The Internal Metamorphic Zone of Pyrenees: an allochtonous unit ?
Abstract
The localization and strike of compressional structures developed in the Pyrenees during the Iberian-Eurasian
collision are strongly controlled by structures inherited from earlier tectonic events. This structural inheritance
results in the first place from the Variscan orogeny, then from the Cretaceous rifting episode. The consequences
of this extension are particularly well developed in the North Pyrenean Zone (NPZ), a relatively narrow domain
that focussed strong localization during subsequent shortening. The position of the Internal Metamorphic Zone
(IMZ) that makes the southern part of the NPZ is debated. The IMZ displays very specific structure and lithologies
(foliated marbles, sometimes brecciated, peridotites...) within a narrow stripe with a width limited to a few
kilometers, from the Agly massif in the east, all the way to the Spanish Basque massifs where it forms the "Nappe
des Marbres". These marbles result from the metamorphic transformation of the carbonate series of extensional
basins presumably of Jurassic to lower Cretaceous age. The HT-LP metamorphism reached temperatures of
550-600C in the IMZ and the Nappe des Marbres and surrounding formations, represented by the Paleozoic
basement or Mesozoic series are not exempt of metamorphism, with measured maximum temperatures up to
350C. The temperature contrasts measured within the NPZ and the deformation intensity contrasts imply late
post-metamorphic movements bringing the high-temperature and low-temperature domains into close contacts.
This observation and the geometrical relations observed in the field suggest the allochthony of the IMZ. We
present a new interpretation of the 3D geometry of the NPZ following this hypothesis and discuss the implications
for the overall geometry of the Pyrenees.