doi: 10.1111/j.1365-3121.2007.00770.x
The Hospitalet gneiss dome (Pyrenees) revisited: lateral flow
during Variscan transpression in the middle crust
Yoann Denèle,1 Philippe Olivier,1 Gérard Gleizes1 and Pierre Barbey2
1
LMTG-Université de Toulouse-CNRS-IRD-OMP, 14 Avenue E. Belin, 31400 Toulouse; 2CRPG-CNRS, Universite´ de Nancy, BP 20, 54501
Vandœuvre-le`s-Nancy, France
ABSTRACT
A new structural and kinematic study of the Hospitalet dome
(Pyrenees) is presented. This dome corresponds to the eastern
half of an EW-trending antiformal structure made of an
orthogneissic core intruded by granitoids, partly covered by
Upper Proterozoic to Lower Ordovician metapelites. Its Variscan
evolution can be split into four stages: (i) development of a
strong high temperature pervasive deformation associated with
subhorizontal foliations and lineations, and with non-coaxial
top-to-the-east kinematics; (ii) formation of a south-verging
overturned megafold; (iii) emplacement of calc-alkaline grani-
Introduction
Gneiss domes are common tectonic
structures in exhumed orogens since
Archaean times. The first interpretations of dome formation favoured
diapirism (Eskola, 1949). During the
1980–1990s, the importance of the
tectonic regime was emphasized, first
in compression (Burg et al., 1984;
Amato et al., 1994) or in extension
for the metamorphic core complexes
(Davis and Lister, 1988; Chen et al.,
1990), and recently, in transpression
(Allen et al., 2001; Valentino et al.,
2004). These studies show why the
formation of domes is still a subject of
debate, their interpretation being frequently limited by the difficulty to link
their thermal and tectonic histories
(Whitney et al., 2004; Yin, 2004).
The Pyrenees afford several examples of gneiss domes that developed in
the middle crust (Fig. 1a) during the
late Variscan orogeny (320–300 Ma).
They form the ÔinfrastructureÕ (Zwart,
1979) around which a high-T (HT)
and low-P (LP) metamorphism developed in the amphibolite facies (Barnolas and Chiron, 1996). These domes
are characterized by shallowly dipping
foliations. In-between the domes, crop
Correspondence: Dr Yoann Denèle,
LMTG, Université de Toulouse, CNRSIRD-OMP, 14 Avenue E. Belin, Toulouse
31400, France. Tel.: +33 5 61 33 26 62;
fax: +33 5 61 33 25 60; e-mail: denele@
lmtg.obs-mip.fr
2007 Blackwell Publishing Ltd
toids; and (iv) formation of mylonitic bands on the southern
border of the dome, with reverse dextral kinematics. The flat
lying pervasive high-T deformation is interpreted as a large
lateral flow developed in a dextral transpressive regime
inducing an important uncoupling between the middle and
upper crusts. The next stages happened in a progressive
deformation in the same transpressive regime.
Terra Nova, 19, 445–453, 2007
out Palaeozoic low-grade metasediments characterized by steep foliations forming the ÔsuperstructureÕ
(Zwart, 1979) intruded by numerous
calc-alkaline plutons.
The structural and chronological
relationships between the infra- and
superstructure are still poorly understood. Contrasting interpretations
were proposed: (i) for Zwart (1979),
the deformations of the infrastructure
and the superstructure are coeval, the
former recording a flat shearing and
the latter a N–S shortening; (ii) for
Verhoef et al. (1984), the structures of
the infra- and superstructure are not
coeval, the doming of the gneisses
being superimposed onto a vertical
cleavage; (iii) for van den Eeckhout
and Zwart (1988) and Vissers (1992),
the domes result from a late extension
dominated by shearing and subhorizontal EW-trending lineations; (iv) for
Soula et al. (1986), the domes were
formed diapirically during an overall
N–S shortening; and (v) for Carreras
and Capella (1994), there is a gradual
evolution from an early compressive
crustal shortening producing a flatlying foliation in the deep levels, to a
late transpressive event responsible for
the upright folding of the upper levels
and for the dome formation.
