Franke 1989; Matte 1991; Quesada 1991).
and late Carboniferous, and stricter constraints are Palaeomagnetic data further demonstrate that there
available from the geological record. Zircon, horn- was large-scale differential rotation and deformation
blende and mica ages from metamorphic rocks relating throughout the belt (Hirt et al. 1992; Ries and Shack-
to the suture zone cluster around 380 370 Ma, which leton 1976; Tait et al. 1994b).
indicates rapid uplift and cooling, probably due to Palaeomagnetic data for the Devonian of Baltica
collision of the Tepla-Barrandian and the Saxothurin- are sparse. Early Devonian poles from the Ukraine
gian (i.e. between northern and southern Armorica; and Spitsbergen (Smethurst and Khramov 1992)
593
wards the south (Floyd 1984; Thirlwall 1988). This
basin probably extended eastwards into the Rhenoher-
cynian zone of Germany, where thick accumulations of
Devonian shelf and haemipelagic sediments associated
with bimodal volcanism, MORB-type mafic rocks,
clearly suggest extension and opening of the Rhenoher-
cynian basin along the suture zone of the former Rheic
Ocean. This oceanic basin must have remained fairly
limited, however, as it cannot be recognised in the
palaeomagnetic record. The onset of convergence is
documented by the late Devonian (Frasnian/Famen-
nian) flysch and greywacke deposition, and foreland
basin-type sedimentation throughout the entire
Rhenohercynian belt, from Portugal to Moravia
(Franke and Engel 1986).
One of the most pronounced features of the Euro-
pean Variscan fold belt is that of the Ibero-Armorican
arc. Ries and Shackleton (1976) concluded from strain
analyses that this arc is similar to a buckle fold with
tangential longitudinal strain, and hence is secondary
in nature. Previously, however, Julivert (1971) and Jul-
ivert and Marcos (1973) had demonstrated that the arc
was to some extent primary in nature, but had under-
Fig. 7 Palaeogeographic reconstruction for Devonian using the
gone secondary tightening and development of radial
data of MacNiocall and Smethurst 1994 (Laurentia), Van der Voo
folds during the Variscan orogeny (late Carboniferous).
1993 (Gondwana), Torsvik et al. 1992 (Baltica), Trench et al. 1992
(Avalonia) and Bachtadse et al. 1983 (Armorica)
This model was subsequently supported by Julivert and
Arboleya (1984, 1986) and Pe´ rez-Estaun (1988) from
´
studies of kinematic indicators and thrust sheet em-
demonstrate an equatorial position for Baltica, but for placement mechanisms. The third model considers cur-
mid and late Devonian times there are very few reliable vature of the arc as resulting from indentation of a rigid
palaeomagnetic poles for Baltica as most results have block, either as a separate microplate (Lorenz 1976;
been shown to comprise overprints (Torsvik et al. Lorenz and Nicholls 1984; Riding 1974) or collision of
1990). Nevertheless, it is clear that by this time the irregularly shaped continents, i.e. indentation of an
oceans separating Baltica from Laurentia, Avalonia African promontory (Julivert and Martinez 1987,
and Armorica had all closed (McKerrow et al. 1991; Matte and Ribeiro 1975). Several palaeomagnetic stud-
Torsvik et al. 1990; Torsvik et al. 1993; Van der Voo ies have been carried out in the Asturian arc, and in
1993) thus forming the Old Red Continent (Fig. 7). most the declination deviation follows the shape of the
The palaeomagnetic data set for Armorica and Ava- arc, indicating the secondary nature of the orocline
lonia during the late Devonian is rather sparse. Per- (Bonhommet et al. 1981; Hirt et al. 1992; Lowrie and
roud et al. (1985) have shown that the data of Jones Hirt 1986; Perroud 1983; Ries et al. 1980; Ries and
et al. (1979) from Normandy fail the fold test, and only Shackleton 1976).
