From rifting to mountain building: The Pyrenean Belt

Abstract

The Pyrenees are again under the spotlights! Today, anincreasing number of research groups are focusing on thisapparently simple double-verging orogeny. These recent(and ongoing) studies using a broad range of disciplinaryapproaches have greatly improved our understanding ofthe geological structure and tectonostratigraphic evolutionof the Pyrenees and sometimes also challenged wellestablishedmodels.

Full text

Foreword
From rifting to mountain building: The Pyrenean Belt
The Pyrenees are again under the spotlights! Today, an
increasing number of research groups are focusing on this
apparently simple double-verging orogeny. These recent
(and ongoing) studies using a broad range of disciplinary
approaches have greatly improved our understanding of
the geological structure and tectonostratigraphic evolution
of the Pyrenees and sometimes also challenged well-
established models.
Over the years, the Pyrenees have generated numerous
controversies due to their specific geological and geophys-
ical characteristics and their occurrence in a highly
debated plate kinematic framework. The uncertainties
on the size of their crustal root, along with the occurrence
of a HT–BP metamorphic belt and numerous bodies of
mantle peridotites made the interpretation of the initiation
and formation of this orogen particularly challenging. In
spite of these shortcomings, the Pyrenees have been used
as the main model for the studies of tectonic–sedimenta-
tion relationships, notably on their southern flank. As a
consequence, the northern flank and the central region, the
axial zone, have been little explored during the past
20 years.
Recent models proposing pre-orogenic mantle exhu-
mation have contributed to the renewing interest within
the geosciences community for the Pyrenean orogeny.
Also, the recent discovery of ongoing extension of
continental passive margins in remote deep-sea regions
requires, today, to better understand their on-land
analogues. This new research on the pre-orogenic evolu-
tion of the Pyrenees triggered a renewed interest in the
processes of tectonic inversion and tectonic wedge
development. For these reasons, the Pyrenees have become
the focus of a series of national and international research
projects (e.g., PYRTEC, PYROPE, PYRAMID, Orogen...).
In this thematic issue, we gather contributions from
various approaches and methods focusing on a wide
variety of topics, thus illustrating the renewed interest for
all the facets of this orogen. This volume stems from a
special session at the RST 2014 held in Pau (France), which
brought together the scientific communities from both
sides of the Pyrenees and where these new results were
discussed and debated for the first time. The articles of
this dossier dedicated to the Pyrenees are organized
according to the geological time scale from Present to
oldest periods.
Genti et al. (2016) use numerical modelling to evaluate
the role of a set of geophysical and topographic parameters
in the development of normal faults to the North of the
Central–Western Pyrenees and investigate the mecha-
nisms controlling seismicity in this area. They show that
seismicity is strongly related to flexural rebound induced
by surface processes.
The morphotectonic evolution of the Pyrenees during
the Cenozoic is studied by Monod et al. (2016), based on
the analysis of thick weathered horizons still preserved on
flat, high-elevation surfaces. Mapping these horizons
allows a 3D reconstruction of the ancient palaeosurface
postdating the Eocene–Oligocene denudation, thus out-
lining its post-orogenic character.
These high-elevation surfaces are also studied by Bosch
et al. (2016a), who test between two opposite interpreta-
tions regarding their evolution. The presence of a thick
crustal root at the base of the surface remnants and the
similar level of erosion in both the Central and Eastern
Pyrenees favour an interpretation where the rise of the
efficient base level of the chain induced the progressive
inhibition of erosion and the smoothing of the relief before
the Late Miocene.
Nivie
`re et al. (2016) use the cosmogenic nuclide
10
Be to
date an alluvial terrace in the foothills of the northwestern
Pyrenees. This terrace was abandoned at 18
2 kyr, more
recently than previously thought, thus leading to a renewed
interpretation of the geomorphological and climatic evolu-
tion of this region. In addition, dating suggests reactivation of
a major thrust in this tectonically active area of the western
Pyrenees.
Solo et al. (2016) measured magnetic fabric in
apparently undeformed Lower to Middle Miocene sedi-
ments of the Ebro basin and show the presence of a subtle
non-sedimentary magnetic fabric, which could be associ-
ated with a very weak north–south compressional
deformation of the same age.
C. R. Geoscience 348 (2016) 169–171
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10.1016/j.crte.2016.04.002
1631-0713/ß2016 Acade
´mie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).

