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403
The Geological Society of America
Memoir 200
2007
Space and time in the tectonic evolution of the northwestern Iberian
Massif: Implications for the Variscan belt
José R. Martínez Catalán
Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain
Ricardo Arenas
Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain
Florentino Díaz García
Departamento de Geología, Universidad de Oviedo, 33005 Oviedo, Spain
Pablo González Cuadra
Área de Geodinámica Externa, Universidad de León, 24007 León, Spain
Juan Gómez-Barreiro
Jacobo Abati
Pedro Castiñeiras
Javier Fernández-Suárez
Sonia Sánchez Martínez
Pilar Andonaegui
Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain
Emilio González Clavijo
Alejandro Díez Montes
Instituto Geológico y Minero de España, Azafranal 48, 37001 Salamanca, Spain
Francisco J. Rubio Pascual
Instituto Geológico y Minero de España, Ríos Rosas 23, 28003 Madrid, Spain
Beatriz Valle Aguado
Departamento de Geociências, Universidade de Aveiro, 3810-193 Aveiro, Portugal
ABSTRACT
Recent advances in geochemical studies of igneous rocks, isotopic age data for
magmatism and metamorphism, quantitative pressure-temperature (P-T) estimates
of metamorphic evolution, and structural geology in the northwestern Iberian Massif
are integrated into a synthesis of the tectonic evolution that places the autochthonous
Martínez Catalán, J.R., Arenas, R., Díaz García, F., González Cuadra, P., Gómez-Barreiro, J., Abati, J., Castiñeiras, P., Fernández-Suárez, J., Sánchez Martínez,
S., Andonaegui, P., González Clavijo, E., Díez Montes, A., Rubio Pascual, F.J., and Valle Aguado, B., 2007, Space and time in the tectonic evolution of the north-
western Iberian Massif: Implications for the Variscan belt, in Hatcher, R.D., Jr., Carlson, M.P., McBride, J.H., and Martínez Catalán, J.R., eds., 4-D Framework of
Continental Crust: Geological Society of America Memoir 200, p. 403–423, doi: 10.1130/2007.1200(21). For permission to copy, contact editing@geosociety.org.
©2007 The Geological Society of America. All rights reserved.
404 Martínez Catalán et al.
and allochthonous terranes in the framework of Paleozoic plate tectonics. Because
northwestern Iberia is free from strike-slip faults of continental scale, it is retrode-
formable and preserves valuable information about the orthogonal component of
convergence of Gondwana with Laurentia and/or Baltica, and the opening and clo-
sure of the Rheic Ocean.
The evolution deduced for northwest Iberia is extended to the rest of the Variscan
belt in an attempt to develop a three-dimensional interpretation that assigns great
importance to the transcurrent components of convergence. Dominant Carbonifer-
ous dextral transpression following large Devonian and Early Carboniferous thrust-
ing and recumbent folding is invoked to explain the complexity of the belt without
requiring a large number of peri-Gondwanan terranes, and its ophiolites and high-
pressure allochthonous units are related to a single oceanic closure.
Palinspastic reconstruction of the Variscan massifs and zones cannot be achieved
without restoration of terrane transport along the colliding plate margins. A sche-
matic reconstruction is proposed that involves postcollisional strike-slip displacement
of ~3000 km between Laurussia and Gondwana during the Carboniferous.
Keywords: Variscan belt, exotic terranes, accretionary history, strike-slip tectonics,
Iberian Massif.
INTRODUCTION
A paleogeographic continental reconstruction for the late
Paleozoic (Fig. 1) shows that three important Paleozoic belts,
the northern Appalachians, the British and Scandinavian Cale-
donides, and the North German–Polish Caledonides, meet rela-
tively close to the NW corner of the Iberian Massif. These belts
mark the collision of three continental masses, Laurentia, Baltica,
and Avalonia, which closed the Iapetus and Tornquist Oceans
(which had opened during the Late Proterozoic and early Paleo-
zoic). Avalonia likely formed close to Gondwana, because of its
fauna (Cocks and Fortey, 1988), and it could also be considered a
part of the Pan-African assemblage. It is viewed as a microconti-
nent or terrane assemblage detached from Gondwana in the Late
Cambrian–Early Ordovician. It drifted away, creating the Rheic
Ocean, and its subsequent closure during the Devonian gave rise
to the Variscan belt in central and western Europe and northern
Africa and to the Alleghanian orogen in North America (Hatcher,
1989, 2002; Winchester et al., 2002; van Staal et al., 1998).
Oceanic closure, which ultimately led to the formation of
Pangea, occurred in several steps and gave rise to three groups
of orogenic episodes, which are themselves diachronous. The
fi rst convergence-related events took place between the Late
Cambrian and the Middle Ordovician. They have been described
in the Scandinavian and British Caledonides (Finnmarkian and
Grampian, respectively) and in the Appalachians (Taconic and
Penobscottian), and they were related to arc-continent collisions
in both sides of the Iapetus realm (Kelling et al., 1985; Hossack
and Cooper, 1986; Stephens and Gee, 1989; van Staal, 2005; van
Staal et al., 1998, this volume).
The second group of events took place between the Early
Silurian and Late Devonian and was induced by the closure of
Iapetus and the collision of Laurentia with Baltica (Scandian) in
the north, and Laurentia with Avalonia in the south (Soper, 1988;
Rey et al., 1997). Details of the latter include the accretion of
Ganderia—an arc formed at the Avalonian side of Iapetus—to
Laurentia in the Early Silurian (Salinic), the collision of Avalo-
nia in the Late Silurian–Early Devonian (Acadian), and subse-
quent dextral transpression during the late Early Devonian–Early
Carboniferous (Neoacadian) (van Staal et al., 1998; Winchester
et al., 2002; van Staal, 2005). Deformation and metamorphism
occurred at the same time in exotic terranes that were later incor-
porated to the Variscan belt. They have been referred to as Lige-
rian (Cogné, 1977; Faure et al., 1997) and Eo-Variscan or early
Variscan events, and they have been described in the Bohemian
Massif (Franke, 2000; Franke and Zelazniewicz, 2002), the
French Massif Central (Santallier et al., 1994), the Armorican
Massif (Ballèvre et al., 1994), the Alps (von Raumer and Neu-
bauer, 1993, 1994), and northwestern Iberia (Gómez Barreiro et
al., 2006, 2007; Fernández-Suárez et al., this volume).
The third group of events spanned the Carboniferous and
Early Permian, and it gave rise to intense deformation in Europe
and Africa (Variscan) and the Appalachians (Alleghanian). These
events resulted from the closure of the Rheic and Theic Oceans
and collision of Gondwana with the previously formed Laurussia
continent (Laurentia, Baltica, and Avalonia; Lefort, 1989).
The Variscan-Appalachian belt is linear but sinuous,
with several oroclinal bends. In Europe (Fig. 2), the belt runs
between two arcuate structures, the Bohemian Massif (Franke
and Zelazniewicz, 2002) and the Iberian-Armorican arc (Bard
et al., 1971; Ribeiro et al., 1995). An additional feature of the
Variscan-Appalachian belt is the extent of transcurrent move-
ments that have been active during most, if not all of the orogeny,
and that have resulted in the terrane dispersion that hinders paleo-
geographic restoration (Gates et al., 1986; Hatcher, 1989, 2002;
Martínez Catalán, 1990; Shelley and Bossière, 2000, 2002).