two pole positions from the Central European Harz Plotting the European Carboniferous and Devonian
Mountains and the Franconian Forest (Bachtadse et al. declination data on the map, a systematic pattern of
1983) are believed to represent primary magnetisations. declination deviation becomes apparent, which is most
The resulting palaeolatitudes of 13° south indicate the pronounced around the Bay of Biscay, the Ibero-Ar-
proximity of Armorica to Baltica (Fig. 7) and the clos- morican arc. These deviations have been identified as
ure of any intervening ocean. being a direct function of the change in general strike
An interesting feature of the European Variscides is along the northern margin of the Variscan mountain
that during this period of overall convergence and belt as discussed by Bachtadse and Van der Voo (1986)
compression, there was a phase of extensional tectonics and Perroud (1986) and Bachtadse (1990). One possible
and basin development in the Rhenohercynian zone interpretation of this declination pattern is that it re-
(Behr et al. 1984; Engel et al. 1983; Floyd 1995). In flects large-scale thrust rotations (especially around the
Germany, lower-Devonian acid volcanics are followed Ibero-Armorican arc, the Rhenish massif and the Ar-
by mid-Devonian intra-plate continental tholeiites, dennes) and/or pervasive strike-slip tectonics in the
whereas in southwest England basic and acid volcanics more internal parts of the orogen (Harz Mountains,
occur throughout the Devonian. These suites in south- Franconian Forest, Massif Central). Indentation of the
west England have been interpreted as a rifted passive irregularly shaped northern margin of Gondwana into
continental margin which became truly oceanic to- Europe during the final stages of suturing between
594
Gondwana the northern continents is one possible ex- results of Hailwood (1974) has been questioned repeat-
planation for this deformation pattern (Bachtadse and edly, mainly on the basis of palaeoclimatological
Van der Voo 1986). This interpretation is supported in (Scotese and Barrett 1990; Wendt 1985) and radiomet-
the geological record, by the presence of the South- ric evidence (Salmon et al. 1986). However, more re-
Armorican and Badajoz-Cordoba shear zones, whose cently high-quality palaeomagnetic data for the late
respective dextral and sinistral displacements fit the Devonian of cratonic Australia (Hurley and Van der
general model (see also Matte 1986). Voo 1987), northern (Bachtadse and Briden 1991) and
Combining all the palaeomagnetic data available for southern Africa (Bachtadse et al. 1987) are in strong
the Ibero-Armorican arc demonstrates that there is support of the central African pole position. When the
a strong correlation between change in strike and decli- palaeolatitudes for the northern coast of Africa, based
nation (Bachtadse and Van der Voo 1986; Eldredge on this pole, are compared with the palaeolatitudes for
et al. 1985; Tait et al. 1996). However, it is not a 1 : 1 the southern margin of the Old Red Continent (Fig. 7),
correlation as would be expected for perfect oroclinal an ocean of up to 4500-km width is indicated for mid-
bending, indicating that the arc had a primary curva- Devonian times. Palaeomagnetic evidence shows that
ture, as originally proposed by Julivert (1971), prior to by the late Devonian this ocean had closed somewhat,
tightening during the latter stages of the Variscan oro- and collision is considered to have been initiated in the
geny. Furthermore, in the most recent palaeomagnetic early Carboniferous. The evidence for this ocean from
study by Hirt et al.(1992), the authors demonstrate that palaeobiogeographic data is ambiguous. Evidence
the vertical axis rotations observed in the thrust sheets against the presence of an ocean is to be found in the
are most compatible with the indentation model. Such Emsian rugose corals of North Africa which bear
a model was first proposed by Riding (1974) who sug- strong similarities to those of Armorica and Avalonia,
gested that northern Spain, comprising the Cantabrian, suggesting that there was no major barrier between
Pyrenean and Montagne Noire regions, formed either these regions (Pedder and Oliver 1990). The presence of
an extension of Africa or a separate microplate prior to reef carbonates in North Africa may also be taken as
Westphalian collision. This model has subsequently evidence that the northern margin of Gondwana was at
gained support from the geological record (Feist and fairly shallow palaeolatitudes ((45° south). On the
Echtler 1990; Julivert and Martinez 1987), but data are other hand, climates in the mid to late Devonian were
still sparse due to the tectonically complex nature of exceptionally warm and reef habitats widespread (Cop-
these regions. per 1986). Positive support for the ocean comes from
Oroclinal bending is also a major feature in the analysis of Devonian vertebrate faunas (Young 1990b).
