Extreme thinning of the continental crust and mantle exhumation at passive margins. Constraints from the Pyrenean analog
Abstract
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 anté-Cénomanienne » described by Pyrenean geologists
since 1930.