More recently, several studies were
performed in the superstructure of the
Pyrenees, which demonstrated that
the structures of the metasediments
(Evans et al., 1997) and the emplacement of the calc-alkaline plutons
(Druguet and Hutton, 1998; Gleizes
et al., 1998, 2001; Auréjac et al., 2004)
were related to a late Variscan dextral
transpressive phase. Moreover, U ⁄ Pb
zircons datings on different plutons
yielded emplacement ages during the
312–305 Ma time span (Roberts et al.,
2000; Maurel et al., 2004; Gleizes
et al., 2006). However, the formation
of the infrastructure is still poorly
understood and it is important to
determine if the transpression was
also responsible for its structural,
magmatic and metamorphism features. The Hospitalet dome constitutes a good example for studying this
question and, more generally, to constrain a model of the Variscan deformation of the Pyrenees. Finally, this
study improves the understanding of
the thermal and tectonic histories of
gneiss dome formed in transpressional
regime.
Geological setting
The Hospitalet dome, located in the
Axial Zone of the Pyrenees (Fig. 1a)
between the Mérens and the Soldeu
thrust faults (Fig. 1b), consists of an
orthogneissic and granitic core partly
covered by Upper Proterozoic to
Lower Ordovician metapelites which
underwent a HT-LP metamorphism
under amphibolite facies conditions
(Barnolas and Chiron, 1996; Mezger,
2005). The orthogneisses, derived
from a thick (>2500 m) monzogranitic concordant massif (van den
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(a)
(b)
Fig. 1 Geological maps showing (a) the Variscan formations of the Pyrenees (NPF = North Pyrenean Fault); (b) the structures
and kinematics of the Aston-Hospitalet area (from Barnolas and Chiron, 1996; modified). Structures in the plutons are from
Gleizes et al. (1991) for Bassiès, Bouchez and Gleizes (1995) for Mont-Louis, Leblanc et al. (1996) for Trois-Seigneurs, Auréjac
et al.(2004) for Quérigut. Structures in the country rocks are from Gleizes (1992) for Mont-Louis, Leblanc et al. (1996) for TroisSeigneurs, Evans et al. (1997) for Bassiès, Auréjac et al.(2004) for Quérigut, Mezger (2005) for the Western Aston massif. U-Pb
zircon datings of the plutons are from Paquette et al. (1997) for Bassiès, Roberts et al. (2000) for Quérigut and Maurel et al. (2004)
for Mont-Louis.
Eeckhout, 1986), are similar to the
Canigou orthogneisses (Fig. 1), which
are dated at 475 ± 10 Ma (Deloule
et al., 2002) and interpreted as a
laccolith (Barbey et al., 2001). The
Hospitalet orthogneisses are intruded
by different types of granitoids
(Fig. 2): two kilometre-scale bodies
and smaller lenses of Variscan peraluminous granite in the eastern part of
the dome, and lenses of calc-alkaline
granitoids similar to those forming the
large plutons of the Pyrenees in the
southern border of the dome.
Around the dome (Fig. 1b), most of
the formations belong to the superstructure, which records an important
deformation in a dextral transpressive
446
regime (see references in Fig. 1b caption). The metasediments are characterized by EW-trending upright folds
with horizontal axes, a vertical cleavage and a horizontal stretching lineation with a systematically dextral
component. The plutons display
S-sigmoids of foliations and lineations, bands of solid-state deformation corresponding to reverse dextral
movements and assymetrical neutral
points of foliation.
Structural data
This study is based on 180 sampling
sites in the gneissic core (Fig. 2a).
