Geological patterns of the pre-Mesozoic substratum of the Paris Basin revealed by recent regional geophysical data
Résumé
The northwestern basement of France experienced at least 3 orogenic cycles: the Paleoproterozoic Icartian cycle (2000 Ma) still poorly documented, the Cadomian orogeny (760-540 Ma), and the poly-orogenic Variscan one (500-300 Ma). It is now recognized that these cycles are punctuated by sequences of compressional and extensional tectonic episodes. However, the general geodynamic setting, the succession of stress fields as well as a detailed chronology of these episodes are still under debate. In the field, it is often difficult to distinguish between neoformed structures from inherited ones, as each cycle partly reactivated earlier structures. Moreover, the Cadomian and Variscan structures are partly hidden underneath the Mesozoic sedimentary cover of the Paris Basin, which is itself was structured during the Mesozoic and Cenozoic eras. Compilation, processing, and interpretation of new aeromagnetic surveys combined with up-to-date gravity data of a large western part of the Paris Basin offer an unprecedented insight into the Paris Basin pre-Mesozoic buried substratum. Running on from the geology of the Armorican Massif, our first observations reveal, for instance, the continuity of late Cadomian post-orogenic plutons, and Cambro-Ordovician magmatism on a large part of the eastern extension of the buried north Armorican domain. Another outstanding structural pattern is represented by a series of regional NW-SE striking faults which appear to have played a significant role in the opening of the late-Variscan Permian basins, and also probably during the Mesozoic, as a result of the opening of the North Atlantic Ocean. These preliminary observations complemented by structural and petrophysical analyzes, seismic data, as well as deep borehole information, provide new insights on the lithological and structural nature of the pre-Mesozoic rocks, and their structural history at the junction between the Armorican Massif, Massif Central, and eastern part of Paris Basin.