Is the basin formation in the Pyrenees different from the Alps ?
Résumé
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 kind of
dogma, is used to understanding and modeling the formation and evolution of sedimentary basins. The study
of the thermal evolution, coupled with other tectonic models, and its consequences have never been really
studied in detail. Petrological changes, related to temperature evolution, modify rock density and then the
subsidence history of the basin.
Recent studies of continental passive margins collectively describe a great variety of processes accounting
for the extreme thinning of the continental crust. Among all the parameters that may act during crustal
stretching, the thermal state of the system and the temporal evolution of the heat distribution during thinning
appear of major importance.
Constrained by the new field data from the north Pyrenean basins about thermal evolution of early margin
formation, we explore effect of different thermal evolution on petrological changes and their consequences
on the geophysical signature of rifted zones associated to different thermal evolution and the consequences
on the subsidence of the basins. We will also present numerical models quantifying mineralogical and
physical changes inside the whole lithosphere during rifting processes. In the light of these models, we
discuss the consequences of different thermal evolution on the subsidence processes as well as on
gravimetric and seismic velocities signature of passive margins.
We are able to distinguish two types of margins according to their thermal evolution:
- An Alpine-type basin in which the temperature rise is 50 to 100 Ma older than the tectonic extension,
leading to the "cold" opening of the ocean.
- A Pyrenean type basin in which temperature changes are synchronous with basin formation, leading to a
crustal boudinage and to the formation of a “anomalous” geophysical layer at the OCT