Fabrics were measured either in the
field or using AMS measurements
(Borradaile
and
Henry,
1997;
Bouchez, 1997, 2000) at places where
no linear structure could be observed,
especially in the granitic rocks (89
sites). Microstructural and kinematic
analyses were realized out of 59 XZ
thin sections (X = stretching lineation; Z = foliation pole) among
which 40 yielded well defined senses
of shear.
The orthogneisses show an intense
and pervasive HT deformation with
microstructures characterized by
chessboard quartz subgrains. The fabric is generally rather prolate with a
conspicuous
stretching
lineation
except in the northern and southern
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Y. Denèle et al. • The Hospitalet gneiss dome revisited
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Fig. 2 (a) Structural and kinematics map of the Hospitalet dome (stereonets: Schmidt lower hemisphere, contour intervals 1%);
AAÕ, BBÕ, CCÕ and DDÕ: cross-sections of Fig. 3. (b) Stereographic representation demonstrating the existence of a rotation axis
connecting the structures of the normal and the overturned parts of the southern limb of the dome.
limbs of the dome where the fabric
becomes rather oblate with a faint
lineation. The foliation planes outline
a south-verging, overturned, kilometre-scale fold. Along an E–W crosssection (Fig. 3), the foliations, horizontal to the west, steepen to the east
up to 30 in the eastern periclinal
end of the dome. Therefore, foliation
trajectories outline a half-dome with a
mean axis at 79 ⁄ 16 (stereoplot in
Fig. 2). Mineral lineations shallowly
plunge (<10) to the east in the
western part and progressively steepen
up to 30 at the eastern end. The
deformation is non-coaxial with systematic top-to-the-east displacements
(27 thin sections) except in the southern overturned limb which displays
top-to-the-SW displacements (nine
thin sections).
The peraluminous granites display
HT solid-state deformation microstructures, and fabrics mostly subparallel to those of the surrounding
orthogneisses characterized by folia 2007 Blackwell Publishing Ltd
tion poles forming a girdle around an
axis at 90 ⁄ 24 and lineations concentrating at 97 ⁄ 24 (Fig. 2).
The calc-alkaline granitoids (monzogranite, granodiorite and gabbrodiorite) display submagmatic fabrics
(Fig. 4a) with a mean foliation at
94S57 and a mean lineation at
99 ⁄ 8. These fabrics are oblique with
respect to the pervasive structures of
the orthogneisses, which belong to
the overturned part of the southern
limb, with a mean foliation at
95N70 and a mean lineation at
67 ⁄ 48 (Fig. 4b). In addition, localized bands of mylonites are observed
in both the granitoids and the orthogneisses with a mean foliation at
104N63 and a mean lineation at
317 ⁄ 46 (Fig. 4e). In these mylonites,
the feldspar porphyroclasts are outlined by S and C planes where biotite
mica-fishes attest to MT conditions of
recrystallization (Fig. 4c,d). This
deformation is strongly non-coaxial
with reverse dextral movements, i.e.
upward displacement of the northern
compartment.
Discussion
Succession of tectonic and magmatic
events in the Hospitalet dome
The HT pervasive lineations measured
both in the orthogneisses and the
peraluminous granites are subparallel
to the axis of the southward overturned megafold. This could be
interpreted as contemporaneous structures. Nevertheless, the following facts
show that the lineation pre-date the
megafold: (i) the peraluminous granitic sills, whose emplacement is coeval
with the formation of the pervasive
lineations, are bent by the megafold
and (ii) there is a mean axis of rotation
oriented at 72 ⁄ 14 (Fig. 2b) that
allows foliations and lineations to
pass, through a clockwise rotation of
62 looking down the rotation axis,
from a top-to-the-east sense of shear in
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Fig. 3 3D overview of the structure and kinematics of the Hospitalet dome from the four cross sections located in Fig. 2a.
the normal part of the southern limb
to a top-to-the-SW sense of shear in
the inverted part of this limb. Moreover, arguments from other domes of
the central Pyrenees confirm that the
HT lineations pre-date the folding. In
the Aston massif, similar lineations
with a top-to-the-east kinematics display a 15–20 obliquity with respect
to axes of EW-trending megafolds
around which these lineations are
rolled up (Fig. 1b). In the Bossòst
dome (Fig. 1a), Mezger and Passchier
(2003) have evidenced NW–SE trending lineations with top-to-the-SE kinematics coeval with the emplacement of
a peraluminous granite. These lineations are also oblique (40) and
rolled up around an E–W megafold
axis.