The northwestern corner of the Iberian Massif includes the
Spanish regions of Galicia and the Cantabrian Mountains, as well
Space and time in the tectonic evolution of the northwestern Iberian Massif 405
as northern Portugal, and it is located at the hinge zone of the
Iberian-Armorican arc (Figs. 2 and 3). Coherent with its setting
inside the Variscan mobile belt, the region preserves relics of one
of the oceanic realms that once separated the early Paleozoic con-
tinents and recorded large amounts of orogenic shortening dur-
ing their amalgamation. Strike-slip faults and shear zones exist
but are not of continental scale, which implies that the Galician–
northern Portugal section, including the Cantabrian Mountains,
is retrodeformable and provides information about the orthogo-
nal components of Gondwana-Laurussia plate convergence.
The northwestern Iberian basement consists of plutonic
and metamorphic rocks with grades ranging from very low to
catazonal, and a clear separation can be established between
autochthonous and allochthonous terranes. The autochthon con-
sists of a thick metasedimentary sequence deposited in northern
Gondwana during the Late Proterozoic and Paleozoic, whereas
the allochthon consists of the remnants of a huge and structur-
ally complex nappe pile preserved in the core of late Variscan
synforms. Both are separated by a thrust sheet, several kilome-
ters thick, consisting of metasediments and volcanics derived
IBERIAN
MASSIF
RHENISH
MASSIF
BOHEMIAN
MASSIF
MASSIF
CENTRAL
ARMORICAN
MASSIF
Alpine
Front
Alpine
Front
NEWFOUNDLAND
BRITISH
ISLES
NOVA
SCOTIA
MOROCCO
SCANDINAVIA
GREENLAND
NORTHERN
APPALACHIANS
ALLOCHTHONOUS TERRA-
NES AND OPHIOLITES
PARAUTOCHTHON /
LOWER ALLOCHTHON
AUTOCHTHON
FORELAND
THRUST BELT
EXTERNAL THRUST BELT
AND FOREDEEP BASIN
VARISCAN BELT
Alpine Front
Variscan and
Alleghanian Fronts
Caledonian and
Appalachian Fronts
Tor nquist Fault
Front
Variscan
Front
Caledonian Front
Front
REGUIBAT
SHIELD
Caledo
nian
Appalachian
?
VARISCAN -
ALLEGHANIAN BELTS
CALEDONIAN -
ACADIAN BELTS
FINNMARKIAN -
GRAMPIAN - TACONIC -
PENOBSCOTTIAN BELTS
GONDWANA
GONDWANA
BALTICA
LAURENTIA
LBM
Fig. 3
500 750 1000 Km
0 250
Figure 1. Sketch showing the position of Iberia in relation to the Appalachian, Caledonian, and Variscan belts at the end of Variscan convergence
(modifi ed from Martínez Catalán et al., 2002). LBM—London-Brabant Massif.
406 Martínez Catalán et al.
Figure 2. Subdivision of the Variscan belt showing the allochthonous terranes and the main transcurrent shear zones. Abbreviations: B—Buçaco; BAOC—Beja-Acebuches
ophiolitic complex; BCSZ—Badajoz-Córdoba shear zone; BF—Black Forest; C—Crozon; CCR—Catalonia Coast Ranges; CIZ—Central Iberian zone; CO—Corsica; CZ—Can-
tabrian zone; ECM—External crystalline massifs of the Alps; GTMZ—Galicia-Trás-os-Montes zone; JPSZ—Juzbado-Penalva shear zone; LC—Lizard Complex; LLF—Layale-
Lubine fault; MF—Midi fault; MGCZ—Mid-German crystalline zone; MM—Maures Massif; MN—Montagne Noire; MT—Moldanubian thrust; MZ—Moldanubian zone;
NASZ—North Armorican shear zone; NEF—Nort-sur-Erdre fault; NPF—North Pyrenean fault; OMZ—Ossa-Morena zone; PTSZ—Porto-Tomar shear zone; PY—Pyrenees;
RHZ—Rhenian-Hercynian zone; S—Sardinia; SASZ—South Armorican shear zone (N and S—northern and southern branches); SH—Sillon Houillier; SPZ—South Portuguese
zone; STA—Silesian terrane assemblage; STZ—Saxo-Thuringian zone; TBZ—Teplá-Barrandian zone; VF—Variscan front; VM—Vosges Massif; WALZ—West Asturian-Le-
onese zone.
Space and time in the tectonic evolution of the northwestern Iberian Massif 407
Figure 3. Geological sketch map of northwestern Iberia, showing the allochthonous complexes and their units. For location, see
Figures 1 and 2. The locations of cross sections in Figure 4 are indicated.
408 Martínez Catalán et al.
from the outer margin of Gondwana (Farias et al., 1987), often
described as a parautochthon (Ribeiro et al., 1990). However,
because stratigraphic continuity with the autochthon is broken
(Valverde-Vaquero et al., 2005), it will be referred to here as the
lower allochthon. The allochthonous terranes, together with the
lower allochthon, are included in the so-called Galicia-Trás-os-
Montes zone (Farias et al., 1987).
TECTONIC SETTING OF ALLOCHTHONOUS
TERRANES
There are three allochthonous complexes in Galicia (Cabo
Ortegal, Órdenes, and Malpica-Tui), and two in northern Portu-
gal (Bragança and Morais). They consist of a pile of allochtho-
nous units characterized by unique lithologic associations and
tectonometamorphic evolution. These units are separated from
each other by faults, either thrusts or extensional detachments
(Figs. 3 and 4). Three groups of allochthonous units can be rec-
ognized from bottom to top in ascending structural order: basal,
ophiolitic, and upper units.
The basal units form a rather continuous thrust sheet consist-
ing of schists and paragneisses alternating with felsic and mafi c
igneous rocks, of which granitic and peralkaline orthogneisses
have yielded Rb-Sr and U-Pb ages of 490–470 Ma (Van Cal-
steren et al., 1979; García Garzón et al., 1981; Santos Zalduegui
et al., 1995). The bimodal, partially alkaline magmatism refl ects
Ordovician rifting (Ribeiro and Floor, 1987; Pin et al., 1992).
Since the basal units are not separated from the lower allochthon
by ophiolites, it is assumed that they were part of Gondwana.
Because the ophiolitic units overlie them, they are viewed as
fragments of the most external edge of the Gondwanan continen-
tal margin. The Early Ordovician magmatism, partly peralkaline,
probably resulted from the drift of a broken-away peri-Gondwa-
nan terrane.
The ophiolitic units crop out discontinuously surrounding
the upper units (Fig. 3) and form part of a formerly continuous
and strongly imbricated nappe stack with at least two different
types of ophiolite (see Arenas et al., this volume; Sánchez Mar-
tínez et al., this volume). Ophiolitic units occupying a relatively
higher structural position represent the basal section of an ophio-
lite sequence that contains serpentinized harzburgitic ultramafi c
rocks, pegmatitic gabbros, and diabase dikes. Their geochemistry
indicates a suprasubduction character, whereas zircons from leu-
cogabbros yield a concordant U-Pb age of 395 Ma (Díaz García
et al., 1999; Pin et al., 2002), providing evidence for oceanic crust
generation, and consumption, in Early Devonian time. The struc-
turally lower ophiolitic units consist of greenschist-facies volca-
nic and plutonic mafi c rocks (greenstones) and metapelites, with
rare felsic orthogneisses, serpentinites, and cherts—all strongly
deformed.
The upper units occupy the core of the allochthonous com-
plexes (Fig. 4) and have been subdivided according to their meta-
morphic evolution into high-pressure (P) and high-temperature
(T) upper units, below, and intermediate-P upper units, above.