eastern extremity of the Variscan fold belt as demon- In early to mid Devonian times, Gondwana had a ver-
strated by Tait et al. (1996). Geological similarities tebrate fauna distinct from that of Eurasia and Amer-
between the Moravo-Silesian zone to the southeast of ica, but by the late Devonian there was faunal
the Bohemian massif and the Rhenohercynian of cen- communication between these areas. It has also been
tral Europe have long been recognised, but it was suggested that closing of the ocean between the Old
unclear whether the present configuration was a pri- Red Continent and Africa in the late Devonian was one
mary or secondary feature (Burchette 1981; Franke of the main causes for the late Devonian (Frasnian/
1989). However, the new palaeomagnetic data demon- Fammenian) faunal crisis (Copper 1986). Many geol-
strate a close correlation between declination and ogists, however, argue further that the Silurian to Car-
strike deviation and hence the secondary nature of the boniferous metamorphism, and general south- or
arc. The mechanism involved to produce the curvature southeast-directed tectonic transport in the Massif
of the Rhenohercynian zone around the northeastern Central and South Armorican domain, are indicative of
flanks of the Bohemian massif may again be one of a prolonged subduction/collision story between Ar-
indentor tectonics. Geological evidence for this, how- morica and a southern opponent Gondwana. Thus,
ever, is not so clear. Whereas the dextral transpressive the problem with regard to the relationship between
Moldanubian thrust flanks the southeastern margin of Gondwana and the northern continents remains
the Bohemian massif, there is little evidence for sinistral enigmatic and a matter of debate between many
shear along the northwest flank. All the major shear palaeomagnetists and geologists.
zones to the north of the Moldanubian s.st.(southern
and northern margin of the Tepla-Barrandian, north-
ern margin of the MGCH) have dextral displacements. Summary
Tectonic rotation of the Moravo-Silesian zone, there-
fore, remains problematic. Avalonia and the Armorican Terrane Assemblage are
The position of Gondwana in the Devonian is again characterised by Cadomian basement, derived from the
controversial, with major apparent discrepancies northern margin of Gondwana either North Africa
between the palaeomagnetic and palaeoclimatological (Armorica) or, possibly, South America (Avalonia),
indicators. The central African location of the late as indicated by zircon ages. Palaeomagnetic data, the
Devonian palaeo-south pole which was based on the onset of calc-alkaline magmatism in Avalonia in the
595
Tremadoc and development of endemic faunas all indi- The other main unanswered questions regard the
cate that Avalonia rifted from Gondwana in the early drift history of Gondwana, and whether or not there
Ordovician and started to move northwards, with sub- was an ocean separating Gondwana from the northern
duction of the intervening Tornquist Sea along a south- continents in mid-late Devonian times. Additional
ward-dipping subduction zone. Palaeomagnetic data problems not addressed in this paper regard the south-
for Armorica (Bohemian Massif) indicates by the late ern flanks of the Variscan fold belt. Several lower
Ordovician Armorica had also drifted northwards, and Palaeozoic sequences crop out along the southern mar-
away from Gondwana. The timing of rifting, however, gin of Europe, e.g. the central Pyrenees, Montagne
is not clear. The oldest known calc-alkaline rocks re- Noire, Greywacke zone, Carnic Alps and the Karawan-
lated to closure of the Rheic between Avalonia and ken. However, these sequences suffered varying degrees
Armorica are Upper Ordovician in age. By early Silur- of deformation during the Alpine orogeny, making
ian times, Avalonia had collided with Baltica, and the determination of their tectonic affinities from structural
Iapetus Ocean between Laurentia and Baltica had nar- and stratigraphic analysis alone complicated. Although
rowed considerably. Closure of this longitudinal ocean palaeomagnetic studies of these sequences are also
was diachronous. Deformation in Scandinavia peaked problematic, preliminary results from the greywacke
in mid-Silurian times (Roberts 1988), in Scotland colli- zone of the eastern Alps have yielded encouraging
¨
sion is marked by Upper Silurian deformation, and results (Schatz et al. 1996), and it is hoped that con-
calc-alkaline volcanism continued into the Lower tinued research of these massifs over the coming years
Devonian (Smith 1995; Thirlwall 1988). In the Ap- will provide further insight and constraints on the plate
palachians, subduction below Laurentia continued into tectonic affinities of these regions.
the Devonian as Avalonia moved in towards the
Laurentian margin (McKerrow et al. 1991).
Acknowledgements This project was supported by the German Re-
The Armorican Terrane Assemblage continued to
search Council (DFG), project numbers So72/52 3 and Ta193/1 2,
and is a contribution towards the Special Research Project Oro-
drift northwards during the Silurian, with closure of the
genic Processes funded by the DFG. Many thanks also to Profes-
Rheic Ocean between Avalonia and Armorica in late
sors Weber and Torsvik for their helpful comments on the manu-
Silurian times. This suture was to reopen in the early
script.
Devonian with formation of a relatively narrow rift
generated Rhenohercynian basin that was sufficiently
attenuated in its internal zone to produce oceanic crust
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