Architecture of ductile-type, hyper-extended passive margins: Geological constraints from the inverted Cretaceous basin of the North-Pyrenean Zone (‘Chaînons Béarnais’, Western Pyrenees)
Résumé
Sub-continental lithospheric mantle rocks are exhumed at the foot of magma-poor distal passive margins as a
response to extreme stretching of the continental crust during plate separation. Remnants of the Northern Iberian
paleo-passive margin are now exposed in the North-Pyrenean Zone (NPZ) and represent field analogues to study
the processes of continental crust thinning and subcontinental mantle exhumation. The NPZ results from the
inversion of basins opened between the Iberia and Europa plates during Albo-Cenomanian times. In the western
NPZ, the ‘Chaînons Béarnais’ ranges display a fold-and-thrust structure involving the Mesozoic sedimentary
cover, decoupled from its continental basement and associated with peridotite bodies in tectonic contact with
Palaeozoic basement lenses of small size. Continental extension developed under hot thermal conditions, as
demonstrated by the syn-metamorphic Cretaceous ductile deformation affecting both the crustal basement and the
allochthonous Mesozoic cover.
In this study, we present structural and geochemical data providing constraints to reconstruct the evolution of
the northern Iberia paleo-margin. Field work confirms that the pre-rift Mesozoic cover is intimately associated
to mantle rocks and to thin tectonic lenses of crustal basement. It also shows that the pre-rift cover was detached
from its bedrock at the Keuper evaporites level and was welded to mantle rocks during their exhumation at
the foot of the hyper-extended margin. The crust/mantle detachment fault is a major shear zone characterized
by anastomosed shear bands defining a plurimetric phacoidal fabric at the top of the serpentinized mantle. The
detachment is marked by a layer of metasomatic rocks, locally 20 meters thick, made of talc-chlorite-pyrite-rich
rocks that developped under greenschist facies conditions. Raman Spectroscopy on Carbonaceous Materials
(RSCM), performed on the Mesozoic cover reveal that the entire sedimentary pile underwent temperatures ranging
between 200C and 480C.
We show that: (i) at the site of mantle rocks exhumation, the boudinaged pre-rift sediments have undergone drastic
syn-metamorphic thinning with the genesis of a S0/S1 foliation and, (ii) the Paleozoic basement has been ductilely
deformed, into thin tectonic lenses that remained welded to the exhumed mantle rocks. Therefore the overall
crustal rheology appears dominated by shallow levels having a ductile behavior. This rheology is related to the
presence of a thick pre- and syn-rift decoupled cover acting as an efficient thermal blanket. This new geological
data set highlights important characteristics of ductile-type hyper-extended passive margin that cannot be obtained
from the study of seismic lines. Finally, we stress that studying field analogues represents a major tool to better
understand the mechanisms of extreme crustal thinning associated with mantle exhumation and their structural
inheritance during tectonic inversion.