Both groups consist of terrigenous metasediments, orthogneisses,
and metabasites, with the additional presence of ultramafi c rocks
in the high-P and high-T upper units. The metabasites include
metagabbros, eclogites, high-P and high-T mafi c granulites, and
amphibolites. The gabbros and orthogneisses have yielded U-Pb
ages around 500 Ma, whereas detrital zircons in the metasedi-
ments indicate a maximum depositional age of 480 Ma for the
uppermost, greenschist-facies metagraywackes (Fernández-
Suárez et al., 2003), and 507 Ma for the structurally lower, high-
P and high-T paragneisses (Schäfer et al., 1993).
Most of the mafi c rocks are metagabbros with tholeiitic
compositions. Their geochemical signature has been compared to
mid-ocean-ridge basalt (MORB) (Gil Ibarguchi et al., 1990) and
related to continental rifting in the case of the high-P and high-T
upper units (Galán and Marcos, 1997), whereas the intermedi-
ate-P upper units have arc-tholeiitic affi nities (Andonaegui et al.,
2002; Castiñeiras, 2003). The additional presence of intermediate
plutonic rocks, such as diorites and tonalites, in the intermediate-
P upper units, reinforces the interpretation that these rocks were
generated in an arc setting. Furthermore, geochemical studies
of the ultramafi c rocks of Cabo Ortegal are consistent with this
hypothesis (Santos et al., 2002).
In spite of the arc affi nities shown by some of them, the
upper units seem to represent a terrane that had drifted away from
Gondwana. The oldest ages obtained from upper intercepts and
inherited zircons from the upper units (between 2.7 and 1.8 Ga;
Kuijper, 1980; Peucat et al., 1990; Dallmeyer and Tucker, 1993;
Schäfer et al., 1993) and the basal units (1.8 Ga; Santos Zalduegui
et al., 1995) are similar to those found in the orthogneisses of the
autochthon (Lancelot et al., 1985; Gebauer, 1993), and they are
also similar to those of the West African craton. These ages point
to a common Gondwanan basement for the upper and basal units
and for the Iberian autochthon. Moreover, detrital zircon ages
have been determined for graywackes from low-grade metasedi-
ments of an intermediate-P upper unit in the Órdenes Complex,
which yielded three age populations of 2.5–2.4 Ga, 2.1–1.9 Ga,
and 610–480 Ma (Fernández-Suárez et al., 2003), which also
record the major events in the West African craton of northern
Gondwana.
On the other hand, Late Cambrian to Early Ordovician
magmatism is widespread, not only in the allochthonous units,
but also in the autochthon, and is a little older in the upper units
(ca. 500 Ma; Dallmeyer and Tucker, 1993; Abati et al., 1999) than
in the basal units or the autochthon (490–470 Ma; Van Calsteren
et al., 1979; García Garzón et al., 1981; Vialette et al., 1987; San-
tos Zalduegui et al., 1995; Gebauer, 1993; Valverde Vaquero and
Dunning, 2000). Magmatism has calc-alkaline and arc affi nities
in the autochthon (Ortega et al., 1996), in some of the allochtho-
nous upper units, and in many granitoids of the allochthonous
basal units, but some granites and mafi c rocks of the latter are
alkaline to peralkaline (Floor, 1966; Pin et al., 1992).
To reconcile the tectonic stability registered by the Early
Ordovician passive-margin sediments of the autochthon (Pérez-
Estaún et al., 1991), the rift-related magmatism of the basal units,
Space and time in the tectonic evolution of the northwestern Iberian Massif 7
Figure 4. Geological sections across central and eastern Galicia and northern Portugal showing the main Variscan structures. Isograds of Variscan regional metamorphism have been drawn
in sections 1, 2, and 3 to show their relationships to folding. See Figure 3 for locations. Sections are based on the following contributions: 1—Marcos et al. (1984) and Arenas (1988);
2—Bastida et al. (1982) and Martínez Catalán et al. (2003); 3—González Lodeiro et al. (1981); 4—Martínez Catalán et al. (2002); 5—Martínez Catalán et al. (2004); 6—Ribeiro (1974)
and González Clavijo and Martínez Catalán (2002).
410 Martínez Catalán et al.
the widely accepted early Paleozoic terrane dispersion in the
peri-Gondwanan realm, and the calc-alkaline and arc affi nities in
the upper units and the autochthon, Valverde Vaquero and Dun-
ning (2000) suggested that the rifting was located in a back-arc
setting behind a subduction zone. This hypothesis is supported
in reconstructions of peri-Gondwanan terranes by Stampfl i et al.
(2002), Winchester et al. (2002), von Raumer et al. (2003), and
van Staal et al. (1998). Rollback of the subducting slab may have
pulled Avalonia apart from Gondwana, and also the terrane par-
tially preserved in the Galician upper allochthonous units. This
terrane was possibly part of a discontinuous continental ribbon
in the eastern continuation of Avalonia (Gómez Barreiro et al.,
2007), and it recorded active-margin arc-related magmatism dur-
ing separation (Fig. 5).
OROGENIC EVOLUTION: THE CROSS-SECTION
VIEW
Early Variscan Accretionary History
In the upper allochthonous units, there is an increase in
metamorphic grade from top to bottom, showing a transition
from the uppermost epizonal units to mesozonal and catazonal
units below. U-Pb dating of metamorphism has yielded two main
age populations (Schäfer et al., 1993; Santos Zalduegui et al.,
1996; Abati et al., 1999; Ordóñez Casado et al., 2001; Fernán-
dez-Suárez et al., 2002a, this volume). The older, dated around
500–490 Ma on monazite, whole zircon grains, and magmatic
domains in zircon, is coeval with widespread magmatism and
probably refl ects high-T and low- to intermediate-P metamor-
phism in the magmatic arc. The younger age population, dated
between 410 and 390 Ma in monazite, zircon, and metamorphic
overgrowths of zircon, is linked to initial Variscan convergence
and high-P and high-T metamorphism. In fact, start of Variscan
convergence should be older, because 410 Ma would be the age
of decompression melting associated with the onset of exhuma-
tion (Fernández-Suárez et al., this volume). Actually, 40Ar/39Ar
data, which give a ca. 425 Ma age for retrogressive amphibolite-
facies foliation in high-P and high-T units, suggest an even older
age for the high-P and high-T metamorphism (Dallmeyer et al.,
1997; Gómez Barreiro, 2004; Gómez Barreiro et al., 2006).
This compressional event of Silurian to Early Devonian
age produced a thick metamorphic pile, and the deep parts reg-
ister pressures of 1.8 GPa or higher (Gil Ibarguchi et al., 1990;
Mendia Aranguren, 2000). Thickening of the upper units and the
subduction of some of them probably refl ect their underthrusting
following accretion to a large continental mass, either Baltica or
Laurentia (Fig. 5). Accretion was followed by retrograde amphib-
olite-facies metamorphism in the lower parts of the accretionary
wedge at 390–375 Ma (Dallmeyer et al., 1991, 1997; Valverde
Vaquero and Fernández, 1996) related to the beginning of exhu-
mation. The fact that units with differences of more than 1.2 GPa
in peak pressure occur presently in a sheet less than 10 km thick
indicates that the original pile has been largely attenuated. Actu-
ally, the different upper units are separated from each other by
extensional faults (Figs. 4 and 5), interpreted as detachments
developed in different stages of their stacking and emplacement
(Martínez Catalán et al., 2002).