Thus, the succession of events that
led to the formation of the Hospitalet
dome can be depicted as follows:
1 development of the HT pervasive
deformation affecting the ortho448
gneisses, more-or-less coeval with
the emplacement and the HT solidstate deformation of the peraluminous granites;
2 formation of the south verging
overturned megafold;
3 intrusion of the calc-alkaline granitoids in the inverted limb of the
southern border of the dome;
4 formation of the mylonites with
reverse dextral kinematics developed at medium temperature
(c. 500 C) mostly in and around
the calc-alkaline granitoids. These
mylonites resemble those that
affect the granitoids of the Mérens
shear zone (Saillant, 1982; Carreras and Cirés, 1986). Although
not dated, this mylonitic event is
obviously Variscan in age because
temperature in the cores of the
Aston and Hospitalet domes has
not exceeded 350 C since the end
of the Variscan times (Maurel
et al., 2004).
Regime of the E–W stretching
deformation
In order to constrain the Variscan
evolution of the Pyrenees, it is necessary to determine which phase and
regime of deformation the EW-trending lineations of the first event
observed in the Hospitalet must be
ascribed to. These lineations were
attributed to a late extension by van
den Eeckhout and Zwart (1988) and
Vissers (1992). In the Bossòst dome
(Fig. 1a), Mezger and Passchier (2003)
have interpreted similar lineations,
with top-to-the-SE kinematics, as the
result of a crustal extension in an
overall compressional regime placed
in-between a thickening event and a
late compression.
In the Hospitalet dome, as in the
Bossòst dome, the HT lineations
developed early with respect to the
megafold, but there are no direct
arguments to relate the E–W-trending
lineations either to an extension or to
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Y. Denèle et al. • The Hospitalet gneiss dome revisited
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(a)
(b)
(c)
(d)
(e)
Fig. 4 Structural and kinematic section (in map view) of the southern overturned limb of the Hospitalet dome along the Ariège
valley. (see location on Fig. 2a). Stereonets: Schmidt lower hemisphere. Scale bar: 2 mm.
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Fig. 5 Schematic model showing the stages of the tectonic evolution of the Hospitalet Massif in a dextral transpressive regime:
(a) formation of the flat foliation and the E–W stretching lineation corresponding to an eastward directed lateral flow in the middle
crust during the beginning of the dextral transpression. The peraluminous granite emplacement is coeval with this stage and
induced a HT-LP metamorphism. (b) Development of the south verging E–W trending megafold inducing the formation of the
dome which corresponds to a large-scale chain-parallel folding of the infrastructure. In the superstructure, the folds developed
progressively and became upright and tight, and stretching lineations subparallel to the folds axes developed because of the dextral
component of the transpression. (c) By the end of the doming, the southern inverted limb of the dome became a transfer zone
for calc-alkaline magmas. ML corresponds to the Mont-Louis pluton whose U-Pb age is 305 ± 3 Ma (Maurel et al., 2004).
