Many large-scale dynamic processes, from continental rifting to plate subduction, are intimately linked to metamorphic
reactions. This close relation between geodynamic processes and metamorphic reactions is, in spite of
appearances, yet poorly understood. For example, during extension processes, rocks will be exposed to important
temperature, pressures and stress changes. Meanwhile less attention has been paid to other important aspects of
the metamorphic processes. When reacting rocks expand and contract, density and volume changes will set up in
the surrounding material.
While several tectonic models are proposed to explain the formation of extensive basins and passive margins (
simple shear detachment mantle exhumation .... ) a single thermal model (McKenzie , 1978), as a dogma, is used
to understanding and modeling the formation and evolution of sedimentary basins . This model is based on the
assumption that the extension is only by pure shear and it is instantaneous. Under this approach, the sedimentary
deposits occur in two stages. i) A short step , 1 to 10 Ma , controlled by tectonics. ii) A longer step , at least 50 Ma
as a result of the thermal evolution of the lithosphere.
However, most stratigraphic data indicate that less thermal model can account for documented vertical movements.
The study of the thermal evolution , coupled with other tectonic models , and its consequences have never been
studied in detail , although the differences may be significant and it is clear that the petrological changes associated
with changes in temperature conditions , influence changes reliefs.
In addition, it seems that the relationship between basin formation and thermal evolution is not always the same:
- Sometimes the temperature rise above 50 to 100 Ma tectonic extension. In the Alps, a significant rise in geothermal
gradient Permo -Triassic followed by a cold extension , leading to the opening of the Ligurian- Piedmont
ocean, from the Middle Jurassic .
- Other examples show that temperature changes are synchronous with basin formation . For example, extensive
ponds Cretaceous North Pyrenean clearly indicate that the cooking of contemporary sediment deposit.
In the light of new models, we discuss the consequences of the formation of LP-granulites during rifting on deformation
and the subsidence processes.
We present an overview of the Mid-Cretaceous hyper-extended rift system
exposed in the northern part of the Pyrenean mountain range. Its
inversion during the Pyrenean orogeny allows for precious observations
of the deep-seated processes occurring at the foot of distal margins.
Some peculiar aspects characterizing the pre-Alpine hyper-extended domain
are examined throughout the 400 km long suture that is fossilized
in the North Pyrenean Zone (NPZ). The high temperature/low pressure
(HT/LP) thermal imprint of the extensional event is studied thanks
to a dataset of more than hundred peak temperature measurements
by Raman spectroscopy of the carbonaceous material (RSCM). The
pre- and syn-rift metasediments are characterized by an intense, synmetamorphic
ductile deformation. Focusing on several key-localities
of the NPZ, we examine the emplacement conditions of the peridotites
in response to this extreme crustal thinning. Geological evidences
such as the occurrence of peridotite bodies directly underlying metamorphic
pre-rift sediments indicate an early attenuation of the rifted continental
crust. Moreover, syn-rift, Albian-Cenomanian flysch sequences
were deposited synchronously with the syn-metamorphic ductile deformation
of the pre-rift sequences. All along the Internal Metamorphic
Zone (IMZ), the base of the flysch deposits also recorded the HT tectonic
event. Such a synchronicity between geological events, which are
generally separated in time, is not common in mountain belts. We examine
how tectonics, metamorphism and sedimentation may be active
in a single basin during extension. We propose an original mechanism
for the evolution of the basins involving continuous basal extraction of
the pre-rift metamorphic sediments. This early HT deformation event
relates to the « phase ante-Cenomanienne » described by Pyrenean geologists
since 1930.