The ophiolitic units were stacked in several slices (Díaz
García et al., 1999) and reached metamorphic conditions ranging
between 625 °C and 680 °C and 1.1 and 1.2 GPa. Amphibolite-
facies prograde metamorphism, dated at 390–380 Ma (Dallmeyer
and Gil Ibarguchi, 1990; Dallmeyer et al., 1991, 1997), was coeval
with retrograde metamorphism in the upper units, thus suggest-
ing that underthrusting and imbrication of oceanic lithosphere
caused exhumation of the overlying upper units (Fig. 5).
The basal units record a high-P regional metamorphic event
not found in the lower allochthon or in the autochthon. Peak pres-
sures reached 1.5–1.7 GPa in a west-directed subduction zone (in
present coordinates), as deduced from the pressure gradient along
both limbs of a huge recumbent anticline (Fig. 4, section 4; Are-
nas et al., 1995; Martínez Catalán et al., 1996). Subduction may
have started ca. 380 Ma and ended ca. 365 Ma (Van Calsteren et
al., 1979; Santos Zalduegui et al., 1995; Rodríguez et al., 2003).
Variscan Collisional Deformation: Thrust and Nappe
Tectonics
The structural evolution of the autochthon and lower alloch-
thon is relatively simple, and the structures are related to three
main compressional events that developed during convergence
following collision (Pérez-Estaún et al., 1991). The fi rst event
(D1) produced recumbent folds with east vergence and axial
planar cleavage (S1), which is the oldest penetrative fabric rec-
ognized. Large D1 folds occur in the Mondoñedo nappe (Fig. 4,
section 2): the Mondoñedo-Lugo-Sarria anticline and the Vilaou-
driz syncline (Matte, 1968; Bastida et al., 1986). Their common
overturned limb reaches 15–30 km due to late horizontal shearing
related partly to thrusting and partly to late orogenic extension.
To the west, recumbent folds are common in low-grade areas, but
they are smaller, and their reverse limbs rarely attain 5 km.
The youngest deposits preserved in the core of one of the
recumbent synclines are Early Devonian (Fig. 4, section 5), and
40Ar/39Ar dating of regional S1 cleavage yielded ages of 359 Ma
close to the allochthonous complexes and 336 Ma to the east,
far away from them and adjacent to the Cantabrian zone (Dall-
meyer et al., 1997). When compared with deformation ages in the
allochthonous units, S1 in the autochthon developed immediately
after the greenschist-facies foliation in the structurally lower
ophiolites (363–367 Ma; Dallmeyer et al., 1997) and the end of
subduction-related metamorphism in the basal units (365 Ma;
Rodríguez et al., 2003). It seems that once continental subduction
of the outermost edge of Gondwana became locked, shortening
began in inner parts of its continental platform, giving rise fi rst to
recumbent folds (D1), and then to large thrust sheets, described as
the second deformational event (D2).
Four large thrusts developed in the internal zones of north-
western Iberia (Figs. 4 and 5). The Lalín-Forcarei thrust carried
Space and time in the tectonic evolution of the northwestern Iberian Massif 411
Figure 5. Proposed stages in the tectonic evolution of northwest Iberia. A: Individualization of a peri-Gondwanan terrane by slab rollback, and
arc-related magmatic activity during the Late Cambrian to Early Ordovician. B: Drifting of the peri-Gondwanan terrane away from Gondwana
and spreading of the Rheic Ocean. C: Building of an accretionary wedge by underthrusting and imbrication of the peri-Gondwanan terrane during
the Silurian to Early Devonian. D: Closure of the Rheic Ocean and imbrication of oceanic slices. E: Subduction of outer edge of the Gondwanan
continental margin in the Middle to Late Devonian. F: Thrusting of allochthonous units over the lower allochthon during the Early Carbonifer-
ous. G: Development of out-of-sequence thrusts in the Early to Middle Carboniferous. H: Thrusting in more external parts of the belt during the
Middle Carboniferous. I: Collapse and extension of Gondwanan thickened crust with formation of extensional detachments and domes. J: Late
upright folding, strike-slip faulting, and thin-skinned tectonics in the foreland during the Late Carboniferous. K: Extensional collapse migrates
to the east, probably in response to crustal thickening induced by underthrusting of basement to the west during thin-skinned shortening in the
foreland. Note that W-E coordinates correspond to present. Past coordinates might have varied from NE-SW to NW-SE during the time interval
covered by the cartoon, according to the paleoposition of continental masses in the reconstruction by Winchester et al. (2002). Abbreviations:
CZ—Cantabrian zone; GD—Guitiriz dome; LAT—lower allochthon thrust; LD—Lugo dome; LFT—Lalín-Forcarei thrust; MBT—Mondoñedo
basal thrust; MVSZ—Malpica-Vigo shear zone; OST—out-of-sequence thrusts; PRSZ—Palas de Rei shear zone; PSD—Pico Sacro detachment;
PTSZ—Porto-Tomar shear zone; VF—Viveiro fault.
412 Martínez Catalán et al.
the basal units over the lower allochthon after 346 Ma, which
is the age of migmatization in allochthonous paragneisses (U-
Pb in monazite and rutile; Abati and Dunning, 2002). The S2
cleavage in the underlying lower allochthon, which developed
during emplacement of the Lalín-Forcarei thrust, was dated at
340 Ma (40Ar/39Ar; Dallmeyer et al., 1997). The upper and ophi-
olitic allochthonous units subsequently moved over the basal
units and the lower allochthon, becoming strongly imbricated
along the sole thrust. As they had been previously stacked and
internally imbricated in sequence, the new thrust system devel-
oped out of sequence (Martínez Catalán et al., 2002). The age
of the out-of-sequence thrusts is constrained between 340 Ma,
age of the Lalín-Forcarei thrust, and 323 ± 11 Ma, age of the
Palas de Rei granodiorite (Bellido et al., 1992), or 317 ± 15 Ma,
age of the Espenuca granite (Ortega Cuesta, 1998), both of
which postdate nappe emplacement and predate the Pico Sacro
extensional detachment. A 325 Ma 40Ar/39Ar age obtained by
Dallmeyer et al. (1997) in an ultramylonite from a high-P and
high-T upper unit may be a representative age for the out-of-
sequence thrusts.
The isotopic data are consistent with the age and structural
relationships of synorogenic fl ysch deposits that crop out close
to the eastern boundary of the Bragança Complex in Portugal,
in the Alcañices synform in Zamora, and in a narrow synform
in central Galicia (Fig. 3; Fig. 4, sections 5 and 6). They con-
sist of low-grade slates, graywackes, and conglomerates with
plant debris and metamorphic pebbles (Riemer, 1966; Matte,
1968; Martínez García, 1972; Pérez-Estaún, 1974; Ribeiro and
Ribeiro, 1974) cropping out in imbricates inside and in front of
the lower allochthon thrust sheet (González Clavijo and Mar-
tínez Catalán, 2002; Martínez Catalán et al., 2004). The synoro-
genic deposits are turbiditic and have been dated as Late Devo-
nian (Frasnian) in Portugal, using palynomorphs (Pereira et al.,
1999), and as early Namurian using the age of the youngest
detrital zircon in the Sil synform in central Galicia (Martínez
Catalán et al., 2004). Here, zircon age populations are more
compatible with those of the allochthonous terranes than with
the autochthon because they lack the 1.1–1 Ga Mesoprotero-
zoic population, which is well represented in the autochthonous
succession (Fernández-Suárez et al., 2000b, 2002b; Martínez
Catalán et al., 2004). This suggests that the synorogenic turbi-
dites were deposited in a trough that developed in front of the
allochthonous terranes during their emplacement by thrusting
(González Clavijo and Martínez Catalán, 2002; Martínez Cata-
lán et al., 2004). Their age is older near the Bragança Complex
than in central Galicia, which refl ects the advance of the alloch-
thonous sheet.