(d) Development of mylonitic bands with reverse dextral movements at the southern boundary of the dome between gneisses and
micaschists, and at the northern boundary corresponding to the Mérens shear zone.
a transpression. However, several
indirect arguments lead us to consider
that these lineations were formed during the late Variscan dextral transpressive phase: (i) the stretching
lineations observed in the Hospitalet
gneisses have the same orientation and
are kinematically compatible with the
E–W-trending lineations of the superstructure (Fig. 1b); (ii) in the nearby
Aston massif, the lineations have
comparable orientations from the
deepest migmatitic facies (Mezger,
2005) to the greenschist cover (Evans
et al., 1997), and the shear senses are
compatible with the transpressive
event whatever the foliation dips;
(iii) WNW-plunging lineations with a
top-to-the-ESE sense of shear can be
observed in other gneisses of the
Pyrenees, for instance in the Trois–
Seigneurs massif (Fig. 1a), from the
migmatitic gneisses to the greenschist
metapelites, where they were interpreted as resulting from the dextral
component of a transpressive phase
(Leblanc et al., 1996); (iv) sedimentological studies in the foreland basins
of the Pyrenees point to a structural
evolution in an overall compressive
regime during the Carboniferous time
between 340 and 305 Ma (Delvolvé
et al., 1998).
All these arguments strongly suggest that the infrastructure and the
superstructure were progressively
deformed in a transpressive regime,
without appreciable change in the
principal stress directions.
Lateral flow in the Variscan middle
crust and development of the
Hospitalet dome
The structural differences of the infrastructure and the superstructure attest
to a large-scale uncoupling between
these two levels during the transpression. In the amphibolite-facies orthogneissic middle crust intruded and
heated by numerous sills of granitoids,
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the highly ductile behaviour induced
the formation of the flat foliation and
the E–W stretching lineation that we
interpret as an eastward directed
lateral flow. In contrast, the rheologically distinct greenschist metapelites
of the superstructure acted as a moreor-less rigid block moving to the east.
This early stage of lateral flow in
the middle crust is illustrated on the
schematic model of the Fig. 5a. The
next tectonic stages of our model,
formation of the dome, upwelling of
calcalkaline magmas through a crustal
fault and development of mylonitic
bands localized along the southern
border of the dome, are detailed on
Fig. 5b–d and corresponding captions.
This view of the dynamics of the
Variscan crust of the Pyrenees is
consistent with recent studies of hot
continental lithospheres submitted to
compression, either naturally (Gapais
et al., 2005) or experimentally (Cagnard et al., 2006). Such studies demonstrate that regions characterized by
flat foliations, as commonly observed
in Precambrian orogens, correspond
to zones of oblique convergence where
the middle crust underwent a lateral
flow in a direction showing an important angle with respect to the direction
of convergence. In such systems, the
competition between crustal thickening fed by magmatic injections and
lateral flow of the ductile crust,
allowed the development of shallowly
dipping fabrics with stretching directions parallel to the range. The formation of this type of structure is not
specific of the Precambrian lithosphere but, more generally, of lithospheres affected by a high thermal
gradient (Cagnard et al., 2006) as
observed in the Pyrenees.
Conclusion
This study leads to a new model where
the late Variscan evolution of both the
infrastructure and superstructure of
the Pyrenees appears as a progressive
deformation in a dextral transpressional regime. This regime induced an
uncoupling of the hot middle crust
recording first a lateral flow and then
a large folding, and of the colder
upper crust recording upright folding.
Our model complements the previous
models in compression (Carreras and
Capella, 1994), but is incompatible
with the models based on an early
(Soula et al., 1986) or a late (van den
Eeckhout and Zwart, 1988) extension
to explain the HT-LP metamorphism.
An important consequence of our
study is that the HT-LP metamorphism was at least partly caused by
the emplacement of the peraluminous
granites, and therefore pre-dated the
doming. Our results highlight the
relationships between dome formation, plutonism and HT-LP metamorphism of the middle crust in chains
submitted to transpression.
Acknowledgements
We thank M. de Saint-Blanquat and J.L.
Bouchez for constructive discussions, K.
Benn and two anonymous reviewers for
their detailed reviews, D. Gill for proofreading the manuscript and F. de Parseval,
J.F. Mena for technical assistance.
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