New chrono- and lithostratigraphy for the central
Aquitaine foreland basin combined with subsidence
analyses are used by Rougier et al. (2016) to construct a
sequentially restored, 120-km-long cross section through
the North Pyrenean retrowedge. Two prograding cycles of
basin fill are identified, controlled by very modest tectonic
subsidence associated with gentle north-directed shorten-
ing in the Pyrenean retroforeland.
Roige
´et al. (2016) study the Eocene clastic systems of
the Jaca foreland basin (southern Pyrenees). They outline
changes in sediment composition during the tectonos-
tratigraphic evolution of the basin and show that the shift
from Middle Lutetian deep-marine conditions to a
Priabonian alluvial system records a main reorganization
in the active Pyrenean pro-wedge.
Bosch et al. (2016b) provide new apatite (U–Th)/He
(AHe), apatite fission track (AFT) and zircon (U–Th)/He
(ZHe) data to unravel the timing of exhumation and
thrusting in the western Axial Zone of the Pyrenees and the
adjacent North Pyrenean Zone including the Chaı
ˆnons
Be
´arnais, thus providing insights into the tectonic and
relief evolution of the chain. These data better constrain
the timing of motions along the main thrusts of the Axial
Zone.
Teixell et al. (2016) present a new crustal section of the
Pyrenees through the Jaca basin, the western Axial Zone
and the Chaı
ˆnons Be
´arnais. They consider feedbacks
between the mid-Cretaceous hyperextension postulated
recently and the Pyrenean inversion. Their reconstruction
allows identifying the ancient continental margins of the
Iberian and European plates and a suture domain where
mantle rocks have been exhumed at the foot of both
passive margins.
Saint Blanquat, (de) et al. (2016) provide new con-
straints on the modes of mantle exhumation in the pre-
orogenic Pyrenees by studying the peridotites located east
of the Lherz body. They reveal the petrographic variability
and the heterogeneity in the serpentinization degree of the
exposed mantle rocks. These results imply a mode of
exhumation, which favoured merging mantle fragments
from different structural levels. As in the Lherz area, these
bodies are indisputable witnesses of a hyper-extended
Cretaceous passive margin.
The mechanisms of mantle exhumation in the distal
portion of the palaeomargins are also investigated by
Corre et al. (2016) in the western North Pyrenean Zone.
New mapping shows that mantle rocks have been
exhumed during Albian–Cenomanian times and that
extreme crustal thinning and mantle exhumation oc-
curred along a detachment associated with the boudinage
of both the mantle rocks and the ductile continental
basement.
Based on structural analysis and Crystal Preferred
Orientations (CPO) measured in marbles of the Lherz
region, Lagabrielle et al. (2016) show that the prerift
sediments of the North Pyrenean Zone display an evolution
from ductile to brittle deformation under a very high
thermal gradient. They describe exhumation processes
ending with the sedimentary reworking of both the
deformed Mesozoic metasediments and the exhumed
ultramafic rocks.
The oxygen and carbon isotopic compositions of
quartz–calcite veins in the Boucheville basin are investi-
gated by Boulvais (2016) and show that the fluids that
circulated throughout the Boucheville Basin were gener-
ated within the basin itself. Therefore, these fluids are the
consequence, not the cause, of the Pyrenean HT–LP
Cretaceous metamorphism.
New sedimentological and structural data of the
Boucheville basin were collected by Chelalou et al.
(2016) and are used to produce synthetic stratigraphic
columns of different portions of the basin and to restore
selected cross-sections. They show the asymmetrical
geometry of the basin. Raman spectroscopy on carbona-
ceous material (RSCM) reveals homogeneity in the
temperatures between 500 8C and 600 8C.
Munoz et al. (2016) present the first detailed study of
the base-metal vein-type mineralizations exposed within
the Axial Zone metasediments. The Pb–Zn deposits are
located in Devonian terranes south of the Mesozoic Aulus
basin and are interpreted to have been emplaced under an
extensional setting by low-temperature NaCl–CaCl
2
brines. Isotopic
208
Pb/
204
Pb and
206
Pb/
204
Pb ratios acquired
on galena suggest an emplacement during the Middle–Late
Triassic and the Late Jurassic.
The post-Variscan evolution of the Anayet Basin is
investigated by Rodriguez-Me
´ndez et al. (2016). This
evolution is characterized by a trans-tensional regime,
which began at least in Stephanian times and lasted until
the Late Permian. During Middle Eocene times, the Alpine
Orogeny inverted and deformed the Anayet Basin.
Acknowledgments
We express our sincere thanks to all the reviewers who
have participated in evaluating the articles published in
this special issue. We thank He
´le
`ne Paquet for her support
throughout the edition of this thematic issue.
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Yves Lagabrielle
a,
*, Michel de Saint Blanquat
b
,
Marguerite Godard
c
a
Ge
´osciences Rennes, UMR CNRS 6118, OSUR, Universite
´
Rennes-1, 35042 Rennes cedex, France
b
Ge
´osciences Environnement Toulouse (GET), OMP, CNRS,
IRD, & Universite
´de Toulouse, 14, avenue E
´douard-Belin,
31400 Toulouse, France
c
Geosciences Montpellier, Universite
´de Montpellier, place
Euge
`ne-Bataillon, 34095 Montpellier cedex 6, France
*Corresponding author
E-mail addresses: yves.lagabrielle@univ-rennes1.fr
(Y. Lagabrielle)
michel.desaintblanquat@get.obs-mip.fr
(M. de Saint Blanquat)
Marguerite.Godard@umontpellier.fr
(M. Godard)
Foreword / C. R. Geoscience 348 (2016) 169–171
171