The emplacement of the lower allochthon, carrying the
allochthonous terranes piggyback, took place along a nearly
horizontal detachment, the lower allochthon thrust. It has an
apparent displacement of nearly 200 km and is a wonderful
example of thin-skinned tectonics in the hinterland of a col-
lisional belt (Fig. 4, sections 5 and 6). Silurian carbonaceous
slates were the weak layer that accommodated the detachment,
and they became strongly phyllonitized (Farias et al., 1987;
Farias Arquer, 1990; González Clavijo and Martínez Catalán,
2002; Marcos and Llana Fúnez, 2002). The low amplitude of
the previous (D1) recumbent folds left the stratigraphic sequence
nearly undisturbed, allowing the thrust surface to utilize the
graphite-rich Silurian slates. Fan-like imbricates at the lower
allochthon thrust front are well preserved in the Alcañices syn-
form (González Clavijo and Martínez Catalán, 2002). In the Sil
synform, the younger detrital zircon (dated 324 ± 7 Ma) con-
strains the age of the lower allochthon thrust as late Visean–
early Namurian (Martínez Catalán et al., 2004). Its motion was
nearly synchronous with the later out-of-sequence thrusts, or
somewhat younger.
The fourth large fault, the Mondoñedo basal thrust, devel-
oped further east, carrying the large recumbent folds previously
formed and possibly enlarging their amplitude by ductile fl ow
concentrated at its basal shear zone (Bastida et al., 1986; Aller
and Bastida, 1993; Martínez Catalán et al., 2003). The precise
age of the Mondoñedo basal thrust is unknown, but two thrust
faults east of the Mondoñedo basal thrust were dated by Dall-
meyer et al. (1997) at ca. 320 Ma.
Several other minor thrust faults developed to the east of the
Mondoñedo basal thrust, and thrusting became very important
in the Cantabrian zone (Figs. 2 and 5), a thin-skinned foreland
thrust belt where these structures are the main ones responsible
for orogenic shortening (Pérez-Estaún et al., 1988).
Variscan Orogenic Collapse and Late Variscan Folding
Closely following the emplacement of the allochthonous
terranes and thrust imbrication of the autochthon, relatively
deep parts of the crust underwent a temperature increase asso-
ciated in part with decompression. A pervasive subhorizontal
tectonic foliation developed in the middle and lower parts of
the autochthonous section, from the biotite zone down to the
deepest accessible parts of the crust, cropping out in the core of
late-orogenic extensional domes. The foliation is a crenulation
cleavage in the upper parts, but it passes quickly to a schistos-
ity and a gneissose banding downward, in the sillimanite–K-
feldspar zone.
Extension is demonstrated by the thinning and even disap-
pearance of some of the previously metamorphic zones at several
map-scale shear zones, equivalent to ductile detachments, and
by the metamorphic evolution on both sides of them: isobaric
heating at their hanging wall and isothermal decompression at
their footwall (Escuder Viruete et al., 1994; Díez Balda et al.,
1995; Arenas and Martínez Catalán, 2003; Martínez Catalán et
al., 2004). Kinematic criteria demonstrate a noncoaxial compo-
nent of deformation with a sense of shear that varies from one
detachment to other and indicate extension normal, oblique, and
parallel to the orogenic trend.
Extension occurred under the allochthonous terranes, and
also to the east, in the Ollo de Sapo antiform and in the Mon-
doñedo nappe. For instance, the Pico Sacro detachment developed
Space and time in the tectonic evolution of the northwestern Iberian Massif 413
between the Órdenes Complex and the migmatite and granite
assemblage below (Fig. 4, section 4; Fig. 5). The Guitiriz dome
is also an extensional structure, which, as many others, evolved
into a dome (Fig. 4, section 3; Fig. 5).
The Lugo dome (Fig. 4, section 2) developed in the internal
parts of the Mondoñedo nappe, which has footwall units that crop
out in two tectonic windows related to doming. There, internal
extension and two extensional ductile detachments stretched the
nappe and its relative autochthon and are responsible for most
of the stretching undergone by the reverse limb of the two larg-
est recumbent folds (Fig. 5). One of the extensional detachments
affected the footwall unit, whereas the other strongly attenuated
the thrust sheet and evolved into a brittle structure, the Viveiro
fault, which cuts across the whole Mondoñedo nappe and its
footwall unit (Arenas and Martínez Catalán, 2003; Martínez
Catalán et al., 2003). The difference in peak pressure between
both sides of the upper extensional detachment and the Viveiro
fault has been estimated to be 0.4–0.5 GPa (Reche et al., 1998),
roughly equivalent to a subtraction of 15–19 km by the shear
zone and the fault.
One of the main characteristics of the geological map of
northwestern Iberia is the alternation of domes and basins (Mar-
tínez et al., 1988), which in many cases implies crustal-scale
boudinage enhanced by deep crustal fl ow. Heat accumulation
due to crustal thickening and some advection of mantle-derived
rocks (Galán et al., 1996) caused partial melting, lowering the
viscosity of the middle and lower crust, and facilitated viscous
fl ow that accommodated extension of the whole crust, prob-
ably in response to gravitational forces. High-grade autoch-
thonous rocks—the youngest parageneses of which are high-T
and low-P—crop out in the domes, accompanied by abundant
Variscan granitoids, whereas the basins are occupied by low-
grade autochthonous metasediments and, in fi ve cases, by the
remnants of the allochthonous terranes preserved as klippen
(Fig. 3).
Late upright folds are related to the third compressional
event (D3) and are associated with a crenulation cleavage (S3). D3
macrostructures interfere with D1 recumbent folds and are easy
to identify because they fold the regional metamorphic isograds.
Large D3 folds commonly nucleated in previously developed
domes and basins, and they vary from open to tight. Variations
in fl attening are due to heterogeneous strain associated with sub-
vertical, transcurrent ductile shear zones (Iglesias Ponce de León
and Choukroune, 1980).
The main phase of gravitational collapse and extension
occurred between 320 and 310 Ma, which are the ages of many
synkinematic granitoids (Fernández-Suárez et al., 2000a). This part
of collapse is considered intra-orogenic for two reasons. One is that
the extensional domes and basins were overprinted by upright folds
and transcurrent ductile shear zones, as in the case of the Guitiriz
dome (Fig. 4, section 3; Fig. 5). The other is that extension in the
internal zone was followed by shortening in the Cantabrian zone
(Figs. 2 and 5), a thin-skinned foreland thrust belt that developed to
the east between 312 and 300 Ma (Pérez-Estaún et al., 1988).
Upright D3 folds have been dated by synkinematic granit-
oids at 314 ± 6 Ma (Capdevila and Vialette, 1970; Ries, 1979),
whereas strike-slip shear zones closely related to their develop-
ment moved between 315 and 305 Ma (Regêncio Macedo, 1988;
Valle Aguado et al., 2005).
