GEOLOGICAL HISTORY OF THE MORVAN
EVOLUTION IN WESTERN EUROPE
Besides classical studies involving stratigraphy and tectonics
and run since the begining of this century, news tools allow to
reconstruct the geodynamic
evolution of the Variscan belt :
- Relationships between the petrologic
characters of the magmatic suites and
the geotectonic sites in terms of plate tectonics;
- Radiometric dating of the protoliths and of the various stages of metamorphism;
- Cinematic analysis and chronology of the
tectonic deformations (folds, nappes, faults);
- Knowledge of the deep structures of the continental crust of
Western Europe (deep seismic reflection profiles).
Nevertheless the geodynamic interpretation of
the Variscan orogenic belt remains uncertain, fragments of
oceanic and continental crust disappearing during particular
stages of the orogeny.
The geologic history of the Paleozoic series
cropping out in the Morvan region is a part of the evolution of
the Variscan orogen, which is characterized by a succession of
probably repeated events including :
- Sea floor spreading and opening of oceanic
- Subduction/obuction and oceanic closure;
- Convergence and collision between a North
European continent and a south Gondwana continent;
- Intra-continental shortening.
The presence of microcontinents and subsequent
irregularities on the Gondwana margin are responsible of the
abundance of major strike-slip faults separating the main
structural units or zones that are defined in the Variscan belt.
A such succession of tectonic events and their
related geologic units are encountered in the Paleozoic
formations of the Morvan region :
- A gneissic basement of limited extension
but having a complex tectono-metamorphic history
extending from Cambrian (550 Ma) to Middle Devonian (380
- A belt of volcano-sedimentary
rocks deposited from Upper Devonian to Middle Permian (360 Ma to 280 Ma) and
largely intruded by Early Carboniferous granites.
In the Western Europe and more particularly in
France, five main periods are distinguished in the building of
the Variscan orogenic belt [REF], [REF].
period (550 to 450 Ma): Cambrian to
Under a generalized extension, the North European continent (NE) is separeted
from the Gondwana continent (G) by a large oceanic domain (O), the Rheic
ocean [REF]. Belonging to the
northern margin of Gondwana a small continental block, the
Cadomian block (C), is characterized by the extension of the Armorican
Sandstones of Arenig age. In Germany, the Barrandian block is a
- Between Cadomian block (C) and
Gondwana continent (G) Paleotethys is a shallow water sea with a
continental floor under distension. In the Early Cambrian,
the crust thinning allows the outpourings inside graywacky sediments of tholeiitic magmas (basalt and gabbro) associated with an acidic magmatism. The
various characters observed in the tholeiitic magmas
either of oceanic origin or of crustal extension and its
occasional association with a calc-alkaline
magmatism, are representative of intra-crustal rifts domains of small extension, back-arc basins [REF] which rarely reach
oceanic stage before closure [REF]. In this domain, future Moldanubian zone (Arverno-vosgian
zone in France), magmatic rocks yield radiometric ages
ranging from 500 to 470 Ma (KO).
- Epicontinental sedimentation prevails on
Cadomian block (C) [REF]. After Cambrian infilling of the Brioverian
basement depressions with conglomerate and local
acidic volcanic rocks, the Arenig sandstones outline a generalized trangression of the Ordovician sea.
Frequent lateral variations of rocks facies and thickness exhibited by alternating shales and sandstones are the expressions of tectonic
movements coeval of more intense orogenic deformations
outside of the Cadomian block.
- The Medio-European sea or Rheic ocean
extending between Cadomian block and North-European
continent (NE) opens during this period. This is a sea with
an oceanic crust and is in part the site of the future
Rhenohercynian zone. On the North-European margin,
continental deposits grades southward in the marine
foredeep to a thick series of fine-grained clastic sediments accumulated continuously from
Cambrian to Silurian (K-S). A continental margin
type bimodal submarine
volcanism occurs in the sedimentary series [REF].
Eo-Variscan period (450 to 400 Ma): Plates convergence,
subduction and burying from Late Ordovician to Silurian.
- Existence and location of subduction zones [REF] remain uncertain, but presently only this type
of geodynamic structure
can explain the high-grade metamorphism (high pressure
and relatively high temperature) undergone by series
deposited in the Paleotethys between Cambrian and Early
Ordovician (K-O). Volcano-sedimentary series engaged in the accretion wedge are progessively subducted in the lithospheric mantle (M) by convection currents. Rocks are deformed,
granitised and high grade metamorphosed. Eclogites which characterized the leptyno-amphibolic group indicate pressures comprised between 12
and 20 Kbs and temperature of 750 °C corresponding to a
burying of 60 km inside the lithosphere.
- On Cadomian block (C) [REF] , Ordovician sedimentation grades continuously
to Upper Silurian where appear sea regression facies in
relation with the Ardennian orogenic phase (415 Ma).
- On North-European continental margin (NE) [REF], the Ordovician and Silurian coarse clastic
sedimentation (K-S) grades southward to a very thick series of
shales and quartzo-phyllades down to the oceanic-type floor (O) of the
Rheic ocean. In this Rhenohercynian zone under tectonic
building, geologic formations are progessively deformed
during the Ardennian orogenic phase between 450 and 415
Ma in a system of northward thrusted and faulted folds. A
partial sea regression occurs.
Medio-Variscan period (400 to 380-350 Ma),
continental collision from Early
Devonian to Middle Devonian.
- During this period, the collision between
Gondwana plate and Cadomian block and possible eastern
prolongation in the Mid German Crystalline Rise continue
to deform Moldanubian zone nappes (K-S).