However, the Lugo dome developed later, as demonstrated
by 40Ar/39Ar cooling ages around 300 Ma (Dallmeyer et al.,
1997), and also because the continuation to the south of the
Viveiro fault, which bounds its western fl ank, cuts and deforms
a late-kinematic granodiorite massif with a Rb-Sr age of 286
± 6 Ma (Román-Berdiel et al., 1995; Ortega et al., 2000), which
belongs to the same series as others dated by U-Pb at ca. 295 Ma
(Fernández-Suárez et al., 2000a). Late development of the Lugo
dome was probably a consequence of migration of the extension
to the external zones of the orogen with time (Fig. 5), in the same
way that compressional episodes D1 and D2 had done before, as
shown by the diachronous character of their associated cleavages
(Dallmeyer et al., 1997).
The amount of extension undergone by the orogenic crust
is diffi cult to estimate, but it seems to be very important given
the abundance of extensional detachments and the high strains
associated with them and the accompanying regional fabrics.
Consequently, the apparently huge displacement shown by the
main Variscan thrusts in the allochthonous terranes, including
~200 km for the lower allochthon thrust, may to a large extent be
a consequence of late orogenic extension and would have been
originally much less (Fig. 5).
GLOBAL VIEW: THE THIRD AND FOURTH
DIMENSIONS
The Pieces of the Puzzle
Iberian geology can be correlated with that of central Europe
by comparing its stratigraphic, metamorphic, and magmatic fea-
tures with those of the different zones of the European massifs.
The zoning of the Variscan belt was fi rst established in central
Europe by Kossmat (1927), and in Iberia by Lotze (1945), and
correlations are being continuously updated as more information
becomes available (Bard et al., 1971; Julivert et al., 1972; Toll-
man, 1982; Franke, 1989; Martínez Catalán, 1990; Matte, 2002).
In Figure 2, a correlation has been attempted using a few simple
criteria. The autochthonous Central Iberian zone can be compared
with the central domain of the Armorican Massif in France based on
continuity across the Iberian-Armorican arc and strong stratigraphic
similarities (Robardet et al., 1990; Young, 1990). The Ossa-Morena
zone of southern Iberia is usually correlated with the northern
domain of the Armorican Massif based on the presence of a strong
Cadomian imprint and also on stratigraphic grounds (Cogné, 1974;
Eguíluz et al., 1984, 2000; Chantraine et al., 1994). It is important to
note that the sedimentary and faunal records in Iberia indicate that
these zones were part of the northern Gondwanan shelf, distal in
the case of the Ossa-Morena zone and proximal in the case of the
Central Iberian zone (Robardet and Gutiérrez-Marco, 2004).
414 Martínez Catalán et al.
The correlation can be continued to the Bohemian Mas-
sif based on the presence of Cadomian crust in the Mid-Ger-
man crystalline zone and the Saxo-Thuringian zone, and also
based on the presence of a Cambrian-Ordovician rift sequence
in the latter, which shows a stronger similarity to the Central Ibe-
rian zone than to the Ossa-Morena zone (Franke, 1989, 2000;
Linnemann and Romer, 2002; Linnemann et al., 2003; Robardet
and Gutiérrez-Marco, 2004). The Teplá-Barrandian zone has also
evident affi nities with the autochthonous terranes of northern
Gondwana (Franke, 2000), and it includes some of the best-pre-
served Cadomian basement in Europe. This fact and the Paleo-
zoic succession and faunal similarities (Gutiérrez-Marco et al.,
1999, 2001) suggest a connection with the Iberian Ossa-Morena
zone or with a zone transitional between the Ossa-Morena zone
and the Central Iberian zone.
Exotic terranes with Paleozoic ophiolites, remnants of
Cambrian-Ordovician volcanic arcs, and early Variscan high-P
metamorphism similar in age and evolution to the northwestern
Iberian terranes exist along the whole length of the Variscan belt
(Fig. 2). They occur in the southern domain of the Armorican
Massif (Hanmer, 1977; Marchand, 1981; Balé and Brun, 1986;
Ballèvre et al., 1994), in the French Massif Central (Burg and
Matte, 1978; Girardeau et al., 1994; Ledru et al., 1994a, 1994b),
the Vosges and Black Forest massifs (Wimmenauer and Lim,
1988; Eisbacher et al., 1989; Franke, 1989, 2000), the Saxo-
Thuringian and Moldanubian zones of the Bohemian Massif
(Tollman, 1982; Behr et al., 1982, 1984; Franke, 1989, 2000;
Crowley et al., 2002), and the Polish Sudetes (O’Brien et al.,
1997; Kröner and Hegner, 1998; Timmermann et al., 2000; Alek-
sandrowski and Mazur, 2002; Floyd et al., 2002). Furthermore,
the exotic terranes seem to continue to the south in the External
crystalline massifs of the Alps and the Maures Massif in southern
France, Corsica, and northern Sardinia (Bourrouilh et al., 1980;
Frisch et al., 1984, 1987; Becker et al., 1987; Ménot et al., 1988;
Vauchez and Bufalo, 1988; von Raumer and Neubauer, 1993,
1994; von Raumer et al., 2002).
Finally, the Rhenian-Hercynian zone wraps around the other
zones and can be traced from the Bohemian Massif to the south-
ern British Isles and to southern Iberia, where it is represented
by the South Portuguese zone (Oliveira et al., 1979). The Rhe-
nian-Hercynian zone is an external thrust belt and also a foredeep
basin that developed during the Middle Devonian and Carbon-
iferous, possibly on Avalonian crust adjacent to the developing
Variscan mountain belt.
The correlation shown in Figure 2 by itself does not provide
a straightforward interpretation of the history of terrane evolution,
convergence, and collision. One of the most important problems to
be solved is the paleoposition and origin of the allochthonous ter-
ranes. Considering the paleogeographic continental reconstruction
for the late Paleozoic (Fig. 1), the ophiolites of northwest Iberia and
similar units in central Europe seem to witness an oceanic realm
between Gondwana, represented by the autochthon, and Avalonia,
represented by the London-Brabant Massif and the eastern Appala-
chians (Rast and Skehan, 1983; Williams and Hatcher, 1983).
According to current reconstructions (Scotese, 2001; Win-
chester et al., 2002), that ocean would have been the Rheic. How-
ever, it is unrealistic to postulate that the ca. 500 Ma arc-type
magmatism preserved in the upper allochthonous units occurred
inside the Rheic Ocean when it was beginning to open. Actu-
ally, arc development at that time was widespread in the Iapetus
Ocean, on the northern side of Avalonia (van Staal et al., 1998;
Winchester et al., 2002; van Staal, 2005). Therefore, correlation
of the upper allochthonous units with, for instance, an arc occurring
outboard of the Iapetus margin of Avalonia is more reasonable.
These facts can be reconciled if the upper units are rem-
nants of a peri-Gondwanan continental block that was drifting
at the same time as Avalonia (Gómez Barreiro et al., 2007) or
that was detached from its Iapetus margin. These units would
have registered active-margin magmatism and, later, would have
docked to Laurussia, facing the Rheic Ocean, with Gondwana
at the opposite margin, and without any intervening Avalonian
terrane (Fig. 5). This possibility would imply that Avalonia did
not form a continuous ribbon between Gondwana and Laurussia,
at least, not for its easternmost part. Moreover, it is possible that
the unstable ocean that drove the slab rollback that detached the
upper allochthonous units from Gondwana was not the Iapetus,
but the Tornquist Ocean (Fig. 5).
A major problem in interpreting the Variscan belt is how
many peri-Gondwanan terranes were involved and how many
oceans developed among them. Some interpretations suggest that
nearly every ophiolitic unit represents a suture, so that several
microcontinents, arcs, and oceans were involved (Matte, 1986,
1991, 2002; Franke, 1989, 2000; Franke and Zelazniewicz, 2002).
However, different ophiolites and associated allochthonous units
occur in terranes separated from each other by strike-slip shear
zones, which suggests that different possible sutures could in
fact be the same, repeated by wrench tectonics. The correlation
among the Variscan exotic terranes was explored by Martínez
Catalán (1990), who concluded that all of them could be rem-
nants of a single gigantic, tongue-shaped allochthonous sheet and
that a single ocean might account for all terranes of oceanic affi n-
ity. Although possible on purely geometrical grounds, the tongue
shape of the allochthon seems mechanically unreasonable and
can be replaced by a rather continuous strip along the northern
Gondwana platform if the transpressional character of the orogen
is considered.
Strike-Slip Tectonics
One of the clues for any interpretation of the Variscan belt
resides in the relationship between northwest Iberia and the
Armorican Massif, on both sides of the Iberian-Armorican arc.
There is a close stratigraphic similarity between the Central Ibe-
rian zone and the central Armorican Massif, which has Ordovi-
cian and Devonian sections that are identical in Buçaco (west-
ern Portugal) and Crozon (western Armorican Massif; Henry
et al., 1974; Paris and Robardet, 1977; Robardet et al., 1990;
Young, 1990; Paris, 1998), precluding the possibility that both
Space and time in the tectonic evolution of the northwestern Iberian Massif 415
were separated by an oceanic domain. However, the Central
Iberian zone and the central domain of the Armorican Massif
lie on different sides of the allochthonous terranes (Fig. 2), sug-
gesting that they were separated by a suture, the Massif Central
suture of Matte (1991).
This apparent contradiction may be solved from an Iberian
perspective. In the southern Armorican Massif, the allochtho-
nous terranes occur adjacent to the southern branch of the South
Armorican shear zone, which is considered to overprint the root
there (Ballèvre et al., 1994). However, in Iberia, the allochthonous
terranes overlie the Central Iberian zone, and their root zone lies
outside, to the north, west, or south of the Central Iberian zone. It
seems reasonable that the Armorican terranes are also allochtho-
nous and do not root in the southern domain of the massif, in spite
of the fact that they seem to root there because they have been
overprinted and masked by subvertical shear zones. Therefore, the
suture would be rootless in both domains, and the present terrane
distribution may be a consequence of wrench tectonics.
Shelley and Bossière (2000, 2002) developed the hypothesis
that the terrane collage was essentially due to dextral transpres-
sion induced by sliding of Laurentia along the northern margin
of Gondwana. Their interpretation relies largely on continental
reconstructions by Dalziel et al. (1994) and Dalziel (1997), and in
well-established evidence for pervasive Devonian-Carboniferous
dextral shearing in the Variscan-Appalachian belt (Gates et al.,
1986; Rolet et al., 1994; van Staal and De Roo, 1995; Franke and
Zelazniewicz, 2002; Hatcher, 2002).
Shelley and Bossière were right in stressing the importance
of strike-slip motion, but paid little attention to the orthogonal
component of convergence. However, the importance of orthogo-
nal components is suggested by the subduction and subsequent
exhumation of high-P allochthonous units and by the large dis-
placement of allochthonous terranes in northwest Iberia and in the
Bohemian Massif. Figure 6 is a simplifi ed attempt to incorporate
both orthogonal and transcurrent components into a model for
the development of the Variscides in a way that is geometrically
feasible. It avoids the need to invoke “extra” peri-Gondwanan
terranes and intervening oceans.
Our foundations are the tectonic evolution of northwestern
Iberia, the evidence that its section is retrodeformable, and the
fact that the allochthonous terranes in the Galicia-Trás-os-Mon-
tes zone contain a rootless suture.
Both the Galicia-Trás-os-Montes zone and the Central Ibe-
rian zone are truncated to the west by the Porto-Tomar dextral
shear zone (Ribeiro et al., 1980), which continues into the South
Armorican shear zone (Fig. 2). The Galicia-Trás-os-Montes zone
might root at a possible cryptic suture at the boundary between
the Central Iberian zone and the Ossa-Morena zone, or farther
south, between the latter and the South Portuguese zone, where
a true suture exists.
The fi rst possibility was explored by Simancas et al. (2002),
and it is supported by the presence of a unit comparable to the
basal units of the Galicia-Trás-os-Montes zone in the Badajoz-
Córdoba sinistral shear zone, which represents the Central Iberian
zone–Ossa-Morena zone boundary. However, no ophiolites have
been found here, and it is not clear whether these units actually
root there or are a narrow klippe pinched at the Badajoz-Córdoba
sinistral shear zone. Furthermore, the existence of an oceanic
domain between the Central Iberian zone and the Ossa-Morena
zone in the Paleozoic is not favored by faunal studies (Robardet,
2002, 2003; Robardet and Gutiérrez-Marco, 2004).
Conversely, ophiolites occur in the Beja-Acebuches ophiol-
itic complex, at the Ossa-Morena zone–South Portuguese zone
boundary (Crespo-Blanc, 1991; Fonseca and Ribeiro, 1993;
Quesada et al., 1994; Figueiras et al., 2002). The age of these
ophiolites is unknown, but their position in the Variscan belt is
similar to those of Lizard in south Cornwall (Fig. 2), dated at
390–400 Ma (U-Pb; Clark et al., 1998; Nutman et al., 2001), so
that the Beja-Acebuches ophiolitic complex may be coeval with
and perhaps linked to the upper ophiolites of the Galicia-Trás-os-
Montes zone, dated at 395 Ma (see also Sánchez Martínez et al.,
this volume).
Vergences are opposite in the Beja-Acebuches ophiolitic
complex suture and the Galicia-Trás-os-Montes zone allochtho-
nous terranes: while emplacement of the Galicia-Trás-os-Mon-
tes zone has an eastward component in present coordinates, the
recumbent folds and thrusts in the South Portuguese zone, Beja-
Acebuches ophiolitic complex, and Ossa-Morena zone show
a southwest-directed motion (Silva et al., 1990; Crespo Blanc,
1991; Onézime et al., 2002; Expósito et al., 2002, 2003; Siman-
cas, 2004). This does not necessarily imply that both sutures
represent two different oceans, as the opposite vergences may
indicate a change in subduction polarity along the plate boundary
of a single ocean.
Faunal evidence indicates that the Central Iberian zone and
the Ossa-Morena zone were never separated from each other by
an ocean during the Paleozoic (Robardet, 2002, 2003; Robardet
and Gutiérrez-Marco, 2004), and we assume that both the Beja-
Acebuches ophiolitic complex and the Galicia-Trás-os-Montes
zone sutures represent the closure of the Rheic Ocean by colli-
sion between Gondwana and Laurussia. However, the evolution
of each suture was very different. To the east, closure of the Rheic
Ocean built an accretionary wedge in the active Laurussia mar-
gin, which was later emplaced as a gigantic thrust over the north-
ern Gondwana continental platform (Figs. 5 and 6).
To the west, south-directed subduction of oceanic lithosphere
created the Rhenian-Hercynian zone (Kossmat, 1927), where an
accretionary wedge developed locally during the Early to Middle
Devonian (Silva et al., 1990; Eden and Andrews, 1990; Onézime
et al., 2002). Afterward, terrigenous sediments and volcanics were
deposited in a foredeep basin during the Late Devonian to Middle
Carboniferous (Oliveira, 1990), and they deformed closely fol-
lowing deposition, forming a thin-skinned thrust belt (Franke,
2000; Oncken et al., 2000; Onézime et al., 2002). A transform
fault might have separated the two parts of the Laurussia-Gond-
wana plate boundary with opposite subduction polarities.