Quick uplift, estimated to 3 or 4 mm/year [REF] of the leptyno-amphibolic complex is responsible of retrograde
metamorphism in amphibolite facies (7 Kbs and 700°C) and
anatectic melting of
acidic rocks (migmatites) radiometricaly dated about 380
Ma [REF]. Mechanism
responsible of the exhumation of previously deeply
subducted rocks is not well understood. The admitted
hypothesis is rapid uplift of the high metamorphic rocks
along slices of underlying granitic crust during plate
convergence. Intense degradation of the reliefs
accompanies montain building; an eroded thickness of 20
km of metamorphic terranes is infered in the Massif
Central [REF].The main structure of the Moldanubian zone and
of the French Massif Central is built during this
collision and is characterized by a pile of two main
gneissic nappes separated by mylonitic thrust planes :
- An upper gneissic unit comprises
a thick series of para-derived gneisses and anatectites, the leptyno-amphibolic group and abondant
eclogites and granulites remnants near the nappe bottom; mesozonal metamorphism ends the nappe deformation.
- A lower gneissic unit is
composed of metamorphosed graywackes and shales (paragneiss)
and intrusions of aluminous granites emplaced between 540
and 430 Ma and orthogneissified. The tectonic setting of the lower gneiss unit
is not well known, possibly overthrusting less
metamorphosed autochthonous Brioverian to Cambrian series.
In the French Massif Central, from Marche to
Monts du Lyonnais, through Haut Allier region, various segments
of these two nappes are southward thrusted. Similar tectonic
structures are disclosed by the Sioule nappes with a possible
northeastern prolongation in Montjeu gneisses that contain
retromorphosed eclogites [REF].
- In Cadomian block [REF] (C) Lower Devonian shales and sandstones succeed
to Silurian similar sediments. Middle and Upper Devonian
shales and limestones disclose a slow sea regression
ended by the Bretonic orogenic phase at 360 Ma that
induces large and gentle folds in series deposited since
- In the Rhenohercynian zone [REF] Lower Devonian phyllades occur in the southern part (K-D),
while a transgression of the sea on the northern part lay
down conglomerates and graywackes. Middle Devonian coral
reef limestones follow the northward sea transgression.
Neo-Variscan period (380 to 300 Ma), from
Late Devonian to Late Carboniferous.
- In the northern part of the Moldubian zone
and in the Morvan region the tectono-metamorphic
evolution ends around 380 Ma (Middle Devonian). The Late
Devonian Brévenne series deposits in a crustal extension area opened in the new crystallophyllian
basement. The volcano-plutonic association of serpentinite, gabbro
and tholeiitic pillow-lava basalts represents an oceanic crust of limited
extension [REF]. The associated acidic volcanism indicates
crustal contamination. This oceanic rift is closed with
the Bretonic orogenic phase (360 Ma). Oceanic crust
fragments are obducted and occur in ophiolitic complex remnants [REF]. The acidic calc-alkaline
volcanism intercalated in Lower Carboniferous clastic
sequences oversteps with an angular unconformity on the
In Vosges and Morvan regions, calc-alkaline
volcanism [REF], [REF] extruded form Late Devonian to Early Carboniferous [REF] may be related to a subduction zone whose the site
remains uncertain, probably located on the southern edge of the
Rhenohercynian zone i.e. the Phyllitic zone [REF].
In South Massif Central, the southward
progradation of the orogenic front is responsible of folding
Lower Visean metamorphic nappes in
Rouergue region and Namurian-Westphalian nappes in Montagne Noire region.
Early and Middle Visean alternating limestones
and conglomerates extend in the area between Brévenne series and
Morvan region (Forez, Montagne Bourbonnaise).
It is inferred that in Morvan region Late
Devonian and possibly Middle Devonian sediments overlap with an
angular unconformity the gneissic basement although the contact
has never been observed.
The geologic history of Morvan region is
detailed in various chapters see : Abstract
- In Cadomian block (C) [REF] a Dinantian conglomeratic transgression occurs on folded
Devonian terranes; clastic deposits contain
intercalations of basic lavas and rhyolitc tuffs. Alternating Tournaisian shales and
volcanic clasts rich sandstones and Visean shales
represent Culm facies. This
Dinantian sedimentation ends with coral reefs limestones
or with Namurian sandy shales deposits. Westphalian sediments
are lacking, the Asturian orogenic phase (300 Ma) locally
induces epizonal to mesozonal metamorphism in folded series. Numerous
intrusions of granodiorite, granite and leucogranite emplaced
during this tectonic event. Then central part of Cadomian
block remains stable, without further hercynian
deformations. Stephanian coal basins and Permian basaltic flows rest horizontally.
- In the Rhenohercynian zone
[REF] the calcareous marine sedimentation continues
to Frasnian, but the Famennian shales and sandstones indicate a sea regression
(K-D) . Remote movements of the Bretonic orogenic
phase can induce local break in the Strunian
sedimentation (uppermost Famennian/lowermost
Carboniferous). To a large sea trangression during
Dinantian times marked by Tournaisian schists and
calcschists succeeds rapidly northward thick limestone
stratas of Middle and Upper Dinantian (Visean). Breccias and local emersions reflect light movements of
the Sudetic orogenic phase widely expressed southward.
There, only a swamp environment occurs, cut by Namurian sea incursions leaving shales and sandstones
deposits arising from the erosion of neighboring reliefs
and then organic matters issued from vegetal cover
forming Westphalian coal measures. Tectonic deformations of
Asturian orogenic phase (300 Ma) intensify previous
recumbent and faulted folds northward overthrusted and
consequently a break in the Stephanian
orogeny part 3