We begin our cartoon (Fig. 6) at the Early Carboniferous,
when the Rheic Ocean had been closed, the northern Gondwana
416 Martínez Catalán et al.
Figure 6. Map view of the proposed evolution of the Variscan belt during the Early and Middle Carboniferous, explaining the different alloch-
thonous terranes as remnants of a huge nappe stack thrust onto the northern platform of Gondwana. Compare with Figure 2 to locate the main
massifs and zones and their evolution during strike-slip motion. Note in particular the original neighborhood of Buçaco (B), Crozon (C), and
the Teplá-Barrandian zone (TBZ), characterized by similar Paleozoic successions and closely related faunal assemblages. The inset shows the
clockwise rotation of Laurentia and Baltica, which provided the convergent component of transpression, and the Carboniferous dextral shearing
along the northern margin of Gondwana (after Shelley and Bossière, 2002).
Space and time in the tectonic evolution of the northwestern Iberian Massif 417
platform had recorded the early deformation events related to
collision, and the allochthonous terranes, including ophiolites
and previously subducted rocks, had been obducted onto it. The
complexities of this stage were described in a previous section
and are sketched in Figure 5. The Rhenian-Hercynian basin had
developed already and was the site of synorogenic sedimenta-
tion and deformation. This is the starting point of our model in
the third dimension, which explains how the allochthonous ter-
ranes and the Gondwanan autochthon became subsequently dis-
membered by strike-slip shear zones and faults that produced
~3000 km of dextral displacement, and how the Rhenian-Her-
cynian zone came to be placed to the north of the allochthonous
terranes, duplicating the Rheic suture and producing the double
vergence characteristic of the Variscan belt.
Probably, wrench components did not appear then for the
fi rst time. In fact, dextral transpression has been identifi ed in the
Appalachians since the Early Devonian (van Staal and De Roo,
1995), and Late Devonian–Early Carboniferous dextral displace-
ments have been suggested for the central European Variscides
by Franke (2000) and for the Armorican Massif by Rolet (1994).
It is possible that oblique convergence was active from the begin-
ning of the Variscan cycle, when the Rheic Ocean began to close,
and it may have been responsible for the orogen-parallel stretch-
ing lineations so common in many allochthonous units (Burg,
1981; Quinquis and Choukroune, 1981; Vauchez and Bufalo,
1988; Ribeiro et al., 1990; Llana-Fúnez, 2002).
The Iberian-Armorican arc has been interpreted as an oro-
clinal bend that developed from rigid-plastic indentation during
collision (Matte, 1986). The arc is rather tight when consider-
ing its inner zones, which have a paleogeographic signifi cance
(Lotze, 1945; Julivert et al., 1972, 1980). Conversely, it is more
open when affecting the main dextral wrench system (Porto-
Tomar dextral shear zone–South Armorican shear zone). Even
admitting the existence of a primary open arcuate structure, as
indicated by paleomagnetism (Perroud and Bonhommet, 1981;
Perroud, 1982; Hirt et al., 1992), Figure 2 suggests that oroclinal
bending was coeval with late orogenic stages, and also with the
concomitant wrenching. This may explain the sinistral motion of
shear zones, which limited the Ossa-Morena zone in the southern
branch of the arc, and the formation of fan imbricates and strike-
slip duplexes such as those apparently present in the Armorican
Massif, which could have formed by the master fault cutting
across oroclinal bends (Fig. 6).
The timing of the different steps of the proposed sequence
remains imprecise, but the strike-slip activity is bracketed
between 345 and 300 Ma, the interval of motion of dextral
shear zones in the Armorican Massif (Diot et al., 1983; Peucat
et al., 1984; Rolet, 1994), which is similar to ages in Iberia,
including 342 Ma amphibolite-facies deformation in the Beja-
Acebuches ophiolitic complex (40Ar/39Ar age; Dallmeyer et al.,
1993). It is coeval with or predates sinistral movement, dated at
315–305 Ma for the Porto-Tomar and Juzbado-Penalva shear
zones (Rb-Sr ages, Regêncio Macedo, 1988; U-Pb ages, Valle
Aguado et al., 2005).
When comparing cross-section and map-view evolution, it is
clear that orthogonal shortening and strike slip were active dur-
ing most of the Carboniferous. This may explain repeated thrust-
ing in northwest Iberia (Fig. 5) and perhaps northwest-directed
thrusting of the allochthonous terranes in the Bohemian Massif
(Franke, 1989, 2000; Collins et al., 2000). Thrust tectonics refl ect
the orthogonal component of transpression and tend to hide or
delete the traces of previous along-strike components (Johnston,
2001). Therefore, well-preserved strike-slip structures are those
formed after thrust and nappe tectonics have ceased in a region.
Both orogenic mechanisms refl ect a partition of deformation
during oblique convergence and may act at the same time, and
northwest Iberia is a clear example of such behavior: the Porto-
Tomar and Juzbado-Penalva shear zones (Fig. 2) moved between
315 and 305 Ma (Regêncio Macedo, 1988; Valle Aguado et al.,
2005), when thin-skinned tectonics were active in the Cantabrian
zone (Fig. 5).
CONCLUSIONS
The northwestern Iberian Massif offers a clue to understand-
ing the evolution of the Variscan belt, mainly because strike-slip
structures are subordinate inside the section, which makes it
retrodeformable. A key aspect of the section is the presence of
exotic terranes that form a huge and complex allochthonous sheet
emplaced upon the sequences deposited on the passive margin
of northern Gondwana. The geochemistry of igneous rocks and
isotopic age data show that the exotic terranes include parts of a
peri-Gondwanan terrane that evolved as a Late Cambrian–Early
Ordovician island arc, suprasubduction ophiolites, and pieces
of the outermost edge of the Gondwanan continental margin. A
precise matching between the ages of metamorphic fabrics and
development of large structures has been attempted and the struc-
tures have been interpreted in the context of a two-stage evolu-
tionary model consisting of an early Variscan accretionary stage,
related to the closure of the Rheic Ocean, followed by a Variscan
collisional stage.
Correlation of Iberian allochthonous terranes with those in
central European massifs, and also of strike-slip structures along
the Variscan belt, have resulted in the elaboration of a transpres-
sional tectonic model for the Variscides that avoids the multi-
plication of microcontinents and narrow oceanic domains. The
model explains the whole belt in terms of the closure of a single
ocean, the Rheic, and the subsequent oblique collision between
Gondwana and Laurussia. Overprint of a gigantic allochthonous
sheet by strike-slip shear zones and faults with a total displace-
ment of ~3000 km may account for the apparent multiplicity of
sutures and peri-Gondwanan terranes.
Given the importance of dextral transpression in the Variscan-
Appalachian belt, the paleoposition of Iberia along the Gondwa-
nan margin might have been more to the east than suggested by
late Paleozoic reconstructions. It is possible that northwest Iberia
faced the Tornquist Ocean during the early Paleozoic, that early
Variscan deformation was a consequence of Gondwana-Baltica
418 Martínez Catalán et al.
convergence, and that the early Variscan accretionary prism pre-
served in the allochthonous terranes developed in the southern
margin of Baltica.
ACKNOWLEDGMENTS
This contribution has been funded by the Spanish government
agency Dirección General de Investigación, through projects
BTE2001-0963-C02 and CGL2004-04306-CO2/BTE. The paper
has benefi ted from constructive reviews by R.D. Hatcher Jr., D.T.
Secor, and C.R. van Staal, whose criticisms and suggestions are
kindly acknowledged.
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